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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?

Flimzy
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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. – SIMEL May 19 '14 at 18:29
  • @IlyaMelamed: I have the same experience, but anecdotes don't make good answers :) – Flimzy May 19 '14 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.](http://cooking.stackexchange.com/tags/food-safety/info) – Oddthinking May 19 '14 at 21:57
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    sounds like it should be posted on The Onion. – user541686 May 20 '14 at 00:18
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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 '14 at 07:32
  • Haha, I read the title too quickly and thought the second word was "opinions". I just came here to share my toxic ones. – Dawood ibn Kareem May 20 '14 at 07:40
  • any setup that claims a cut onion "creates" bacteria, harmful or not, would automatically raise some suspicion as to its veracity. i don't think this is an issue with grammar, as much as it is a piece of sensationalism. – alt May 29 '14 at 15:58
  • @alt: I can give leeway for someone saying "creating" bacteria, when what they really mean, from a technical standpoint, is attracts and/or cultivates bacteria... But you're right. If onions could truly create bacteria, that would be a claim all of its own, and quite the biological breakthrough if we could determine how. – Flimzy May 29 '14 at 16:19

2 Answers2

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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.

Community
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SIMEL
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    Is the Nationl Onion Association a bias-free source about onions? – Erel Segal-Halevi May 20 '14 at 05:49
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    @Erel, *The Onion* would probably disagree. – Simon Richter May 20 '14 at 06:28
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    Snopes [presents a similar story](http://www.snopes.com/food/tainted/cutonions.asp). The Wayback Machine [has several snapshots of mentioned blog](https://web.archive.org/web/*/http://www.dinnerwithzola.com/), however with some quick searches I couldn't find mentioned source though. Maybe someone else can spot it? – Jeroen May 20 '14 at 07:32
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    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 '14 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/

user19338
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    Whilst this may theoretically answer the question, [it would be preferable](http://meta.stackoverflow.com/q/8259) to include the essential parts of the answer here, and provide the link for reference. – Jamiec May 21 '14 at 11:54