STANDARDIZING CONDITIONS FOR PILOT SCALE PRODUCTION AND STORAGE OF COW MILK DAHI USING STREPTOCOCCUS THERMOPHILUS STRAINS AS STARTER CULTURE SOME MICROBIOLOGICAL ASPECTS
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Date
1990
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AAU, Anand
Abstract
This study was planned and conducted to evaluate the comparative performance of three promising strains of Streptococcus thermophilus namely D-3 (C,), MD-2 (C2) and MD-8 (Cg) isolated from market samples of dahi and to assess their feasibility for commercial manufacture of dahi from standardised cow milk, adjusted to three levels of solids. These strains were also examined for antibiotic sensitiveness and their antibacterial ability against various food poisoning and spoilage microorganisms. for studying - different attributes, dahi was manufactured on pilot scale from whole cow milk adjusted to 12 (T1), 15 (T2) and 18 (T3) per cent total solids by adding condensed cow skim milk. The fat content in these milks were maintained at 3 per cent. After standardization these three lots of milk were preheated to 60°C and then homogenized at a pressure of 140.7 kg/cm2 in a single stage homogenizer. The homogenized lots of standardized milk were preheated to 85°C for 15 min and then cooled to 40°C. The standardized milk having individual level of milk solids was divided further into three equal parts. To each of these D-3, MD-2 and MD-8 strains were individually added as inoculum at the rate of 2 per cent. The inoculated milk samples were filled in polysterene cups having lids and then incubated at 40 ± 1°C. Por assessing microbiological changes during incubation, a set of cups from each lot of milk were drawn at 0, 2, 4 and 6 hour periods. For analysis of cow milk ,dahi during storage, samples were transferred to refrigerator (5-7°C) when the curd samples attained an acidity of 0.70 per cent and kept for about 12 hour. The starting of storage period at 37°C and in the refrigerator was considerate the end of 12 hour period indicated above. For assessing microbial changes in dahi at ambient temperature of storage (37 ± 1°C) the samples were drawn at 0, 12, 24 and 48 hours of storage and in case of refrigerated storage (5-7°C) at an interval of 0, 6, 12 and 18 days of storage. To (18 per cent T.S.) level of total solids gave significant (P < 0.05) result than other level of total solicJs with respect to change in titratable acidity, shift in pH, extent of lactose degradation, change in lactic count and 6-ga lactosidase content, during incubation with all the three strains. C2 and C3 showed higher acid production upto 4 hour than C1 and dahi with desired level of titratable acidity can be produced with these two cultures within . 4.5 hr of incubation. C2 gave highest g-galactosidase activity followed by C3 and it was lowest with C1 culture . During ambient and refrigerated temperature storage also T3 showed significant (P< 0.05) result with respect to change in titratable acidity, shift in pH, change in lactic count, extent of lactose degradation, B-galactosidase content and standard plate count. Dahi showed a shelflife of 12 days at refrigerated temperature and only 24h at ambient temperature, coliform count was found within the limit throughout the storage while yeast and mold showed a count within prescribed limit only upto 24h in case of room temperature and 12 days at refrigerated storage, with products showing high count after 48h and 18 days at which period it was not acceptable organoleptically. Among the three strains which were used in this study, MD-2 showed higher resistance to a number of antibiotics compared to MD-8 and D-3. however, all three strains to be were found /resistant: to penicillin, carbenicillin and nalidixic acid.
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DAIRY MICROBIOLOGY, A STUDY