IMPORTANCE OF pH ON ANTIBROWNING ACTIVITY OF OXALIC ACID

Abstract

The importance of maintaining stable pH when studying antibrowning activity of oxalic acid in a catechol-polyphenol oxidase model system was demonstrated. Enzyme inhibition in the reaction medium showing a decrease in pH caused by the addition of increasing oxalic acid concentrations was compared with the inhibition of the same oxalic acid concentrations at a stable pH of 5.0. Relative antibrowning activity of oxalic acid was significantly lower at stable pH conditions than at variable pH conditions. The main reason for this was because of the very slight alterations around the pH used for the experiments, which were sufficient to induce significant differences in the catalytic activity of the enzyme. Oxalic acid at pH 5.0 was found to be a noncompetitive inhibitor of the mixed-type with an inhibitor constant (Ki) of 0.8 mM. PRACTICAL APPLICATIONS Fruits and vegetables are prone to polyphenol oxidase (PPO)-catalyzed browning reactions during handling and processing, and the food industry is interested in methods to prevent browning for minimally processed crops. Although not an approved food additive, oxalic acid is a common component of several foods from plant origin. There are few reports on the inhibition of PPO by oxalic acid in reaction model systems. The importance of maintaining a stable pH when studying the antibrowning activity of oxalic acid in a catechol-PPO model system was demonstrated. Small additions of oxalic acid into the assay system induced significant pH variation. Minute changes in the degree of acidity severely affected PPO activity and resulted in significant errors in measuring inhibition. Identifying how environmental changes in assay conditions affect activity and inhibition are important in interpreting how this enzyme is controlled on foods by inhibitors.