13Soybean Oil

Earl G. Hammond, Lawrence A. Johnson, Caiping Su,Tong Wang, and Pamela J. White

Iowa State University

Ames, Iowa

1. INTRODUCTION

The amounts of soybeans and total vegetable oil crops have been rising for a

number of years. World production of soybeans in 2003 was estimated to be

184.49 million MT out of 317.89 million MT total for vegetable oil crops, making

soybeans the world’s largest oilseed crop, rivaled only by palm oil (1). The 2003

crop of soybeans was expected to yield 29.85 million MT of soybean oil out of a

total of 91.79 million MT of vegetable oil worldwide. The U.S. production of

soybean oil was estimated at 8.59 million MT for 2002, of which 7.86 million

MT was consumed domestically. During 2002–2003, Brazil produced 4.90 million

MT and Argentina 4.12 million MT of soybean oil (2). The U.S. price of crude

soybean oil has varied from $0.24/kg to $0.62/kg over the past 5 years with the

lower prices being more recent (1).

Soybeans owe their dominance of the oilseed market to the value of their protein,

which is much greater than that of other oilseeds. Of the oilseed meals produced in

2003, 129.58 million MT out of a total of 185.69 milllion MT was soybean meal

(1). Of the money made on extracting soybeans, the meal accounted for between

51% and 76% of the total in the last 10 years. Soybean oil of typical composition

performs well as a salad oil, but it is usually hydrogenated for use as a margarine

stock or frying oil. Soybean oil’s stability to oxidation also is limited by its content

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

577

of linolenic acid. Recent decades have witnessed numerous attempts to manipulate

the fatty acid composition of soybean oil to help it compete better in various uses,

but the cost of growing, segregating, and testing special varieties and resistance to

genetically modified oils have limited the appeal of these altered varieties.

2. COMPOSITION OF SOYBEANS

Table 1 shows the average composition of soybean seed (oil, protein, and some

amino acids) grown in the United States during recent years (3). Aside from varietal

differences, the composition is affected by various geographic/environmental fac-

tors. According to Hurburgh (5), ‘‘oil is much more variable than protein from year-

to-year. States most distant from the center of the Corn Belt (probably those with

the greatest weather extremes) experience the most variability in composition.’’

Table 2 lists some of the environmental and cultivation practices that have an

TABLE 1. Typical Composition (wt% � std. dev.)

of Soybeans (dry weight basis) (3).

Protein 40.69 � 0.51

Lysine 2.56 � 0.11

Methionine 0.57 � 0.03

Cysteine 0.72 � 0.06

Tryptophane 0.52 � 0.05

Threonine 1.54 � 0.07

Oil 21.38 � 0.64

Ash 4.56 � 0.34 (4)

Carbohydrate 29.4 � 3.29 (4)

TABLE 2. Soybean Protein and Oil Responses to Various

Environmental and Cultivation Practices (3).

Variable Protein Oil

High temperatures ?a

Early season drought – þLate season drought þ –

Early frost/cold temperature – –b

Additional soil nitrogen þ –

Increased fertility (P,S) þ þLate planting þ –

Insect defoliation – –

Insect depodding þ ?

Rhizobium inoculation þ –

a? ¼ inconclusive; þ¼ increase; �¼ decrease.bOil is reduced because of refining loss to remove chlorophyll.

578 SOYBEAN OIL

observable effect on soybean protein and oil percentages. Maestri et al. (6) grew

soybean cultivars in several regions of Argentina and concluded that the protein

and oil contents were positively correlated with altitude. Protein was negatively cor-

related with latitude and precipitation, and oil was negatively correlated with tem-

perature and precipitation. Oil content in soybeans tends to be negatively correlated

with protein, but breeding soybeans for high protein while maintaining oil content

has been a priority of the U.S. soybean producers, and some progress has been

achieved (7, 8). The variety Prolina reportedly produces 22.7% oil and 45.5% pro-

tein on a dry weight basis. There also has been interest in reducing the oligosac-

charides that cause flatulence and reduce the digestibility and nutritive value of

soybeans.

Isoflavones are minor constituents of soybeans whose consumption is believed to

have beneficial effects (9–11). The benefits of isoflavones have encouraged the

direct consumption of soy protein in the United States. The concentration of

isoflavones changes with variety and growing conditions and has been reported

to be 1.2–2.5 mg/g in U.S. beans (9), 0.5–2.3 mg/g in Korean beans (10), and

0.2–3.5 mg/g in Japanese beans (11).

Table 3 shows the typical composition of the lipid phase of soybeans. Triacyl-

glycerols are the primary component. The 3.7% phospholipids content in the soy

beans is higher than that usually found in hexane-extracted oil, which is typically

TABLE 3. Typical Composition of Crude Soybean Oil.

Component % Std. Dev.

Triacylglycerol 94.4a 1.4

Phospholipids 3.7b 1.2 (12)

Unsaponifable matter (13–15) 1.3–1.6

Sterolsc (16) 0.236 0.053

Campesterol 0.059 0.018

Stigmasterol 0.054 0.013

ß-Sitosterol 0.123 0.027

�5-Avenasterol (17) 0.005

�7-stigmasterol (17) 0.005

�7-avenasterol (17) 0.002

Tocopherols (16) 0.123 0.040

Alpha 0.0093 0.0044

Beta 0.0018 0.0028

Gamma 0.0834 0.036

Delta 0.029 0.010

Hydrocarbons (14, 15) 0.38

Free fatty acids (18) 0.3–0.7

Trace metals (18) ppm

Iron 1–3

Copper 0.03–0.05

aBy difference.bBased on 23 varieties chosen to represent a wide fatty acid composition.cBased on 13 varieties chosen to represent a wide range of composition.

COMPOSITION OF SOYBEANS 579

1.85–2.75% (19). Of the unsaponifiable matter of soybean oil, typically about

1.45% of the oil, 16% is sterols, 8.5% is tocopherols, and 26% is hydrocarbons.

The remaining 50% of the unsaponifiable matter consists of other minor and uni-

dentified products. The sterols are about 52% ß-sitosterol, 25% campesterol, and

23% stigmasterol. Maestrl et al. (4) reported similar proportions on the three major

sterols but also reported 5.4% � 0.82 of �5-avenasterol, 3.8% � 0.76 of �7-

stigmasterol, 1.3% � 0.42 of �7-avenasterol, and traces of cholesterol in the total

sterols. The tocopherols are about 7.6% a, 1.5% b, 67.8% g, and 23.6% d. There is

considerable variation among plant varieties in the amounts and proportions of

molecular species of sterols and tocopherols (4, 16, 20). Vlahakis and Hazebroeck

(21) also have investigated the effects of planting locations and temperature on the

sterol and total tocopherol contents of a number of soybean varieties. They found

that growth temperature can cause as much as a 2.5-fold difference in sterol

content, with higher temperatures favoring higher amounts of sterols, increasing

the campesterol/ß-sitosterol ratio and decreasing total tocopherols. McCord et al.

(22) examined a number of soybean lines with low and normal contents of linoleate.

The low-linolenate lines averaged about 6% lower in tocopherol than the high-

linolenate lines, but some reduced-linolenate lines were not significantly different

from normal-linolenate lines in tocopherols. The a- and g-tocopherols tend to be

concentrated in the soybean germ, whereas d-tocopherol is concentrated in the

endosperm (23). The hydrocarbon fraction of soybeans consists of n-hydrocarbons

of chain length 14 to 33 plus squalene and small amounts of hexahydrofarnesyla-

cetone (14, 15). The squalene content is reported to be about 0.014% of the oil.

There seems to be considerable variation in the distribution of the hydrocarbon

chain lengths with plant variety, judging from the two examples in the literature.

Free fatty acids vary considerably with the age and soundness of the beans

but are seldom lower than the 0.1% of the crude oil (18). Damaged beans can

contain 1–8% free fatty acid as well as elevated iron and copper, 3–7 ppm and

0.08–0.18 ppm, respectively.

Refined oil usually retains little phospholipid, but damaged beans can have a sig-

nificant content of phosphatidic acid, and the amount of iron in the oil is related to

the amount of phosphorus (24). During deodorization, considerable amounts of

sterol and tocopherol may be removed from the oil. The proportion removed

depends on deodorization conditions, but a 30% to 40% decrease is not unusual

(25). Much of the hydrocarbons and squalene are lost to the deodorizer distillate

as well. Free fatty acids in fully refined oil are required to be <0.05% and unsapo-

nifiable matter <1.5% (26).

Table 4 shows the percentages and standard deviations of the methyl esters of 21

typical refined soybean oil samples. This composition is typical of most presently

commercial soybean varieties. The typical composition probably has been selected

through plant breeding because it is associated with good yield and other important

agronomic properties. It has been possible to change the composition of soybean oil

considerably, and Table 4 also shows the ranges of percentages that have been

reported for each methyl ester. Many of the changes in composition can be achieved

without great losses in yield or oil content, but lines with high or low palmitate

580 SOYBEAN OIL

percentages tend to have reduced oil contents (33, 40, 41). Lines with high stearate

percentages suffer from low yields and sporadically from poor germination. Wang

et al. (42) tested lines with elevated palmitate or stearate in a number of tests of

germination and seedling vigor at three temperatures and found that, although

the high-saturate seed did well in these tests, vigor was negatively correlated

with saturate percentage. Most of the changes reported in Table 4 were attained

by traditional plant breeding or use of mutagenic agents. The high-oleic mutant

is an exception and was attained by direct genetic manipulation (37). High-oleate

lines developed by traditional plant breeding have been reported, but their oleate

percentage varies widely with growth environment, which limits their commercial

value (38).

The fatty acid composition of soybean oil changes considerably with maturity

and with seed oil deposition (15, 35, 43, 44). In typical soybean triacylglycerols,

the palmitate and linolenate tend to decrease with maturity, whereas linoleate

increases. Oleate tends to increase to a maximum and then decline slightly. Soy-

beans selected for atypical fatty acid compositions show quite different patterns

of change with maturity from typical soybeans.

Seitz (31) and Wesolowski (32) measured the saponification and iodine values of

a number of samples from various geographic locations, and their ranges and typi-

cal values are shown in Table 4.

Harp and Hammond (45) explored the stereospecific distribution of acyl groups

on the three positions of the glycerol molecule for soybean triacylglycerols with a

wide range in fatty acid composition. They found that the amount of an acyl group

TABLE 4. The Averages and Standard Deviations of Methyl Esters from Typical Soybean

Oils and the Range Reported for each Methyl Ester.

Methyl Ester Typical Value %a (27) Range Achieved %

Myristate 0.04 � 0.5 (27) trace–0.03 (4)

Palmitate 10.57 � 0.43 (27) 3.2–26.4 (33, 34)

Palmitoleate 0.02 � 0.04 (27) trace–0.7 (29)

Stearate 4.09 � 0.34 (27) 2.6–32.6 (33, 35)

Oleate 22.98 � 2.01 (27) 8.6–79.0 (36, 37)

Linoleate 54.51 � 1.54 (27) 35.2–64.8 (35–37)

Linolenate 7.23 � 0.78 (27) 1.7–19.0 (38, 39)

Arachidate 0.33 � 0.14 (27) trace–0.7 (28)

Gondoate 0.18 (28) trace–0.6 (4)

Behenate 0.25 � 0.20 (27) trace–1.0 (4)

Lignocerate 0.1 (29) —

Furanoid IIb 0.014 � 0.0086 (30) 0.0033–0.0290 (30)

Furanoid IIIc 0.015 � 0.0076 (30) 0.0084–0.0272 (30)

Saponification Value 190.4 (31, 32) 188.5–201.6 (31, 32)

Iodine Value 132.7(31, 32) 114.0–138.5 (31, 32)

aBased on 21 commercial samples.b10,13-epoxy-11,12-dimethyloctadeca-10,12-dienoate.c12,15-epoxy-13,14-dimethyloctadeca-12,14-dienoate.

COMPOSITION OF SOYBEANS 581

on a particular position was linearly related to the amount of that acyl group in the

whole triacylglycerol. At low concentrations of palmitate, stearate, oleate, and

linoleate in the total triacylglycerols, the amounts on the sn-1 > sn-3, but the

reverse was true at higher total concentrations. Palmitate and stearate were confined

to the sn-1 and sn-3 positions, whereas the oleate concentration was similar on all

three positions. Linoleate concentrations at the sn-2 position were generally greater

than those at the sn-1 and sn-3 positions, but the amount of linoleate on the sn-2

position seemed to be strongly and negatively correlated with the amounts of satu-

rates on the sn-1 and sn-3 positions. Plots of linolenate concentrations at particular

positions versus the concentrations in the whole triacylglycerol showed consider-

ably more scatter than plots for the other acyl groups, but they generally showed

the amount of linolenate on sn-2 > sn-1 > sn-3. The saturate percentages also

seemed to influence the amounts of linolenate on the sn-2 position positively and

on the sn-3 position negatively. Table 5 shows the stereospecific distribution of typi-

cal soybean triacylglycerols.

Theoretically, stereospecific data can be used to predict the acylglycerol struc-

ture using the 1-random-2-random-3-random distribution theory (47), if one

assumes the fatty acid composition of the three glycerol positions are individually

controlled but that the combinations of the three positions are random. However,

the change in fatty acid composition with maturity, described in the previous

paragraph, shows that the triacylglycerol composition is unlikely to be truly random

in the combination of the three glycerol positions. In addition, soybeans from the

same plant or pod can have slightly different acyl group compositions, so pooled oil

from many seeds and plants is unlikely to be exactly random in its glycerol position

combinations. Thus, such a calculation can lead only to approximate compositions.

Neff et al. (48, 49) partially separated the triacylglycerols of soybean oils

with a wide range of fatty acid compositions using high-performance liquid

TABLE 5. Stereospecific Distribution of Acyl Groups in the Triacylglycerols, Phospha-

tidylcholine, Phosphatidylethanolamine, and Phosphatidylinositol of a Typical Soybean

(45, 46).

Compound/Acyl group 16:0 18:0 18:1 18:2 18:3

Triacylglycerols 11.8 4.6 29.4 47.1 7.2

sn-1 19.3 7.5 25.4 39.6 7.8

sn-2 2.9 0.8 27.9 61.1 7.4

sn-3 14.9 6.4 34.8 38.9 5.0

Phosphatidylcholine 11.2 11.9 8.6 58.6 9.9

sn-1 16.0 22.6 7.3 38.3 6.0

sn-2 4.1 3.7 9.9 71.1 11.2

Phosphatidylethanolamine 16.0 8.3 6.8 57.3 11.7

sn-1 28.5 17.1 5.0 42.4 7.1

sn-2 3.5 2.4 8.7 73.8 11.7

Phosphatidylinositol 22.2 19.3 6.1 43.4 9.3

sn-1 45.1 35.2 5.3 17.1 2.4

sn-2 4.6 3.4 5.9 70.9 15.3

582 SOYBEAN OIL

chromatography, and their results are shown in Table 6. These data show how the

amounts of the triacylglycerol species change with fatty acid composition.

The primary phosphatides of soybean oil are phosphatidylcholine, phosphatidyl-

ethanolamine, and phosphotidylinositol, which generally make up 55.3%, 26.3%,

and 18.4% of the total phosphatides, respectively (50). The stereospecific distribu-

tion of the acyl groups in these phospholipids for a typical soybean lipid is shown in

Table 5. In all the phospholipids, the saturated acyl groups are concentrated in the

TABLE 6. Acyl and Triacylglycerol Composition in mol% of Soybean Oils Having a Wide

Range of Fatty Acyl Compositions (48, 49).

Sample Number

——————————————————————————————

Acyl group 1 2 3 4 5

Palmitate (P) 3.9 21.4 23.6 28.2 8.5

Stearate (S) 3.3 3.3 19.0 3.9 26.5

Oleate (O) 28.5 23.6 9.3 13.9 18.0

Linoleate (L) 61.8 49.0 38.0 43.8 38.9

Linolenate (Ln) 2.5 2.7 10.0 10.2 8.2

Triacylglycerol Species

LnLL 2.0 1.2 1.4 3.2 2.6

LnLnO — 0.1 0.1 0.2 0.1

LnLnP — — 0.4 0.6 0.1

LLL 30.0 11.5 3.7 9.6 6.5

LnLO 1.7 1.5 0.8 2.0 1.9

LnLP 0.4 1.7 6.9 9.4 2.2

LLO 26.9 14.4 3.6 8.7 7.1

LnOO 0.4 0.5 0.1 0.3 0.3

LLP 6.4 20.7 17.5 21.4 11.6

LnOP 0.1 1.0 1.4 2.1 0.4

LnPP 0.1 — 1.5 2.0 0.1

LOO 13.9 7.3 1.3 3.1 2.5

LLS 3.6 2.6 7.7 2.0 13.0

LOP 3.7 16.3 7.4 12.2 6.4

PLP 0.8 8.6 13.8 14.8 2.0

OOO 4.6 2.1 1.0 0.8 1.1

LOS 2.6 2.3 3.9 1.3 11.8

POO 0.9 3.2 0.6 1.0 0.5

SLP 0.8 2.2 — — —

LnSS — — 16.0 3.0 8.8

POP 0.2 1.7 1.2 1.5 0.3

PPP — — 0.1 — —

SOO 0.7 0.5 0.3 0.2 2.1

SLS 0.3 0.2 6.5 0.3 12.3

SOP 0.1 0.4 1.3 0.2 1.4

PPP — — 0.6 — 0.6

SOS — — 0.6 — 3.4

PSS — — 0.2 — 0.1

SSS — — — — 0.1

COMPOSITION OF SOYBEANS 583

sn-1 position and the unsaturated acyl groups, especially linoleate, on the sn-2 posi-

tion. Phosphatidylinositol tends to be richest in palmitate and stearate, whereas

phosphatidylcholine has the least palmitate. Wang et al. (51) reported the stereospe-

cific distribution of the acyl groups in the various phospholipids types and the

amounts of particular acyl combinations for soybean lipids with a wide variety

of fatty acid compositions. Some phosphatidic acid and lysophospholipids also

may be present as a result of hydrolysis of the phospholipids (52). The amounts

of the hydrolytic products usually increase with age and damage to the beans (53).

Soybeans also contain 170 � 47 ppm of cerebrosides in which the sugar is glu-

cose and the chief fatty acid is 2-hydroxy palmitic acid (54). Traces of ceramides

also are present. These are believed to play a role in cell signaling in the soybean

plant.

Crude soybean oil contains about 1.9 ppm of Vitamin K1 or phylloquinone (55).

This vitamin plays a role in blood coagulation and bone metabolism. During refin-

ing, some Vitamin K1 may be lost (56, 57), especially during deodorization. Hydro-

genation of the fat converts some of the Vitamin K1 to 20,30-dihydrovitamin K1 (58).

Wilson et al. (7, 8, 39, 59) have reviewed the genetic control of fatty acid bio-

synthesis in soybeans and discussed the advantages of soybean oil with special

compositions. Oil with reduced palmitate is available presently in a limited market.

The commercial introduction of low-linolenate soybeans has been inhibited by the

availability of corn oil, which has a composition like very low-linolenate soybean

oil. The price differential between these oils often is smaller than the costs of con-

tract growing, segregating, and processing low-linolenate soybeans. High-oleate

soybean oil is stable under frying conditions, but this trait alters the flavor of the

fried products (60). The acceptance of high-oleate soybean oil also suffers from

public concern about the growth and consumption of plants produced by direct

genetic modification.

There are small amounts of two acyl groups containing furan rings in soybeans

(30). These oils are reported to be the sources of the odorous compound 3-methyl-

2,4-dione by photo-oxidation (61), but Kao et al. (62) were not able to find differ-

ences in flavor of photo-oxidized varieties with high and low content of these acyl

groups.

3. PHYSICAL PROPERTIES OF SOYBEAN OIL

The physical properties of fatty acids vary with their chain length, unsaturation, and

other substituents and change with temperature. Numerous attempts have been

made to develop equations that will predict these properties. Soybean oil’s proper-

ties should reflect its constituents and, especially, its fatty acid composition, and

physical properties have frequently been measured for typical soybean oils, but

there have been fewer measurements of soybean oils with modified fatty acid com-

positions.

Table 7 shows the values of physical properties of soybean oil of typical com-

position. Seitz (31) examined 77 samples of soybean oil from various parts of the

584 SOYBEAN OIL

world over a seven-year period and reported densities at 20�C ranging from

0.9165 g/mL to 0.9210 g/mL. Wesolowski (32) examined the density of 53

Polish soybean oils at 19.9�C and reported values ranging from 0.9202 g/mL to

0.9165 g/mL. The following correlations of density and other variables were found:

with refractive index 0.62, with iodine value �0.64, with saponification value 0.34,

and with acid value 0.59. Yokota and Tachimori (77, 78) also reported a close

relation between density and iodine value. Halvorsen et al. (79) and Rodenbush

et al. (80) developed equations to predict the density of vegetable oils that took their

fatty acid compositions into account and predicted densities of soybean oils with

<0.1% error. The density of vegetable oils changes approximately linearly with

temperature, and Kravchenko et al. (65, 66) found the density decreased

0.000668 g/mL�C between 0�C and 100�C, whereas Alvarado (63, 64) found a

value 0.000643 between 20�C and 70�C, and Noureddini et al. (67) found

0.0006674 between 23.9�C and 110�C.

The densities of soybean oil-solvent mixtures at various temperatures are impor-

tant for engineering calculations and have been reported for hexane, ethylene

dichloride, and tricholoroethylene at 25�C, 37.8�C, and 50�C (81); Skellysolve B

at �20�C, �10�C, 0�C, 10�C, 25�C, and 40�C (82); dichloromethane at 25�C (83);

and hexane at 25�C (84).

The specific heat capacity of soybean oil was measured by Clark et al. (85)

and varied from 0.448 cal/g�C to 0.666 cal/g�C between 1�C and 271�C. Specific

heat increased linearly with temperature at 0.00070 cal/g�C. Tochitani and Fujimo-

to (68) measured the specific heat capacity of soybean oil from about the

TABLE 7. Some Physical Properties of Typical Soybean Oil.

Density 20�C 0.9165 to 0.9261 g/mL (31, 32) Decreases 0.000643 to

0.000668 g/mL�C (63–67)

Specific Heat Capacity 20�C 0.448 cal/g�C Increases 0.000616 cal/g�C (68)

Melting Point 0.6�C (35)

Cloud Point �9�C (69)

Pour Point �12 to �16�C (69, 70)

Heat of Combustion 9450-9388 cal/g (71)

9135 � 91 cal/g (72)

Heat Transfer Coefficient 269.7 watts/�K M2 at 180�C (73)

Surface Tension 30�C 27.6 dyne/cm Decreases 0.077 dyne/cm�C (63, 64)

Viscosity 20�C 58.5–62.2 cP (31)

Refractive Index nD20�C 1.4733–1.4760 (32)

Vapor Pressure 1m at 254�C (74)

Heat of Vaporization 44,200 cal/mol (74)

Electrical Resistivity 24�CDry 23.7 Tohm �cm (75)

Water Saturated 7.25 Tohm �cm (75)

Smoke Point �245�C (76)

Flash Point �324�C (76)

Fire Point �360�C (76)

PHYSICAL PROPERTIES OF SOYBEAN OIL 585

approximate melting point to 150�C and found a linear increase that fit the follow-

ing equation:

Sp: Heat Capacity in cal=g�C ¼ 0:4353 þ 0:000616 T; ð1Þ

where T is the temperature in �C. Their data agreed closely with those of Clark et al.

(85) but were slightly higher than those reported by Kasprzycka-Guttman et al.

(86), who made measurements between 70�C and 140�C. Wang and Briggs (87)

estimated the heat capacities of soybean oils of various compositions based on

an equation by Morad et al. (88). They calculated that high-oleate oils should have

a slightly higher heat capacity and low-saturate oils a slightly lower heat capacity

than typical soybean oil, and the change with temperature should be 0.00057 cal/

g�C. Their equation agreed with their experimental values within �5%.

Miller et al. (73) determined the heat-transfer coefficient for soybean oil at fry-

ing temperatures and found that they varied from 261.3 watts/�KM2 to 276.2 watts/�KM2 between 170�C and 190�C, where M2 is square meters of surface.

The melting of natural fats and oils usually occurs over a considerable tempera-

ture range, and soybean oil’s typical melting range is below 0�C. The availability of

differential scanning calorimetry (DSC) at low temperatures has made information

on melting of soybean oil available, and interest in using vegetable oils as fuels has

also sparked measurements of their cloud and pour points. Table 8 (41) gives the

temperatures of onset, maximum, and end of melting for various types of soybean

oil. Table 7 gives the cloud and pour points of typical soybean oil. Wang and Briggs

(87) also gave DSC curves for the melting of high-oleate, low-saturate, and low-

linolenate soybean oil. Hagura and Suzuki (89, 90) used the change in electrical

capacitance of oil samples to obtain the melting range of soybean oil and found

the results agreed with those obtained by DSC.

Seitz (31) measured the viscosity at 20�C of 77 soybean oils from four

geographic locations, and the range of variation was 58.1cP to 62.2cP (Table 7).

Viscosity decreases with temperature, and the relation is not linear. Kinematic

values (viscosity/density) have been reported at 20�C and 80�C by Chioffi (91)

and by Miller et al. (73) at frying temperatures (170–190�C); dynamic viscosities

have been reported between 0�C and 100�C by Kravchenko et al. (65, 66), between

23.9�C and 110�C by Nourreddini et al. (67), between 20�C and 70�C by Alvarado

TABLE 8. The Onset, Maximum Rate, and Termination of Melting Temperatures of

Soybean Oil with Various Fatty Acid Compositions % (41).

Class Onset Maximum Termination 16:0 18:0 18:1 18:2 18:3

Typical �39.6 �9.4 �0.6 11.4 4.2 26.1 50.3 7.9

18:0 " �13.7 18.3 20.7 10.1 22.8 17.3 42.2 7.7

16:0 & 18:0 " �17.1 16.8 18.9 24.6 18.7 8.6 37.5 10.7

16:0 " �21.8 8.4 11.6 28.0 4.7 13.8 42.1 11.4

16:0 # �46.1 �13.8 �8.1 3.4 2.6 18.0 64.8 11.2

586 SOYBEAN OIL

(63, 64), and between 1�C and 60�C by Arissen (92). Dahlberg et al. (93) were able

to predict the viscosity of soybean and other oils from the Fourier transform infra-

red spectra. Rodenbush et al. (80) calculated the viscosity of oils by relating visc-

osity to a function they termed the reduced density, which they could calculate from

the fatty acid composition.

Several authors have fit their viscosity-temperature data to equations (63–67, 87,

94). Some of these come with a claim of theoretical significance, but all have

enough variables to fit the data well. One of Alavarado’s equations (63, 64) is

ln m ¼ lnm0 þ E=RT; ð2Þ

where E/R was 3262 and ln m0 was �6.997 for soybean oil. Wang and Briggs (87)

reported graphically the change of viscosity with temperature from 10�C to 90�Cfor soybeans with altered fatty acid compositions. They found the viscosity of high-

oleic soybean oil higher and low-saturated soybean oil lower than that of typical

soybean oil.

Miller et al. (73) determined the kinematic viscosity of soybean oil at temp-

eratures of 170�C, 180�C, and 190�C, and obtained values of 3.151 cm2/sec,

2.880 cm2/sec, and 2.614 cm2/sec, respectively. The viscosities of soybean oil-hex-

ane (Skellysolve B) mixtures at temperatures between �20�C and 40�C were inves-

tigated by Magne et al. (84). Ibemesi and Igwe (95) examined the reduced viscosity

(viscosity/concentration) of solutions of soybean oil in toluene, xylene, cyclohex-

ane, and tetrahydrofuran. They found an anomalous reduced viscosity increase at

concentrations below about 0.12 g/mL that they attributed to clustering of the fat

molecules in the solvent. Erhan et al. (96) determined the kinematic viscosity of

blends of typical soybean oil with polyalphaolefins and isobutyrl oleate and

high-oleic soybean oil with isotrideceyl adipate and mineral oil to achieve viscos-

ities suitable for lubricants.

The surface tension of soybean oil at 20 –70�C was reported by Alvarado (63, 64)

and is given in Table 7. The surface tension decreased linearly with temperature at

0.077 dyne/cm�C.

Wesolowski (32) examined the refractive index of 53 samples of soybean oil

from Poland, and the range is given in Table 7. Sietz (31) reported average values

for samples from several geographic locations, and these values (1.4747–1.4752)

fall near the mean of Weslowski’s samples. Refractive index depends on chain

length and unsaturation (97) and often has been used to follow hydrogenation

(98–102). Refractive index also has been used to follow autoxidation (103). A clo-

sely related quantity, the dielectric capacitance also has been used to assess the

quality of frying oil (104). Perry et al. (74) measured the vapor pressure of soybean

oil at various temperatures and found that the data fit the equation:

log P ¼ 18:3 � 9650=T; ð3Þ

where P is the pressure in microns and T is in K. The also estimated the heat of

vaporization (Table 7).

PHYSICAL PROPERTIES OF SOYBEAN OIL 587

Tomoto and Kusano (105, 106) measured the solubility of carbon dioxide,

nitrogen, hydrogen, and oxygen in soybean oil between 0.2 atm and 1 atm and

between 30�C and 70�C. The Bunsen coefficient (volume of gas at standard condi-

tions / volume of soybean oil at 760 mm) at 30�C was 1.018 for carbon dioxide,

0.086 for nitrogen, and 0.048 for hydrogen. The Bunsen coefficient of oxygen at

30�C was 0.141 but increased with temperature, probably because of oxidation

during the measurement. The Bunsen coefficient decreases linearly to zero at

zero gas pressure. The natural logarithm of the Bunsen coefficient versus 1/T in

K is linear, and the constant is the heat of solution of the gases divided by the

gas constant. These heats of solution are �2.42 kcal/mol for carbon dioxide,

�2.58 kcal/mol for nitrogen, and �3.86 kcal/mol for hydrogen. From this relation,

one can calculate the solubility at any temperature and pressure in the range of the

study. Comparison of the values for soybean oil with olive and linseed oil suggested

that the Bunsen coefficients are influenced by the degree of unsaturation of the oil.

The viscosity of soybean oil decreased with the amount of carbon dioxide

dissolved, but dissolved nitrogen slightly increased the viscosity.

Loncin (107) reviewed the data on the solubility of water in fats and oils. For

typical soybean oil, the solubility of water was 0.11% by weight at 22�C and

rose to 0.19% at 60�C. The solubility of water decreases with fatty acid chain

length and increases with the percentage of free fatty acids.

The vapor pressures of soybean oil-hexane mixtures between 75�C and 120�Cwere reported (108, 109), and similar data for soybean oil with commercial hexanes

was reported by Smith (110). Arnold and Breuklander (83) measured the boiling

point of dichloroethylene-soybean oil mixtures and found the log (V.P.) was a linear

relation of the mole fraction of oil. Kusano (111, 112) measured the vapor pressure

(P) of soybean oil-solvent mixtures that included hexane, benzene, and carbon tet-

rachloride between 20�C and 50�C and found linear relations between log P and 1/

T. Anikin et al. (113, 114) measured the vapor pressure of mixtures of soybean oil

with the khladon 113 (trichlorotrifluoroethane) between 30�C and 100�C. Aeber-

hard and Spekuljak (115, 116) measured the vapor pressure of hexane in hexane-

soybean oil mixtures and found the vapor pressure at 25�C could be predicted by

the equation

P ¼ 9128x � 0:2807x2 þ 0:004695x3; ð4Þ

where P is the vapor pressure in Torr and x is the weight percentage of solvent in the

mixture.

Tekin and Hammond (75) measured the resistivity of soybean oil and found it

decreased logarithmically with temperature from about 100 Tohm �cm at �5�C to

0.251 Tohm �cm at 100�C. The resistivity was decreased by saturating the oil with

water and the addition of oleic acid, a-tocopherols, b-carotene, phospholipids, and

monoacylglycerol.

The smoke, flash, and fire points of soybean oil have been determined by the

Cleveland Cup method and show considerable variation. Dickhart (117) reported

a smoke point of 138�C while Detwiler and Markley (76) reported 241–250�C.

588 SOYBEAN OIL

Detwiler and Markley (76) found that the smoke point varied considerably with the

degree of refining, especially the removal of free fatty acids, and also with the mode

of oil extraction. Yen et al. (118) found a smoke point of 191�C, which was raised

several degrees by the addition of phenolic antioxidants. The flash point of soybean

oil, the temperature at which vapors coming from the oil will catch fire from an

ignition source, were reported as 304�C (117), 326–331�C (76), 174�C (69),

318�C (70), and 320�C (119). The low value reported by Ali et al. (69) was

obtained by using a Pensky-Martens closed tester and ASTM method 093-90.

The flash points of hexane-soybean oil mixtures were determined and correlated

with headspace gas chromatography data (120).

Fire points or self-ignition temperatures (SITs) for soybean oil by using the

Cleveland Cup method, which uses a brass cup, were reported to be 356–363�C

(76) and 400�C using a stainless-steel cup apparatus (71). The burning rate of soy-

bean oil was 4.3 g/m2sec, flame height 129 mm, and irradiance 0.153 kW/m2 (71).

Kowalski (119) studied the self-ignition temperature in a differential scanning

calorimeter heated at rates of 40–90�C/min and under 800–2800 kPa of oxygen

pressure and found values of 260–290�C for soybean oil. He found the addition

of copper wire to the sample decreased the self-ignition temperature by 5–15�C.

The self-ignition temperature was inversely related to oxygen pressure. Wakakura

(121, 122) used a scanning calorimeter at an oxygen pressure of 980 kPa with

soybean oil spread on glass wool and in bulk and found self-ignition temperatures

of 147�C and 376�C, respectively.

4. GRADING

To facilitate soybean marketing, the U.S. Federal Grain Inspection Service (FGIS)

established grading standards for soybeans (Table 9) (123), and the FGIS website

(124) provides much more detailed information than can be provided here (124).

TABLE 9. Official Grades and Grade Requirements of the Federal Grain Inspection

Service, United States Department of Agriculture.

Minimum

Maximum Limits

—————————————————————————

Damaged KernelsTest —————————— Soybeans

Weight Heat Foreign of Other

per Bushel Damaged Total Material Splits Colors

Grade (lbs) (%) (%) (%) (%) (%)

U.S. No. 1 56.0 0.2 2.0 1.0 10.0 1.0

U.S. No. 2 54.0 0.5 3.0 2.0 20.0 2.0

U.S. No. 3 52.0 1.0 5.0 3.0 30.0 5.0

U.S. No. 4 49.0 3.0 8.0 5.0 40.0 10.0

U.S. Sample Grade

GRADING 589

Soybeans are classified into two classes based on color, Yellow Soybeans and

Mixed Soybeans. There are four numerical grades (U.S. No. 1, 2, 3, and 4) and a

U.S. Sample Grade for each class. Sample Grade designates those soybeans that do

not meet the requirements of any of the numerical grades. Six factors are consid-

ered in assigning a grade designation: test weight, amounts of beans that are

damaged or heat damaged, and amounts of foreign material, splits, and soybeans

of other colors. Although important to processors because they affect yields and

qualities of finished products, the FGIS official grades do not consider moisture,

protein, and oil contents, but these factors may be specified on contracts in some

markets. Near infrared transmission (NIT) spectroscopy is widely used to rapidly

estimate (within less than 2 min after sampling and without any sample preparation

required) moisture, protein, and oil contents. Brumm and Hurburgh (125) devel-

oped a computer program to estimate the process value of soybeans based on their

composition and selling prices of oil and meal. In some cases, price premiums are

offered for soybeans high in oil content or high in both oil and protein contents, and

details of the program are available on the Internet (126).

Beans low in test weight may contain less oil. Test weight is the weight in

pounds of grain per Winchester bushel (35.2 L) and is determined by using an Offi-

cial Test Weight Apparatus and a 11/4-quart (1.18 L) sample before removing for-

eign material. All other grading factors are measured as percentages of total sample

weight. Foreign material, which is other grains, weed seeds, pods, leaves, stems,

etc., reduces oil and protein contents and storage life. Foreign material is deter-

mined by sieving a sample. All materials, including soybeans and soybean pieces

that readily pass through an 8/64-inch (3.2-mm) round-hole sieve and all material

other than soybeans remaining on the sieve after sieving are considered to be for-

eign matter. Split soybeans, which result from mechanical damage during handling

and over drying, reduce storage life and oil yield, and increase losses during oil

refining. Splits (typically the cotyledon splits into two halves) and broken beans

(more than two pieces) increase free fatty acid (FFA), phosphatides, iron, and per-

oxide contents of the crude oil. Heat-damaged beans have high-FFA content and

darken the oil color, both changes in oil quality increase refining loss (127). Splits

are defined as beans with more than one-fourth of the bean removed and are not

damaged. Splits are determined by sieving a portion of the grain after removing

the foreign material. Damaged beans reduce the storage life of the beans and oil

yield in processing, cause the oil to be dark-colored and poor in flavor, and increase

losses during oil refining (128). Soybeans and soybean pieces that are badly

damaged by the ground, weather, frost, heat, insects (stinkbug-stung kernels are

considered at one-fourth the actual percentage), mould, or sprouting are considered

to be damaged. Damaged beams are determined by hand picking after removing

foreign material. Soybeans of other colors may affect oil color by contributing

undesirable pigments and are those beans that are green, black, brown, or have mul-

tiple colors.

Almost 27 million MT of soybeans were exported from the United States during

the 2002 crop year, of which 4.8% was U.S. No. 1, 94.6% was U.S. No. 2, 0.4% was

U.S. No. 3, and 0.1% was U.S. No. 4. By comparison, Brazilian soybeans are

590 SOYBEAN OIL

typically slightly higher in oil content (6-yr average of 1.2% higher oil content),

foreign matter, damage, free fatty acid, and moisture contents and lower in test

weight (129).

5. RECOVERY OF OIL FROM SOYBEANS

Soybeans are economically important because of their high qualities and quantities

of oil and protein. From one bushel of soybeans (60 lb, 27.2 kg), crushers typically

recover 11.1 lb (5.0 kg) of crude oil, 44.3 lb (20.1 kg) of meal (48% protein), and

3.3 lb (1.5 kg) of hulls with the remainder being shrinkage. According to the U.S.

Department of Agriculture statistics, the oil accounts for about one-third of the

returns in processing soybeans with the protein in the form of meal accounting

for the remainder (130). Over the past five years, the meal (48% protein) has ranged

in yearly average prices of $153–289/MT (6.9–13.1 cents/lb), whereas the oil has

ranged $311–569/MT (14.1–25.8 cents/lb). Hulls have limited outlets, mostly in

cattle feeds, and sell for about $66/MT (3 cents/lb) and return $4.04/MT of

soybeans ($0.11/bu). During the same period, the average price of soybeans in

the United States ranged from $167–270/MT ($4.54–7.35/bu) and crushing

margins, the difference in soybean price and crusher returns, averaged $23.1–

56.2/MT ($0.63–1.53/bu).

Farmers often store their soybeans in metal bins on the farm or in concrete silos

at local elevators for a fee. This allows farmers to sell their crop later in the year

when prices usually increase. Soybeans should be stored at less than 13% moisture

to assure safe storage and preservation of the quality. This moisture content is

usually achieved by drying in the field before harvesting. Lower moisture contents

increase the tendency of soybeans to split during handling to form two half pieces

of cotyledon. Higher moisture content during storage can lead to mold damage or

heating damage due to seed respiration (131). These forms of damage can affect

soybean grade and oil quantity and quality when processed.

The processing of soybeans has been described in more detail elsewhere than

can be done here (132–134). Oil is recovered today by either mechanical means

or through the use of organic solvents. In the preindustrial revolution period, soy-

beans were merely pressed with lever or animal-driven screw-operated batch

presses. Around the turn of the Twentieth Century, when soybeans became a viable

commercial crop in the United States, steam-powered hydraulic batch presses were

used. Today, electric-powered continuous screw-presses, often referred to as

expellers (but this is a trademarked name for screw presses manufactured by one

supplier), or continuous countercurrent solvent extractors are used.

In either case, soybeans are pretreated prior to oil recovery to either make oil

recovery easier or more complete, or to increase the value of the defatted solids

known as meal. Usually, soybeans arriving from the farm or elevator are cleaned

to remove stems, leaves, pods, broken grain, dirt, stones, and extraneous seeds

using shaker screens and aspirators. It is usually advantageous to remove the major

portion of the hulls because they are low in oil (<1%) and protein. The hulls of

RECOVERY OF OIL FROM SOYBEANS 591

soybeans account for 7–8% of the weight. Dehulling reduces the material going

downstream into costly operations and increases the protein content of the meal.

Dehulling raises the meal protein content by about four percentage points (i.e.,

from 44% for undehulled solvent-extracted soybean meal to 48–49%) and reduces

fiber content (from 7.0% to <3.3%). The formulated feed market prefers high-pro-

tein and low-fiber meal, especially in manufacturing swine and poultry feeds. The

hulls are relatively easy to remove from soybeans compared with those of other oil-

seeds, simply cracking the bean into 6–8 pieces to free the hull using corrugated

roller mills and aspirating the hulls away from the oil- and protein-rich cotyledon,

known as meat, is effective. Consistent bean size is important to proper cracking

and drought-caused shrinking and wrinkling make dehulling much more difficult

and less efficient (135). Often, the aspirated hulls go to gravity tables to scavange

any small meats aspirated with the hulls. Usually, cleaned soybeans are conditioned

prior to cracking to improve dehulling efficiency by heating and drying the beans to

about 9.5% moisture and allowing the moisture to equilibrate for 1–7 days within

the bean to loosen the hull. Various hot-dehulling schemes have also been devised to

increase dehulling efficiency, and are often used in northern latitudes where the pro-

tein contents of soybeans, and, consequently, meal protein levels, may be lower and

specified protein levels cannot be achieved without more complete hull removal.

In the 1930s, soybeans were widely processed by screw pressing after cooking

the seed. A typical process diagram for screw pressing soybeans is shown in

Figure 1 and a plant photo is shown in Figure 2. The beans are heated and the

oil is squeezed out. The pressed oil usually goes to settling basins to reduce fine

Foreign Matter

Soybeans

DRYINGOR COOKING

SCREW PRESSING

Foots

Crude Oil

SETTLING

CRACKING

( optional)

POLISH FILTERING

CLEANING

ASPIRATING Hulls

Meats

Foots Cake

Partially DefattedMeal

MEAL COOLING

MEAL GRINDING

Figure 1. Process flow diagram for screw pressing soybeans.

592 SOYBEAN OIL

solids content, with the fines being recycled to the screw press. The oil then goes to

polish filters before being placed into storage for shipment to a refinery. Today, in

the United States, there are less than a half-dozen traditional screw press plants

(excluding extrusion-expelling, which will be discussed later). Only one screw-

press plant crushing more than 800 MT/day exists, and it is located in Ralston,

IA. Under optimum processing, the meal can contain as low as 4–6% residual

oil, which contributes metabolizable energy to livestock consuming screw-pressed

meal. As a result of the heat treatment during cooking and screw pressing,

increased rumen-bypass characteristics improves feed efficiency in high producing

dairy cattle. Thereby, the meal may sell for premium prices over solvent-extracted

meal when adequate numbers of dairy animals are located nearby. As this meal is

used to feed ruminants, the beans are not usually dehulled.

Direct solvent extraction is the most widely used oil-recovery method for

soybeans, but it also requires considerable capital and large scale to compete. In

actual practice, solvent extraction is used to crush over 98% of the soybean pro-

cessed in the United States. Process flow diagrams are shown in Figures 3 and 4.

Most soybean solvent-extraction plants process more than 2,500 MT/day (Figure 5),

and some are capable of processing as much as 5,000 MT/day (especially newly

constructed plants in Brazil). Direct-solvent-extraction plants smaller than 1,000

MT/day have difficulty competing in the United States. At various times, soybeans

have been extracted commercially with petroleum distillate fractions that resemble

gasoline, acetone, carbon disulfide, ethanol, trichloroethylene, and even water,

Figure 2. Photograph inside a modern soybean screw-press plant (courtesy of West Central

Cooperative, Ralston, IA). (This figure is available in full color at http://www.mrw.interscience.

wiley.com/biofp.)

RECOVERY OF OIL FROM SOYBEANS 593

which is not a true solvent but facilitates oil separation by creaming. A petroleum

distillate containing a mixture of hexane isomers having a typical boiling range of

65�C to 71�C is the only solvent used today. These products typically contain 45%

to 70% n-hexane. n-Hexane is considered a neurotoxin in the United States and has

proven toxicity at high concentrations. The U.S. Occupational and Safety Admin-

istration has set the maximum workplace exposure level at 500 ppm and a time-

weighted average not to exceed 50 ppm (136). In recent years, there has been con-

siderable interest by the soybean industry in alternative solvents to hexanes because

of increasing environmental and safety concerns. Alternative solvent technologies

have been extensively reviewed (137–139).

Foreign Matter

Soybeans

CONDITIONING

SOLVENTEXTRACTING

Enzyme-activeFlour

Toasted Meal

CRACKING

CLEANING

ASPIRATING Hulls

GRINDING

GRAVITYTABLING

Meats

Cracked Meats

Flakes

EXPANDING

FLAKING

EVAPORATING

Miscella(oil and solvent)

Solvent

Crude OilSolvent

FLASHDESOLVENTIZING

MEALDESOLVENTIZING

TOASTING

Marc(solids and solvent)

GRINDING

COOLING

White Flakes

STRIPPING

Collets

( optional)

( optional)

( optional)

Figure 3. Process flow diagram for direct solvent-extracting soybeans.

594 SOYBEAN OIL

Figure 4. Depiction of equipment and process flow diagram for direct solvent-extracting soybeans (courtesy of French Oil Mill Machinery Co., Piqua, OH).595

Cleaned and dehulled soybeans are conditioned by heating to 74�C to soften the

meat prior to flaking using smooth roller mills. Proper cracking and conditioning

are important to achieve the desired cell distortion or cell rupture that is necessary

for efficient extraction and to prevent production of excessive amounts of fine meat

particles that impede proper flaking or extraction. Highly distorted cells are desired

(140) so that cell walls and pseudo-membranes around oil bodies are sufficiently

ruptured, and the oil can be easily contacted by the solvent and leached out. Soy-

beans are typically flaked to 0.25 mm (10–12 thousandths of an inch) to achieve the

desired distortion (141). The flakes may be conveyed directly to the extractor or to

an expander. In recent years, expanders have been adopted to achieve increased cell

distortion and to produce an easily extractable porous pellet (collets) that is more

dense than flaked soybeans. Thereby, more mass of material can be placed into the

fixed volume of the extractor, the oil is more quickly extracted reducing extraction

time, and the solvent drains more completely reducing the load on meal desolven-

tizing equipment. All of these factors increase plant throughput capacity (142–144).

Plants vary in the amounts of flakes that are expanded, typically about one-third of

the flake production, but in a few cases, all flakes are expanded. Although there is

not universal agreement, expanding may also improve oil quality by quickly inac-

tivating phospholipases, which cause phospholipids to become nonhydratable. In

the author’s opinion, adoption of expanders is the most significant change in solvent

extraction during the past quarter century.

Figure 5. Photograph of a modern soybean-extraction plant (courtesy of Bunge North America,

Council Bluffs, IA). (This figure is available in full color at http://www.mrw.interscience.wiley.

com/biofp.)

596 SOYBEAN OIL

Soybeans are exclusively extracted in the percolation mode as opposed to the

immersion mode used during early days of soybean extraction. A photograph of

a modern chain extractor is shown in Figure 6. The percolating solvent flows by

gravity through the bed. Solvent is always passed countercurrent to the transport

of meal solids. There are several different types of extractors, including chain

and basket types, and shallow- and deep-bed types. Soybean flakes or collets are

extracted for 30 –45 min in six or more stages.

The best quality oil, low in non-triacylglycerole components, is extracted first,

and with more exhaustive extraction, poorer quality oil is recovered. Thus, at low-

residual oil levels, the proportions of phosphatides, free fatty acid, and pigments

that are extracted are greater and so is the refining loss. However, the current indus-

try practice strives for the most complete extraction possible, typically in the range

or 0.5% to 1.25% residual oil. For this reason, exhaustive laboratory devices, such

as a Soxhlet extractor with ground material, are not very useful when trying to

achieve oil that is representative of that produced by a commercial extractor,

and, for best results, the solvent should be percolated in stages through a bed of

flaked material.

The full miscella (oil-rich extract) containing 20–30% oil drains from the fresh-

est flaked or expanded meats and is sent to solvent-recovery operations. The opera-

tions include two-stage evaporators and an oil stripper. The oil content exiting the

first-stage evaporator is 65–70% oil and is heated with vapors from the desolventi-

zer-toaster. After the second-stage evaporator, the oil content is 90 –95% oil. The oil

stripper uses steam-injection vapor, high heat, and high vacuum to remove the

Figure 6. Photograph inside a modern direct solvent-extraction plant processing soybeans

(courtesy of Crown Iron Works, Minneapolis, MN). (This figure is available in full color at

http://www.mrw.interscience.wiley.com/biofp.)

RECOVERY OF OIL FROM SOYBEANS 597

solvent to less than 0.2% remaining in the oil. The temperature of the oil in the

stripper should not exceed 115�C to prevent scorching the oil and causing dark col-

or. Flash point determination is an easy method to assure that the solvent-evapora-

tion equipment is operating as it should and the flash point should exceed 150�C.

All evaporated solvent is recycled to the extractor. The oil should be sent to a

vacuum dryer to remove any residual stripping steam condensate and the dry oil

immediately cooled prior to placing into storage.

As a result of natural antioxidants (i.e., phoshpahtides, tocopherols), crude soy-

bean oil can be stored for a long time in large tanks provided the oil is first cooled to

ambient temperature and has limited access to air. The crude oil should be low in

moisture to prevent hydrolysis. Gummy deposits of phosphatides may spontaneously

form in the bottoms of storage tanks and tank cars used for shipping crude oil.

There has been much speculation about using supercritical carbon dioxide

because using this technology eliminates safety issues as carbon dioxide is not

flammable and the oil is better quality (139), but no such plants have been con-

structed to process soybeans. This is due to the absence of a commercially feasible

means of continuously feeding soybean flakes into a high-pressure vessel and

removing the spent flakes. Recently, one company has developed a screw press in

which supercritical carbon dioxide is injected into the barrel. This equipment has been

successfully used to produce soybean meal with lower residual oil contents than typi-

cally produced by screw pressing and with little heat denaturation of the protein.

The spent flakes or collets are sent to a meal desolventizer-toaster (DT). Newer

equipment incorporates countercurrent steam usage. The Schumacher-type deso-

lventizer/toaster/dryer/cooler has become widely accepted in the soybean industry,

and, with this equipment, residual levels of hexane should be less that 500 ppm.

Both indirect and direct steam heating are used. Steam vapor and a modest vacuum

carry away the solvent vapors for condensing. Condensed solvent is recycled to the

extractor after separating water from the hexane. A desolventizer-toaster is a series

of trays through which the meal flows. Soybean meal is unique in that it must be

toasted to inactivate protease inhibitors (especially trypsin inhibitor) that would

reduce feed efficiency if not denatured and inactivated. Urease activity is used as

a measure of adequate heating. The toasted meal typically has low-protein solubi-

lity as measured by protein dispersibility index (typically 45 PDI). The meal is then

sent to a dryer-cooler to reduce the meal temperature for safe storage. The moisture

content should be about 12% and the residual fat content less than 1.5%. The free

extractable oil after extraction is less than 1.0%, but heating during desolventizing-

toasting frees some bound fat that previously was not extractable with hexane.

Overtoasting may reduce digestibility and nutritional value of the meal. The meal

is then ground with a hammer mill to produce meal with uniform particle size.

If dehulling is employed, as is typical for plants in the United States, the meal

will contain around 48% protein. Additionally, dehulling reduces the fiber content

of the meal by over 50%. In some plants, a portion of the soybean hulls may be

added back to the meal prior to grinding to adjust and precisely control meal protein

content. Livestock feeders are concerned about having uniform protein and fiber

contents in order to formulate minimum-cost feeds for maximum feed efficiency.

598 SOYBEAN OIL

The meal is generally ground so that 95% passes a U.S. 10-mesh screen and a max-

imum of 3% to 6% passes through a U.S. 80-mesh screen.

Some plants divert part of their spent flake production away from a desolventi-

zer-toaster to a flash desolventizer, which is designed to produce white flakes with

high-protein solubility (PDI 70 –90). White flakes are used as the starting material

for producing protein isolates or concentrates, which contain >90% and 65% pro-

tein, respectively, and are used as food ingredients.

Some soybean extraction plants also degum their oil before shipping to centra-

lized refineries. There is not sufficient market to make it profitable to recover all of

the soybean phosphatides and market them as soy lecithin. The gums are added

back to the meal in the toaster to evaporate the water. The gums contribute to

the metabolizable energy content of the meal and the soybean crusher can get

meal prices for crude phosphatides.

Quality standards and trading rules for solvent-extracted soybean meal and oil

are designated by the National Oilseed Processors Association and are available

at a website (145). Soybean products are remarkably uniform in their quality char-

acteristics compared with alternative sources of oil and meal.

Recently, a third process, known as extruding-expelling (or Express Systems as

trademarked by the equipment manufacturer), was developed (Figures 7 and 8)

(146, 147). In this process, a dry extruder, which generates heat solely through fric-

tion of the beans in the extruder, replaces steam generating and steam heating the

beans. The heated beans then go to a screw press and the rest of the process is

the same as in screw pressing. The plants typically process 5–50 MT/day.

Foreign Matter

Soybeans

EXTRUDING

EXPELLING(screw pressing)

Foots

Crude Oil

SETTLING

CRACKING

( optional)

POLISH FILTERING

CLEANING

ASPIRATING Hulls

Meats

Foots Cake

Partially DefattedMeal

MEAL COOLING

MEAL GRINDING

Figure 7. Process flow diagram for extruding-expelling soybeans.

RECOVERY OF OIL FROM SOYBEANS 599

Approximately 70 extruding-expelling plants have been built over the past 10 years

for crushing soybeans. Usually, these plants are farmer-owned and provide meal to

nearby livestock feeders (148). The oil is sold to the large oil refineries, often at a

discount despite the oil being of excellent or superior quality because high costs are

incurred in handling small lots of oil. These plants are ideally suited to identity-

preserved processing. There are niche opportunities for these plants to market

certified organic or nonGMO soybean oil, for which there is a lucrative market

in some countries. Other opportunities reside with genetically enhanced soybean

oils and meals, such as low-linolenate, high- and low-saturates, and high-oleate

oils. This process has even been proposed for producing soybean products during

interplanetary exploration (149). NASA plans to grow soybean in space because

some missions, such as Mars exploration, cannot be supported without growing

food in space.

6. QUALITIES OF SOYBEAN OILS AND MEALS EXTRACTEDBY DIFFERENT METHODS

Wang and Johnson (150) compared the qualities of soybean oils and meals obtained

by the three processing methods. Soybean oil and meal samples were collected

at three times within a one-year period from 13 extruding-expelling plants, eight

Figure 8. Photograph inside a modern extruding-expelling plant processing soybeans. (This

figure is available in full color at http://www.mrw.interscience.wiley.com/biofp.)

600 SOYBEAN OIL

solvent-extraction plants, and one continuous screw-press plant. Their results are

shown in Tables 10 and 11. Solvent extraction is by far the most efficient method

of recovering oil from soybeans, typically only about 1.2% residual oil is left in the

meal. Screw-pressing is slightly more efficient in recovering oil than is extruding-

expelling, leaving 6.3% oil in screw-pressed meal compared with a mean of 7.2%

for extruded-expelled meals. Most solvent-extraction plants dehull soybeans to pro-

duce soybean meal with 48% or more protein and carefully control the moisture

content at 12%. Solvent-extracted soybean meal is highly uniform, often much

more so than either screw-pressed or extruded-expelled meal. The high-protein

and low-fiber contents of solvent-extracted soybean meal are desired when feeding

poultry and swine, which consume 46% and 25% of the soybean meal produced,

respectively. Most extrusion-expelling and screw-press plants have not invested

in dehulling equipment, as their meal generally goes into feeding ruminant animals.

Protein dispersibility indices, a measure of protein denaturation that is used in

the food industry, are lower for extruded-expelled and screw-pressed meals. Protein

TABLE 10. Quality Characteristics of Soybean Meals Produced by Different Oil-Extrac-

tion Processes.

Processing Method

—————————————————————————————

Property Solvent Extraction Screw-Pressing Extruding-Expelling

Moisture, % 11.65 11.03 6.94

Residual oil1, % 1.2 6.3 7.2

Protein1, % 48.8 43.2 42.5

Urease, �pH 0.04 0.03 0.07

Protein solubility in KOH, % 89.1 61.6 88.1

Protein dispersibility index 44.5 10.6 18.1

Rumen-bypass protein, % 36.0 48.1 37.6

Hunter ‘‘L’’ color 69.1 51.5 65.8

Trypsin inhibitor, mg/g 5.46 0.3 5.52

Trypsin inhibitor, TIU/g 5280 2000 12,250

1 Reported at 12% moisture basis.

TABLE 11. Quality Characteristics of Soybean Oils Recovered by Different Processes.

Processing Method

—————————————————————————————

Property Solvent Extraction Screw-Pressing Extruding-Expelling

FFA, % 0.31 0.33 0.21

Phosphorus, ppm 277 463 75

Tocopherols, ppm 1365 1217 1257

Moisture, % 0.08 0.05 0.08

PV, meq/kg 0.96 1.76 1.73

AOM stability, h 39.8 36.2 23.9

Lovibond color, red 11.1 17.5 10.2

QUALITIES OF SOYBEAN OILS AND MEALS EXTRACTED 601

solubilities in potassium hydroxide solution, a measure of protein denaturation and

an indicator of overcooking that is used in the feed industry, are similar for

extruded-expelled and solvent-extracted meals, but higher than that of screw-

pressed meal (62%). Rumen-bypass protein values are higher for the screw-pressed

meals, indicating that more protein escapes the rumen and is not converted to

microbial protein that has a lower nutritive value than the original soybean protein.

All meals examined by Wang and Johnson (151), regardless of the processing meth-

od employed, had low-trypsin-inhibitor activity, which is important to proper pro-

tein digestion. Soybean trypsin inhibitors, especially in unheated soybeans, can

inhibit the protease enzymes trypsin and chymotrypsin, reducing protein hydrolysis

during digestion. There are two trypsin inhibitors in soybeans, Kunitz inhibitor and

Bowman-Birk inhibitor. The Kunitz inhibitor is relatively easily inactivated by

moist heat, comprises about 85% of the inhibitory activity, and acts only on trypsin;

the Bowman-Birk inhibitor is much more stable to heat (due to six disulfide cross

linkages) and acts on both trypsin and chymotrypsin. The activity of the enzyme

urease (easily measured as pH change) is often used as a quick and easy indicator

of adequate cooking. A valuable resource for characteristics of soybean meal is

http://www.stratsoy.uiuc.edu/epv/.

Oil properties vary considerably between different types of plants (Table 11) and

among plants of the same type and sampling times. The free fatty acid (FFA) con-

tent, a measure of hydrolytic degradation during seed storage and oil extraction, of

extruded-expelled oil is significantly lower than that of solvent-extracted oil, which

may be due to the rapid inactivation of lipases during extrusion. Screw-pressed

soybean oil typically contains 0.33% FFA, which is similar to that of typical sol-

vent-extracted oil. The amounts of phospholipids in the oils after settling are much

lower in extruded-expelled oil (75 ppm phosphorus) than in solvent-extracted oil

(277 ppm phosphorus). Screw-pressed oil has much higher phospholipid content

(463 ppm phosphorus) than does solvent-extracted oil. The phospholipid in

extruded-expelled oil is readily hydratable and easy to settle, which are attributed

to the rapid heat inactivation of the phospholipases. The tocopherol contents of

crude extruded-expelled oils are slightly lower than those of crude solvent-extracted

oil.

Peroxide values (PVs), a measure of primary lipid oxidation products, are sig-

nificantly higher for crude extruded-expelled oil than for crude solvent-extracted

oil, which is attributed to the high temperature used in extruding-expelling, the

long period typically allowed for oil cooling, or the often poor oil-storage condi-

tions and longer storage times at extruding-expelling plants. Oxidative stability,

as measured by the Active Oxygen Method (AOM), of extruded-expelled oil is sig-

nificantly lower than that of solvent-extracted oil, probably because of the higher

PV value and lower contents of phosphorus (phosphatides) and tocopherols in crude

extruded-expelled oil. The colors of extruded-expelled and solvent-extracted oils

are significantly different. Although solvent-extracted oil tends to be slightly darker

than extruded-expelled oil, screw-pressed oil is much darker in color than are the

other two types, probably because of the more severe heat treatment of the screw-

pressed oil before pressing.

602 SOYBEAN OIL

7. SOY PROTEIN INGREDIENTS

Defatted soybean meal (white flakes) may be heated to produce a variety of solu-

bility and enzyme-activity characteristics, ground and sized to produce grits or

flour, and used as a food ingredient in bakery products, soymilk, and meat products.

A historical accounting of the development of these products was published by

Johnson et al. (151, 152). Soy flour may be relecithinated or refatted with refined,

bleached, and deodorized oil to achieve desirable functional properties. Soy flour

can also be texturized by using an extruder to produce meat-like products called

TVP (texturized vegetable protein) that are often used to extend ground meat.

Enzyme-active soy flour is used in bread at 0.5% of the wheat flour. Lipoxygenase

in the soy flour bleaches the carotenoids of wheat flour to produce a whiter crumb

and improves dough-mixing properties. White flakes may be processed into soy

protein isolates or concentrates (132, 153). Soy protein is poorly soluble in water

at pH 4.5, the isoelectric point, and highly soluble at pH >8.0. These solubility

characteristics can be used to isolate or concentrate soy protein.

Untoasted and flash-desolventized meal in which the protein is undenatured and

highly soluble (>70 PDI and preferably >90 PDI) is the preferred starting material

in manufacturing soy protein isolates. Under some conditions, extruded-expelled

meal can be used, but the yield of soy isolate is reduced. The meal is ground in

water adjusted to pH 8.0 with sodium hydroxide and centrifuged to remove insolu-

ble fiber. The soluble fraction is acidified to pH 4.5, and the protein precipitates.

The precipitated protein curd is separated from the soluble sugars by centrifuging.

The protein curd may be washed, neutralized, and spray-dried.

High protein solubility is not needed for protein concentrates and heating to

insolubilize the protein and facilitate extracting the solubles (mostly sugars) with

water is one way that has been used to prepare soy protein concentrates. Concen-

trates today, however, are normally made by extracting the sugars with either acid

(pH 4.5) or aqueous ethanol (60–80%). Aqueous ethanol is most frequently used

because it produces the blandest product, but ethanol denatures the protein and

leaves the protein with reduced functional properties unless the product is refunc-

tionalized by jet cooking (154, 155) or by homogenizing under alkaline conditions

(156). Soy protein concentrate must contain >65% protein on a dry basis.

The soybean storage proteins glycinin and b-conglycinin, which often are recog-

nized in the older literature as 11S and 7S proteins, respectively, based on their

sedimentation during ultra centrifuging, comprise 65–80% of the protein. Methods

have even been developed to separate soy protein into fractions rich in individual

proteins (157, 158). Some believe b-conglycinin has greater health benefits than

glycinin.

Soy protein isolates are used in dairy analogs (milk replacers and beverage pow-

ders), meat-pumping solutions, luncheon meats, and infant formulas, whereas soy

protein concentrates are used in dairy analogs (milk replacers, beverage powders,

cheeses, coffee whiteners, frozen desserts, whipped toppings), baked goods, and

meat products (156). These protein products are used for their functional properties

such as solubility, water absorption and binding, viscosity control, gelation,

SOY PROTEIN INGREDIENTS 603

cohesion-adhesion, elasticity, emulsification, fat absorption and binding, foaming,

and color control. The solubility and thermal properties of these products were

recently compared by Lee et al. (159). Some products have high solubility even

though they were largely denatured.

Many health benefits have been attributed to soy protein products, either because

of the proteins or accompanying phytochemicals, such as isoflavones, saponins, etc.

There is a growing body of evidence that soy protein products may impact hyper-

tension and heart disease, osteoporosis and bone health, and certain cancers. The

perception of such nutritional benefits is driving an increased interest by food com-

panies in the incorporation of soy protein products. In October 1999, the U.S. Food

and Drug Administration (FDA) authorized a health claim for soy protein in cardi-

ovascular disease. U.S. food labeling laws now permit a statement on the label that

‘‘Diets low in saturated fat and cholesterol that include 25 grams of soy protein a

day may reduce the risk of heart disease. One serving of (name of food) provides

(list number) grams of soy protein.’’ The health claim allowance is reported in the

Federal Register (160) and is posted on the FDA website (161).

8. BASIC PROCESSING OPERATIONS

As discussed in the previous section on soybean oil composition and Table 11,

crude soybean oil can contain phospholipids, free fatty acids, lipid oxidation

products, and unsaponificable matter, which includes chlorophyll and carotenoid

pigments, tocopherols, sterols, and hydrocarbons. Some of these components nega-

tively affect oil quality, and some may play positive roles in nutrition and function-

ality. The goal of oil refining is to remove the undesirable components so that a

bland, stable, and nutritious product can be obtained. The basic processing opera-

tions in oil refining are (1) degumming, (2) neutralization, (3) bleaching, (4) hydro-

genation, (5) deodorization, and (6) winterization or crystallization. These steps are

outlined in a flow chart as shown in Figure 9.

8.1. Degumming

Crude soybean oil contains a relatively high concentration of phospholipids com-

pared with other vegetable oils. Degumming is a process of removing these com-

ponents from crude soybean oil to improve its physical stability and facilitate

further refining. Phospholipids can lead to dark-colored oils and they can also serve

as precursors of off-flavor (162) compounds. Free fatty acids, pigments, and other

impurities are also partially removed by degumming. Soybean oil can also be neu-

tralized directly without degumming if gum or lecithin recovery is not desired. Con-

ventional belief holds that the loss of neutral oil in refining crude oil by direct

neutralization is less than the combined losses of degumming and caustic refining

of the degummed oil.

The quality of crude soybean oil influences the efficacy of degumming. Phos-

pholipids can exist in a hydratable form, which can be readily removed by addition

604 SOYBEAN OIL

of water, or in a nonhydratable form, which cannot be easily hydrated and removed.

The nonhydratable phospholipids are considered to be the calcium and magnesium

salts of phosphatidic acids, which are formed by enzymatic hydrolysis of the origi-

nal phospholipids. This degradation can result from seed damage during storage

and improper handling. List et al. (53) studied the factors promoting the formation

of nonhydratable phospholipids in soybeans and showed that they are promoted by

four interrelated factors: (1) moisture content of beans or flakes, (2) phospholipase

D activity, (3) heat applied to beans or flakes prior to and during extraction, and

(4) disruption of the cellular structure by cracking or flaking. These results suggest

that a nonhydratable-phosphatide formation can be minimized by control of the

moisture of beans or flakes entering the extraction process, inactivation of phospho-

lipase D, and optimizing the temperature during conditioning of cracked beans or

flakes. Normal quality soybean oil from the conventional solvent extraction

contains about 90% hydratable and 10% nonhydratable phospholipids. Phosphoric

or citric acid can be used as a pretreatment to achieve more complete removal of

nonhydratable phospholipids, but their presence in the gum will darken it and

reduce its quality. The total phospholipid content in crude soybean oils ranges

from 1.85% to 2.75% (19) and partially depends on the seed preparation and extrac-

tion methods employed. Use of an expander or the Alcon process to cook the flakes

prior to extraction will increase total phospholipids content in the crude oil and the

phosphatidylcholine percentage in the gum (163).

Crude Soybean Oil

Foots

NEUTRALIZING

CENTRIFUGING

VACUUM DRYING

BLEACHING

FILTERING

Alkali

Water

BleachingEarth

Water

Soapstock(free fatty acids,phosphatides)

Wash-water(residualsoapstock)

Moisture

Spent BleachingEarth (color, residualsoapstock)

Salad & Cooking Oils

DEODORIZING

POLISH FILTERING Deodorizer Distillate(off-flavor compounds, minor

volatiles, free fatty acids)

CENTRIFUGING

WASHING Lecithin

CENTRIFUGINGGUMS HYDRATING

FILTERING

GUMS DRYING

Steam

Moisture

DISTILLATE CONDENSING

Figure 9. Diagram of conventional soybean oil refining.

BASIC PROCESSING OPERATIONS 605

Degumming can be achieved in a batch or continuous fashion. In batch degum-

ming, soft water at the same percentage as total phospholipid is added to oil heated

to 70�C and mixed thoroughly for 30–60 min, followed by settling or centrifuging.

In continuous water degumming, heated oil is mixed with water by an in-line

proportioning and mixing system and the mixture is held in a retention vessel for

15–30 min before centrifugation. The phosphorus content is typically lowered to

12–170 ppm (164). A well-degummed soybean oil should contain less than

50 ppm of phosphorus, which is well below the 200 ppm level specified in the

National Oilseed Processors Association (165) trading rules for crude degummed

soybean oil. Degumming for physical refining, as opposed to alkali refining of soy-

bean oil, requires more complete removal of the phospholipids to prevent darkening

during fatty acid distillation. For more complete phospholipid removal, several

modified degumming methods can be employed (166, 167).

Recently, polymeric ultrafiltration membranes were used for degumming crude

soybean oil and removing phospholipids from the crude oil/hexane miscella (168).

Crude soybean oil also can be de-acidified by methanol extraction of the free fatty

acids and the extract separated into fatty acids and solvent by a membrane filter

(169). A surfactant-aided membrane degumming also has been applied to crude

soybean oil, and the degummed oil contained 20–58 ppm of phosphorus (170).

Supercritical carbon dioxide extraction was shown to be an effective means of

degumming (171). In this process, soybean oil countercurrently contacted supercri-

tical carbon dioxide at 55 MPa and 75�C. The phosphorus content of the oil was

reduced from 620 ppm to less than 5 ppm. Ultrasonic degumming was also success-

fully used to reduce the gum content of soybean oil (172).

8.2. Neutralization

Neutralization is also referred to as de-acidification and alkali or caustic refining.

Neutralization is achieved by treating the soybean oil with aqueous alkaline solu-

tion (most commonly, sodium hydroxide) to neutralize the free fatty acids in a batch

or continuous system. The soap formed in the reaction also adsorbs natural pig-

ments, the gum and mucilaginous substances not removed by degumming. Natural

settling or centrifugation is used to remove the soap. Crude soybean oil also can be

netralized directly without degumming. When this is practiced, the oil commonly is

pretreated with 300–1000 ppm of 75% phosphoric acid to facilitate removal of

phospholipids. The percentage of excess sodium hydroxide solution required for

crude oil is higher than that for degummed oil (173).

The quality changes, such as lipid oxidation and reduction of tocopherols and

phytosteols during neutralization, are considerable compared with the other proces-

sing steps as shown by Wang and Johnson (174), and also as presented in Table 12.

The further phospholipid removal (below 2 ppm phosphorus) also reduces the oxi-

dative stability of soybean oil (175) due to the antioxidant property of these phos-

pholipids.

One of the new developments in neutralization is the use of silica-based adsor-

bent to remove the residual soap instead of using water washing. Water usage and

606 SOYBEAN OIL

waste generation is greatly reduced by this practice. Sodium silicate also was used

as a mild neutralizing agent to refine specialty oils (176). Its agglomerating ten-

dency allowed the removal of the soap by filtration, and its low alkalinity mini-

mized saponification of neutral oil and loss of minor nutrients. Other adsorbents,

such as magnesium silicate, also were shown to be effective in reducing free fatty

acids, as well as reducing primary and secondary oxidation products in the treated

oil (175, 177).

Physical refining or steam refining is a process similar to steam deodorization.

Steam distillation is typically used for oil with a high free-fatty acid content to

reduce the refining loss, which would be significant if caustic refining was used.

Acid-aided degumming produces soybean oil with very low phosphorus content

and makes the distillation of free fatty acids possible. Nevertheless, the relatively

difficult task of removing sufficient phospholipids from soybean oil has prevented

extensive use of this technique in the United States. Physical refining, however, has

virtually replaced caustic refining of palm oil in Malaysia.

8.3. Bleaching

Bleaching is a process designed not only to remove the oxidation-inducing pig-

ments such as chlorophylls, but more importantly to decompose the peroxides pro-

duced by oxidation into lower molecular weight carbonyl compounds that can be

removed by subsequent deodorization. Bleaching also removes other impurities

such as soap and metal ions. In soybean oil refining, color reduction occurs at

each step, nevertheless, the most significant reduction of chlorophylls occurs in

the bleaching step. Acid-activated bleaching clay is most effective in adsorbing

chlorophylls and decomposing peroxides, and it is commonly used for soybean

oil. The chlorophyll content in normal crude soybean oil (1–1.5 ppm) can be

reduced by 25% by alkali refining, and bleaching with acid earth further reduced

chlorophylls to 15 ppb (178) The subsequent hydrogenation and deodorization

remove or degrade red and yellow pigments more than chlorophyll, so incomplete

chlorophyll removal by bleaching will cause the refined oil to appear greenish. The

refined and bleached oil is particularly susceptible to oxidation and is less stable

than the crude, degummed, refined, or deodorized oils (178).

TABLE 12. Effect of processing on content of tocopherols, sterols, and squalene

in soybean oil (25).

Tocopherols Sterols Squalene

Processing —————————— —————————— —————————

Step ppm % Loss Ppm % Loss ppm % Loss

Crude 1132 — 3870 — 143 —

Degummed 1116 1.4 3730 3.6 142 0.7

Neutralized 997 11.9 3010 22.2 140 2.1

Bleached 863 23.8 3050 21.2 137 4.2

Deodorized 726 35.9 2620 32.3 89 37.8

BASIC PROCESSING OPERATIONS 607

The desired bleaching endpoint is typically zero peroxide, although a color spe-

cification is often used as an important measure. The amount of bleaching earth

should be adjusted based on the quality of oil to be bleached, and it usually ranges

from 0.3% to 0.6% for a typical soybean oil. Low contents of phosphorus (5–10

ppm P) and soap (10–30 ppm) in the neutralized oil are essential to maximize

the bleaching effect. Successful bleaching can be achieved by atmospheric batch

bleaching, vacuum batch bleaching, or continuous vacuum bleaching at tempera-

tures between 100�C and 120�C for 20–30 min. More details of soybean oil bleach-

ing are described by Erickson (179).

Recently, silica-based synthetic materials have been used in bleaching. The nat-

ural bleaching earth, fuller’s earth, a hydrated aluminum silicate, mostly has been

replaced by acid activated clays, which are sulfuric- or hydrochloric-acid-treated

bentonites or montmorillonites. Manufacturers continuously improve the quality

and develop new bleaching earths to meet the market’s needs. Higher activity

and filterability are the main focuses of such development.

8.4. Hydrogenation

The high degree of unsaturation, particularly the relatively high content of linole-

nate, of soybean oil significantly limits its food applications because of low oxida-

tive stability. Hydrogenation is used to improve oxidative stability as well as to

increase the melting temperature of soybean oil. A great proportion of soybean

oil is hydrogenated to produce cooking oil, bakery/confectionery fats, and shorten-

ing.

When oil is treated with hydrogen gas in the presence of a catalyst (typically

nickel) and under appropriate agitation and temperature conditions, it becomes

more saturated and forms a semisolid or plastic fat that is suitable for many food

applications. Selectivity is a term used to describe the relative reaction rate of the

fatty acids from the more unsaturated to the more saturated forms. Perfect selectiv-

ity would provide sequential elimination of linolenate, linoleate, and then oleate. To

completely hydrogenate linolenate while minimizing changes in the other acyl

groups, a high ratio of the reaction rates of linolenate to linoleate compared with

linoleate to oleate is desirable. Generally, selectivity increases with temperature and

catalyst concentration and with decreases in hydrogen pressure and agitation rate

(180). The effect of pressure on hydrogenation selectivity of soybean oil was

reported by List et al. (181), who found that the linoleate-containing triacylglycer-

ols were reduced at a slower rates than the linolenate-containing triacylglycerols

under selective condition. At higher pressures (500 psi), the reaction was truly non-

selective; whereas at 50 psi, the reaction became selective. Impurities in soybean

oil, such as phosphorus, oxidation products, carotene, and metal ions can poison

the catalyst and cause slower hydrogenation (182). A particular limitation with

nickel catalyst is its low selectivity for linolenate over linoleate, and copper-con-

taining catalysts have greater selectivity for linolenate acid than the conventional

nickel catalysts (183). The use of copper catalyst can produce soybean oil that

has a low degree of hydrogenation (iodine value of 110–115) but has less than

608 SOYBEAN OIL

1% linolenate. However, copper catalysts are not as active as nickel catalysts; they

are also easily poisoned (184). Furthermore, any trace of residual copper in the fully

processed oil will promote lipid oxidation.

The most common tests for degree of hydrogenation are congeal point and the

iodine value as determined by refractive index. Refractive index is a valuable tool

for iodine values above 95, but when the oil is further hydrogenated, refractive

index becomes an inadequate measurement for melting prediction because

increased amount of trans-isomers results in harder oil than the refractive index

would indicate (185). For margarine or shortening, the solid fat index (SFI), as

determined by dilatometry, or solid fat content (SFC), determined by nuclear mag-

netic resonance, is the most appropriate method to measure the consistency of the

hydrogenated oil. These indices predict the workability and creaming ability at a

particular temperature.

Double-bond isomerization or trans-fatty acid formation is the most important

side-reaction that occurs during hydrogenation, and it has a strong impact on the

physical and possibly the nutritional properties of the products. Trans-double bonds

are thermodynamically a more favorable configuration than their cis-counterpart; so

trans-bonds are produced in significant quantities if the hydrogenation does not go

to completion. The trans-fatty acids have a much higher melting point than their

cis-isomers, therefore a fat product with considerable trans-acyl groups will have

an elevated melting point, which is desirable in shortening and margarine applica-

tions. A partially hydrogenated soybean oil can have at least 30 different one-, two-,

and three-double-bond isomers that will result in more than 4000 different triacyl-

glycerol molecules. This complexity allows the production of a great variety of oils,

margarines, and shortenings that have a wide range of physical and functional prop-

erties. However, the established relationship between trans-fat consumption and

health has prompted research to minimize trans-double formation in fats and oils.

Hydrogenation of soybean oil may be carried out in a batch or a continuous sys-

tem. In the United States, batch operations are typical. More comprehensive

reviews on hydrogenation and formulation can be found in Erickson and Erickson

(180), Hastert (186), and Kellens (187).

8.5. Deodorization

Deodorization is usually the last step in conventional oil processing. It is a steam-

stripping process in which good quality steam (1–3% of oil) generated from de-aer-

ated and properly treated feed water is injected into soybean oil under high tem-

perature (252–266 �C) and high vacuum (<6 mm Hg) to decompose peroxides

and vaporize the free fatty acids and odorous compounds. Deodorization relies

on the large differences in volatility between the triacylglycerols and other undesir-

able components under certain conditions. The musty and earthy odor produced

from bleaching and the hydrogenation odor and flavor are effectively removed by

deodorization. The free fatty acids, typically ranging from 0.1% to 0.5% in neutra-

lized oil and 0.5% to 5% in oil to be physically refined, are also reduced to below

0.03%, a value used as an indicator for deodorization efficiency. Zero peroxide

BASIC PROCESSING OPERATIONS 609

value is another indicator for effective deodorization. Heat bleaching is achieved by

holding the oil for 15–60 min at high temperature to ensure considerable decom-

position of carotenoid pigments.

During the deodorization process, many desirable reactions take place, but some

undesirable reactions, such as lipid hydrolysis, polymerization, and isomerization,

also occur. Therefore, the deodorization temperature is carefully controlled to

achieve optimum quality of the finished soybean oil product. The effect of refining

condition on trans-fatty acid content in refined vegetable oils was investigated by

Okamoto et al. (188). Trans-fatty acid contents of deodorized oils increased with

prolonged exposure to high temperature, and trans-formation was higher in

oils containing greater proportions of polyunsaturates. The isomerization rate of

linolenate was 6.5- to 16.3-fold higher than that of linoleate in soybean oil. Kemeny

et al. (189) studied kinetics of the formation of trans-linoleic acid and trans-

linolenic acid in vegetable oils deodorized at temperatures from 204–230�C for

2–86 h. Their data can give good estimates of the trans-level of refined oils for

given deodorization conditions. Deodorization has also been modified to retain

more nutrients and prevent other undesirable reactions. Mathematical models

have been established describing the influence of different process parameters

such as time, temperature, steam rate, and pressure on tocopherol stripping, produc-

tion of oxidized and polymeric triacylglycerols, and trans-fatty acid formation dur-

ing physical refining of soybean oil (190). Tocopherol removal was mainly

influenced by processing temperature and steam rate, whereas oxidized and poly-

merized triacylglycerols were not significantly affected by any of the investigated

process parameters.

There are three types of deodorization operations. The batch process is the least

common because of its low efficiency and inconsistent product quality. The

semicontinuous and continuous deodorizers have improved processing efficiency.

There are several configurations of the continuous deodorizer, including the

single-shell cylindrical vessel type, the vertically stacked-tray type, and the thin-

film packed-column type. The thin-film system provides excellent fatty acid

stripping with minimum use of steam, but it does not achieve the desired heat

bleaching or effective deodorization because of its relatively short retention time.

A retention vessel held at high temperature has to be used after the column distillation to

achieve bleaching (191).

The overall oil quality change during refining of soybean oil was examined by

Jung et al. (178), and their results are shown in Table 13. A study of oxidative

TABLE 13. Effect of Processing Steps on Quality of Soybean Oil (178).

Phosphorus Iron Chlorophyll Peroxide Value Tocopherol Free Fatty

Refining Step (ppm) (ppm) (ppm) (meq/kg) (ppm) Acid (%)

Crude 510 2.9 0.30 2.4 1670 0.74

Degummed 120 0.8 not available 10.5 1579 0.36

Refined 5 0.6 0.23 8.8 1546 0.02

Bleached 1 0.3 0.08 16.5 1467 0.03

Deodorized 1 0.3 0.00 0.0 1138 0.02

610 SOYBEAN OIL

stability of soybean oil at different stages of refining indicated that crude oil was the

most stable and highly purified oil was the least stable (192). The influence of the

refining steps on the distribution of free and esterified phytosterols in soybean and

other oils was reported by Verleyen et al (193). A significant reduction in free ster-

ols was found after neutralization. Deodorization removed free sterols and also pro-

moted steryl ester formation when the oil was physically refined due to a heat-

promoted esterification reaction between free sterols and free fatty acids.

8.6. Fractionation and Winterization

Fractionation or winterization is a process in which the more saturated molecular

species in the oil are solidified and removed by a low-temperature treatment, which

increases the cold storage physical stability of the oil. Partially hydrogenated soy-

bean oil with 110–115 iodine value (IV) that is intended for use as salad and cook-

ing oil should be fractionated. By doing so, the more saturated molecules and some

high-melting trans-isomers are removed to produce clear oil that meets low-tem-

perature storage requirements. The formation of large and easily filterable crystals

and the removal of the crystallized fraction from the liquid oil can be challenging

tasks. The temperature of the oil should be lowered slowly to prevent small crystal

formation. Nucleation occurs when the oil is supercooled to a temperature that is

much lower than the thermodynamic equilibrium temperature. Heterogeneous

nucleation, i.e., the formation of nuclei on to foreign substances, typically takes

place around dust particles or on the walls of the crystallizer. The crystal growth

rate depends on the degree of supercooling and polymorphic form. In order to

have continuous and uniform crystallization, an intense but nondestructive agitation

is required. To produce salad oil with good cold stability, soybean oil is usually

hydrogenated to an iodine value of 100–110 (linolenate content of 2–3%) and win-

terized at 2–3�C. To produce a cooking and frying oil, hydrogenation to an iodine

value less than 90 (linolenate content of less than 0.5%) is more desirable, and the

stearine fraction obtained from winterization of such oil is a good shortening and

margarine base. Crystal separation can be done by filtering, centrifuging, or decan-

tating. More details about these systems are presented by Krishnamurthy and

Kellens (194).

9. ALTERNATIVE REFINING METHODS

Although oil extraction by mechanical pressing of soybeans accounts for a very

small percentage of soybean processing, it is used by many farm cooperatives or

family-owned on-farm operations in the United States, primarily for using protein

meals as animal feed. There is an increasing use of extrusion-expelling technology

to produce identity-preserved soybean oil and protein products for niche market.

The advantages of small tonnage requirement, no flammable solvent used, low initi-

al capital investment, and unique products have made this processing technology

very appealing for many soybean growers and processors.

ALTERNATIVE REFINING METHODS 611

Alternative techniques are being developed for refining soybean oil produced by

mechanical means. Simple refining methods were explored to process extruded-

expelled (E-E) soybean oils with various fatty acid compositions (174, 177). E-E

oils can be easily water degummed to very low phosphorus levels. Free fatty

acid content was reduced to 0.04% by adsorption treatment with Magnesol1, a

commercial magnesium silicate product from Dallas Group of America (Jefferson-

ville, IN). This material also adsorbed primary and secondary oil oxidation pro-

ducts. A mild steam deodorization as the last processing step produced good-

quality soybean oil. This adsorption refining procedure was much milder than con-

ventional refining, as indicated by little formation of primary and secondary lipid

oxidation products and less loss of tocopherol during refining.

10. COPRODUCTS AND UTILIZATION

10.1. Lecithin

Soybean lecithin is the predominant source of food and pharmaceutical lecithin

because of its availability and outstanding functionality. The composition of crude

soy lecithin is shown in Table 14. As a result of the presence of a large amount of

neutral oil, crude lecithin is usually de-oiled to improve its functionality. De-oiling

is based on the solubility difference of neutral and polar lipids in acetone, in which

the phospholipids are precipitated and separated. Alcohol fractionation of de-oiled

lecithin can further separate lecithin into an alcohol-soluble fraction that is enriched

with phosphatidylcholine and an alcohol-insoluble fraction enriched with phospha-

TABLE 14. Composition of Commercial Soy Lecithin in Comparison with Egg Lecithin,

wt % (195).

Compounds Soy Lecithin Egg Lecithin

Phosphatidylcholine 10–15 65–70

Phosphatidylethanolamine 9–12 9–13

Phosphatidylinositol 8–10 –

Phosphatidylserine 1–2 –

Phosphatidic acid 2–3 –

Lysophosphatidylcholine 1–2 2–4

Lysophosphatidylethanolamine 1–2 2–4

Phytoglycolipids 4–7 –

Phytosterines 0.5–2.0 –

Other phosphorus-containing lipids 5–8 –

Sphingomyelin – 2–3

Carbohydrate 2–3 –

Free fatty acids max 1 max 1

Mono-, diacylglycerols max 1 Trace

Water max 1.5 max 1.5

Triacylglycerols 35–40 10–15

612 SOYBEAN OIL

tidylinositol. The phosphatidylcholine-enriched fraction is an excellent oil-in-water

emulsifier, and the phosphoinositol-enriched fraction is a good water-in-oil emulsi-

fier that is often used in the chocolate industry. The typical composition of de-oiled

and fractionated lecithin products is shown in Table 15.

Supercritical CO2 extraction also has been used to selectively extract phospha-

tidylcholine from de-oiled soybean lecithin (197). The effects of temperature, pres-

sure, and amount of ethanol on phosphatidylcholine extraction were examined, and

a high-purity product could be produced with optimized conditions.

Lecithin recovered from solvent-extracted soybean oil had different phospho-

lipid class compositions from those produced by mechanical pressing (198). The

percentage of phosphatidylcholine was considerably higher in lecithin recovered

from extruded-expelled oil than from solvent-extracted oil. The phosphatidylcho-

line- and phosphatidylinositol-enriched fractions produced by ethanol extraction

of the crude lecithin also showed different functional properties (199).

Soybean lecithins can be chemically altered to modify their emulsifying proper-

ties and improve their dispersibility in aqueous systems. Phospholipids may be

hydrolyzed by acid, base, or enzyme to achieve better hydrophilic and emulsifica-

tion properties. Hydroxylation of lecithin improves its oil-in-water emulsification

property and water dispersibility. Acetylation creates improved fluidity and emul-

sification, water dispersion properties, and heat stability (200).

10.2. Deodorizer Distillate

Deodorizer distillate is the material collected from the steam distillation of oils. It is

a mixture of free fatty acids (especially during physical refining) tocopherols, phy-

tosterols and their esters, hydrocarbons, and lipid oxidation products. The quality

and composition of deodorizer distillate depends on the feedstock oil composition

and processing conditions. Tocopherols and sterols are the most valuable compo-

nents that can be recovered from the distillate, and they are used in the nutrition

supplement and pharmaceutical industries (201). Typical soybean deodorizer distil-

late contains about 33% unsaponifiable matters, of which 11% is tocopherol and

18% sterol (202).

TABLE 15. Typical Composition (%) of Commercially Refined Soy Lecithin

Products (196).

Lecithin Lecithin Lecithin

Oil-Free Alcohol-Soluble Alcohol-Insoluble

Phosphatidylcholine 29 60 4

Phosphatidylethanolamine 29 30 29

Phosphatidylinositol and glycolipid 32 2 55

Neutral oil 3 4 4

Others 7 4 8

Emulsion type favored w/o or o/w o/w w/o

COPRODUCTS AND UTILIZATION 613

Soybean tocopherols are the major source of natural fat-soluble antioxidants and

Vitamin E. The Vitamin E activity of natural d-a-tocopherol is much greater that

that of synthetic Vitamin E, which is a mixture of eight stereoisomers (203). Phy-

tosterols are used as raw materials for over 75% of the world’s steroid production.

The more recent application of phytosterol, phytostanol, and their fatty acid esters

in margarine and table spreads is based on the blood cholesterol-lowering effect of

these compounds (204, 205). The recent development of functional foods containing phy-

tosterols has been reviewed by Hollingsworth (206) and Hicks and Moreau (207).

The preparation of high-purity tocopherols and phytosterols involves steps such

as molecular distillation, adduct formation, liquid-liquid extraction, supercritical

fluid extraction, saponification, and chromatography (175). The extraction of toco-

pherols from soybean oil deodorizer distillate by urea inclusion and saponification

of free fatty acids resulted in good recovery of tocopherols (208). To improve the

separation of sterols and tocopherols, Shimada et al. (209) used a lipase to esterify

sterols with free fatty acids. Then the steryl esters and tocopherols were separated

better by molecular distillation. Chang et al. (210) used supercritical fluid CO2

extraction to recover tocopherols and sterols from soybean oil deodorizer distillate.

A patent by Sumner et al. (211) advocated treatment of the distillate with methanol

to converted free fatty acids and other fatty acid esters to methyl esters that can then

be removed by a stripping operation. Then separation of sterols and tocopherols

could be carried out by molecular distillation.

10.3. Soapstock

Soap is recovered from alkaline neutralization of the crude or degummed soybean

oil. Soap consists of water, free fatty acids, neutral oil, phospholipids, unsaponifi-

able matter, proteins, and mucilaginous substances. Its composition depends on

seed quality and oil extraction and refining conditions. Soapstock is the least valu-

able byproduct from oil processing, and it is generated at a rate of about 6% of the

volume of crude soybean oil refined (212), amounting to as much as 0.8 million MT

in the United States annually. The majority of the soap or acidulated soap is used as

a feed ingredient contributing metabolizable energy. Soybean oil can be refined

using potassium hydroxide and acidulated with sulfuric acid, followed by neutrali-

zation with ammonia rather than sodium hydroxide to produce a fertilizer (213).

Soybean oil methyl esters can also be produced from soapstock (214–218) for bio-

diesel applications.

11. FOOD AND BIOBASED PRODUCT USES OF SOYBEAN OIL

11.1. Distribution of Soybean Oil Utilization

In 2001–2002, when 8.32 million MT (18,300 million pounds) of soybean oil was

used in the United States, over 97% (8.09 million MT, 17,800 million pounds) was

used for food, with the remainder used in nonfood products (219). Among the food

614 SOYBEAN OIL

uses, about 48% (3.89 million MT, 8,570 million pounds) was for shortening, 43%

(3.58 million MT, 7,897 million pounds) for cooking and salad oils, 7% (0.56 mil-

lion MT, 1,237 million pounds) for margarine, and 1% (0.06 million MT, 125 mil-

lion pounds) for other food uses. Soybean oil is used to produce about 95% of the

total margarine and 83% of the total shortening consumed in the United States.

Among the 0.24 million MT (519 million pounds) used in nonfood products,

about 16% (0.04 million MT, 85 million pounds) was for resins and plastics,

12% (0.03 million MT, 60 million pounds) for paint and varnish, 13% (0.03 million

MT, 68 million pounds) for fatty acids, and 59% (0.14 million MT, 306 million

pounds) for a myriad of other inedible uses. The use of soybean oil in lubricants

(220), oleochemicals (221), and bioplastics (222), and the production of methyl

soyate for environmentally friendly solvents (223, 224) and for blending with diesel

fuel to produce biodiesel (20% methyl soyate/80% diesel fuel) (225) are significant

parts of the soy oil used in nonfood applications (226). Usage of soybean oil to

make biodiesel is likely to increase in future years because several new plants

are planned for construction as a result of the recent Farm Bill of 2002 providing

financial incentives for producing biobiesel. Some states, notably Minnesota, have

enacted legislation that provides biodiesel tax incentives. Biodiesel interests have

become organized as the National Biodiesel Board (Jefferson City, MO) and the

Renewable Fuels Association (Washington, DC), and exercise considerable politi-

cal influence. During 2002, 57 million liters (15 million gal) of biodiesel were pro-

duced in the United States (227), almost three times that which was produced in

2001.

The usage of soybean oil in food products is similar to other oils, and these uses

and products are discussed in more detail for all oils in other chapters of this edi-

tion. This chapter will focus on specifics of soybean oil in those uses. The major

products in which soybean oil is consumed are cooking and salad oils, frying oils

and fats, baking shortenings, and margarine. Only minor amounts of soybean oil are

used in vegetable dairy products and confectionery products.

11.2. Trading Rules for Crude and Refined Soybean Oils

As the U.S. government does not have trading rules, the National Oilseed Proces-

sors Association (NOPA, Washington, D.C.) has established them, including quality

specifications, to facilitate trade and marketing of three types of oils: crude

degummed, once-refined, and fully refined soybean oils (Table 16). These rules

are also available on the Internet (228). Factors that impact grade of crude

degummed and once-refined soybean oils are moisture and volatile matter content,

flash point, free fatty acid content, smoke point, unsaponifiable matter content,

green color, phosphorus content, and refined bleached color. The flash point reflects

the presence of residual hexane, and the other factors reflect expected refining loss.

For fully refined soybean oils, the flavor, cold test values, peroxide value, and AOM

(Active Oxygen Method) are additional considerations that reflect crystallization

at low temperatures and stability to oxidation. Crude soybean oil is sold as

degummed oil because the gums tend to spontaneously hydrate and settle out during

FOOD AND BIOBASED PRODUCT USES OF SOYBEAN 615

transportation and storage, which cause numerous handling problems. Once-refined

soybean oil is seldom traded anymore because most buyers do their own refining or

purchase fully refined oil. End-users typically have their own specifications for fully

refined soybean oil and use the NOPA values as bases for their more stringent spe-

cifications (136).

11.3. Cooking and Salad Oils

In most parts of the world, both cooking and salad oils from soybeans are refined to

have bland taste and light color. For other oils, distinct flavors and dark colors may

be acceptable. Important distinctions between salad oils, cooking oils, and frying

TABLE 16. Trading Specifications for Crude Degummed, Once-Refined and Fully Refined

Soybean Oils (228).

Methods of

Factor Crude Degummed Once-Refineda Fully Refineda Analysisb

Moisture and volatile

matter and

0.3 max.c 0.10 max. 0.10 max.d Ca 2d-25

insoluble impurities (%) (up to 0.15 with

discount)

(up to 0.15 with

discount)

Ca 3a-46

Flash point (�C) 121 min. 121 min. Cc 9c-95

Free fatty acids

(% as oleic)

0.75 max. 0.10 max. 0.05 max. Ca 5a-40

(up to 1.25 with

discount)

(up to 0.15 with

discount)

Unsaponifiable

matter (%)

1.5 max. 1.5 max. 1.5 max. Ca 6a-40

Presence of fish and

marine animal oils

Neg. 28.121

Phosphorus (%) 0.02 max. Ca 12-55

(up to 0.025 with

discount)

Refined bleached color 3.5 Red max. 20 Yellow, 2.0 Cc 8e-63

(Lovibond) Red, max. Cc 13b-45

Green color None

Flavor Bland

Cold test (hr) 5.5 min. Cc 11-53

Peroxide value

(meg/kg)

2.0 max. Cd 8-53

AOM Stability

(hr to 35 PV)

8 min. Cd 12-57

aThe oil shall be clear and brilliant in appearance at 21–29�C (70–85�F) and free from settlings in this

temperature range.bAnalyses in accordance with the Official and Tentative Methods of the American Oil Chemist’s Society except

for presence of fish and marine animal oils in accordance with Association of Official Analytical Chemists

methods.cIncludes insoluble impurities as determined by AOCS Method Ca 3-46.dOil shall be free of settlings or foreign matter of any kind.

616 SOYBEAN OIL

oils, however, reflect their differences in oxidative and thermal stabilities. Cooking

and frying oils need to be more stable to oxidation than salad oil because of the

higher temperatures to which cooking oils are exposed. Temperature stability is

especially required in fats and oils used in deep-fat frying. Salad oils must be phy-

sically stable so that they do not crystalize at refrigerated temperatures.

As soybean oil contains relatively great amounts of the polyunsaturates, notably

unstable linoleate (61%) and linolenate (7.8%), partial hydrogenation is customary

to make cooking or salad oils more stable to oxidation. Typical specifications for

different cooking and salad oils are shown in Table 17.

Synthetic antioxidants, such as butylated hydroxyanisole (BHA), butylated

hydroxytoluene (BHT), propyl gallate (PG), ascorbyl palmitate, and tertiary-butyl-

hydroquinone (TBHQ), are used in soybean cooking oils and frying fats (230).

These antioxidants are typically added at 0.01% for one antioxidant and 0.02% total

for two or more. Natural antioxidants, derived from sage, rosemary, and green tea,

are increasingly popular because of consumer preferences for natural food ingredi-

ents (231).

Salad oils differ from cooking oils in their tolerance to cold temperatures with-

out crystallizing. Salad oils must not crystallize, cloud, or leave deposits of any kind

when stored at refrigerator temperatures (4.4�C) and are defined as such. Soybean

oil used as a salad oil should not cloud or produce any visible crystals and remain

brilliant and clear for a minimum of 5.5 hr at 0�C. Fully refined soybean oil can be

directly used as salad oil because it will normally meet this specification, whereas

other oils, such as sunflower and corn, must be dewaxed before they can meet typi-

cal salad oil specifications. Soybean oil may be partially hydrogenated and then

winterized to achieve greater oxidative stability and still not crystallize nor lose

proper emulsion properties when refrigerated, although most of the soybean oil

used in commercial dressings is not hydrogenated.

New nutrition-oriented salad and cooking oils have been developed in recent

years. LoSatSoy is an oil low in saturated fatty acids that was developed at Iowa

State University, licensed to Pioneer Hybrid International (Johnston, IA), and com-

mercialized as a salad or cooking oil. This specialty soybean oil has one-half the

amount of saturated fatty acids in normal soybean oil (7% versus 15%); therefore, it

is promoted as having improved nutritional and health benefits.

Other specialty soybean oils, low (<2% or <1%) in linolenate and with

improved oxidative stabilities in salad and cooking oil applications, are comparable

with typical soybean oil that is partially hydrogenated. Today, low-linolenic-acid

soybean oil is an attractive alternative to hydrogenated oil that contains trans-fatty

acids. Beginning in 2006, labeled food products must disclose both the grams of saturated

fat and grams of trans-fat per serving (232). This is inducing food companies to

eliminate or significantly reduce trans-fatty acid contents of their products.

All specialty soybean oils require identity-preserved soybean production, crush-

ing, and refining systems. As financial incentives are needed all along the produc-

tion process to compensate for increased costs of identity preservation, specialty

soybean oils command premium consumer prices and have been slow to impact

soybean oil markets.

FOOD AND BIOBASED PRODUCT USES OF SOYBEAN 617

TABLE 17. Trading Specifications for Soybean Cooking and Salad Oils (229).

Cooking and Salad Oil

—————————————————————————

Factor Refined, Fully, Fully, HWb Analytical

Deodorized Refineda Refineda Soybean Methodc

Source of specifica-

tions

Fedd,e NSPA ASCSf Fedd

Moisture and volatile 0.06 max. 0.10 max. 0.10 max. 0.06 max. Ca 2d-25

matter (max) (%) (0.14 with

discount)

Unsaponifiable con-

tent (%)

– 1.5 max. – – Ca 6a-40

Flash point, �C – – 228 min. – Cc 9b-55

Free fatty acids (wt%)

as oleic

0.05 max.g,h 0.05 max. 0.05 max. 0.05 max.g,h Ca 5a-40

Red color (Lovibond) 4 max. 2.0 max. 2.0 (2.6 with

discount)

2.0 max. Cc 8b-52

Cc 8e-63

Cc 13b-45

Yellow color (Lovi-

bond)

35 max. 20 max. 20 max. 20 max. Cc 8b-52

Cc 8e-63

Cc 13b-45

Peroxide value (meg/

kg)

1.0 max.h 2.0 max. 0.5 max.

(1.0 with

discount)

1.0 max.h Cd 8-53

Fat stability by AOM

methodi

(a) Peroxide value

after 8 hr

– 35 max. 35 max. – Cd 12–57

(b) Peroxide value

of 100 or less at

indicated no. of hr

15 min.h – – 25 min.h Cd 12-57

Cold test (hr)

Free from sediment

and foreign matter of

any kind

Yesj Yes Yes Yesj Ca 3-46

Clear and brilliant at

21–29�CYes Yes Yes Yes –

Fish oil and marine

animal oil test

– Neg. – – k

Iodine value – – – 105–115 Cd 1-25

Linolenic acid (wt%) – – – 3.0 max. by Cd 7-58

or 3.5 max.

by

Cd 1-62

Odor and flavor 1 1 1 1 m

618 SOYBEAN OIL

11.4. Frying Oils and Fats

In addition to its use as a common household cooking oil, soybean oil is used

widely in home and commercial deep-fat frying procedures. The popularity of fried

foods among U.S. consumers has created a large market for stable frying oils and

for fast-food establishments. Typical untreated cooking and salad oils, including

soybean oil, are not suitable for frying applications because they oxidize too

quickly. Thus, the oils must be altered to make them stable to the frying treatment.

Heat treatments, such as commercial and household frying, accelerate autoxida-

tion. The heat itself causes oxidation and breakdown of the fat. In addition, when

TABLE 17. (Continued )

Additives

/preservative

n o p n, q –

Permitted/required

aTypically a refined, bleached, and deodorized oil.bRefined, bleached, partially hydrogenated, winterized, and deodorized, pure soybean oil.cAnalyses in accordance with the Official and Tentative Methods of the American Oil Chemist’s Society

Champaign, Illinois, unless indicated otherwise.dFederal specifications No. JJJ-S-30G dated March 24, 1978, issued by U.S. General Services Administration,

Washington, D.C.eThe salad oil may contain properly refined and deodorized cottonseed, corn, peanut, soybean, sesame,

sunflower, or safflower vegetable oils or a mixture of these oils. Olive oil shall not be used. Edible vegetable

oils not specified may also be used provided they are in accordance with good commercial practice.fSpecifications per announcement PV-50–1 dated June 17, 1976, issued by Agricultural Stabilization and

Conservation Service, U.S. Department of Agriculture, Shawnee Mission, Kansas.g 0.05% will be acceptable if propyl gallate has been added as an antioxidant or as a component in an

antioxidant.hDetermination will be made within 7 days after packaging each lot.iActive oxygen method.jExclusive of particles of resinous flux material from can manufacture.kAssociation of Official Analytical Chemists’ Method No. 28.107.lThe oil after heating shall be bland and free from beany, rancid, painty, musty, soapy, fishy, metallic, and other

undesirable or foreign flavors and odors when tested by the method prescribed in footnote m within 7 days

after packaging each lot.mApproximately 50 g of the finished product shall be placed in a clean 150-mL Pyrex glass beaker and heated

to a temperature 177 � 3�C. The oil shall be examined for odor at this temperature, and for flavor, each

cooling to approximately 38�C. From Federal Specification JJJ-S-30G.nHeavy metal scavengers, antifoaming agents, and antioxidant materials may be added to improve the

keeping quality and use performance of the oils. The ASCA specifications also permit the addition of

oxystearin. Such additives should be of a kind and at levels permitted in edible oil products under the federal

Food, Drug, and Cosmetic Act and regulations promulgated thereunder.oPreservatives ‘‘generally recognized as safe’’ are permitted.pDuring the cooling stage of deodorization, 0.005% of citric acid or 0.006% of monoisopropyl citrate shall be

added to the oil.qThe packaging gas shall be of food-grade quality and may consist of pure nitrogen or a mixture of nitrogen

and approximately 10% of carbon dioxide plus other inert gases in the atmosphere, but it shall contain no

more than 0.005% oxygen. Maximum permissible oxygen content of the headspace gas within 15 min after

the oil is packaged is 0.50% as measured at standard temperature and pressure. Measurement shall be made

at time of packaging or within 15 min thereafter. For method of analysis, see Bulletin 916, issued in 1963 by

American Dry Milk Institute, Chicago, IL.

FOOD AND BIOBASED PRODUCT USES OF SOYBEAN 619

fats are heated in the presence of moisture, as often is the case in food applications,

fatty acids are released via hydrolysis of the ester linkages (233). The free fatty

acids, in turn, can accelerate oxidation of the oil. Decomposition and condensation

of hydroperoxides also produces a multitude of nonvolatile monomeric products,

including di- and tri-oxygenated esters, and dimeric and polymeric materials, espe-

cially at elevated temperature. Many of these dimers and polymers are known to be

rich sources of volatile carbonyl compounds and decrease the flavor and oxidative

stability of soybean oil (234). These high-molecular-weight materials also can pro-

duce a series of physical and chemical changes to the oil and food products, includ-

ing increased viscosity, polarity, free-fatty acid content, development of dark color,

and an increased tendency of the oil to foam (233).

A typical soybean oil shortening is generally hydrogenated to enhance its

stability, making it suitable for frying procedures. In addition, polydimethylsiloxane

is routinely added at a level of 0.02–2 ppm as an antifoaming agent, which greatly

extends the frying life of soybean oil (235). The antioxidants mentioned in the

subsection on Cooking and Salad Oils provide oil stability prior to frying and

can enhance the oxidative stability of the fried food. Even though most antioxidants

are volatile at frying temperatures, with their concentration decreasing during

frying, some antioxidant is transferred to and retained in the food (carry through),

thus providing antioxidant protection in the food during storage. In tests,

heated palm olein with no frying lost 70% of its original BHT and 60% of the

original BHA after 8 hr (236). TBHQ being the highest molecular weight (lowest

volatility) of the typical antioxidnts, provides the greatest carry-through benefit

(237).

Extensive hydrogenation produces flaked fats or shortening-like products for fry-

ing applications, which offer convenience in filling fryers and excellent frying sta-

bility. Unfortunately, the process of hydrogenation creates trans-fatty acids as

byproducts of the reaction As noted elsewhere in this chapter, recent concerns about

the presence of trans-fatty acids in our diets, and the subsequent new labeling

requirements for trans-fatty acids (232), have prompted food manufacturers and

oil producers to explore alternative treatments to create soybean oil that is stable

to frying.

One procedure to increase stability without creating trans-fatty acids involves

adding a small amount of a fully hydrogenated oil (hardstock) to a typical soybean

oil. The blended oil is then interesterified to create a stable frying oil without

trans-fatty acids. In a recent study, the low-linolenate soybean oil noted in the

subsection on Cooking and Salad Oils, when blended with 5% of a soybean oil

hardstock, was as stable as a traditional trans-fat-containing soybean oil that

had been stabilized for deep-fat frying, while still retaining excellent flavor

characteristics (238). Another approach to enhance frying stability of soybean

oils is to increase the oleate concentration in the soybean oil created by the

plant, either through traditional plant breeding or biotechnological methods. The

resulting oil, however, when used in frying, creates a fried food with a stale,

waxy-like flavor that lacks the desirable flavor components typical of a fried

food (239, 240).

620 SOYBEAN OIL

11.5. Mayonnaise and Salad Dressing

In the United States, mayonnaise, salad dressing, and French dressing are defined

by Standards of Identity issued by the U.S. Food and Drug Administration (FDA;

Code of Federal Regulations, Section 21, 169.140) (241). The Food, Drug and Cos-

metic Act of 1930 and later revisions and amendments were promulgated to prevent

adulteration and misrepresentation of certain food products by establishing

Standards of Identity.

Mayonnaise is defined as a semisolid food prepared with not less than 65% vege-

table oil, and egg yolk and vinegar. Most mayonnaise in the United States, however,

contains 75–82% oil, to get the proper texture (242). Soybean oil is usually used in

mayonnaise but winterized cottonseed, corn, and canola and hydrogenated soybean

oil also can be used. Mayonnaise is an oil-in-water emulsion with oil droplets mea-

suring 1–2 mm in diameter. The higher the oil content, the more tightly the oil dro-

plets are packed in the continuous water phase and thus, the greater the viscosity

and rigidity. Mayonnaise production is partly an art because of the difficulty of pro-

ducing an oil-in-water emulsion in which the dispersed phase has seven times more

volume than the continuous phase. The protein in the egg yolk solids is the only

emulsifier allowed and processing conditions play critical roles in achieving

high-quality and high-stability mayonnaise.

Salad dressings are also oil-in-water emulsions and were developed as alterna-

tives to mayonnaise. The Standard of Identity (21 CFR, 1699.150) requires that sal-

ad dressings contain not less than 30% vegetable oil (but most contain 35–50% oil),

vinegar, 4% egg yolk, and starch. For texture and viscosity, salad dressings rely

on starch, in contrast to mayonnaise, which depends on greater oil content. The oils

used in salad dressings are selected using the same criteria for mayonnaise.

The qualities of mayonnaise and salad dressing are determined by the physical

and oxidative stability of its lipid components. Phase separation or emulsion

breakdown is caused by mechanical shock, agitation, extreme temperatures, or

fat crystallization. Oxidation of vegetable oil and egg lipid also can occur. As the

quality of oil plays a major role in the flavor stability of these products, only the

best quality salad oil should be used. It is particularly important to use salad oils

with long cloud point times (high cold test hours). If fat crystals form during storage

at refrigerated temperatures, the emulsion will break and the product will become

unsightly with visible free oil. Crystal inhibitors, such as oxystearine, lecithin, and

polyglycerol esters, are allowed to prevent crystallization and emulsion breakdown.

Although mayonnaise and salad dressings are spoonable products due to their

high viscosity, French dressing is a pourable oil-in-water dressing. French dressing

must contain 35% oil as defined by a Standard of Identity (21 CFR, 169.115). Egg

products are optional. Other dressings, such as Thousand Island, are not subject to

Standards of Identity, and any ingredients can be used. Pourable dressings can be in

two different finished forms; emulsion or two phases depending on whether the pro-

duct is homogenized. The oil used in these products is predominantly soybean salad

oil in the United States. In Canada and Europe, other salad oils are often used,

depending on the availability and costs of those vegetable oils in each specific region.

FOOD AND BIOBASED PRODUCT USES OF SOYBEAN 621

As the oil contents of mayonnaise, salad dressings, and French dressing are high,

it is important to prepare them from salad oils that taste bland and are relatively

stable to oxidation. Peroxide values of the oil should be <2 meq/kg. Even early

stages of oxidation can be detected in mayonnaise and salad dressings as ‘‘grassy’’

and ‘‘beany’’ flavors. Packaging with an inert headspace is important to prevent

oxidation during distribution, retailing, and consumer storage. Storage under refrig-

eration is important once the package is opened and the headspace gas becomes

replaced with air.

11.6. Margarine

Margarine was first produced in 1869 by the French chemist Hippolyte Megge

Mouries. During the Franco-Prussian War, he was awarded a prize and patent for

his invention of a butter substitute. It was not until the 1940s, however, that mar-

garine became widely used. Until then, the powerful dairy industry in the United

States prevented the sale of colored margarine in many states, and consumers did

not readily accept white table margarine. Today, more than twice as much margar-

ine is consumed as butter per capita in the United States, and margarine is no longer

considered a cheap imitation of butter. Unlike butter, margarine can be formulated

from a variety of fats and oils to give a variety of physical and functional properties,

which are needed in many food applications today.

In the United States, margarine or oleomargarine is also controlled by an FDA

Standard of Identity (21 CFR, 166.110), requiring at least 80% fat. Soybean oil is

predominantly used in the United States, followed by cottonseed and corn oils. The

other 20% of the margarine formulation may be made up of water and other

optional ingredients, including milk products, soy protein isolate, salt, selected

emulsifiers (up to 0.5%), mold inhibitors, antioxidants, color additives, flavorings,

and acidulants. Margarine is a water-in-oil emulsion.

The traditional retail form of margarine is stick margarine, but margarine is now

also marketed as pourable and soft tub products. Margarine may also be sold as a

whipped product in which air or an inert gas is incorporated. Still other margarine-

like forms, including polyunsaturated and low-fat spreads, have been developed to

satisfy consumer demands for improved convenience and reduced saturated fat and

calories. In addition to the traditional use as a table food, margarine is also widely

used in baking applications such as in cookies and as roll-in fats for puff and Danish

pastries.

A significant recent consumer trend is increased demand for margarine-like

spreads that are not controlled by a Standard of Identity and that contain much

less fat. Most spreads contain 40–60% fat with 40% fat spreads being more popular

in Europe and 60% in the United States. During the past 15 years, however, very

low-fat spreads containing less than 20% fat have been introduced. As a result of

these trends, there are significantly fewer 80%-fat margarine products available in

the United States today than in the previous decades. Stabilizing these high levels of

aqueous phases in such a small amount of fat as the continuous phase requires spe-

cial equipment to generate the necessary shear and higher amounts of emulsifiers.

622 SOYBEAN OIL

Moustafa (243) reports that the aqueous droplets must no longer be spherical but

rather polyhedral when loading levels of the aqueous phase exceed 74%.

Margarine processing includes blending the fats separately from the aqueous

phase ingredients and water, dispersing and emulsifying the aqueous phase within

the fat phase, chilling to solidify the fats, pin working the solidified mass, resting,

forming, and packaging. The ingredients are emulsified before being fed into a

swept-surface heat exchanger for crystallizing. The mass emerging from the cool-

ing tubes is partially solidified, and it is further crystallized in the working unit. The

texture of the product is further modified in the resting tube before the margarine is

packaged.

Margarine and shortening have fat crystal networks in which liquid oil is

entrained. As a result, they exhibit a yield stress that must be exceeded before

the product begins to flow as a viscous fluid. The yield stress is related to spread-

ability. The rheological properties of margarine have been discussed by Segura

et al. (244).

In North America, margarines may be composed of blends of hydrogenated soy-

bean oil and palm oil, partially hydrogenated soybean oil and cottonseed oil, liquid

soybean oil and partially hydrogenated soybean oil, liquid corn oil and hydroge-

nated corn oil, or simply hydrogenated soybean oil. Most oil blends contain high

levels of soybean oil to keep costs competitive. Table 18 shows some typical com-

positions and properties of margarine.

The most important functional properties of margarines and spreads are spread-

ability and hardness, oiliness, and melting characteristics. These properties relate to

fat level, proportion of solid fat, fat melting point, and crystal form. Diverse tex-

tures and functionalities can be achieved by varying the extent of hydrogenation.

Consistency and emulsion stability depend on the amount and type of crystallized

fat. Spreadability and hardness can be predicted by the solid fat index and penetra-

tion measurements. A cone penetrometer is typically used to determine margarine

hardness (245). Typical margarines should be spreadable at refrigeration tempera-

tures, remain semisolid at ambient temperatures, and melt at less than body

temperature. Oil-off refers to the separation of liquid fat when the fat crystals no

longer form a network able to hold the liquid oil.

TABLE 18. Compositions and Properties of Hydrogenated and Interesterified Soybean

Margarine Oils (187).

Melting Point

——————————————— Trans IV

Soybean Oil Type 10�C 21.1�C 33.3�C (�C) (%) (calc)

Hydrogenated Stick margarine 28.6 18.9 5.3 46 31.0 92.1

Hydrogenated Tub margarine 15.6 8.8 1.3 46 23.2 108.0

Hydrogenated Tub margarine 7.1 4.5 2.0 46 12.9 121.8

Interesterified 90:10a 1.7 1.3 0.2 40 1.7 123.8

Interesterified 85:15a 4.3 2.2 0.9 46 2.1 116.6

Interesterified 80:20a 8.0 3.5 2.2 47 1.6 109.4

FOOD AND BIOBASED PRODUCT USES OF SOYBEAN 623

Fats exhibit polymorphism in which they can exist in different crystalline forms

depending on how the triacylglycerols pack in the crystal and a, b0, and b poly-

morphs are known. The preferred polymorphic form for margarine is b0, which

gives a smooth, pleasing mouthfeel and proper spreadability. Despite hydrogenated

soybean oil’s tendency to form b crystals, it is used in over 90% of all margarines

and table spreads in the United States. The less heterogeneous the fatty acid com-

position of the hydrogenated fats, the more it is b tending. Hydrogenated fats richer

in trans-isomers are less b tending and tend to produce margarines with smoother

textures. Blending small amounts of b0-tending base fats (palm and cottonseed

oils) or different soybean base oils increase fatty acid heterogeneity favoring b0

crystal stability. Blending unmodified oils with oils that have been hydrogenated

to various degrees allows the production of margarines with desirable texture.

The greater the number of base stocks available, the greater the flexibility to pro-

duce a wide range of products and the higher the tolerance to processing conditions.

Different procedures for designing good margarine from various base stocks were

evaluated by Cho et al. (246).

Base oils for margarine must be hydrogenated to achieve the desired solid-fat

content with the consequential isomerization of some fatty acids. The new regula-

tions requiring reporting of trans-fats content on labels may dissuade some consu-

mers from using traditional margarine. Emken (247) reported that some traditional

margarines may have as much as 21% trans-fatty acids while Kellens (187) found

as much as 31%, and D’Souza et al. (248) reported that the high-melting acylgly-

cerols contained in hydrogenated base stocks used for formulating North American

margarines have 33.1–45.0% trans-fatty acid content in stick margarine and 22.4–

30.1% trans-fatty acid content in soft margarine. Trans-acyl groups contribute to

the firmness of margarine. A recent comprehensive review concluded that consum-

ing more than 4% of total calories as trans-fatty acids may raise plasma lipid levels

(249) and may cause heart disease (250, 251).

Some companies are producing low-trans- or zero trans-margarines by random

(252) or directed interesterification of mixtures of unhydrogenated and fully hydro-

genated soybean oils and other fats (253). To produce these products, a liquid oil

and completely hydrogenated hardstock are interesterified, so that proper plasticity

can be obtained. Oils that contain considerable amounts of palmitic acid favorably

influence crystallization and polymorphic form of the interesterified fat blends

(254).

Chemical interesterification is conveniently achieved by using alkali metal

methylates as a catalyst. Microbial lipases are also used as biocatalysts in enzy-

matic interesterification. In contrast to the chemical process, the enzymatic process

can be more selective if an enzyme with positional specificity is used, but this

reaction is usually much slower and more sensitive to reaction conditions. Recent

developments in lipase-catalyzed interesterification have resulted in new industrial

applications of this process (255). Nevertheless, the high costs of enzymes and pro-

cess equipment may limit widespread adoption of this process.

In developing trans-free fat, various methods for laboratory-scale, pilot plant,

and commercial batch reaction were described by Erickson (256). List et al.

624 SOYBEAN OIL

(252) developed a zero-trans margarine by interesterifying 80% refined, bleached,

and deodorized (RBD) soybean oil with 20% fully hydrogenated soybean oil. The

resulting product has a solid fat index comparable with that of conventional pro-

ducts. The randomly interesterified low- [zero-] trans-soybean margarines crystal-

lize in the more favorable b0 crystal form (252) but tend to crystallize slowly after

chilling and result in a product that is harder than desired (257). Addition of 20%

liquid soybean oil to the interesterified oil yielded a softer, more desirable product.

Table 19 presents a typical example of the combined use of hydrogenation, inter-

esterification, and fractionation to produce low-trans fats with physical properties

comparable with partially hydrogenated soybean oil with high trans content.

Alternatively, recent research has focused on soybeans bred for high contents of

saturated fatty acids, some with as much as �43% saturates, 23% palmitate, and

20% stearate compared with the normal �15 % saturates, 11% palmitate, and

4% stearate. Soybeans only produce cis-fatty acids and, thus, there are no sources

of trans-fatty acids in the blends. List et al. (258, 259) showed that soybean oil from

soybeans bred to produce 30–40% saturates was not sufficiently solid to make good

margarine, but soybean oil with elevated saturated fatty acid contents (17–38%)

could be blended with high-melting oils, such as palm oil, interesterified palm

oil, interesterified palm and soybean oils, and cottonseed and soybean hardstocks,

to make a good margarine. Kok et al. (260) used blends (50:50) of traditional soy-

bean oil and oil from soybeans bred to produce oil high in saturated fat (�43% satu-

rates, 23% palmitate, 20% stearate). The blend was then interesterified to produce

oil that was made into soft tub margarines. The small differences in sensory proper-

ties observed in comparisons with other tub margarines indicated the interesterified

product should be quite acceptable to most consumers. List et al. (259) also report

randomly interesterifying (randomizing) neat soybean oil high in saturated fatty

acids (10% palmitate, 18% stearate) gave good margarine without graininess

(SFI values of 5–8 at 10�C, 2–3 at 21.1�C, and 1–2 at 33.3�C).

TABLE 19. Example of Combined Hydrogenation, Interesterification, and Fractionation

to Produce Low Trans-Margarine Fat (187).

Solid Fat Content (%, at �C)

Melting

Iodine Value Point (�C) 10 20 30 40

Soybean oil (SBO) feedstock 134 �7 0 — — —

Fully hydrogenated SBO (FHSBO) 1 71 95 94 94 93

Blending SBO and FHSBO (60:40) 81 63 44 42 39 35

Random interesterification of SBO 81 53 38 33 20 11

and FHSBO (60:40)

Fractionation of the interesterified oil

Soft fraction 91 24 25 1 0 0

Hard fraction 63 58 60 58 45 32

FOOD AND BIOBASED PRODUCT USES OF SOYBEAN 625

11.7. Shortenings

Shortenings are fats of vegetable or animal origin used in baking, but the term

shortening also has been accepted as a term to describe semisolid fats for frying

and cooking. Just as in margarine, the solid fat exists as a tight network of small

crystals, which trap liquid oil. Plastic shortenings differ from margarine in that

shortening is not an emulsion; it is all lipid material and may contain emulsifiers.

Prior to the development of hydrogenation, lard and tallow were the principle short-

ening fats, but these fats lack the diversity of texture and functionality required for

many products. Today, most shortenings contain at least some soybean oil, largely

because it is the least expensive oil that can confer adequate functionality. Short-

ening is available in many forms: plastic and semisolid (cubed, sheeted, and

printed), pourable fluid (with suspended solids), encapsulated powder, and flaked.

Most plastic shortenings are produced by blending oils with hydrogenated fats and

often emulsifiers and solidifying or crystallizing and plasticizing the blend. The

shortening is packaged and tempered by holding it in a quiescent state for several

days at 30�C. During solidification, 10–25% air is often incorporated to improve the

color and texture. Pourable and fluid shortenings are produced by blending appro-

priate oils and emulsifiers. They are crystallized by cooling the fluid mass and stir-

ring the suspended crystals for 4–6 hr at precise temperatures so that large crystals

do not develop, and the fluid becomes stabilized.

Shortenings are added to baked goods to shorten or tenderize them by interrupt-

ing the gluten structure. Shortenings improve mouthfeel and eating qualities, add

lubricity, improve dough-handling properties, contribute flavor and structure, and

promote desirable crumb grain and texture (261). Shortening and tenderizing

effects are especially important in cakes, piecrusts, pastries, cookies, and crackers.

Generally, solid fat indices that change little with temperature are desired for most

shortening applications. Table 20 shows plasticity and melting properties of differ-

ent commercial shortenings. Typical shortening levels are 2–5% in bread, 5–25% in

cake, 20–30% in sweet goods, 30–40% in puff pastry, and 20–35% in piecrusts.

Many plastic shortenings are packaged in 50-lb polyethylene-lined boxes, pri-

marily for use in retail bakeries, e.g., in grocery stores. These are difficult to handle

in large, automated wholesale bakeries. Sometimes, 190-kg drums are used, but are

still difficult to manage and use in the bakery where large amounts are needed.

Pourable and pumpable fluid shortenings were developed to avoid these problems

and are based on soybean oil. However, liquid oils do not cream and aerate well.

The addition of small amounts of hardfats, known as stearine, and various emulsi-

fiers can impart good functional properties to the liquid shortening.

Although adequate quality bread and rolls can be produced without shortening

by using the sponge-and-dough or straight-dough methods, the inclusion of short-

ening increases volume by as much as 25% compared with breads with no short-

ening. This volume increase often is referred to as oven spring, and it reduces

firmness throughout the products storage life. The largest volume of bread is

made by the continuous-mixing method in the United States and shortening is cri-

tical to good quality bread manufactured when using this method. Shortening

626 SOYBEAN OIL

TABLE 20. Typical Compositions and Properties of Baking Shortenings.

Solid Fat Index Melting Point

————————————————————————

Type Composition 10�C 21.1�C 26.7�C 33.3�C 37.8�C (�C)

Cookie and pie dough shortening Partially hydrogenated soybean and palm oils

(unemulsified)

26–30 18–22 16–20 12–15 9.5–13 46–48

Cake and icing shortening Partially hydrogenated soybean and cottonseed oils

(mono and diglycerides)

23–27 16–19 15–18 12–15 9–12 48–50

Yeast-raised sweet goods Partially hydrogenated soybean and palm oils (mono and

diglycerides)

24–29 14–18 9–12 44–47

Fluid cake shortening Partially hydrogenated soybean oil (mono and diglycer-

ides, triglycerol monostearate, sodium stearoyl 2-

lactylate)

High volume cream filling and

icing

Partially hydrogenated soybean and palm oils (mono and

diglycerides, polysorbate 60)

25–28 19–22 18–21 14–17 11–14 47–49

Biscuit shortening Partially hydrogenated soybean and palm oils

(unemulsified)

25–30 16–20 7.5–

11.5

44–47

Roll-in margarine for yeast-raised

sweet goods

Partially hydrogenated soybean and palm oils (mono and

diglycerides)

25–30 15–19 6–9 41–42

Fluid bread shortening Partially hydrogenated soybean oil (mono and

diglycerides)

Fluid bread shortening Partially hydrogenated soybean oil (mono and

diglycerides, sodium stearoyl 2-lactylate, ethoxylated

mono and diglycerides)

delays starch gelatinization and allows the dough to expand more before the struc-

ture is set. Maximum loaf volume, which is a desirable trait in the United States, is

achieved with 6% of emulsified shortening, based on flour weight, but, in practice,

3–5% is normally used. Hardfats in bread shortenings are important in reducing col-

lapse of the loaf’s sidewall. At least 4% hydrogenated lard stearine is desired in

many bread shortenings. Refined, bleached, and deodorized soybean oil is used

in most commercial white pan breads.

Bread shortenings should crystallize in the b form. The base fat of a typical plas-

tic bread shortening is comprised of 90% partially hydrogenated soybean oil (70

IV) and 10% lard stearine (<5 IV); whereas the base fat of a typical fluid bread

shortening is comprised of 95% partially hydrogenated soybean oil (95 IV) and

5% lard stearine. Mono-and diglycerols, are added to reduce staling rate and

more functional emulsifiers, such as sodium steroyl-2-lactylate or ethoxylated or

succinylated mono-and diglycerols, are added as dough conditioners to impart

greater mixing tolerance to enable the bread to withstand abuse without loss of

loaf volume (262).

Using emulsified shortening in layer cakes, cake doughnuts, and muffins

increases volume and reduces air cell size and produces a fine internal grain.

Creaming is defined as the mixing of the shortening over wheat flour particles

and incorporating of air nuclei into the fat. The air nuclei can become sites for

gas bubble formation, which is important in cakemaking. The large number of min-

ute air bubbles incorporated into shortening improves the leavening in baked goods.

For the shortening used in cakes and icings, small (�1 mm) needle-like b0 crystals

are preferred to the larger (5–15 mm) b crystals because the b0 shortenings appear

smooth, provide good aeration, and have better creaming properties (263).

Typically, partially hydrogenated soybean oil is blended with cottonseed or palm

oil hardstock to obtain b0 crystals. Most cake shortenings contain mono- and digly-

cerides to decrease the size of entrained air cells during creaming, to produce finer

air cells and grain in the cake crumb, and obtain a larger volume per unit weight of

batter (specific volume). To achieve proper aeration of fluid cake shortenings, how-

ever, partially hydrogenated soybean oil with b-tending soybean hardstock is

balanced with a-tending emulsifiers, which are typically mono- and di-glycerides

and glyceryl-lacto fatty esters.

Generally, plastic baking shortenings should be firm and plastic, but not brittle or

too soft and oily. Hardfat is added to soybean oil to achieve proper texture, plasti-

city, and creaming properties. Plastic shortenings should be soft and plastic at low

temperatures and still remain semisolid at body temperature.

Soybean oil is excellent for preparing hydrogenated base stocks from which a

wide array of shortenings is made. Up to 50% soybean hardfats are blended with

partially hydrogenated soybean oil in some shortenings. Soybean hardfats, however,

crystallize in the b polymorph unless blended with an equal or greater amount of b0

hardfat, such as hydrogenated palm or cottonseed oil. Partial hydrogenation of the

base soybean oil improves the oxidative stability of the shortening. The amount of

hardstock is varied to achieve the desired texture for the specific product applica-

tion. Various kinds of baked goods need varied shortening functionalities and

628 SOYBEAN OIL

plasticities to produce optimum quality. Plasticity is controlled by achieving the

proper solid fat content or solid fat index. Typical plastic shortenings should

have a relatively flat solid fat index, with solids content in the range of 15% to

30% over the temperature range of 15�C to 32�C (264). One means of getting these

properties is blending 10% hardstock from two sources to get the proper crystal

structure with 90% partially hydrogenated soybean oil (IV 65–80).

11.8. Confectionery and Imitation Dairy Products, and Low-Calorie FatSubstitutes

Very little soybean oil is used to manufacture the hard butters used in confectionery

products or imitation dairy products. For imitation chocolate, enrobing fats, coffee

whiteners, whipped toppings, imitation cheese, frozen desserts, and filled milk,

coconut and palm kernel oils are preferred because of their sharp melting points.

It is important in these applications to have very low solids at body temperature

to prevent a waxy mouthfeel. A few fractionated specialty blends of hydrogenated

soybean oil and hydrogenated cottonseed oils (265) or soybean oil that has been

hydrogenated by using sulfur-treated nickel catalysts to achieve high selectivity

(266) occasionally may be used. These fats, however, are also high in trans-fatty

acids (>40%) and new trans-fat labeling requirements discourage their use. The

advantages of imitation dairy and chocolate products are improved functionality

compared with natural products. Thus, freeze-thaw stability in whipped toppings

and melting properties can be customized for specific applications (267).

As a result of widespread concern about weight control, the production of lipid

materials with reduced or zero calories has been of special interest recently. The

lipid-based fat replacers are esters that resist enzymatic hydrolysis, are poorly

absorbed, have relatively low-energy content, or have different modes of metabo-

lism. Many of these materials can be made from soybean oil or contain soybean oil

fatty acids. Sucrose polyester or other synthetic esters and diacylglycerol oils are

examples of these low-calorie fat substitutes (268–274).

12. OXIDATIVE QUALITY OF SOYBEAN OIL

The oxidative stability of soybean oil is affected by its composition, handling of

beans prior to extraction, processing conditions, and additives. Important composi-

tional factors in soybean oil stability include its fatty acid composition and the pre-

sence of free fatty acids, phospholipids, natural antioxidants, and pigments (275).

Important handling and processing factors include excessive bean moisture,

damage, and temperature; exposure to oxygen; contamination by pro-oxidant

metals; and exposure to light (276).

12.1. Flavor Reversion

Soybean oil has poor oxidative stability, which is a major problem for the soybean

industry. Crude soybean oil has a characteristic ‘‘green-beany’’ flavor, which is

OXIDATIVE QUALITY OF SOYBEAN OIL 629

eliminated during refining, bleaching, and deodorization, to produce a bland-tast-

ing, light-colored oil. During storage, however, refined soybean oil develops a char-

acteristic flavor that often is called ‘‘reversion flavor’’ (277). Prior to the 1940’s,

some believed that soybean oil ‘‘reverted’’ to its unrefined flavor after being refined

and deodorized. Soybean oil was considered extremely light sensitive, and it was

believed to revert if one carried the freshly deodorized oil past the light of a north

window. This reversion was not considered an oxidative phenomenon (278). Actu-

ally, the term ‘‘reversion’’ is a misnomer, because (1) soybean oil does not revert to

its original crude-oil flavor, (2) the effect of light is real but was greatly exagger-

ated, and (3) the off-flavor development is indeed an oxidative reaction (278). Pro-

cedures available for following oxidation prior to the 1940’s involved an iodometric

titration to obtain a peroxide value, but this method was too insensitive to measure

the low degree of oxidation that could be detected in soybean oil by sensory exam-

ination. With the support of more sensitive methods, we now know that upon oxi-

dation, soybean oil develops ‘‘beany and grassy’’ flavors at the early stages (i.e.,

peroxide value 10 or below), rancidity at higher levels of oxidation (peroxide value

of 10 or more), and ‘‘fishy’’ or ‘‘painty’’ flavors at the more advanced stages. These

flavor deterioration characteristics are common to all unsaturated oils containing

significant amounts of linolenate (279). It is now widely accepted that flavor dete-

rioration of soybean oil is an oxidative phenomenon, and that linolenate is the most

important precursor of flavor reversion of soybean oil.

The technology to handle soybean oil’s off-flavor was discovered by an interest-

ing set of circumstances. Near the end of World War II, Warren Goss, who was

commissioned to learn the secrets of the German oilseed industry, found that a

Dr. Tassusky and his daughter Ilona had patented a process involving multiple

washes of crude soybean oil with water or sodium silicate solution and the addition

of 0.01% citric acid to the deodorizer (278). This process worked, not because of

the washings, but because of the addition of citric acid. Now we know that trace

metals accelerate flavor deterioration and that treatment with citric acid or other

metal deactivators is a practical and effective means of improving flavor stability

(274).

12.2. Studies on Oil Oxidation

Extensive work has been done to clarify the mechanism of oil oxidation. It is a free-

radical chain reaction catalyzed by light, heat, and metals, in which molecular oxy-

gen reacts with unsaturated fatty acids to produce hydroperoxides. (280). An impor-

tant factor in initiating the oxidation of unsaturated fats is by exposure to light in

the presence of oxygen and a sensitizer. The activation of ordinary triplet oxygen in

this way forms singlet oxygen, which reacts readily with unsaturated fatty acids

(281). Oxygen is quite soluble in soybean oils (282), which frequently contain nat-

ural photosensitizers, such as chlorophylls or pheophytins. Singlet oxygen readily

reacts with the double bonds of unsaturated fatty acids; for example, singlet oxygen

reacts with methyl linoleate at a rate of at least 1500 times faster than normal triplet

oxygen (282). Once oxidation is initiated by singlet oxygen, the hydroperoxides

630 SOYBEAN OIL

that result can decompose to yield free radicals, and the reaction mode quickly

becomes autocatalytic in the presence of triplet oxygen. A study by Carlsson et

al. (283) found that the photo-oxidation of various unsaturated vegetable oils was

not retarded by known free-radical scavengers, but was retarded by compounds

known to quench singlet oxygen. Furthermore, the degree of retardation apparently

paralleled the singlet oxygen-quenching ability of these compounds.

Commonly, the fatty acids in food lipids are exposed to heat during oil

processing and food manufacture. Once peroxides are formed, they can decompose

and generate free radicals, and the rate of peroxide decomposition increases with

temperature. Such reactions are of extreme importance to both consumers and

processors, because of their flavor significance, and under frying conditions

they can affect the physical, nutritional, and toxological properties of the fried

food.

Enzymes native to plants and animals can initiate oxidation reactions. The most

important and best known of these enzymes is lipoxygenase (linoleate:oxygen oxi-

doreductase, E.C. 1.13.11.12) (LOX) (284, 285). Enzymatic oxidations in plant sys-

tems are mediated by lipoxygenases that use molecular oxygen to catalyze the

oxidation of lipids containing a cis, cis-1,4-pentadiene moiety, such as linoleate

and linolenate. The reaction leads to the formation of hydroperoxides, giving the

same isomers as those formed during autoxidation of linoleate and linolenate. Soy-

beans are a rich source of lipoxygenase isozymes known as LOX-1, LOX-2, and

LOX-3, and their activity is associated with the development of off-flavors, espe-

cially green-beany flavors, in soybean products (285).

Monohydroperoxides are the primary products of lipid oxidation. A variety of

hydroperoxides with positional and geometrical isomers are formed depending

on the position and number of double bonds of the unsaturated fatty acids and

the oxidation mechanism. A number of reviews have been published on the com-

position of isomeric hydroperoxides formed from oxidation of oleate, linoleate, and

linolenate (286, 287–291). The hydroperoxides formed are odorless, but they are

relatively unstable and are the precursors of a variety of volatile and nonvolatile

scission products that are important to the oxidized flavor.

Secondary volatile scission products from primary hydroperoxide decomposition

include aldehydes, alkanes, alkenes, alkynes, alcohols, and hydrocarbons. There are

considerable differences, however, in the flavor significance of these volatile com-

pounds. When estimating the impact of volatile oxidation products on flavor, it is

necessary to know not only their relative concentration, but also their relative

threshold values. One way of evaluating flavor impact is to divide the concentration

by the threshold concentration, although the relative flavor impact may change with

absolute concentration (292). Also, interactions among flavor compounds in the

olfactory response may be important. The relative volatility also may play a role

if a compound must be in the gas phase to reach the olfactory organ. Lee et al.

(293) created equations to relate the flavor impact of individual volatiles, dispersed

in an oil-water emulsion, to a specific concentration of 2-heptanone (Table 21). By

this method, in a fresh and oxidized soybean oil, nonanal contributed the greatest

individual effect on the flavor intensity, followed by trans, trans- and trans,

OXIDATIVE QUALITY OF SOYBEAN OIL 631

cis-2,4-heptadienal, and 2-heptenal. Hexanal produced a large GC peak, but its

effect on flavor intensity was relatively small. More recently, Kao et al. (294)

suggested that particles formed in the oral cavity could transport entrained triacyl-

glycerols to the olfactory epithelium, allowing the triacylglycerols themselves to

impart flavor, thus implying that compounds in oxidized soybean oil do not need

to be volatile to contribute to flavor. They noted that the nutty flavor of fresh

soybean oil could only be observed when the lips were parted or the tongue drawn

away from the palate, both being conditions that generated particles. Liu and

Hammond (295) did further work to support the hypothesis that oral particles

strongly influence flavor perception of ketones typically found in oxidized soybean

oils and of flavor compounds in other foods.

Numerous studies have shown that the off-flavor intensity of soybean oil is cor-

related with its concentration of linolenate. Although the concentrations of both

linoleate and linolenate, which can reach 60–65% in typical soybean oil, undoubt-

edly contribute to soybean oil’s instability, it is not clear why the much smaller

amount of linolenate has such a strong effect on soybean oil flavor. Linolenate is

expected to oxidize about twice as fast as linoleate, but there is seven to eight times

more linoleate than linolenate in typical soybean oil. The flavor compounds pro-

duced by linolenate do not seem to have much lower thresholds than those produced

from linoleate. Possibly flavor interactions in olfaction may account for these

effects.

TABLE 21. Concentrations (ppb in emulsion) of 2-Heptanone Perceived to Have

the Same Flavor Intensity as the Components Isolated from Commercial Soybean

Oil Oxidized at 35�C Under Fluorescent Light for up to 11 Days (293).

Day

——————————————————————————————

Component 0 4 7 11

1-Penten-3-one 0.21 0.46 1.14 1.00

Pentanal 0.27 0.27 0.34 0.35

t-2-pentenal 0.27 0.18 0.24 0.35

Toluene 0.80 0.44 0.37 1.24

Hexanal 2.57 4.67 6.21 6.42

Heptanal 12.35 15.50 17.47 16.19

t-2-Heptenal 5.37 16.02 28.55 38.86

1-Octen-3-one 1.58 1.86 2.43 2.47

1-Octen-3-ol 1.41 3.98 8.74 12.60

t,c-2,4-Heptadienal 17.09 29.30 42.66 48.05

2-Pentylfuran 3.54 4.51 5.26 5.52

t,t-2,4-Heptadienal 20.80 34.27 45.10 48.18

2-Octenal 5.14 8.07 10.82 12.09

Nonanal 76.58 108.18 113.70 101.90

t,c-2,4-Decadienal none none 15.62 26.24

t,t-2,4-Decadienal none none 25.09 43.30

Total 148.0 227.8 323.8 364.8

632 SOYBEAN OIL

12.3. Control/Stabilization Measures

Selective hydrogenation to lower the concentrations of linolenate or linolenate and

linoleate has been practiced to improve the oxidative stability of soybean oil. The

linolenate concentration of soybean oil also can be altered by mutation breeding

and genetic engineering (296).

Autoxidation can be inhibited or retarded by adding low concentrations of chain-

breaking antioxidants that interfere with either chain propagation or initiation

(286). Chain-breaking antioxidants include phenolic and aromatic compounds hin-

dered with bulky alkyl substituents. Common synthetic chain-breaking antioxidants

used in food lipids include butylated hydroxyanisole (BHA), butylated hydroxyto-

luene (BHT), tert-butylhydroquinone (TBHQ), and propyl gallate (PG). This class

of antioxidants react with peroxy free radicals to terminate reaction chains. The

antioxidant radical (A�) formed in Equation 5 should be relatively stable and unable

to initiate or propagate the oxidation chain reaction.

ROO� þ AH ! ROOH þ A� ð5Þ

The phenolic antioxidants achieve stability by forming resonance hybrids (Figure 10)

(297). A radical intermediate, such as semiquinone, can undergo a variety of

reactions, including dismutation, to form a stable quinone and can regenerate the

original hydroquinone (Figure 11). However, these antioxidants generally lose their

efficiency at elevated temperatures, and they are most effective during the induction

period. Once the antioxidant is consumed, oxidation accelerates (297).

Preventive antioxidants reduce the rate of the chain initiation. The most impor-

tant initiation suppressors are metal deactivators that chelate metal ions. Metal

deactivators used for stabilizing edible fat and lipid-containing foods include citric,

phosphoric, tartaric acids, and phospholipids. Peroxide destroyers also are preventive

ROO +

OH

OH

ROOH +

O

O

O

OH

O

OH

Figure 10. The formation of resonance hybrids by phenolic antioxidants.

+

O

OH

O

OH

OH

OH

O

O

+

Figure 11. This dismutation of a semiquinone radical intermediate.

OXIDATIVE QUALITY OF SOYBEAN OIL 633

antioxidants; for example, the sulfur compounds, phosphates, and phosphines

reduce hydroperoxides to more stable alcohols (286).

Ultraviolet light deactivators can prevent oxidation by absorbing irradiation

without the formation of radicals. Examples include pigments such as carbon black,

phenyl salicylate, and a-hydroxybenzophenone. A significant synergistic antioxida-

tive effect can be achieved when chain-breaking and preventive antioxidants are

used together, because they suppress both initiation and propagation. The synergis-

tic effect of common antioxidants in combination with metal inactivators in foods

has been known for some time (33). Loliger (298) showed that the tertiary antiox-

idant system of Vitamin E, Vitamin C, and phospholipids provided the best protec-

tion against oxidative degradation when compared with the two antioxidants used

alone or in combination.

Light deterioration is also an important factor in the storage stability of soybean

oils. Refining and bleaching remove not only natural photosensitizers, but also sing-

let oxygen quenchers such as carotenoids. The restoration of the removed carote-

noids may protect lipids effectively against singlet oxygen deterioration, but the

resulting yellow coloration may be objectionable to consumers. Another approach

to protecting stored oils from light is the use of a package or container that absorbs

the light necessary for photosensitization or that prevents light from reaching the

oil.

Avoiding metal contamination is also very important, as metals such as copper

and iron are strong pro-oxidants for soybean oil. Copper or iron-containing alloys,

except stainless steel, should never be used for equipment involved in direct contact

with soybean oil. Soybean oil may be stored in containers made from carbon steel

that is coated on the interior with an epoxy or polyurethane lacquer, in stainless

steel, or in fiberglass-reinforced polyester.

Displacement of oxygen in container headspaces by nitrogen or carbon dioxide

to � 2% has been shown to reduce oxidation effectively in vegetable oil (299).

Therefore, nitrogen or other inert gas protection should be considered whenever

the oil is to be stored for an extended period or held in the hot, liquid state.

12.4. Evaluation of Finished Oil Quality

Regardless of the official specifications for soybean oil and its products, the ulti-

mate ‘‘proof of the pudding is in the eating’’; that is, sensory evaluation of the odors

and flavors of soybean oil and its products is the ultimate method to assess oil qual-

ity and stability. Sensory evaluation cannot be replaced fully by any chemical or

instrumental analysis, although some methods can correlate fairly well with sensory

results. Sensory evaluation of oils usually is done by a panel of experts or a trained

panel, and often the method recommended by the American Oil Chemists’ Society

(300) is used. During the evaluation, the panel is asked to score the overall flavor

quality, as well as the intensity of many individual off-flavors. Although chemical

and physical tests are more reproducible and less time consuming than sensory eva-

luations, oxidative rancidity and off-flavor evaluation of soybean oils are best done

by sensory tests. Correlations established between sensory evaluation scores and

634 SOYBEAN OIL

various chemical tests, however, can be used to predict the sensory quality of fin-

ished oil products.

Peroxide value, expressed as milliequivalents of peroxide per kilogram of oil,

measures the primary oxidation products of oils—the hydroperoxides. The peroxide

value has shown a particularly good correlation with sensory flavor scores of soy-

bean oil, and its use during storage is quite common. The peroxide value is an index

to the oxidative state of an oil. Soybean oil is considered ‘‘fresh’’ with a peroxide

value <1.0 mEq/kg, to have low oxidation with 1.0–5.0 mEq/kg, to have moderate

oxidation at 5.0–10.0 mEq/kg, to have high oxidation at >10.0 mEq/kg, and to have

poor flavor quality at >20 mEq/kg (6). Several methods (300–303) can be used to

measure the peroxide value of an oil depending on the specific circumstance.

One of the first steps in the oxidation of polyunsaturated fatty acids is a shift in

the position of double bonds, resulting in the formation of conjugated hydroperox-

ides. The conjugated structure absorbs strongly at a wavelength of 232–234 nm.

The conjugated diene value (300) is expressed as the percentage of conjugated die-

noic acid in the oil and is an indication of initial or primary oxidation products.

Conjugated diene value can be used as a comparative method only when the oils

have the same initial fatty acid composition, because the greater the amount of

polyenoates in an oil, the greater the potential rise in the conjugated diene value.

As a result, this method should be used as a relative measurement of oxidation in an

oil only if the fatty acid composition is known (303).

As aldehydes and some ketones have long been identified as oxidation and

breakdown products of fats, their determination also has been common in soybean

oil quality control. The p-anisidine value (300) measures light absorbance of alde-

hydes, primarily 2-alkenals, and 2,4-dienals at 350 nm. However, this measure is

not entirely specific, because the color intensity developed depends not only on

the concentration but also on the structure of the aldehyde. Therefore, the results

are comparable only within oils of similar type and treatment (304).

Free fatty acid (305), polar compounds (300), viscosity, and color analyses are

often performed to determine the degree of abuse that oils receive during heating or

frying. They are important indicators of frying oil quality, because these compo-

nents affect the quality of the fried food. The free fatty acid increase during frying

indicates released from triacylglyceride ester linkages via hydrolysis (233). Thus, it

is an important marker for oil quality. Abused frying oil should be discarded if it

contains >27% total polar compounds, according to a German standard of frying

oil quality (306). Changes in viscosity and color of the frying oil also are used as

indicators of the extent of frying oil degradation.

There are many other methods for measuring lipid oxidation and quality by che-

mical means. Among the best-known procedures are the thiobarbituric acid (TBA)

test, carbonyl value, and headspace oxygen analysis. These methods have been

reviewed and discussed elsewhere (287, 307).

The volatile carbonyl compounds formed during oxidation of fats and oils are

major contributors to off-flavor development. Therefore, there have been significant

efforts at identifying and quantifying these compounds. It is difficult to analyze

these compounds in fats and oils for several reasons. First, it is difficult to remove

OXIDATIVE QUALITY OF SOYBEAN OIL 635

them quantitatively from the fats and oils. Second, widespread contamination by

carbonyls in solvents, glassware, and other laboratory materials may cause artifacts.

Finally, hundreds of volatile compounds may be formed in fats and oils during oxi-

dation causing difficulties in the interpretation. Today, the use of efficient gas chro-

matography (GC) columns and proper means of identification has made reliable

volatile compound analysis become possible.

Three basic GC procedures are generally employed (300), including static head-

space, dynamic headspace, and direct injection. Static headspace involves equili-

bration of gases from the area above a liquid sample; a set volume of the

headspace gas from the sample is then injected directly into the GC for separation

and quantification. The dynamic headspace method, also known as purge and trap,

employs a sorbent, such as Tenax GC, Chromosorb, or Porapak Q, to collect vola-

tile compounds that are swept from a heated sample with an inert gas such as

helium or nitrogen. After trapping, the sorbent may be extracted with solvent, or

transferred directly to the GC inlet port. In direct injection, an oil sample may be

injected directly into the port of the GC through a silanized glass wool plug. Each

of these methods has their own advantages and disadvantages (287).

Recently, the method of gas chromatographic solid-phase microextraction (GC-

SPME) has been developed (308–310). This method uses fibers coated with various

polymers to extract volatile compounds from a food system. The method can be

used in solid, liquid, and gaseous systems. It is fairly easy to evaluate volatile com-

pounds by this analysis and to maintain consistent conditions.

Evans et al. (311) and Scholz and Ptak (312) used GC analysis of n-pentane as a

measurement of rancidity of vegetable oils. Dupuy et al. (313, 314) determined the

volatile carbonyl compounds from soybean oil using a modified gas chromato-

graphic inlet tube and found good correlations between the volatile profile analysis

and sensory scores. The Flavor Quality and Stability Committee of the AOCS eval-

uated GC volatile profiling as a standard method of flavor evaluation (275). As a

result, they wrote two Recommended Practices, entitled ‘‘Volatiles in Fats and

Oils by Gas-Liquid Chromatography’’ Cg 4-94, 1997 (300) and ‘‘Correlation of

Oil Volatiles with Flavor Scores of Edible Oils’’ AOCS method Cg 1-83, 1997

(300). These AOCS methods were validated in an AOCS collaborative study on

sensory and volatile analyses, in which three methods of volatile compound ana-

lyses were compared with sensory analyses by using the AOCS flavor scales

(315). Despite agreement on the usefulness of these methods, the committee

stressed that only humans can measure flavor, thus these volatile GC methods mea-

sured features such as oxidative stability and compound breakdown—not sensory

perceptions per se.

Not surprisingly, heat treatment, such as commercial and household frying,

accelerates autoxidation. In addition to undergoing autoxidation, when fats are

heated in the presence of moisture, as often is the case in food applications, fatty

acids are released via hydrolysis of the ester linkages (233). The free fatty acids can

accelerate oxidation of the oil. During heat treatment, the formation of dimeric and

cyclic compounds seems to be the predominant thermolytic reaction of unsaturated

fatty acids. In the presence of oxygen during heat treatment, however, oxidative

636 SOYBEAN OIL

polymerization also can occur (233). Obviously, temperature, heating time, avail-

ability of oxygen, etc. can largely influence the extent to which these thermal and

oxidative polymerization reactions occur.

Decomposition and condensation of hydroperoxides also produces a multitude

of nonvolatile monomeric products, including di- and tri-oxygenated esters, and

dimeric and polymeric materials, especially at elevated temperature. Many of these

dimers and polymers are known to be rich sources of volatile carbonyl compounds

and to decrease the flavor and oxidative stability of soybean oil (316). These high-

molecular-weight materials also can produce a series of physical and chemical

changes to the oil and food products, including increased viscosity, polarity, free

acid content, development of dark color, and an increased tendency of the oil to

foam (233).

12.5. Storage and Handling

Production of good quality soybean oil requires close control from harvesting of the

soybeans, during bean storage, during and after oil processing, through consump-

tion of the finished oil products to guard against oxidative, enzymatic, and micro-

biological deterioration. Good processing measures include careful control of

refining temperature, vacuum bleaching, and inert gas blanketing. Heat accelerates

the reaction of atmospheric oxygen with edible oils, therefore, localized overheat-

ing is detrimental to final oil quality. After processing, soybean oil should be stored

at as low a temperature as possible and practical, and with protection from light.

Vacuum conditions are very important during bleaching, because oxidation can

readily occur by exposure of a large surface area to air at elevated temperatures.

During storage, a package containing the maximum amount of oil is preferable,

because oxygen availability is lower with a lower headspace-to-oil ratio. Peroxide

formation also is a linear function of surface-to-volume ratio (275). According to

List (317), in field storage tanks, the oil is also subjected to conditions that cause

development of sizable temperature gradients that can produce considerable inter-

nal oil movement. Such movement would be expected to increase the quantity of oil

available at the surface and to accelerate oxygen diffusion. Therefore, soybean oil

stored in filled tanks should be at as low a temperature as possible to avoid such

conditions.

12.6. Special Processing for Off-Specification Oil

Oils from field-, frost-, moisture-, and storage-damaged beans usually have higher

levels of free fatty acids and iron, lower levels of phosphorous, darker colors, and

poorer flavor and oxidative stability in the finished products than do oils from unda-

maged beans. Such beans are difficult to process, and standard processing methods

usually do not produce finished oils that can meet soybean oil specifications for

trading or domestic consumption.

The National Soybean Processors’ Association (318) trading rules specify that

prime crude oils, after refining and bleaching by an official method (300), must

OXIDATIVE QUALITY OF SOYBEAN OIL 637

meet a Lovibond color level of 6.0. Frost-damaged oils often will not meet this

requirement. Oils from frost-damaged beans tend to have an undesirable green col-

or in the crude oils caused by compounds related to, but not identical to, chlorophyll

(includes pheophytin) or some of its derivatives, according to Stern and Grossman

(319). When bleaching such oils, acid-activated clays are more efficient than neutral

clays and increased amounts of bleaching earth make the removal of the green color

more effective. According to Stern and Grossman (319), pretreatment with charcoal

(0.4–1.0%) at 90�C or treatment of a cold hexane-oil mixture with charcoal is effec-

tive in partly removing the green pigment. When charcoal pretreatment is combined

with additional treatment from sugars and activated bleaching clays, complete

removal of green pigments is possible. Hydrogenation can also be used to remove

green color from soybean oil. According to Beal et al. (175), a green oil (IV 132)

hydrogenated to IV 110 in the presence of 1% copper chromate catalyst was no

longer green after cooling and filtration. However, the use of copper chromate is

not a common practice.

When soybeans are exposed to rain or humid weather in the field, the beans tend

to sprout and decay, and the oil from these beans develops a dark-brown color and

chalky texture (312). Drought stress affects protein and oil content of soybeans but

seldom damages oil quality significantly. According to List (317), off-specification

oils from field-, frost-, heat-, and moisture-damaged soybeans result in high refining

losses during processing, poor refined-bleached color, and lowered flavor and oxi-

dative stability. High refining losses may be partly overcome by use of phosphoric

acid or acetic anhydride degumming. Color problems of oils from damaged beans

may be alleviated, in part, by use of acidic bleaching earths, increased amounts of

bleaching earths, and higher bleaching temperatures. Overall, however, the best

practice for producing high-quality oil is to segregate the bad beans and not include

them in the processing.

13. DIETARY FATTY ACIDS AND THEIR HEALTH EFFECTS

13.1. Cholesterol and Heart Disease

Heart disease is still the number one cause of death for both men and women in the

United States. High-blood-cholesterol levels increase the risk of getting heart dis-

ease (319), so, generally, serum (blood) cholesterol is measured to determine a per-

son’s risk of developing heart disease. Although some cholesterol is essential in

forming the body’s cell membranes and synthesizing hormones and bile acids,

too much cholesterol is associated with heart disease. The fat eaten can affect

the blood-cholesterol level. In addition to monitoring total blood cholesterol, the

ratio of high-density lipoproteins (HDL) to low-density lipoproteins (LDL) of the

blood also is important in predicting heart disease. As cholesterol, a waxy sub-

stance, does not mix with water, it needs help circulating through blood, which

is mostly water. Lipoproteins transport cholesterol throughout the body. Low-den-

sity lipoproteins carry cholesterol from the liver to the body and leave deposits on

artery walls. High-density lipoproteins carry cholesterol back to the liver for

638 SOYBEAN OIL

elimination. If the ratio of low/high-density lipoproteins becomes too large, it is

likely that more cholesterol will be deposited in the arteries than is removed. So

the low/high-density lipoprotein ratio also may be used to predict a person’s

chances of developing heart disease. A ratio greater than 3 can indicate above aver-

age risk. The most important dietary influences on blood cholesterol levels are satu-

rated fat, total fat, and dietary cholesterol.

13.2. Saturated Fat and Health Effects

Saturated fat has more impact on raising blood cholesterol levels than anything else

in the diet. The most effective way to reduce the blood cholesterol level is to reduce

the amount of saturated fat in the diet. Animal products are a major source of satu-

rated fat in the average American diet. A very few vegetable oils, including coco-

nut, palm kernel, and palm oils, are rich in saturated fat. Other vegetable oils,

including soybean oil, can become saturated by hydrogenation. Consumption of

too much saturated fat has been associated with the development of heart disease,

some cancers, and other health problems. As soybean oil is the major edible oil

consumed in the United States, lowering its saturated fat could help reduce heart

disease in this country, even though its total saturated fatty acid composition is

only about 15% to 16%. As noted, the major saturates in soybean oil are palmitate

and stearate. Palmitate is responsible for about 70% of the total saturated fat in soy-

bean oil. Substitution of palmitate for carbohydrates or monounsaturates in the diet

increased levels of serum low-density lipoproteins and total cholesterol (320). Stea-

rate has been found to be relatively neutral in its effects on blood lipids, and some

researchers (321, 322) showed that dietary stearate actually lowered serum low-

density lipoproteins and total cholesterol levels; thus, many people recommend

that this saturate not be included in the category of hypercholesterolemic acyl

groups. It was for these reasons that Iowa State University scientists developed

LoSatSoyTM, a soybean oil with half the saturated fat of conventional soybean

oil, with reduction of palmitate to <�3%.

13.3. Unsaturated Fat and Health Effects

Unsaturated fats, classified as either monounsaturated or polyunsaturated, can help

lower the cholesterol levels in blood when substituted for saturated fats. Sources of

monounsaturated fat include nuts, olive oil, and canola oil. Sources of polyunsatu-

rated fat include corn, safflower, sesame, soybean, and sunflower oils.

Soybean oil contains about 21% of the monounsaturate oleate. Studies have

shown that the oxidation rate of oleate is much slower than that of the polyunsatu-

rates, linoleate and linolenate, which oxidize quickly and are the major contributors

to the poor stability of soybean oil (287, 323). A diet high in monounsaturates may

help to reduce elevated levels of total plasma cholesterol without reducing the high-

density lipoprotein-cholesterol level (324). Therefore, high-oleate soybean oil is not

only more stable than conventional soybean oil (275), but also has enhanced nutri-

tive value.

DIETARY FATTY ACIDS AND THEIR HEALTH EFFECTS 639

In both clinical trials and population studies, polyunsaturated fats in the diet

have been shown to actively lower serum cholesterol levels. Soybean oil is consid-

ered to have good nutritive value mainly because of its high concentration of essen-

tial polyunsaturates. As noted previously, it contains about 55% linoleate and 8%

linolenate, both recognized as essential fatty acids. Ingestion of approximately

1–2% of daily calories as linoleate is widely accepted as the amount needed to

meet the essential fatty acid requirement of rodent species and humans (325).

The physiological effects of linoleate have been well characterized. Various

deficiency symptoms include depressed growth, scaly dermatoses, increased skin

permeability, fatty liver, kidney damage, and impaired reproduction. The 8%

linolenate of soybean oil, makes it not only an excellent source of essential fatty

acids, but also a member of the n-3 fatty acid group (the third carbon atom from

the terminal end of the hydrocarbon chain is involved in a double bond). A number

of health benefits have been associated with the consumption of foods or oils that

contain n-3 fatty acids. These associations originally derived from epidemiological

studies of Eskimos who consumed high levels of n-3 fatty acid from seals and

coldwater fish (326). Compared with Danish counterparts, these Eskimos were

found to have a low incidence of heart disease and immune system diseases,

although a somewhat higher level of hemorrhagic stroke. Still today, large-scale

epidemiological studies suggest that individuals at risk for CHD benefit from the

consumption of plant- and marine-derived n-3 fatty acids (327).

13.4. Trans-Fatty Acids and Their Health Effects

The process of catalytic hydrogenation of vegetable oils was discovered in 1897 to

reduce the polyunsaturates and to improve flavor stability, versatility, and perfor-

mance of vegetable oils in salad dressings, during cooking, in deep-fat-frying,

and for the manufacture of margarines, shortenings, and other baking and snack

food applications (328). A side reaction that occurs during hydrogenation is the for-

mation of positional and geometrical isomers of the unsaturated sites that are left

unsaturated. Formation of trans-isomers is rapid and extensive (320). Although

hydrogenation can improve soybean oil oxidative stability and performance versa-

tility, the presence of the trans-fatty acids may make hydrogenated oils nutritionally

undesirable. In particular, the role of partially hydrogenated soybean oil in nutrition

has been under scrutiny because of the health concerns over the presence of trans-

acyl groups in our diets (329); however, the biological significance of these trans-

acyl groups is unclear. The formation of trans-acyl groups in vegetable oils also can

occur, to a small extent, during deodorization (330, 331) and during frying (332,

333). The 9-cis,12-trans-linoleate is present in most vegetable shortenings in

much greater quantities than the 9-trans,12-trans-linoleic acid (334). In heated

vegetable oils, the isomers just mentioned have been reported, plus trans-, cis-iso-

mers of linolenate (330, 332, 335). Trans-isomers are essential fatty acid antago-

nists, especially when the linoleate and linolenate are limited in the diet. For

example, the cis, cis, trans-isomer of 18:3 is elongated and desaturated to form

n-3 trans-isomers of 20:5n-3 and 22:6n-3 in rats (336); isomers that also have

640 SOYBEAN OIL

been found in human platelets (337). The 9-cis,12-trans-linoleic acid can be con-

verted to 20:4n-6 containing a trans-double bond. Unfortunately, this trans-isomer

of 20:4n-6 inhibited the formation of prostaglandins from all-cis-20:4n-6 (338).

Mensink and Katan (250) reported that a diet high in trans-acyl groups raised total

and low/high-density lipoprotein cholesterol ratio compared with a diet high in cis-

acyl groups may be more cholesterolemic than saturates (339), and were linked to

an increased risk of breast cancer development (340).

The estimated trans-acyl group intake by typical U.S. consumers is 11.1–27.6 g/

person/day (341). A comprehensive review concluded that trans-acyl groups con-

sumed at 4.0% or more of total calories may raise plasma lipid levels (342). As a

result of health concerns over the presence of trans-acyl groups in our diet, mod-

ifying fatty acid composition of soybean oil to improve its oxidative and flavor

stability in ways similar to that obtained by hydrogenation, but without trans-

formation, has become an objective of plant breeders.

13.5. Total Fat and Its Health Effects

Excessive intake of any fat is not healthy. According to Klurfeld and Kritchevsky

(342), the enhancement of tumor growth by dietary fat may result, in part, from the

caloric contribution of this nutrient. Significant reduction of tumor incidence with

consumption of 25% less energy was seen consistently in rat tumor systems induced

by chemicals. Currently, most American children get about 34% of their calories

from fat (318). It is recommended, however, that healthy children’s intake of fat

average no more than 30% of calories. Experts also suggest lowering children’s

saturated fat intake to less than 10% of calories. Similar recommendations have

been made for adults (343).

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