Chemistry Reference
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indicating a depletion flocculation mechanism (Silletti
et al
., 2007).
Flocculation was also observed for Na-caseinate emulsions in mix-
ing with both saliva as well as a model mucin (pig gastric mucin). A
sharply defined mucin concentration was required, while the observed
aggregates were reversible upon dilution, an indication of a depletion
flocculation mechanism (Vingerhoeds
et al
., 2005). The concentration
of model mucin required to induce flocculation was much higher (0.4
wt%) than the average concentration in saliva (0.02 wt%). The floccula-
tion phenomena that have been observed upon mixing with saliva could
also be attributed to the presence of large aggregates in salivary mucin
(Wickstrom
et al
., 2000; Soares
et al.
, 2004). Upon mixing of emul-
sions (stabilised by Tween 20, WPI- and β-lg) with saliva from certain
individuals, irreversible flocculation was observed, implying that other
flocculation mechanisms such as bridging might be of importance.
Besides the effect of the saliva-induced flocculation on the structure
of protein-stabilised emulsions, it has been postulated that these floccu-
lation events also have an effect on the emulsions' sensory attributes. As
flocculation is a result of mixing with saliva, it will be more pronounced
close to the oral surfaces, where saliva concentrations are higher. This
would lead to a locally higher viscosity that, in turn, could be respon-
sible for the smooth, velvety perception characterising these emulsions
(van Vliet
et al.
, 2009).
9.3.4
Interaction with oral surfaces
Although mouthfeel and a number of sensory attributes have been related
to emulsion bulk rheological properties, interactions with oral surfaces
might also have a profound effect (Kokini, 1987; van Aken
et al
., 2007).
One of the reasons for this is that all the oral processes that determine
mouthfeel, such as temperature changes, mixing and interaction with
saliva, will be more pronounced close to the oral surfaces. Efforts have
been devoted to studying interactions of food formulations with oral
surfaces. Watson
et al
. (2002) and Adams
et al
. (2007) have developed
a fluorescent-based endoscopy technique to study the retention of food
materials
in vivo
. Results have demonstrated that increasing the viscosity
of CMC solutions resulted in an increase in the amount retained in the
mouth after swallowing. Salivary flow rate was also found to influence
the time at which material was consecutively removed.
One of the important attributes of multiphase (fat-containing) food
products is that they cover oral surfaces, resulting in a thin, fat-based
film. The existence of this thin film is believed to be responsible for part
of the sensory characteristics attributed to many emulsion-based sys-
tems. Dresselhuis
et al
. (2008c) have studied the mechanisms by which
emulsions stabilised with WPI reduce friction, and the resultant effect of
