Mechanisms involved in the formation and structure of soya protein cold-set gels: A molecular and supramolecular investigation

Molecular structure, microstructure, rheological properties and the fractal dimension (Df) of soy protein isolate cold-set gels made with 10 or 20 mM CaCl2 were investigated. Particle size and Fourier transform-infrared (FT-IR) spectroscopic analysis revealed that native globular proteins are denatured and aggregated molecules are produced by preheating, which constitutes the first step of the gelling process. Similar FT-IR spectra were obtained after incorporation of CaCl2, suggesting that aggregates formed during preheating constitute the structural base of the gel. TEM and SEM showed that gels formed at 10 mM CaCl2 were composed of homogeneous fine filamentous structures while gels formed at 20 mM CaCl2 were composed of irregular bead-like fused masses. Rheological analyses revealed that the higher CaCl2 concentration resulted in an increase in the storage modulus (G′ value). Df values determined from rheological data using scaling models of Wu and Morbidelli, were lower (2.00) for 10 mM calcium gel than for 20 mM calcium gel (2.64). Based on the molecular and supramolecular comparison of the two gel types, a schematic representation of the mechanism of soy protein isolate cold-gelation was proposed.