Glass transition, structural relaxation and stability of spray-dried amorphous food solids: A review

Amorphous products have been used from ancient times not only in our daily life but also in some laboratory experiments. Many food products exist in a partly or fully amorphous state, such as hard candy and many food powders. The physical state and molecular mobility of amorphous food solids are affected by temperature and also by composition of food solids, which are related to the stability and functionality of food systems. A better understanding of the concept of amorphous food solids, its phase transition, stability, functionality, and application is important and essential to food industry and scientists. This review discussed the concept of amorphous state and its molecular arrangement. Two main factors affecting the stability of amorphous food solids were discussed. The methods used to determine glass transition and structural relaxation of amorphous solids were also been reviewed. Molecular mobility and free volume show significant change at and above the glass transition temperature (Tg). As water plasticises the material and the Tg approaches T, the material will clearly start to change structure, which results in a collapse and densification prior to crystallisation. Moreover, increasing molecular mobility of amorphous solids could result in serious problems, such as crystallization, caking, and sticking, which cause poor flowability and solubility, and destroy encapsulation matrix.