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Why do cut apples turn brown?

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Quick answer

Cut apples turn brown because cutting breaks open cells and lets an enzyme called polyphenol oxidase meet the fruit's natural phenolic compounds and oxygen in the air. The enzyme converts those compounds into quinones, which link together into brown melanin pigments. Lemon juice, cold water or covering the slices slows the reaction.

Cut apples turn brown because an enzyme called polyphenol oxidase, released from broken cells, uses oxygen from the air to convert the fruit's natural phenolic compounds into quinones. The quinones then link together into brown pigments called melanins, which discolour the exposed flesh within minutes.

Step by step

  1. A knife cuts through the apple, rupturing cells and exposing their contents on the surface.
  2. Polyphenol oxidase from damaged cell compartments mixes with phenolic compounds stored in the cell vacuoles.
  3. Oxygen from the air dissolves into the wet surface and lets the enzyme oxidise phenolics into quinones.
  4. Quinones react with one another and with proteins, building larger molecules that absorb light.
  5. These brown melanin pigments accumulate within minutes, darkening the exposed flesh while the apple's skin stays unchanged.

Key things to understand

  1. In an intact apple, phenolic compounds and the enzyme polyphenol oxidase are kept apart in different compartments of each cell.
  2. Cutting, biting or bruising ruptures cell walls and membranes, allowing the enzyme and phenolics to mix in the exposed flesh.
  3. With oxygen from the air, polyphenol oxidase converts the phenolic compounds into reactive molecules called quinones.
  4. The quinones react with one another and with proteins, linking into large brown pigments similar to melanin.
  5. Acid, cold, lack of oxygen and heat all slow or stop the enzyme, which is why lemon juice and refrigeration help.

Labelled diagram

  1. Intact cell

    Enzyme and phenolic compounds are stored apart inside separate compartments.

  2. Cut surface

    Slicing ruptures the cells, mixing enzyme and phenolics together.

  3. Oxygen

    Air supplies the oxygen the enzyme needs to oxidise the phenolics.

  4. Quinones

    Reactive products form and begin linking with each other and proteins.

  5. Brown pigment

    Melanin-like polymers build up, colouring the exposed flesh brown.

The enzyme reaction that browns a cut apple

Real-world examples

  • Indian cooks drop cut brinjal and raw banana into water before cooking, limiting oxygen so the pieces brown less.
  • Tea leaves in Assam and Darjeeling factories are rolled to break cells so the same enzyme darkens them into black tea.
  • Cut avocado and guacamole turn brown at the surface, while the layer underneath stays green.
  • Fruit salads are often tossed with lemon or orange juice so apple and pear pieces stay pale longer.

Common mistake

✗ Myth: Cut apples turn brown because the iron in them rusts.

✓ Fact: Apples contain only traces of iron, and the browning is an enzyme reaction. Polyphenol oxidase uses oxygen to turn phenolic compounds into brown pigments, which is why heat-treated apple does not brown.

People also ask

How to stop apples from turning brown?
Slow the enzyme or remove oxygen. Brushing slices with lemon or another juice rich in vitamin C works well, as does dipping them in lightly salted or honeyed water. Chilling slows the reaction, and wrapping slices tightly or keeping them submerged limits air. Briefly blanching destroys the enzyme but softens the fruit.
Why does a bitten apple turn brown?
Biting crushes and ruptures the apple's cells exactly as a knife does. The enzyme polyphenol oxidase and the fruit's phenolic compounds mix at the damaged surface, and oxygen from the air lets the enzyme turn the phenolics into quinones, which then form brown pigments within minutes.
Why do bananas and potatoes also turn brown?
Bananas, potatoes, avocados, pears and aubergines contain the same family of polyphenol oxidase enzymes and phenolic compounds as apples. When their cells are cut or bruised, the reaction proceeds the same way, producing brown pigments. A banana peel darkens in the fridge because cold damages the cells and releases the enzyme.
Does lemon juice really stop apples browning?
Yes, for a while. Lemon juice is acidic, which slows polyphenol oxidase, and its vitamin C converts quinones back into colourless phenolics before they can form brown pigments. Once the vitamin C is used up, browning resumes.
Are brown apple slices safe to eat?
Yes. Enzymatic browning changes colour and can slightly alter flavour and texture, but it is not a sign of spoilage or toxins. A bruised or brown apple is safe unless it also shows rot, mould or a fermented smell.
Why do some apple varieties brown faster than others?
Varieties differ in how much polyphenol oxidase and phenolic compounds they contain. Apples high in both brown quickly, while those with lower levels or more natural vitamin C stay pale longer after cutting.
Is there an apple that does not brown?
Yes. Arctic apples, approved in the United States in 2015, were engineered to make much less polyphenol oxidase, so their cut flesh stays pale. Gene silencing reduces the enzyme rather than adding any preservative.

Part of the guide: Food & Kitchen Science →