Development and properties of functional yogurt enriched with postbiotics produced by yogurt cultures using cheese whey and skim milk

The objective of the research was to find the ideal conditions for postbiotic production of Streptococcus thermophilus and Lactobacillus bulgaricus (yoghurt starters) and to develop a method for fortifying yogurt with postbiotic-containing powders to enhance its nutritional content. For this purpose, different temperatures and incubation times were evaluated. The ideal temperature and incubation time for S. thermophilus in cheese whey were determined to be 39.6°C and 68 hours, while in skim milk, they were 40°C and 68 hours, respectively and for L. bulgaricus, the optimal temperature and incubation time in cheese whey was found to be 42.1°C and 68 hours, and in skim milk, they were 46°C and 64 hours, respectively, to efficiently produce postbiotics. Therefore, yoghurt batches were prepared by adding 3%, 4%, and 5% of postbiotic-containing powders to the milk. Among these variations, the yoghurt fortified with 3% of postbiotic-containing powders received the highest satisfactory score in the sensory evaluation. Physicochemical changes (acidity, water-holding capacity, radical scavenging activity, yoghurt starter culture viability), as well as sensory changes occurring during 22-day refrigerated storage, were determined. Fortification of yoghurt was found to significantly (p < 0.05) influence pH, water holding capacity, syneresis, DPPH and sensory profiles. The use of cheese whey and skim milk improved the physicochemical and sensorial quality of yoghurt. Additionally, this supplementation contributed to the yogurt's ability to decrease in pH levels. LB-CW showed the highest DPPH activity with 16.64 % on the 22-day of storage. Therefore, the viability of starter cultures was higher in yoghurt samples fortified with ST-SM and LB-SM. According to sensory evaluation results, there was a notable distinction (p < 0.05) observed across all samples except for flavor and mouthfeel (p > 0.05) and LB-SM received the highest score in terms of flavor, mouthfeel and texture. The same treatment acquired the highest acceptability among panelist. The development of fortified yogurt has a significant impact on its shelf life, quality, and health benefits and it can pave the way for the integration of fortification in other dairy products. Keywords: postbiotics, yoghurt starters, cheese whey, skim milk, Streptococcus thermophilus, Lactobacillus bulgaricus