Degradation kinetics of ascorbic acid during ohmic heating with stainless steel electrodes

Ascorbic acid degradation experiments were performed in buffer solution at pH 3.5 using a batch ohmic heater with uncoated stainless steel electrodes. The electrical conductivity of the buffer solution was adjusted using sodium chloride. The concentration of ascorbic acid was found using an HPLC technique. Kinetics of degradation can be described adequately by a first order model for both conventional and ohmic treatments, but unlike conventional heating, the temperature dependence of degradation for some ohmic treatments cannot be represented by the Arrhenius relation. During ohmic heating, power, temperature and NaCl content affect the degradation rate. A number of reactions, including electrode reactions, electrolysis of the solution, as well as reactions between electrode materials and electrolysis products may influence the reaction mechanism as well as kinetic parameters. At the highest power and salt content, citrate complexation and a significant loss of buffering capacity were noted, resulting in an increase in pH. The results underline the importance of inert electrode coatings, or the use of high frequency power for control of electrochemical reactions.