Caking of sucrose: Elucidation of the drying kinetics according to the relative humidity by considering external and internal mass transfer

Hygroscopic powders are prone to caking when they are submitted to variations of ambient relative humidity (RH) during process, transport and/or storage. The increase of RH above a characteristic threshold induces the formation by direct sorption of a saturated aqueous solution through a process called deliquescence, forming liquid bridges between the particles. When the RH decreases below this threshold, these liquid bridges recrystallize into solid bridges to form an agglomerate. In this work, the thermodynamics and kinetics of sucrose deliquescence and efflorescence were determined by gravimetric analysis. The influence of the drying RH on efflorescence was studied. In particular, two drying mechanisms were highlighted and modeled according to the imposed RH. An innovative method based on the representation of the water uptake/loss rates as a function of the imposed RH was established to determine the deliquescence RH.