(and an equilibrium water distribution has been reached, which may take several
days), we also know that the water content of the lactose in the powder is the same as
it is in the lactose–water system. In other words, it may still becWthat is
determinant (as is assumed by theorists).
Question 2
Assume that you want to develop a crispy breakfast cereal in the form of flakes in
which the main component is wheat starch. To start with, you use the simplest model
system of native starch and water. How would you proceed, and in particular what
should be the course of water content and temperature to obtain a product with the
desired physical properties? Tip: Also consult Section 6.6.2.
Answer
A glance at Figure 16.5 shows that to obtain a crispy product, i.e., one for whichTg
is well above room temperature, thecWvalue should be below about 0.17. In order
to obtain a coherent dough (and also to obtain a product that is well digestible), the
starch should be gelatinized. To that end, you should add water to acWvalue of
about 0.5 and heat to above the gelatinization temperature (about 70 8 C). Then the
mass should be cooled down to a lower temperature, but aboveTg, to obtain a dough
consistency that allows making flakelike shapes. The flakes are then heated to
evaporate water, taking care that the temperature remains betweenTgandTm
(why?), until the desired water content is obtained. Then cool to room temperature.
16.2 THE SPECIAL GLASS TRANSITION
State Diagrams. As mentioned, a glass transition point can be
reached in a solution byfreeze concentration. An example is given in Figure
16.6, a state diagram of the sucrose–water system. What will happen when a
solution is slowly cooled has been discussed in relation to Figures 15.14 and
- Assuming equilibrium, ice starts to crystallize upon reaching the phase
boundary curveTf. The solution becomes concentrated due to further
freezing, and its composition followsTf. When the eutectic point E is
reached, sucrose should start to crystallize. However, considerable
supersaturation can occur if cooling proceeds quickly. The system then
further followsTf, which now is not an equilibrium curve. At some stage,
the line for homogeneous nucleation of sucrose will be crossed, but the
viscosity of the system may by then be high enough virtually to prevent
nucleation. Nevertheless, freezing of water goes on. (As mentioned, water