PPO requires กำหนด copper ions to be active (Du, Dou & Wu, 2012). Thus, the presence of a substance capable of binding divalent cations present in the medium reduces the enzymatic activity of PPO. There are several chelators in the literature. The principal chelating agents are kojic acid, citric acid E330 and EDTA E385. The legislation is very elusive on kojic acid. Usually citric acid is used for its chelating role, but also for acidifying the medium.