Enzymatic browning is one of the most important reactions that takes place in most fruits and vegetables as well as in seafood. These processes affect the taste, color, and value of such foods. Generally, it is a chemical reaction involving polyphenol oxidase (PPO), catechol oxidase, and other enzymes that create melanins and benzoquinone from natural phenols. Enzymatic browning (also called oxidation of foods) requires exposure to oxygen. It begins with the oxidation of phenols by polyphenol oxidase into quinones, whose strong electrophilic state causes high susceptibility to a nucleophilic attack from other proteins. These quinones are then polymerized in a series of reactions, eventually resulting in the formation of brown pigments (melanosis) on the surface of the food. The rate of enzymatic browning is reflected by the amount of active polyphenol oxidases present in the food. Hence, most research into methods of preventing enzymatic browning has been directed towards inhibiting polyphenol oxidase activity. However, not all browning of food produces negative effects.
Examples of beneficial enzymatic browning:  
* Developing color and flavor in coffee, cocoa beans, and tea.
* Developing color and flavor in dried fruit such as figs and raisins.
Examples of non-beneficial enzymatic browning:
* Fresh fruit and vegetables, including apples, potatoes, bananas and avocados.
* Oxidation of polyphenols is the major cause of melanosis in crustaceans such as shrimp.