ucleation generally influence growth rate in much the same way.
The specific effects of each parameter depend, to some extent, on
the mechanism(s) of growth.
Supersaturation. Sucrose crystal growth has been studied extensively,
as summarized by Hartel and Shastry (1991). For the most
part, sucrose crystal growth rate increases linearly with increasing
supersaturation at concentrations below the point where molecular
mobility becomes a limiting factor.
As with nucleation, crystal growth rate increases as supersaturation
increases, up to the point where decreased molecular mobility
limits diffusion and growth rate decreases. For example, at room
temperature, an increase in sweetener concentration above the saturation
concentration initially results in increased growth rate as
supersaturation increases. At some point, however, the increased
concentration leads to a decrease in the ability of molecules to
diffuse and growth rate begins to decrease. When the sweetener
concentration is high enough (water content low enough) so that
the glass transition temperature of the mixture reaches room temperature,
mobility is sufficiently decreased and growth of sugar
crystals is shut down completely.
Temperature. Growth rate in concentrated sugar solutions is influenced
by changes in temperature in much the same manner as
nucleation rate.When a sweetener solution with constant concentration
is cooled, there is a temperature where growth rate reaches
a maximum, with slower growth at temperatures both above and
below that optimal point. Initially, the growth rate increases as the
solution is cooled below the solubility point due to the increased
supersaturation. However, the increased driving force induced by
cooling is offset by the decreased molecular mobility of sweetener
molecules.
Molecular mobility/viscosity. As noted for nucleation, mobility is
governed primarily by temperature and solution concentration.
Diffusivity, or the ability for molecules to move in space, generally
correlates inversely with viscosity. Diffusivity decreases (reduced
molecular mobility) as concentration increases and temperature
decreases (Zhymria 1972).
When molecular mobility is near zero, as in the glassy state,
growth is effectively stopped. Any crystals that nucleate prior to
formation of the glassy state remain unchanged, without growing,
within the metastable glass matrix. Commercial hard candies can
potentially contain crystals since a portion of the manufacturing
process falls within the crystallization boundary (see Figure 7B).
One study estimated that hard candy contains, on average, 2%
to 3% crystals (Smidova and others 2004). Any crystals imbedded
within the hard candy glass matrix do not grow despite the highly
supersaturated condition because of the lack of mobility (Hartel
and others 2008).
Agitation. Agitation of the solution phase in the presence of
growing crystals generally enhances the crystal growth rate, particularly
in systems like sugars where growth is strongly influenced
by mass transfer (Hartel 2001). Increased agitation enhances mass
transfer by convection, bringing molecules more quickly to the
growing interface. However, once any mass transfer limitation has
been completely alleviated, further increases in agitation have little
effect on growth (Van Hook 1945).
Formulation factors. Many of the ingredients used in confections
influence the rate of molecular incorporation into the crystal lattice.
Additives may affect crystal growth either due to a change
in the thermodynamic driving force (effect on solubility concentration)
or due to specific growth inhibition effects of the
individual molecules (Mullin 2001). Growth inhibition from foreign
molecules may have 3 causes (Hartel 2001): (1) the foreign
molecules may impede diffusion of the sweetener to the crystal
surface and thereby slow growth, (2) the foreign molecules may
adsorb to the crystal surface (most likely due to hydrogen bonding)
and inhibit the ability of molecules to incorporate into the
lattice, and (3) the foreign molecules may adsorb so strongly that
they actually become incorporated into the lattice itself, thereby
preventing addition of new sweetener molecules to the lattice.
To properly study the effects of ingredient additives on crystal
growth, it is necessary to account for any changes in solubility induced
by the additive. As noted previously, many of the common
additives in confections (such as invert sugar and corn syrup) also
decrease solubility, causing a change in driving force for crystallization.
As shown in Table 3, replacement of sucrose with corn
syrup in a formulation changes both the amount of sucrose present
and its solubility in the remaining water, thus changing the supersaturation.
Unfortunately, not all studies have accounted for this
difference.