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et al
., 2005; Akhtar
et al
., 2006), but also by its lubricating properties.
This is consistent with the observations from a study by Lillford (2000),
in which it is indicated that, in the mouth, food is subject to not only
a range of shear rates, but also to extensional flow; for more theory on
this matter, the reader is directed to Steffe (1992).
On the other hand, astringency seems to be related to an increase in
the friction coefficient (Malone
et al
., 2003a; de Hoog
et al
., 2006). In
recent work (Bongaerts
et al
., 2007; Stokes
et al
., 2008), the injection
of a typically astringent compound, epigallocatechin gallate (ECGC),
a polyphenol extracted from green tea, was used while measuring the
friction coefficient of saliva and was found to increase the friction co-
efficient from around 0.1 to 20. Under similar conditions, addition of
water to the system also increased the friction coefficient, but to a lesser
extent. The main difference rising from the introduction of ECGC is
the rate at which the friction coefficient increases. Such data seems to
confirm the original work from Malone
et al
. (2003a), in which astrin-
gency is related to the flocculation of the material in contact with the
oral mucosa.
In conclusion, the various attempts made to formulate low-fat prod-
ucts by only matching their viscosity (to that of the full-fat equiva-
lent) have failed because of the lack of understanding of oral process-
ing, as well as the microstructure of the product. Indeed, it appears
that, as suggested by Lillford (2000),
'texture is a consequence of the
microstructure'
.
In terms of rheological response, the mouth is therefore very interest-
ing (Chen, 2009). Here processes that are involved are a combination of
the following: fracture as the material is broken, mixing with saliva, the
viscosity effects that are obtained as the material breaks down and is di-
luted, and thin-film behaviour as the mouth works the material. Each of
these is an interesting process and forms the basis of significant amounts
of past and present research in foods (Aguilera and Lillford, 2007). The
most recent addition to the toolbox of measurement techniques is tri-
bology, which can be used to estimate,
in vitro
, the in-mouth lubricating
properties of a food, allowing the food technologist to partially predict
the performance of a designed formulation, which can be correlated
with consumer response. It is now becoming a routine technique and as
such is a very interesting measurement for the design and product devel-
opment of everyday foods. However, the surfaces need to be adapted to
more closely mimic the bio-surfaces of the mouth. This requirement has
resulted in two different adaptations of the technique: the first is to have
soft rubber surfaces in a pin or ball on plate technique (Malone
et al
.,
2003a) and the other is the use of pig tongues (Dresselhuis
et al
., 2008).
Malone
et al
. (2003a) carried out the original piece of work in this
field, where they studied a range of emulsions with different fat contents.
