Bovine milk .kappa.-casein trypsin digest is a growth enhancer for the genus Bifidobacterium

Bifidobacterium bifidum and Bifidobacterium longum, both isolated from infant feces, were used as assay organisms to evaluate bifidobacterial growth-promoting activities of bovine milk casein components. K-casein was found to be the most potent growth enhancer when digested by trypsin. Its glycomacro-peptide had no activity whatever. Activity was lost when the disulfide bonds in K-casein were modified by performic acid oxidation, reduction-alkylation, dinitrophenylation, or combination in a mixed disulfide linkage with 2-mercaptopyridine. Synthetic K-casein peptides containing cysteine or other cysteine compounds could not substitute for the K-casein digest. Similar behavior was observed with yeast extract and hog gastric mucin