Surface Topography of Heat-Set Whey Protein Gels: Effects of Added Salt and Xanthan Gum - Food Colloids: Self-Assembly and Material Science - page 476

1000

(a)

100

0.01 0.1 1 10

Speed (mm/s)

2000

(b)

1500

1000

500

0

0 500 1000 1500 2000

Load (Pa)

Figure 2 Surface friction force as a function of (a) sliding speed and (b) surface load for
heat-set gels: J , no added salt; D , 200 mM NaCl

the surface friction of the WPI gel much less speed-dependent but much more load-
dependent. The surface friction of the protein gel containing no added salt was
found to increase by almost 10 times as the sliding speed increased from 0.01 to 10
mm s 1 , while the friction force of the gel containing 200 mM NaCl gave a much
smaller increase over the same range of sliding speeds. The friction forces for both
gels show a linear dependence on the surface load. However, the slope for the salt-
containing gel is 2 2 times higher than that for the protein gel containing no salt.
Classical friction theory interprets the surface friction as the work required to
overcome the energy barrier caused by surface asperities. The magnitude of the
energy barrier depends on the roughness of the surface and on the load.
Therefore, the friction force for every surface has a characteristic relationship
with the surface load. The rougher the surface, the greater is the load depend-
ence. The results in Figure 2(b) suggest a smooth surface for the heat-set WPI
gel without salt, but a relatively rough surface for the gel containing 200 mM