Cold-set whey protein–flaxseed gum gels induced by mono or divalent salt addition

Mixed cold-set whey protein isolate (WPI)–flaxseed gum (FG) gels, induced by the addition of CaCl2 or NaCl at fixed ionic strength (150 mM), were evaluated with respect to their mechanical properties, water-holding capacity (WHC) and SEM microscopy. They were prepared by mixing FG and thermally denatured (90 °C/30 min) WPI solutions at room temperature, but the gels were formed at 10 °C using two methods of salt incorporation: diffusion through dialysis membranes and direct addition. The mixed systems formed using dialysis membranes showed phase separation with the development of two (axial) layers, and the CaCl2-induced gels presented radial phase separation. In general the CaCl2-induced gels were less discontinuous, stronger, and showing lower WHC and deformability than the NaCl-induced gels. An increase in the FG concentration reduced the gel strength and WHC for both systems, which was associated with a prevailing phase separation between the biopolymers over the gelation process. Using direct salt addition, apparently none of the mixed gels showed macroscopic phase separation, but the NaCl-induced gels showed much higher hardness and elasticity than the CaCl2-induced gels. Since the gelation process occurred more quickly by direct salt addition, and more effectively for the divalent salts, the more fragile structure of the CaCl2-induced gels was a consequence of disruption of the cross-link interactions of the aggregates during the agitation used to homogenize the salt added.