Some observations on slow and fast acid-producing variants of strains of Streptococcus cremoris and Str. lactis used as cheese starters

Abstract

Improved growth of Streptococcus cremoris 924 was obtained in the presence of CO2.When fast and slow acid-producing variants of Str. cremoris 924 were mixed and the mixture subcultured daily the resulting cultures were all active acid producers, irrespective of the initial ratio of fast to slow cells. A balance is established between the two types of cell; probably this will vary with the cultural conditions and vary between strains of Str. cremoris and Str. lactis for any particular conditions of maintenance.Providing the strains are correctly selected, single-strain starter cultures can be maintained by daily subculture in litmus milk, apparently indefinitely. Activity varies with the present method within wide limits and this variation seems to be a function of the culture.Freeze drying is a satisfactory method of maintaining active single-strain starters.The author wishes to thank Miss Shelagh Gibbs for technical assistance, Miss H. R. Chapman for making the cheese, and Messrs. Cow and Gate, Ltd., for supplying the dried milk powder for these experiments.