Associative Growth and Proteolysis of Streptococcus thermophilus and Lactobacillus bulgaricus in Skim Milk

Abstract

Proteolytic activities of nine strains of Streptococcus thermophilus and nine strains of Loctobacillus bulgaricus cultures incubated in pasteurized reconstituted NDM at 42°C as single and mixed cultures were studied. Lactobacilli were highly proteolytic (61.0 to 144.6 μg of tyrosine/ml of milk) and S. thermophilus were less proteolytic (2.4 to 14.8 μg of tyrosine/ml of milk). Mixed cultures, with the exception of one combination, liberated more tyrosine (92.6 to 419.9 μg/ml) than the sum of the individual cultures. Mixed cultures also produced more acid (lower pH). Of 81 combinations of L. bulgaricus and S. thermophilus cultures, only one combination was less proteolytic (92.6 μg tyrosine/ml) than the corresponding L. bulgaricus strain in pure culture (125 μg tyrosine/ml). © 1990, American Dairy Science Association. All rights reserved.