Understanding lactose behaviour during storage by monitoring surface energy change using inverse gas chromatography

The purposes of this research were to use inverse gas chromatography (IGC) to examine surface energy changes to coarse lactose (CL) and micronized lactose (ML) during storage at high humidity and to relate these changes to powder properties. Surface energies, work of cohesion and surface heterogeneity were determined by IGC. Surface morphology, particle size distributions and amorphous content were determined by scanning electron microscopy, laser diffraction and time-of-flight particle sizing and dynamic vapour sorption, respectively. Surface energies (dispersive, polar and total) were higher for ML than for CL. Surface heterogeneity profiles indicated a greater number of energy sites on ML. No detectable amorphous content was present in lactoses. After storage at 75% relative humidity, the particle size increased and the span of distribution decreased for ML, indicating the formation of small agglomerates. The ability of agglomerated ML to disperse decreased after storage, indicating the formation of strong agglomerates during storage. The dispersive surface energies of CL and ML significantly decreased after storage (P < 0.05), while the polar surface energies significantly increased (P < 0.05). The total surface energy and work of cohesion of ML increased. IGC was useful to distinguish between lactose powders; the total surface energy and work of cohesion of ML were higher than those of CL. While the increase in total surface energy and work of cohesion of ML after storage was in good agreement with the formation of stronger agglomerates, these changes may have been associated more with moisture adsorption than with inherent surface energy changes to lactose.