Determining the physical stability and water–solid interactions responsible for caking during storage of alpha-anhydrous glucose

Abstract

Typically, a crystalline powder is considered reasonably stable below it deliquescence point (RH0), however, caking has been reported for some materials below their RH0. The critical relative humidity (RH) values for caking and hydrate formation in alpha-anhydrous glucose (α-AG) and α-AG partitioned into three particle sizes were assessed using saturated salt slurries ranging from 0 to 84 % RH at 25 °C for 20 weeks. The degree of caking was determined by a five-point visual physical stability scale, from free flowing with minimal clumping (1) to fully caked (5), and X-ray powder diffraction was used to determine the composition of the samples. Caking was observed in α-AG during storage at 68 % RH at 25 °C and the severity of caking increased with increasing RH. Fine particle α-AG caked during storage at 64 % RH, whereas medium and large particle α-AG caked at 68 and 75 % RH, respectively, at 25 °C. Caking was observed in the absence of deliquescence, amorphous content, and hydrate formation; therefore, it is proposed that capillary condensation leads to caking in α-AG below its RH0. Capillary condensation caking occurs at a specific RH (termed RHcc) where direct condensation of moisture into confined spaces, such as particle contact points or surface defects, causes the formation of liquid bridges, which may solidify over time without changes in RH or temperature. To avoid caking, α-AG should be stored below its RHcc, which is highly dependent on particle size; and to avoid conversion to glucose monohydrate, α-AG, regardless of particle size, should be stored below 64 % RH at 25 °C.