Foams and emulsions of β-casein examined by interfacial rheology

The objective of this research work is the study of foams and emulsions stabilised by a protein, β-casein, and its relation to the dilatational properties of the adsorbed layers at the air–water and the oil–water interface. For this purpose, we have taken the dilatational properties of adsorbed layers at the air–water and tetradecane–water interfaces previously measured in Maldonado-Valderrama et al. [J. Maldonado-Valderrama, V.B. Fainerman, M.J. Gálvez-Ruiz, A. Martín-Rodríguez, M.A. Cabrerizo-Vílchez, R. Miller, J. Phys. Chem. B 109 (2005) 1608–1616]. In order to facilitate the correlation, the interfacial coverage of bubbles and drops was chosen similar to that of the dilatational experiments. The formation and stabilising mechanisms of foams and emulsions obtained in this manner are discussed separately and linked to the dilatational properties of the adsorbed layers. The interfacial viscosity of the protein adsorbed layer appears to be a key factor in the formation of the foam whereas the interfacial elasticity seems to control both the stability of emulsions and foams. Moreover, a detailed comparative analysis of the different stabilising mechanisms and the corresponding dilatational properties is given.