Investigating the moisture-induced crystallization kinetics of spray-dried lactose

Gravimetric water sorption experiments were performed to study the crystallization behavior of amorphous spray-dried lactose over a wide range of temperature and humidity conditions. Experiments performed at 25 °C between 48 and 60% relative humidity (RH) showed that the onset time to crystallization increased dramatically with decreasing humidity. At 55% RH and above, crystallization occurred in a single detectible step, while below a two-step process was observed. Experiments performed at 51% RH between 22 and 32 °C indicated the induction time to crystallization onset increased with decreasing temperature. Above 25 °C at 51% RH, crystallization occurred in one measurable step, while below crystallization occurred in two steps. The constant RH with varying temperature results were modeled to determine the crystallization mechanism. Above 25 °C a mechanism consisting of two competing reaction sequences fit the data with a 0.9997 correlation coefficient. Both reaction sequences have two steps: an auto-catalytic first step is followed by a three-dimensional diffusion controlled water loss step.