| Food & Ingredients | | | Educ. Serv. | | | Glossary | | | References | | | Images | FAQ |
|---|
Mechanism of Enzymatic Oxidation of Phenolic Compounds |
The fundamental step in enzymatic browning is the oxidation of phenolic compounds to ortho-quinones. Frequently, the catechol-related substrate is formed by hydroxylation of a monohydroxyphenol to the ortho-dihydroxyphenol. Mason (1955) proposed that this step is catalyzed by a monophenol oxidase. This action is called the cresolase activity. Kertesz and Zito (1962) suggested that the ortho-quinone produced by the oxidation of ortho-diphenol causes the hydroxylation of monophenols, thus:
Catecholase activity is more important than cresolase action in foods because most of the phenolic substrates in foods are dihydroxyphenols (Mathew and Parpia, 1971). Ortho-quinones can undergo further autooxidation, condensation, and rearrangement to form brown or black polymers. Singleton (1972) characterized this polymerization as a random free-radical type process even though initiated by enzymatic oxidation. He theorized that polymerization is a stabilizing process whereby labile chromophores are incorporated into one molecule. Because these chromophores are of varying shapes, strains are produced and the polymers appear brown or black.
In the reactions outlined about, copper plays an important role. An aged polyphenol oxidase apoenzyme can be reactivated by addition of fresh cupric copper which is reduced to the active cuprous form (Kertesz, 1966). Mason (1956) suggested that the hydroxylation of monophenols occurs after phenolase with copper in the cuprous state combines with molecular oxygen. The monophenol then combines with this phenolase-oxygen complex to yield phenolase in the cupric state and diphenol.
The overall oxidation reactions catalyzed by the phenolase complex is shown in the following model by Mason (1957):
This model implies that the phenolase complex is an oxygen transferase. Dawson and Tarpley (1951) suggested that polyphenol oxidase may function as a terminal oxidase in respiration.
Updated: Wednesday, June 20, 2007.
OSU Disclaimer.