Towards an improved understanding of glass transition and relaxations in foods: molecular mobility in the glass transition range

Recent research ha's contributed to a better understanding of the glass-liquid transition (GLT) and its relationship with relaxation processes in the. material. This paper reviews models and theories that are currently used to describe and explain the physical changes in the GLT temperature range (T-g); ageing below T-g, changes in mechanical properties above T-g, and the concept of fragility are described. Measurements of the GLT temperature are now practised routinely in many food laboratories, but lack of information on the experimental conditions may lead to discrepancies between results. Several examples from the food domain are reported, illustrating that the GLT has been mainly used to interpret, with more or less success, changes in low moisture foods and biomaterials. Taking the temperature of GLT into consideration alone cannot sufficiently explain changes as a function of temperature or water content, particularly when chemical/biochemical reactions are concerned. The relationship between molecular mobility and the GLT is discussed. More measurements of the various types of molecular motions are necessary, specialty in close vicinity to the GLT and in the glassy state. (C) 2000 Elsevier Science Ltd. All rights reserved.