Combined effects of incubation temperature, type of starter bacteria and final pH of fermentation on microbiological, biochemical and sensory characteristics of probiotic Doogh (Iranian drink based on fermented milk)

Viability of probiotic microorganisms in the final product is the most important qualitative parameter which is affected by many factors, including type of starter calture, pH, acidity, redox potential, incubation temperature and refrigerating temperature and time. In this study, the combined effects of three important factors including; type of starter calture, incubation temperature and final pH of fermentation on viability of two probiotic species; Lactobacillus acidophilus and Bifidobacterium lactis and also the biochemical and sensory characteristics of their product, doogh, were investigated.

Different treatments of probiotic Doogh were prepared with yoghurt starter culture and two probiotic strains; Lactobacillus acidophilus La-5 (A) and Bifidobacterium lactis Bb-12 (B) using reconstituted skim milk powder with 6% of nonfat solid milk by incubating at 38°C or 44°C until pH 4.0 or 4.5 and keeping in 4°C or 8°C storage temperature for 21 days. The count of probiotic bacteria and biochemical characteristics were performed during fermentation and over the refrigerating period. The sensory attributes of treatments were determined at the end of fermentation and refrigeration.

In general, the greatest drop in mean pH rate, increase in mean acidity rate, increase in mean redox potential rate and shortest incubation time were observed in treatments with incubation in 44°C and final pH of 4.5. Among the selected treatments regarding the taste acceptance, ABY3-38-4.5 treatment revealed the greatest viability of the two probiotic bacteria in both storage temperature of 4°C and 8°C. After 21 days of refrigeration, ABY1-38°C-4.0 and ABY2-38°C-4.0 achieved the highest and lowest total acceptance scores, respectively.

All of the variables in this research significantly affected the qualitative properties of probiotic yogurt. Either the fermentation temperature of 38°C (in spite of need for prolonged fermentation) or the final pH of 4.5 resulted in increased viability of the probiotics, although the final pH of 4 resulted in better sensory evaluation.