Are Leftover Onions Bad For You? Will They Give You Food Poisoning?

How often do you use half of an onion and store the other half in a plastic bag inside the refrigerator? You may have heard the claim that leftover cut onions are a particular hazard and cause of food poisoning. This claim states that cut onions are somehow a magnet for bacteria and that you should never save leftover onions but should discard them immediately. Are onions “poisonous” and dangerous? Should you never save leftover onions for later? Well, this claim came from famous blog post back in 2008 that has since been removed. However, the belief has persisted and has been spread through social media and other internet posts. It is a myth! There is nothing inherently dangerous about leftover onions and you can safely store them in the refrigerator for up to seven days.

According to Snopes, the earliest example they could find of this nonsense was in an online post from March 2008 written by Sarah McCann who wrote on a website called “Dinner with Zola” under the pen name Zola Gorgon, coined off gorgonzola. Zola, who claimed to have written several cookbooks, wrote that she had toured an outfit called Mullins

Mullins Food Products where she spoke to a chemist about food poisoning, who turns out to be tour-guide and co-owner “Ed.” Ed was asked if there is any need to worry about mayonnaise and he claimed, according to Zola, that mayonnaise is perfectly safe and doesn’t even need to be refrigerated, although there’s no harm in doing so. The pH in mayonnaise is such that no bacteria could survive in it, although everyone always blames the mayonnaise when people get sick from the potato salad at the picnic. Ed’s right, mayonnaise is quite acidic, and you are not very likely to get food poisoning from commercial jarred mayonnaise even when it’s sitting outdoors inside some potato salad. Homemade mayonnaise made with raw eggs is another story.

*Warnings about cut onion like this one have circulated online for years.*

Ed went on to claim that when food poisoning is reported, the first thing officials suspect is onions, and asked where the victims last at onions. It’s the onions that spoil in the potato salad, says Ed. And if it’s not the onions, it’s the potatoes. This is because, elaborates Ed, “Both the onions and the moist potato in a potato salad, will attract and grow bacteria faster than any commercial mayonnaise will even begin to break down.”

This little anecdote may well have been the spark that created the myth about cut and leftover onions being bacteria magnets, although the website itself no longer exists. In reality, though, the story was a hash job compared to what Mullins Food actually explained to the people on the tour. In a nutshell, what they said that it’s the other ingredients in the potato salad that are likely to be the cause of foodborne illness, not the mayonnaise. In another exchange, and example of condiments for hotdogs being left out was given, referring to cut and already contaminated onions being used as a topping for the hot dogs. It’s true that you are much more likely to get sick from onions and other ingredients that have been chopped and left sitting out to use on a freshly heated hot dog than the hot dog itself. Mullins food did claim that already cut onions were notorious culprits for causing food poisoning outbreaks.

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Onions do not contain any compounds that are dangerous when the onions are cut or stored. Likewise, onions do not attract bacteria or present a more favorable environment for bacteria compared to other foods. Despite the cautions that Mullins expressed, this myth was essentially made up and presented on the web by one source, from where it was disseminated. As Snopes pointed out, chopped up wet foods, especially since they may have been contaminated from microbes already on the surface of the food, are a great environment for bacteria to thrive but the idea that cut onions in particular are “magnet for bacteria” is not credible.

Two compounds in onions, allicin and quercetin, have been shown to have antibacterial activity. Other compounds like propenesulfenic acid, flavonoids, and fructo-oligosaccharides, kaempferol, leteolin and quercetin derivatives may contribute to the anitbacterial activity of onions. Onion juice or onion extracts may be useful as bacterial inhibitors. ¹,²

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How To Store Leftover Onions

Both fresh and cooked onions can be safely stored in the refrigerator. The peel of an onion may contain more of certain antibacterial compounds. However, the peel may also be contaminated with dirt and bacteria. So, it is best to remove the peel before storing leftover onions. When storing half of an onion, store the peeled onion in a sealed plastic bag in the refrigerator. You can also wrap the onion in plastic wrap or place it in a sealed storage container. Half an onion will keep well for around 5 to 7 days in the fridge. If the edges of the cut onion look dry when you want to use it, simply slice off the dry part.

Store sliced onions in an airtight container or plastic bag. Sliced or chopped onions will not last as long as an intact half onion but can be safely stored in the refrigerator for a few days.

scallions or green onions at farmer's market

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Before placing leftover onions in the refrigerator, make sure they are sealed tightly or the odor from the onions will escape into the air inside the refrigerator, potentially impacting the smell and taste of other foods.

Cooked onions can be stored in the fridge for 4 to 5 days in a tightly sealed container.

Freezing Onions for Storage

When freezing onions, it is best not to a freeze whole or halved onion. Before placing onions in the freezer for storage, chop the onions to the size you want to use and store them in a freezer bag. Use within six months for best results.