Question 2
Milk fat as it occurs in milk, i.e., in globules of a fewmm in diameter, yields upon
cooling always very small crystals. Can you (a) explain this and (b) think of a method
to obtain larger crystals in the globules?
Answer
(a) Because of the dispersed state of the fat, considerable undercooling is needed to
ensure nucleation. This implies that crystal growth occurs at high supersaturation,
which means in turn extensive secondary nucleation and hence small crystals. (b)
Heat the cooled milk to slightly below theb^0 clear point, say 31 8 C. Most of the fat
will melt, and only a few crystals will remain. Subsequent slow cooling will then
cause these crystals to grow and become large. (Fast cooling would, again, induce
secondary nucleation.)
Note The temperature treatment just mentioned makes the globules quite
unstable to partial coalescence (Section 13.5).15.5 RECAPITULATION
It is mostly water, sugars, salts, and triglycerides that crystallize in foods
during processing or storage. It nearly always concerns crystallization from
a supersaturated solution, not from the melt. The building blocks of these
crystals are molecules or ions. The forces holding them together are van der
Waals interactions, hydrogen bonds, or ionic bonds.
The Crystalline State. In principle, the packing of molecules in a
crystal shows perfect order. Seven different packing types or crystal systems
can be distinguished, based on the geometry of the unit cell. This is the
smallest volume element that comprises all geometric information; it usually
contains one or two molecules. A unit cell is characterized by the length of
its three axes and by the angles between them. The shape (habit) of a crystal
results from the stacking of units cells. In this way the crystal obtains
various faces, which are identified by their Miller indices. Which faces are
formed and how large they become is determined by the relative rate of
growth of each. The shape of a crystal can thus vary widely for one and the
same unit cell, but the angles between faces are constant. Some properties of
a crystal, particularly its birefringence, are determined by the shape of the
unit cell.
Several substances showpolymorphism, which means that they can
crystallize in various systems (or with various unit cells in the same system).