Pepsin-induced changes in the size and molecular weight distribution of bovine casein during enzymatic hydrolysis

Bovine casein was digested with pepsin at pH 2.0 in a batch-stirred tank reactor. To investigate the effect of peptic digestion on the aggregate size and molecular weight distribution of bovine casein, the resulting hydrolysates were examined by size-exclusion chromatography coupled with multiangle laser light scattering and dynamic light scattering. Casein was resolved by size-exclusion chromatography into 2 major peaks corresponding to aggregates and monomers, both of which showed a continuous decrease as hydrolysis proceeded. However, the ratio of aggregates to monomers was maintained at almost 1 (2:2.5) during the initial 30-min hydrolysis, indicating that the caseins in solution were in a type of equilibrium between aggregates and monomers. Upon peptic hydrolysis, casein aggregates increased in size and molecular weight, and exhibited a decrease in intermolecular repulsion. This finding was confirmed by dynamic light scattering measurements, which traced the changes in the hydrodynamic radii and light scattering intensities of casein hydrolysates. In addition, the release kinetics of peptide fractions with different molecular weights was also examined. It was concluded that the increase in hydrophobic attraction and the reduction in intermicellar repulsion might promote the growth in aggregate size of bovine casein during the limited hydrolysis.