Why Do Cut Apples Turn Brown?

In an intact apple, cell structures keep many substances apart. Cutting or bruising breaks those cells, allowing polyphenol oxidase to meet phenolic compounds and oxygen from the air. The enzyme helps oxidise the phenols into highly reactive quinones. Further reactions then produce the brown material that becomes visible on the cut surface.

This process is called enzymatic browning. It is different from the Maillard reaction, in which sugars and amino acids react during heating, and it does not by itself mean that the apple has spoiled. The colour shows that damaged tissue has undergone chemical change, but browning alone does not determine whether the fruit is unfit to eat.

Lemon juice slows browning mainly because an acidic environment reduces the activity of polyphenol oxidase; its ascorbic acid can also reduce some oxidation products. Keeping the cut surface close to a covering, or briefly placing it in water, reduces contact with oxygen. These methods slow the reaction rather than restoring the damaged cells. Browning is therefore more than an apple simply “rusting” in air: it follows when broken cellular boundaries allow an enzyme, its substrates and oxygen to meet.

https://cen.acs.org/food/food-science/Periodic-Graphics-Kitchen-chemistry-hacks-explained/101/i27
https://pubs.acs.org/doi/10.1021/jf0705724