Many foods show the special glass transition upon freezing. For most
of these,Tg^0 ranges between10 and 408 C, whereasc^0 Wis generally about
0.2. Frozen storage at a temperature belowTg^0 then greatly enhances
stability. Physical changes do not occur anymore, and most chemical
changes become negligible; some reactions, e.g., lipid oxidation, may
proceed very slowly. It is often difficult to determine the special glass
transition point with accuracy.
Freezing of Foods. Freezing of foods has many ramifications. The
decrease in temperature causes virtually all chemical reactions to decrease in
rate. Globular proteins tend to unfold at very low temperatures, e.g.,
causing enzymes to become inactive. Moderate freeze-concentrating may
increase reaction rates, because the concentration of reactants, or that of a
catalyst, increases. Also the ionic strength increases, if salts are present,
which can cause salting out of proteins and possibly irreversible changes
(e.g., in muscle tissue). At deeper freezing, the viscosity of the remaining
solution greatly increases, slowing down all reactions, and most of them
stop belowTg^0.
Freezing of tissues (vegetable or animal) generally causes some
damage. At temperatures above 108 C, ice crystals only form outside the
cells. This causes freeze concentration of the extracellular liquid, hence to an
osmotic pressure difference between intra- and extracellular liquid and
hence to osmotic dehydration of the cells (plasmolysis). At lower
temperatures, ice crystals tend to penetrate the cells, whereby intracellular
crystallization occurs. This reduces plasmolysis but tends to increase
mechanical damage, possibly leading to a soft texture of the tissue after
thawing.
Cryoprotectants are additives that can reduce adverse effects of
freezing. One of these can be a high ionic strength. Water activity and
osmotic pressure depend on temperature only, as soon as ice has formed.
For instance, at 238 C,aW¼0.80, independent of food composition. By
adding nonionic solutes, such as sugars, the extent of freeze concentration at
a given temperature is thus reduced, and thereby the ionic strength. This
reduces damage to proteins. The smaller the molar mass of the solute, the
more effective a given weight of solute is.
Solutes can also decrease mechanical damage, since less ice is formed
at any given subfreezing temperature. Freezing causes a volume increase,
which can cause local pressure differences and hence mechanical damage.
Less ice and smaller crystals tend to give less damage. Substances that
reduce crystal size, like some biopolymers (that give a strong gel in the
freeze-concentrated solution) and antifreeze peptides, also reduce damage.
Moreover, large crystals may give an undesirable texture.