The Nutritional Requirements of Lactic Streptococci Isolated from Starter Cultures. III. Variation in Growth-Promoting Properties of Fresh Whole Milks

Previous investigations (1, 2) demonstrated marked stimulation of lactic streptococci in synthetic media and milk after addition of liver fraction L and enzymatic digests of milk. Since liver L is considered an abundant source of peptides and similar compounds are liberated by enzymatic hydrolysis of milk proteins, the results suggested stimulation by peptides. It also was considered possible that a similar factor might be involved in the variation in rate of acid production frequently observed for lactic streptococcus starter cultures in different individual herd or cow samples of milk. This study, therefore, represents an attempt to correlate rate of activity of lactic streptococci with concentration in such milks of protein degradation products in the form of peptides, peptones, and proteoses. For the sake of brevity and to avoid misunderstanding in nomenclature in the following descriptions and discussion, the term peptides will be used to designate all the protein fragments occurring between amino acids and" the complete proteins (caseins, albumins, and globulins).