Water and the glass transition--dependence of the glass transition on composition and chemical structure: special implications for flour functionally in cookie baking

What is a glass? What is a glass transition? Why is the temperature at which a glass transition occurs (Tg) so important to the processing and storage stability of so many different types of foods? Why is the effect of water as a plasticizer on Tg of such widespread relevance to food products and processes? Why are considerations of non-equilibrium glassy solid and rubbery liquid states in foods, rather than equilibrium phases, more germane to issues of food quality and safety? Why are the kinetics of heat/moisture processes for foods and of deteriorative changes in food systems during storage more often appropriately interpreted in terms of the Williams-Landel-Ferry (WLF), rather than the Arrhenius, equation? What is the 'food polymer science' approach, with its central concepts of 'glass dynamics' and 'water dynamics', and why has this research approach proved so useful to the study of glasses and glass transitions in foods? And why has there been, since the early 1980s, such interest in these questions, and such increasing research activity, especially in the last few years, in this area of food science and technology? These are the questions addressed in this paper. The answers to these questions are illustrated by a review that emphasizes, in the first part, (a) theoretical principles from the field of synthetic polymer science that are applicable to studies of glasses and glass transitions in aqueous food systems, and (b) a broad compilation focusing primarily on the most recent experimental studies by a number of groups that have been especially active in this growing field of research. In the second part of this paper, a general discussion of the effects on Tg of (a) structure and molecular weight of water-compatible solutes, and (b) plasticization by water, is highlighted. This is followed by specific illustrations of the application of the food polymer science approach as an interpretive and experimental framework for studies of (a) structure-function relationships of polymeric flour components, and (b) flour functionality in cookie baking. The paper describes how results of these studies have demonstrated the major opportunity offered by the food polymer science approach to expand not only quantitative knowledge but also, of broader practical value in the context of industrial applications, qualitative understanding of (a) the functionality of flours and flour components, and (b) the importance of glassy and rubbery states, in commercial cookie products and processes.