From gel structure to digestion: the egg white model

Egg white is a common gelling ingredient that can produce gels with varying macro- and microstructures depending on the conditions (pH and/or ionic strength) employed, without the need to change the chemical composition. Consequently, egg white gels provide interesting models for investigating the food matrix effect on food digestion. In the present study, 4 gels were prepared by heating egg white at 80°C for 1h with pH varying from 2 to 9. Multi-scale characterization of the gels was performed using infra-red analysis (FTIR), electronic microscopy (SEM), and texturometry techniques. Each gel was subjected to static in vitro digestion using a two-step model (gastric step for 60 min, followed by an intestinal step for 30 min). Digest samples were taken throughout the digestion period and analyzed for the extent and rate of proteolysis using SDS-PAGE; the concentration of soluble peptides was determined by RP-HPLC. The amino acid sequence (for identification) and relative amount of each peptide was then determined using nano-LC-MS/MS. The macroscopic differences between the gels resulting from the different gelation conditions were related to textural differences. For example, gel hardness was maximum for the pH 5 gel and minimal for the pH 2 gel, cohesiveness and resilience decreased when gel pH decreased from 9 to 2. Furthermore, SEM observations revealed smooth microstructures for the pH 9 and pH 2 gels, whereas the pH 5 and 7 gels were comprised of aggregates. Concurrently, different in vitro digestion kinetics were observed: the pH 7 gel was digested more quickly and more extensively than the other gels. Moreover, quantitative peptidomics and multivariate data analysis of the in vitro digests enabled peptide clustering based on the nature and relative amount of the peptides generated; this highlighted the impact of gel structural and textural characteristics on the digestion process. Combining a food structure characterization and peptidomic approach, the present study provides original results that contribute to the understanding of the relationship between food matrix structure and digestion.