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a-Lactose monohydrate is the carrier of choice for dry powder inhaler formulations but problems during dry powder inhalation development occur, mainly due to differences in the surface texture and surface energy of lactose received from different sources, but also due to batch-to-batch variability.
It was the aim of this study to investi-gate the influence of the relative humidity during re-crystallization and the amount of amorphous material present on the physical form of the re-crystallized lactose. Spray-dried lactose and conditioned a-lactose monohydrate are used as reference materials for completely amorphous and fully crystalline lactose. Blends of these lactose qualities were re-crystallized in an isothermal microcalorimeter at different relative humidity (RH) and subsequently analysed by differential scanning calorimetry (DSC).
It was found that the relative humidity during re-crystallization influences the physical form of the re-crystallized lactose. Re-crystallization at 100 % RH led to the formation of a-lactose monohydrate whereas re-crystallization at lower RH led to an increasing amount of ß-lactose. In addition, it could be shown that the content of amorphous lactose in a powder blend at fixed RH also influences the re-crystallization result. The higher the amorphous content was the more ß-lactose was formed.