I've noticed that a banana in the refrigerator will turn pitch black in just a few hours. Why is this?

2 Answers 2


Your fridge speeds production of polyphenols, which speeds the blackening process. Unrefrigerated bananas ripen by ethylene gas which will also eventually turn them black, but the cold short circuits that process.

  • 2
    And there you have it! Although I'm curious how it speeds production of the polyphenals since normally cold will slow any sort of chemical process. Any ideas? Aug 20, 2010 at 1:18
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    I think the answer is something along the "blah, blah, blah oxidation, blah, blah" lines. I know that the same kind of chemical compounds are produced when aging whiskey in oak and then oxidized. I don't think anybody has a study on bananas specifically but I would guess that the cold does retard the chemical process that retards the production of the polyphenals in the first place, thus letting the compound that makes them oxidise first. Aug 20, 2010 at 1:56
  • Actually heres a paper but it's behind a pay wall. sorry. Aug 20, 2010 at 1:59
  • how to know when you cannot eat a black banana? -Well, it is getting long comment. I moved the question here.
    – user2954
    Jul 30, 2011 at 17:26

It is because, the water that is present inside the veins of bananas get expanded when is cooled from 4 deg celsius to 0 deg. at refrigerator. As the expansion of veins reaches climax it can't be holded any more and finally bursts causing blackening of bananas. It is included in the topic ANOMALOUS EXPANSION OF WATER!!!!

  • Sorry, but while it's true that water does expand slightly as it's cooled from 4C to 0C, it's a very very tiny amount of expansion (about 0.01%). In order for it to expand enough to actually burst veins or cells in fruit, you have to freeze it.
    – Cascabel
    Aug 12, 2015 at 16:46

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