Application of D-optimal Design for Determination of the Influence of Water Content on the Thermal Degradation Kinetics of Ascorbic Acid at Low Water Contents

The kinetics of thermal degradation of ascorbic acid was studied in a Maltodextrin matrix at different temperatures between 75 and 140°C. Isothermal experiments were performed with samples earlier equilibrated in environments of known a_w at 4°C, enclosed in hermetically sealed vials, with water contents (w) between 0·17 and 1·68 g water/g solids. Sampling times were selected according to an optimal experimental design in terms of minimum confidence regions of the parameters estimated, using the Bigelow model and estimates from preliminary experiments. The results showed a quadratic dependence of the reference D-value and linear dependence of the z-value with moisture content: D_r(140°C) = 150·07-290·73w+269w² (min); z = 12·14+22·99w (°C) and showed the applicability of D-optimal designs for determining kinetic parameters in complex situations, with limited experimental requirements. The dependence of the rate constants with water content could not be described by the WLF model in qualitative terms, although in absolute values this model could be used with constants similar to those expected from glass transition theory. The thermodynamic analysis of the results showed a good application of the compensation theory in the whole range of water contents.