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The Distribution of Certain Amino Acids in Soluble Fractions of Milk Cultures of Streptococcus Lactis^1,2

Department of Dairy Industry, Iowa State College

ABSTRACT

Microbiological assays for ten amino acids performed on tungstic and lactic acid filtrates of skimmilk cultures of five strains of Streptococcus lactis after extended incubation revealed marked increases over values for the uninoculated control filtrates. Higher assay values obtained on the lactic acid filtrates were attributed to the presence of a higher proportion of the lower fractions of protein degradation which contain amino acid residues available to the assay organisms.

Assays for leucine and phenylalanine on both unhydrolyzed filtrates prepared from cultures of two strains of S. lactis after various periods of incubation indicated that active strains are able to effect a marked increase in the microbiologically available forms of these two amino acids during the first 3 days of incubation and a slower increase through the fourteenth day. Simultaneous increases in the assay values of the hydrolyzed lactic acid filtrates indicated that the total concentration of protein degradation products increased throughout the 14-day incubation period.

Evidence is presented which indicates the presence of an intermediate fraction in sterilized skimmilk which is probably proteose or peptone in nature. This fraction contains a considerable amount of leucine but very little phenylalanine.

The role of S. lactis in cheese ripening is discussed in light of the present work.

¹ Journal paper no. J-1873 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project no. 1050.

² This paper was prepared from data presented in a thesis by the senior author in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Iowa State College.

³ Present address: Department of Animal Industries, University of Connecticut, Storrs.