Effects of pH and aging on the texture and physicochemical properties of extruded pea protein isolate

Abstract

Pea protein isolate (PPI) matrices were extruded at a water content of 50%w/w with the pH of the hydrated PPI adjusted to values between 5 and 9. The impact of the pH on the microstructure, mechanical and swelling of the extrudates was investigated by scanning electron microscopy (SEM), texture analysis and water imbibition experiments. We find that the hardness of the extruded PPI matrices as determined by texture profile analysis (TPA) decreases by ∼50% when the pH is increased from 5 to 9, while the water holding capacity (WHC) is observed to increase linearly from 126% ± 6% w/w on dry basis at pH = 5–442% ± 10% w/w on dry basis at pH = 9. The hardness of the extruded matrices at their WHC is observed to linearly decrease with the WHC. This is explained by a gradual loosening of the matrix as the balance of intermolecular forces shifts from hydrophobic to electrostatic with increasing pH. The extruded PPI matrices are swelling in an anisotropic manner when exposed to pH-adjusted water, with the longitudinal swelling strain being significantly larger than the transversal swelling strain, in particular towards lower values of the pH. Significant aging of the extruded matrices was observed for the first two days following extrusion. Owing to aging, the hardness of the matrices was observed to increase by between 18% (pH = 9) and 23% (pH = 7). Thus, the pH as well as aging are two novel parameters that can be exploited in optimizing the mechanical properties and water holding capacity of extruded PPI matrices, for instance for use in meat analogues.