Surface structure smoothing effect of polysaccharide on a heat-set protein particle gel

This work investigates surface properties of a protein particle gel and effects of polysaccharide on the surface microstructure of such a protein gel. Whey protein isolate (WPI) was used as the primary gelling agent, and a polysaccharide (xanthan) was investigated for its surface smoothing effects. The surface properties of heat-set WPI gels with and without the presence of xanthan (0, 0.05, and 0.25%) were characterized using a surface friction technique. The surface friction force of a gel against a stainless steel substrate was found to be highly dependent on the sliding speed for all three gel samples, and the addition of xanthan caused a general reduction of surface friction. The gel containing no xanthan has the largest surface friction and behaved in the most load-dependent manner, whereas the gel containing 0.25% xanthan has the lowest surface friction and showed the least load dependency. It was inferred that the WPI gel containing no xanthan has the roughest surface among the three samples and the presence of xanthan leads to a smoother surface with probably a thinner layer of surface water. Surface features derived from surface friction tests were confirmed by surface microstructure observation from confocal laser scanning microscopy (CLSM) and environmental electron scanning microscopy (ESEM). Surface profiles from CLSM images were used to quantify the surface roughness of these gels. The mean square root surface roughness R(q) was calculated to be 3.8 +/- 0.2, 3.0 +/- 0.2, and 1.5 +/- 0.2 microm for gels containing 0, 0.05, and 0.25% xanthan, respectively. The dual excitation images of protein and xanthan from CLSM observation and images from ESEM observation indicate a xanthan-rich layer at the surfaces of the xanthan-containing gel samples. We speculate that the creation of the outer surface of a particle gel is based on a different particle aggregation mechanism from that leading to network formation in the bulk.