Digestion behaviour of protein-rich soft foods produced using milk protein, soy protein, and their blends under simulated elderly gastrointestinal tract

The digestibility and protein quality of soft-textured foods are fundamental to ensuring adequate protein intake and overall nutritional status in older individuals. This study investigated the digestion behaviour of custard-type soft foods formulated with milk protein concentrate (MPC), soy protein isolate (SPI), and their blends, prepared with or without transglutaminase. A dynamic human stomach–intestine model designed to closely mimic digestion in the elderly, was used to conduct in vitro digestion tests. Gastric emptying rate, particle size, zeta potential, elastic modulus, protein hydrolysis, free amino acid profile, and in vitro Digestible Indispensable Amino Acid Score (DIAAS) were measured to assess protein quality and digestibility. Custards based on MPC showed superior digestibility, with smaller particle sizes, fewer aggregates, and greater protein hydrolysis after 180 min of digestion. In contrast, SPI-based custards exhibited larger particles, higher aggregation, and lower digestibility. A faster gastric emptying rate was observed in the MPC-based custards, corresponding to a shorter half-disintegration time (t 1 / 2 ) (46.08 min) compared to SPI (50.29 min) and MPC–SPI blends (56.57 min). Micro-texture profile analysis revealed that gastric digestion led to increased firmness and elastic modulus in all samples due to protein coagulation prior to transition into the intestinal phase. Threonine was identified as the limiting amino acid across all samples based on amino acid profiling and in vitro DIAAS calculations. These findings highlight the nuanced considerations required when selecting protein sources and underscore the importance of assessing digestion kinetics and amino acid bioavailability in the development of protein-rich soft foods for older individuals, particularly those with swallowing difficulties. • Milk protein, soy protein, and blended custards were studied for elderly digestion. • Milk protein custard showed smaller particles, higher stability, and better digestibility. • Soy protein custard exhibited larger aggregates, lower gastric empty rate and digestibility. • Gastric digestion caused short term increased of hardness across all formulations. • In vitro DIAAS and amino acid profiling identified threonine as the limiting amino acid.