Assessing the interaction of particle size distribution and moisture content on the mechanical behaviour of artificial gastric food boluses

The mechanical disintegration of food boluses in the stomach is a key determinant of nutrient release, satiety, and postprandial health outcomes. However, most digestion models oversimplify boluses as fluids, overlooking their initial behaviour as cohesive soft solids. In this study, we investigated how intrinsic bolus properties—namely particle size distribution and moisture content—govern the deformation and breakdown of artificial gastric boluses derived from food products. Using uniaxial compression and shear rheometry at physiologically relevant strain rates, we show that bolus mechanics are strongly hydration-dependent and exhibit the characteristics of pressure-sensitive viscoplastic soft solids. Under lower hydration, finer particles enhanced shear resistance but had no effect on compressive strength, whereas under higher hydration, they increased compressive strength without altering shear behaviour, revealing distinct deformation mechanisms across hydration regimes. Across all conditions, boluses exhibited pronounced rate dependence in their stress–strain response but rate-independent fracture strains. Their constitutive behaviour was captured by two distinct models, a von Mises formulation for the less hydrated boluses and an exponential Drucker–Prager law for the high moisture content boluses. These findings provide a mechanistic link between bolus structure and breakdown, and suggest appropriate constitutive models that are essential for simulating gastric digestion.