Sequential release of amino acids and peptides during in vitro digestion of casein and rapeseed proteins

An in vitro proteolysis method was used to study the kinetics of digestion of casein and rapeseed proteins. After a pepsin predigestion, the protein sources were submitted to a 24-hour pancreatin hydrolysis, with the continuous removal of digestion products by dialysis. Dialysates were collected at 3-hour intervals, and analysed for content of nitrogen, amino acids and low molecular weight peptides. At the end of the experimental period, casein had a higher overall digestibility (97%) than rapeseed (83%). The difference was mostly due to a high rate of digestion during the first 3 hours for casein, as nitrogen release was similar for both proteins at subsequent intervals. Much more variation was observed in individual amino acid digestibility. In casein, arginine, tyrosine and methionine were the most rapidly released, being more than 95% digested after 12 hours. In rapeseed, the amino acids were generally hydrolysed more gradually. Only tyrosine and methionine were more than 95% released by 24 hours. In general, highly digestible amino acids were mostly released as free amino acids. But differences were also noted between proteins for the distribution of individual amino acids in low molecular weight peptides. With casein, isoleucine, valine and threonine were found in large proportion in peptides, and arginine was mostly released as a free amino acid. With rapeseed, histidine was preferentially released in peptides, while methionine appeared mostly in the free form from 6 to 12 hours. These findings give new insight into the effect of protein nature upon the kinetics and form of release of digestion products. Differences in sequential hydrolysis also suggest that subsequent absorption and utilisation of dietary amino acids in the living organism may be largely dependent on the luminal digestion process.