Why do apples turn brown after you cut them?
Quick answer
Cut apples turn brown because slicing breaks their cells and exposes them to oxygen. An enzyme called polyphenol oxidase (PPO) then reacts with natural compounds called polyphenols, turning them into brown pigments. This is called enzymatic browning. It’s harmless; the fruit is still safe to eat.
The reaction in three steps
| Step | What happens |
|---|---|
| Cutting | Breaks cells, mixing the PPO enzyme with polyphenols |
| Oxygen | Air reaches the exposed flesh |
| Browning | PPO turns polyphenols into quinones, which link up into brown melanin-like pigments |
The same kind of pigment, melanin, colors human skin and hair.
Fruits and vegetables that brown
- Quick to brown: apples, pears, bananas, avocados, peaches, potatoes, mushrooms
- Slow or no browning: citrus fruits, tomatoes, melons, strawberries
Fruits rich in vitamin C or acids tend to resist browning.
How to keep slices fresh
| Method | Why it works |
|---|---|
| Lemon or lime juice | Acid slows the enzyme; vitamin C reacts with oxygen first |
| Salt water (a pinch in a cup of water) | Salt slows the enzyme |
| Honey water | Contains natural antioxidants |
| Cold water / fridge | Cold slows reactions; water blocks air |
| Airtight wrap | Keeps oxygen out |
| Blanching (briefly boiling) | Heat destroys the enzyme, but softens the fruit |
A non-browning apple
The Arctic apple, approved in the US and Canada in 2015, was genetically modified to make much less PPO, so slices stay white.
Browning on purpose
Enzymatic browning isn’t always unwanted. It helps give tea leaves their dark color, and contributes to the flavor of cocoa and dried fruits such as raisins and dates.
It’s different from the heat-driven browning of cooking; see why toast turns brown. For a fruit with a surprising seed arrangement, read which fruit has its seeds on the outside.
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