Polyphenol oxidase (PPO) enzymes catalyse the o-hydroxylation of monophenols (phenol molecules in which the benzene ring contains a single hydroxyl substituent) to o-diphenols (phenol molecules containing two hydroxyl substituents). They can also further catalyse the oxidation of o-diphenols to produce o-quinones. It is the rapid polymerisation of o-quinones to produce black, brown or red pigments (polyphenols) that is the cause of fruit browning. The amino acid tyrosine contains a single phenolic ring that may be oxidised by the action of PPOs to form o-quinone. Hence, PPOs may also be referred to as tyrosinases.
Enzyme nomenclature differentiates between monophenol oxidase enzymes (tyrosinases) and o-diphenol:oxygen oxidoreductase enzymes (catechol oxidases). Therefore please refer to the tyrosinase and catechol oxidase articles for more information on polyphenol oxidase enzymes.
Polyphenol oxidase is found in fruits and is the enzyme responsible for them turning brown.
Enzymatic browning is not unique to apples. PPO — a mixture of monophenol oxidase and catechol oxidase enzymes — is present in nearly all plant tissues, and can also be found in bacteria, animals, and fungi. In fact, browning by PPO is not always an undesirable reaction; the familiar brown color of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing.