Food Lipids: Chemistry, Nutrition, and Biotechnology

Generally, it is accepted that both homogeneous and heterogeneous forms of
nucleation occur. The former occurs in the absence of any foreign particle surfaces.
The latter type occurs in practical systems when nuclei develop on the surface of
solid impurities that are present. Chemical nucleation has also been referred to as a
third, distinct type of nucleation [4]. Unlike heterogeneous nucleation, chemical nucleation
results when organic agents that were added to a system dissolve and chemically
react, activating the polymer. The ionic chain ends, which develop from polymer
chain scission, aggregate and form nuclei. Therefore, it appears that chemical
nucleation simply refers to chemical induction of heterogeneous nuclei rather than
the more direct physical addition of solid heterogeneous nuclei.
Both primary and secondary nucleation have been described by many authors
[1–3]. Secondary nucleation is relatively simple to understand. It results from fracture
of growing crystals into smaller stable crystal nuclei. Nuclei stability is a function
of nuclei solubility. This solubility, in turn, depends on the nuclei size at a given
temperature. Nucleation theory addresses the concept of critical size regarding the
development of stable primary nuclei.
An elegant discussion of these concepts was presented by Timms [2]. Two
opposing actions exist when molecules attempt to aggregate. The first, energy evolution
due to the heat of crystallization, favors crystallization as energy is released
from the aggregating embryo. The second action, that of molecular surface enlargement,
requires energy input to overcome surface tension or pressure. A stable nuclei
will form only when the heat of crystallization is greater than the energy required
to overcome surface energy. This relationship is represented in Eq. (3):
3
2 4�r �Gv �G embryo =4�r � � (3)
3Vm where the first term on the right represents the surface energy contributions and
the second depicts the volume contributions of the heat of fusion. Defining terms,
�G embryo is the Gibbs free energy of the embryo, r is the radius of the nucleus, � is
the surface free energy per unit surface area, �G v is the molar free energy change
resulting from the melt–solid phase change, and V m is the molar volume [3]. The
overall free energy will reach a maximum at some critical embryo size. Free energy
will then tend toward a minimum in all embryos. This occurs through melting of
smaller embryos or continued growth of embryos greater than the critical size.
C. Crystal Growth
Crystal growth continues as the properly configured crystallizing molecule diffuses
to the proper place on the growing crystal surface. The rate of growth is directly
proportional to supercooling and varies inversely with viscosity, since molecular
diffusion is reduced as melt viscosity increases. Equation (2) predicts that small
amounts of supercooling will lead to larger crystals versus greater supercooling leading
to smaller crystals. These crystals will be relatively perfect, since the crystal
attachment surface can become more precisely configured with the slower crystallization
rate. Greater supercooling, by contrast, affords faster crystallization (provided
the viscosity remains adequate for molecular diffusion) but also results in more
crystal faults. Faults can arise as molecules from the melt attach to the crystal surface
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.