Crystallization during drying of thin sucrose films

Abstract

Crystallization in thin films (50–55 μm) of sucrose solution during drying was studied. Computer aided video microscopy was used to measure the growth of sucrose crystals during the early stages of drying, and the results were compared with predicted values based on solution of the heat and mass transfer equations. Concentration profiles, as predicted by heat and mass transfer equations, showed that sucrose concentrations at the surface of drying films increased rapidly to levels where nucleation was inhibited. When the thin films were seeded with small sugar crystals, a linear increase in crystal size with drying time (up to 15 min) was observed. Crystal growth rate, as determined from the slope of the crystal size history curve, increased significantly with increase in air temperature from 25 to 30 °C and agreed well with predicted values based on solution of the heat and mass transfer equations. Both experimental and predicted values showed that an increase in initial sucrose concentration from 70 to 76% (W/W) caused no significant changes in sucrose crystal growth. Sucrose crystal growth rate was significantly decreased in the absence of drying. However, an increase in air velocity from 4.7 to 9.4 ms −1 during drying had no effect on crystal growth rate according to both experimental and predicted results.