How enzymatic browning occurs

A number of changes take place to convert the colorless polyphenols to the brown discoloration. These include enzymatic oxidation, rearrangement of groups, nonenzymatic oxidation, and polymerization. The enzyme responsible for enzymatic browning, officially ortho-diphenol: oxygen oxidoreductase, is also known as catecholase, tyrosinase, phenolase, and polyphenol oxidase. (Paul and Palmer, 1972)

Polyphenol oxidase (PPO) is present in most foods . It is a mixed function oxidase that participates only in the beginning phase of oxidation as it catalyzes the change of monophenols to diphenols which are then changed to highly reactive , colored o-quinones. These then react with other o-quinones, amino acids, reducing sugars, etc., to form polymers that precipitate, leaving the dark discoloration. (Ho, et.al., 1992)

The substances that PPO react with are known as substrates. These are different phenols present in fruits and vegetables at the same time as the polyphenol oxidase. But only when the tissues are disrupted or destroyed do the compounds come in contact with each other and cause oxidative browning.

The browning of fruits and vegetables is undesirable and so each is handled carefully to avoid tissue injury. But in some instances, such as with cocoa and tea, browning is important to flavor and color development and so the tissues of these plants are purposely damaged.

Example of the browning reaction in bananas

ENZYMATIC BROWNING INHIBITORS: FACTORS WHICH INTERFERE OR PREVENT POLYPHENOLOXIDASE FROM OXIDIZING PHENOLIC COMPOUNDS IN TISSUES AND CREATING BROWNISH POLYMERS. June 3, 1996 by Connie Robertson and Gina Christensen

Updated: Sunday, March 30, 2008.