Role of the fat globules in the inhibition of certain strains of Streptococcus cremoris in pasteurized milk

Abstract

Pasteurized whole milk was separated by various means. Strains of Str. cremoris exhibiting susceptibility to the milk as a whole were more active in the skim-milk fraction. All strains of Str. cremoris were markedly inhibited by the cream fraction and this inhibition could be transferred to sterilized skim-milk by the addition of cream, to a degree influenced by the amount of cream added. It is suggested that high dilutions of cream are active only against susceptible strains, and low dilutions against all strains. This was attributed to a shortage of appropriate nutrients. In cream resuspended in skim-milk and water and subsequently inoculated with susceptible and resistant strains the susceptible cells rose with the fat globules and remained in the cream layer, whereas the resistant strains were uniformly distributed between both layers. Inhibition could be prevented in milk samples by constant rotation to prevent creaming, in the cream fraction by washing three times with sterile water and in skim-milk by adsorbing the inhibitory factors on to washed cream.