Tribology as a Tool to Study Emulsion Behaviour in the Mouth - Food Colloids: Self-Assembly and Material Science - page 455

relate to perception taking place in the mouth upon oral processing of an
emulsion: 17 (i) the interaction between emulsion components and/or saliva,
(ii) the interaction between emulsion components and the mucous layer cover-
ing the tongue, and (iii) the rubbing and squeezing of the emulsion against the
palate. This rubbing of the product against the palate generates a friction force
which can be sensed by mucosal mechanoreceptors. 18 As surface characteristics
are of great relevance in high-pressure processing, it is likely also that they are
of importance in the low-pressure in-mouth processing of food. In our work we
have characterized a pig's tongue surface and have compared it with poly-
dimethylsiloxane (PDMS), a surface often used as a mouth-mimicking surface.
Pig's tongue was chosen as the surface mimicking human tongue since humans
consume roughly the same type of diet as pigs. 19 Based on observations made
during preparation of the pig's tongue, we expect that there is no longer a
mucous layer present on the pig's tongue samples used in our measurements. 19
We have characterized the surfaces with respect to roughness, deformability,
and wetting characteristics.
The roughness of the pig's tongue was determined using stereomicroscopy
and confocal laser scanning microscopy (CLSM). The filiform papillae, giving
the tongue its roughness, are on average 300 times longer than the average
asperity height on the PDMS surface we used (see Figure 2). The roughness of
the PDMS surface is likely to be similar to the roughness of the steel, PTFE,
Zirconia, and rubber surfaces used by Malone, de Wijk and Lee. 13-15 The mean
asperity height on PDMS was around 5 nm, allowing hydrodynamic lubrica-
tion by a food product to occur. This suggests that, based on solely the
difference in the observed roughness between tongue and PDMS, the friction
forces in the mouth are of larger magnitude than those measured in a tribo-
meter with, say, rubber and steel as the tribopair.
As well as roughness, the elastic moduli of the two sliding surfaces are of
importance in friction force measurements. Friction depends on the contact
pressure, which is related to the reduced elastic modulus of the two sliding
surfaces. In our work the elastic modulus of PDMS, a relatively soft rubber, is
being compared with pig's tongue tissue. We have shown 19 that the elastic
modulus of PDMS is two orders of magnitude larger than that of pig's tongue.
These results indicate that most of the oral simulating tribological experiments
using rubber surfaces are conducted at a contact pressure that cannot be
reached in the mouth.

Figure 2 Illustration of macroscopic roughness of pig's tongue: (A) stereomicroscopy

image, scale bar ¼ 1000 m m; (B) stereomicroscopy image, scale bar ¼ 200 m m;
and (C) 3-D CLSM image, 300 500 m m 2