Food Lipids: Chemistry, Nutrition, and Biotechnology

and have insufficient time to become optimally arranged before new attachments are
made.
Boistelle [1] described one effect of the variable consequences resulting from
different amounts of supercooling—the formation of ‘‘metastable phases.’’ These are
simply thermodynamically unstable phases formed as a consequence of kinetic preference.
In the case of crystallizing materials, the stable phase rarely forms first; rather,
the free energy of the first crystal is closest to the free energy of the original melt.
The kinetic preference for the unstable phase is a consequence of a lower difference
in surface free energy [� in Eq. (3)] between the melt and the crystal surface compared
to the difference in the stable phase versus the melt. As with nucleation, the
rate of growth is proportional to the degree of supersaturation. Since the solubility
of a stable phase is less than that of a metastable phase, the nucleation and growth
rates of the stable phase are expected to be faster. However, as discussed earlier,
solubility curves are developed from thermodynamic data, whereas the initial nucleation
and growth are governed by kinetics.
As crystallization continues, the degree of supersaturation in the system decreases.
This then causes the critical crystal size to become greater. Therefore smaller
crystals will dissolve and only larger crystals will grow. Eventually, only one crystal
size will be stable. This process, called Ostwald ripening, occurs over a very long
time possibly for years.
Sintering has been described as the formation of crystal bridges between preformed
crystals in a semisolid dispersion of crystals and liquid [5]. In fats, the bridges
are fat crystals having different thermodynamic and kinetic crystallization parameters
relative to the precrystallized material. The bridge material can result from fractional
crystallization and can consist of triacylglycerols or minor lipid compounds that fail
to crystallize with the predominant lipids. Sintering has been measured in flocculation
studies that assumed the adhesion between preformed crystals (sintering) resulted in
greater flocculate volumes [5]. Sintering is important especially relative to the final
texture and consistency of fat-based foods.
The process of sintering, like Ostwald ripening, occurs over a long time. As
such it and Ostwald ripening are often called postcrystallization processes. Since,
however, each describes crystallization phenomena, each is more precisely a crystal
maturation process.
D. Crystal Geometry
Crystals are solids with atoms arranged in a periodic three-dimensional pattern [6].
Representation of the arrangement as a ‘‘point lattice’’ (Fig. 2) depicts each point
having identical surroundings. All cells of a particular lattice are identical; therefore,
any one cell’s dimensions and angles describe the lattice constants or lattice parameters
of the unit cell (Fig. 3). The unit cell is the repeating unit of the whole structure.
This compares to the concept of the subcell. As the name implies, a subcell is a
smaller periodic structure within the real unit cell which defines the repeating unit
of the whole structure [7]. This has particular relevance to long chain hydrocarbons,
where the real unit cell is large and the subcell geometry is measured with X-ray
diffraction. Only seven different cells are necessary to include all the possible point
lattices. These correspond to the seven crystal systems into which all crystals can be
classified (Table 1).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.