Modification of soy protein isolates using combined pre-heat treatment and controlled enzymatic hydrolysis for improving foaming properties

The foaming properties of soy protein hydrolysates modified with a combination of pre-heat treatment and controlled enzymatic hydrolysis by pepsin were investigated. Degree of hydrolysis, soluble protein percentage, peptide profile, molar mass distribution, surface properties at air-water interface and interfacial rheological properties of hydrolysates were measured to gain insights into the relationships between molecular composition and foaming properties. It was found that pre-heat treatment of soy protein at 55 °C for 30 min promoted the hydrolysis of hydrophilic acidic subunits and increased the ratio of 7S/11S. As a result, it resulted in the best interfacial properties and foam capacity as well as stability amongst all treatments. Although pre-heat treatment above 65 °C before soy protein isolate hydrolysis increased degree of hydrolysis, higher proportion of large protein aggregates in the hydrolysates with increasing temperature deteriorated their foaming capacity and stability due to the lower storage modulus (G′) of the adsorbed layers and less surface excess concentration (Γ) compared to the hydrolysate with pre-heat treatment at 55 °C. Without pre-heat treatment, unfolded soy protein did not provide sufficient accessible sites for pepsin hydrolysis. As a result, foam capacity and stability of non-preheat treatment hydrolysate were inferior to those of 55 °C pre-heat treatment hydrolysate. This study provided the foundation for the application of soy protein isolate in aerated food products with desirable foaming properties.