Bisulfite Degrades Aflatoxin: Effect of Citric Acid and Methanol and Possible Mechanism of Degradation

Abstract

Citric acid retarded degradation of aflatoxins B1 and G1 by bisulfite. Replacement of potassium acid phthalate-NaOH with citric acid-NaOH in the mixture of buffer (50 ml, 0.035 M)-methanol (0.65 ml)−0.8 g of K2SO3, pH 5.5, at 25 C resulted in a decrease from 9.76 × 10−3h− to 2.47 × 10−3h− and from 1.19 × 10−2h− to 4.32 × 10−3h− in rate of degradation of aflatoxin B1 and G1, respectively. Methanol also retarded degradation of aflatoxin by bisulfite. Increasing the methanol concentration from 1.3 to 10.0% (v/v) in 50 ml of 0.035 M KHP-NaOH buffer, pH 5.5, plus 0.40 g of K2SO3, resulted in a decrease in rate constants from 4.26 × 10−3h− to 2.16 × 10−3h− for aflatoxin B1 and from 5.54 × 10−3h− to 2.98 × 10−3h− for aflatoxin G1, Presence of citric acid and various concentrations of methanol also reduced rates at which free bisulfite concentrations changed. From these observations, known effects of methanol and probable effects of citric acid on bisulfite oxidation, we suggest that degradation of aflatoxin by bisulfite is dependent on bisulfite oxidation. 14C-labelled aflatoxin B1 was treated with K2SO3 at pH 5.5 and allowed to react for 96 h. Most of the degradation product(s) were in the water soluble phase, indicating that a structural modification of aflatoxin occurred.