Influence of added β-casein on air-water surfaces, bubbles and foams stabilized by Hydrophobin

Class II hydrophobin (HFBII) is a highly surface active molecule and in the context of aeration can be considered to be an structuring protein conferring exceptional stability to foams for periods far in excess of that obtained with any other commonly used protein. This is of interest to the food industry, since producing shelf stable foams in food formulation is very difficult. Although HFBII has proved to be very promising in terms of foam stability when used alone, it is still unknown whether HFBII will be able to maintain its functionality when other surface active agents are present, such as in real food systems.

In this project, rheological properties were investigated in for mixitures of HFBII and β-casein at various ratios and how this can relate to bubble and foam stability. The addition of β-casein up to a certain ratio increased the surface shear viscosity significantly whilst the dilatational elasticity was less affected. This was accompanied by an increase in stability for air bubbles and foams as they showed more resistant towards coalescence and disproportionation and allowed the formation of extremely stable smaller sized air bubbles. Confocal laser microscopy images of these bubbles stabilised, suggested that the enhanced stability is due to highly unusual and complicated interfacial packing phenomena plus local bubble curvature effects that requires further investigation. Overall, there is a suggestion of some kind of synergy between the two proteins. However, the exact nature of this interaction is unknown. ζ- potential measurements, the effect of heat and Brewster angle microscopy indicated that the type of interaction is not electrostatic or hydrophobic but more likely to be of a weak nature and is possibly due to the way the proteins pack together at the interface since on expansion of the interfacial films completely disintegrated.