Lactic Acid Production by Streptococcus lactis and Streptococcus cremoris in Milk Precultured with Psychrotrophic...

Psychrotrophic organisms isolated from raw milk were inoculated into fresh raw milk and incubated at 7 C for 5 days. Half of each sample was autoclaved, the other half, pasteurized. Streptococcus lactis and Streptococcus cremoris were individually inoculated into the milks and incubated at 21 C for 20 h. Acid production was measured at 2-h intervals. Lactic acid production was greater in milks precultured with added psychrotrophs or the normal flora than in pasteurized control milks. In most instances, pasteurized precultured milks had more acid than did autoclaved precultured milk. More titratable acid was produced in autoclaved rather than pasteurized control milks. Milks with the psychrotroph and lactic streptococci added simultaneously had more titratable acid than did control milk. When cell-free filtrates of psychrotrophic cultures were used instead of cells of psychrotrophs, results resembled those obtained with the bacteria. Simultaneous inoculation of milk with lactic acid bacteria and cell-free filtrate gave results comparable to those from milks precultured for 5 days with the cell-free filtrate.