33-6 |
The glass transition and food product stability |
L. SLADE, Kraft-Nabisco, 200 Deforest Ave., PO Box 1944, East Hanover, NJ 07936-2833 and H. Levine, Nabisco, Inc., 200 Deforest Ave., PO Box 1944, East Hanover, NJ 07936-2833. The science of sugar glasses has been studied for over 70 years. Much more recently (22 years ago), the 'food polymer science' (FPS) approach was developed to study glasses and glass transitions, and their effects on processing, product quality and storage stability, in foods. Since 1980, this 'new' approach to food research -- beyond that of the 'water activity' concept of moisture management -- has been used to understand structure-function relationships, the effects of plasticization by water -- as illustrated by means of state diagrams -- on thermal, mechanical, rheological and textural properties, and physical (meta)stability in the non-equilibrium glassy solid state, vs. instability in the rubbery or viscous liquid state. Key concepts of the FPS approach, especially as it has been widely applied in the decade of the 90's, include a practical understanding of the significance of the glass transition temperature, Tg, and its temperature range, how Tg is defined for multi-component, aqueous amorphous blends, and how Tg relates to the relative mobilities of individual components, including water, in such blends. Due to water's well-known plasticizing effect, Tg decreases monotonically (curvilinearly) with increasing moisture content, but the (inverse) linear relationship is between Tg and measured system relative humidity (% RH), rather than moisture content. Like the glass transition itself, plasticization is a kinetic process, so the mere presence of water is not a guarantee that softening or other changes in rheological properties have already occurred. Selected examples of recent applications of the FPS approach and the uses of calorimetry in studies of food product stability will be described, which illustrate the significant progress made in recent years, based on amorphous product technology, as applied to methods for producing and then maintaining the freshness and improving the shelf-life of high-solids foods.
Session 33, Applications of calorimetry in food and biological materials
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