10-5 |
Investigation of intermolecular interactions in dextran systems by thermal analysis |
D. ICOZ, C. I. Moraru, and J. L. Kokini. Dept. of Food Science, Rutgers, The State Univ. of New Jersey, 65 Dudley Rd., New Brunswick, NJ 08901-8520 Food materials are composed of several molecular species with different chemical structures. Compatibility/miscibility of these molecular entities is critical for control of the resulting phase behavior. In turn phase behavior affects the texture, functionality, processability and stability of foods. Many food biopolymers are immiscible, since even small differences in their chemical structure can cause incompatibility. Glass transition temperature (Tg) can be used as an important criterion in determining the boundary for mobility of mixed biopolymer systems. The objective of this study was to characterize the miscibility of dextran systems of similar chemistry by studying their phase behavior. The effects of method of preparation and molecular weight on the Tg of dextran mixtures were investigated using thermal analysis. Seven dextrans with weight-average molecular weights (Mw) ranging from 970 to 2,000,000 were used. Samples were prepared both directly, in powder form, and by freeze-drying 30% (w/w) dextran solutions. All samples were equilibrated at a water activity (Aw) of 0.33. Tg values were determined by Differential Scanning Calorimetry (DSC) in the temperature range of 30-130°C, at a heating rate of 10°C/min. At Mw<43,000, an increase in Tg was observed with increasing Mw. At Mw>43,000, no significant effect of Mw on Tg was found. Mixtures of two powder dextrans showed phase-separation (2-Tg blend), whereas samples obtained by freeze-drying solutions of two dextrans showed miscibility (1-Tg blend). Small amounts of low Mw dextrans had a profound effect on decreasing Tg of high Mw dextrans by reducing the number-average molecular weight (Mn) of the mixture. These results suggest that Mn rather than Mw controls the glass transition of the dextran mixtures. The direct implication is that it may be possible to control texture and stability of food systems by manipulating Mw of the ingredients rather than their moisture content/Aw.
Session 10, Carbohydrate: General
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