have a number of increasing ‘‘final’’ melting points (2 to 6), for each
polymorph formed.
In the third place, triglycerides of similar molecular structure can form
compound crystals. In multicomponent fats these can take the form of solid
solutions, where one type of crystal can contain a continuously varying
range of components. This occurs readily in thea-form, where the packing
of the molecules is not very dense, but this form is generally short-lived. It
also occurs, though with less variation in composition, in theb^0 -form, and
hardly or not at all in theb-form. Polymorphic transitions thus imply a
change in crystal composition and must therefore proceed via the liquid
state. Owing to the formation of solid solutions, the temperature history of
the fat has a marked effect on the proportion solid and on the shape of the
melting curve.
Altogether, partially solid fatsare almost never in thermodynamic
equilibrium and show slow recrystallization on storage. Two extremes can
be distinguished. A fat of relatively simple composition, like cocoa butter,
can almost behave as a system of one or two pure triglycerides crystallizing
in oil. Recrystallization primarily involves polymorphic transitions. In a
multicomponent fat, like milk fat and some margarine fats, compound
crystals of a wide compositional range are formed, which composition will
change during storage and temperature fluctuation; transitions to another
polymorph tend to be slow.
Fat crystallization tends to be very slow, unless the supersaturation is
high; the latter can generally be realized. Nucleation is for the most part
heterogeneous, where existing fat crystals are very effective catalytic
impurities for other crystals. In a multicomponent fat, considerable
secondary nucleation occurs, which results in quite small crystals. Slow
recrystallization can lead to the formation of large crystals, and possibly to
oil separation from the mass of crystals.
BIBLIOGRAPHY
The basics of crystallography and crystal properties are treated in most textbooks on
physical chemistry. A useful introduction into many aspects of crystallization is in
R. H. Doremus. Rates of Phase Transformations. Academic Press, Orlando, FL,
1985.
See especially Chapter 9, Crystal growth from solution. An authoritative and
detailed description of several aspects of crystal growth is
P. Bennema. Growth and morphology of crystals. In: D. T. J. Hurle, ed. Handbook
of Crystal Growth. Vol. 1. Elsevier, Amsterdam, 1993.