Isolation and Typing of Various Strains of Streptococcus Thermophilus and Lactobacilus Bulgaricus from Local Dahi Samples and Their Efeects on Various Physico-Chemical Properties of Milk During Fermentation

Abstract

A total of 246 samples of indigenous made dahi were collected and analyzed for the isolation and identification of different strains of S. Thermophilus and L. Bulgaricus on the basis of their morphological, growth, on specific media, biochemical characteristics and production of titrable acidity in milk. 318 isolates of S. Thermophilus and 275 isolates of L. bulgaricus were isolated in the present study. All the isolates of both the organisms showed uniform morphological, cultural and growth characteristics but difference were observed in sugar fermentation a percentage of titrable acidity production. The isolates of both the organism were classifies in 3 groups i.e. A, B and C on basis of their sugar fermentation and titrable acidity production in milk at 45OC in 5 hours. Out of 318 isolates of S. thermophilus of group A (93 isolates) fermented glucose, fructose, lactose and saccharose, but showed week reaction in arabinose and raffinose. None of them fermented xylose, maltose, trehalose, mannitol and sorbitol sugars in group B (103 isolates) fermented same number of sugar but in addition they showed mild reaction in maltose and mannitol, while in group C (122 isolates) fermented maltose but showed wee reaction in mannitol. (Table 2). Out of 275 isolates of L. Bulgaricus, in group A 942 isolates) fermented glucose, fructose, galactose and lactose, in group B (122 isolates) fermented mannose with mild reaction while in group C (121 isolates) fermented mannose completely in addition to fermentation of above sugars (table 3) The percentage of titrable acidity produced in milk at 45OC in 5 hours by strains of three groups viz: A, B and C of S. thermophilus were 0.54, 0.49 and 0.44 percent respectively. The three groups of L. bulgaricus produced titrable acidity 0.60, 0.55 and 0.46 percent respectively. While mixed cultures of S. thermophilus and L. bulgaricus viz; A: A, B: B and C:C produced 0.95, 0.87 and 0.71 percentage of titrable acidity, respectively, (Tables 4,5 and 6). 126 samples of yogurt were prepared from mixed cultures of 42 isolates of S. thermophilus and L. Bulgaricus of A: A B: B and C: C groups. All the samples yogurt were presented to the panel of three judges for the evaluation of organoleptic properties, pH, titrable acidity percentage and volatile fatty acids were also analyzed. The results of the present study showed that the highest score for physical appearance, consistency, flavor and aroma were given to the samples prepared from the mixed cultures of A: A group followed by B: B and C: C groups of the isolates. of both the organisms. (Table 7 and fig 5). According to the statistical analysis the highest titrable acidity 0.957 + 0.019 were determined from the dahi samples made from mixed culture of A: A groups of S. thermophilus and L. bulgaricus followed by group B: B 0.928 + 0.28 and C: C 0.877 + 0.014 percent. All the groups of both the organism showed same patterns of volatile fatty acids production in milk. The maximum VFA 2.151 + 0.295 mg/ 2ml of sample were produced by mixed culture of A: A group of the S. thermophilus and L. bulgaricus followed by group B: B, 1.947 + 0.201 and C: C which produced lowest 1.619 + 0.421 mg/ 2ml of samples. (Table 8, Fig.6).