Sorption and diffusional studies of extruded waxy maize starch-glycerol systems

It is believed that glycerol decreases the water activity of food systems due to its hygroscopic nature. The work presented here focuses on water vapour sorption and diffusion characteristics of thermomechanically extruded waxy maize starch (WMS) and glycerol samples. The different mixtures were prepared with similar specific mechanical energies (SME ∼50 Wh/kg). The levels of glycerol ranged from 0-20% dry weight basis. At equilibrium relative humidities RH > 70% at 25°C, glycerol contributed to an increase in moisture but, for RH < 70% the moisture content decreased. The values of the monolayer (m _o) and heat of sorption (C) from Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-de Boer (GAB) equations significantly decreased in the presence of the polyol, suggesting a reduction in sorption sites availability. The diffusivities (D) of these mixtures were estimated, assuming a Fickian mechanism. The D values increased with RH but, there was an apparent reduction in this value for the RH > 60% when glycerol was present. Microscopic observations indicated formation of agglomerates and loss of particle integrity, which coincided with the marked change in the diffusivity value and the mixture's transition from the glassy to rubbery state. Therefore understanding of the amount and speed of water uptake into complex systems is important if their shelf life properties and texture are to be understood.