16
Glass Transitions and Freezing
Many foods of low water content are wholly or partly in a glassy (vitreous)
state. This is of great importance for the mechanical properties and the
physical and chemical stability of the food. A glassy state can also form in
foods of high water content when the food is frozen, causing removal of
liquid water by freeze concentration. Moreover, freezing can cause other
changes that affect properties and stability. These phenomena are the
subject of this chapter.
Some basic aspects are found in Chapters 5 (diffusion and rheology), 8
(water relations), 14 (nucleation of crystals), and 15 (crystallization of water
and phase diagrams).
16.1 THE GLASSY STATE
16.1.1 Fundamentals
Definition. Aglassis anamorphous solidshowing aglass transition.
Here a solid is defined as a material having an apparentviscosity(at the time
scale involved) larger than a specified value, often 10^12 Pa?s, i.e., 10^15 times
that of water. A solid can be crystalline or amorphous. An amorphous
material does not show aregular periodicityin atom or molecule density, as
illustrated in Figure 16.1. This can be established by x-ray diffraction: the