Lactic Acid Production by Streptococcus thermophilus and Lactobacillus bulgaricus in Milk Precultured with...

Raw milk was incubated at 7 C for 5 days after it was inoculated with psychrotophic bacteria previously isolated from other raw milks. Then a portion of each sample of milk was pasteurized; the remainder was autoclaved. Streptococcus thermophilus and Lactobacillus bulgaricus were individually inoculated into all pasteurized and autoclaved milks which were then incubated at 37 C and titratable acidity was measured at 2-h intervals. Pasteurized milks precultured with psychrotrophic bacteria supported more acid production than did pasteurized control milks. S. thermophilus and especially L. bulgaricus produced more acid in autoclaved than in pasteurized milks. Cell-free filtrates from cultures of psychrotrophic bacteria were added to raw, pasteurized, and autoclaved milks before they were inoculated with the lactic acid bacteria. Results were similar to those obtained when psychrotrophic bacteria were added to milk. When the cell-free filtrate was added to milks simultaneously with starter cultures, less than 10 h were required to produce conditions in milk favorable for enhanced lactic acid production.