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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.

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The texture properties of crispy breads were studied as a function of water content using compression tests, acoustic measurements and sensory analysis. The addition of water slowly lowers the characteristics associated with crispness up to 9%, after which they steeply decrease. Dynamic Mechanical Thermal Analysis (DMTA) measurements were carried out, at room temperature and at 5 Hz, on the same bread samples. The evolution of the textural properties of crispy bread are accompanied by an increase of the loss factor prior to the one associated to the glass transition. It was therefore proposed that the effect of water on the brittle character, on crispness and on the intensity of the sound emitted at fracture were due to the onset of molecular motions preceding or accompanying the glass transition.