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

Abstract

A free flowing crystalline powder can develop severe caking if held under humid and/or high temperature storage. In addition, a crystalline hydrate material can experience compositional changes due to hydrate formation or loss if held at too high or low of a storage RH, respectively. Thus, the critical RH values for caking and compositional changes of glucose monohydrate (GM) and GM partitioned into three particle sizes were assessed using saturated salt slurries ranging from 0 to 84 % RH at 25 °C for 20 weeks. Caking was measured using a five-point visual physical stability scale, from free flowing to fully caked, and sample composition was determined using X-ray powder diffraction. Caking was observed in GM during storage at 53–84 % RH at 25 °C and fine particle GM caked at lower RH values than medium and large particle GM. For all GM samples, hydrate loss (via conversion of GM to alpha-anhydrous glucose) occurred at 0 and 11 % RH and hydrate formation (via mutarotation of beta-anhydrous glucose to alpha-anhydrous glucose and conversion to GM) occurred at 53–84 % RH at 25 °C. Particle size did not affect compositional changes during GM storage, but greatly affected caking. Neither deliquescence nor amorphous content were detected in this study and hydrate formation and β-AG content did not cause caking in GM, therefore, decreased flowability was the result of capillary condensation caking. At 25 °C, GM should be stored above 11 % RH to avoid hydrate loss and below 53 % RH to avoid caking.