Rheological behaviour of spray-dried egg yolk/xanthan gum aqueous dispersions

This paper provides information on the microstructure of, and reports particle size distributions and rheological results for, aqueous dispersions of spray-dried egg yolk and xanthan gum prepared on a laboratory scale using two types of homogenisers. Laser light scattering results demonstrated that higher energy input during homogenisation yielded a dispersion with a lower average particle size and a wider polydispersity, slightly influencing the linear dynamic viscoelastic functions due to the low concentration of egg yolk particles. These dispersions exhibited weak gel properties at the composition studied. The mechanical spectrum and the corresponding relaxation spectrum were dominated by the xanthan/gum-water matrix which controls the structure of the continuous phase. This fact explained the lack of any wall depletion effects. Several controlled-rate and controlled-stress rotational rheometers and a capillary rheometer were used to obtain information on flow properties. The shear rate dependence of steady state viscosity was determined through twelve decades, and was fitted using the Carreau equation. The kinetics of structural recovery after steady-state shear was studied by start-up at the inception of shear and flow interrupted experiments under controlled shear history. The results were analysed in terms of the ratio of a time-dependent amount of overshoot to the amount of overshoot of the original sample, using the addition of two first order equations. Additionally, combined steady state flow properties at fixed shear stress/oscillatory shear experiments were also used. The increase of the storage modulus with time, checking a linear viscoelastic response, tracked the structural recovery after steady shear. Laser light scattering of sheared samples helped gain a better understanding of the role of egg yolk particles on the rheology of these dispersions. Received: 6 February 2000 Accepted: 5 September 2000