Tribology as a Tool to Study Emulsion Behaviour in the Mouth - Food Colloids: Self-Assembly and Material Science - page 454

The amount of friction between two sliding surfaces is predominantly
determined by the nature of the sliding surfaces, the load applied, and the
speed of shearing. So, for example, the friction forces are considered to be
determined by the roughness, the wetting characteristics, and the deformability
of the two sliding surfaces. Furthermore, the ability of a surface to interact, by
chemical or physical means, with the opposing surface and with the lubricant
confined between the surfaces will also influence the friction force. In order to
reduce the friction between a pair of sliding surfaces, a lubricant should be able
to wet the surfaces, preferably forming a thick viscous layer on each of the
surfaces. When a viscous layer with a thickness exceeding the mean asperity
height is present, a hydrodynamic pressure can build up within this viscous
layer at high sliding speeds (hydrodynamic lubrication) resulting in low friction
forces. In the case of oral processing, it is likely that the oral mucosa is 'mixed
lubricated', implying that both the viscosity of the food and the surface
characteristics determine the in-mouth friction.
In their experiments designed to relate sensory perception of emulsions
to tribological data, Malone et al. 14 used oil-in-water emulsions of similar
viscosity but varying in fat content. The emulsion droplets were stabilized by a
polyglycerol ester. Tribological measurements on the emulsions were con-
ducted with a steel ball on a silicone rubber disk. Malone and co-workers 14
concluded that the measured difference in frictional properties could be corre-
lated to the objective perception of fat. De Wijk and Prinz 15 used a home built
set-up consisting of a rotating rubber band and a metal cylinder to test the
lubricational behaviour of various fat-containing products. Like Malone et al.,
they concluded 15 that friction is closely associated with fat-related texture

attributes.
Lee and co-workers 13 used tribological measurements to predict the differ-
ence in perception among chocolate samples varying in particle size, compo-
sition, and processing. In the 'pin-on-disk' set-up used to measure the
lubricational behaviour of the various chocolate samples, Lee et al. 13 used
different pins and disks in order to study the effect of deformation of the
surface on lubricational behaviour. In analogy with findings from high-pressure
tribological measurements, the elastic modulus of the surfaces was also found
to be crucial for measurements at relative low pressure. 13
The various experiments described above were all designed to predict the
in-mouth sensation of an emulsion. Yet, all of the experiments were conducted
with artificial surfaces. The question then remains: if measurements are per-
formed with surfaces so different from the oral mucosa, can they really mimic
in-mouth friction and thereby predict sensory perception?

31.4 Importance of Surface Characteristics
Depending on whether it is liquid, solid, or semi-solid, the food goes through
several stages before being swallowed and digested. Emulsions are considered
to be semi-solid food systems. There are three phenomena that are thought to