OXIDATION OF CHLOROGENIC ACID, CATECHINS, AND 4-METHYLCATECHOL IN MODEL SOLUTIONS BY APPLE POLYPHENOL OXIDASE

The oxidation of chlorogenic acid (CG), (+)-catechin (CA), (-)-epicatechin (EC), and 4-methylcatechol (4MC) catalyzed by apple polyphenol oxidase (PPO) was followed between pH 4 and 5 by polarography, spectrophotometry, and HPLC. In the presence of cysteine, the stoichiometry O2/phenol consumed was always close to 0.5 regardless of the time of measurement, the initial O2/phenol ratio, and the phenols tested at pH between 4 and 5. In the absence of cysteine, the observed stoichiometries greatly varied with the preceding factors. Moreover, the o-quinone reactivities widely differed from one phenol to another. Two pathways have been proposed for the degradation of 4MC o-quinones. The first one, favored by acid pH, corresponds to an hydroxylation followed by a coupled oxidation of another molecule or o-quinone leading to the formation of 2-hydroxy-5-methyl-p-benzoquinone and the regeneration of 4MC. The second pathway corresponds to polymerization reactions which were favored by higher pH values. The same pathways have been observed with CG, although the polymerization reactions seemed to be dominant. Due to the high instability of EC o-quinones, this study was not carried out with EC. Nevertheless, the different pathways of o-quinone degradation and their relative importance which varied with the pH, the phenol, and the relative concentrations of the reactants (phenol, o-quinone, and O2) could explain the variation in stoichiometry.