Effect of subcritical water treatment on the structure and foaming properties of egg white protein

The effect of conventional heat treatment and subcritical water (SW) treatment on the structure and foaming properties of egg white protein (EWP) were investigated. Compared with unheated EWP, SW treatment enhanced the foaming capacity by 3-fold and induced the transition from α-helix structure (from 29.8% to 13.6%) to β-sheet structure (from 18.2% to 26.2%) and random coil structures (from 39.7% to 46.3%). Meanwhile, heating at 90 and 120 °C both remarkably increased the surface hydrophobicity and aggregation of EWP. The particle size of EWP (unheated, heated at 90 and 120 °C) was 104.37 nm, 438.5 nm and 311.6 nm, respectively. Combined with the results of sodium dodecyl sulfate-polyacrylamide gel (SDS-PAGE) electrophoresis, we speculated that SW treatment could hydrolyze protein aggregates formed at 90 °C into small molecular weight aggregation. Scanning electron microscopy revealed that SW treatment could induce the surface of EWP rougher, looser and more fragile. The viscosity and surface tension proved that SW treatment could improve the flexibility of EWP. These were all beneficial to the protein adsorption at the air-water interface and enhancement of foaming capacity. It might be of great significance for us to re-recognize the influence of high temperature on the foaming properties of egg white protein.