Cold-Set Gelation of Soybean and Amaranth Proteins by Hydration of Freeze-Dried Protein Previously Denatured in the Presence of Calcium

The gelation of soybean and amaranth proteins through a three-step-strategy: heat-induced denaturation at low protein content (2 or 4 wt%) in the presence of calcium (0.075–0.250 mmol Ca/g protein) and at pH 7.0, followed by freeze drying, and rehydration at higher protein content (10 or 13 wt%) was evaluated for mixtures 80:20 (soybean:amaranth) and for soybean proteins alone. Gelation was favored by high protein contents during denaturation and rehydration, and by a Ca²⁺:protein ratio of 0.100 mmol Ca/g protein. Gels were soft (hardness from texture profile analysis was 0.26 N) and self-supporting and exhibited excellent water-holding capacity (99% upon centrifugation at 20,000xg). The aggregates formed during denaturation were weakly associated upon rehydration and were mostly extractable with water, which partially explained the softness of gels. The appropriate Ca²⁺:protein ratio would lead to a particular distribution of Ca²⁺ between free in solution and bound to proteins, which in turn balanced associations and repulsions allowing gelation. The presence of 20% amaranth proteins led to a more brownish color, a higher adhesiveness and a lower cohesiveness (texture), lower storage modulus, apparent viscosity, consistency index, and area of hysteresis (rheology) when compared to gels containing only soybean proteins. The mechanical differences suggest that soybean proteins dominated the three-dimensional matrix while amaranth proteins were less engaged and acted as a filler.