Kinetic models of ascorbic acid thermal degradation during hot air drying of maltodextrin solutions

The kinetic parameters of models that describe the joint influence of water content and temperature on the thermal degradation of ascorbic acid were estimated from drying experiments using maltodextrin DE 12 as a base material. Two models were used: a first-order decay model with (1) a TDT temperature dependency and a polynomial dependency of the D_r and z values with water content, and (2) a WLF dependency of the D value on glass transition temperature, with the Gordon-Taylor equation describing the influence of water content on T_g. Experiments were performed in a drying oven with dry bulb temperatures between 100 °C and 160 °C, with most of the drying occurring with the sample temperature below the oven temperature. Both models provided good fits of the experimental data. Other data from literature for a very different temperature range (4 °C-68 °C) and for maltodextrin DE 25 were handled in the same way and while the polynomial/TDT model yielded a different moisture dependence structure, the WLF/Gordon-Taylor model yielded similar parameters, thus showing a better predictive power.