The breakdown of food during gastric digestion is an important process that has not been extensively studied. Although the gastric emptying rate and glucose absorption have been measured for many food products, their breakdown processes have yet to be quantified. This knowledge is essential for future optimization of functional food products with specific digestive characteristics, such as increased satiety, optimized energy release, or controlled glucose, nutrient, or drug absorption.;The physical and chemical breakdown of four rice types was quantified over a 2-8 hr postprandial period in the proximal and distal regions of pig stomachs as a model for the adult human. The rate of mixing of both gastric secretions with the meal bolus, as well as the mixing of the solid meal particles was quantified in brown and white rice (Calrose variety) meals over an 8 hr postprandial period, and quantitative mixing indices were calculated to determine mixing rate constants.;It was observed for all four rice types that the protein content of the brown rice chyme increased in the distal stomach over longer digestion times, suggesting an accumulation of bran layers that had been "peeled off" by the antral contraction waves. This data was supported by particle size distributions, which showed larger particles in brown rice gastric chyme in the distal region at longer digestion times, and images showed the presence of the outer bran layer fragments.;Gastric secretions were found more prevalently in the distal stomach, as quantified by a lower pH and higher moisture content to the proximal stomach in all four rice types. This suggests that the gastric secretions, although mainly secreted in the proximal stomach, move to the distal stomach to be mixed by the antral contraction waves, as they can only slowly diffuse into the meal bolus in the proximal stomach.;Rice chyme physical properties varied across the stomach regions, with the distal region showing more particle breakdown (i.e. lower rheological properties, decreased grain firmness/hardness) than the proximal region, supporting the classical viewpoint of stomach functionality. However, mixing was observed between the regions, as the physical properties did not change only in the distal region.;Mixing of the gastric secretions with the rice meal, as well as mixing of the solid meal portions were quantified using a statistically-based mixing index. Mixing rate constants were determined, and it was observed that white rice mixed at a faster rate than brown rice, both solid-liquid mixing and solid-solid mixing. This increased rate of mixing may be due to a larger amount of gastric secretions, faster white rice particle breakdown, absence of dietary fiber to change gastric viscosity, and the amount of particles present in the gastric chyme.;This work has quantified both the physical and chemical changes that occur during gastric digestion in vivo in an attempt to understand the food breakdown process. Additionally, the solid-liquid and solid-solid mixing has been quantified, and rates of mixing have been calculated for brown and white rice.;The information gathered in this study is essential to further our understanding of the food breakdown process during gastric digestion. Once the process and mechanisms of food breakdown are understood, future foods can be designed with specific digestive characteristics, such as increased satiety or controlled glucose release.
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