Kinetics of peptide fraction release during in vitro digestion of casein

The charges and sizes of peptides released during in vitro casein digestion were investigated. Casein was hydrolysed sequentially by pepsin and pancreatin and, during hydrolysis, the products of digestion were removed by dialysis. The undigested residues were separated by ion-exchange chromatography into basic-neutral (BN), lightly acid (LA) and acid (A) fractions. Each of these fractions was further resolved by sequential ultrafiltration (MWCO 10 and 1 kDa) into two retentates (>10 and 10-1 kDa) and one permeate (<1 kDa). The polypeptides (>10 kDa) produced by pepsin hydrolysis were degraded into small molecules during the first 2 h of pancreatin hydrolysis; less than 100 mg g^-1 of the total N remained undigested. Most of the material produced was in the BN10-1 (ie basic-neutral, 10-1 kDa) fraction, with three times as much N as the A10-1 fractions. As digestion progressed a decrease in the proportion of N in the residues retained by the dialysis membrane was observed. This decrease was particularly rapid in the BN10-1 fraction. Large proportions of leucine (Leu), lysine (Lys), arginine (Arg), phenylalanine (Phe) and tyrosine (Tyr) were found as peptides smaller than 1 kDa, both in the dialysates and retentates, while glutamine (Glu), threonine (Thr), serine (Ser) and asparagine (Asp) appeared mostly in the A10-1 and A > 10 fractions. After 6 h of pancreatin hydrolysis most of the proline (Pro) content was in the BN10-1 fractions. The mechanisms behind and the implications of these results are discussed.