Proteolysis of ewe's caseins and whey proteins during fermentation of yogurt and storage.: Effect of the starters used

Yogurts are mostly produced from cow's milk and to a very limited extent from ewe's milk. Changes in the caseins and whey proteins in ovine milk subjected to different thermal treatments (63C/30 min; 73C/15 min; 85C/10 min or 96C/5 min) were followed during fermentation of yogurt, using two different starters, and during their storage up to 14 days. One starter (YC-183) contained mixed strain culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus at a ratio 1:1. The other starter (ABT-3) is a well-defined mixed-strain culture containing Sc. thermophilus TH4, Lb. acidophilus LA5 and Bifidobacterium Bb12 at a ratio 1:1:1. The level of free amino groups in the yogurts increased gradually during the fermentation up to a maximum after 4 or 6 h fermentation in the case of ABT-3 and YC-183, respectively. A large decrease in the amount of free amino groups was observed after 4-6 h fermentation. During storage of the yogurts up to 14 days, the amount of free amino groups increased with the duration of storage time up to a maximum after 7 days both in the case of YC-183 and ABT-3 starters. A large decrease in the concentration of free amino groups was observed in the first 24 h and between 7 and 14 days of storage in the case of yogurt made with the two starters indicating that microorganisms continue to grow in low temperatures. During the fermentation and the storage of both yogurt types, alpha-lactalbumin was hydrolyzed to a slightly greater extent than beta-lactoglobulin. During fermentation and the storage, beta-casein was degraded slightly than alpha(S1)- + alpha(S2)-caseins. Generally, a more intense heat pretreatment led to greater degradation of whey proteins and caseins during fermentation and storage. Differences in proteolytic activity between the two starters used (proteins degraded more by ABT-3 than by YC-183) may lead to the improvement in production and formulation of yogurts differing in their physico-chemical and rheological properties.