Effect of Water Activity of Milk upon Growth and Acid Production by Mixed Cultures of Streptococcus thermophilus and Lactobacillus bulgaricus

Acid production and growth rates were determined in pure cultures of Streptococcus thermophilus and Lactobacillus bulgaricus and cocci/ bacilli ratios in mixed cultures of both species, using milk with modified a_w as culture medium. Maximal growth rates were always at a_w lower than 0.992 for all strains whcn a_w of milk was adjusted with glycerol, while maximal acid production rates were at a_w 0.992-0.983. Glucose had an inhibitory effect additional to that caused by lowering a_w. The cocci/bacilli ratio in mixed cultures increased as a_w decreased in most cases, because of the higher susceptibility of L. bulgaricus to a_w decrease. The behavior of a mixed culture could not always be predicted from data of the component strains in a pure culture, showing some kind of symbiotic compatibility between species.