Polyphenol oxidase (PPO) is an enzyme found in many fruits, vegetables, and plants, where it plays a key role in the enzymatic browning process. This enzyme catalyzes the oxidation of phenolic compounds to quinones, which subsequently polymerize to form dark pigments known as melanins. The activity of polyphenol oxidase is triggered when plant tissues are damaged, such as when fruits are cut, bruised, or exposed to air.
While the browning caused by PPO is a natural defense mechanism in plants, deterring pests and pathogens, it is often undesirable in food preparation and processing, as it can affect the color, flavor, and overall appeal of fresh produce. Controlling PPO activity through methods like reducing oxygen exposure, adding antioxidants, or altering pH by adding acid is a common practice in the food industry to maintain the quality of fruits and vegetables.
Home chefs can inhibit fruit browning using similar techniques. Oxygen exposure can be reduced by placing cut fruits and vegetables under water, and pH can be reduced by adding lemon juice or vinegar. The technique of placing cut vegetables in acidulated water combines both of these.