Casein derived from cow's colostrum contains 0·85% N-acetyl neuraminic acid and liberates 2·95% nitrogen soluble in 12% trichloroacetic acid (NPN) under the action of rennin. It therefore appears to contain a large proportion of κ-casein (calculated to be approximately 35%).
The smaller the micelles of casein in colostrum the larger is the amount of N-acetyl neuraminic acid they contain. Soluble casein, i.e. 25% of the casein in colostrum, contains 1·52% of this acid and gives 6·24% NPN under the action of rennin.
Casein and particularly the soluble casein of colostrum appear to be important sources of κ-casein and caseino-glycopeptide.