Decomposition and discoloration kinetics of L-ascorbic acid powders in superheated steam

The decomposition and discoloration of powdery L-ascorbic acid in superheated steam were observed in the temperature range of 140-180 °C. For comparison, the decomposition and discoloration were also measured in the temperature range in hot air and at 180 °C in nitrogen. L-Ascorbic acid decomposed faster in superheated steam than in hot air. Discoloration was also more remarkable for the L-ascorbic acid treated in the superheated steam. The Weibull equation was applicable to the decomposition in both superheated steam and hot air. The early stage of discoloration was assumed to be expressed by zeroth-order kinetics. In the latter stage, fading occurred and the process was expressed by first-order kinetics which was modified to level off at a certain value.