Chemistry Reference
In-Depth Information
seemed to have no effect on the range of the tested samples. Similarly
for smoothness, Kokini (1987) correlated the thin film rheology and
friction of foods to creaminess and thickness (viscosity and large-scale
rheology).
Physical measurements on the structure of emulsions can be greatly
affected in the case of a time-dependent behaviour. The stability of an
emulsion is an important factor in categorising its behaviour. Measure-
ments of interfacial rheology in emulsions stabilised by proteins have
been shown to relate to droplet stability, to creaming and coalescence
phenomena, but also to the actual texture of the emulsion (Murray and
Dickinson, 1996). Links to actual perception of bulk rheological prop-
erties are not yet established. However, work in understanding these
interfaces could lead to a better understanding of emulsion behaviour
and even sensory characteristics (Moore
et al
., 1998; Murray, 2002).
Recent work in emulsions has related fat perception, in the form
of slipperiness or mouth lubrication, to properties measured using
thin-film rheology and tribology, since bulk rheology failed to give re-
liable predictions for the overall mouthfeel of these systems. Tribology
measurements consist of measuring the friction between two surfaces
(with either one or both of these in motions) separated by a thin film
of lubricating material. Commonly in food research, soft surfaces such
as Poly-di-methyl-siloxane (PDMS) or silicone are used to give com-
parable 'oral-like' surfaces for friction measurement (de Vicente
et al
.,
2006; Bongaerts
et al
., 2007a). The most frequently used configuration,
during tribology experiments, consists of a rotating sphere loaded at an
angle onto a rotating disc, creating a small contact point between the
surfaces. Traditionally, lubrication properties are represented using a
Stribeck curve, where the measured friction (or traction) coefficient data
are plotted for a range of entrainment speeds of a lubricant (Fig. 9.3).
The Stribeck curve can be divided into three main sections, which
correspond to different lubrication regimes and also to films (as formed
between the two surfaces) of different thicknesses. At low entrainment
speeds, the system is within the boundary regime, where very little or
no lubricant is entrained by the rotating surfaces and so the measured
(high) friction is mainly due to surface-to-surface contacts. As speed is
increased, the system enters the mixed regime, where now a thin film of
material is present between the surfaces, partly separating them, and as a
result, the measured friction is significantly reduced. Further increases in
speed will finally move the system into the elasto-hydrodynamic regime,
where the surfaces become completely separated from each other
and the bulk (rheological) properties of the lubricant now determine
the measured friction (Czichos, 1978). The behaviour, schematically
represented in Fig. 9.3, is only exhibited by materials where there is
no physical change taking place. For time-dependant materials, such as
