Enzymatic browning is a natural process that occurs in some foods, mostly fruits and vegetables, causing them to turn brown. It is a chemical reaction that involves polyphenol oxidase (PPO), catechol oxidase, and other enzymes that create melanins and benzoquinone from natural phenols. This reaction requires exposure to oxygen, and it affects the taste, color, and value of such foods. Enzymatic browning is one of the most important reactions that takes place in most fruits and vegetables as well as in seafood.

The enzymatic browning process occurs over several steps. The enzyme responsible for the browning is called polyphenol oxidase (PPO). In the presence of oxygen, the PPO enzyme changes substances known as phenolic compounds (through a process of oxidation) into different compounds called quinones. The quinones then react with other compounds to form melanin, which is the same dark brown pigment that colors hair, skin, and the irises of our eyes. It also turns fruit and vegetables brown.

Enzymatic browning affects the color, flavor, and nutritional value of foods, causing huge economic loss when not sold to consumers on time. It is estimated that more than 50% of produce is lost as a result of enzymatic browning. However, enzymatic browning can also be useful, as it is responsible for the color quality of some food products, such as tea and chocolate.

To slow down enzymatic browning, one can remove oxygen from the cut surface of fruits and vegetables, as this reaction requires exposure to oxygen. Other methods to inhibit the browning reaction include using natural extracts and genome-editing.