Surface Topography of Heat-Set Whey Protein Gels: Effects of Added Salt and Xanthan Gum - Food Colloids: Self-Assembly and Material Science - page 479

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Figure 4 Surface height profiles (in z-direction) determined from CLSM images as
function of position in x-direction for heat-set whey protein gels. (a) No added
salt; (b) 200 mM NaCl

Figure 5 shows (a) the speed dependence and (b) the load dependence of the
surface friction for whey protein gels (14 wt% protein, 200 mMNaCl) containing
different amounts of xanthan gum (0, 0.05 and 0.25 wt%). It is noticeable that
the addition of the polysaccharide causes a lowering of surface friction over the
whole range of sliding speed. The friction force for the gel containing 0.25 wt%
xanthan has the highest speed-dependence over the tested speed range; this can
probably be attributed to the additional contribution of adsorption/desorption
of xanthan chains at the surface. However, the gel containing no xanthan
becomes highly speed-dependent at high sliding speeds (> 1 mm s 1 ), indicating
probably the great importance of hydrodynamic flow at the surface under these
conditions. The load-dependence of the surface friction becomes reduced in the
presence of xanthan. These data suggest that the xanthan has a significant
'surface smoothing' effect on the heat-set whey protein gels.
The surface smoothing effect of the added hydrocolloid is also clearly evident
from the 3-D surface images obtained by CLSM observation. Figure 6 shows
protein-stained surface profiles for gels containing (a) no added xanthan,
(b) 0.05 wt% xanthan, and (c) 0.25 wt% xanthan. It should be noted that
these images have the same length scales for the x- and y-coordinates, but
slightly different magnifications in the z-direction. The surface of the gel
containing no xanthan appears to be the most rough, with high peaks and
deep valleys spread out over the whole surface. In contrast, the surface for the
gel containing 0.25 wt% xanthan appears much smoother. The calculated
values of the average surface roughness, R q , are (a) 3.8, (b) 3.0, and (c) 1.5 mm.
This estimated surface roughness is based on the surface profile of just the
protein aggregates, i.e., with the distribution of the polysaccharide not included
in the calculation. Even so, it is obvious that the presence of the xanthan causes
a dampening effect on protein aggregate morphology at the surface, thereby
leading to a smoother surface.