concentrated until the eutectic point (ce,Te) is reached, where also the
solute will freeze. At high initial concentration, the solute will crystallize
first, etc. The relation between temperature and time during heat removal is
complicated and variable, since nucleation requires (a variable extent of)
undercooling. Some solutes can markedly decrease ice crystal growth rate,
such as gums that form a firm gel, and some proteins adsorbing onto ice
(especially antifreeze peptides).
Crystallization of mostsugars(but not sucrose) gives some complica-
tions because (a) crystals with or without water can form, and (b) because of
the presence of two crystallizable anomers (aandb) showing mutarotation.
The sugar will crystallize in the form that shows the highest supersaturation,
which depends on temperature and composition. The presence of another,
not crystallizing, anomer can greatly retard crystal growth by competition,
and also strongly affect the crystal shape obtained. Lactose is a typical
example.
Fat Crystallization. The crystallization of fats, i.e., mixtures of
triglycerides, shows a number of complications. In the first place, fats have a
range of similar componentsof various melting properties, where the high
melting species can dissolve in the low melting ones. The solutions show
near ideal behavior, and the Hildebrand equation well predicts solubility.
Melting temperature and heat of fusion are closely related. In pure
triglyceride they depend mainly on (a) the chain length of the fatty acid
residues, (b) the number and configuration (cis or trans) of double bonds in
the chain, and (c) distribution of fatty acid residues in the triglyceride
molecules, a more uneven and asymmetric distribution giving a lower
melting point. Consequently, fats have a melting range. Natural fats vary
greatly (a) in their clear point (final melting point), below 0 8 C for most oils
and up to 40 8 C in plastic fats; and (b) in the wideness of the melting range,
where milk fat (melting range from40 to 40 8 C) and cocoa butter (most of
the fat melting between 22 and 32 8 C) are extreme examples. Moreover, fats
can be modified, the most common treatment being hydrogenation
(saturation of double bonds) of oils.
In the second place, triglycerides can crystallize in a number of
monotropic polymorphs. These can vary in (a) the manner of packing of the
paraffinic chains, and (b) the conformation and packing of the whole
molecules. This gives rise to a bewildering variety of polymorphs, but the
main types—primarily varying in chain packing—area,b^0 ,andb. This is
also the order of increasing melting point, heat of fusion, density, and
stability, and transitions can occur to a more stable form. Thea-form is
mostly short-lived, but theb^0 -form can persist for longer times; in some fats,
theb-form generally does not form. All this means that a natural fat may