MANUFACTURE OF FROZEN YOGURT WITH ULTRAFILTERED MILK AND PROBIOTIC LACTIC ACID

BACTERIA'

G.A. ORDONEZ, I.J. JEON' and H.A. ROBERTS

Department of Animal Sciences and Industry Kansas State University Manhattan, KS 66506

Accepted for Publication August 18, 1999

ABSTRACT

Utilization of ultrafiltered (UF) milk for the manufacture of frozen yogurt was investigated. Three basic frozen yogurt mixes (0, 2, and 4% milk fat) were formulated using ultrafiltered skim milk as a base material and fermented with Lactobacillus acidophilus, Bifidobacterium bifidum, and a mixed yogurt culture of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus. The cultured mixes were frozen using a continuous freezer and evaluated for product stability. Results suggest that culture organisms grew well during fermentation and produced acids continuously in the UF mixes, which became excessively viscous at a TA of 0.40% or higher. Therefore, all mixes were fermented to 0.30% TA ( - 4 h) and rapidly cooled to 10C or lower to prevent excessive viscosity development. The finished products contained high levels of protein and calcium as well as goodflavor and textural quality. In addition, the culture organisms were stable during six weeks of frozen storage.

INTRODUCTION

Consumers have shown a great interest in frozen yogurt because of its potential as a low fat replacement for ice cream and the perceived probiotic effects of the lactic acid bacteria used in its manufacture (Hughes and Hoover 1991). Total frozen yogurt production in 1996 was over 447 million liters in the United States (USDA 1997). No current federal regulations specify the amount or type of organisms to be used in frozen yogurts. However, many states have stipulated their own regulations, and the common practice is to utilize the

' Contribution No. 98-277-5 from the Kansas Agricultural Experiment Station ' Send correspondence to: Dr. Ike J . Jeon, Kansas State University, 139 Call Hall, Manhattan,

Kansas 66506. Tel. (785)532-1211; Fax (785)532-5681; E-mail < ijjfdsc@ksu.edu > .

Journal of Food Processing Preservation 24 (2000) 163-176. All Rights Reserved. 'DCopyright 2000 by Food & Nutrition Press, Inc., Trumbull. Connecticut. 163

164 G.A. ORDONEZ, I . J . JEON and H.A. ROBERTS

traditional yogurt cultures of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus. Because of the perceived probiotic value (Hoover 1993; Gilliland 1989), other species such as Bifidobacterium bifidum and Lactobacillus acidophilus also are used. Hekmat and McMahon (1992) developed a probiotic ice cream using commercial cultures of B. bifldum and L. acidophilus and reported that the culture population decreased from lo8 to lo6 CFU/mL after 17 weeks of frozen storage at -29C. Holcomb et al. (1991) reported that L. acidophilus and B. bifidum grew well at pHs as low as 5.4 and were stable in soft-serve frozen yogurt. Laroia and Martin (1991) observed that over 90% of the cultures of B. bifidum and L. acidophilus in a frozen yogurt mix (pH 5.6) survived after freezing and decreased by less than two log cycles during eight weeks of frozen storage. Mashayekh and Brown (1992) reported that loss of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus in frozen yogurt at pH 4.9 was less than 0.5 log cycle of viable colony counts after freezing and one month of frozen storage.

Use of ultrafiltered (UF) skim milk retentates in the manufacture of lowfat frozen dairy desserts has been shown to favorably affect sensory attributes. The UF milk-based products are higher in protein but lower in lactose than conventional frozen dairy desserts (Jensen et al. 1989). Geilman and Schmidt (1992) reported that ice cream made from UF milk had 2.24 times as much protein and 65% less lactose than that made with nonfat dry milk (NFDM). Grow (1990) reported increases in protein concentration of 38 to 118% and decreases in lactose of 49 to 63% in UF-based ice milks. The author also reported that ice milks manufactured with UF milk had higher flavor and body scores than the control ice milks in which solids nonfat were supplied by NFDM. The effect of the high protein content of UF milk on B. bifidum also has been studied. Ventling and Mistry (1991) compared rates of acidity development and growth of B. bifidum cultured in UF skim milk versus nonUF skim milk and reported higher populations of B. bifidum and greater acidity development in UF milk.

The objectives of this study were to develop manufacturing procedures for frozen yogurts by fermenting UF milk-based mixes with probiotic culture organisms and to evaluate quality and stability of the finished products during frozen storage.

MATERIALS AND METHODS

Preparation of Ultrafiltered Skim Milk

Fresh pasteurized skim milk was obtained from the Kansas State University Dairy Processing Plant and subjected to ultrafiltration utilizing an Abcor Model

MANUFACTURE OF FROZEN YOGURT 165

111 Sanitary Pilot UF unit (Koch Membrane Systems Inc., Wilmington, MA). The unit was fitted with an Abcor spiral wound polysulfone membrane (Koch's Part No. 0700290) with a molecular cut-off of 5000 daltons. All milk samples were circulated through the membrane cartridge until the total solids of 20% were achieved as measured by refractive index (Grow 1990) using an Abbe refractometer (Bausch & Lomb Optical Co., Rochester, NY). The UF retentates were stored in a walk-in cooler (1.6C) until used.

Mix Formulations

Three mix formulations were used in this study with different fat levels (0, 2.0, and 4.0%) to represent a fat-free, a low-fat, and a regular fat product (Table 1). In each formulation, milk solids nonfat (MSNF) was furnished by the UF retentate as described previously by Grow (1990) and Sullivan (1992). The UF milk needed to satisfy the MSNF in the formulations was 66 to 68% (Table 1). The milk fat was supplied by heavy cream (38% fat) that was obtained from the KSU Dairy Plant. The other ingredients used and their sources were: granulated sugar from Holly Sugar Corp. (Colorado Springs, CO); corn syrup

TABLE 1 THE INGREDIENT FORMULAS USED FOR FROZEN YOGURT MIXES'

Mix 1 Mix 2 Mix 3 (0% fat) (2% fat) (4% fat)

Ingredient kg % kg % kg %

UF milk2 24 87 68.7 24.90 68.9 2405 6 6 5

Cream' 0 0 1.64 4.5 3 60 9.9

Sugar 4.36 12.1 4 36 12.1 4.36 12 1

Corn syrup solids 1.82 5.0 I 82 5.0 I 82 5.0

Stabilizer' 0.14 0.4 0.14 0.4 0 14 0.4

Water' 5.00 13.8 3.30 9.1 2.21 6 1

' Batch size 36 kg

' Heavy cream (38% fat) to provide fat ' A mixture o f stabilizers and emulsifiers containing microcrystalline cellulose. cellulose

gum, locust bean gum, carrageenan, and mono- and diglycerides plus 2% carrageenan (Gold Star NFB; Danisco Ingredients USA Inc., New Century, KS) Variable according to the amount o f UF milk and fat used for each formulation

20% solids that provide 13.5% milk solids nonfat

166 G.A. ORDONEZ, I .J . JEON and H.A. ROBERTS

solids (Star-Dri 35R) from A.E. Staley Mfg. (Decatur, IL); and stabiliz- edemulsifier (GoldStar NFB) from Danisco Ingredients USA, Inc. (New Century, KS). The laboratory-scale yogurt mixes were 400 g/batch, whereas plant-scale mixes were 36 kg/batch. All mix calculations were made utilizing the algebraic method described by Sommer (195 1).

Pasteurization of Mixes

All mix batches were prepared in a 75-liter kettle (Groen Mfg. Co., Chicago IL) by mixing preblended dry ingredients into the UF milk. The mixes were pasteurized at 74C for 30 min with gentle agitation, cooled to 63C, and homogenized with a single-stage homogenizer (Creamery Package Co., Chicago, IL) at 10.4 MPa (1500 psi). The pasteurized and homogenized mixes were cooled to 5C in stainless steel milk cans (38 L) with ice water and stored in a walk-in cooler (1.6C). All mixes were used for fermentation within 12 h of preparation.

Fermentation of Mixes With Starter Cultures

Each batch (36 kg) of mixes was inoculated at 40C with bulk cultures of three commercial, concentrated, freeze-dried cultures (CFDC) of Bijidobacteri- urn bifidum 10 L, Lactobacillus acidophilus 10 L, and the mixed yogurt culture of Streptococcus thermophilus and Lactobacillus delbrueckii ssp. bulgaricus CH-150. All CFDC were from Chr. Hansen Laboratories (Milwaukee, WI). The bulk cultures were prepared from CFDC with 10% NFDM by incubating aerobically at 40C to a pH of 5.5 (attained in 5 to 5.5 h). The cultures then were stored overnight at 7C and used for mix inoculations. The following inoculation levels were used according to the preliminary study done on a laboratory scale: 5% (wt/wt) of B. bifidus, 1 % (wt/wt) of L. acidophilus, and 0.1 % (wt/wt) of the mixed yogurt culture.

Fermentation and Mix Viscosity Measurement

To determine the effect of acidity development on mix viscosity during fermentation, a 2 kg batch of each mix was inoculated and fermented (as described above) for up to 0.5% TA. Viscosity readings were taken at 0.05% TA intervals by a Brookfield viscometer model LVF (Brook field Engineering Laboratories, Inc., Stoughton, MA). Two spindles were used: a #2 spindle for readings up to 1000 centipoise and a #4 spindle for readings above that level.

Fermentation of Mixes €or Frozen Yogurts

Based on the preliminary work done above on viscosity, all mixes were fermented to a final titratable acidity (TA) of 0.30% (-4 h) and immediately

MANUFACTURE OF FROZEN YOGURT 167

cooled to 1OC in ice water with gentle stirring (-30 min). The mixes were stored in a walk-in cooler (1.6C) for approximately 12 h before freezing.

Freezing and Hardening

Prior to freezing, pure vanilla extract (E.B. Weber & Co., Wheeling, IL) and annato fat soluble vegetable color (Chr. Hansen’s Labs., Milwaukee, WI) were added to each batch of the mix at 0.45 and 0.01 % (wtlwt), respectively. The mixes were frozen using a Vogt continuous ice cream freezer (Cherry Burrel Corp., Cedar Rapids, IA) with a target of 80% overrun. The frozen mixes were packaged into 1.89 L (‘/2 gal) paper cartons and ,hardened at -29C.

Compositional Analysis

Proximate analysis of the frozen yogurts was done by the following AOAC (1990) methods. Total solids and fat were determined by the Mojonnier method described for ice cream. Protein content was measured by the macro-Kjeldahl method. Calcium was determined with a Varian Model AA-1475 Atomic Emission Spectrophotometer (Varian Instrument Group, Sunnyvale, CA). Lactose was determined by high performance liquid chromatography (Kwak and Jeon 1986) using a Beckman system (Beckman Instruments, Inc., Fullerton, CA) fitted with a Water’s carbohydrate column (Waters Assoc., Milford, MA) and a refractive index detector. The titratable acidity (TA) was determined as % lactic acid for ice cream mixes (Arbuckle 1986), and pH was measured using a Radiometer pH meter (Radiometer, Copenhagen, Denmark). All analyses were done in duplicate.

Sensory Analysis

To access quality changes during storage, frozen yogurt samples were coded randomly and evaluated weekly by a nine-membered trained panel for six weeks. The panelists consisted of five past members of dairy product judging teams and four KSU faculty and staff who had dairy product judging experience. A modified ice cream score card from American Dairy Science Association was used. The frozen yogurts were evaluated for three major attributes: overall flavor, bodyltexture, and quality. A 10-point scale was used with 10 being excellent (or no defect) and 1 being extremely poor. The products also were evaluated for the following flavor defects: high or low acidity, acetaldehyde flavor, cooked, lacks freshness, oxidized, high or low sweetness, lacks fine flavor, and unnatural flavor. They also were evaluated for the following bodyhexture defects: coarselicy, gummy, soggy, weak, and crumbly. A scale of 1 (none) to 5 (pronounced) was used to score these defects. The frozen mixes were tempered in a cooler at 1OC for 24 h prior to sensory evaluation.

168 G.A. ORDONEZ, I . J . JEON and H.A. ROBERTS

Stability Test for Culture Organisms during Frozen Storage

The populations of B. bifidum, L. acidophilus, and the mixed culture of S . thermophilus and L. delbrueckii ssp. bulgaricus in frozen yogurts were determined selectively. Lithium chloride-sodium propionate (LP) agar (Lapierre et al. 1992) was used for the enumeration of BiBdobacteria, and MRS agar (Difco Laboratories, Michigan, IL) plus 2 % Oxgall (Difco Laboratories, Michigan, IL) was used for L. acidophilus (Gilliland and Speck 1977). Total counts were performed using Ellikers agar (Difco Laboratories, Michigan, IL). The population of the mixed yogurt culture was estimated by the difference between total counts and count for the two organisms. The Bifidobacteria and total count plates were incubated at 40C in an anaerobic chamber (Model 1024, Forma Scientific, Marietta, OH). The L. acidophilus plates were incubated aerobically at 40C. The microbial analyses were done every two weeks for eight weeks during frozen storage (-29C). Frozen yogurt samples were thawed in a cooler at 1OC before duplicate analysis.

Statistical Analysis

The entire experiment was repeated three times. Statistical analyses were carried out using SAS statistical software version 6.03 (SAS Institute, Cary, NC). Data variability for sensory scores due to panelist errors were analyzed using the analysis of variance (ANOVA) procedure. The weekly scores given by the panel for each treatment (0 , 2, and 4% fat) were compared using the LSMEANS procedure for differences in overall quality, flavor, and bodykexture, as well as for defects. To test the effect of frozen storage conditions (-29C) on product quality, the sensory scores obtained from different batches also were compared using the LSMEANS procedure.

RESULTS AND DISCUSSION

Composition of Frozen Yogurt Mixes

Finished product analysis showed that all three formulations contained fat and total solids close to the target levels (Table 2). Mix 1, which had no added fat, contained 0.52% of fat. We assumed that this fat originated from skim milk that was used for the UF retentate in the formulation. Mix 2 had slightly less fat (1.86%) than anticipated, whereas mix 3 contained the targeted amount of fat (4.04%). Unique features of the product were the high levels of protein (8.78-9.01 %) and calcium (241.3-243.9 mg/lOO g mix) when compared to regular frozen yogurts on the market. These were due to the use of UF milk retentate as a base material. According to a survey done by Tieszen and Baer

MANUFACTURE OF FROZEN YOGURT 169

(1989), commercial frozen yogurts (vanilla) had average contents of 3.69% fat, 3.74% protein, and 31.47% total solids. Vanilla ice milk (31.38% solids), which is similar in composition to frozen yogurt, had 134 mg calcium per 100 g of product (USDA 1976), whereas a 4% milk fat ice cream contained 5.1 % protein and 186 mg calcium per 100 g (Arbuckle 1986). The lactose content in our product was relatively low (Table 2) because of the use of UF milk retentate. This level of lactose may not be a serious concern to consumers who have a lactose maldigestion problem, because the probiotic cultures used in this process exhibited fairly good P-galactosidase activities (Ordonez and Jeon 1995).

TABLE 2. COMPOSITION (%) OF FINISHED FROZEN YOGURTS'

Mix Fat2 Protein' Lactose' Solids' Calcium (mg)"

1 0 52' 9 O l d 4 43( 32 33h 241 30 '

2 1 86b 8 86d 4 69' 34 30h 241 30'

3 4 04' 8 78d 4 75' 35 40' 243 92'

'Means ofthree trials. Means (within a column) with the same letter are not significantly different.

'-'By Mojonnier method for ice cream mixes (AOAC, 1992) 'By macroKjeldahl method 'By high performance liquid chromatography 6By Atomic Emission Spectrophotometry (md100 g of product)

Titratable Acidity as a Control Point

As shown in Table 3 , small increases in mix viscosity occurred between TAs of 0.25 and 0.35% at which the respective viscosities were 630 and 723 centiposes. However, as TA increased from 0.35 to 0.40% (pH 6.4 to 6.2), the viscosity increased about four times, which made the mixes too viscous for processing. Therefore, we considered a TA of 0.30% as a control point in this process. To avoid an excessive viscosity development, the fermentations were stopped at 0.30% TA, and the mixes were cooled rapidly to 1OC or lower to maintain a maximum TA of 0.35%. We assumed that the starter cultures used in this study would not grow or produce acids at this temperature (Scardovi 1986; Hardie 1986; Kandler and Weiss 1986). We observed that cooling the mixes to 1OC in 1 h resulted in an 0.04% increase of TA.

1 70 G.A. ORDONEZ, I .J . JEON and H.A. ROBERTS

TABLE 3. VISCOSITY OF MIXES AT VARIOUS TITRATABLE ACIDITY

DURING FERMENTATION

Viscosity (Centipoise)'

Titratable Acidity Mix 1 (0.5 %fat) Mix 2 (1.8 Ye fat) Mix 3 (4 0 %fat)

0.25 630

0.30 687

0.35 713

0.40 2733

0.45 3200

0.50 4067

643

690

717

2867

3400

4261

670

707

723

3100

3667

4400

'Measured with Brookfield viscometer model LVF (Brookfield Engineering Labs , Stoughton, MA); #2 spindle for readings up to 1000 centipoise and #4 spindle for above 1000 centipoise. Average of triplicates.

Although viscosity has long been considered an important property of a frozen dessert mix for proper whipping and retention of air, the desirable level has not been identified (Arbuckle 1986). In unfermented dairy mixes, viscosity is due mainly to the quality and type of ingredients. In fermented dairy mixes, however, the effect of the amount of acid on the stability of the milk proteins (predominantly casein) plays a significant role in mix viscosity. As pH of the milk lowers, casein micelles begin to aggregate and form a complex with calcium that contributes to the increase in viscosity. We speculated that high levels of protein and calcium in UF skim milk might have contributed to the abrupt increase in viscosity at 0.4% TA. Calcium cross-linkages in the casein matrix or the charge-neutralizing effect of the calcium may allow proteins to interact through hydrophobic bonding (Brown 1988). High protein in UF-based dessert mixes also increases the water-binding capacity (Geilman and Schmidt 1992).

MANUFACTURE OF FROZEN YOGURT 171

Acid Production and Growth of Culture Organisms

The target acidity of 0.30% in all mixes was achieved easily within 4 h of fermentation. The levels of probiotic culture organisms used apparently were sufficient to develop the target acidity within a reasonable time for this type of processing. The levels of inoculation were equivalent to approximately lo7 CFU/mL for B. bfldus, lo6 CFU/mL for L. acidophilus, and lo5 CFU/mL for the mixed yogurt culture. One of the reasons we used a low level (0.1 % , wt/wt) of the yogurt culture was that the mixed cultures have a tendency to grow faster in a multiculture fermentation and thereby inhibit the growth of L. acidophilus (Mital and Garg 1992). They probably also inhibit B. bifidum cultures. Our data suggest that the levels of fat (0.52 to 4.04%) and solids (32.23 to 35.40%) used had no apparent effect on culture growth or acidity development in the mixes. In fact, the combined cultures steadily increased the acidity in all three mixes to 0.51 % during 8 h of fermentation at 40C (data not shown). We also observed that the growth rates and generation times of these organisms were not different (P > 0.05) within cultures (data not shown). Although we did not attempt to estimate the degree of acidity contributed by individual cultures in the mix, we can assume that the greatest contribution, relative to the inoculation level, came from the mixed culture of S. thermophilus and L. delbrueckii ssp. bulgaricus, which are known for high acid production.

Our results generally agree with other reports. Hekmat and McMahon (1992) reported that B. bifidum and L. acidophilus had good growth and acid development in an ice cream mix with 12% fat and 40% total solids when both were inoculated at a level of lo8 CFU/mL. According to their report, pH of the mixes decreased from 6.5 to 5.5 in 5 h. In our experiment, we observed that the rate of decrease in pH during mix fermentation averaged 0.074 unit per hour. Final pH for all mixes was between 6.47 and 6.50. At 4 h of mix fermentation, mean populations had increased to greater than log 7.5 for B. bifidum, log 6.5 for L. acidophilus, and log 5.7 CFU/mL or the mixed yogurt culture (Table 4).

Microbiological Stability during Frozen Storage

Survivability of culture organisms during frozen storage is shown in Table 4. The initial decrease after two weeks of storage was near one log count for all organisms. This decrease probably was due mostly to the temperature shock caused by the freezing process itself, as well as the residual oxygen from the air incorporated during freezing (overrun). After the initial decrease, however, all culture populations remained relatively stable for eight weeks of frozen storage although steady decreases occurred (Table 4). The total counts averaged log 6.7 after eight weeks. Hekmat and McMahon (1992) reported similar stability (approximately one log cycle loss) of L. acidophihs and B. bifldum during 17 weeks of frozen storage. Holcomb et al. (1991) reported that the reduction of

172 G.A. ORDONEZ, I.J. JEON and H.A. ROBERTS

B. bifldum and L. acidophilus was less than 1 log count for soft-serve frozen yogurt after freezing. Mashayekh and Brown (1992) also reported similar results for the survivability of S . thermophilus and L. delbrueckii ssp. bulgaricus in frozen cultured ice cream.

TABLE 4. MEAN POPULATION COUNTS OF CULTURE ORGANISMS IN FROZEN YOGURTS

DURING 8 WEEKS OF STORAGE AT -29C

Culture organisms (log CFUlmL)’

Storage (wk) B. bijidum L. acidophilus Mixed yogurt2 Total counts -~ ~~~

0 7.51‘ 6.56‘ 5.69’ 7.57’

2 6.79b 5.89b 5. 26b 6.90b

4 6.6gb 5.82b,‘ 5. 17b.‘ 6.83”‘

6 6.62b 5.72‘ 5.14b.c 6.75b,d

8 6.60b 5.70’ 5 . lo’ 6.72d

I

’ Mean value from three mixes representing the average of three trials. Same superscript letters within a column are not significantly different (P > 0.05). Streptococcus thermphilus and Lactobacillus delbrueckii ssp. bulgaricus

Sensory Quality

Table 5 shows mean sensory scores of overall quality for three frozen yogurt mixes during 6 weeks of storage at -29C. The results suggest that the mixes with higher amounts of fat yielded better sensory quality. Mix 3 with 4.04% fat received the highest mean scores ranging from 7.99 to 8.63, which were equivalent to “good” to “moderately good” on the sensory scale. Mix 2 with 1.86% fat yielded slightly lower scores ranging from 7.52 to 8.25 which also were equivalent to “good” to “moderately good”. Mix 1 with 0.5% fat received the lowest scores ranging from 6.87 to 6.36, which were equivalent to “marginally good” to “good”. In addition, mixes 2 and 3 showed no significant changes in quality (P > 0.05) during storage, whereas mix 1 showed a slight but significant decrease in quality after 4 weeks.

MANUFACTURE OF FROZEN YOGURT 173

TABLE 5. MEAN SENSORY SCORES OF OVERALL QUALITY FOR THREE MIXES OF

FROZEN YOGURT DURING 6 WEEKS OF STORAGE AT -29C

Storage time (wk)

Mix2 1 2 3 4 5 6

1 (0.5% fat) 6.87*’.2 1.04’’ 6.93” 6.50’2.’ 6.21” 6.36“

2 (1.8% fat) 7.6gb’ 7.90b’.’ 8.25b’ 7.7Ib’ 7.52b‘ 7.56b’

3 (4.0% fat) 8.32‘ ’.’ 8.13b’ 8.63b2 8.26“.’ 8.07‘‘ 7.99“

‘Averages of three trials. Means in column with different superscript letters significantly different (PC0.05). Means in row with different superscript numbers significantly different (P < 0.05). Scales: 1 to 10 with 10 being excellent.

’Fat levels in finished products.

The overall flavor of all three mixes was acceptable (Table 6). However, the flavor of mix 1 was rated “fair” to “good” (6.34-7.11). This low quality can be attributed to the low milk fat content (0.5% fat). O’Donell (1992) reported that when milk fat in an ice cream mix was reduced from 10 to 3%, sensory perception of vanilla flavor decreased significantly. On the other hand, the flavor quality for mixes 2 and 3 was rated “good” to “very good” (6.58 to 8.19 for mix 2 and 8.01 to 8.61 for mix 3). No significant decreases in flavor quality occurred during storage (Table 5) . A mild but consistent criticism received for all three formulations was lack of “acid” flavor. As a fermented product, frozen yogurt should have some tartness.

Overall scores for body and texture suggested that the formulations with higher amounts of fat were better in textural quality (Table 7). Overall scores for mixes 2 and 3 ranged from 7.47 to 8.30 and 8.03 to 8.94, respectively, which were equivalent to “good” to “very good” on the sensory scale. Mix 1 received the lowest scores ranging from 6.29 to 7.33 (“marginally good“ to “good”). No serious body defects were detected by the sensory panel. Among body and texture defects, “crumbly body” was the only criticism received with any frequency. Scores for “crumbly body” were generally not different (P>O.O5) between mixes 2 and 3.

174 G.A. ORDONEZ, I.J. JEON and H.A. ROBERTS

TABLE 6. MEANSENSORYSCORESOFOVERALLFLAVORFORTHREEMIXESOFFROZEN

YOGURT DURING 6 WEEKS OF STORAGE AT -29C ~~ ~

Storage time (weeks) Mix2

1 2 3 4 5 6

I (0.5% fat) 7.11'' 6.87" 7.08" 6.47" 6.34" 6.51"'

2 (1.8% fat) 7.71b' 7.60" 8.19" 7.8Ib' 7.73" 7.58''

3 (4.0% fat) 8.01"' 8.19" 8.61'' 8.22'' 8.0Sb' 8.11bl

'Averages of three trials. Means in column with different superscript letters significantly different (P <0.05). Means in row with different superscript numhers significantly different (P < 0.05). Scales: 1 to 10 with 10 being excellent.

2Fat levels in finished products.

TABLE 7. MEAN SENSORY SCORES OF OVERALL BODY AND TEXTURE FOR THREE MIXES O F

FROZEN YOGURT DURING 6 WEEKS OF STORAGE AT -29C ~~~ _ _ _ _ _ _ _ _ ~

Storage time (weeks)

1 2 3 4 5 6 Mixes2

1 (0.5% fat) 7.18.' 7.33" 7.21'' 6.96.' 6.85" 6.2Y2

2 (1.8% fat) 7.87b 8.10b' 8.30b' 7.86b'.2 7.47b2 7 S b 2

3 (4.0% fat) 8.32' 8.2Ib 8.94c2 8.52".' 8.22".' 8.03"

'Averages of three trials. Means in column with different superscript letters significantly different (P< 0.05). Means in row with different superscript nurnhers significantly different (P< 0.05). Scales: 1 to 10 with 10 being excellent.

2Fat levels in finished products.

These sensory results indicate that the mixes with higher amounts of fat were consistently better in all categories of quality attributes including flavor and texture. This is well documented in the literature. Arbuckle (1986) stated that milk fat in ice cream contributes to a subtle flavor quality, is a good carrier and a synergist for added flavor compounds, and promotes desirable textural qualities.

MANUFACTURE OF FROZEN YOGURT 175

CONCLUSIONS

The probiotic organisms B. bifidum and L. acidophilus in addition to the standard yogurt cultures of S. thermophilus and L. delbrueckii ssp. bulgancus grew well in the UF-based mixes. The culture organisms were stable to the freezing process, and populations remained high during 8 weeks of storage at -29C. The process requires a rigid, control of acidity development in mixes in order to avoid a high viscosity problem. The fermentation should be stopped at 0.30% TA, and the mixes should be cooled rapidly to 1OC or lower. The final mix TA should not exceed 0.35%. The finished products contained high levels of protein and calcium and had good flavor and body characteristics. This process can be adapted to larger production scales.

REFERENCES

AOAC. 1990. Ofjicial Methods of Analysis, 15th Ed. Association of Official Analytical Chemists, Washington, DC.

ARBUCKLE, W.S. 1986. Ice Cream, 4th Ed. AVI Van Nostrand Reinhold, New York.

BROWN, R.J. 1988. Milk coagulation and protein denaturation. In Fundamen- tals of Dairy Chemistry, 3rd Ed. (N.P. Wong, ed.) pp. 585-586, Van Nostrand Reinhold, New York.

GEILMAN, W.G. and SCHMIDT, D.E. 1992. Physical characteristics of frozen desserts made from ultrafiltered milk and various carbohydrates. J. Dairy Sci. 75, 2670-2675.

GILLILAND, S.E. 1989. Acidophilus milk products: A review of potential benefits to consumers. J. Dairy Sci. 72, 2483-2494.

GILLILAND, S.E. and SPECK, M.L. 1977. Instability of Lactobacillus acidophilus in yogurt. J. Dairy Sci. 60, 1394-1398.

GROW, K.P. 1990. Evaluation of ice milk containing ultrafiltered skim milk retentate. M.S. Thesis, Kansas State University, Manhattan.

HARDIE, J.M. 1986. Genus Streptococcus. Bergey’s Manual of Determinative Bacteriology, 9th Ed. William and Wilkins, Baltimore.

HEKMAT, S. and MCMAHON, D.J. 1992. Survival of Lactobacillus acidophilus and Bifidobacterium bifldum in ice cream for use as a probiotic food. I. Dairy Sci. 75, 1415-1421.

HOLCOMB, J.E., FRANK, J.F. and MCGREGOR, J.U. 1991. Viability of Lactobacillus acidophilus and Bifidobacterium bifidum in so ft-serve frozen yogurt. Cult. Dairy Prod. J. 26(3), 4-5.

HOOVER, D.G. 1993. Bifidobacteria: activity and potential benefits. Food Technol. 47(6), 120-124.

176 G.A. ORDONEZ, I.J. JEON and H.A. ROBERTS

HUGHES, D.B. and HOOVER, D.G. 1991. Bifidobacteria: Their potential use in American dairy products. Food Technol. 45(4), 74-83.

JENSEN, L.A., TONG, P.S. and HARRIS, L. 1989. Characteristics of frozen desserts containing retentate from ultrafiltration of skim milk. I . Mix composition and freezing. J. Dairy Sci. Abs. 72 (Suppl. l), 129.

KANDLER, 0. and WEISS, N. 1986. Regular, nonsporing gram-positive rods. Bergey's Manual of Determinative Bacteriology, 9th Ed., Williams and Wilkins, Baltimore, MD.

KWAK, H.S. and JEON, I.J. 1986. Effect of various conditions on the formation of oligosaccharides in milk treated with P-galactosidase. J. Dairy Sci. 69, 2785-2790.

LAPIERRE, L., UNDELAND, P. and COX, L.J. 1992. Lithium chloride-sodi- um propionate agar for the enumeration of bifidobacteria in fermented dairy products. J. Dairy Sci. 75, 1193-1197.

LAROIA, S. and MARTIN, J.H. 1991. Extended survival of bifidobacteria in a frozen fermented dairy product. J. Dairy Sci. Abs. 72 (Suppl. l) , 116.

MASHAYEKH, M. and BROWN, R.J. 1992. Stability of Lactobacillus delbrueckii ssp. bulgaricus, Streptococcus salivarius ssp. thermophilus and 6-galactosidase activity in frozen cultured ice cream. Cult. Dairy Prod. J.

MITAL, B.J. and GARG, S.K. 1992. Acidophilus milk products: Manufacture

O'DONELL, C. 1993. Benefiting from a healthy image. Dairy Foods 94(4),

ORDONEZ, G.O. and JEON, I.J. 1995. Evaluation of P-gaclactosidase activities associated with probiotic lactic acid bacteria by high performance liquid chromatography. Cultured Dairy Prod. J. 30(4), 29-32.

SCARDOVI, V. 1986. Bifidobacterium. Bergey 's Manual of Determinative Bacteriology, gth Ed., Williams and Wilkins, Baltimore.

SOMMER, H.H. 1951. Theory and Practice of Ice Cream Making, 6'h Ed., Published by the Author, Madison, WI.

SULLIVAN, J.S. 1992. The Development of Lowfat Ice Milk Utilizing Ultrafiltered Skim Milk Retentate. M.S. Thesis, Kansas State University, Manhattan.

TIESZEN, K.M. and BAER, R.J. 1989. Composition and microbiological quality of frozen yogurts. Cultured Dairy Prod. J. 24(4), 11, 13- 14.

USDA. 1976. Composition of Foods: Dairy and Egg Products. United States Dept. of Agriculture Handbook No. 8-1.

USDA. 1997. Dairy Products: 1996 Summary. United States Dept. of Agricul- ture NASS Da 2-1(97)a.

VENTLING, V.L. and MISTRY, V.V. 1991. Growth and activity of bifidobacteria in ultrafiltered milk. J. Dairy Sci. Abs. 72 (Supp. l ) , 82.

27(1), 4-8.

and therapeutics. Food Rev. Intl. 8, 347-389.

93-100.