D. Champion

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

Abstract Recent research has 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, specially in close vicinity to the GLT and in the glassy state.

Water in Dairy Products

During the last 50 years, our knowledge of the properties and roles of water in foods has progressed very significantly; at the beginning of this period, the emphasis was on the binding of water to other constituents, which was supposed to impart to it special properties, different from those of bulk water. These concepts of free and bound water were used widely, although most often poorly defined. They can now be supplemented by much more precise descriptions of the properties of water present in food products, in terms of thermodynamics and molecular mobility. The concept of bound water in foods (as well as in biological systems) originated in various observations, such as increasing difficulty to dehydrate the materials and increasing irreversibility of the dehydration. The concept was backed up by the knowledge of the unique properties of the water molecule. The dipolar structure of the molecule and its ability to interact with various chemical groups of the other constituents actually are at the basis of the most important role of water in some sensory properties of foods and in many of the changes that occur during processing and storage.