In silico investigation of hydrodynamics of low viscosity liquids for INFOGEST gastric phase

The INFOGEST network has proposed standardised semi-dynamic in vitro models by expanding upon the INFOGEST static model protocol to incorporate the kinetics of the gastric phase. This includes gradual acidification, secretion of fluids and enzymes, gastric emptying, and stirring. Continuous stirring is expected to disperse added gastric fluids and enzymes for digestion while engaging the fluid in motion. How effectively advective mixing produced by the INFOGEST stirrer represents incomplete mixing of gastric fluid with food substrates in vivo is not yet understood. We present the first in silico prediction of hydrodynamics around the INFOGEST stirrer based on the Smoothed Particle Hydrodynamics (SPH) numerical method. This is used to investigate digesta circulation and dispersive transport of injected gastric fluid and enzymes across a range of viscosities that represent different stages of gastric digestion for a low viscosity Newtonian liquid food following a standardised semi-dynamic protocol. Three sets of conditions were considered corresponding to the start, one hour, and two hours into gastric digestion. Flow behaviour reveals strong azimuthal swirl, complex radial and axial recirculation and mixing. Transport is affected by viscosity and fill level. Higher viscosities (0.05 Pa.s) during the initial stage were found to hinder radial dispersion of gastric fluid and enzymes, potentially slowing early digestion. Fluid beneath the stirrer remains stagnant throughout all digestion stages, indicating denser digesta that settle to the bottom may remain undigested. The model predictions for gastric digesta mixing using the INFOGEST stirrer give encouraging evidence that, at least for simple liquid foods, the stirrer design can produce the extent of spatial variation in digesta composition and hydrolysis rates required by the semi-dynamic protocol and similar to the incomplete mixing observed in vivo.