DEGRADATION KINETICS OF ASCORBIC ACID AT HIGH TEMPERATURE AND WATER ACTIVITY

The kinetics of ascorbic acid degradation were studied in an intermediate moisture model food system as a function of water activity (0.69‐0.90) and temperature (61‐105°C). The disappearance of ascorbic acid in each case followed a zero order kinetic model. Rates of ascorbic acid degradation ranged from 1.5‐10.5 mg/100g solids/min, while the activation energy was in the range of 14—17 Kcal/mole. It is suggested that dissolved oxygen concentration was limiting above 92°C, resulting in a rate decrease between 92 and 105°C. Rates of ascorbic acid degradation were found to increase with increasing a_w, except at 105°C, where the opposite was observed. An equation derived from the integrated zero order rate law was used to predict ascorbic acid losses during an unsteady state heating process approximating a linear temperature rise, with good results. The same equation was much less accurate (predictions were 2‐4 times larger), when used to predict ascorbic acid losses during extrusion processing, most likely due to the difficulty in obtaining an accurate temperature history of the extruded product.