Measuring and modelling of diffusivities in carbohydrate-rich matrices during thin film drying

Abstract

Knowledge about moisture diffusivity in solid matrices is a key for understanding drying behaviour of for example probiotic or enzymatic formulations. This paper presents an experimental procedure to determine moisture diffusivity on the basis of thin film drying and gravimetric analysis in a Dynamic Vapour Sorption (DVS) system. The extraction of moisture diffusivity is based on the “regular regime approach”. The method was explored and verified for its assumptions. It provided insight in the effect of moisture content and temperature on moisture diffusivity. Moreover, it was found that moisture diffusivity in different carbohydrate systems was similar and decreased with moisture content. The latter was explained by similar molecular interactions in carbohydrate systems and formation of a percolating network at low moisture content that affects water mobility. Subsequently, measured moisture diffusivities were compared to model predictions based on the generalised Darken relation. It was found that predicted moisture diffusivities were in fair agreement with these, including the effect of moisture content and temperature on moisture diffusivity. At low moisture content the model overestimated the sensitivity of moisture diffusivity towards temperature. This was explained by the fact that the different water–solid interactions at lower moisture content (including relaxation behaviour in the glassy state) are not considered in the modelling. Finally, the methodology was successfully evaluated to other solid matrices such as glycerol, skimmed milk and casein, providing different moisture diffusivities as function of moisture content.