Summary -A method to prepare bifidus milk on a commercial basis was standardised by addition of 10% inoculum of Bifidobacterium bifidum NDRI to 9% reconstituted skim milk (0.5% fat, 8.7% non-fat solids) heated at 95 "C for 30 min and incubated at 37 oC for 18 h. The product was able to meet ail the criteria regarding technological and dietetic properties required in a quality product. The effect of various factors influencing the antibacterial activity of bifidus milk against 4 test organisms viz Escherichia coli, Shigella dysenteriae, Staphylococcus aureus and Bacillus cereus was determined. There was a significant variation (P < 0.05) in the antibacterial activity of bifidus milk made from various types of milk (reconstituted skim milk, cow milk, buffalo milk, reconstituted infant food) and with various types of heat treatment. Reconstituted skim milk was recommended for preparation of bifidus milk. Heating of milk to sterilization temperature or at 95 "C for 30 min had a maximum effect on antibacterial activity. There was no significant variation (P > 0.05) in the antibacterial activity at different levels of inoculum (5.10 and 15%) or due to varying concentrations of sugar. Incubation at 37 oC showed highest inhibitory activity. Bifidus milk had a minimum storage Iife of 3 weeks under refrigeration, retaining a sufficiently good taste and the requisite amount of microbial population (10 8 cfu/g) to be potentially beneficial.

The effect of incubation temperature on acid and flavor production by 8 lactic starter cultures was studied. Higher titratable acidity was produced at 22 than at 30 C by Streptococcus cremoris, while a reverse trend was observed with Streptococcus diacetilactis. With Streptococcus thermophilus, Lactobacillus casei, and Lactobacillus helveticus, increasing the incubation temperature from 37 to 42 C retarded acid production by 8.54, 31.8, and 37.4%, respectively. Volatile acidity increased at higher incubation temperatures in some cultures, while in others, a significant decrease was observed. Higher diacetyl production was noted when S. diacetilactis was incubated at 22 C, as compared to 30 C, while the reverse occurred with S. thermophilus. Acetoin production by S. diacetilactis was enhanced 7-fold by increasing the incubation temperature from 22 to 30 C. No significant difference was noted in proteolytic activity of all cultures, regardless of incubation temperature.

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