At the end of “The Mango That Ripened All the Bananas”, one mystery was left on the table. Rohan’s three green bananas, hidden in Mummy’s mango box, had turned yellow, spotty and sweet in two days, set off by the ethylene gas the ripening mangoes gave off. But the bunch of green grapes he hid right next to them, in the same box, in the same cloud of ethylene, came out exactly as green and sour as they went in. Mummy’s explanation was short: grapes are picked when they’re ready, and that’s that. She was right, and the reason divides the fruit in your kitchen into two quite different kinds.
Botanists call them climacteric and non-climacteric fruits. The climacteric ones include bananas, mangoes, apples, tomatoes, papayas and avocados. The non-climacteric ones include grapes, oranges, lemons, strawberries, cherries and pineapples. The difference is in how they ripen, and in particular, in whether they can do it on their own.
A climacteric fruit, once it’s mature, contains everything it needs to ripen. Above all, it has a store of starch, packed into its cells while it was growing on the plant. When ripening begins, the fruit undergoes a dramatic change called the climacteric: its rate of respiration, the burning of sugar for energy, suddenly shoots up, and at the same time it starts producing ethylene in large amounts. As the original piece described, this ethylene production feeds itself. Ethylene switches on the genes for making more ethylene, so once the process starts, it races ahead. The ethylene also switches on enzymes that break the stored starch into sugar, soften the cell walls and change the colour. A green banana is mostly starch; a ripe one is mostly sugar. All of that sugar came from inside the banana itself.
This is why a climacteric fruit can be picked green and still ripen later, in a box, a shop or a ripening room. It has brought its own lunch. Some don’t even begin until they’re picked: an avocado will hang on the tree for months, firm and unripe, and only starts to soften once it’s been cut off.
Grapes are different. They don’t store much starch at all. As they ripen on the vine, their sweetness arrives from outside: the vine’s leaves make sugar by photosynthesis and pump it, as sucrose, along tiny tubes in the stems and into the berries. The grape is filled up with sugar from the plant, day by day, until it’s ready. And there’s no climacteric burst: grapes don’t suddenly start churning out ethylene, and they don’t have the self-feeding ripening programme that bananas do.
So the moment Rohan’s bunch was cut from the vine, long before it reached the fruit seller, its supply line was cut too. The grapes had no store of starch to convert, no burst of ripening to set off, and no vine to keep sending sugar. The ethylene in the mango box had nothing to work with. The sweetest those grapes would ever be was the moment they were picked. Mummy was right: grapes are picked when they’re ready, because they can’t get ready afterwards.
Non-climacteric fruits aren’t completely deaf to ethylene. Some respond in small ways. Oranges grown in warm places often stay green-skinned even when they’re ripe inside, because the warm nights don’t break down their chlorophyll. Growers sometimes treat them with ethylene, which clears away the green and lets the orange show through. But the treatment changes only the colour. The orange is no sweeter than before, because the sweetness was never going to come from the orange itself.
This is the reason behind a lot of kitchen wisdom. Bananas and mangoes can be bought firm and left to ripen at home. Strawberries and grapes should be bought ripe, because the ones that look pale in the shop will stay that way. A sour orange will stay sour. A hard pineapple, despite the old trick of pulling a leaf from its crown, won’t get sweeter on the counter; it’ll just get softer and, eventually, mouldy.
Rohan has taken this in his own way. He now hides bananas in his school bag, as the original piece reported, and grapes in the fridge, where at least they stay cold and sour. Mummy has not told him that the mango box was the best ripening chamber in the house. Some knowledge is best kept in the family’s older generation.