Impacts of citric acid concentration and pH value on mechanism and rheological properties of cold-set whey protein fibrils hydrogels

Whey protein fibrils (WFs) gelation is dependent on protein conformation which can be affected by citric acid (CA) and pH, but the related mechanism is unclear. CA concentrations (0–400 mmol/L) were applied to induce WFs gelation at acidic (pH 2), neutral (pH 7) and alkaline (pH 10) conditions, to investigate how CA concentration and pH value co-affect the mechanism and rheological properties of cold-set WFs hydrogels. CA promoted hydrophobic interactions of WFs molecules. At acidic condition, CA facilitated forming hydrogen bonds within WFs hydrogels in a level-dependent manner, high CA concentration was beneficial for gelation. At non-acidic condition, CA induced covalent cross-linking of WFs molecules by amide bonds and disulfide bonds, resulting in higher modulus of neutral WFs hydrogels. However, it also increases their sensitivity to deformation by reducing WF molecular flexibility. Excessive cross-linking induced by high CA concentration (400 mmol/L) was detrimental to gels formed at non-acidic condition. Despite the differences in gelation mechanism, the three types of WFs hydrogels exhibited similar physical changes and mouthfeel during oral processing (friction coefficient of 0.2–0.35). This study provided new insights into the gelation mechanism of CA-mediated cold-set WFs hydrogels and into designing an easy swallowing soft food gel based on WFs.