Sucrose crystallization kinetics in thin films at elevated temperatures and supersaturations

Sucrose crystal growth from stagnant sucrose syrups in thin films was monitored by videomicroscopy at different temperatures and concentrations in order to obtain sucrose crystal growth rates as a function of sucrose total solids at four temperatures (40, 55, 70, and 82 degreesC). Total solids were varied from 70% to 95%, creating supersaturation values (based on concentration units in weight percent) of 1.02-1.3 at each temperature. Growth rates for individual crystals at each temperature under stagnant growth conditions in thin films were monitored by an image analysis system for up to 30 min. Additionally, theoretical growth rates based on diffusion-limited sucrose growth models were obtained at the same temperatures. A concentration gradient model was compared to a model based on chemical potential differences. At 40, 55, and 70 degreesC, growth rates decreased at very high supersaturation values, whereas at 82 degreesC growth rates continued to increase with supersaturation within experimental limits). The diffusion-limited growth rates calculated using either the chemical potential driving force or the concentration-based driving force approximated the actual growth rates very well. This analysis confirms that concentration gradients are an acceptable approximation to chemical potential gradients under these conditions.