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The use of model systems containing purified anthocyanins, chlorogenic acid (CG), and blueberry polyphenol oxidase (PPO) and of different analysis methods (HPLC and polarographic methods) have allowed the study of the mechanism of anthocyanin degradation by PPO. Cyanidin 3-glucoside (Cy 3-glc) was not oxidized by PPO since no molecular oxygen was consumed. However, the presence of CG induced pigment degradation. The quinone of CG, formed enzymatically, was involved in the degradation of Cy 3-glc by coupled oxidation mechanisms with partial regeneration of CG. This means that part of the CG was incorporated into the degradation products of Cy 3-glc. The ratio of degraded Cy 3-glc to oxidized CG was equal to 2. An anthocyanin degradation mechanism is proposed.

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The most efficient method for polyphenol oxidase (PPO) extraction from Highbush blueberry fruits was the preparation of an acetone powder. No activity was detected after direct extraction with phosphate buffer (pH 6·5) and detergents such as Triton X‐100. PPO has been purified 19‐fold, by ultrafiltration, ammonium sulphate precipitation and hydrophobic chromatography. Native‐PAGE of the purified fraction revealed the presence of two isoforms. PPO has an observed optimum pH at 4·0, followed by a shoulder at pH 5·0. Caffeic acid is the best substrate (100%), followed by chlorogenic acid (60%) and pyrocatechol (32·5%). No activity was detected towards catechin, protocatechuic acid, resorcinol and monophenols. © 1997 SCI.