The role of metal ions in emulsion characteristics and flocculation behaviour of phosvitin-stabilised emulsions

Phosvitin (Pvt) is an egg yolk and a natural metal-chelating protein presenting excellent emulsion-stabilising properties. During emulsification process, proteins are able to adsorb at the oil droplet interface decreasing the interfacial tension and, at the same time, stabilise the colloidal system by the formation of an interfacial film. Even if Pvt is a good stabilising agent, it could be susceptible to the presence of metal ions usually found in food systems. In this work, we have measured the emulsifying capacity of Pvt in the presence of Mg and Fe ions. The influence of the latter was evaluated before and after the emulsion preparation. Pvt-stabilised emulsions, which naturally hold Mg, were highly flocculated at pH 6 and 7, as confirmed by rheology measurements. Flocculation was partially reversed by subsequent EDTA addition, and was totally reversed when EDTA was added before emulsion preparation. The addition of Fe ions to Pvt emulsions flocculates the system at pH values between 4 and 7, and especially at pH 6 and 7. EDTA (4.8 mM) was able to partially de-flocculate the emulsion. Similarly, SDS alone was not adequate to completely disaggregate oil droplet flocs, and only the presence of SDS and EDTA at the same time successfully de-flocculated the system. When Fe was added before emulsion formation, the Fe–Pvt complexes presented less emulsifying capacity than the crude preparation, and they exhibited a reduced flocculation compared with emulsions where Fe ions were added after the emulsion formation.