Mummy keeps raw mangoes wrapped in newspaper in a cardboard box under the bed, and for most of my life I assumed the newspaper was for keeping them clean. It is not, as my brother proved, by accident, while trying to hide food from me.
Rohan had been given three green bananas and a bunch of green grapes after school, and he knew from experience that anything left on the dining table belonged to whoever saw it first. So he hid his in the mango box. Then he forgot about them, because he is eleven.
Two days later Mummy pulled the box out to check the mangoes and found three bananas that were bright yellow, spotted with brown, soft and smelling like a sweet shop. The rest of the bunch they had come from, still on the table, was green. The grapes, sitting right next to the bananas in the same box, were exactly as green and sour as when he had put them in. Rohan was furious that someone had found his bananas. I was more interested in why the box had done that.
The answer is a gas called ethylene. It’s a tiny molecule, just two carbon atoms joined by a double bond with four hydrogens around them, and to a fruit it is not just a gas but a message. Plants use it as a hormone. When a mango starts to ripen, it begins making ethylene and releasing it into the air around it, and any fruit nearby that can detect it reads the signal as: it’s time.
What the signal switches on is a whole programme of chemistry. Enzymes start breaking down the long chains of starch packed into the fruit and chopping them into sugar, so the fruit gets sweeter. Other enzymes loosen pectin, the glue that holds the fruit’s cell walls stiffly together, so the flesh softens. The green chlorophyll in the skin is dismantled, and the yellow pigments that were there underneath all along finally show. Even the smell is part of it: ripening fruit makes new scent molecules that announce, to anything with a nose, that it is ready to eat.
The cleverest part is that the system feeds on itself. Ethylene tells a fruit to ripen, and one of the things a ripening fruit does is make more ethylene. So a ripe mango sets off a banana, the banana starts producing its own ethylene, and now there are two fruits pumping the signal into a closed box. That is what the box is for. Out on the table, the gas drifts off into the room and is diluted to almost nothing. Inside a box, under newspaper that lets just enough air through to stop mould, it builds up. The fruit sit in a cloud of each other’s instructions.
It’s a very sensitive system, too. Many fruits respond to ethylene at less than one part per million, a few molecules in every million in the air. That’s why one ripe fruit really can spoil a bowl: the old saying about one bad apple is plain chemistry.
None of which explained the grapes. They had sat in the same cloud of ethylene for two days and done nothing at all. When I asked Mummy, she said grapes are picked when they’re ready and that’s that, which turned out to be true, and turned out to have a far more interesting reason behind it than I expected.
Rohan, meanwhile, has learned his lesson, which is that the mango box is a terrible place to hide bananas. He now hides them in his school bag. We’ll see how that goes.
Try it yourselfThe ripening race
- Put one green fruit in a paper bag with the ripe banana or apple, and fold the top closed.
- Put the second green fruit in a paper bag on its own, also folded closed.
- Leave the third out on the table.
- Check all three each day and note the colour and how soft they feel.
What's going on?
The fruit sharing a bag with a ripe one usually wins. The ripe fruit gives off ethylene; the bag traps it, so the green fruit gets a strong signal to start ripening, and soon adds ethylene of its own. The fruit alone in the bag is trapped with only its own ethylene, and the one on the table loses its gas to the room. Paper lets enough air through to stop mould; plastic would make the fruit sweat.