Chemistry Reference
In-Depth Information
For locust bean gum, there is a decrease in salt release when the con-
centration exceeds 0.5%, which, not surprisingly, is consistent with the
change in mixing efficiency shown in Fig. 8.4 and the change in flavour
perception for hydrocolloids once c
*
is exceeded as shown in Fig. 8.2.
An additional factor which may be important is the low viscosity of
gelatin solutions as evidenced by the low value of intrinsic viscosity
compared to that of polysaccharide gelling agents (Wulansari
et al
.,
1998). This may result in more effective distribution of biopolymer
molecules from the gel surface into the bulk solution on melting. This
is a slightly different idea from the mixing of high-viscosity solutions
which the main part of this discussion has addressed.
8.5
BEYOND SHEAR RHEOLOGY
For liquid foods, the shear viscosity is not sufficient to predict flavour
perception in viscous solutions as is shown by the result in Fig. 8.3. It
also does not provide a complete basis for the prediction of mouthfeel.
In this chapter, it has been argued that the ease with which a liquid
mixes with saliva is an important driver for both taste perception and
mouthfeel. It seems, therefore, reasonable to postulate that the resistance
to break up a viscous fluid droplet when in contact with water or saliva
prevents rapid mixing. The hypothesis here is that a liquid-thickened
food comprised of swollen granules suspended basically in water posses
a comparatively lower resistance to break up and therefore mix more
readily in the mouth.
Droplet break-up has been extensively investigated both experimen-
tally and theoretically for immiscible fluids, although data on suspen-
sions are scarce. For homogeneous polymer solutions, the rheological
properties of both immiscible fluids and the interfacial characteristics
control droplet deformation and break up at given flow stresses. The
rheological properties of the polymer solutions are of course imparted
by the molecular conformation and the state of entanglement. Similar
rules should also apply to suspension droplets; however, the boundary
layer inside the droplet adjacent to immiscible continuous fluid phase
is depleted of particles. Hence, the flow stresses act on the suspension
medium, and the behaviour of the total system is not akin to the be-
haviour classically observed for a pair of homogeneous immiscible flu-
ids. This was investigated by following the behaviour of microscopically
small HPMC and physically cross-linked waxy maize starch suspension
droplets using a flow shear cell described by Vervoort and Budtova
(2003).
As can be seen in Fig. 8.9, the pattern of droplet break-up is very
different for these two thickener systems. Under these conditions, the
