At high solute concentration and low temperature, the viscosity of the
system may be greatly enhanced, whereby the diffusion coefficient of
water is reduced.Diffusion limitationmay now occur.
Gums present in small concentrations can reduce growth rate by one
to two orders of magnitude, but only if they form a firm gel. Gums
that merely increase viscosity have very little effect.
Some solutes may causeinhibition, i.e., reduce growth at quite low
concentrations. Several, generally polymeric, substances can adsorb
onto ice crystals and thereby reduce the freezing rate. This includes
many proteins. Especially theantifreeze peptides, found in many
plants and cold-blooded animals that have been subjected to cold
stress, strongly decrease growth rate at fairly low concentrations.
The presence of solutes and particulate material can also affect the
shape of the ice crystals.In practice, the rate of ice formation during freezing of foods is nearly
always limited by the rate ofheat removal. Even at a few degrees of
undercooling, a considerable portion of the water can freeze, and this
produces as much as 334 J of heat per g water frozen. Moreover, the system
will soon become more or less solid, preventing convection and agitation.
Hence the undercooling at the ice crystal surface is reduced to small values
(often much below one kelvin). Linear crystallization rate then is rarely
more than some micrometers per second; this is, however, still fast
compared to the rate obtained for crystallization of most substances from
solution.
Dendritic Growth. Occasionally, ice crystals exhibit the formation
of slender protrusions ordendrites. A crystal surface will often be slightly
irregular, and a local bulge may occur. In most cases, heat of crystallization
will be removed predominantly through the ice, because of its relatively high
thermal diffusivity. This means that the bulge will be at a somewhat higher
temperature than the adjacent flat surface, causing its growth to become
retarded; thus the interface will become flat again. However, if the ice crystal
face is in contact with undercooled water (or solution), an opposite
temperature gradient is formed. At the crystal surface the temperature
equalsTeq; further into the liquid it is lower by a few kelvins. Now a bulge
will grow faster, because its heat of crystallization can be more readily
removed than that at the flat surface. This means that the bulge develops
into a longer protrusion, thereby further increasing the difference in
temperature and hence in rate of growth. In this way several dendrites are