Interplay of egg white gel pH and intragastric pH: Impact on breakdown kinetics and mass transport processes

Egg white gels have been utilized as a model system to study protein breakdown kinetics based on physical and biochemical breakdown processes during in vitro gastric digestion. Additionally, the impact of regulating intragastric pH on the breakdown kinetic processes was investigated. The present study evaluated the impact of gel pH (based on the pH of protein dispersion prepared at pH 3, 5 and 7.5) and intragastric pH regulation (with or without adjustment to pH 2 during in vitro gastric digestion) on the effective diffusion of gastric juice components (water and HCl), gel softening kinetics during gastric digestion, microstructural analysis using micro- computed tomography and protein hydrolysis in the liquid and solid fraction of egg white gel digesta. Egg white gels were subjected to 30 s oral digestion and 15, 30, 60, 120, 180 or 240 min gastric digestion in a static in vitro gastric digestion model, with or without gastric pH adjustment to pH 2. The gel pH affected all the properties measured during gastric digestion and each gel pH represented a specific driving mechanism for protein breakdown. A lower gel pH (pH 3) demonstrated a higher diffusion of moisture and acid, resulting in faster softening (p < 0.05). An intermediate pH (pH 5) showed greater protein-protein interactions due to the proximity to the isoelectric point of egg white proteins, resulting in very slow softening during digestion (p < 0.05), and a higher pH (pH 7) resulted in higher acid diffusion, intermediate gel hardness and very slow softening kinetics (p < 0.05). The gastric pH adjustment during digestion of egg protein gels affected (p < 0.05) the equilibrium moisture and acid contents as well as protein hydrolysis. The study confirmed that there is an interplay between initial gel pH and the intragastric pH which affected the breakdown kinetics of egg white gels during the gastric digestion process.