Toensure stability, it is often desirable to keep the frozen product at a
temperature belowTg^0. Approximate values are
Fruits and fruit juices 408 C
Starchy vegetables (maize, potato) 128 C
Green vegetables 15 to 258 C
Muscle (fish, red meat) 128 C
Ice cream 328 C
However, these values vary with product source and also with pretreatment
and freezing rate. In heterogeneous foods,Tg^0 may vary with location.
Freezing Damage. As discussed in Section 15.3.1, the freezing of
water causes anincrease in volume. Consequently, ice crystals formed in a
disperse system can cause locally increased pressures, which can in turn
cause mechanical damage. This occurs, for instance, when freezing an oil-in-
water emulsion, especially if freezing is slow, the volume fraction of oil is
large, and the oil droplets are not very small. At the prevailing low
temperature, part of the oil will generally crystallize, which then means that
partial coalescence can occur when fat globules are pressed together (Section
13.5). This becomes manifest upon thawing of the emulsion, when clumps of
fat globules appear that will melt to form large oil droplets at higher
temperature.
Innatural foods, i.e., in animal or vegetable tissues, the situation is
more complicated. If undercooling is at most by a few degrees, which
generally means slow freezing, ice crystals are nearly always formed outside
the cells. This means that the extracellular liquid will be freeze-concentrated,
whereby its osmotic pressure increases. That will causeplasmolysis, i.e., the
osmotic dehydration of the cell. The outer cell membrane or plasmalemma is
permeable to water, but not, or poorly, to most solutes. Hence water will
leave the cells, which will shrink considerably: see Figure 16.13. The
intracellular liquid becomes highly concentrated, although the ice formation
occurs outside the cell. This causes all the changes due to freeze-
concentration, and it can possibly lead to damage of the cell membrane.
It appears that most cells do not contain internal surfaces that act as
catalytic impurities for ice nucleation, until the temperature is as low as a
few degrees above the homogeneous nucleation temperature (about 408 C).
In tissues, however, ice crystals are observed inside cells at temperatures
below 10 to 158 C. This is presumably due to the penetration of
extracellular crystals through the plasmalemma into the cell. This will