How does browning in fruit involve oxidation?

The browning of fruit, such as apples, bananas, and pears, is a biochemical process known as enzymatic browning. This phenomenon is a direct result of an oxidation-reduction reaction triggered when the fruit's tissue is damaged or cut, exposing it to atmospheric oxygen.12

The process involves the following steps:

• Enzyme Activation: When fruit is cut or bruised, the enzyme polyphenol oxidase (PPO)—also referred to as phenolase—is exposed to oxygen.12
• Oxidation: This enzyme catalyzes the oxidation of polyphenols (or specific amino acids like tyrosine) present in the fruit's flesh. In chemical terms, this oxidation involves the addition of oxygen or the removal of hydrogen atoms from the molecules.12
• Formation of Pigments: The resulting products, known as o-quinones, are highly reactive. They subsequently react with amino acids to produce brown-colored pigments called melanins.12

To inhibit this oxidation, one can use antioxidants such as vitamin C (ascorbic acid), which acts as a reducing agent to bind free oxygen, or acids like citric acid to inactivate the enzymes.12