Why cut fruit turns brown — Chemistry, 4–6 years
A cut apple does not become dirty at once. The air starts a slow chemical change on its fresh surface.
The air changes the cut surface
When an apple or banana is cut, air reaches parts that were hidden inside. Substances in the fruit react with oxygen, and a brown colour slowly appears; this is a chemical change, not dirt.
Why does it happen?
A whole fruit keeps its inner substances separated from much of the air. Cutting breaks that protection, so the fruit’s colour-changing reaction can begin; learning this helps explain why food can look different while still being safe.
An apple in three steps
First, cut one apple into two equal pieces. Leave one piece on a plate and brush the other with lemon juice. After about 20 minutes, the uncovered piece is usually browner, while the juice has slowed the air’s reaction.
Brown does not always mean rotten
It is reasonable to think brown fruit must be dirty or rotten, because mould and decay can also change food’s appearance. But browning soon after cutting is often only the fruit reacting with air; smell, softness and mould give better clues.
In a lunch box
People sometimes add lemon juice to cut apples or keep cut fruit covered. These actions reduce contact with air, so the fruit stays lighter for longer; they slow the change rather than making the fruit unchanged forever.
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