Particle degradation and nutrient bioavailability of soybean milk during in vitro digestion

This work studies the structural breakdown and nutrient release during in vitro digestion of soybean milk reconstituted from soybean flour with different particle sizes (> 100 mesh, 50–100 mesh, 28–50 mesh, 14–28 mesh and < 14 mesh). A clear effect of the particle size is noticed on the functional characteristics of the colloidal food during the gastric and intestinal digestion stages. In vitro gastric digestion significantly improves the release of polyphenols (21.4 ± 1.1–35.6 ± 3.2%, compared with the undigested one). In that stage, the antioxidant capacity increases between 20.8–38.5%, and is related to the particle size, hence free surface area, of soybean flour. Confocal microscopy shows that the proteins released from the soybean flour particles aggregate during gastric digestion, while the lipid droplets decompose down into smaller colloidal entities. Examination of the effect of particle size on the release of phenolic compounds, small molecule proteins/peptides, and fatty acids, showed that the highest amounts of the above substances existed when the particle size of soybean flour lies in the region of 50–100 mesh. This particle size also shows the highest bioaccessibility of the contained bioactive substances (polyphenols, 124.5 ± 0.5%; flavonoids, 49.1 ± 1.5%). These findings add insight on the role of the colloidal properties of the soybean flour with different particle sizes on the functionality of soybean milk.