Oxidative browning (or enzymatic browning) is a biochemical process that occurs in fruits and vegetables when they are exposed to oxygen, resulting in a brown coloration. This reaction is primarily catalyzed by the enzyme polyphenol oxidase (PPO), which oxidizes phenolic compounds present in the fruit into quinones. These quinones then polymerize to form brown pigments known as melanins.
Oxidative browning is commonly observed in fruits such as apples, bananas, and avocados after they are cut or bruised. While this browning can affect the visual appeal and perceived freshness of the fruit, it does not necessarily indicate spoilage or a loss of nutritional value. However, in commercial food processing and culinary preparation, enzymatic browning is often undesirable as it can lead to a negative perception of the food's quality.
Various methods are employed to control or prevent enzymatic browning, such as reducing the fruit's exposure to oxygen, lowering the pH with acidic substances like lemon juice, or applying heat to denature the enzymes.
A simple home technique for reducing oxidative browning is to place cut fruits or vegetables into acidulated water, or to apply a few drops of lemon juice or vinegar to cut foods.