Breakdown mechanisms of whey protein gels during dynamic in vitro gastric digestion

Breakdown of solid foods during gastric digestion plays a major role in the release and absorption of nutrients in the gastrointestinal tract. The breakdown mechanisms of foods during gastric digestion may be influenced by composition, particle geometry, and the resulting moisture uptake and gastric emptying. The extent of breakdown may have implications on the pH, pepsin activity, and subsequent protein hydrolysis. This study aims to identify the influence of particle geometry on pH, buffering capacity, and breakdown mechanisms during in vitro dynamic gastric digestion. Whey protein gels made in different geometries (small, medium, and large cubes with side lengths of 3.1, 5.2, and 10.3 mm, respectively, and spheres with a diameter of 6.5 mm) were subjected to gastric digestion using the Human Gastric Simulator (HGS) over a 180 min period. Particle size in the bulk digesta showed the breakdown mechanism of spheres was primarily by erosion, whereas breakdown of cubes was by fragmentation at the beginning of digestion, followed by erosion. Moisture uptake and gastric emptying of dry matter were significantly influenced by digestion time, particle geometry, and their interaction (p < 0.001). Initial buffering capacity of the gels was highest in small cubes and lowest in large cubes, causing the pH to decrease faster in large cubes. There was a higher pepsin activity in the liquid phase of the digesta in large cubes compared to the rest of the treatments, which was hypothesized to be due to a diffusion limitation of pepsin, resulting in less diffusion into large cubes due to their lower total specific surface area. Further work is needed to develop quantitative connections between food initial properties, breakdown mechanisms, and their implications on pH, pepsin activity, and nutrient digestibility for future food design.