Impact of a thermal treatment at different pH on the adsorption behaviour of untreated and enzyme-modified egg yolk at the oil-water interface.

Adsorption properties of egg yolk proteins were investigated with or without enzymatic modification with phospholipase A(2) (PLA(2)) and after a thermal treatment at pH 4 and 6.5. Heat treatment was assessed from 60 to 90 degrees C resulting in a wide range of protein denaturation degrees, which were correlated with the interfacial protein load of emulsion droplets. It was shown that denatured proteins, i.e. protein aggregates are able to adsorb at the oil-water interface and that, most notably, granule constituents play an important role in stabilisation of the oil-water interface at pH 4. Moreover, it was shown that after a thermal treatment of untreated egg yolk (not enzyme treated) an even higher protein load is reached than when PLA(2)-treated egg yolk is used for emulsification. Heat-treated natural egg yolk is therefore supposed to be an appropriate alternative for PLA(2)-treated egg yolk for producing stable emulsions. The SDS-Page profiles show that the composition of the interfacial film is highly influenced by a modification with PLA(2) at pH 4 while at pH 6.5 the interfacial composition of adsorbed proteins is less influenced.