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About a week ago, I purchased two limes, a lemon, a couple of kiwi fruits, and some oranges. When I got home, I placed them in a bowl. One of the limes ended up at the bottom of the bowl but there were still enough gaps between the fruit that it wasn't completely hidden, and there weren't enough fruits above it that the lime was in danger of being squashed.

I've since used up all the fruit but the limes. This afternoon, when I picked up the lemon to use, I noticed that the lime that was partly underneath had changed color from green to yellow. Here is a photograph of the two limes:

two limes side by side. the one on the right has turned yellow. The one on the right is still green.

Why has the lime on the right turned yellow? It was purchased at the same time as the one on the left and stored in the same bowl.

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Ethylene, most likely.

You've said that that lime was at the bottom of the bowl, mostly covered with other fruit. I'll bet it was very close to some kiwis.

Fruit -- particularly fruit like kiwi which ripens after being picked -- produces and releases ethylene. Ethylene serves a key role in fruit ripening. This is why you're advised to keep bananas in a closed paper bag to ripen them: the ethylene gas produced by the bananas builds up and ripens them faster. In open air, ethylene concentrations (in the fruit, not just around it) remain lower, and ripening is retarded.

In fact, citrus producers commonly use ethylene gas to artificially ripen ("de-green") the rind of citrus fruit. This would normally be done for lemons and oranges, which are commonly picked when still slightly green.

While you didn't have a fully closed container, it sounds like the lime was deep enough in the bowl for some concentration of ethylene, produced largely by the kiwis, to build up around it, ripening at least the rind of the lime. (Yes, some limes are yellow when fully ripe -- limes at the store are picked unripe because consumers like the look of green ones.)

Congratulations on your accidental biochemistry experiment.

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  • 3
    Fascinating. The other lime was in the same bowl ... right next to the kiwis. Just that the one that turned yellow was below the kiwis and so had less leeway to escape the ethylene, I guess.
    – verbose
    Mar 10 at 10:28
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    Makes sense. Ethylene gas is much heavier than air. It'll build up in a depressed area like a bowl unless there's sufficient air movement to blow it away.
    – Sneftel
    Mar 10 at 10:33
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    @Sneftel, it gets even better, limes are climacteric ripeners, so they won't ripen further after picking... unless you expose them to ethylene. This one was exposed to higher concentrations of ethylene, so ripened quicker than the other. Bananas are non-climacteric so are actually triggered to ripen before they are placed in the shops. Picked green, kept in constant air-flow to reduce ethylene concentration, they won't ripen, but supply a small bit of ethylene and they will start the process and then it can't be stopped, you can only speed it up with more ethylene.
    – bob1
    Mar 10 at 19:54
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    @Sneftel My sources say density of ethylene is 1.18 kg per cubic meter, while density of air is 1.225 kg per cubic meter.
    – njuffa
    Mar 11 at 0:44
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    @RedSonja were you worried you might have been .. sold a lemon?
    – pjc50
    Mar 11 at 10:16

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