Moisture sorption, compressibility and caking of lactose polymorphs

The aim of this study was to conduct storage studies on the moisture sorption and caking properties of lactose powder containing different polymorphs (i.e. α-monohydrate, α-anhydrous unstable, α-anhydrous stable, β-anhydrous) and spray-dried lactose. The dry sample was compacted using a texture analyzer in paper cylinders and stored at relative humidity (RH) of 33%, 43%, 57% and 75% (25 °C, for 3 months). The samples were monitored for weight gain, moisture content, α/β balance and hardness. A simple new method of powder compression for measuring the degree of hardness of caked lactose was developed using a texture analyzer. Clear distinctions were found in the storage behavior of the five different samples. Storage at various RHs caused severe caking to β-lactose anhydrous and spray-dried lactose. The β-lactose anhydrous was hygroscopic at 75% RH. The spray-dried lactose, which contained some amorphous lactose, was hygroscopic at all RHs studied. Its moisture sorption behavior differed from that of its major component, α-lactose monohydrate, by initially absorbing moisture then desorbing. α-Lactose monohydrate was less hygroscopic at 75% RH and it formed friable cakes. The α-lactose anhydrous stable was hygroscopic at 75% RH and initially formed hard cakes which became friable during storage. The unstable form of anhydrous α-lactose was hygroscopic at all levels of RH studied but did not cake.