The use of dynamic vapour sorption methods for the characterisation of water uptake in amorphous trehalose

Water uptake by amorphous sugars is an issue of high importance for the food and pharmaceutical industries. However, while the processes associated with sorption-induced crystallisation have been widely studied, little is known regarding the uptake mechanisms associated with pre-crystallisation water levels. In the present investigation we use dynamic vapour sorption to study the water uptake mechanisms associated with amorphous trehalose. More specifically, we have prepared spray-dried amorphous trehalose with three initial water contents and studied water uptake as a function of time and relative humidity. We model the data obtained prior to crystallisation and suggest two mechanisms that are based on Type II diffusion (using the Peleg equation), which predominates under high humidity conditions (50% RH and above), while we use a Type I Fickian diffusion expression to model uptake under low relative humidity conditions (40% RH and below). The model allows prediction of equilibrium sorption values which correlate well with previously published data calculated from equilibrium vapour pressure data. We also note that the water content following recrystallisation is greater than that predicted by the stoichiometric ratio. A novel model is suggested whereby recrystallisation onset times may be estimated from the vapour sorption data. In conclusion the study has demonstrated that pre-crystallisation water sorption may be associated with two mechanisms depending on the humidity conditions and that such modelling allows insights into both the mechanisms of uptake and the storage behaviour of amorphous sugar samples.