Chemistry Reference
In-Depth Information
are entrained into the gap. As they are reasonably soft, they are dis-
torted by the applied normal force and so, in effect, their presence is
a narrowing of the gap, i.e. they allow less flow of the liquid through
the gap. This results in the friction remaining constant for a significant
range of rotation speeds (in this example from approximately 10 to 35
mm/s). As the speed is increased further, the force of fluid and fluid gel
entering the gap causes the ball-and-plate gap to widen and the friction
decreases. On decreasing the rotational speed of the plate, gel particles
are being entrained at all stages so the friction is lower than the up
ramp when they were excluded. At the higher polymer concentration,
this hysteresis is not observed and the up and down ramps overlay each
other. It would seem that this is due to the modulus of the gel particle: at
an applied normal force of 4 N, the particles are not distorted, so when
they are entrained, they immediately lead to the lowering of the friction.
This finding has been shown to be reproducible and, as such, is very
interesting as it raises questions about what happens within the mouth
and whether soft particles will still have lubrication properties.
So how is this used to replace fat in a product? We turn our attention
again to mayonnaise. If the majority of the oil droplets in a mayonnaise
are replaced by soft elastic spherical gel particles, then the overall
bulk rheology of the mayonnaise can be matched (as in Fig. 10.15). In
this figure, we have shown a flow curve for a full-fat mayonnaise and
compared it with a reduced-fat (3% compared to 80% in the full-fat
version) emulsion in which the oil droplets have been replaced by
sheared agar gel (5%), assembled as a particulate gel. This combination
400
Sheared agar
Mayonnaise
300
200
100
0
0
50
100
150
200
250
300
Shear rate (per second)
Fig. 10.15
Stress/shear rate curve for a 5% sheared agar (
◦
) with 3% phase volume of
oil droplets (droplet size
∼
1
µ
m) compared to a full-fat Hellman's mayonnaise (
)usinga
roughened cone-and-plate geometry on a Rheometrics instrument.
