Why does celery sticks turn brown when they are cut?

Celery sticks turn brown when cut due to a process called enzymatic browning. When the plant cells are damaged, enzymes like polyphenol oxidase are activated, leading to the oxidation of phenolic compounds in the celery. This reaction produces brown pigments known as melanins. The browning can be minimized by reducing exposure to air or applying acidic substances like lemon juice.