Physical and Sensory Properties of Yogurt Stabilized with Milk Proteins1,2

Physical and Sensory Properties of Yogurt Stabilized with Milk Proteins 1,2 H. W. MODLER, 3 M. E. LARMOND, 4 C. S. LIN, s D. FROEHLICH, 3 and D. B. EMMONS s Agriculture Canada, Research Branch Ottawa, Ontario K1A 0C6

ABSTRACT

INTRODUCTION

Eighteen skim milk yogurts, prepared from all combinations of six protein types (three casein- and three whey-based products) and three protein concentrations (.05, 1.0, and 1.5% added protein), were compared by four physical and four sensory characteristics. Addition of increasing amounts of protein increased gel firmness and decreased syneresis. The gelatin control (.5%) had the least syneresis, followed by sodium caseinate (1.5%). Generally, the casein-based yogurts were firmer with less syneresis than yogurts based on whey protein. The casein-based proteins, particularly sodium caseinate, produced yogurts that were generally inferior to gelatin for smoothness and appearance. Whey protein concentrates, at 1.0 and 1.5% of protein addition, produced yogurts generally superior to casein-based products for both appearance and smoothness. Treatment effects were significant for all variables except pH. The interaction between protein type and protein percent was significant for measured gel firmness, titritable acidity, smoothness and appearance but was not significant for syneresis, pH, sensory firmness, and sensory acidity. The correlation between sensory firmness and syneresis was - . 8 2 for the 18 experimental treatments, and the correlation between measured gel firmness and syneresis was - . 6 6 .

Preparing yogurt and other cultured dairy products that are free from defects in body, consistency, and syneresis continues to be a problem in the dairy industry. The appearance of whey detracts from consumer appeal and is usually indicative of improper formulation, culturing, or processing (5). Many yogurt formulations incorporate plant or animal hydrocolloids to impart desired thickening, stabilizing, or gelling effect. Inclusion of these compounds can lead to flavor problems even at .3 to .5% (5). This, combined with a trend to produce "all natural " yogurt, suggests replacement of hydrocolloids with milk based proteins and milk solids. In a recent review by Tamine and Deeth (6) a variety of ingredients are listed as stabilizing and fortification agents. These include 3 to 4% skim milk powder (SMP), whey powder (1 to 2%), sodium caseinate, and milk concentrated by ultrafiltration (UF) or reverse osmosis (RO) (18 to 20% total solids). Use of these ingredients can present problems, however. Whey powder imparts undesirable flavor, and fortification with SMP or whole milk powder can cause excessive acid production and taste deviations (6). According to Abrahamsen and Holmen (1), good quality yogurt can be produced from RO and UF concentrates of skim milk. Objectives of our investigation were to determine the influence of various milk-based protein additives on physical-chemical and sensory properties of yogurt. MATER IALS AND METHODS Preparation of Yogurt

Received April 5, 1982. a Contribution No. 4 7 7 from the Food Research Institute. 2Contribution No. 1-427 from the Engineering and Statistical Research Institute. 3 Food Research Institute. 4 Institutes and Programs Coordination Directorate. SEngineering and Statistical Research Institute. 1983 J Dairy Sci. 66:422--429

Fresh milk was obtained from the show herd on the Experimental Farm of Agriculture Canada and was separated at 40°C by a Westfalia separator (Model LWA-205) to yield skim milk containing .05 to .07% milk fat (F). The skim milk was pasteurized at 88°C for 25 min in a Groen processing kettle (Model TDC/TA-20SP)

422

MILK PROTEIN STABILIZER equipped with a scraped-surface agitator. The pasteurized product was cooled to 45 to 46°C and inoculated with high concentrations of starter to increase the rate of acid production and accentuate syneresis (2% Streptococcus thermophilus and 2% Lactobacillus bulgaricus). Following inoculation, the product was stirred for 5 min to ensure uniform distribution of the cultures. A "Brewer" automatic pipet was used to portion 120-ml aliquots of the product into 150-mt plastic tubs (4.2 cm high x 6.7 cm diameter). The tubs were sealed and incubated at 43°C until the pH decreased to 4.80 to 4.85. This normally required 2.5 to 2.75 h. Following incubation the yogurt was placed in a forced-air storage at 3°C to cool the product and terminate acid development. Sodium easeinate, milk protein concentrate (MPC), SMP, and three whey protein concentrates (WPC) described in Table 1 were added to cold skim milk in sufficient quantities to increase protein in the finished yogurt by .5, 1:0, and 1.5% (wt/wt). The control yogurt consisted of fluid skim milk stabilized with .5% gelatin (225 Bloom strength). Yogurts manufactured with unstabilized fluid skim milk and with added whey powder were judged unacceptable by the sensory evaluation panel in preliminary testing and were eliminated. Skim milk yogurt suffered from extreme syneresis, and whey powder produced a highly objectional off-flavor.

423

The control treatment and 2 or 3 of the 18 product treatments were processed in 7-kg batch sizes 1 day a week for 15 wk. The number of replications was 15 for control and 2 for product treatments. All product treatments were prepared once before the second replication commenced. Analytical Procedures

Nitrogen, solids, and titritable acidity were measured by AOAC procedures (2). Fat was measured by the Babcock method, and pH was measured by a Model 26 Radiometer, pH meter equipped with a combination Jena Thalamid ® electrode (Sargent Welch 5 30073-15). Gel firmness was measured in triplicate by a curd firmness meter with a Cherry-Burrell knife (7). The resistance to passage of the knife through the gel w~s recorded in grams. After this measurement, the same tubs were inverted on a 120-mesh stainless steel screen placed on top of a long-stemmed funnel. The bottom of the container then was punctured with a hot electric soldering iron to allow air entry and facilitate whey drainage. After 2 h of draining at 3°C the quantity of whey collected was read directly from a 100-ml graduated cylinder. This reading was an index of syneresis. All analyses were on yogurt that was 7 to 10 days old.

TABLE 1. Ingredients for stabilization of yogurt. Methods of preparation

Product name

Code

Supplier

Nutricase

Caseinate MPC

Casein converted to sodium caseinate Ultrafiltration

91.5

Milk protein concentrate (experimental sample)

Montreal Casein Ltd., Montreal, Quebec Sodispro Technology Ltd., St. Hyacinthe, Quebec

Skim milk powder (low heat)

SMP

Ault Foods Ltd., Winchester, Ont.

Spray dried

36.3

Enrpro 50

UF-WPC

Ultrafiltration

53.2

(Experimental sample)

IE-WPC

Stauffer Chemical Co., Westport, CT Dean Foods Ltd., Rockford, IL

Ion exchange

33.4

Foretein 35

ED-WPC

Electrodialysis/ lactose crystallization

34.4

Foremost Foods Co., San Francisco, CA

Protein (%)

40.5

Journal of Dairy Science Vol. 66, No. 3, 1983

424

MODLER ET AL. 50-

Sensory Evaluation

Eleven judges with sensory experience evaluated the yogurts for appearance, gel firmness, smoothness, and acidity by descriptive analysis with the scaling method (4). The judge recorded the perceived intensity of each attribute on an unstructured 15-cm line with anchor points 1.5 cm from either end. The descriptive terms assigned to the anchor points from left to right were: gel firmness (weak-firm), smoothness (smooth-coarse), acidity (weakstrong), and appearance (smooth-coarse). Because of the number of characteristics evaluated, only two or three experimental samples and the control were evaluated each day. Statistical Analysis

Because the 19 treatments were nonorthogonal for days and number of replicates, data were analyzed by the method of least squares using the fitting constants. The model for the measured physical characteristics (gel firmness, syneresis, pH, and acidity) was:

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Yik = u + d i + t k + elk where Yik is the mean of the kth treatment on the ith day and u, d, t, and e represent the overall mean, day, treatment, and residual effects. The four sensory characteristics (smoothness, firmness, acidity, and appearance) were based on the following model: Yijk = u + d i + Jj + t k + eij k where Yijk is the score given by the jth judge for the kth treatment in the ith day, and J is the judge effect. For both models, the 18 experimental treatments were investigated further for factorial effects of type and percent of protein. The treatment means in the tables are estimates of # + tk. Because the experiment was not orthogonal, the standard error (SE) of treatment means and that of treatment differences were not equal for each treatment and each paired comparison. However, for convenience of presentation, only the largest SE is given for treatments in the tables. Whenever treatment effects are significant by F test, results of pair-wise t tests are also in the tables. Journal of Dairy Science Vol. 66, No. 3, 1983

RESULTS A N D DISCUSSION

Throughout the 15 wk of experimentation the protein content of the skim milk was 3.5 + .05%. This meant the final yogurt preparations contained 4.0, 4.5, and 5.0% total protein + .05% for fortification of .5, 1.0, and 1.5%• During initial sensory evaluation, yogurt preparation was changed to yield a less acidic product by culturing to a final pH of 4.14 to 4.41, which is about .2 units higher than most commercial yogurts (3). To achieve this pH, incubation was terminated at pH 4.80 to 4.85. A typical temperature profile for yogurt production appears in Figure 1. Approximately 50 to 60 rain elapsed before the product was cooled to 27°C, which is considered critical for acid production (3). Analysis of Variance

Analysis of variance for physical characteristics and sensory characteristics are in Tables 2 and 3, respectively- Differences among

MILK PROTEIN STABILIZER

425

TABLE 2. Hierarchical analyses of variance for physical characteristics. Mean squares pH

Acidity

109.62"*

.0288**

217.87"*

521.26"*

123.63"*

.0057

99.19"*

975.70** 1190.14"* 2365.48** 14.81 160.27"* 1186.31"* 32.58

39.47* 802.73** 1605.46"* .00 29.90 9124.82"* 12.89

.0060 .0066 .0121 .0012 .0053 .0003 .0054

120.46"* 328.76** 647.80** 9.72 42.68* 1973.30"* 16.34

Source

df

Gel firmness

Days Treatments Among experimental treatments Protein type (T) Protein percent (L) Linear Quadratic T× L Control vs. others Residual

14 18

423.37** 17 5 2 10 1

18

Syneresis

*P<.05. **P<.Ol.

19 t r e a t m e n t effects were highly significant ( P < . 0 1 ) for all c h a r a c t e r i s t i c s in Tables 2 a n d 3 w i t h t h e e x c e p t i o n of pH. F o r t h e 18 e x p e r i m e n t a l t r e a t m e n t s intera c t i o n b e t w e e n p r o t e i n t y p e a n d p e r c e n t (T × L) was h i g h l y significant ( P < . 0 1 ) for m e a s u r e d

gel firmness, acidity (Table 2), a n d for s m o o t h ness a n d a p p e a r a n c e ( T a b l e 3). F o r syneresis, s e n s o r y firmness, a n d s e n s o r y acidity, intera c t i o n was n o t significant, b u t b o t h m a i n effects were significant. T h e r e s p o n s e t o p r o t e i n p e r c e n t was linear.

TABLE 3. Hierarchical analyses of variance for sensory characteristics. Mean squares Source Days Judges Treatments Among experimental treatments Protein type (T) Protein percent (L) Linear Quadratic TXL Control vs. others Residual1

df 13 10 18 17

10 1 458

Smoothness

Firmness

12.77"* 101.05"*

21.72"* 109.80"*

16.52"* 62.66**

19.32"* 63.15"*

53.23**

39.28**

13.23"*

64.62**

149.16"* 3.06 6.02 .11 15.30"* 721.95"* 4.08

33.28** 228.95**

21.57"* 23.76* 34.13" 13.39 6.94 209.87** 5.16

182.58"* 30:56** 55.93** 5.18 12.45"* 393.33** 4.74

457.47**

.43 3.89 4.37 4.68

Acidity

Appearance

I Residual df is 453 for smoothness. *P<.05. **P<.01. Journal of Dairy Science Vol. 66, No. 3, 1983

426

MODLER ET AL.

TABLE 4. Adjusted means of measured gel firmness and syneresis in yogurt prepared with six types of dairy proteins at three concentrations of addition and with gelatin. Gel firmness Type of protein 1

.5%

1.0%

Caseinate MPC SMP UF-WPC IE-WPC ED-WPC Gelatin SE Experimental treatments 2 Gelatin

56.7 de 52.7efg 54.1efg 56.0 e 45.7efg 40.5g 52.4efg

89.6 b 70.2 cd 72.2 c 55.7 e 42.5fg 45.7efg

Syneresis (ml) 1.5%

.5%

1.0%

117.9 a 77.1 c 79.7 bc 78.9 bc 54.5 ef 49.5efg

45.4 ab 47.2 a 44.4 abc 41.1 abcd 38.1 abcde 43.1 ahc 3.7J

33.1 def 35.0 def 30.9efg 34 0 def 3714 cdef 37.6 bcde

(g)

1.5%

(ml)

3.36 1.07

11.9 i 23.0 h 20.7 h 23.1g h 28.8fgh 33.7 def

5.34 1.70

a'b'c'd'e'f'g'h'iMeansnot followed by same letter are different (t test, P<.05). l Caseinate, nutricase; MPC, milk protein concentrate; SMP, skim milk powder; WPC, whey protein concentrate; UF, uhrafihration; IC, ion exchange; ED, electrodialysis. 2Only highest SE reported.

Physical Characteristics Yogurt p r e p a r e d with increasing a m o u n t s o f p r o t e i n w e r e generally f i r m e r for each o f t h e six p r o d u c t s t e s t e d (Table 4). Casein-based p r o d u c t s (caseinate, SMP, and MPC) t e n d e d t o p r o d u c e

f i r m e r gels t h a n WPC f o r t i f i e d y o g u r t with n o t a b l e e x c e p t i o n o f UF-WPC. Y o g u r t containing 1.5% caseinate h a d a m e a n gel s t r e n g t h o f 117.9 g, which is a p p r o x i m a t e l y t w o times t h a t o f t h e gelatin c o n t r o l . T h e MPC and SMP

TABLE 5, Adjusted means of pH and titratable acidity of yogurt prepared with six types of dairy proteins at three concentrations of addition and with gelatin. pHI

Titratable acidity 2

Type of protein 3

.5%

1.0%

1.5%

.5%

1.0%

1.5%

Caseinate

4.29

4.14

4.31

MPC SMP UF-WPC IE-WPC ED-WPC Gelatin SE Product treatments4 Gelatin

4.32 4.23 4.30 4.24 4.25 4.29

4.29 4.22 4.31 4.41 4.34

4.30 4.39 4.34 4.29 4.32

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1.03 def 1.19 ab 1.08 cde

.04 .01

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MILK PROTEIN STABILIZER at 1.0 and 1.5%, caseinate at 1.0% and 1.5%, and UF-WPC at 1.5% had measured gel firmness significantly higher than the relatin control. Gel firmness of the remaining treatments did not differ significantly (P>.05) from control. Generally, syneresis decreased with increasing protein concentration (Table 4). All experimental treatments had more syneresis (P<.05) than the gelatin control. Caseinate at 1.5% protein addition had syneresis that was significantly less than the remaining 17 treatments. Gelatin-stabilized yogurt was significantly better (P<.05) than any of the experimental treatments with a mean syneresis of 3.7 ml. During processing, the pH of the various preparations of yogurt was controlled such that there was no significant difference of type or of percent protein. There were significant differences among the titratable acidities of the various treatments in Table 5. Yogurt stabilized with 1.5% MPC, IE-WPC, and SMP had the highest acidity, and gelatin, caseinate (.5%), and ED-WPC (.5 and 1.0%) had the lowest titratable acidities. When individual treatments are compared in Table 5, 15 of 18 products had acidities significantly higher than gelatin (P<.05) with MPC at 1.5% being the highest. The titratable acidity of the three remaining treatments did not differ from gelatin (P<.05). The higher acidities with the higher percents of added protein, particularly with SMP, would be expected because of the higher buffer capacity. Sensory Characteristics

Sensory evaluation data for smoothness, gel firmness, acidity, and appearance of the 19 treatments are in Table 6. Generally, the casein-based products yielded a coarser yogurt than WPC (Table 6). The smoothness of IE-WPC (1%) and ED-WPC (1 and 1.5%) did not differ significantly from the gelatin reference sample (P>.05), and the remaining 15 samples were significantly coarser than gelatin. Sensory firmness increased with increasing protein (Table 6). Ten of the 18 product treatments were as firm as the gelation control, three were firmer, and five were softer. Data related to sensory evaluation of yogurt for acidity also are summarized in Table 6. Two-thirds of the product treatments were

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Journal of Dairy Science Vol. 66, No. 3, 1983

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Figure 2. Relationship between syneresis (ml) and measured gel firmness (g) • r = - . 6 6 for experimental treatments (n = 36) and - . 0 9 when gelatin control is included (n = 50). MPC, milk protein concentrate; SMP, skim milk powder; WPC, whey protein concentrate, UF, ultrafiltration; IE, ion exchange; ED, electrodialysis.

significantly more acidic than gelatin whereas the remaining one-third had acidities similar to gelatin. All casein-based products had a coarser appearance (higher score) than the gelatin control (Table 6). In addition, all of the WPC's at the .5% fortification were also coarser. Of the six remaining experimental WPC treatments (1.0 and 1.5%), five had appearance scores similar to gelatin, but only IE (1.0%) was significantly ( P < . 0 5 ) smoother. Correlations

There was no significant correlation b e t w e e n measured gel firmness and syneresis for the 19 treatments in Table 4. When the control product was excluded, correlation was - . 6 6 (N = 36) for the remaining 18 experimental treatments. A plot of means indicates that gelatin behaves in a manner different from m i l k proteins as a stabilizer (Figure 2). A correlation --.82 (n = 36) was established b e t w e e n sensory firmness and syneresis (Figure 3). This correlation drops to - . 4 6 (n = 50) w h e n gelatin is included. Again, a plot o f the Journal of Dairy Science Vol. 66, No. 3, 1983

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FIRMNESS- SENSORY Figure 3. Relationship between syneresis (ml) and sensory firmness • r = --.82 for experimental treatments (n = 36) and - . 4 6 with gelatin control (n = 50). MPC, milk protein concentrate; SMP, skim milk powder; WPC, whey protein concentrate, UF, ultrafiltration; [E, ion exchange; ED, electrodialysis.

means indicated that gelatin has the ability to bind large amounts of water w i t h o u t increasing firmness as m u c h as protein stabilizers (Figure 3). No relationships were established b e t w e e n appearance and syneresis or appearance and firmness. CONCLUSIONS

Skim milk yogurt fortified with dairy based proteins suffered from one o f more defects not observed w h e n gelatin was used as stabilizer. S o d i u m caseinate (1.5%) was m o s t effective in increasing gel strength and reducing syneresis, but the products were not as s m o o t h in texture as gelatin stabilized yogurt. Also, the appearance was coarser than gelatin stabilized yogurt. The MPC and SMP also contributed to an undesirable texture and appearance at higher fortification. In addition, these casein-based proteins were not able to reduce syneresis to the same degree as sodium caseinate or gelatin. The three WPC's produced yogurt that was generally acceptable for texture and appearance

MILK PROTEIN STABILIZER at higher fortification but suffered f r o m excessive syneresis. These proteins should n o t be ruled o u t as stabilizers for y o g u r t as partial r e p l a c e m e n t for h y d r o c o l l o i d s and starch. The syneresis m a y n o t be as serious in y o g u r t containing higher solids and added milkfat. In addition, these e x p e r i m e n t s were designed to accentuate syneresis, and this defect m a y n o t be as serious when y o g u r t is processed under m o r e ideal conditions.

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ACKNOWLEDGMENTS

The authors are grateful to Alan Payne, G. Larocque, and E. O'Brien for their skillful technical assistance in this project. We also wish to thank Doug Bullock (University of Guelph) and G. A s h t o n for their efforts in reviewing this manuscript. REFERENCES

1 Abrahamsen, R. K., and T. B. Holmen. 1980.

5 6 7

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Yogurt from hyperfiltrated and ultrafiltrated and evaporated milk and from milk with added milk powder. Milchwissenschaft 35 : 399. Association of Official Analytical Chemists. 1975. Official methods of analysis. 12th ed., Washington, DC. Chambers, J. V. 1977. Culture and processing techniques important to the manufacture of good quality yogurt. Cult. Dairy Prod. J. 14: 28. Larmond, E. 1977. Laboratory methods for sensory evaluation of food. Pub., 1637. Canada Dep. Agric., Research Branch, Ottawa, Ont. Rasie, J. L., and J. A. Kurmann. 1978. Yoghurt: Part V, Ch. 19. Published by authors. Distr. Tech. Dairy Publ. House, Copenhagen, Denmark. Tamime, A. Y., and H. C. Deeth. 1980. Yogurt: Technology and biochemistry. J. Food Prot. 43:939. Emmons, D. B., D. C. Beckett, and E. Larmond. 1972. Physical properties and storage stability of milk-based puddings made with various starches and stabilizers. Can. Inst. Food Sci. Technol. J. 5:72.

Journal of Dairy Science Vol. 66, No. 3, 1983