Isolation and Characterization of Streptococcus thermophilus from Traditionally Prepared Sour Curd

Aims: Dahi, or curd, is a common fermented milk product in West Bengal and throughout India. In the unorganised business, the "back-slopping" approach is still used to produce it using an unidentified range of strain starters, which could result in batch variation that affects safety and shelf life. Lactobacillus bulgaricus and Streptococcus thermophilus are thermophilic microorganisms employed in curd. The goal of this research work is to increase the techno-functional qualities of sour curd by isolating and characterizing the thermophilic lactic acid bacteria that produce cocci from traditionally prepared sour curd. Study Design: Phenotypic and Biochemical observations were done through consideration of 3 replications in each case. Place and Duration of Study: The study on collected sour curd samples were evaluated at Department of Dairy Microbiology, F/Dairy Technology, WBUAFS in duration of November, 2022 to March, 2023. Methodology: Twenty-one sour curd samples were collected from various local sweet shops of Nadia and North 24 Parganas district of West Bengal. The pH, titratable acidity, total lactic counts of the samples were observed. Coliforms and yeasts and moulds were also found. The analysis on catalase test, negative staining and microscopic study were also followed. The esculin hydrolysis and sugar fermentation profiles were measured to recognize the Streptococcus thermophilus. Results: The pH, titratable acidity, total lactic counts varied from 3.94-4.65, 0.72-1.05% lactic acid and 7.96-9.90 log cfu/ml, respectively. The samples were heavily loaded with coliforms and yeasts & moulds with a range of 2.70-6.73 log cfu/ml and 3.04-6.40 log cfu/ml, respectively. Isolation of bacteria was done by pour plating using skim milk agar. 41 isolates were catalase negative and gram-positive cocci among 91 colonies of samples. The 41 isolates were cocci which showed positive growth at 45°C but negative in other purity tests except two. 20 representative cocci isolate from 39 isolates showed positive growth at 45°C. The esculin hydrolysis and sugar fermentation profiles were utilized to identify the Streptococcus thermophilus. Conclusion: The proper isolation and characterisation of S. thermophilus upgrade the culture quality.