High cell wall-associated proteinase activity of some Streptococcus thermophilus strains (H-strains) correlated with a high acidification rate in milk

Dairy starter strains Streptococcus thermophilus CNRZ 385 and CNRZ 703 coagulatad low heat skim milk in 3 h while the raference strain CNRZ 302 required more than 10 h.The high acidification rates of these 2 strains (H-strains) were correlated with the presence of a 10-and 7-fold increase in proteinase activity as compared to 13 other randomly chosen strains (L-strains), including the reference strain CNRZ 302.The level of proteinase activity in M17 broth was similar ta the activity of the Prt+ Lactococcus lactis ssp lactis strain CNRZ 1076.Proteinase activity of S thermophilus H-strains was found ta be cell wall-associated and was not released in the absence of CaCI 2 as in the case of L lactis strains.The relevance of these proteinase activities in relation to high acid production rates was confirmed by a proteinase-negative mutant whose growth and proteinase activity were reduced to levels observed for the L-strains.