Proteolytic activity of lactic acid bacteria in skim milk with special reference to the biodegradation of casein fractions.

Streptococci and lactobacilli were assayed for their proteolytic activity in pasteurised (95 degrees C for 30 min) fresh Friesian cows' skim milk incubated at 30 degrees C for 48 h. Lactobacilli were more proteolytic than the streptococci except S. faecalis subsp. liquefaciens. S. faecalis and S. thermophilus followed S. lactis subsp. diacetylactis in the proteolytic activity. Electrophoretic analysis of the precipitated casein revealed K-, pre-beta- and the small slow band of alpha s1-casein to be the most fractions and beta-casein the least fraction susceptible to the biodegradation. S. faecalis subsp. liquefaciens was the only organism able to degrade beta-casein to 3 fractions within 48 h. S. lactis with its subsp. diacetylactis was characterized by its inability to degrade gamma-casein. Addition of 2.0 g glucose, 0.5 g yeast extract, 2.0 mmol Mg2+, 0.5 mmol Mn2+ and 0.1 mmol Fe2+/l of skim milk culture of each L. casei and L. plantarum increased acid formation but decreased proteolysis. 6% NaCl was inhibitory to both. After 60 days at 18 degrees C, a fraction, probably derived from beta-casein, was noticeable. The large intense band of alpha s1-casein appears to be degraded gradually to only small peptides which in turn are transferred immediately into the cells. The hydrolysis of gamma-casein and beta-casein also appears to be inhibited by exogenously supplied NaCl and nutrients, respectively.