Why do apples turn brown, and how do you stop it?

Cutting an apple ruptures cells, allowing an enzyme called polyphenol oxidase to contact both oxygen and phenolic compounds that were previously kept separate. The enzyme catalyzes their conversion into quinones, which then link up into melanins, the brown pigments. The same process browns bananas, avocados, pears, potatoes, and mushrooms, and it is also what turns tea leaves dark during processing and gives cider its color, so it is not spoilage. Browned fruit is still safe to eat, though texture and flavor degrade over longer periods.

Every effective countermeasure attacks one of the three requirements. Acid lowers pH below the enzyme's optimal range and slows it dramatically, which is why lemon, lime, pineapple juice, or a splash of vinegar work, and why the effect is a real chemical intervention rather than a coating. Cold slows the reaction, so refrigeration buys time. Blocking oxygen works, whether by submerging slices in water, wrapping tightly, or vacuum sealing. Heat denatures the enzyme permanently, which is why cooked apples do not continue browning and why blanching is standard before freezing many vegetables. A brief soak in lightly salted water is effective and, if rinsed afterward, leaves no detectable taste.

Two practical notes. Varieties differ substantially in how fast they brown because they carry different levels of the enzyme and of phenolic compounds, and some cultivars have been bred or genetically modified specifically for reduced browning. And the compounds involved are related to the fruit's antioxidant content, so heavy browning correlates with some loss of certain nutrients, though the effect on overall nutrition is modest and not a reason to discard the fruit. For a lunchbox, a quick dip in a solution of water with a squeeze of lemon, drained, keeps slices presentable for hours without a noticeable citrus taste.