Abstract
The permeability coefficients of potato and rice starches were studied at different relative humidities and related to the diffusion and solubility rate over a range of temperatures (0–90°C). Inflections in the permeability and diffusivity plots versus 1/ T(K) provide an indirect indication of the glass transition; these values are in satisfactory agreement with the results of differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) measurements. Both uniaxially drawn and undrawn films were evaluated and the permeability values were correlated with the % crystallinity (determined by wide angle X-ray scattering and DSC) that developed on storage. The incorporation of sugars (fructose, glucose and sucrose) in the starches and their subsequent storage at 20°C was related to the process of crystallization. Fructose and glucose, for example, induce an increase in the rate of crystallization but decrease the permeability, diffusivity and solubility, whereas sucrose, by retarding the crystallization, has precisely the contrary effects. The synergistic action of sugars and water in lowering the glass transition ( T g) by plasticizing the potato and rice starch films is explicable in terms of the Couchman-Karasz equation if not complicated by phase separation. The values of the activation energies determined by experiment coincide with those calculated theoretically from given models where the % crystallinity is an important factor, which confirms the significance of crystallinity in the permeability of such films.
Published Version
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