a b s t r a c tMilk supplementation with milk proteins in four different levels was used to investigate the effect on acidification and textural properties of yogurt. Commercial skim milk powder was diluted in distilled water, and the supplements were added to give different enriched-milk bases; these were heat treated at 90 C for 5 min. These mixtures were incubated with the bacterial cultures for fermentation in a water bath, at 42 C, until pH 4.50 was reached. Chemical changes during fermentation were followed by measuring the pH. Protein concentration measurements, microbial counts of Lactobacillus bulgaricus and Streptococcus thermophilus, and textural properties (G 0 , G 00 , yield stress and firmness) were determined after 24 h of storage at 4 C. Yogurt made with milk supplemented with sodium caseinate resulted in significant properties changes, which were decrease in fermentation time, and increase in yield stress, storage modulus, and firmness, with increases in supplement level. Microstructure also differed from that of yogurt produced with milk supplemented with skim milk powder or sodium caseinate.

PRACTICAL APPLICATIONSProbiotic cultures should grow quickly in milk, provide adequate sensory and rheological properties to the product, and remain viable during storage. Commercially, it is very common to use yogurt starter culture (i.e. Streptococcus thermophilus [ST] and Lactobacillus delbrueckii ssp. bulgaricus) in 3 Corresponding

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a b s t r a c tThis study aimed to optimize the rheological properties of probiotic yoghurts supplemented with skimmed milk powder (SMP), whey protein concentrate (WPC) and sodium caseinate (Na-Cn) by using an experimental design type simplex-centroid for mixture modeling. It included seven batches/trials: three were supplemented with each type of the dairy protein used, three corresponding to the binary mixtures and one to the ternary one in order to increase protein concentration in 1 g 100 g À1 of final product. A control experiment was prepared without supplementing the milk base. Processed milk bases were fermented at 42 C until pH 4.5 by using a starter culture blend that consisted of Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, and Bifidobacterium animalis subsp. lactis. The kinetics of acidification was followed during the fermentation period as well the physico-chemical analyses, enumeration of viable bacteria and rheological characteristics of the yoghurts. Models were adjusted to the results (kinetic responses, counts of viable bacteria and rheological parameters) through three regression models (linear, quadratic and cubic special) applied to mixtures. The results showed that the addition of milk proteins affected slightly acidification profile and counts of S. thermophilus and B. animalis subsp. lactis, but it was significant for L. delbrueckii subsp. bulgaricus. Partially-replacing SMP (45 g/100 g) with WPC or Na-Cn simultaneously enhanced the rheological properties of probiotic yoghurts taking into account the kinetics of acidification and enumeration of viable bacteria.

This study aimed to evaluate the quality of stirred-type skim milk probiotic yogurt fortified by partially replacing skim milk powder (SMP) with whey protein concentrate (WPC) and sodium caseinate (Na-CN) during cold storage for 28 d compared with nonfortified yogurt. The rheological properties (as measured using dynamic oscillation) and sensory profiles of probiotic yogurts were greatly enhanced when SMP (i.e., 45%) was replaced with WPC and Na-CN. Higher values of mechanical parameters related to storage and loss modulus and consistent microstructure were found in the fortified yogurts. The acidification profile was not affected by supplementation of the solids in the milk base, and the viable counts of probiotic microbiota were high and satisfactory. These positive characteristics of probiotic yogurts were maintained until the end of the storage period. The microstructure of the fortified yogurt showed some differences compared with the nonfortified product, which were due to changes in chemical composition of the milk base in addition to the colloidal characteristics of the product.