Long-term stabilisation potential of poly(vinylpyrrolidone) for amorphous lactose in spray-dried composites

The aim of this study was to investigate the potential of poly(vinylpyrrolidone) (PVP) to inhibit the crystallisation of amorphous lactose during storage of the composites up to 6 months. Short-term stability was assessed by microcalorimetry over 10 days and long-term stability by storage in desiccators with different relative humidities for 3 and 6 months. The solid-state structure of the particles after storage was analysed by differential scanning calorimetry. It was found that the presence of PVP increased the critical relative humidity (RH) for crystallisation relative to the pure lactose and both the proportion and the molecular weight of the PVP affected the stabilisation of the amorphous phase. The difference in critical RH between the materials increased over time. The T(g) of the materials was generally reduced due to the absorption of water and it is suggested that the inhibiting effect therefore is related mainly to a specific interaction between lactose and PVP, rather than to a counteracting effect of the polymer on the moisture induced depression of T(g).