The viscosity and emulsifying properties of egg yolk submitted to a heat treatment at 68 degrees C for up to 11 min were investigated in a standard mayonnaise recipe. Heating of the egg yolk prior to emulsification was found to lead to a reduction of the average oil droplet size in mayonnaise of up to 40% compared to non-heated egg yolk. This led to an increased viscosity and greater pseudoplasticity of the mayonnaise prepared with heated egg yolk. In order to investigate the effect of egg yolk heating on the rheological properties of the mayonnaise independently from the oil droplet size, emulsions having various median oil droplet diameters were prepared with non-heated and heated egg yolk. This work shows that heating of egg yolk prior to emulsification significantly impacts the rheological properties of mayonnaise when the median oil droplet diameter exceeds 5 mu m. The presence of thermally unfolded and interacting proteins in the aqueous lamella of continuous phase surrounding each oil droplet of the mayonnaise may explain the impact of heating on the rheological properties of the product. The increased thixotropy observed in mayonnaise prepared with heated egg yolk may reflect the formation of a network of oil droplets connected by weak bonds formed between proteins adsorbed at the interface and within the continuous phase. (c) 2005 Elsevier Ltd. All rights reserved.