Apple Polyphenol Oxidase Activity in Relation to Various Phenolic Compounds

SUMMARY— The role of various polyphenolic compounds in oxidations catalyzed by apple polyphenol oxidase (PPO) was investigated. Quercetin, quercitrin, rutin, cyanidin chloride, phloroglucinol, and resorcinol were neither substrates nor inhibitors of apple PPO. Phloroglucinol and resorcinol increased the rate of PPO catalyzed oxidation of chlorogenic acid. Esculetin and dihydroquercetin were found to be substrates of PPO. No synergistic effect was detected in the browning rates of an esculetinchlorogenic acid mixture or a dihydroquercetin‐chlorogenic acid mixture. Ferulic acid, fisetin, and p‐coumaric acid were generally non‐competitive inhibitors, although ferulic acid inhibited competitively in one test. It was hypothesized that the conjugated system in flavones prevents these compounds from being apple PPO substrates.