Principles of Food ChemistryThird Edition

John M. deMan, PhDProfessor Emeritus

Department of Food ScienceUniversity of Guelph

Guelph, Ontario

A Chapman & Hall Food Science Book

AN ASPEN PUBLICATION®Aspen Publishers, Inc.

Gaithersburg, Maryland1999

INTRODUCTION

It has been difficult to provide a definitionfor the class of substances called lipids. Earlydefinitions were mainly based on whether thesubstance is soluble in organic solvents likeether, benzene, or chloroform and is not solu-ble in water. In addition, definitions usuallyemphasize the central character of the fattyacids—that is, whether lipids are actual orpotential derivatives of fatty acids. Every def-inition proposed so far has some limitations.For example, monoglycerides of the short-chain fatty acids are undoubtedly lipids, butthey would not fit the definition on the basisof solubility because they are more soluble inwater than in organic solvents. Instead of try-ing to find a definition that would include alllipids, it is better to provide a schemedescribing the lipids and their components, asFigure 2-1 shows. The basic components oflipids (also called derived lipids) are listed inthe central column with the fatty acids occu-pying the prominent position. The left col-umn lists the lipids known as phospholipids.The right column of the diagram includes thecompounds most important from a quantita-tive standpoint in foods. These are mostlyesters of fatty acids and glycerol. Up to 99percent of the lipids in plant and animal

material consist of such esters, known as fatsand oils. Fats are solid at room temperature,and oils are liquid.

The fat content of foods can range fromvery low to very high in both vegetable andanimal products, as indicated in Table 2-1. Innonmodified foods, such as meat, milk, cere-als, and fish, the lipids are mixtures of manyof the compounds listed in Figure 2-1, withtriglycerides making up the major portion.The fats and oils used for making fabricatedfoods, such as margarine and shortening, arealmost pure triglyceride mixtures. Fats aresometimes divided into visible and invisiblefats. In the United States, about 60 percent oftotal fat and oil consumed consists of invisi-ble fats—that is, those contained in dairyproducts (excluding butter), eggs, meat, poul-try, fish, fruits, vegetables, and grain prod-ucts. The visible fats, including lard, butter,margarine, shortening, and cooking oils,account for 40 percent of total fat intake. Theinterrelationship of most of the lipids is repre-sented in Figure 2-1. A number of minorcomponents, such as hydrocarbons, fat-solu-ble vitamins, and pigments are not includedin this scheme.

Fats and oils may differ considerably incomposition, depending on their origin. Bothfatty acid and glyceride composition may

Lipids

CHAPTER 2

result in different properties. Fats and oilscan be classified broadly as of animal or veg-etable origin. Animal fats can be further sub-divided into mammal depot fat (lard andtallow) and milk fat (mostly ruminant) andmarine oils (fish and whale oil). Vegetableoils and fats can be divided into seed oils(such as soybean, canola), fruit coat fats(palm and olive oils), and kernel oils (coco-nut and palm kernel).

The scientific name for esters of glyceroland fatty acids is acylglycerols. Triacylglyc-erols, diacylglycerols, and monoacylglycer-ols have three, two, or one fatty acid esterlinkages. The common names for these com-pounds are glycerides, triglycerides, diglyc-erides, and monoglycerides. The scientificand common names are used interchange-ably in the literature, and this practice is fol-lowed in this book.

Figure 2-1 Interrelationship of the Lipids

FATTYALDEHYDES

PHOSPHORIC ACIDAMINO ALCOHOLS

FATTYALCOHOLS

Plasmalogens

Phosphatidylesters

Waxes

FATTYACIDS

GLYCEROL

Etheresters

Glycer ylether

Mono, Di, Tr iglycerides

Sterol estersSTEROLS

Sphingomyelin

Cercbrosides

SPHINGOSINE

HEXOSES

Table 2-1 Fat Contents of Some Foods

Product Fat (%)

Asparagus 0.25Oats 4.4Barley 1.9Rice 1.4Walnut 58Coconut 34Peanut 49Soybean 17Sunflower 28Milk 3.5Butter 80Cheese 34Hamburger 30Beef cuts 10-30Chicken 7Ham 31Cod 0.4Haddock 0.1Herring 12.5

SHORTHAND DESCRIPTION OFFATTY ACIDS AND GLYCERIDES

To describe the composition of fatty acidsit is sometimes useful to use a shorthanddesignation. In this convention the composi-tion of a fatty acid can be described by twonumbers separated by a colon. The firstnumber indicates the number of carbonatoms in the fatty acid chain, the secondnumber indicates the number of doublebonds. Thus, 4:0 is short for butyric acid,16:0 for palmitic acid, 18:1 for oleic acid,etc. The two numbers provide a completedescription of a saturated fatty acid. Forunsaturated fatty acids, information aboutthe location of double bonds and their stereoisomers can be given as follows: oleic acid(the cis isomer) is 18:lc9; elaidic acid (the

trans isomer) is I8:lt9. The numbering ofcarbon atoms in fatty acids starts normallywith the carboxyl carbon as number one. Insome cases polyunsaturated fatty acids arenumbered starting at the methyl end; forinstance, linoleic acid is represented as18:2n-6 and linolenic acid 18:3n-3. Thesesymbols indicate straight-chain, 18-carbonfatty acids with two and three methyleneinterrupted cis double bonds that start at thesixth and third carbon from the methyl end,respectively. These have also been describedas 006 and co3. The reason for this type ofdescription is that the members of eachgroup n-6 or n-3 are related biosyntheticallythrough processes involving desaturation,chain elongation, and chain shortening(Gunstone 1986) (Figure 2-2).

Triglycerides can be abbreviated by usingthe first letters of the common names of thecomponent fatty acids. SSS indicates tri-stearin, PPP tripalmitin, and SOS a triglycer-ide with two palmitic acid residues in the 1and 3 positions and oleic acid in the 2 posi-tion. In some cases, glyceride compositionsare discussed in terms of saturated and unsat-urated component fatty acids. In this case, Sand U are used and glycerides would be indi-cated as SSS for trisaturated glyceride andSUS for a glyceride with an unsaturated fattyacid in the 2 position. In other cases, the totalnumber of carbon atoms in a glyceride isimportant, and this can be shortened to glyc-erides with carbon numbers 54, 52, and soon. A glyceride with carbon number 54could be made up of three fatty acids with 18carbons, most likely to happen if the glycer-ide originated from one of the seed oils. Aglyceride with carbon number 52 could havetwo component fatty acids with 18 carbonsand one with 16 carbons. The carbon numberdoes not give any information about satura-tion and unsaturation.

COMPONENT FATTY ACIDS

Even-numbered, straight-chain saturatedand unsaturated fatty acids make up thegreatest proportion of the fatty acids of natu-ral fats. However, it is now known that manyother fatty acids may be present in smallamounts. Some of these include odd carbonnumber acids, branched-chain acids, andhydroxy acids. These may occur in naturalfats (products that occur in nature), as well asin processed fats. The latter category may, inaddition, contain a variety of isomeric fattyacids not normally found in natural fats. It iscustomary to divide the fatty acids into dif-ferent groups, for example, into saturated andunsaturated ones. This particular division isuseful in food technology because saturatedfatty acids have a much higher melting pointthan unsaturated ones, so the ratio of satu-rated fatty acids to unsaturated ones signifi-cantly affects the physical properties of a fator oil. Another common division is intoshort-chain, medium-chain, and long-chainfatty acids. Unfortunately, there is no gener-ally accepted division of these groups. Gen-

erally, short-chain fatty acids have from 4 to10 carbon atoms; medium-chain fatty acids,12 or 14 carbon atoms; and long-chain fattyacids, 16 or more carbon atoms. However,some authors use the terms long- and short-chain fatty acid in a strictly relative sense. Ina fat containing fatty acids with 16 and 18carbon atoms, the 16 carbon acid could becalled the short-chain fatty acid. Yet anotherdivision differentiates between essential andnonessential fatty acids.

Some of the more important saturated fattyacids are listed with their systematic andcommon names in Table 2-2, and some ofthe unsaturated fatty acids are listed in Table2-3. The naturally occurring unsaturatedfatty acids in fats are almost exclusively inthe c«-form (Figure 2-3), although trans-acids are present in ruminant milk fats and incatalytically hydrogenated fats. In general,the following outline of fatty acid composi-tion can be given:

• Depot fats of higher land animals consistmainly of palmitic, oleic, and stearicacid and are high in saturated fatty acids.

Figure 2-2 The n-3 Family Polyunsaturated Fatty Acids Based on Linolenic Acid. The heavy arrowsshow the relationship between the most important n-3 acids through desaturation (vertical arrows) andchain elongation (horizontal arrows)

16 : 3 24 : 3

16 : 4 «- 18 : 4 -»-20 : 4 -* [22 : 4] -» 24 : 4

18 : 5 «- 20 : 5 -»-22 : 5 -> 24 : 5 -> 26 : 5 -* [28 : 5] -> 30 : 5

22 : 6 -> 24 : 6 -> 26 : 6

The total content of acids with 18 carbonatoms is about 70 percent.

• Ruminant milk fats are characterized bya much greater variety of componentfatty acids. Lower saturated acids with 4to 10 carbon atoms are present in rela-tively large amounts. The major fattyacids are palmitic, oleic, and stearic.

• Marine oils also contain a wide varietyof fatty acids. They are high in unsatur-ated fatty acids, especially those unsatur-ated acids with long chains containing20 or 22 carbons or more. Several ofthese fatty acids, including eicosapen-taenoic acid (EPA) and docosahex-aenoic acid (DHA), have recently re-

ceived a good deal of attention becauseof biomedical interest (Ackman 1988b).

• Fruit coat fats contain mainly palmitic,oleic, and sometimes linoleic acids.

• Seedfats are characterized by low con-tents of saturated fatty acids. They con-tain palmitic, oleic, linoleic, and linolenicacids. Sometimes unusual fatty acidsmay be present, such as erucic acid inrapeseed oil. Recent developments inplant breeding have made it possible tochange the fatty acid composition of seedoils dramatically. Rapeseed oil in whichthe erucic acid has been replaced by oleicacid is known as canola oil. Low lino-lenic acid soybean oil can be obtained, as

Table 2-2 Saturated Even- and Odd-Carbon Numbered Fatty Acids

Systematic Name

n-Butanoicn-Hexanoicn-Octanoicn-Decanoicn-Dodecanoicn-Tetradecanoicn-Hexadecanoicn-Octadecanoicn-Eicosanoic/7-Docosanoicn-Pentanoicn-Heptanoic/i-Nonanoicn-Undecanoicn-Tridecanoicn-Pentadecanoicn-Heptadecanoic

Common Name

ButyricCaproicCaprylicCapricLaurie

MyristicPalmiticStearic

ArachidicBehenicValeric

EnanthicPelargonic

Margaric

Formula

CH3-(CH2J2-COOHCH3-(CH2J4-COOHCH3-(CH2J6-COOHCH3-(CH2J8-COOHCH3-(CH2J10-COOHCH3-(CH2J12-COOHCH3-(CH2J14-COOHCH3-(CH2J16-COOHCH3-(CH2J18-COOHCH3-(CH2J20-COOHCH3-(CH2J3-COOHCH3-(CH2J5-COOHCH3-(CH2J7-COOHCH3-(CH2J9-COOHCH3-(CH2J11-COOHCH3-(CH2J13-COOHCH3-(CH2J15-COOH

ShorthandDescription

4:06:08:010:012:014:016:018:020:022:05:07:09:011:013:015:017:0

can sunflower and linseed oils with moredesirable fatty acid composition.

The depot fats of higher land animals,especially mammals, have relatively simplefatty acid composition. The fats of birds aresomewhat more complex. The fatty acidcompositions of the major food fats of this

group are listed in Table 2-4. The kind offeed consumed by the animals may greatlyinfluence the composition of the depot fats.Animal depot fats are characterized by thepresence of 20 to 30 percent palmitic acid, aproperty shared by human depot fat. Many ofthe seed oils, in contrast, are very low inpalmitic acid. The influence of food con-

Figure 2-3 Structures of Octadec-cw-9-Enoic Acid (Oleic Acid) and Octadec-Jraws-9-Enoic Acid(Elaidic Acid)

Table 2-3 Unsaturated Fatty Acids

Systematic Name

Dec-9-enoicDodec-9-enolcTetradec-9-enoicHexadec-9-enoicOctadec-6-enoicOctadec-9-enoicOctadec-11-enoicOctadeca-9: 1 2-dienoicOctadeca-9: 1 2: 1 5-trienoicOctadeca-6:9: 1 2-trienoicOctadeca-9: 11:1 3-trienoicEicos-9-enoicEicosa-5:8:1 1 :14-tetraenoicEicosa-5:8:1 1:1 4:17-

pentaenoic acidDocos-13-enoicDocosa-4:7:10:13:16:19-

hexaenoic acid

CommonName

MyristoleicPalmitoleicPetroselinicOleicVaccenicLinoleicLinolenicy-LinolenicElaeostearicGadoleicArachidonicEPA

ErucicDHA

Formula

CH2=CH.(CH2)rCOOHCH3.CH2.CH=CH.(CH2)rCOOHCH3.(CH2)3.CH=CH.(CH2)rCOOHCH3.(CH2)5.CH=CH.(CH2)rCOOHCH3.(CH2)10.CH=CH.(CH2)4.COOHCH3.(CH2)7.CH=CH.(CH2)7-COOHCH3.(CH2)5.CH=CH.(CH2)9.COOHCH3.(CH2)4.(CH=CH.CH2)2.(CH2)6.COOHCH3.CH2.(CH=CH.CH2)3.(CH2)6.COOHCH3.(CH2)4.(CH=CH.CH2)3.(CH2)3.COOHCH3.(CH2)3.(CH=CH)3.(CH2)rCOOHCH3.(CH2)9.CH=CH-(CH2)7.COOHCH3.(CH2)4.(CH=CH.CH2)4.(CH2)2.COOHCH3.CH2.(CH=CH.CH2)5.(CH2)2.COOH

CH3.(CH2)7-CH=CH-(CH2)1 1 -COOHCH3.CH2(CH=CH.CH2)6.(CH2).COOH

ShorthandDescription

10:112:114:116:118:118:118:118:2co618:3co318:3(0620:320:120:40)620:5(03

22:122:6(03

sumption applies equally for the depot fat ofchicken and turkey (Marion et al. 1970; Jenet al. 1971). The animal depot fats are gener-ally low in polyunsaturated fatty acids. Theiodine value of beef fat is about 50 and oflard about 60. Iodine value is generally usedin the food industry as a measure of totalunsaturation in a fat.

Ruminant milk fat is extremely complex infatty acid composition. By using gas chro-matography in combination with fractionaldistillation of the methyl esters and adsorp-tion chromatography, Magidman et al.(1962) and Herb et al. (1962) identified atleast 60 fatty acids in cow's milk fat. Severaladditional minor fatty acid components havebeen found in other recent studies. About 12fatty acids occur in amounts greater than 1percent (Jensen and Newburg 1995). Amongthese, the short-chain fatty acids frombutyric to capric are characteristic of rumi-nant milk fat. Data provided by Hilditch andWilliams (1964) on the component fattyacids of some milk fats are listed in Table 2-5.Fatty acid compositions are usually reportedin percentage by weight, but in the case offats containing short-chain fatty acids (orvery long-chain fatty acids) this method maynot give a good impression of the molecularproportions of fatty acids present. Therefore,in many instances, the fatty acid composi-

tion is reported in mole percent, as is thecase with the data in Table 2-5. Accordingto Jensen (1973) the following fatty acids arepresent in cow's milk fat: even and odd satu-rated acids from 2:0 to 28:0; even and oddmonoenoic acids from 10:1 to 26:1, with theexception of 11:1, and including positionaland geometric isomers; even unsaturatedfatty acids from 14:2 to 26:2 with some con-jugated geometric isomers; polyenoic evenacids from 18:3 to 22:6 including some con-jugated trans isomers; monobranched fattyacids 9:0 and 11:0 to 25:0—some iso andsome ante-iso (iso acids have a methylbranch on the penultimate carbon, ante-isoon the next to penultimate carbon [Figure2-4]); multibranched acids from 16:0 to28:0, both odd and even with three to fivemethyl branches; and a number of keto,hydroxy, and cyclic acids.

It is impossible to determine all of the con-stituents of milk fatty acids by a normalchromatographic technique, because many ofthe minor component fatty acids are eithernot resolved or are covered by peaks of othermajor fatty acids. A milk fat chromatogramof fatty acid composition is shown in Figure2-5. Such fatty acid compositions as re-ported are therefore only to be considered asapproximations of the major component fattyacids; these are listed in Table 2-6. This

Table 2-4 Component Fatty Acids of Animal Depot Fats

Fatty Acids Wt %

Animal

PigBeefSheepChickenTurkey

14:0

14311

16:0

2425212420

76:7

35266

18:0

13192566

18:1

4136344038

18:2

10451724

78:3

1Trace

312

table reports the most recent results of themajor component fatty acids in bovine milkfat as well as their distribution among the sn-1, sn-2, and sn-3 positions in the triacylglyc-erols (Jensen and Newburg 1995).

In most natural fats the double bonds ofunsaturated fatty acids occur in the cis con-figuration. In milk fat a considerable propor-tion is in the trans configuration. Thesetrans bonds result from microbial action inthe rumen where polyunsaturated fatty acidsof the feed are partially hydrogenated. Cata-lytic hydrogenation of oils in the fat industry

also results in trans isomer formation. Thelevel of trans isomers in milk fat has beenreported as 2 to 4 percent (deMan anddeMan 1983). Since the total content ofunsaturated fatty acids in milk fat is about34 percent, trans isomers may constituteabout 10 percent of total unsaturation. Thecomplexity of the mixture of different iso-mers is demonstrated by the distribution ofpositional and geometric isomers in themonoenoic fatty acids of milk fat (Table 2-7)and in the unconjugated 18:2 fatty acids(Table 2-8). The iodine value of milk fat is

Figure 2-4 Examples of Iso- and Ante-Iso-Branched-Chain Fatty Acids

Table 2-5 The Component Fatty Acids of Some Milk Fats in Mole %

Fatty Acid

4:06:08:010:0Total short chain12:014:016:018:020:010-12 unsaturated16:118:118:220-22 unsaturated

Cow

9.54.10.83.2

17.62.9

11.526.77.61.81.14.3

22.43.11.0

Goat

7.54.74.3

12.829.36.6

11.824.14.70.41.42.2

16.52.80.2

Sheep

7.55.33.56.4

22.74.59.9

21.610.30.81.02.0

21.64.31.3

Source: From TP. Hilditch and P.M. Williams, The Chemical Constitution of Natural Fats, 4th ed., 1964, JohnWiley & Sons.

in the range of 30 to 35, much lower thanthat of lard, shortening, or margarine, whichhave similar consistencies.

Marine oils have also been found to con-tain a large number of component fatty acids.Ackman (1972) has reported as many as 50or 60 components. Only about 14 of theseare of importance in terms of weight percentof the total. These consist of relatively fewsaturated fatty acids (14:0, 16:0, and 18:0)and a larger number of unsaturated fattyacids with 16 to 22 carbon atoms and up to 6double bonds. This provides the possibilityfor many positional isomers.

The complexity of the fatty acid composi-tion of marine oils is evident from the chro-matogram shown in Figure 2-6 (Ackman1994). The end structure of the polyunsatu-

rated fatty acids is of nutritional impor-tance, especially eicosapentaenoic acid(EPA), 20:5co3 or 20:5 n-3, and docosa-hexaenoic acid (DHA), 22:6co3 or 22:6 n-3.The double bonds in marine oils occurexclusively in the cis configuration. EPAand DHA can be produced slowly fromlinolenic acid by herbivore animals, but notby humans. EPA and DHA occur in majoramounts in fish from cold, deep waters,such as cod, mackerel, tuna, swordfish, sar-dines, and herring (Ackman 1988a; Simo-poulos 1988). Arachidonic acid is theprecursor in the human system of pros-tanoids and leukotrienes.

Ackman (1988b) has drawn attention tothe view that the fatty acid compositions ofmarine oils are all much the same and vary

Figure 2-5 Chromatogram of Milk Fat Fatty Acid Composition Analyzed as Butyl Esters on a 30-mCapillary Column. Source: Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved andTechnological Advances in Alternative Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 298, ©1994, Aspen Publishers, Inc.

HEXA

NE

BUTA

NOL

only in the proportions of fatty acids. Thepreviously held view was that marine oilswere species-specific. The major fatty acidsof marine oils from high-, medium-, andlow-fat fish are listed in Table 2-9 (Ackman1994).

The fatty acid composition of egg yolk isgiven in Table 2-10. The main fatty acidsare palmitic, oleic, and linoleic. The yolkconstitutes about one-third of the weight ofthe edible egg portion. The relative amountsof egg yolk and white vary with the size ofthe egg. Small eggs have relatively higheramounts of yolk. The egg white is virtuallydevoid of fat.

The vegetable oils and fats can be dividedinto three groups on the basis of fatty acidcomposition. The first group comprises oilscontaining mainly fatty acids with 16 or 18

carbon atoms and includes most of the seedoils; in this group are cottonseed oil, peanutoil, sunflower oil, corn oil, sesame oil, oliveoil, palm oil, soybean oil, and safflower oil.The second group comprises seed oils con-taining erucic (docos-13-enoic) acid. Theseinclude rapeseed and mustard seed oil. Thethird group is the vegetable fats, comprisingcoconut oil and palm kernel oil, which arehighly saturated (iodine value about 15),and cocoa butter, the fat obtained fromcocoa beans, which is hard and brittle atroom temperature (iodine value 38). Thecomponent fatty acids of some of the mostcommon vegetable oils are listed in Table2-11. Palmitic is the most common satu-rated fatty acid in vegetable oils, and onlyvery small amounts of stearic acid arepresent. Oils containing linolenic acid, such

Table 2-6 Major Fatty Acids of Bovine Milk Fat and Their Distribution in the Triacylglycerols

Fatty Acids(mol%)

4:06:08:010:012:014:015:016:016:117:018:018:118:218:3

Bovine Milk Fat

TG

11.84.61.9

373.9

11.22.1

23.92.60.87.0

24.02.5

Trace

sn-1

1.41.94.99.72.0

34.02.81.3

10.330.0

1.7

sn-2

0.90.73.06.2

17.52.9

32.33.61.09.5

18.93.5

sn-3

35.412.93.66.20.66.41.45.41.40.11.2

23.12.3

Source: Reprinted with permission from R.G. Jensen and D. S. Newburg, Milk Lipids, in Handbook of Milk Compo-sition, R.G. Jensen, ed., p. 546, © 1995, Academic Press.

as soybean oil, are unstable. Such oils canbe slightly hydrogenated to reduce the lino-lenic acid content before use in foods.Another fatty acid that has received atten-tion for its possible beneficial effect onhealth is the n-6 essential fatty acid,

gamma-linolenic acid (18:3 n-6), whichoccurs at a level of 8 to 10 percent inevening primrose oil (Carter 1988).

The Crucifera seed oils, including rape-seed and mustard oil, are characterized bythe presence of large amounts of erucic acid

Table 2-8 Location of Double Bonds in Unconjugated 18:2 lsomers of Milk Lipids

CIS, CIS

11,1510,159, 158, 15 and/or 8, 127, 15 and/or 7, 126, 15 and/or 6, 12

cis, trans or trans, c/s

11, 16 and/or 11, 1510, 16 and/or 10, 159, 15 and/or 9, 168, 1 6 and/or 8, 1 5

and/or 8, 12

trans, trans

12, 1611, 16and/or11, 1510, 16 and/or 10, 159, 16 and/or 9, 15

and/or 9, 13

Source: From R.G. Jensen, Composition of Bovine Milk Lipids, J. Am. Oil Chem. Soc., Vol. 50, pp. 186-192, 1973.

Source: From R.G. Jensen, Composition of Bovine Milk Lipids, J. Am. Oil Chem. Soc., Vol. 50, pp. 186-192,1973.

Table 2-7 Positional and Geometric lsomers of Bovine Milk Lipid Monoenoic Fatty Acids (Wt%)

Position ofDoubleBond

5678910111213141516

c/s lsomers

14:1

1.00.80.90.696.6

16:1

Tr1.35.6Tr

88.7Tr2.6Tr

17:1

3.42.120.171.3Tr2.9Tr

trans lsomers

18:1

1.795.8Tr2.5

16:1

2.27.86.75.032.81.710.612.910.6

18:1

1.00.83.210.210.535.74.110.59.06.87.5

(docos-13-enoic) and smaller amounts ofeicos-11-enoic acid. Rapeseed oil of the vari-ety Brassica napus may have over 40 percentof erucic acid (Table 2-12), whereas Bras-sica campestris oil usually has a much lowererucic acid content, about 22 percent.Because of possible health problems result-ing from ingestion of erucic acid, new vari-eties of rapeseed have been introduced inrecent years; these are the so-called low-eru-cic acid rapeseed (LEAR) varieties, whichproduce LEAR oil. When the seed is alsolow in glucosinolates, the oil is known ascanola oil. Plant breeders have succeeded inreducing the erucic acid level to less than 1percent and as a result canola oil has a very

high level of oleic acid (Table 2-12). Thebreeding of these varieties has in effectresulted in the creation of a completely newoil. Removal of the erucic and eicosenoicacids results in a proportional increase in theoleic acid content. The low erucic acid oil isa linolenic acid-containing oil and is there-fore similar in this respect to soybean oil.The fatty acid composition of mustard oil isgiven in Table 2-12. It is similar to that of B.campestris oil.

Vegetable fats, in contrast to the oils, arehighly saturated, have low iodine values, andhave high melting points. Coconut oil andpalm kernel oil belong to the lauric acid fats.They contain large amounts of medium- and

Figure 2-6 Chromatogram of the Fatty Acid Composition of Fish Oil (Menhaden). Analysis ofmethyl esters on a 30-m capillary column. Source: Reprinted from R.G. Ackman, Animal and MarineLipids, in Improved and Technological Advances in Alternative Sources of Lipids, B. Kamel and Y.Kakuda, eds., p. 308, 1994, Aspen Publishers, Inc.

short-chain fatty acids, especially lauric acid(Table 2-13). Cocoa butter is unusual in thatit contains only three major fatty acids—palmitic, stearic, and oleic—in approxi-mately equal proportions.

Table 2-10 Fatty Acid Composition of Egg Yolk

Fatty Acid %

Total saturated 36.214:0 0.316:0 26.618:0 9.3

Total monounsaturated 48.216:1 4.018:1 44.1

Total polyunsaturated 14.718:2 13.418:3 0.320:4 1.0

COMPONENT GLYCERIDES

Natural fats can be defined as mixtures ofmixed triglycerides. Simple triglycerides arevirtually absent in natural fats, and the distri-bution of fatty acids both between and withinglycerides is selective rather than random.When asymmetric substitution in a glycerolmolecule occurs, enantiomorphic forms areproduced (Kuksis 1972; Villeneuve andFoglia 1997). This is illustrated in Figure 2 -7. Glycerol has a plane of symmetry or mir-ror plane, because two of the four substitu-ents on the central carbon atom are identical.When one of the carbon atoms is esterifiedwith a fatty acid, a monoglyceride resultsand two nonsuperimposable structures exist.These are called enantiomers and are alsoreferred to as chiral. A racemic mixture is amixture of equal amounts of enantiomers.Asymmetric or chiral compounds are formedin 1-monoglycerides; all 1, 2-diglycerides; 1,

Table 2-9 Total Fat Content and Major Fatty Acids in High-, Medium-, and Low-Fat Fish

Total fatFatty acid14:016:016:118:120:122:120:5n-322:6A?-3Total

High Fat

Capelin

14.1

7.19.9

11.013.416.312.68.66.7

85.6

Sprat

12.9

5.517.55.8

18.07.4

12.87.4

11.786.1

Medium Fat

BlueWhiting

7.4

3.911.56.1

14.810.712.410.412.682.4

Capelin

4.0

7.39.78.3

14.513.610.49.2

11.084.0

Loiv Fat

Dogfish

1.7

1.615.34.9

20.811.27.96.0

15.584.8

Saith,Gutted

0.4

1.712.42.7

13.15.93.5

12.730.682.6

Source: Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved and Technological Advances in Alter-native Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 302, 1994, Aspen Publishers, Inc.

3-diglycerides containing unlike substitu-ents; and all triglycerides in which the 1- and3- positions carry different acyl groups.

The glyceride molecule can be representedin the wedge and slash form (Figure 2-8). Inthis spatial representation, the wedge indi-cates a substituent coming out of the planetoward the observer, and the slash indicates asubstituent going away from the observer.The three carbon atoms of the glycerol are

then described by the stereospecific number-ing (STZ) with the three carbon atoms desig-nated sn-l from the top to sn-3 at the bottom.

When a fat or oil is characterized by deter-mination of its component fatty acids, therestill remains the question as to how theseacids are distributed among and within theglycerides. Originally theories of glyceridedistribution were attempts by means of math-ematical schemes to explain the occurrence

Table 2-1 1 Component Fatty Acids of Some Vegetable Oils

Fatty Acid Wt%

Oil

CanolaCottonseedPeanut*OliveRice branSoybeanSunflowerSunflower high oleicPalmCocoa butter

16:0

42713101611544426

18:0

22322455434

18:1

56183878422220813935

18:2

26514173753698113

18:3

10TraceTrace

18

Total C18

96738390848995965474

'Peanut oil also contains about 3% of 22:0 and 1% of 22:1 .

Table 2-12 Component Fatty Acids of Some Crucifera Seed Oils (Wt%)

Fatty Acid

Seed Oil

Rapeseed (B. campestris)Rapeseed (B. napus)Canola (LEAR)Mustard (B. juncea)

16:0

4344

18:0

212

18:1

33175522

18:2

18142624

18:3

991014

20:1

1211212

22:1

2245

Enantiomers

Figure 2-7 Plane of Symmetry of a GlycerolMolecule (Top) and Mirror Image of Two Enan-tiomers of a Mono-Acylglycerol (bottom).Source: Reprinted with permission from P. Ville-neuve and TA. Foglia, Lipase Specificities:Potential Application in Bioconversions, Inform,8, pp. 640-650, © 1997, AOCS Press.

thesis. Hilditch proposed the concept of evendistribution (Gunstone 1967). In the rule ofeven (or widest) distribution, each fatty acidin a fat is distributed as widely as possibleamong glyceride molecules. This means thatwhen a given fatty acid A constitutes about35 mole percent or more of the total fattyacids (A + X), it will occur at least once in alltriglyceride molecules, as represented byGAX2. If A occurs at levels of 35 to 70 molepercent, it will occur twice in an increasingnumber of triglycerides GA2X. At levelsover 70 percent, simple triglycerides GA3 areformed. In strictly random distribution theamount of GA3 in a fat would be propor-tional to the cube of the percentage of Apresent. For example, at 30 percent A therewould be 2.7 percent of GA3, which underrules of even distribution would occur onlyat levels of A over 70 percent (Figure 2-9).

of particular kinds and amounts of glyceridesin natural fats. Subsequent theories havebeen refinements attempting to relate to thebiochemical mechanisms of glyceride syn-

Table 2-13 Component Fatty Acids of Some Vegetable Fats (Wt %)

Fatty Acid

Vegetable Fat

CoconutPalm kernelCocoa butter

6:0

0.5

8:0

9.02.7

10:0

6.87.0

12:0

46.446.9

14:0

18.014.1

16:0

9.08.8

26.2

18:0

1.01.3

34.4

18:1

7.618.537.3

18:2

1.60.72.1

Figure 2-8 Stereospecific Numbering of theCarbons in a Triacylglycerol

Mirror

Symmetric

Mirrorplane

The theory of restricted random distribu-tion was proposed by Kartha (1953). In thistheory the fatty acids are distributed at ran-dom, but the content of fully saturated glycer-ides is limited to the amount that can remainfluid in vivo. This theory is followed by the1,3 random, 2 random distribution hypothesisof Vander WaI (1964). According to this the-ory, all acyl groups at the 2-positions of theglycerol moieties of a fat are distributedtherein at random. Equally, all acyl groups atthe 1- and 3-positions are distributed at ran-dom and these positions are identical. Appli-cation of this theory to the results obtainedwith a number of fats gave good agreement(Vander WaI 1964), as Table 2-14 shows.

In vegetable fats and oils, the saturatedfatty acyl groups have a tendency to occupythe 1- and 3- positions in the glycerides andthe unsaturated acyl groups occupy the 2-

position (Figure 2-10). Since these fats con-tain a limited number of fatty acids, it is cus-tomary to show the glyceride composition interms of saturated (S) and unsaturated (U)acids. The predominant glyceride types inthese fats and oils are S-U-S and S-U-U.Lard is an exception—saturated acyl groupspredominate in the 2-position. The glyceridedistribution of cocoa butter results in a fatwith a sharp melting point of about 30 to340C. It is hard and brittle below the meltingpoint, which makes the fat useful for choco-late and confectionery manufacture. Otherfats with similar fatty acid composition, suchas sheep depot fat (see Table 2-4), have agreater variety of glycerides, giving the fat ahigher melting point (about 450C) and awider melting range, and a greasy and softappearance.

Brockerhoff et al. (1966) studied the fattyacid distribution in the 1-, 2-, and 3-posi-tions of the triglycerides of animal depot fatsby stereospecific analysis. The distributionamong the three positions was nonrandom.The distribution of fatty acids seems to begoverned by chain length and unsaturation.In most fats a short chain and unsaturationdirect a fatty acid toward position 2. Thedepot fat of pigs is an exception, palmiticacid being predominant in position 2. In thefats of marine animals, chain length is thedirecting factor, with polyunsaturated andshort-chain fatty acids accumulated in the 2-position and long chains in the 1- and 3-positions. In the fats of birds, unsaturationseems to be the only directing factor andthese acids accumulate in the 2-position.

The positional distribution of fatty acids inpig fat (lard) and cocoa butter is shown inTable 2-15. Most of the unsaturation in lardis located in the 1- and 3-positions, whereasin cocoa butter the major portion of theunsaturation is located in the 2-position. This

Figure 2-9 Calculated Values for GlycerideTypes in Random Distribution (Solid Lines) andEven Distribution (Dotted Lines). Source: FromRD. Gunstone, An Introduction to the Chemis-try of Fats and Fatty Acids, 1967, Chapman andHall.

Acid (A)(TL mot}

Gly

ceri

de

(%fn

oD

difference accounts for the difference inphysical properties of the two fats (deMan etal. 1987).

Milk fat, with its great variety of fattyacids, also has a very large number of glyc-erides. It is possible, by, for example, frac-tional crystallization from solvents, to sep-arate milk fat in a number of fractions withdifferent melting points (Chen and deMan1966). Milk fat is peculiar in some respects.Its short-chain fatty acids are classifiedchemically as saturated compounds but

behave physically like unsaturated fattyacids. One of the unsaturated fatty acids, theso-called oleic acid, is partly trans and has amuch higher melting point than the cis iso-mers. In the highest melting fraction frommilk fat, there is very little short-chain fattyacid and little unsaturation, mostly in thetrans configuration (Woodrow and deMan1968). The low melting fractions are high inshort-chain fatty acids and unsaturation (cis).The general distribution of major fatty acidsin whole milk fat is as follows (Morrison

Figure 2-10 Fatty Acid Distribution in the Triacylglycerols of Vegetable Oils

2-random

1,3-random

unsaturated

saturated+

long-chainmono-

unsaturated

Table 2-14 Comparison of the Glyceride Composition of Some Natural Fats as DeterminedExperimentally and as Calculated by 1,3 Random, 2 Random Hypothesis

Molecular Species

Fat

LardLardChicken fatChicken fatCocoa butterCocoa butter

Method

ExperimentCalculatedExperimentCalculatedExperimentCalculated

SSS(Mole %)

863355

SUS(Mole %)

O210106669

SSU(Mole %)

292991072

USU(Mole %)

36361293O

UUS(Mole %)

151238362022

UUU(Mole %)

1215283212

Source: From RJ. Vander WaI, Triglyceride Structure, Adv. LIpId Res., Vol. 2, pp. 1-16, 1964.

1970): 4:0 and 6:0 are located largely in pri-mary positions; 18:0 and 18:1 are preferen-tially in primary positions; 10:0, 12:0, and16:0 are distributed randomly or with aslight preference for the secondary position;and 14:0 is predominantly in the secondaryposition. The distribution of milk fat tria-cylglycerols according to carbon numberand unsaturation has been reported byJensen and Newburg (1995) and is presentedin Table 2-16.

PHOSPHOLIPIDS

All fats and oils and fat-containing foodscontain a number of phospholipids. Thelowest amounts of phospholipid are presentin pure animal fats such as lard and beef tal-low. In some crude vegetable oils, such ascottonseed, corn, and soybean oils, phos-pholipids may be present at levels of 2 to 3percent. Fish, Crustacea, and mollusks con-tain approximately 0.7 percent of phospho-lipids in the muscle tissue. Phospholipidsare surface active, because they contain alipophilic and hydrophilic portion. Sincethey can easily be hydrated, they can beremoved from fats and oils during the refin-

ing process. In some cases they may beremoved by separation of two phases; forexample, if butter is melted and filtered, thepure oil thus obtained is free from phospho-lipids. The structure of the most importantphospholipids is given in Figure 2-11. Afterrefining of oils, neutralization, bleaching,and deodorization, the phospholipid contentis reduced to virtually zero. The phospholip-ids removed from soybean oil are used asemulsifiers in certain foods, such as choco-late. Soybean phospholipids contain about35 percent lecithin and 65 percent cephalin.The fatty acid composition of phospholipidsis usually different from that of the oil inwhich they are present. The acyl groups areusually more unsaturated than those of thetriglycerides. Phospholipids of many vegeta-ble oils contain two oleic acid residues. Thephospholipids of milk do not contain theshort-chain fatty acids found in milk fat tri-glycerides, and they contain more long-chain polyunsaturated fatty acids than thetriglycerides. The composition of cow'smilk phospholipids has been reported byJensen (1973), as shown in Table 2-17. Thedifference in composition of triglyceridesand phospholipids in mackerel is demon-

Table 2-15 Positional Distribution Fatty Acids in Pig Fat and Cocoa Butter

Fatty Acid (Mole %)

Fat

Pig fat

Cocoa butter

Position

123123

14:0

0.94.1O

16:0

9.572.30.4

34.01.7

36.5

16:1

2.44.81.50.60.20.3

18:0

29.52.17.4

50.42.1

52.8

18:1

51.313.472.712.387.48.6

18:2

6.43.3

18.21.38.60.4

7s w" C

o55'

gQ.

^-*

Tl

M 2?

% P

I?

-O Oi

I

J° i. a Q I $ >

strated by the data reported by Ackman andEaton (1971), as shown in Table 2-18. Thephospholipids of flesh and liver in mackerelare considerably more unsaturated than thetriglycerides.

The distribution of fatty acids in phospho-lipids is not random, with saturated fattyacids preferentially occupying position 1 andunsaturated fatty acids position 2.

UNSAPONIFIABLES

The unsaponifiable fraction of fats consistsof sterols, terpenic alcohols, aliphatic alco-hols, squalene, and hydrocarbons. The distri-bution of the various components of theunsaponifiable fraction in some fats and oilsis given in Table 2-19. In most fats the majorcomponents of the unsaponifiable fractionare sterols. Animal fats contain cholesterol

and, in some cases, minor amounts of othersterols such as lanosterol. Plant fats and oilscontain phytosterols, usually at least three,and sometimes four (Fedeli and Jacini 1971).They contain no or only trace amounts ofcholesterol. The predominant phytosterol is(3-sitosterol; the others are campesterol andstigmasterol. In rapeseed oil, brassicasteroltakes the place of stigmasterol. Sterols arecompounds containing the perhydrocyclo-penteno-phenanthrene nucleus, which theyhave in common with many other naturalcompounds, including bile acids, hormones,and vitamin D. The nucleus and the descrip-tion of the four rings, as well as the systemof numbering of the carbon atoms, areshown in Figure 2-12A. The sterols are sol-ids with high melting points. Stereochemi-cally they are relatively flat molecules,usually with all trans linkages, as shown in

Table 2-16 Distribution (wt %) of Milk Fat Triacylglycerols According to Carbon Number and Unsaturation

Number of Double Bonds

Carbon Number

3436384042444648505254Total

O

4.85.04.62.01.51.01.31.62.62.72.2

29.3

1

1.44.96.94.62.42.82.12.23.45.71.4

37.8

2

2.62.93.12.12.92.22.22.71.90.3

22.9

3

3.11.21.21.01.01.00.80.4

9.7

Source: Reprinted with permission from R.G. Jensen and D. S. Newburg, Milk Lipids, in Handbook of Milk Compo-sition, R.G. Jensen, ed., p. 550, © 1995, Academic Press.

Figure 2-11 Structure of the Major Phospholipids

Table 2-17 Composition of the Phospholipids of Cow's Milk

Source: From R.G. Jensen, Composition of Bovine Milk Lipids, J. Am. OH Chem. Soc., Vol. 50, pp. 186-192,1973.

Phosphatidylcholine(lecithin)

Phosphatidylethanolamine(cephalin)

Phosphatidylserine

Phosphoinositides

Phospholipid

PhosphatidylcholinePhosphatidylethanolaminePhosphatidylserinePhosphatidylinositolSphingomyelinLysophosphatidylcholineLysophosphatidylethanolamineTotal choline phospholipidsPlasmalogensDiphosphatidyl glycerolCeramidesCerebrosides

Mole (%)

34.531.83.14.725.2TraceTrace59.7

3TraceTraceTrace

Figure 2-12B. The ring junction betweenrings A and B is trans in some steroids, cis inothers. The junctions between B and C and Cand D are normally trans. Substituents thatlie above the plane, as drawn in Figure 2 -12C, are named p, those below the plane, a.The 3-OH group in cholesterol (Figure 2 -12C) is the p-configuration, and it is thisgroup that may form ester linkages. Thecomposition of the plant sterols is given inFigure 2-13. Part of the sterols in natural

fats are present as esters of fatty acids; forexample, in milk fat, about 10 percent of thecholesterol occurs in the form of cholesterolesters.

The sterols provide a method of distin-guishing between animal and vegetable fatsby means of their acetates. Cholesterol ace-tate has a melting point of 1140C, whereasphytosterol acetates melt in the range of 126to 1370C. This provides a way to detect adul-teration of animal fats with vegetable fats.

Table 2-19 Composition of the Unsaponifiable Fraction of Some Fats and Oils

Oils

OliveLinseedTeaseedSoybeanRapeseedCornLardTallow

Hydrocarbons

2.8-3.53.7-14.0

3.43.88.71.4

23.811.8

Squalene

32-501.0-3.9

2.62.54.32.24.61.2

Aliphatic Alcohols

0.52.5-5.9

4.97.25.02.12.4

Terpenic Alcohols

20-2629-30

23.29.26.77.15.5

Sterols

20-3034.5-52

22.758.463.681.347.064.0

Source: From G. Jacini, E. Fedeli, and A. Lanzani, Research in the Nonglyceride Substances of Vegetable Oils,J. Assoc. Off. Anal. Chem., Vol. 50, pp. 84-90, 1967.

Table 2-18 Triglycerides and Phospholipids of Mackerel Lipids and Calculated Iodine Values for MethylEsters of Fatty Acid from Lipids

Triglycerides

Light fleshDark fleshLiver

In Lipid(%)

89.574.279.5

In Tissue(%)

9.110.714.4

Ester IodineValue

152.3144.3130.9

Phospholipids

In Lipid(%)

4.711.39.3

In Tissue(%)

0.51.61.7

Ester IodineValue

242.9208.1242.1

Source: From R.G. Ackman and C.A. Eaton, Mackerel Lipids and Fatty Acids, Can. lnst. Food ScL Technol. J.,Vol.4, pp. 169-174,1971.

The sterol content of some fats and oils isgiven in Table 2-20. Cholesterol is the mainsterol of animal, fish, and marine fats and oils.

The hydrocarbons of the unsaponifiableoils are members of the it-paraffins as well asof the branched-chain paraffins of the iso andante-iso configuration. The composition ofhydrocarbon constituents of some vegetableoils has been reported by Jacini et al. (1967)and is listed in Table 2-21.

The structure of squalene is presented inFigure 2-14, which also gives the structureof one of the terpenic alcohols, geranyl

geraniol; this alcohol has been reported to bea component of the nonglyceride fraction ofvegetable oils (Fedeli et al. 1966).

AUTOXIDATION

The unsaturated bonds present in all fatsand oils represent active centers that, amongother things, may react with oxygen. Thisreaction leads to the formation of primary,secondary, and tertiary oxidation productsthat may make the fat or fat-containing foodsunsuitable for consumption.

Figure 2-12 Sterols. (A) Structure of the Steroid Nucleus, (B) Stereochemical Representation, and (C)Cholesterol

C

B

A

Me

Me R

The process of autoxidation and the result-ing deterioration in flavor of fats and fattyfoods are often described by the term rancid-ity. Usually rancidity refers to oxidative dete-rioration but, in the field of dairy science,rancidity refers usually to hydrolytic changesresulting from enzyme activity. Lundberg(1961) distinguishes several types of rancid-ity. In fats such as lard, common oxidativerancidity results from exposure to oxygen;this is characterized by a sweet but undesir-able odor and flavor that become progres-sively more intense and unpleasant asoxidation progresses. Flavor reversion is theterm used for the objectionable flavors thatdevelop in oils containing linolenic acid.This type of oxidation is produced with con-siderably less oxygen than with commonoxidation. A type of oxidation similar toreversion may take place in dairy products,where a very small amount of oxygen may

result in intense oxidation off-flavors. It isinteresting to note that the linolenic acid con-tent of milk fat is quite low.

Among the many factors that affect the rateof oxidation are the following:

Table 2-20 Sterol Content of Fats and Oils

Fat Sterol (%)

L a r d 0 / 1 2Beef tallow 0.08Milk fat 0.3Herring 0.2-0.6Cottonseed 1.4Soybean 0.7Corn 1.0Rapeseed 0.4Coconut 0.08Cocoa butter 0.2

/3-Sitosterol

Brassicasterol

Stigmasterol

Campesterol

Figure 2-13 Structures of the Plant Sterols

• amount of oxygen present• degree of unsaturation of the lipids• presence of antioxidants• presence of prooxidants, especially cop-

per, and some organic compounds such asheme-containing molecules and lipoxi-dase

• nature of packaging material• light exposure• temperature of storage

The autoxidation reaction can be dividedinto the following three parts: initiation,propagation, and termination. In the initia-tion part, hydrogen is abstracted from an ole-fmic compound to yield a free radical.

RH -> R* + H"

The removal of hydrogen takes place at thecarbon atom next to the double bond and can

be brought about by the action of, forinstance, light or metals. The dissociationenergy of hydrogen in various olefinic com-pounds has been listed by Ohloff (1973) andis shown in Table 2-22. Once a free radicalhas been formed, it will combine with oxy-gen to form a peroxy-free radical, which canin turn abstract hydrogen from another unsat-urated molecule to yield a peroxide and anew free radical, thus starting the propaga-tion reaction. This reaction may be repeatedup to several thousand times and has thenature of a chain reaction.

Table 2-21 Hydrocarbon Composition of Some Vegetable Oils

Oils

CornPeanutRapeseedLinseedOlive

n-Paraffins

Cl 1-31C-1 1-30Cl 1-31Cl 1-35

Cl 1» GI 3-30

iso- and/or ante-isoParaffins

Cl 1-21G! 1-23

C-1 1-17> 1̂9-21Cl 1-21

Unidentified

87676

TotalHydrocarbons

404036

43-4529

Source: From G. Jacini, E. Fedeli, and A. Lanzani, Research in the Nonglyceride Substances of Vegetable Oils, J.Assoc. Off. Anal. Chem., Vol. 50, pp. 84-90, 1967.

Figure 2-14 Structure of Squalene and Geranyl GeraniolGeranyl geraniol

Squalene

Source: From G. Ohloff, Fats as Precursors, in Func-tional Properties of Fats in Foods, J. Solms, ed., 1973,Forster Publishing.

The propagation can be followed by termi-nation if the free radicals react with them-selves to yield nonactive products, as shownhere:

carbonyls, which are the most important. Theperoxides have no importance to flavor dete-rioration, which is wholly caused by the sec-ondary oxidation products. The nature of theprocess can be represented by the curves ofFigure 2-15 (Pokorny 1971). In the initialstages of the reaction, the amount of hydro-peroxides increases slowly; this stage istermed the induction period. At the end ofthe induction period, there is a suddenincrease in peroxide content. Because perox-ides are easily determined in fats, the perox-ide value is frequently used to measure theprogress of oxidation. Organoleptic changesare more closely related to the secondaryoxidation products, which can be measuredby various procedures, including the benzi-dine value, which is related to aldehydedecomposition products. As the aldehydesare themselves oxidized, fatty acids areformed; these free fatty acids may be consid-ered tertiary oxidation products. The lengthof the induction period, therefore, depends

Table 2-22 Dissociation Energy for theAbstraction of Hydrogen from OlefinicCompounds and Peroxides

CompoundAE (tea//

mole)

The hydroperoxides formed in the propaga-tion part of the reaction are the primary oxi-dation products. The hydroperoxide mech-anism of autoxidation was first proposed byFarmer (1946). These oxidation productsare generally unstable and decompose intothe secondary oxidation products, whichinclude a variety of compounds, including

VA

LUE

Figure 2-15 Autoxidation of Lard. (A) peroxidevalue, (B) benzidine value, (C) acid value.Source: From J. Pokorny, Stabilization of Fats byPhenolic Antioxidants, Can. Inst. Food ScL Tech-nol J., Vol. 4, pp. 68-74, 1971.

TIME

R-R

RO2R

(R02)n

R" + R"

R' + RO2*

nR02 '

In addition to the changes in double bondposition, there is isomerization from cis totrans, and 90 percent of the peroxidesformed may be in the trans configuration(Lundberg 1961).

From linoleic acid (a'si-c/5'-9,12-octadeca-dienoic acid), three isomeric hydroperoxidescan be formed as shown in the next formula.In this mixture of 9, 11, and 13 hydroperox-ides, the conjugated ones occur in greatest

quantity because they are the more stableforms. The hydroperoxides occur in the cis-trans and trans-trans configurations, thecontent of the latter being greater with highertemperature and greater extent of oxidation.From the oxidation of linolenic acid (cis, cis,ds-9,12,15-octadecatrienoic acid), six iso-metric hydroperoxides can be expectedaccording to theory, as shown:

on the method used to determine oxidationproducts.

Although even saturated fatty acids may beoxidized, the rate of oxidation greatly de-pends on the degree of unsaturation. In theseries of 18-carbon-atom fatty acids 18:0,18:1, 18:2, 18:3, the relative rate of oxidationhas been reported to be in the ratio of1:100:1200:2500. The reaction of unsatur-ated compounds proceeds by the abstractionof hydrogen from the a carbon, and the

resulting free radical is stabilized by reso-nance as follows:

1 2 3 1 2 3

-CH-CH=CH- ^=^ -CH=CH-CH-• •

If oleic acid is taken as example of a mono-ethenoid compound (c/.s-9-octadecenoicacid), the reaction will proceed by abstrac-tion of hydrogen from carbons 8 or 11,resulting in two pairs of resonance hybrids.

This leads to the formation of the followingfour isomeric hydroperoxides:

Hydroperoxides of linolenate decomposemore readily than those of oleate andlinoleate because active methylene groupsare present. The active methylene groups arethe ones located between a single doublebond and a conjugated diene group. Thehydrogen at this methylene group couldreadily be abstracted to form dihydroperox-ides. The possibilities here for decomposi-tion products are obviously more abundantthan with oleate oxidation.

The decomposition of hydroperoxides hasbeen outlined by Keeney (1962). The firststep involves decomposition to the alkoxyand hydroxy free radicals.

R-CH(OOH)-R » R—CH-R + "OH

O*

The alkoxy radical can react to form alde-hydes.

R — C H - R » R' + RCHO

O*

This reaction involves fission of the chainand can occur on either side of the free radi-cal. The aldehyde that is formed can be ashort-chain volatile compound, or it can beattached to the glyceride part of the mole-cule; in this case, the compound is nonvola-

tile. The volatile aldehydes are in great partresponsible for the oxidized flavor of fats.

The alkoxy radical may also abstract ahydrogen atom from another molecule toyield an alcohol and a new free radical, asshown:

R—CH-R + R1H » R—CH—R + R1'

O" OH

The new free radicals formed may partici-pate in propagation of the chain reaction.Some of the free radicals may interact withthemselves to terminate the chain, and thiscould lead to the formation of ketones as fol-lows:

R — C H - R + R1' > R—C—R + R1HI Il

O' O

As indicated, a variety of aldehydes havebeen demonstrated in oxidized fats. Alcoholshave also been identified, but the presence ofketones is not as certain. Keeney (1962) haslisted the aldehydes that may be formed frombreakdown of hydroperoxides of oxidizedoleic, linoleic, linolenic, and arachidonicacids (Table 2-23). The aldehydes are pow-erful flavor compounds and have very lowflavor thresholds; for example, 2,4-decadie-nal has a flavor threshold of less than onepart per billion. The presence of a doublebond in an aldehyde generally lowers the fla-vor threshold considerably. The aldehydescan be further oxidized to carboxylic acids orother tertiary oxidation products.

When chain fission of the alkoxy radicaloccurs on the other side of the free radicalgroup, the reaction will not yield volatilealdehydes but will instead form nonvolatilealdehydo-glycerides. Volatile oxidation prod-ucts can be removed in the refining process

during deodorization, but the nonvolatileproducts remain; this can result in a loweroxidative stability of oils that have alreadyoxidized before refining.

The rate and course of autoxidation dependprimarily on the composition of the fat—itsdegree of unsaturation and the types ofunsaturated fatty acids present. The absence,or at least a low value, of peroxides does not

necessarily indicate that an oil is not oxi-dized. As Figure 2-16 indicates, peroxidesare labile and may be transformed into sec-ondary oxidation products. A combinedindex of primary and secondary oxidationproducts gives a better evaluation of the stateof oxidation of an oil. This is expressed asTotox value: Totox value = 2 x peroxidevalue + anisidine value. (Anisidine value is a

"Only the most active methylene groups in each acid are considered.

Source: From M. Keeney, Secondary Degradation Products, in Lipids and Their Oxidation, H.W. Schultz et al.,eds., 1962, AVI Publishing Co.

Table 2-23 Hydroperoxides and Aldehydes (with Single Oxygen Function) That May Be Formed inAutoxidation of Some Unsaturated Fatty Acids

Fatty Acid

Oleic

Llnoleic

Linolenic

Arachidonic

MethyleneGroup

Involved"

11

8

11

14

11

13

10

7

lsomeric Hydroperoxides Formed fromthe Structures Contributing to the

Intermediate Free Radical ResonanceHybrid

1 1-hydroperoxy-9-ene9-hydroperoxy-1 0-ene8-hydroperoxy-9-ene1 0-hydroperoxy-8-ene1 3-hydroperoxy-9,1 1 -diene1 1-hydroperoxy-9,12-diene9-hydroperoxy- 1 0, 1 2-diene1 6-hydroperoxy-9, 1 2, 1 4-triene1 4-hydroperoxy-9, 1 2, 1 5-triene1 2-hydroperoxy-9, 1 3, 1 5-triene13-hydroperoxy-9, 11,1 5-triene1 1 -hydroperoxy-9,1 2,1 5-triene9-hydroperoxy- 1 0,1 2,1 5-triene1 5-hydroperoxy-5,8,1 1 ,1 3-tetraene1 3-hydroperoxy-5,8, 11,1 4-tetraene1 1 -hydroperoxy-5,8, 12,1 4-tetraene1 2-hydroperoxy-5,8, 1 0, 1 4-tetraene1 0-hydroperoxy-5,8, 11,1 4-tetraene8-hydroperoxy-5,9,1 1 ,1 4-tetraene9-hydroperoxy-5,7, 11,1 4-tetraene7-hydroperoxy-5,8, 11,1 4-tetraene5-hydroperoxy-6,8,1 1 ,1 4-tetraene

Aldehydes Formed by Decom-position of the Hydroperoxides

octanal2-decenal2-undecenalnonanalhexanal2-octenal2,4-decadienalpropanal2-pentenal2,4-heptadienal3-hexenal2,5-octadienal2,4,7-decatrienalhexanal2-octenal2,4-decadienal3-nonenal2,5-undecadienal2,4,7-tridecatrienal3,6-dodecadienal2,5,8-tetradecatrienal2,4,7,1 0-hexadecatetraenal

measure of secondary oxidation products.)Removal of oxygen from foods will preventoxidation, but, in practice, this is not easy toaccomplish in many cases. At high tempera-tures (100 to 14O0C) such as those used inthe accelerated tests for oil stability (activeoxygen method), formic acid is produced,which can be used to indicate the end of theinduction period. The formation of formicacid results from aldehyde decomposition.Peroxidation of aldehydes establishes a reso-nance equilibrium between two limitingforms.

The second hybrid ties up oxygen at the acarbon to yield the cc-hydroperoxy aldehydeas follows:

Breakdown of oxygen and carbon bondsyields formic acid and a new aldehyde.

Per

oxid

e V

alue

InductionPeriod

Free RadicalInitiationPhase

Free RadicalPropagation

"PeroxideFormationPhase"

Free RadicalTermination Phase

PeroxideStabi I ization Perox ide

DecompositionPhase

Time (hours)

Figure 2-16 Peroxide Formation and Decomposition as a Function of Time

deMan et al. (1987) investigated this reactionwith a variety of oils and found that althoughformic acid was the main reaction product,other short-chain acids from acetic to caproicwere also formed. Trace metals, especiallycopper, and to a lesser extent iron, will cata-lyze fat oxidation; metal deactivators such ascitric acid can be used to reduce the effect.Lipoxygenase (lipoxidase) and heme com-pounds act as catalysts of lipid oxidation.Antioxidants can be very effective in slowingdown oxidation and increasing the inductionperiod. Many foods contain natural antioxi-dants; the tocopherols are the most importantof these. They are present in greater amountsin vegetable oils than in animal fats, whichmay explain the former's greater stability.

Antioxidants such as tocopherols may benaturally present; they may be induced byprocesses such as smoking or roasting, oradded as synthetic antioxidants. Antioxi-dants act by reacting with free radicals, thusterminating the chain. The antioxidant AHmay react with the fatty acid free radical orwith the peroxy free radical,

AH+R--»RH+A-AH+R02'->RO2H + A'

The antioxidant free radical deactivated byfurther oxidation to quinones, thus terminat-ing the chain. Only phenolic compounds thatcan easily produce quinones are active asantioxidants (Pokorny 1971). At high con-centrations antioxidants may have a prooxi-dant effect and one of the reactions may beas follows:

A- + RH->AH+R-

Tocopherols in natural fats are usuallypresent at optimum levels. Addition of anti-

oxidant beyond optimum amounts may resultin increasing the extent of prooxidant action.Lard is an example of a fat with very low nat-ural antioxidant activity and antioxidant mustbe added to it, to provide protection. Theeffect of antioxidants can be expressed interms of protection factor, as shown in Figure2-17 (Pokorny 1971). The highly active anti-oxidants that are used in the food industry areactive at about 10 to 50 parts per million(ppm). Chemical structure of the antioxidantsis the most important factor affecting theiractivity. The number of synthetic antioxi-dants permitted in foods is limited, and thestructure of the most widely used compoundsis shown in Figure 2-18. Propyl gallate ismore soluble in water than in fats. The octyland dodecyl esters are more fat soluble. Theyare heat resistant and nonvolatile with steam,making them useful for frying oils and inbaked products. These are considered to havecarry-through properties. Butylated hydroxy-anisole (BHA) has carry-through propertiesbut butylated hydroxy toluene (BHT) doesnot, because it is volatile with steam. Thecompound tert-butyl hydroquinine (TBHQ)is used for its effectiveness in increasing oxi-dative stability of polyunsaturated oils andfats. It also provides carry-through protectionfor fried foods. Antioxidants are frequentlyused in combination or together with syner-gists. The latter are frequently metal deactiva-tors that have the ability to chelate metal ions.An example of the combined effect of antiox-idants is shown in Figure 2-19. It has beenpointed out (Zambiazi and Przybylski 1998)that fatty acid composition can explain onlyabout half of the oxidative stability of a vege-table oil. The other half can be contributed tominor components including tocopherols,metals, pigments, free fatty acids, phenols,phospholipids, and sterols.

PHOTOOXIDATION

Oxidation of lipids, in addition to the freeradical process, can be brought about by atleast two other mechanisms—photooxida-tion and enzymic oxidation by lipoxygenase.The latter is dealt with in Chapter 10. Light-

induced oxidation or photooxidation resultsfrom the reactivity of an excited state of oxy-gen, known as singlet oxygen (1O2). Ground-state or normal oxygen is triplet oxygen(3O2). The activation energy for the reactionof normal oxygen with an unsaturated fattyacid is very high, of the order of 146 to 273

Figure 2-18 Structure of Propyl Gallate (PG), Butylated Hydroxyanisole (BHA), Butylated HydroxyToluene (BHT), and Tert-Butyl Hydroquinone (TBHQ)

PG BHA BHT TBHQ

Figure 2-17 Determination of Protection Factor. (A) lard, (B) lard + antioxidant. Source: From J.Pokorny, Stabilization of Fats by Phenolic Antioxidants, Can. Inst. Food ScL TechnoL /., Vol. 4, pp.68-74, 1971.

TIME

PVPF=S^St1

kJ/mole. When oxygen is converted from theground state to the singlet state, energy istaken up amounting to 92 kJ/mole, and inthis state the oxygen is much more reactive.Singlet-state oxygen production requires thepresence of a sensitizer. The sensitizer isactivated by light, and can then either reactdirectly with the substrate (type I sensitizer)or activate oxygen to the singlet state (type IIsensitizer). In both cases unsaturated fattyacid residues are converted into hydroperox-ides. The light can be from the visible orultraviolet region of the spectrum.

Singlet oxygen is short-lived and revertsback to the ground state with the emission oflight. This light is fluorescent, which meansthat the wavelength of the emitted light ishigher than that of the light that wasabsorbed for the excitation. The reactivity ofsinglet oxygen is 1,500 greater than that ofground-state oxygen. Compounds that canact as sensitizers are widely occurring foodcomponents, including chlorophyll, myoglo-bin, riboflavin, and heavy metals. Most of

these compounds promote type II oxidationreactions. In these reactions the sensitizer istransformed into the activated state by light.The activated sensitizer then reacts with oxy-gen to produce singlet oxygen.

hvsen ^- sen*

sen* + O 2 *- sen + 1O2

The singlet oxygen can react directly withunsaturated fatty acids.

1O2 + RH ^ ROOH

The singlet oxygen reacts directly with thedouble bond by addition, and shifts the dou-ble bond one carbon away. The singlet oxy-gen attack on linoleate produces fourhydroperoxides as shown in Figure 2-20.Photooxidation has no induction period, butthe reaction can be quenched by carotenoids

Figure 2-19 Effect of Copper Concentration on Protective Effect of Antioxidants in Lard. (A) lard +0.01% BHT, (B) lard + 0.01% ascorbyl palmitate, (C) lard + 0.005% BHT and 0.05% ascorbyl palmi-tate. Source: From J. Pokorny, Stabilization of Fats by Phenolic Antioxidants, Can. Inst. Food ScLTechnol J., Vol. 4, pp. 68-74, 1971.

L O G CCu

PV

A B C

that effectively compete for the singlet oxy-gen and bring it back to the ground state.

Phenolic antioxidants do not protect fatsfrom oxidation by singlet oxidation (Yasaeiet al. 1996). However, the antioxidant ascor-byl palmitate is an effective singlet oxygenquencher (Lee et al. 1997). Carotenoids arewidely used as quenchers. Rahmani andCsallany (1998) reported that in the photoox-idation of virgin olive oil, pheophytin Afunctioned as sensitizer, while p-caroteneacted as a quencher.

The combination of light and sensitizers ispresent in many foods displayed in transpar-ent containers in brightly lit supermarkets.The light-induced deterioration of milk hasbeen studied extensively. Sattar et al. (1976)

reported on the light-induced flavor deterio-ration of several oils and fats. Of the five fatsexamined, milk fat and soybean oil weremost susceptible and corn oil least suscepti-ble to singlet oxygen attack. The effect oftemperature on the rate of oxidation of illu-minated corn oil was reported by Chahineand deMan (1971) (Figure 2-21). Theyfound that temperature has an importanteffect on photooxidation rates, but evenfreezing does not completely prevent oxida-tion.

HEATED FATS—FRYING

Fats and oils are heated during commercialprocessing and during frying. Heating during

Figure 2-20 Photooxidation. Singlet-oxygen attack on oleate produces two hydroperoxides; linoleateyields four hydroperoxides

Methyl linoleate

Methyl linoleate

Methyl oleate

Methyl oleate

processing mainly involves hydrogenation,physical refining, and deodorization. Tem-perature used in these processes may rangefrom 12O0C to 27O0C. The oil is not in con-tact with air, which eliminates the possibilityof oxidation. At the high temperatures usedin physical refining and deodorization, sev-eral chemical changes may take place. Theseinclude randomization of the glyceride struc-ture, dimer formation, cis-trans isomeriza-tion, and formation of conjugated fatty acids(positional isomerization) of polyunsaturatedfatty acids (Hoffmann 1989). The trans iso-mer formation in sunflower oil as a result ofhigh temperature deodorization is shown inFigure 2-22 (Ackman 1994).

Conditions prevailing during frying areless favorable than those encountered in theabove-mentioned processes. Deep frying,where the food is heated by immersion in hot

oil, is practiced in commercial frying as wellas in food service operations. The tempera-tures used are in the range of 16O0C to1950C. At lower temperatures frying takeslonger, and at higher temperatures deteriora-tion of the oil is the limiting factor. Deep fry-ing is a complex process involving both theoil and the food to be fried. The reactionstaking place are schematically presented inFigure 2-23. Steam is given off during thefrying, which removes volatile antioxidants,free fatty acids, and other volatiles. Contactwith the air leads to autoxidation and the for-mation of a large number of degradationproducts. The presence of steam results inhydrolysis, with the production of free fattyacids and partial glycerides. At lower fryingtemperatures the food has to be fried longerto reach the desirable color, and this resultsin higher oil uptake. Oil absorption by fried

STORAGE TIME (HOURS)

Figure 2-21 Effect of Temperature on Rate of Oxidation of Illuminated Corn Oil. Source: From M.H.Chahine and J.M. deMan, Autoxidation of Corn Oil under the Influence of Fluorescent Light, Can.Inst. Food ScI Technol J.t Vol. 4, pp. 24-28, 1971.

PE

RO

XID

E

VA

LU

E

(meq

uiv.

/Kg)

foods may range from 10 to 40 percent,depending on conditions of frying and thenature and size of the food.

Oils used in deep frying must be of highquality because of the harsh conditions dur-ing deep frying and to provide satisfactoryshelf life in fried foods. The suitability of anoil for frying is directly related to its contentof unsaturated fatty acids, especially lino-lenic acid. This has been described by Erick-son (1996) as "inherent stability" calculatedfrom the level of each of the unsaturatedfatty acids (oleic, linoleic, and linolenic) andtheir relative reaction rate with oxygen. Theinherent stability calculated for a number ofoils is given in Table 2-24. The higher theinherent stability, the less suitable the oil isfor frying. The liquid seed oils, such as soy-bean and sunflower oil, are not suitable fordeep frying and are usually partially hydro-genated for this purpose. Such hydrogenatedoils can take the form of shortenings, which

may be plastic solids or pourable suspen-sions. Through plant breeding and geneticengineering, oils with higher inherent stabil-ity can be obtained, such as high-oleic sun-flower oil, low-linolenic canola oil, and low-linolenic soybean oil.

The stability of frying oils and fats is usu-ally measured by an accelerated test knownas the active oxygen method (AOM). In thistest, air is bubbled through an oil samplemaintained at 950C and the peroxide value ismeasured at intervals. At the end point theperoxide value shows a sharp increase, andthis represents the AOM value in hours. Typ-ical AOM values for liquid seed oils rangefrom 10 to 30 hours; heavy-duty fryingshortenings range from 200 to 300 hours.AOM values of some oils and fats deter-mined by measuring the peroxide value andusing an automatic recording of volatileacids produced during the test are given inTable 2-25 (deMan et al. 1987).

Figure 2-22 Trans Isomer Formation in Sunflower Oil as a Function of Deodorization Temperature.Source: Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved and TechnologicalAdvances in Alternative Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 301, 1994, Aspen Publish-ers, Inc.

Hours

trans c

onte

nt

(w/w

%)

As shown in Figure 2-23, oil breakdownduring frying can be caused by oxidation andthermal alteration. Oxidation can result in theformation of oxidized monomeric, dimeric,and oligomeric triglycerides as well as vola-tile compounds including aldehydes, ketones,alcohols, and hydrocarbons. In addition, oxi-dized sterols may be formed. Thermal degra-

dation can result in cyclic monomeric tri-glycerides and nonpolar dimeric and oligo-meric triglycerides. The polymerization reac-tion may take place by conversion of part ofthe cw-cw-1,4 diene system of linoleates tothe trans-trans conjugated diene. The 1,4 and1,3 dienes can combine in a Diels-Alder typeaddition reaction to produce a dimer as

Figure 2-23 Summary of Chemical Reactions Occurring During Deep Frying. Source: Reprinted withpermission from FT. Orthoefer, S. Gurkin, and K. Lui, Dynamics of Frying in Deep Frying, in Chem-istry, Nutrition and Practical Applications, E.G. Perkins and M.D. Erickson, eds., p. 224. © 1996,AOCS Press.

dlmerscyclic compounds

HEATING

acids hydrocarbons

dlmerst rimersepoxldesalcoholshydrocarbons

ketonesalcoholsaldehydes

DEHYDRATION FREE RADICALSFISSION

hydroperoxides(conjugated dlenes)

colored compoundsfood llpids

SOLUBILIZATION

free fatty acidsdiglyceridesmonoglyceridesglycerine

HYOROLYSIS

steam

FOODOXIDATION

oxygen

AERATION ABSORPTION VAPORIZATION

steamvollflles (smoke!antfoxldants

Table 2-25 Active Oxygen Method (AOM) Time ofSeveral Oils and Fats as Determined by PeroxideValue and Conductivity Measurements

AOM Time AOM TimeOil (POVf (Conductivity)*

Sunflower 6.2 7.1Canola 14.0 15.8Olive 17.8 17.8Corn 12.4 13.8Peanut 21.1 21.5Soybean 11.0 10.4Triolein 8.1 7.4Lard 42.7 43.2Butterfat 2.8 2.0

3At peroxide value 100.bAt intercept of conductivity curve and time axis.

Source: Reprinted with permission from J.M. deMan,et al., Formation of Short Chain Volatile Organic Acidsin the Automated AOM Method, J.A.O.C.S., Vol. 64, p.996, © 1987, American Oil Chemists' Society.

shown in Figure 2-24. Other possible routesfor dimer formation are through free radicalreactions. As shown in Figure 2-25, this mayinvolve combination of radicals, intermolecu-lar addition, and intramolecular addition.From dimers, higher oligomers can be pro-duced; the structure of these is still relativelyunknown.

Another class of compounds formed duringfrying is cyclic monomers of fatty acids.Linoleic acid can react at either the C9 or C12double bonds to give rings between carbons 5and 9, 5 and 10, 8 and 12, 12 and 17, and 13and 17. Cyclic monomers with a cyclopente-nyl ring have been isolated from heated sun-flower oil, and their structure is illustrated inFigure 2-26 (Le Quere and Sebedio 1996).

Some countries such as France require thatfrying oils contain less than 2 percent lino-lenic acid. Several European countries haveset maximum limits for the level of polar

Table 2-24 Inherent Stability of Oils for Use in Frying

Oil

SoybeanSunflowerHigh-oleic sunflowerCornCottonseedCanolaPeanutLardOlivePalmPahn oleinPalm stearinTallowPalm kernelCoconut

Iodine Value

1301209011.09811092608855583550179

Inherent Stability*

7.47.72.06.25.25.44.51.41.81.41.61.00.70.50.4

Inherent stability calculated from decimal fraction of fatty acids multiplied by relative reaction rates with oxygen,assuming rate for oleic acid = 1 , linoleic acid = 10, and linolenic acid = 25.

compounds or for the level of free fatty acidsbeyond which the fat is considered unfit forhuman consumption. In continuous indus-trial frying, oil is constantly being removedfrom the fryer with the fried food and replen-ished with fresh oil so that the quality of theoil can remain satisfactory. This is more diffi-cult in intermittent frying operations.

FLAVOR REVERSION

Soybean oil and other fats and oils contain-ing linolenic acid show the reversion phe-nomenon when exposed to air. Reversionflavor is a particular type of oxidized flavorthat develops at comparatively low levels ofoxidation. The off-flavors may develop in oils

Figure 2-24 Polymerization of Diene Systems To Form Dimers

a) Combination of radicals:

b) lntermolecular addition:

c) Intramolecular addition:

Figure 2-25 Nonpolar Dimer Formation Through Free Radical Reactions

Next page

INTRODUCTION

Proteins are polymers of some 21 differentamino acids joined together by peptidebonds. Because of the variety of side chainsthat occur when these amino acids are linkedtogether, the different proteins may have dif-ferent chemical properties and widely differ-ent secondary and tertiary structures. Thevarious amino acids joined in a peptide chainare shown in Figure 3-1. The amino acidsare grouped on the basis of the chemicalnature of the side chains (Krull and Wall1969). The side chains may be polar or non-polar. High levels of polar amino acid resi-dues in a protein increase water solubility.The most polar side chains are those of thebasic and acidic amino acids. These aminoacids are present at high levels in the solublealbumins and globulins. In contrast, the wheatproteins, gliadin and glutenin, have low levelsof polar side chains and are quite insoluble inwater. The acidic amino acids may also bepresent in proteins in the form of theiramides, glutamine and asparagine. Thisincreases the nitrogen content of the protein.Hydroxyl groups in the side chains maybecome involved in ester linkages with phos-phoric acid and phosphates. Sulfur aminoacids may form disulfide cross-links betweenneighboring peptide chains or between dif-

ferent parts of the same chain. Proline andhydroxyproline impose significant structurallimitations on the geometry of the peptidechain.

Proteins occur in animal as well as vegeta-ble products in important quantities. In thedeveloped countries, people obtain much oftheir protein from animal products. In otherparts of the world, the major portion ofdietary protein is derived from plant prod-ucts. Many plant proteins are deficient in oneor more of the essential amino acids. Theprotein content of some selected foods islisted in Table 3-1.

AMINO ACID COMPOSITION

Amino acids joined together by peptidebonds form the primary structure of proteins.The amino acid composition establishes thenature of secondary and tertiary structures.These, in turn, significantly influence thefunctional properties of food proteins andtheir behavior during processing. Of the 20amino acids, only about half are essential forhuman nutrition. The amounts of these essen-tial amino acids present in a protein and theiravailability determine the nutritional qualityof the protein. In general, animal proteins areof higher quality than plant proteins. Plant

Proteins

CHAPTER 3

Figure 3-1 Component Amino Acids of ProteinsJoined by Peptide Bonds and Character of SideChains. Source: From Northern Regional Re-search Laboratory, U.S. Department of Agricul-ture.

proteins can be upgraded nutritionally byjudicious blending or by genetic modificationthrough plant breeding. The amino acid com-position of some selected animal and vegeta-ble proteins is given in Table 3—2.

Egg protein is one of the best quality pro-teins and is considered to have a biologicalvalue of 100. It is widely used as a standard,and protein efficiency ratio (PER) valuessometimes use egg white as a standard.Cereal proteins are generally deficient inlysine and threonine, as indicated in Table

Table 3-1 Protein Content of Some SelectedFoods

Product Protein (g/1 OO g)

Meat: beef 16.5pork 10.2

Chicken (light meat) 23.4Fish: haddock 18.3

cod 17.6Milk 3.6Egg 12.9Wheat 13.3Bread 8.7Soybeans: dry, raw 34.1

cooked 11.0Peas 6.3Beans: dry, raw 22.3

cooked 7.8Rice: white, raw 6.7

cooked 2.0Cassava 1.6Potato 2.0Corn 10.0

3-3. Soybean is a good source of Iysine butis deficient in methionine. Cottonseed pro-tein is deficient in lysine and peanut proteinin methionine and lysine. The protein ofpotato although present in small quantity(Table 3-1) is of excellent quality and isequivalent to that of whole egg.

Table 3-3 Limiting Essential Amino Acids ofSome Grain Proteins

First SecondLimiting Limiting

Grain Amino Acid Amino Acid

Wheat Lysine ThreonineCorn Lysine TryptophanRice Lysine ThreonineSorghum Lysine ThreonineMillet Lysine Threonine

PROTEIN CLASSIFICATION

Proteins are complex molecules, and classi-fication has been based mostly on solubility indifferent solvents. Increasingly, however, asmore knowledge about molecular composi-tion and structure is obtained, other criteriaare being used for classification. Theseinclude behavior in the ultracentrifuge andelectrophoretic properties. Proteins are di-vided into the following main groups: simple,conjugated, and derived proteins.

Simple Proteins

Simple proteins yield only amino acids onhydrolysis and include the following classes:

• Albumins. Soluble in neutral, salt-freewater. Usually these are proteins of rela-tively low molecular weight. Examples

Table 3-2 Amino AcJd Content of Some Selected Foods (mg/g Total Nitrogen)

Amino Acid

lsoleucineLeucineLysineMethlonineCystinePhenylalanineTyroslneThreonineValineArginineHistidineAlanineAspartic acidGlutamic acidGlycineProlineSerine

Meat (Beef)

30150755616980

275225287313395213365562955304236252

Milk

399782450156

434396278463160214255424

1151144514342

Egg

393551436210152358260320428381152370601796207260478

Wheat

20441717994

159282187183276288143226308

1866245621281

Peas

2674254705770

287171254294595143255685

1009253244271

Com

23078316712097

305239225303262170471392

1184231559311

are egg albumin, lactalbumin, and serumalbumin in the whey proteins of milk,leucosin of cereals, and legumelin inlegume seeds.

• Globulins. Soluble in neutral salt solu-tions and almost insoluble in water.Examples are serum globulins and (3-lac-toglobulin in milk, myosin and actin inmeat, and glycinin in soybeans.

• Glutelins. Soluble in very dilute acid orbase and insoluble in neutral solvents.These proteins occur in cereals, such asglutenin in wheat and oryzenin in rice.

• Prolamins. Soluble in 50 to 90 percentethanol and insoluble in water. Theseproteins have large amounts of prolineand glutamic acid and occur in cereals.Examples are zein in corn, gliadin inwheat, and hordein in barley.

• Scleroproteins. Insoluble in water andneutral solvents and resistant to enzymichydrolysis. These are fibrous proteinsserving structural and binding purposes.Collagen of muscle tissue is included inthis group, as is gelatin, which is derivedfrom it. Other examples include elastin,a component of tendons, and keratin, acomponent of hair and hoofs.

• Histories. Basic proteins, as defined bytheir high content of lysine and arginine.Soluble in water and precipitated byammonia.

• Protamines. Strongly basic proteins oflow molecular weight (4,000 to 8,000).They are rich in arginine. Examples areclupein from herring and scombrin frommackerel.

Conjugated Proteins

Conjugated proteins contain an aminoacid part combined with a nonprotein mate-rial such as a lipid, nucleic acid, or carbohy-

drate. Some of the major conjugatedproteins are as follows:

• Phosphoproteins. An important groupthat includes many major food proteins.Phosphate groups are linked to thehydroxyl groups of serine and threonine.This group includes casein of milk andthe phosphoproteins of egg yolk.

• Lipoproteins. These are combinations oflipids with protein and have excellentemulsifying capacity. Lipoproteins occurin milk and egg yolk.

• Nucleoproteins. These are combinationsof nucleic acids with protein. Thesecompounds are found in cell nuclei.

• Glycoproteins. These are combinationsof carbohydrates with protein. Usuallythe amount of carbohydrate is small, butsome glycoproteins have carbohydratecontents of 8 to 20 percent. An exampleof such a mucoprotein is ovomucin ofegg white.

• Chromopmteins. These are proteins witha colored prosthetic group. There aremany compounds of this type, includinghemoglobin and myoglobin, chlorophyll,and flavoproteins.

Derived Proteins

These are compounds obtained by chemi-cal or enzymatic methods and are dividedinto primary and secondary derivatives, de-pending on the extent of change that hastaken place. Primary derivatives are slightlymodified and are insoluble in water; rennet-coagulated casein is an example of a primaryderivative. Secondary derivatives are moreextensively changed and include proteoses,peptones, and peptides. The differencebetween these breakdown products is in sizeand solubility. All are soluble in water and

not coagulated by heat, but proteoses can beprecipitated with saturated ammonium sul-fate solution. Peptides contain two or moreamino acid residues. These breakdown prod-ucts are formed during the processing ofmany foods, for example, during ripening ofcheese.

PROTEIN STRUCTURE

Proteins are macromolecules with differentlevels of structural organization. The primarystructure of proteins relates to the peptidebonds between component amino acids andalso to the amino acid sequence in the mole-cule. Researchers have elucidated the aminoacid sequence in many proteins. For exam-ple, the amino acid composition and se-quence for several milk proteins is now wellestablished (Swaisgood 1982).

Some proteolytic enzymes have quite spe-cific actions; they attack only a limited num-ber of bonds, involving only particular aminoacid residues in a particular sequence. Thismay lead to the accumulation of well-definedpeptides during some enzymic proteolyticreactions in foods.

The secondary structure of proteins in-volves folding the primary structure. Hydro-gen bonds between amide nitrogen and car-bonyl oxygen are the major stabilizing force.These bonds may be formed between differ-ent areas of the same polypeptide chain orbetween adjacent chains. In aqueous media,the hydrogen bonds may be less significant,and van der Waals forces and hydrophobicinteraction between apolar side chains maycontribute to the stability of the secondarystructure. The secondary structure may beeither the oc-helix or the sheet structure, asshown in Figure 3-2. The helical structuresare stabilized by intramolecular hydrogenbonds, the sheet structures by intermolecular

hydrogen bonds. The requirements for maxi-mum stability of the helix structure wereestablished by Pauling et al. (1951). Thehelix model involves a translation of 0.54 nmper turn along the central axis. A completeturn is made for every 3.6 amino acid resi-dues. Proteins do not necessarily have tooccur in a complete a-helix configuration;rather, only parts of the peptide chains maybe helical, with other areas of the chain in amore or less unordered configuration. Pro-teins with a-helix structure may be eitherglobular or fibrous. In the parallel sheetstructure, the polypeptide chains are almostfully extended and can form hydrogen bondsbetween adjacent chains. Such structures aregenerally insoluble in aqueous solvents andare fibrous in nature.

The tertiary structure of proteins involves apattern of folding of the chains into a com-pact unit that is stabilized by hydrogen bonds,van der Waals forces, disulfide bridges, andhydrophobic interactions. The tertiary struc-ture results in the formation of a tightlypacked unit with most of the polar amino acidresidues located on the outside and hydrated.This leaves the internal part with most of theapolar side chains and virtually no hydration.Certain amino acids, such as proline, disruptthe a-helix, and this causes fold regions withrandom structure (Kinsella 1982). The natureof the tertiary structure varies among proteinsas does the ratio of a-helix and random coil.Insulin is loosely folded, and its tertiary struc-ture is stabilized by disulfide bridges. Lyso-zyme and glycinin have disulfide bridges butare compactly folded.

Large molecules of molecular weightsabove about 50,000 may form quaternarystructures by association of subunits. Thesestructures may be stabilized by hydrogenbonds, disulfide bridges, and hydrophobicinteractions. The bond energies involved in

Figure 3-2 Secondary Structures of Proteins, (A) Alpha Helix, (B) Antiparallel Sheet

3rdturn

2ndturn

1stturn

Rise perresidue

forming these structures are listed in Table3-4.

The term subunit denotes a protein chainpossessing an internal covalent and noncova-lent structure that is capable of joining withother similar subunits through noncovalentforces or disulfide bonds to form an oligo-meric macromolecule (Stanley and Yada1992). Many food proteins are oligomericand consist of a number of subunits, usually2 or 4, but occasionally as many as 24. A list-ing of some oligomeric food proteins isgiven in Table 3-5. The subunits of proteinsare held together by various types of bonds:electrostatic bonds involving carboxyl,amino, imidazole, and guanido groups; hy-drogen bonds involving hydroxyl, amide,and phenol groups; hydrophobic bonds in-volving long-chain aliphatic residues or aro-matic groups; and covalent disulfide bondsinvolving cystine residues. Hydrophobicbonds are not true bonds but have beendescribed as interactions of nonpolar groups.These nonpolar groups or areas have a ten-dency to orient themselves to the interior ofthe protein molecule. This tendency dependson the relative number of nonpolar amino

Table 3-4 Bond Energies of the Bonds Involvedin Protein Structure

Bond Energy*Bond (kcal/mole)Covalent C-C 83Covalent S-S 50Hydrogen bond 3-7Ionic electrostatic bond 3-7Hydrophobic bond 3-5Van der Waals bond 1 -2

These refer to free energy required to break thebonds: in the case of a hydrophobic bond, the freeenergy required to unfold a nonpolar side chain fromthe interior of the molecule into the aqueous medium.

acid residues and their location in the peptidechain. Many food p