Preparation and optimization of soy (Katul cultivar) protein isolate cold-set gels induced by CaCl₂ and transglutaminase

Response surface methodology (RSM) was employed to analyze the effects of three independent variables included of protein isolate dispersions (8%-12% w/v), CaCl₂ (0-0.60 M), and TGase transglutaminase (10-50 U g^-1), to optimize syneresis, turbidity, stiffness, and loss tangent of gels made from soy protein isolate (SPI). It was found that the physical properties of the gels are mainly dependent on processing conditions. With increasing CaCl₂, transglutaminase (TGase), and protein concentrations, gel stiffness and turbidity increased. The effect of protein content on turbidity was more intense than that of CaCl₂ and TGase concentrations. With increasing TGase concentration, gel syneresis decreased. Numerical optimization determined the optimum preparation of soy protein isolate cold-set gels conditions based on the highest stiffness of gels, and the lowest syneresis, turbidity, and loss tangent as being protein content of 10.10%, CaCl₂ of 0.6 M, and TGase concentration of 46 U g^-1. At this optimum point, stiffness, turbidity, syneresis, and loss tangent were found to be 218.85 (g), 0.76, 0.34 (%), and 0.2 (%), respectively. The microstructural properties of the gels were evaluated by cryo-SEM and it was found that the three-dimensional structures of the SPI gel were denser in the presence of both TGase and CaCl₂, compared to those made from the SPI gel with TGase or salt alone.