Rheological behaviour of concentrated maltodextrins describes skin formation and morphology development during droplet drying

Abstract

We have studied the rheological behaviour of concentrated maltodextrins varying in dextrose equivalence value (DE) over a range of temperatures (≤70 °C) and concentrations (≥62% (w/w) solids) using small amplitude oscillatory experiments. Rheological data could be mapped onto master curves for storage and loss modulus as described with a model for transient or entangled polymeric networks. Maltodextrins DE5 and DE12 showed a rubber-to-glass transition, where the rubber-like behaviour is hypothesized to be the consequence of physical entanglements at sufficiently high concentrations of polysaccharides. Maltodextrins DE21 and DE38 mostly consisting of small sugars and oligomers exhibited viscous behaviour over a large range of frequencies and only at high frequencies the onset of a cross-over was visible which could signify the vicinity of the glass transition. An effective diffusion drying model validated with sessile single droplet drying experiments was coupled to rheological models to estimate rheological behaviour during droplet drying. The predicted rheological properties of the skin formed at the surface of the droplet could be related to the occurrence of surface instabilities later during drying, including cavitation for elastic skins and wrinkling for viscous skins. Insight in rheological behaviour at high concentrations may therefore offer help in predicting the morphology of droplets generated with spray drying.