Cooking science

Ethylene and ripening

Ripening fruit gives off ethylene, a gas that triggers ripening in other fruit. That single fact explains why a banana ripens an avocado, why one bad apple spoils the barrel, and why potatoes sprout next to onions.

A gas that tells fruit to ripen

Ethylene is a simple gas produced by many plants. In fruit it acts as a ripening signal: it triggers the enzymes that convert starch to sugar, break down pectin so the flesh softens, and generate the aroma compounds of ripeness.

Critically, fruit responds to ethylene from any source, not only its own. A ripening banana in a bowl is broadcasting a ripening signal to everything around it.

The distinction that matters is between fruit that ripens after picking — climacteric fruit, which produces its own ethylene and responds to it — and fruit that does not. Ethylene will not ripen a strawberry, because a strawberry has no ripening left to do.

Producers and responders

Strong producers

Bananas, apples, avocados, tomatoes, pears, peaches, plums, melons, passion fruit, figs.

Strongly affected

Avocados, bananas, kiwi, pears, mangoes, tomatoes, stone fruit, broccoli, leafy greens, cucumbers, carrots.

Neither

Citrus, grapes, berries, cherries, pineapple, watermelon. These do not ripen further and produce very little.

Vegetables affected badly

Leafy greens yellow, broccoli goes yellow and bitter, carrots go bitter, cucumbers soften.

Potatoes and onions

Store them apart. Onions encourage potatoes to sprout, and both keep better separately.

One bad apple

A damaged or over-ripe fruit produces far more ethylene, which is exactly where the saying comes from.

Using and avoiding it

1. To ripen something faster: put it in a paper bag with a banana or an apple. The bag holds the ethylene around the fruit; paper rather than plastic, so moisture can escape.
2. Check daily. The bag trick works quickly and it is easy to overshoot.
3. To slow ripening: separate the fruit, and refrigerate anything that is already ripe. Cold slows ethylene production and response.
4. Store bananas away from everything else. They are the strongest common producer and the usual culprit.
5. Keep leafy greens and broccoli away from the fruit bowl.
6. Keep potatoes away from onions, in a dark cool place.
7. Remove damaged fruit immediately. A bruised apple produces a great deal of ethylene and shortens everything around it.

What it does not do

• It will not ripen a non-climacteric fruit. A strawberry, a grape or a pineapple will not sweeten in a bag with a banana.
• It will not add sugar. Ethylene triggers the conversion of starch already in the fruit. A fruit picked with little starch cannot become sweet.
• It will not fix a mealy peach. That is cold damage and a different problem.
• It does not work through a fridge. Cold suppresses the whole process, which is why ripening is done at room temperature.
• It is not harmful. It is a naturally occurring plant hormone and is present around all ripening fruit.

Common questions