WATER AND THE GLASS-TRANSITION - DEPENDENCE OF THE GLASS-TRANSITION ON COMPOSITION AND CHEMICAL-STRUCTURE - SPECIAL IMPLICATIONS FOR FLOUR FUNCTIONALITY IN COOKIE BAKING

What is a glass? What is a glass transition? Why is the temperature at which a glass transition occurs (T(d)) so important to the processing and storage stability of so many different types of foods ? Why is th e effect of water as a plasticizer on T(g) of such widespread relevanc e to food products and processes? Why are considerations of non-equili brium glassy solid and rubbery liquid states in foods, rather than equ ilibrium phases, more germane to issues of food quality and safety? Wh y are the kinetics of heat/moisture processes for foods and of deterio rative 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, w ith its central concepts of 'glass dynamics' and 'water dynamics, and why has this research approach proved so useful to the study of glasse s and glass transitions in foods? And why has there been, since the ea rly 1980s, such interest in these questions, and such increasing resea rch activity, especially in the last few years, in this area of food s cience and technology? These are the questions addressed in this paper . The answers to these questions are illustrated by a review that emph asizes, in the first part, (a) theoretical principles from the field o f synthetic polmer science that are applicable to studies of glasses a nd glass transitions in aqueous food systems, and (b) a broad compilat ion focusing primarily on the most recent experimental studies by a nu mber 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 T(g) of (a) structure and molecular weight of water-co mpatible solutes, and (b) plasticization by water, is highlighted. Thi s is followed by specific illustrations of the application of the food polymer science approach as an interpretive and experimental framewor k for studies of (a) structure-function relationships of polymeric flo ur components, and (b) flour functionality in cookie baking. The paper describes how results of these studies have demonstrated the major op portunity offered by the food polymer science approach to expand not o nly quantitative knowledge but also, of broader practical value in the context of industrial applications, a qualitative understanding of (a ) the functionality of flours and flour components, and (b) the import ance of glassy and rubbery states, in commercial cookie products and p rocesses.