Chemistry Reference
In-Depth Information
Boundary
regime
Mixed
regime
Hydrodynamic
regime
Entrainment speed
Fig. 9.3
Schematic of a Stribeck curve showing the different lubrication regimes.
un-stabilised emulsions, tribological experiments under constant speeds
should be carried out instead, as these would allow determination of
the behaviour of these systems under 'mouth-like' conditions.
In vitro
measurements, such as tribology, can also prove extremely
useful for the evaluation of fat-replacement approaches in food products
(via an emulsion route), where the main concern is that the consumer
might perceive the reduction in oil content as a reduction in the overall
'fattiness' of the product. Work carried out by Malone
et al
. (2003)
investigated the tribological behaviour and sensory perception of sim-
ple oil-in-water emulsions of varying fat content and compared these to
pure oil. The obtained Stribeck curves showed differences between sam-
ples of varying oil content. Below 15% oil content, the curves obtained
overlapped that of pure water, thus showing a water-continuous contact
under all stages of lubrication. The behaviour of systems containing
between 15 and 30% oil was only found to, more or less, overlap that of
pure oil in the boundary and mixed regimes and to follow that of pure
water in the hydrodynamic regime. Finally for the emulsions containing
55% of oil, the obtained behaviour matches that of pure oil throughout
the three lubrication regimes.
The sensory part of this work was equally interesting. Malone
et al
.
(2003) showed that samples with oil content between 15 and 30% were
difficult to be distinguished and received similar scores, which is in
agreement with the fact that the friction results for these systems overlap
within the boundary and mixed regimes. This further indicates that it is
these lubrication regions that reflect the lubrication as it occurs between
the tongue and palate during oral processing/assessment. The work also
showed that the 55% oil-containing emulsion received a higher score
