Water sorption, glass transition, and freeze-concentrated glass-like transition properties of calcium maltobionate–maltose mixtures

Abstract

The water sorption, glass transition, and freeze-concentrated thermal transition properties of calcium maltobionate (MBCa)–maltose mixtures were investigated. At lower maltose molar fractions, the water content of the mixtures increased sigmoidally with water activity (aw). At higher maltose molar fractions, maltose preferentially crystallized in an aw condition, and water sorption isotherms deviated from sigmoidal behavior. The glass transition temperature (Tg) decreased with increasing water content because of a water-plasticizing effect, described as a Tg curve. The critical water content (Wc) and critical water activity (awc) (i.e., water content and aw at Tg = 298 K) were determined from the water sorption isotherms and Tg curves. The values of Wc and awc decreased linearly with increasing maltose molar fraction. The DSC curve of the MBCa–maltose aqueous solutions showed a large endothermic shift before the endothermic peak due to melting ice, and freeze-concentrated glass-like transition temperature (Tg′) was evaluated from the endothermic shift. The Tg′ of the MBCa–maltose aqueous solutions decreased with increasing maltose molar fraction. These results are useful for predicting the physical stability (collapse, crystallization of maltose, and powder caking) of MBCa–maltose mixtures during freeze-drying and subsequent storage.