PHYSICO‐CHEMICAL PROPERTIES of POLYPHENOL OXIDASE FROM d'ANJOU and BARTLETT PEARS (PYRUS COMMUNIS L.)

Polyphenol oxidase (PPO) and phenolics were studied in two pear cultivars, d'Anjou and Bartlett. PPO activity, total phenolics and chlorogenic acid concentration differed significantly with respect to cultivar. PPO activity, total phenolics and chlorogenic acid all decreased in overripe fruits. The pH optimum (d'Anjou, 4.7 and Bartlett, 5.5), and optimum temperature (d'Anjou 40C and Bartlett, 20C) for maximum PPO activity were determined. Among substrates, 4-methylcatechol followed by catechol and dopamine, was the most readily oxidized substrate of PPO from both pear cultivars. The enzyme from both cultivars did not show any activity with any of the monohydroxy substrates. K m of 13.33 and 10.92 mM were determined with 4-methylcatechol for d'Anjou and Bartlett pear PPO, respectively. Benzoic and cinnamic acid series compounds were poor inhibitors of PPO. However, L-cysteine, sodium metabisulfite, ascorbic acid and thiourea proved to be effective inhibitors of this enzyme. Heating at 65C for 30 min resulted in a loss of 60-72% of PPO activity.