Tribology as a Tool to Study Emulsion Behaviour in the Mouth - Food Colloids: Self-Assembly and Material Science - page 453

of Kirkmeyer and Tepper 11 support the notion that flavour and mouth-
feel attributes are highly integrated in the creaminess perception, and
they suggest that humans are incapable of easily separating these sensa-
tions. Our own work 12 also confirms that flavour is of importance for the
perception of creaminess. Our sensorial quantitative descriptive analysis has
shown that applying different fat phases in oil-in-water emulsions resulted in a
difference in creaminess ratings. Creaminess was closely related to the rated
perception of thickness and smoothness, but also to flavour intensity and
creamy flavour. 12 This suggests that both the viscosity and the flavour release
are of importance.
Nevertheless, taking into account the effect of flavour still does not fully
explain the fact that rheological measurements cannot be directly related to
creaminess sensations. The inability to make this link is partially due to the
complexity of oral processing. During this processing, a food product is rubbed
and squeezed between tongue and palate, it is mixed with saliva, and the
temperature of the product rises. In contrast to the simple shear applied in
standard rheological equipment, the food in the mouth is subject to a complex
shear flow pattern, which mainly occurs in the thin film present between tongue
and palate. Several authors have indicated 13-15 that it is not the bulk behaviour
of an emulsion as studied with standard rheology that predicts the creamy
sensation of a product, but the thin film rheology as studied in tribology. In the
present work, we focus on studying the ability of tribology to be a tool in
understanding in-mouth behaviour of emulsions and consequently in under-
standing their sensory perception.
31.3 Tribology and Sensory Science
Tribology is the science of the study of friction, lubrication processes, and wear
phenomena caused by surfaces sliding over one another. Tribology is widely
studied in relation to high-pressure production processes, e.g., in the metals
industry. Here friction and wear should be minimized, implying that the
friction forces must be reduced by means of lubrication of the surfaces. We
can distinguish three lubrication regimes. In the hydrodynamic regime the
lubricating layer is thicker than the maximum height of the surface asperities,
resulting in a complete separation of the two opposing surfaces. In boundary
lubrication the lubricating layer is thinner than the surface roughness, typically
only a few molecular layers thick. A third regime is the mixed regime in which
both boundary lubrication and hydrodynamic lubrication occur. 16
Several authors 13-15 have applied this tribology approach to the low-pressure
oral processing of food. Upon swallowing a food product, the tongue is pressed
against the palate, giving rise to a friction force with the food product acting as
a lubricant. From the combination of sensorial evaluation of food systems and
tribological measurements conducted on these same systems, they have in-
ferred 13-15 an inverse relation between lubricational properties of the system
and the sensorially perceived creaminess and fattiness.