Isolation of exopolysaccharide producing Streptococcus thermophilus organism of yoghurt

Abstract

The present study was conducted to isolate the exo-polysaccharide producing Streptococcus thermophilus organism from the available yoghurt in Chittagong areas of Bangladesh and to develop pure culture in the laboratory. A total of 100 samples (25 of each brand) were collected from four different brands namely, A, B, C and D (Food plaza, Banoful, Fulkoli and Genuine, respectively). All the samples were subsequently cultured in nutrient agar for detecting live bacterial growth followed by deMan Rogosa and Sharpe agar to isolate Streptococcus thermophilus and finally Ruthenium red milk plate for the isolation of exopolysaccharide producing Streptococcus thermophilus. Then fermented product, yoghurt was prepared using this isolated organism and sensory evaluation was done by a panel of judges. The study reveals 36% of the Brand C dahi were Streptococcus thermophilus positive. 44.44% cases were exopolysaccharide producing amongst the positive cases.16% of the Brand D yoghurt were Streptococcus thermophilus positive and 20% for the positive case of the Brand B were exopolysaccharide producing. In sensory evaluation, the yoghurt made by using the isolated organism was well acceptable regarding taste, appearance, acidity, flavor. It achieved on average 87% acceptability regarding the selected criteria. In conclusion, the isolated Streptococcus thermophiles might be a good seed for preparing yoghurt having excellent probiotic capability.