Tribology as a Tool to Study Emulsion Behaviour in the Mouth - Food Colloids: Self-Assembly and Material Science - page 458

31.5 In-Mouth Emulsion Behaviour and Perception
The observed differences in coalescence behaviour (Figure 4) and in measured
friction force show that surface characteristics are of importance in low-
pressure processes. The intriguing question now is: what are the implications
of these results for using tribology as a predictive tool for sensory perception of
emulsions? We need to investigate whether a difference in coalescence in the
mouth can influence the perception. To this purpose, emulsions varying in their
sensitivity to coalescence were engineered and then sensorially evaluated.
In our study 12 we have prepared oil-in-water food emulsions, stabilized by
1 wt% WPI (Davisco) and containing 10 wt% sunflower oil (Cargill, Amster-
dam) with either small droplets (d 32 B 2.5 mm).

The larger droplets were expected to be more sensitive to coalescence due to the
lower Laplace pressure. A second group of emulsions was engineered with solid
fat as the fat phase instead of the liquid sunflower oil. Solid fat in emulsion
droplets is partly crystalline, causing those droplets to be more sensitive to
partial coalescence. With increasing droplet size, there is an increase in the
probability that fat crystals are able to penetrate the interfacial layer stabilizing
the emulsion droplet. 20 Furthermore, as mentioned before, the Laplace pres-
sure stabilizing the droplet is lower for larger droplets. A sensory experimental
panel, trained specifically on liquid emulsions, evaluated the emulsions in three
separate sessions. The panel consisted of eight panellists, who evaluated the
emulsions on odour, taste, mouth-feel, and after-feel attributes. 12
The emulsions containing sunflower oil, and differing solely in particle size,
showed no significant difference in the perception of creaminess, thickness, and
coating after-feel. These attributes are also suggested to relate to friction. When
using solid fat as the dispersed phase we did find an effect of the droplet size on
the perception of these attributes. 12 Panellists rated emulsions with larger
droplets containing solid fat as more creamy, and as giving a more coating
after-feel. These results suggest that the expected difference in the occurrence of
partial coalescence between smaller and larger droplets containing solid fat is
indeed sensorial evaluated as more creamy.
As already mentioned in Section 31.2, viscosity has been suggested to relate
to the perception of creaminess. Figure 5 shows that the viscosity of our
samples was roughly the same for all droplet sizes, which assures us that the
differences in the perception of creaminess between the samples was not due in
this case to any differences in viscosity. CLSM observations of the sheared
emulsions did not show conclusive evidence, however, that samples having
initially larger droplets were more vulnerable to coalescence than the smaller
droplets. This is due to the fact that the droplets enlarged by coalescence cream
out of the focal plane of the microscope. 19
Figure 6 shows the friction force data of the emulsions sheared between pig's
tongue and glass. The differences in friction forces between the samples are
small, and not in all cases significant, but they do show a trend. (The variability
between the different tongue samples introduces an error, which is almost equal
in magnitude to the difference in friction force between the samples.) The

1.5 mm) or large droplets (d 32 B