Most low water content or frozen food products are partly or fully amorphous. This review will discuss the extent to which it is possible to understand and predict their behavior during processing and storage, on the basis of glass transition temperature values (Tg) and phenomena related to glass transition. Two main conclusions are provisionally proposed. Firstly, glass transition cannot be considered as an absolute threshold for molecular mobility. Transport of water and other small molecules takes place even in the glassy state at a significant rate, resulting in effective exchange of water in multi-domains foods or sensitivity to oxidation of encapsulated materials. Texture properties (crispness) also appear to be greatly affected by sub-Tg relaxations and aging below Tg. Secondly, glass transition is only one among the various factors controlling the kinetics of evolution of products during storage and processing. For processes such as collapse, caking, crystallization, and operations like drying, extrusion, flaking, Tg data and WLF kinetics have good predictive value as regards the effects of temperature and water content. On the contrary, chemical/biochemical reactions are frequently observed at temperature below Tg, albeit at a reduced rate, and WLF kinetics may be obscured by other factors.

The prediction of the stability of low-moisture products is complex, particularly close to the glass transition temperature. This study demonstrates that the relations used to evaluate the influence of temperature on the viscosity of carbohydrate media cannot be applied to the diffusivity. The translational diffusion coefficient of a fluorescent molecule (fluorescein) is measured in sucrose−water mixtures as a function of temperature. The main result is that the mobility of the fluorescein is not simply coupled to the viscosity of the diffusion medium at temperatures close to the glass transition temperature. Indeed the WLF equation, which gives a good prediction of the viscosity, does not allow the determination of the diffusivity in a temperature range close to T g because the translational diffusion follows a weaker temperature dependence. Different possible explanations for this apparent decoupling between translational diffusion and viscosity are suggested:  a small change in the hydrodynamic radius of the diffusing molecule due to low water content and/or a connection with the β relaxation process.

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