Post-Oxidative Activity of Streptococcus thermophilus and Lactobacillus acidophilus Strains: Developing a Starter Culture for Fermented Baked Dairy Bio-Drink Ryazhenka

Fermented dairy products have a wide range of beneficial effects. As a result, their commercial popularity keeps growing worldwide. Recent years have witnessed a rise in the production of fermented dairy products with prolonged shelf life and probiotic properties. Excessive acidification during storage caused by the ongoing metabolism of lactic acid bacteria is a key defect leading to low product quality. Cultures of S. thermophilus and L. acidophilus were studied based on their acid-forming activity during storage: those possessing low post-acidification activity were used to develop a starter culture for the production of the traditional baked fermented milk (ryazhenka) with a prolonged shelf life. The research featured five strains of thermophilic streptococci (S. thermophilus 55, S. thermophilus 102, S. thermophilus BC 234, S. thermophilus 3t, S. thermophilus 19ch), three strains of Lactobacillus acidophilus (L. acidophilus NK1, L. acidophilus 4900, L. acidophilus 18a), and bio-ryazhenka samples produced using these cultures. The study involved microbiological methods for quantifying lactic acid bacteria (CFU/cm³), along with physicochemical methods for evaluating active acidity (pH), post-acidification activity (ΔpH), water-holding capacity, and conditional viscosity. The post-acidification activity of consortia of S. thermophilus 55 and L. acidophilus NK1 during 28-day storage at (4 ± 2) ºC yielded a starter culture with a ratio of 4.5:0.5 at an inoculum of 5% and a relatively low post-acidification activity (ΔpH 0.33 ± 0.03). The results can serve as a basis for developing new starter cultures used in the production of fermented dairy products.