Spray dried products with controlled crystallinity is desired to realize an improved quality and functionality of the final products. This study used the glass-filament single droplet drying technique to study lactose crystallization behaviour during convective drying. Single lactose droplets with different proportions of alpha- and beta-isomers were subjected to air drying temperatures of 70 degrees C and 110 degrees C. Lactose particles dried at 70 degrees C with higher initial proportions of alpha-isomer showed the lowest dissolution rate. XRD analysis on the freshly dried particles showed that higher crystallinity was achieved with a higher air drying temperature and a higher initial alpha-isomer proportion. This observation was confirmed with the SEM observation of lactose particles that were both freshly dried and with a post-drying crystallization process. Dried lactose particles could have a two-layer morphology where the surface shell and the interior part possess different crystallinity, due to the different crystallization kinetics during drying. The results suggest that during drying there is a critical crystallization stage where both droplet temperature and moisture content are sufficiently high, constituting large T - T-g driving force. For rapid crystallization to occur. The findings provide experimental support for the solid-phase crystallization theory. (C) 2011 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.