Quinones

By definition, a quinone is a cyclohexadienedione. Those with carbonyl groups ortho to each other are called 0-quinones; those with carbonyl groups para to each other are called p-quinones.

Enzymatic (oxidative) browning occurs on the cut surfaces of certain fruits, such as apples, peaches, bananas and pears.

Oxygen becomes available when the cut surface is exposed to air. The enzyme catalyzes the oxidation of the polyphenolic substrate to quinone. Quinones are not dark in color but are readily polymerized to dark-colored compounds.

Quinones are a special class of ketones in which carbonyl groups are a part of an aromatic ring of benzene, anthracene or naphthalene. Among difference types of quinones, methylnaphthoquinone (menadione) and its derivative with either phytyl side chain (phylloquinones) or the side chain having up to 13 isoprenoid units (menaquinones), are vitamin K activity.
Quinones