Estimation of effective diffusivities and glass transition temperature of polydextrose as a function of moisture content

The glass transition temperature (Tg) of polydextrose was determined by differential scanning calorimetry as a function of moisture content, and was fitted to the Gordon–Taylor equation. Tg decreased with an increase in moisture content, confirming the plasticizing effect of water on polydextrose. The effective diffusivity of water in polydextrose (D), which increased with an increase in storage water activity (aw), was estimated at room temperature both above and below the glass transition by applying the method of slopes [Effective water diffusivity in starch materials, MS Thesis, Rutgers University (1987); Effect of gelatinization and sugars on the effective moisture diffusivity in starch materials, MS Thesis, Rutgers University (1988)] to the experimental data. D showed a sharp increase in regions close to the glass transition, where polydextrose changed from the glassy to the rubbery state due to increased mobility. D was also correlated to a model [The effect of water activity on moisture diffusivity in soy flour as determined from a free volume based approach (2002)] based on the free volume theory [J. Appl. Polym. Sci. 22 (1978) 2325; AIChE J. 38 (1992) 405], which captured the change in diffusion mechanism as a function of glass transition.