This study aimed to characterize and determine the disintegration of emulsion gels in a human gastric simulator (HGS) and the physicochemical characteristics of gastric digesta. Using thermal treatment at 90 °C, whey protein emulsion gels with different structures and gel strengths were formed by varying the ionic strength. Simulated boluses of soft (containing 10 mM NaCl) and hard (200 mM NaCl) gels, which had similar particle sizes to those of human subjects, were created for gastric digestion. Soft gels disintegrated faster than hard gels in the HGS. The boluses of both gels gradually disintegrated into particles of size ∼10 μm. With further digestion, the protein matrix of the soft gel particles dissolved, the proteins were disrupted mainly by proteolysis and large quantities of oil droplets were released. In contrast, for the hard gel particles, although all proteins were hydrolysed after 240 min the breakdown of the particles was slow and no oil droplets were released after 300 min. The differences in the breakdown of soft and hard gels in the HGS were attributed to the structures of the emulsion gel, which may result in different sets of peptides in the digestion. In addition, coalescence of the oil droplets was observed only for the soft gel.

A set of whey protein stabilized-emulsion gels with different droplet size distributions (D_4,3 = ∼1, 6 and 12 μm) was produced, and the mechanical properties of the gels in the linear viscoelastic region and at large deformation were measured, along with the physicochemical and structural changes of the gels during oral mastication and gastric digestion. The gels containing 1 μm oil droplets had an aggregated particle structure with proteins coating at oil droplets whereas the gels containing 12 μm oil droplets had a particle-filled structure with spatially continuous matrix. During oral processing, the release of oil droplets from the gels increased as the droplet size increased, with coalescence being seen in gels containing oil droplets of 6 and 12 μm diameter. Under gastric digestion, high degrees of coalescence and phase separation of oil droplets occurred in the gels containing 6 and 12 μm oil droplets because of oil droplet release from the gel matrix; this led to slow gastric emptying. The gels were finally broken down into peptide aggregates and oil droplets (or free oil). The gels, containing 1 μm oil droplets disintegrated into various particles of several to several tens of microns with a low degree of oil droplet release and coalescence. Protein breakdown was slower in these gels, suggesting that the protein structures of the gel matrices were affected by the sizes of the incorporated oil droplets.

The understanding of the disintegration and gastric emptying of foods in the stomach is important for designing functional foods. In this study, a dynamic stomach model (human gastric simulator, HGS) was employed to investigate the disintegration and subsequent emptying of two differently structured whey protein emulsion gels (soft and hard gels).The gels were mechanically ground into fragments to reproduce the particle size distribution of an in vivo gel bolus. The simulated gel bolus was prepared by mixing gel fragments and artificial saliva, and exposed to 5 hours of simulated gastric digestion in the presence and absence of pepsin. Results showed that regardless of pepsin, the soft gel always disintegrated faster than the hard gel. The presence of pepsin significantly accelerated the disintegration of both gels. In particular, it enhanced abrasion of the soft gel into fine particles (<0.425 mm) after 180 min of processing. The emptying of the gels was influenced by the combined effects of the original particle size of the gel boluses and their disintegration kinetics in the HGS. In the presence or absence of pepsin, the larger particles of the soft gel emptied slower than the hard one during the first 120 min of process. However, in the presence of pepsin, the soft gel emptied faster than the hard one after 120 min because of a higher level of disintegration. These findings highlight the role of food structure, bolus properties and biochemical effects on the disintegration and gastric emptying patterns of gels during gastric digestion.