Surface Topography of Heat-Set Whey Protein Gels: Effects of Added Salt and Xanthan Gum - Food Colloids: Self-Assembly and Material Science - page 475

Load cell

Cotton
string

Stainless
steel disc

Low-friction
pulley Gel

sample

Base

Supporting arm

Figure 1 A schematic diagram showing the experimental arrangement for the surface
friction measurements. The friction force is recorded by the load cell and the
sliding speed and distance are controlled by a stepper motor connected to the
load cell

Protein solutions (14 wt %) were prepared with or without addition of salt
(0.2 M NaCl). The required amount of xanthan was added to the protein
solution using a stock xanthan solution. A certain volume of protein solution
(50 ml of protein solution for the salt effect experiments, and 30 ml of degassed
protein solution for the xanthan effect experiments) was carefully transferred
into an aluminium dish (diameter 81 mm, height 11 mm). Each dish was
covered with a lid to stop moisture loss and was transferred to a 901C
waterbath and left for 1 h in order to produce a firm gel. The level of the dish
was carefully checked using a spirit-levelling device. 5 The xanthan was labelled
covalently with a fluorescent dye (FITC) following the procedure given by
Tromp et al. 10
The experimental arrangement for the surface friction measurements is
shown in Figure 1. The friction force between the gel and a stainless steel disc
was measured as a function of sliding speed of the disc (from 10 mms 1 to
10 mm s 1 ) and as a function of surface load (up to 600 Pa). 5,11 The surface
microstructure was observed using a Leica CLSM (Leica Microsystems,
Germany) and a Philips XL30 environmental scanning electron microscope
(ESEM). The procedures adopted for the microstructural observations and
image analysis can be found elsewhere. 6

33.3 Results and Discussion

33.3.1 Effect of Added Salt
Figure 2 shows the surface friction forces for heat-set WPI gels containing no
salt and 200mM NaCl. Large differences in the surface characteristics of the
two gels are apparent. Figure 2(a) shows surface friction as a function of sliding
speed, and Figure 2(b) shows the surface friction as a function of surface load.
We can see that the presence of added salt during heat-induced gelation makes