Why are lemons yellow and limes green?

All citrus fruits are green while they are still growing on the tree. Lemons lose their green colour as they ripen because the chlorophyll pigment is replaced with a chemical called anthocyanin.

Many lime species would also turn yellow if you left them on the tree long enough, but they never get a chance. This is because ripe citrus fruits are too soft to travel well, so farmers always pick the fruits while they are green and under-ripe.

Oranges and lemons will continue to ripen on their way to the supermarket, but a quirk of biology means that limes stop ripening once they are picked.

Read more:

• Why does fruit change colour as it ripens?
• Why do banana skins get thinner as the fruit ripens?
• Is the sugar in fruit bad for us?
• Why does grapefruit juice have an impact on drugs?

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