Surface adsorption alters the susceptibility of whey proteins to pepsin-digestion

An in vitro digestion model mimicking the gastric phase of the human gastrointestinal tract coupled with SDS-PAGE and MALDI-TOF mass spectroscopy was employed to study the hydrolysis profiles of whey proteins in solution and adsorbed at the oil-water interface. The objective of this work was to understand the differences in hydrolysis behaviour of whey protein isolates once adsorbed at the interface, and comparisons were carried out with pure beta-lactoglobulin and alpha-lactalbumin fractions. In solution, while beta-lactoglobulin appeared to be resistant to enzymatic treatment, alpha-lactalbumin was fully degraded. Adsorption of both proteins at the oil-water interface affected their conformational structure and susceptibility to peptic hydrolysis. Adsorbed beta-lactoglobulin was hydrolyzed into small polypeptides and in contrast, the resistance of alpha-lactalbumin to pepsin increased upon adsorption at the interface. In addition, changes in the particle size distribution of the droplets during pepsin hydrolysis mainly depended on the original protein concentration. The results suggested that exchanges occur at the interface between adsorbed and non-adsorbed protein, that is to say that either some protein desorb from the interface and does not fully recover its structure in solution, or that hydrolysis of the protein at the interface induces further adsorption and hydrolysis of the protein in solution. These mechanisms have important implications in the digestibility of the proteins.