Rheological characteristics and physicochemical stability of dressing-type emulsions made of oil bodies–egg yolk blends

Two oil body creams, differing in oil volume fraction and surface protein composition, were obtained by applying alkaline aqueous extraction to comminuted maize germ and then recovering the oil bodies from the extract by centrifugation, either in the presence of 0.5 M sucrose (OB-W) or following isoelectric precipitation at pH 5.0 (OB-A). Oil bodies in the former cream are stabilised by natural oil body surface proteins while in the OB-A cream, exogenous proteins, in addition to natural oil body proteins, are also present. The creams were blended with appropriate amounts of water, NaCl and liquid yolk, and the pH was adjusted to 3.8 to obtain 20% or 45% (w/w) in oil OB-W and 20% (w/w) in oil OB-A model dressing-type emulsions. The physical stability of the emulsions, against creaming and coalescence, was monitored upon storage, while the development of emulsion structure during ageing was probed by applying steady shear and small deformation oscillatory rheometry. The adsorbed to oil bodies’ surface proteins were analysed by applying SDS–PAGE. Since no yolk protein constituents were detected at the oil body surface layer of the emulsions, it is hypothesised that the presence of unadsorbed yolk particles in the emulsion continuous phase results in the intensification of interdroplet interaction effects, due to depletion events, and may therefore have an indirect but nevertheless strong influence on emulsion structure and physical stability.