Physiochemical Properties, Microstructure, and

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Keywords: goats' milk, nonfat yogurt, probiotic, whey protein concentrate. Practical .... concentrate (WPC 80% protein) was acquired from Davisco. Foods Int. Inc.
Physiochemical Properties, Microstructure, and Probiotic Survivability of Nonfat Goats’ Milk Yogurt Using Heat-Treated Whey Protein Concentrate as Fat Replacer Tiehua Zhang, James McCarthy, Guorong Wang, Yanyan Liu, and Mingruo Guo∗

There is a market demand for nonfat fermented goats’ milk products. A nonfat goats’ milk yogurt containing probiotics (Lactobacillus acidophilus, and Bifidobacterium spp.) was developed using heat-treated whey protein concentrate (HWPC) as a fat replacer and pectin as a thickening agent. Yogurts containing untreated whey protein concentrate (WPC) and pectin, and the one with only pectin were also prepared. Skim cows’ milk yogurt with pectin was also made as a control. The yogurts were analyzed for chemical composition, water holding capacity (syneresis), microstructure, changes in pH and viscosity, mold, yeast and coliform counts, and probiotic survivability during storage at 4 °C for 10 wk. The results showed that the nonfat goats’ milk yogurt made with 1.2% HWPC (WPC solution heated at 85 °C for 30 min at pH 8.5) and 0.35% pectin had significantly higher viscosity (P < 0.01) than any of the other yogurts and lower syneresis than the goats’ yogurt with only pectin (P < 0.01). Viscosity and pH of all the yogurt samples did not change much throughout storage. Bifidobacterium spp. remained stable and was above 106 CFU g-1 during the 10-wk storage. However, the population of Lactobacillus acidophilus dropped to below 106 CFU g-1 after 2 wk of storage. Microstructure analysis of the nonfat goats’ milk yogurt by scanning electron microscopy revealed that HWPC interacted with casein micelles to form a relatively compact network in the yogurt gel. The results indicated that HWPC could be used as a fat replacer for improving the consistency of nonfat goats’ milk yogurt and other similar products. Abstract:

M: Food Microbiology & Safety

Keywords: goats’ milk, nonfat yogurt, probiotic, whey protein concentrate

HWPC could be used as a fat replacer for improving the consistency of nonfat goats’ milk yogurt and other similar products. Whey proteins are a byproduct from cheese making. Whey proteins have higher nutrition value compared to many other food proteins. They are more compatible to milk than any other nonmilk based fat replacers.

Practical Application:

Introduction Goat milk is an important source of nutrition for people across the globe, nourishing more people in underdeveloped countries than bovine milk (Haenlein 2004). Goat milk products are gaining in popularity in developed countries in part due to people becoming increasingly sensitive to cows’ milk (Park 1994a), it is higher in many minerals as well as having a greater abundance of certain vitamins (Park 1994b; Ljutovac and others 2008), and its rising appeal to certain segments of the population (Haenlein 1996). However, goats’ milk often forms a weak curd in yogurt due to its lack of α s-1 casein (Guo 2003). Fat, especially saturated fat, has been increasingly criticized for its contributions to certain health issues in developed countries, and the demand for MS 20141583 Submitted 9/22/2014, Accepted 12/21/2014. Authors Zhang and Guo are with Dept. of Food Sciences and Engineering, Jilin Univ., Changchun, Jilin 130062, China. Authors McCarthy and Wang are with Dept. of Nutrition and Food Sciences, The Univ. of Vermont, Burlington, VT 05405, U.S.A. Author Liu is with Food College, Heilongjiang Bayi Agricultural Univ, High-tech Industrial Development Zone, Daqing, 163319, China. Author Guo is also with Dept. of Nutrition and Food Sciences, The Univ. of Vermont, Burlington, VT 05405, U.S.A. Direct inquiries to author Guo (E-mail: [email protected]). ∗ Corresponding



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low fat and nonfat dairy products is on the rise (Sandoval-Castilla and others 2004). The most common methods for improving the weak texture of goats’ milk yogurt include increasing the total solids (TS) content of the milk. This can be achieved by evaporation, the addition of skim milk powder, milk proteins, or thickening agents (Li and Guo 2006; Park and Haenlein 2006), and using microbial transglutaminase to produce an acceptable yogurt (Farnsworth and others 2006). Whey proteins have many functional properties in food systems including gelation, thickening, and water-holding capacity (Bryant and McClements 1998), and heat treatment may affect these properties. Wang and others (2012) investigated using whey protein isolate (WPI) to produce a full-fat goats’ milk yogurt of acceptable quality. WPI is a whey protein of a higher purity than whey protein concentrate (WPC); however, the main constituent, β-lactoglobulin, is the same. When heated above its isoelectric point of 5.2 and held for a period of time the protein molecules unfold and become susceptible to increased electrostatic interactions. In addition, when β-lactoglobulin unfolds more thiol containing amino acids are exposed leading to increased reaction among the cysteine residues to cross-linkages and increasing the aggregate size which further increase the gel strength R  C 2015 Institute of Food Technologists

doi: 10.1111/1750-3841.12834 Further reproduction without permission is prohibited

Properties of nonfat goats’ milk yogurt . . .

Materials and Methods

Table 1–Effects of addition of HWPC, pectin, or both on viscosity and pH of nonfat goats’ milk yogurt. Formulation 1.4%HWPC 1.2%HWPC 1.0%HWPC 0.8%HWPC 0.45%pectin 0.35% pectin 0.25% pectin 0.15% pectin Nonfat goat’s milk yogurt 1.4%HWPC+0.15%pectin 1.4%HWPC+0.25%pectin 1.2%HWPC+0.25%pectin 1.2%HWPC+0.35%pectin 1.0%HWPC+0.35%pectin 1.0%HWPC+0.45%pectin 0.8%HWPC+0.45%pectin

Viscosity (mPa.s)


831.46 ± 35.42d

4.21 ± 0.01i 4.21 ± 0.02i 4.22 ± 0.01i 4.21 ± 0.02i 4.33 ± 0.00k 4.29 ± 0.01j 4.39 ± 0.02k 4.34 ± 0.02k 4.47 ± 0.01h 4.27 ± 0.01j 4.28 ± 0.01j 4.25 ± 0.01i 4.27 ± 0.02j 4.25 ± 0.02i 4.30 ± 0.02j 4.29 ± 0.01j

809.26 ± 12.72d 695.46 ± 22.45c 575.36 ± 10.78b 717.96 ± 24.46c 731.32 ± 13.39c 731.84 ± 23.13c 672.82 ± 32.70c 234.56 ± 28.23a 1434.32 ± 22.30f 1648.65 ± 12.40g 1432.87 ± 18.70f 1650.60 ± 40.20g 1385.23 ± 32.60f 1305.00 ± 34.40e 1240.50 ± 16.90e

Values with different superscript letters indicate significant difference P < 0.05 relative to nonfat goats’ milk yogurt.

produced a gel that did not fully set at room temperature. The WPC solution (12.5%) as a control was also made by blending and refrigerating the WPC powder and made up to 1000 mL without adjusting pH and heating.

Preparation of yogurt R GENU pectin was added to the goat’s milk separately at a 0.15%, 0.25%, 0.35%, and 0.45% (w/v) and blended for 1 min by a hand hold blender. It was then heated to 82 °C to dissolve the pectin and to pasteurize the mix. The prepared HWPC was added to the pasteurized goat’s milk separately at 8%, 10%, 12%, and 14% (w/v) and blended for 1 min. The mixed blends were then cooled in a water bath to 40 °C. Then starter culture slurry was made according to directions provided by Chr. Hansen and added at a rate of 0.04% (v/v). Then the mixed blend was transferred to yogurt cups. The mixture was then incubated at 43 °C for 4.5 h, after which samples were cooled in a refrigerator. Using the same method, the prepared HWPC was mixed to the pasteurized goats’ milk containing pectin. Another yogurt was produced using the above technique only using the WPC solution as a substitute for the HWPC. A 3rd yogurt was made with pectin (0.35%) and starter but without the addition of either HWPC or WPC. A cows’ milk yogurt was also produced using only pectin (0.35%)and starter as a control. A total of 3 trials were carried out for each of the yogurts on separate days and held at 4 °C for 10 wk to analyze chemical composition, probiotic survivability, pH, viscosity, and syneresis of each of the samples. The experimental design was shown in Table 1.

Materials Nonfat goats’ milk was obtained from Oak Knoll Dairy in Winsor, VT. Skim cows’ milk was purchased at a local grocery store. The yogurt starter culture, Yo-Fast 100, was purchased from Chr. Hansen (Milwaukee, Wis., U.S.A.) which contained a combination of the microorganisms Streptococcus thermophilus, Lactobacillus delbrueckiissp, Bulgaricus, L. acidophilus, and Bifidobacterium R spp. The low-methoxy pectin, GENU texturizer type YA-100, was received from CP Kelco (Atlanta, Ga., U.S.A.). Whey protein concentrate (WPC 80% protein) was acquired from Davisco Chemical composition analysis Foods Int. Inc. (Le Sueur, Minn., U.S.A.). The yogurt samples were analyzed for TS, ash, protein, fat, and carbohydrate contents as described in the Association of Official Preparation of HWPC Analytical Chemist procedures (AOAC 2002). TS content was WPC powder (125 g) was blended and dissolved in 700 mL determined by air-drying samples in a Forced Air Oven for 24 purified water and held overnight at 4 °C to fully rehydrate the h. Ash content then was determined by taking the dried samples proteins. The solution was then brought to 1000 mL total volume and using a muffle furnace to burn off all the organic matter at to produce a 12.5% (w/v) solution (containing 10% protein). The 550 °C for 8 to 10 h. Protein content was analyzed using the KjelpH of the solution was then adjusted to 8.5 using 3M NaOH. dahl method and a nitrogen conversion factor of 6.38 was used. Fat The WPC solution was then heated to 85 °C and held for 30 min content in the yogurt was measured using the Mojonnier method. in a water bath. It was then cooled rapidly in an ice bath to bring Carbohydrate content was then obtained by determining the difto room temperature. A pH of 8.5 and a heating time of 30 min ference (Carbohydrate content % = TS%-Ash%-Fat%-Protein%). Vol. 80, Nr. 4, 2015 r Journal of Food Science M789

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(Roeffs and de Kruif 1994). When the pH is lowered below 5.0 during fermentation, decreasing the electrostatic repulsions of the polymerized proteins allow a complex network of the whey proteins and casein to form, strengthening the gel upon cooling (Bryant and McClements 1998). Pectin was used as a thickening agent to further increase the overall consistency of the yogurt (Vardhanabhuti and Foegeding 1999), as well as to improve the mouth feel and to prevent whey separation (Lucey and others 1999). Fat plays an important role in the structural integrity and mouth feeling of yogurt, in large part because it interacts with casein micelles (Everett and Rosiland 2005). Removal of the fat can lead to increased syneresis, unfavorable texture, and weak body (Mistry and Hassan 1992). Many different types of fat replacers have been explored in bovine milk yogurts including the addition of inulin (Aryana and others 2007), β-glucan (Sahan and others 2008), gum tragacanth (Aziznia and others 2008), high milk protein powder (Mistry and Hassan 1992), and WPC (Guzman-Gonz´alez and others 1999; Calleros-Lobato 2004; Sandoval-Castilla and others 2004). Gelling properties of WPC and various plant-based hydrocolloids have been investigated extensively (Beaulieu and others 2001; Mishra and others 2001). Use of WPC as a fat replacer in bovine milk yogurts has been explored (Guzman-Gonz´alez and others 1999; Sandoval-Castilla and others 2004; Aziznia and others 2008). The health benefits and safety of probiotics have been well established (Salminen and others 1998; Vasiljevic and Shah 2008; Saad and others 2013), and the use of yogurt as a delivery system for the microorganisms has been widely accepted (Lourens-Hattingh and Vijoen 2001). Lactobacillus acidophilus (L. acidophilus) and Bifidobacterium spp. have been well studied and it has been suggested that a viable cell count at least 10-6 CFU g-1 is needed for beneficial effects (Guo 2007). With the enhancement of people health consciousness, to reduce fat intake and more choose low-fat foods, there is a need to develop low/non fat goats’ milk yogurt. The objectives of this study were to investigate a novel approach to develop a probiotics nonfat goats’ milk yogurt using heat-treated whey protein concentrate (HWPC) as a fat replacer.

Properties of nonfat goats’ milk yogurt . . . Table 2–Chemical composition of the nonfat goats’ milk yogurt and cows’ milk yogurt (mean ± standard deviation, n = 3). % Total solids Protein Carbohydrate Fat Ash




Nonfat cows’ milk plus pectin

8.86 ± 0.59a

8.84 ± 0.22a

8.37 ± 0.45b

8.37 ± 0.32b 2.96 ± 0.15d 4.33 ± 0.45f 0.35 ± 0.07g 0.37 ± 0.01g

3.79 ± 0.71c 3.97 ± 1.10e 0.34 ± 0.05g 0.76 ± 0.05h

3.80 ± 0.47c 3.98 ± 0.49e 0.33 ± 0.04g 0.73 ± 0.04h

2.95 ± 0.08d 4.34 ± 0.35f 0.34 ± 0.09g 0.74 ± 0.04h

Values with different superscript letters indicate significant difference (P < 0.05)relative to nonfat cows’ milk plus pectin.

Syneresis test Water-holding capacity of the yogurts was examined by centrifugation as described by Li and Guo (2006). A portion of 200 g of each of yogurt (Y) was prepared using a centrifuge cup and weighed before incubation. The samples were then centrifuged at 4 °C for 10 min at 2500 rpm (640 × g). The supernatant (S) layer was poured off and weighed. Three trials of each were examined. Syneresis was calculated by using the following equation:

were dehydrated and frozen in liquid nitrogen and fractured. The pieces were then thawed in ethanol (100%) and dried in CO2 . After being mounted on aluminum SEM stubs, the cubes were then sputter coated with 3 nm of Au/Pd and looked at under an FEI Quanta 200F scanning electron microscope being carried out at 5kV. Several micrographs were taken at various magnifications (Walsh and others 2010).

Statistical analysis The syneresis data and chemical composition were analyzed using one-way ANOVA with a Bonferroni adjustment to determine statistical differences among formulations. The data collected over Survivability of probiotics and microbiological analysis Probiotic enumeration was performed on each of the 4 yogurts the 10-wk period were analyzed using a two-way ANOVA with once a week for a 10-wk period according to the procedures a Bonferroni adjustment posttest to compare statistical differences of Chr. Hansen (2005). L. acidophilus was grown on DifcoTM for individual weeks. Lacobacilli MRS Agar and anaerobically incubated at 37 °C for 3 d. Bifidobacterium spp. was selectively cultivated on the same me- Results and Discussion dia with the addition of L-cysteine hydrochloride, lithium chloride, and Dicloxacilin and anaerobically incubated at 37 °C for 3 d. Preliminary results The level of adding HWPC or pectin for the nonfat milk yogurt Mold and yeast counts were determined every 2 wk by using Yeast and Mold Petrifilm (3MTM PetrifilmTM , St Paul, Minn., U.S.A.) was optimized. The results (Table 1) showed that the yogurt with and stored at room temperature (21 ± 2 °C) for 5 d. Coliform 1.4% and 1.2% HWPC were more viscous than others (P < 0.05). were determined every 2 wk by using Coliform Petrifilm (3MTM All yogurts with different amount of HWPC showed similar pH PetrifilmTM ) and stored at room temperature (21 ± 2 °C) for 3 d. values. Yogurts with different levels of pectin had no significant impact on viscosity (P > 0.05), but when the pectin addition was Three trials of each were examined. 0.35%, the yogurt had lower pH (P < 0.05). The level of HWPC that could be used in conjunction with pH measurement pectin was also explored. Table 1 showed that viscosity of appH values of the yogurt samples were measured once a week proximately 1650 mPa.s could be achieved with 2 different comduring storage using a pH meter (IQ Scientific Instruments Inc., binations of HWPC and pectin. A level of 1.2% HWPC and San Diego, Calif., U.S.A.). 0.35% pectin was chosen since yogurt with lower level of whey Syneresis(%) = (S/Y)X100%


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Viscosity analysis Viscosity of each of the formulations of yogurt was determined once a week during storage. Three trials of each were tested. Each of these tests was performed using yogurt that was brought back up to room temperature (21 ± 2 °C) for 2 h before analysis. Viscosity was measured using a Brookfield Viscometer (Brookfield Engineering Laboratories Inc., Middleboro, Mass., U.S.A.) and stated in mPa·s. A reading was taken after a 30-s period at 100 rpm. Microstructure analysis by scanning electron microscopy (SEM) Microstructure of the yogurt samples was examined by using SEM as described by Walsh and others (2010). Samples placed in cubes of agar were set in 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer (pH 7.2) and allowed to set at 4 °C for 12 h. The cubes were then washed in triplicate for 10 min each in the buffer and postfixed in 1.0% osmium tetroxide. This was followed by an additional 3 rinses in a dilute (50 mM) cacodylate buffer Figure 1–Effects of HWPC and pectin on syneresis of goats’ and cows’ milk (pH 7.2). Using a series of ethanol to 100%, the cubes of agar yogurt. M790 Journal of Food Science r Vol. 80, Nr. 4, 2015

Properties of nonfat goats’ milk yogurt . . .

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protein is technically convenient while still producing a product milk yogurt with only pectin added (P < 0.05) and the cows’ with smooth and creamy texture. milk yogurt (P < 0.05). This is due to the addition of the extra whey proteins during manufacturing of the goats’ milk yogurts. Ash content of the goats’ milk yogurt with HWPC was 0.76%, Chemical composition Chemical composition for the 4 formulations of the yogurts and significantly higher than any of the other yogurts (P < 0.05). It was summarized in Table 2. TS of the goats’ milk yogurt with should be noted that while the properties of the cows’ milk yogurt HWPC and WPC were 8.86% and 8.84%, respectively. There are likely to be consistent because of the practice of year round was no difference in TS between these 2 yogurts (P > 0.05). TS production from large numbers of animals, while the production of the goats’ milk yogurt with HWPC and WPC were signifi- of goats’ milk from smaller herds may vary with the seasons and cantly higher than those of the cows’ milk and goats’ milk yogurt breeds of goat (Haenlein 2004). (P < 0.05); however, there was no difference between the yogurt samples with only pectin. Carbohydrate content in the cows’ Syneresis There was no significant difference in syneresis between the milk yogurt was significantly higher than HWPC and WPC goats’ yogurts (P < 0.05), but there was no significant difference between HWPC goats’ milk yogurt and the WPC goats’ milk yogurt (Figthe goats’ milk yogurt with only pectin added (P > 0.05). There ure 1). However, without the addition of the whey protein to the is no difference in the fat content between any of the yogurts as milk before processing the syneresis of the goats’ milk yogurt was they were all made from nonfat milk. The protein contents were 19.37%, and the water holding capacity for the yogurt with either 3.79% and 3.80% in the HWPC and WPC goats’ milk yogurt HWPC or WPC was significantly lower (P < 0.01) than other yosamples. They were significantly higher compared with the goats’ gurts, most likely due to the difference in protein content (Table 2).

Figure 2–Changes in pH of nonfat goats’ milk yogurts and nonfat cows’ milk yogurt during storage at 4 °C.

Figure 3–Changes in viscosity of nonfat goats’ milk yogurts and nonfat cows’ milk yogurt during storage at 4 °C.

Figure 4–Survivability of L. acidophilus during storage at 4 °C.

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Properties of nonfat goats’ milk yogurt . . . Changes in pH during storage There was some decrease in pH, but no significant changes over the 10-wk period for any of the formulations of the yogurts (P > 0.05). The initial pH in the HWPC and WPC goats’ milk yogurt were 4.13 and 4.15, respectively, and slightly lower than that of yogurt containing WPC. There was no considerable decline in pH for the HWPC goats’ milk yogurt during storage at 4 °C over the 10-wk storage when compared to the other yogurts (Figure 2). The results were in agreement with the findings of Wang and others (2012) who reported a decrease in the pH over the course of 10 wk. The pH of the WPC goats’ milk yogurt was similar to those of the goats’ milk yogurt with pectin and the cows’ milk yogurt (Figure 2).

Changes in viscosity during storage The viscosity of the yogurts did not change considerably over the 10-wk storage. The HWPC goats’ milk yogurt’s viscosity was initially from 1763±67 mPa.s to 1556±66 mPa.s at week 10. It was significantly higher than the goats’ milk yogurt containing WPC (P < 0.01), the pectin goats’ milk yogurt (P < 0.01), and the cows’ milk yogurt (P < 0.01) (Figure 3). This could be due to the aggregation of whey proteins that occurred during heat treatment. When the denatured WPC formed aggregates or particles, they might be less symmetric in shape, and have increased volume fraction than the native molecules. This may result in increased viscosity of the yogurt (Bryant and McClements 1998; Vardhanabhuti and Foegeding 1999). Figure 5–Survivability of Bifidobacterium spp. during storage at 4 °C.

M: Food Microbiology & Safety Figure 6–Scanning electron microscopy (SEM) photographs (×2000 and ×500) of nonfat goats’ milk yogurt with HTWP (A), nonfat goats’ milk yogurt with WPC (B), nonfat goats’ milk yogurt with pectin only (C), and nonfat cows’ milk yogurt with pectin (D).

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The viscosity of the goats’ milk yogurt with pectin was significantly lower than that of either the WPC goats’ milk yogurt (P < 0.05) or the cows’ milk yogurt (P < 0.01). There was no significant difference in viscosity between goats’ milk yogurt with WPC and the cows’ milk yogurt. To exploit the nonfat goats’ milk yogurt, using only pectin to improve fermented goats’ milk viscosity and texture were not effective. The combination of whey protein or modified whey protein with pectin might achieve the similar effect for nonfat milk yogurt. The HWPC seemed to interact with casein micelles to form a network-like structure in the yogurt during fermentation resulting in increased viscosity.

Survivability of L. acidophilus and Bifidobacterium spp. during storage The population of L. acidophilus in the yogurt was only above 106 CFU g-1 for the 1st 2 wk of storage for all of the formulations of yogurt except the goats’ milk yogurt with pectin, where it fell below 106 CFU g-1 after the 1st week. Results showed a gradual decline in populations during storage. The L. acidophilus population of the yogurt sample containing HWPC was slightly higher than those of the other samples (Figure 4). The rapid decline in L. acidophilus population might be due to the production of hydrogen peroxide by L. Bulgaricus during storage, which may inhibit L. acidophilus (Gilland and Speck 1977; Li and others 2012). Counts of the Bifidobacterium spp. declined gradually for all of the formulations; however, they remained above 106 CFU g-1 during the 10 wk of the trial for all of the formulations of yogurt. Similarly, Bifidobacterium spp. seems survived a little better compared with other yogurts (Figure 5). Lourens-Hattingh and Viljoen (2001) suggested that S. thermophilus in the yogurt reducing the oxygen present, creating an anaerobic environment favorable to Bifidobacterium spp.

further strengthen electrostatic interactions, weakening electrostatic repulsions. This allows for an increased interaction between hydrophobic portions of the casein (Phadungath 2005). The goats’ milk yogurt with its absence or very low level of α s-1 casein may not be able to form a strong network, leading to a weaker gel (Park and Guo 2006). The use of heat-treated WPI has previously been shown to increase the water holding capacity, TS, and texture in goats’ milk yogurt (Li and Guo 2006; Wang and others 2012). The use of WPC in goats’ milk yogurt requires that the proteins undergo extensive denaturation through the raising of the pH and heating. β-Lactoglobulin is the primary protein in WPC. In its native form, it contains one free thiol-containing cysteine residue as well as 2 disulfide bridges, the more active residing nearest the N-terminal. Upon heating at high pH, the disulfide bridges can be broken resulting in more available reactive thiol groups. These residues could then interact with each other, forming large aggregates (Roeffs and de Kruif 1994; Alting and others 2000; Nicolai Britton and Schmitt 2011). When added to the milk system and inoculated with starter cultures, the pH begins to decrease, the HWPC might interact with the casein micelles and form aggregated particulates in the presence of calcium ions. The calcium, which is a divalent cation, binds the negative phosphate groups on nearby protein molecules (Bryant and McClements 1998). LM pectin is an anionic hydrocolloid, capable of interacting with the positive charges on casein molecules. The pectin may stabilize the casein micelles, leading to an increase in water holding capacity (Everett and Rosiland 2005) and might also interact with the whey protein aggregates induced by heating. The pectin plays a role in binding calcium as well. If the casein micelles bind too strongly, water holding capacity will decrease. Beaulieu and others (2001) suggest that the pectin should bind enough calcium so that gelation can occur without the presence of too much aggregation.

Mold, yeast, and coliforms There was no mold, yeast, or coliforms detected in any of the Conclusion WPC could be polymerized to form aggregates by the consamples during storage. The absence of these organisms indicated that the yogurts were safe and clean even after storage at 4 °C for trolled heat treatment. Microstructure analysis showed that HWPC aggregates might interact with casein micelles to form a relatively 10 wk. uniformed protein network in the goats’ milk yogurt and resulted in improvement of the goats’ milk yogurt texture and water holdMicrostructure SEM photographs of the 4 yogurt samples at different mag- ing capacity. Mold, yeast, and coliform were not detected in the nifications were shown in Figure 6: (A) goats’ milk yogurt with yogurt. Bifidobacterium spp. in the yogurt remained viable during HWPC and pectin, (B) goats’ milk yogurt with WPC and pectin, the 10-wk storage. Results indicated that HWPC could be used (C) goats’ milk yogurt with only pectin, and (D) cow’s milk yogurt as a fat replacer for nonfat goats’ milk yogurt formulation. with pectin. It can be seen in Figure 6A that the protein network was more uniform and absent of large voids. The voids seen in Acknowledgments Financial support for this project was provided in part by a Figure 6B showed less uniformed matrix and more prominent voids. These voids represent areas filled by the serum phase, an USDA-NIFA Hatch Grant (UVM project nr 027094-2012/13) excess of which will lead to an open or loose texture (Sandoval- and Oak Knoll Dairy in Winsor, VT. It was also supported by Castilla and others 2004) which could lead to the observed lower The National Key Technology Research and Development Proviscosity and weak body of the nonheat treated WPC yogurt. gram for the 12th Five-year Plan (Nr. 2013BAD18B07). The Figure 6C showed the microstructure of the goats’ milk yogurt authors would like to thank Dr. Alan Howard for his assistance in with pectin only. Because of the inherent weak structure of goats’ data analysis. milk due possibly to the lack of α s1 -casein, the sample most likely settled out during the fermentation resulting in the SEM Author Contributions Mingruo Guo designed and directed the study. Tiehua Zhang, photograph which is why the observed picture was so compact James McCarthy, Guorong Wang, and Yanyan Liu carried out all or and grainy. As the pH was decreased, the electrostatic repulsion between the part of experiments, data analyses, figure drawing, and interpreted normally negative casein molecules is lowered. Hydrophobic inter- the results. James McCarthy drafted the 1st version manuscript. actions will increase leading to the formation of the 3-dimensional References protein network comprised of casein strands. Below pH 5.0, col- Alting AC, Hamer RJ, de Kruif CG, Visschers RW. 2000. Formation of disulfide bonds in acid loidal calcium phosphate leaks into the serum from casein micelles induced gels of preheated whey protein isolate. J Agric Food Chem 48:5001–7. Vol. 80, Nr. 4, 2015 r Journal of Food Science M793

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Properties of nonfat goats’ milk yogurt . . .

Properties of nonfat goats’ milk yogurt . . .

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