Effect of Ca²⁺ on Properties and Structure of Heat-Induced and Cold-Induced Pea Protein Gels

In order to improve the gel properties of pea protein isolate (PPI), heat-induced and cold-induced gels were prepared using PPI solution with added Ca²⁺. The effect of Ca²⁺ concentration on the gel strength, water-holding capacity and dynamic rheology of heat-induced and cold-induced gels was investigated. Results showed that the gel strength, water-holding capacity, and viscoelasticity of gels increased first and then decreased with increasing Ca²⁺ concentration. Both gels exhibited the maximum strength (58.82 and 44.30 g) and water-holding capacity (97.71% and 99.62%) at Ca²⁺ concentrations of 15 and 5 mmol/L, respectively. Furthermore, the effect of Ca²⁺ concentration on the intermolecular forces, zeta potential, surface hydrophobicity, free sulfhydryl group content, secondary structure, and microstructure of the gels was investigated. As Ca²⁺ concentration increased, the hydrophobic interactions and the contents of disulfide bonds and hydrogen bonds in the gels increased first, peaking at 15, 15 and 5 mmol/L, respectively, and then decreased. The electrostatic interactions increased gradually. Heat-induced gels showed higher electrostatic interactions, hydrophobic interactions, disulfide bonds, and absolute zeta potential value but lower hydrogen bonds, surface hydrophobicity, and free sulfhydryl content than did cold-induced gels. The β-sheet contents in heat-induced and cold-induced gels added with 15 mmol/L Ca²⁺ were 41.74% and 41.51%, respectively, higher than that observed without Ca²⁺ (31.77%). Scanning electron microscopic (SEM) images showed that the network of heat-induced gels was uniform and ordered, while that of cold-induced gels was dense. In conclusion, Ca²⁺ could enhance the intermolecular force of pea protein and improve the gel properties, being the most effective when used at a concentration of 15 mmol/L. It resulted in higher strength of heat-induced gels and better water-holding of cold-induced gels. The findings of this study provide a theoretical basis for the application of Ca²⁺ in heat-induced and cold-induced gels.