The influence of the anionic surfactant sodium dodecyl sulphate (SDS) on the small-deformation shear rheological behaviour of heat-set beta-lactoglobulin emulsion gels was investigated. Emulsion gels containing surfactant added after homogenization (5-8 wt% beta-lactoglobulin, 38 wt% n-tetradecane, pH 7) were prepared by in situ heat treatment (30 min at 90 degrees C). Storage and loss moduli (frequency 1 Hz) were determined at 30 degrees C. Emulsion gel strength was found to be sensitively dependent on the total protein content and, especially, the surfactant/protein molar ratio R. For low surfactant additions (0 < R less than or equal to 2), the gel strength tends slightly to decrease. At higher SDS concentrations (2 < R less than or equal to 4), the elastic modulus increases strongly with R; this probably corresponds to the concentration range at which SDS-induced unfolding of protein begins to take place. There is a maximum gel strength around R approximate to 6. Large additions of SDS (R > 8) inhibit gel formation. Qualitatively similar behaviour is found in beta-lactoglobulin systems containing no emulsion droplets, albeit at a substantially higher protein content. Taken together with results from complementary electrophoretic mobility and surface shear viscosity measurements, the emulsion gel small-deformation rheological behaviour can be explained in terms of various combinations of interfacial and bulk protein-surfactant interactions. These interactions may produce electrostatic protein-protein repulsion at low R, attractive SDS-mediated protein unfolding and cross-linking at intermediate R, and solubilization of protein into discrete non-associating mixed micelles at high R. Comparison with previous results indicates that surfactant effects on the rheology of heat-set protein emulsion gels cannot readily be generalized. The behaviour appears to be sensitive to specific aspects of the surfactant association structure and the protein-surfactant interactions. (C) 1997 Elsevier Science B.V.
