Chemistry Reference
In-Depth Information
The authors used a simplified squeeze flow to predict clearance times
as a function of viscosity and estimated shear rates as high as 15 000
per second. This has lead to the development of thin film rheology
and tribology in order to evaluate behaviour of structured fluids at high
shear rates (Malone
et al.
, 2003; Davies and Stokes, 2008). This type
of technique will be discussed in more detail in the later parts of this
chapter.
9.3.3
Interactions with saliva
Saliva is a complex biological fluid providing, among other things, lubri-
cation and protection of the oral surfaces. It is mainly composed of water
(99.5%), proteins (0.3%) and inorganic and trace substances (Humphrey
and Williamson, 2001). Over 1050 proteins have been identified in
saliva, including glycoproteins and peptides. The majority of saliva
(90%) is secreted from three pairs of glands, the paired parotid gland,
the sublingual gland and the ubmandibular gland (Aps and Martens,
2005). The remaining 10% is secreted from minor glands such as the
gingival crevicular sulci, Ebner's glands and buccal mucosae. The rate
of saliva secretion ranges from 0.3 to 7 mL/min, depending on the
stimulus type, individual, etc. Characterisation of saliva is extremely
challenging as it is biochemically unstable and can contain relatively
large particles, such as epithelial cells. Saliva has the characteristics of
a weak gel (Glantz, 1997) and has a strong time-dependent viscoelastic
behaviour, which in turn depends, among other parameters, on the type
of stimulation (Stokes and Davies, 2007). The lubrication characteristics
of saliva and its molecular structure have been an area of active research
(Bongaerts
et al
., 2007b; Sajewicz, 2009). Although some of the unique
properties of saliva are attributed to the high molecular weight of mucin
(Strous and Dekker, 1992), it has been also shown that the performance
of a simple mucin solution is different to that of saliva itself (Raynal
et al
., 2002).
Part of the structural changes that emulsions undergo during oral
processing are a direct result of mixing with saliva; for example, floc-
culation of emulsion droplets within time scales relevant to eating have
been observed
in vitro
and
in vivo
(Silletti
et al
., 2007; van Aken
et al
.,
2007; van Vliet
et al
., 2009). Overall, these structural changes are con-
sidered to be a result of depletion flocculation, electrostatic and van der
Waals interactions. Surface charge appears to have a strong effect on the
mechanism of flocculation in emulsions under oral processing (Silletti
et al
., 2007). For highly negatively charged o/w emulsions, stabilised by
sodium dodecyl sulphate (SDS), no flocculation was observed upon mix-
ing with saliva. For weakly negatively charged emulsions (charged with
β-lactoglubulin or Tween 20), reversible flocculation was observed,
