Comparative effect of thermal treatment on the physicochemical properties of whey and egg white protein foams

The optimization of the functionalities of commercial protein ingredients still constitutes a key objective of the food industry. Our aim was therefore to compare the effect of thermal treatments applied in typical industrial conditions on the foaming properties of whey protein isolate (WPI) and egg white proteins (EWP): EWP was pasteurized in dry state from 1 to 5 days and from 60 °C to 80 °C, while WPI was heat-treated between 80 °C and 100 °C under dynamic conditions using a tubular heat exchanger. Typical protein concentrations of the food industry were also used, 2% (w/v) WPI and 10% (w/v) EWP at pH 7, which provided solutions of similar viscosity. Consequently, WPI exhibited a higher foamability than EWP. For WPI, heat treatment induced a slight decrease of overrun when temperature was above 90 °C, i.e. when aggregation reduced too considerably the amount of monomers that played the key role on foam formation; conversely, it increased foamability for EWP due to the lower aggregation degree resulting from dry heating compared to heat-treated WPI solutions. As expected, thermal treatments improved significantly the stability of WPI and EWP foams, but stability always passed through a maximum as a function of the intensity of heat treatment. In both cases, optimum conditions for foam stability that did not impair foamability corresponded to about 20% soluble protein aggregates. A key discrepancy was finally that the dry heat treatment of EWP provided softer foams, despite more rigid than the WPI-based foams, whereas dynamically heat-treated WPI gave firmer foams than native proteins.