Abstract: The activity of polyphenol oxidase (PPO) in sour cherry fruit are determined by spectrophotometer. The effects of optimum pH, substrate concentration, temperature, substrates and inhibitors on PPO are studied and the reaction kinetic equation is established. The results showed that the optimum conditions are pH7.5, 0.1 M substrate and 20°C for PPO assay. However, the PPO is inactivated in boiling water for 80s. The K m, Vmax and kinetic equation are 3.5 mM, 50 U/min and 1/V = 0.0688/[S]+19.915 using catechol as substrate, respectively. Of the inhibitors tested, L-cysteine and ascorbic acid are the most effective
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