Kinetics of Moisture-induced-discoloration of L-Ascorbic Acid Powders

The discoloration of L-ascorbic acid (vitamin C) powders with a water content of 1 to 10% (w/w) during storage at a temperature of 60 to 90°C was investigated by monitoring the absorbance of L-ascorbic acid solutions at 360, 450 and 500 nm. The discoloration reaction of the L-ascorbic acid powders was expressed by the modified Weibull equation under any condition. The rate constant, the shape constant and the maximum absorbance were estimated based on the equation. The temperature dependence of the rate constant was expressed by the Arrhenius equation under every storage condition, and the values of the activation energy and frequency factor increased with the increase in the water content. Although the shape constant depended on the wavelength at which the discoloration was observed, it was independent of the temperature. The maximum absorbance at every wavelength was expressed as functions of the water content of L-ascorbic acid powders and the storage temperature. These results enable us to estimate the discoloration of L-ascorbic acid powders under any condition, while the water content and the temperature would be limited in the ranges of 0 to 10% (w/w) and 60 to 90°C, respectively.