No, pre-chopped onions are not ‘poisonous’ the next day as old myth claims

Social media posts aren’t usually the best source for valid medical information, especially when it comes to ... chopped onions. 

We all know the arduous practice of cutting onions makes us cry like babies – but is consuming day-old chopped onions actually "poisonous"?

That’s what a Facebook post claims:

"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."

The post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

This years-old warning is exaggerated. Don’t be scared to use pre-chopped onions – they are perfectly safe to consume as long as you handle the food properly.

The myth is rooted in the possibility that bacteria can grow on vegetables between the time they are chopped and cooked. But with proper handling and storage, it is unlikely to become "dangerous" overnight or over several nights. 

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In an archived blog post, Joe Schwarcz, director of the McGill Office for Science and Society at McGill University in Canada, writes that onions are not especially prone to bacterial contamination. In fact, he says, it is quite the opposite.

"Cutting an onion triggers the release of enzymes that initiate a chemical reaction producing propenesulfenic acid, which in turn decomposes to yield sulphuric acid. It is the sulphuric acid that makes you cry by irritating the eyes! But sulphuric acid also inhibits the growth of bacteria," Schwarcz says in the post. 

He goes on to explain that because a cut onion’s surface dries out quickly, it reduces the moisture needed for bacteria to multiply. And to have bacteria multiply, there needs to be a source of bacteria on the onion in the first place.

So unless you have sliced your onions on a contaminated cutting board, or handled them with dirty hands, you can safely put them in a plastic bag and store them and there will not be any bacterial contamination," he says. "There is no reason to suggest that onions are in any way more risky than other foods and avoiding onions is not only unnecessary but unhealthy. That’s because onions contain a variety of compounds that have health benefits.

According to the National Onion Association, chopped or sliced onions can be stored in a sealed container in the refrigerator for seven to 10 days. 

An old myth circulating online warns consumers that pre-chopping an onion to cook the following day is dangerous because it "becomes highly poisonous" and "creates toxic bacteria."

Any vegetable can become contaminated with bacteria if it isn’t handled or stored properly. Onions do not attract bacteria more than other vegetables and actually inhibit pathogen growth. 

We rate this claim False.