Impact of a thermal treatment on the emulsifying properties of egg yolk. Part 2: Effect of the environmental conditions

The protein solubility and emulsifying properties of native and heat-treated egg yolk (EY) suspensions were investigated in various environmental conditions. Four distinct conditions were tested by combining two levels of pH, namely pH 4.0 and 6.5, and two levels of ionic strength, namely 0.15 and 0.52 M NaCl, in a model oil-in-water (O/W) emulsion containing 30% oil (v/v). Although the protein solubility was greatly reduced by the thermal denaturation in all tested environmental conditions, the average size of oil droplets obtained in emulsions made with heated EY was observed to be either similar or slightly smaller than that obtained with native EY, depending on the environmental conditions. Using heat-treated EY rather than native EY led to a significant increase of the interfacial protein concentration in all environmental conditions. This increased interfacial protein concentration was shown to have a major impact on the flocculation behaviour of the emulsions, as well as on their rheological properties and stability to creaming. Hypotheses regarding the mechanisms by which insoluble protein aggregates stabilise O/W emulsions at various pH and ionic strengths are discussed.