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I saw a new "health advice" meme on Facebook today:

enter image description here

Please remember it is dangerous to cut an onion and try to use it to cook the next day, it becomes highly poisonous for even a single night and creates toxic bacteria which may cause adverse stomach infections because of excess bile secretions and even food poisoning.

Aside from the terrible grammar and punctuation, the claims at a glance seem largely ridiculous--cooking would kill any bacteria, for example.

Is there any truth to the claim that an onion becomes dangerous, specifically for cooking (as opposed to eating it raw) within a day of being cut open?

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No it isn't, I've eaten plenty of onions that weren't cut at the same day, and my wife even eats them rare. Nothing bad ever happened to us from it. –  Ilya Melamed May 19 at 18:29
    
@IlyaMelamed: I have the same experience, but anecdotes don't make good answers :) –  Flimzy May 19 at 18:33
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"Cooking would kill any bacteria" is an overstatement that our friends at Seasoned Advice are constantly fighting. Cooking is pasteurization, not sterilization. –  Oddthinking May 19 at 21:57
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sounds like it should be posted on The Onion. –  Mehrdad May 20 at 0:18
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A classic use for onions is to include them in marinade for meat, leaving it there for a day or so, and then cooking and eating them along with that meat. If that would make them toxic, I'd be dead by now. –  Peteris May 20 at 7:32
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2 Answers 2

As well as personal experience from eating onions that were cut not on the same day. From the National Onion Association:

Leftover Onion and Cut Onion

Q: Are cut onions or leftover onions poisonous?

A: When handled properly, cut onions are not poisonous. After being cut, onions can be stored in the refrigerator in a sealed container for up to 7 days. A widely circulated claim states uncooked, leftover onions are 'poisonous' because they're 'a huge magnet for bacteria,' thus likely to spoilage. This claim stems from a blog post that dates back to March 2008. While the original post was removed from the internet in 2009, part of that post continues to circulate the internet.

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Is the Nationl Onion Association a bias-free source about onions? –  Erel Segal Halevi May 20 at 5:49
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@Erel, The Onion would probably disagree. –  Simon Richter May 20 at 6:28
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Snopes presents a similar story. The Wayback Machine has several snapshots of mentioned blog, however with some quick searches I couldn't find mentioned source though. Maybe someone else can spot it? –  Jeroen May 20 at 7:32
    
I would accept this answer if it came from a less biased source. Can you find a more neutral source to back this answer? –  Flimzy May 29 at 17:20
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This article seems to suggest quite the opposite effect is to be expected - http://permaculturenews.org/2013/04/12/layers-of-healing-realizing-the-power-of-the-ordinary-onion/

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Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. –  Jamiec May 21 at 11:54
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