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Why are lemons yellow and limes green? © Getty Images

Why are lemons yellow and limes green?

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Asked by: Ken Wang, Manchester

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.

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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.

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luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.

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