3.4. Growth rate dispersion (GRD)
GRD was first seen by White and Wright (1971) in batch sucrose
crystallization. Typically, changing the solution supersaturation or
crystallization temperature will lead to the change of GRD. In this
study, GRD was observed under all conditions with each formulation,
even though they were grown under the same external
conditions. Fig. 6 illustrates the effect of composition on GRD. In
general, a lower degree of GRD was observed with an increase in
the level of corn syrup. Moreover, all the individual crystals had
different growth rates and showed no trend with respect to initial
size of the respective crystals (data not shown).
The contour plots in Fig. 7 show the combined impact of temperature
and concentration on sucrose crystal GRD. The GRD descended
progressively from the dark zone to the light zone in all
directions of changing temperatures and concentrations. These
contour plots were generally in agreement with the growth rate
contour plots, which indicate that the GRD increased proportionally
with increased growth rates. Howell and Hartel (2001) found
a similar trend, where the variability of growth rate increased with
increasing temperature, which was attributed to the extent of
increase in the GRD with higher supersaturation. This finding also
agrees with Liang et al. (1987a), who showed that the GRD of
sucrose crystals in stirred and stagnant situations increased with
the square of the average growth rate. This relationship can also
be seen in Fig. 8 for sucrose/corn syrup solids ratio 78/22; systems
86/14 and 70/30 followed the same trend. Furthermore, based on
informal macroscopic observation of the slides, the differences in
GRD had no apparent influence on the appearance of thin films
between different levels of corn syrup.
By overlaying GRD zones on the state diagram, the competing
effects of supersaturation driving force and molecular mobility
inhibition on sucrose crystal GRD were clearly observed. This graph
can be used to predict optimal coating process conditions based on
different state behaviors to deliver desired sensory attributes and
processing behavior in food systems.