Six polymorphic forms are generally observed, and some properties
are given in Table 15.4. The common notation is from I to VI, in order of
increasing melting point. In the present case, meltingpointis a reasonable
term, since the crystals in one polymorph give a pretty sharp melting peak
(on a DSC scan), almost like that of a single triglyceride; theg-form tends to
give a melting range. I–V can form from the melt and by transition from a
less stable polymorph; the latter is the only route to obtain form VI. It
should be added that cocoa butter also contains 2–3% trisaturated
triglycerides, which crystallize separately from the main group. Most other
triglycerides remain liquid, even below 20 8 C.
Cocoa butter is an essential component of chocolate, and its
crystallization is of substantial importance for the quality of the product,
which should be firm and smooth (both visually and in the mouth) and
melt rapidly to provide a sensation of coolness. A problem can be the
formation of bloom: this consists of whitish patches on the surface, which
are due to the slow formation of largish crystals (consumers may think
that it is mold growth). It arises from slow recrystallization, especially due
to the transition from form IV to V. The way to circumvent this is
tempering. In the present context this implies (a) rapidly cooling the liquid
(the ‘‘chocolate mass’’) to 15 8 C or lower and keeping it there for some
time, which ensures that nucleation is fast and that many small crystals
form; (b) warming to a temperature just below the melting point of form V
and keeping it there for at least an hour; and (c) bringing the chocolate to
room temperature. Such a process leads to small crystals, nearly all of
which are in form V. Some bloom can still slowly form via the transition
V!VI, particularly when the storage temperature is above 25 8 Cor
when temperature fluctuations occur.
TABLE15.4 Polymorphic Forms of
Cocoa ButterCode Unit cell Melting pointa, 8 C
I g 5–18
II a 16–24
III b^02 -L2 20–27
IV b^01 -L2 25–28
V b 2 -L3 26–33
VI b 1 -L3 32–36
aPeak temperature on DSC curve.