Interfacial rheology and stability of air bubbles stabilized by mixtures of hydrophobin and β-casein

Abstract

Class II hydrophobin (HFBII) is a highly surface active molecule and in the context of aeration can be considered to be an air 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 formulations is very difficult. Although HFBII has proven 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. The surface rheology: surface shear viscosity (ηs) and surface dilatational elasticity (ε), for HFBII and β-casein mixes at various ratios is described in this paper and how this relates to bubble stability. The addition of β-casein up to a certain ratio seems to increase ηs significantly, whilst ε is less affected. This is accompanied by improved stability of air bubbles to coalescence and allows the formation of very small air bubbles that remain extremely stable to disproportionation. Overall, there is the suggestion of some kind of synergy between the two proteins. The exact nature of this interaction is unknown. Measurements of the ζ-potential of the proteins suggest that electrostatic interactions are probably not important at the pH investigated (pH 7). Confocal microscopy of individual bubbles over prolonged periods of time, stabilized by HFBII and a fluorescently labeled β-casein, suggests that the enhanced stability is due to highly unusual and complicated interfacial packing phenomena plus local bubble curvature effects that require further investigation.