Chemistry Reference
In-Depth Information
requires that the whole food is heated to at least 121
◦
C for 15 minutes.
For solid and canned foods, this is often performed in static or agitat-
ing retorts. Rapid heating of liquids can be performed in a plate heat
exchanger. Often referred to as Ultra High Temperature (UHT) sterili-
sation processes, they involve heating the liquid to as high as 150
◦
C, in
some cases for 1-2 seconds, and then rapidly cooling. Pasteurisation is a
much less severe heat treatment typically carried out at temperatures be-
low the boiling point of water. For example, batch pasteurisation of milk
requires heating to 62.8
◦
C (145
◦
F) for 30 minutes. High temperature-
short time pasteurisation (HTST) is also commonly used in which the
milk is heated to 71.7
◦
C (161
◦
F) for at least 15 seconds.
It is essential to understand the effect of thermal treatments in terms
of the conformational transition of the xanthan gum, as this can have an
effect on the final solution rheology. In general when there is:
no conformational transition during heat treatment:
◦
low shear viscosity is maintained (see Fig. 5.17a);
◦
degradation at low pH is minimised during heating;
synergy with galactomannans is maximised (see Fig. 5.8).
a conformational transition during heat treatment:
◦
◦
low shear viscosity is reduced (see Fig. 5.17b);
◦
degredation at low pH is a greater risk during heating;
◦
synergy with galactomannans is reduced (see Fig. 5.8).
To maximise the stability of the xanthan gum and minimise any effects
of thermal treatment on its rheology, it is recommended that any salts
present in the formulation are added to the xanthan gum solution prior
to the thermal treatment. As discussed earlier, the conformational tran-
sition of xanthan gum is dependent upon the concentration of salts, so
whether or not the xanthan solution will pass through the conforma-
tional transition will be dependent on the ionic strength of the system
and the temperature of the treatment. Fig. 5.6 can be used as a guide. It
is to be noted that in the vast majority of applications, there are sufficient
salts present, in the form of sodium, potassium and/or calcium, to raise
the transition temperature of the xanthan gum above that of the typical
processing temperatures. This means that the xanthan gum solution vis-
cosity is very stable during heat treatments and can provide suspension
at high temperatures.
5.5.2
Homogenisation
Xanthan gum solutions are very stable during high shear mixing. Xan-
than gum solution viscosity is unchanged after homogenisation in equip-
ments such as colloid mills.
