Effect of the addition of extracts of thermophilic lactobacilli on acid production by Streptococcus thermophilus in milk.

Soluble extracts of 20 strains of thermophilic lactobacilli (Lactobacillus helveticus, L. lactis and L. bulgaricus) were prepared and added to milk for the culture of 10 strains of Streptococcus thermophilus. Acid production was stimulated in 64.5% of cases for 9 of these 10 strains. The L. helveticus extracts were the most stimulatory, but the same extracts did not always strongly stimulate each strain of Str. thermophilus. The stimulatory effects observed varied with the volume of extract and the strain of Str. thermophilus. The exception was Str. thermophilus 385, which was never stimulated. The stimulatory effects observed were due to aminopeptidases present in the lactobacillus extracts and were not related to a general caseinolytic activity. The possible addition of such extracts to milk for cooked hard cheese is discussed.