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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.

Nelson
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verbose
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1 Answers1

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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 "climacteric" 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.

Sneftel
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    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 '21 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 '21 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 '21 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 '21 at 00:44
  • Yellow limes: I bought a little lime tree in a pot, and was very disappointed indeed when the fruit ripened yellow. But they taste and smell like limes, so I suppose I got a yellow lime tree. – RedSonja Mar 11 '21 at 07:19
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    @RedSonja were you worried you might have been .. sold a lemon? – pjc50 Mar 11 '21 at 10:16
  • @njuffa Huh, that's what I get for trusting Wolfram Alpha. Just the effect of air circulation, then. – Sneftel Mar 11 '21 at 10:34
  • @njuffa It gets very complicated because eythlene is denser than nitrogen and air is mostly nitrogen. Air can easily stratify in a small, semi-enclosed space if obstacles prevent sufficient turbulent mixing. – David Schwartz Mar 11 '21 at 16:36
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    @David Schwartz I was specifically commenting on (my emphasis) "Ethylene gas is **much heavier than air**". By no means an expert, but I would assume that in a small closed space, air does not de-mix into layers of nitrogen and oxygen due to ongoing diffusion. – njuffa Mar 11 '21 at 19:55
  • @njuffa Right. But if the fact that nitrogen is less dense than ethylene doesn't matter due to mixing, neither does the fact that ethylene is less dense than air. (I'm agreeing with you. The individual densities either don't really matter due to diffusion or still bring ethylene mostly to the bottom due to it being denser than nitrogen.) – David Schwartz Mar 11 '21 at 19:59
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    @DavidSchwartz - I think in this case it is merely more mixing and less mixing respectively, nothing to do with stratification. The fruit at the top had greater airflow and mixing with lower local concentration, so didn't ripen; the one at the bottom had less airflow and mixing in confined space so greater local concentration and did ripen. – bob1 Mar 11 '21 at 23:44
  • @pjc50 — blimey, what a terrible joke. – myklbykl Mar 16 '21 at 18:47