Influence Of Heat Treatment On Structural And Functional Properties Of Soy Isolate Protein

Heat treatment was widely applied in food industry as a protein modification means as well as an inevitable process during production. The effect of heat treatment on structural and functional properties of soy protein isolate (SPI) has been widely studied by many investigators. However, some results were different from each other. So, a further study on this project would be helpful for the optimization of product processing and the proper application of SPI.In this paper, we mainly investigated the influence of heat treatment by different means, temperatures, heating times and protein concentrations on structural characteristics and functional properties of SPI. Three means were chosen for heat treatments, including heat insulation, spray drying and instant super high temperature sterilizer (UHT). The insulation was performed at 80, 90 and 100℃for15, 30, 60, 100 and 180 min respectively. After the effect of protein concentrations on protein aggregation being studied, two protein concentrations, 2% (w/v) and 5%, with different aggregation characteristics were chosen to study the influence of protein concentrations on the heating endurance of SPI. The structural characteristics of heat treated SPI were characterized by denaturation degree, HPLC, dynamic light scattering, SDS-PAGE, surface hydrophobicity, free sulfhydryl groups and carbonyl groups while functional properties were evaluated by solubility, foaming and emulsifying properties.A different influence of heat treatments on the structural characteristics of SPI was observed at different temperatures and protein concentrations. The surface hydrophobicity of SPI was improve after heat treated by different means. The results of HPLC and dynamic light scattering showed that different heat treatments all resulted in the protein aggregation and dissociation. Higher temperature and protein concentration was applied, more remarkable aggregation was observed. The results of GPC and free amino groups determination showed that the dissociation phenomenon of heated SPI was more obvious at low protein concentration than that at high protein concentration. And the results of carbonyl determination suggested that the aggregation could reduce the velocity of protein oxidation.Resulting from the influence on structural characteristics, a diverse effect of heat treatments on the functional properties was observed. When treated at the concentration of 2%, it was found that the functional properties of SPI were changed insignificantly except the decrease of emulsify stability; at 90 and 100℃, the emulsifying properties decreased remarkably while the foaming properties was limited improved. The emulsifying properties and the solubility of SPI were improved significantly when the concentration of protein reached to 5%, and the foaming stability increased while the foaming capacity decreased. Spray drying had a limited effect on protein aggregation, which resulted in the decrease of solubility, as well as the UHT lead a dramatic dissociation of proteins, resulting in the decrease of emulsifying property and the improvement of solubility and foaming property.On the basis of work before, the relationship between functional properties and structural characteristics was studied. Soluble aggregations had an important effect on the functional properties of heat treated SPI. Different functional properties of heat treated SPI was observed mainly because the content and size of aggregates varied when different heating conditions and protein concentration was applied. When treated at low protein concentration, the small size aggregates was formed, resulting in the improvement of foaming capacity. While at high concentration, the content and size of aggregates was increased, the emulsifying property and foaming stability were improved while the foaming capacity decreased.