Abstract
Yoghurts are mostly produced from cow milk and to a very limited extent from ewe milk. The evolution of caseins and whey proteins in ovine milk submitted to different thermal treatments (63°C/30 min; 73°C/15 min; 85°C/10 min or 96°C/5 min) was followed during fermentation of yoghurts and during their storage up to 14 days, using two different sets of starters. One set of starter LAB was a “ropy” culture (YC‐191( which is a well‐defined mixed strain culture containing Streptococcus thermophilus ST‐143 and Lactobacillus delbrueckii subsp. bulgaricus (LB‐18 and LB‐CH2). The other set of starter bacteria (YC‐460) was a standard yoghurt culture (“non‐ropy”) containing mixed strain culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Contents of free amino groups in produced yoghurts increased gradually during the fermentation, up to a maximal value obtained after 4 h fermentation, then they did not change significantly during storage of yoghurt produced with YC‐191 starter. In contrary, a large drop in the amount of free amino groups was observed in the first 24 h of storage in the case of yoghurt made with YC‐460 indicating that microorganisms continue still to grow in low temperatures. During fermentation and storage of both yoghurt types, α‐lactalbumin was hydrolyzed to a slightly bigger extent than β‐lactoglobulin. During fermentation, β‐casein was slightly more degraded than α~S~‐caseins; however, the opposite was observed during storage up to 14 days. Generally, a more intense heat pretreatment led to a higher degradation of whey proteins and caseins during fermentation and storage. Differences in proteolytic activity between the two starters used (whey proteins more degraded by YC‐191; caseins more degraded by YC‐460) may lead to improvement in production and formulation of yoghurts differing in their physicochemical and rheological properties.