Physical stability of co-freeze-dried powders made from NaCl and maltodextrins – Impact of NaCl on glass transition temperature, water vapour sorption isotherm and water vapour sorption kinetics

Abstract

NaCl is commonly added into food powders as a humectant or taste enhancer with little consideration of its impact on the physical properties and stability of powders. In this study, maltodextrins of dextrose equivalence (DE) 5, 20 and 45 and 10%w/w NaCl were first dissolved in water and then freeze-dried to produce powder mixtures. After freeze drying, maltodextrin DE 20-NaCl and maltodextrin DE 45-NaCl mixtures were amorphous based on powder X-ray diffraction and scanning electron microscopy-energy dispersive X-ray spectroscopy analyses while NaCl partially recrystallized in the maltodextrin DE 5 mixture. Compared to the corresponding pure maltodextrin, freeze-dried maltodextrin DE 5-NaCl blend had a lower glass transition temperature (Tg). Furthermore, the Tg depression effect of NaCl increased with maltodextrin molecular weight. Also, freeze-dried maltodextrin 20/45-NaCl mixtures had greater hygrocapacities than their respective maltodextrins throughout the aw range investigated (0.05–0.50). The impact of molecular mixing via freeze drying versus dry mixing on the sorption behaviour of maltodextrin-NaCl blends was also investigated. Freeze-dried maltodextrin-NaCl absorbed the most moisture (from 20% to 70%RH), followed by dry-mixed blends, which absorbed more moisture than predicted after 60, 50 and 40%RH for maltodextrin DE 5, 20 and 45-NaCl mixtures, respectively. In all, formulating NaCl with maltodextrin caused freeze-dried maltodextrin-NaCl blends to be more temperature- and moisture-sensitive.