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(Question 2: Starch Based Fat Substitutes)
Glass Transition Temperature
Melting is a first-order phase transition where a
crystalline material changes from a solid to a liquid. Some substances
freeze into an amorphous solid called a glass. In this state, they are
solid in the sense that they do not flow readily. Rather, the molecules
are randomly distributed as if they were in the liquid state. When a glass
is warmed it softens and eventually becomes fluid. This is not a first
order transition and therefore occurs over a range of temperatures called
the 'glass transition temperature'. Below are four graphs which demonstrate
how the coefficient of thermal expansion (a), volume (V), heat capacity
(Cp) and enthalpy (H) of a sucrose mixture are effect by temperature. Note
that the characteristic of glass transition is the rapid onset of softening.
Figure 1:
Figure 2 depicts the change in the elastic modulus with temperature
at the glass phase transition. The elastic modulus is a measure of the
stiffness of the material. Note the logarithmic scale of the graph.
Figure 2: Viscosity Curves of amorphous phases of frozen sugar, acid
water.
The final graph estimates the viscosity curves of amorphous phases in
frozen sucrose-, glucose-, fructose-, and citric acid water systems.
Figure 3:
A mixture of ice crystals and an amorphous matrix is a two phase frozen
system. Can you think of any foods which may have this system?
What are the components of the amorphous phase? Answer