16.4 RECAPITULATION
The Glassy State. A glass is an amorphous solid that shows a glass
transition upon heating. A crystalline material melts upon heating, taking up
heat at the melting temperature. A glass changes into a highly viscous liquid or
into a rubbery material upon heating; this also occurs at a pretty sharp glass
transition temperatureTg, where the specific heat of the material suddenly
increases. A glass is not in an equilibrium state, and the value ofTgdepends
somewhat on temperature history. Tg is always below the melting
temperature, often by 100 K or more. It is very difficult to transform a pure
liquid into a glass. Some mixtures, however, particularly those containing
biopolymers (starch, protein), can readily form a glass; polymers may be
partly crystalline, partly glassy. Examples of glassy foods are high-boiled
sweets, dried pasta, hard biscuits, some breakfast cereals, and dried skim milk.
The lower the water content of a food, the higher its glass transition
temperature. This is because water acts as a plasticizer. As long as the
temperature is belowTg, a dried food tends to be brittle and crisp. One
function of the glassy state then is to provide crispness. Crispness can
suddenly disappear on a temperature increase of a few degrees or a mere 1 to
2 %increase in water content.
The other important function of the glassy state is to provide stability
to physical and chemical changes. The viscosity of a glassy material is
extremely high, e.g., 10^14 Pa?s. This effectively stops all molecular motion
and hence all change. However, in a glass of mixed composition, molecules
that are smaller than those of the component(s) responsible for the glassy
state can still diffuse, although greatly hindered. Hence complete chemical
stability is usually not reached, but physical changes, like crystallization, are
generally impossible below the glass transition temperature.
A glass can be made by drying a liquid or semisolid food, including
such methods as baking, extrusion, and freeze-drying.
The Special Glass Transition. Drying can also be achieved by
freeze concentration. When a solution is cooled, water can crystallize,
thereby increasing the solute concentration in the remaining liquid. If
freezing is fast, crystallization of solutes often fails to occur, and the dry
matter content of the solution becomes so high as to reach the glassy state.
This may be called the special glass transition, which occurs in a maximally
freeze-concentrated system. It is characterized by a temperature denotedTg^0
and a mass fraction unfrozen waterc^0 W. This water does not freeze because
its diffusivity is virtually zero, not necessarily because it is ‘‘bound.’’ The
values ofTg^0 , and to a lesser extent ofc^0 W, depend to some extent on the
freezing rate and the initial water content of the system.