Physical Chemistry of Foods

Mechanical damage depends on the extent ofvolume expansioncaused
by ice formation. To reduce mechanical damage, it is often desirable that ice
crystals remain small; small crystals may also be needed to obtain a
desirable texture and mouth feel. To that end, theantifreeze peptidesbriefly
discussed in Section 15.3.1 can be quite effective in a product like ice cream.
They strongly reduce ice crystal growth at low or moderate undercooling,
but they do not impede nucleation. Consequently, more, and thereby
smaller, crystals will form.
In the manufacture of ice cream, biopolymers (polysaccharides,
gelatin) are often added as ‘‘stabilizers.’’ These can affect the size and
shape of the ice crystals formed, presumably via their effect on agitation
during the freezing process, but also during quiescent crystallization. The
stabilizers do not significantly affect nucleation. Neither do they substan-
tially retard crystal growth when present in solution, since they do not
markedly affect the diffusion rate of small molecules (see Figure 5.14).
However, if they can form a gel, i.e., an elastic polymer network, they may
be able to reduce ice crystal growth rate. Locally, the polymer concentration
can be quite high, because of (a) freeze-concentration; (b) phase separation
between the ‘‘stabilizer’’ and the milk proteins present; and (c) local
concentration near a growing crystal face. That will cause the gel to become
quite strong and have quite small pores. This will eventually prevent ice
growing into the network, by mechanical resistance, causing growth to stop
and the crystals to remain small. Moreover, the gel may hinder mutual
contact, hence sintering, of ice crystals. The effect varies among biopolymers
and increases with increasing biopolymer concentration. Although the
mechanisms involved are not yet fully resolved, the general effect is clear.
(Some workers assume that some polysaccharides can adsorb onto ice
crystals and act like the antifreeze peptides just mentioned.)
In ice cream, as well as in other frozen products, considerable
recrystallization can occur, particularly when the storage temperature is
relatively high and fluctuates. It involves the disappearance of small ice
crystals and the growth of large ones. This suggests that the process is due to
Ostwald ripening (Section 13.6), but it seems to be more complex. Gel-
forming biopolymers, as well as antifreeze peptides, can also retard
recrystallization.
In conclusion, the factors involved in cryoprotection are not yet fully
understood. It is often far from clear why one substance works better than a
similar one. Also the effect of the freezing rate on damage is variable. In
some instances fast freezing, and in other cases slow freezing, gives the best
results, and often the freezing rate has little effect. For frozen vegetables,
very fast freezing is generally considered desirable.