Are Leftover Onions "Poisonous," as Claimed on the Internet?

a purple onion being sliced.

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A viral text circulating since April 2008 claims that raw, leftover onions are "poisonous" and should never be kept for re-use, even in a refrigerator, because they are "a huge magnet for bacteria," supposedly, and especially prone to spoilage. However, this is a mostly false rumor, as food scientists do not concur.

Viral Email Example

Email text - November 24, 2009:

I have used an onion which has been left in the fridge, and sometimes I don't use a whole one at one time, so save the other half for later.
Now with this info, I have changed my mind.... will buy smaller onions in the future.
I had the wonderful privilege of touring Mullins Food Products, Makers of mayonnaise.. Mullins is huge, and is owned by 11 brothers and sisters in the Mullins family. My friend, Jeanne, is the CEO.
Questions about food poisoning came up, and I wanted to share what I learned from a chemist.
The guy who gave us our tour is named Ed. He's one of the brothers Ed is a chemistry expert and is involved in developing most of the sauce formula. He's even developed sauce formula for McDonald's.
Keep in mind that Ed is a food chemistry whiz. During the tour, someone asked if we really needed to worry about mayonnaise. People are always worried that mayonnaise will spoil. Ed's answer will surprise you. Ed said that all commercially-made Mayo is completely safe.
"It doesn't even have to be refrigerated. No harm in refrigerating it, but it's not really necessary." He explained that the pH in mayonnaise is set at a point that bacteria could not survive in that environment. He then talked about the quaint essential picnic, with the bowl of potato salad sitting on the table and how everyone blames the mayonnaise when someone gets sick.
Ed says that when food poisoning is reported, the first thing the officials look for is when the 'victim' last ate ONIONS and where those onions came from (in the potato salad?). Ed says it's not the mayonnaise (as long as it's not homemade Mayo) that spoils in the outdoors. It's probably the onions, and if not the onions, it's the POTATOES.
He explained, onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion.. He says it's not even safe if you put it in a zip-lock bag and put it in your refrigerator.
It's already contaminated enough just by being cut open and out for a bit, that it can be a danger to you (and doubly watch out for those onions you put in your hotdogs at the baseball park!)
Ed says if you take the leftover onion and cook it like crazy you'll probably be okay, but if you slice that leftover onion and put on your sandwich, you're asking for trouble. 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.
So, how's that for news? Take it for what you will. I (the author) am going to be very careful about my onions from now on. For some reason, I see a lot of credibility coming from a chemist and a company that produces millions of pounds of mayonnaise every year.'


Versions of this text have been circulating since mid-2008, with the earliest examples attributed to food writer "Zola Gorgon" (aka Sarah McCann), though the exact date or venue of its original appearance cannot be pinpointed.

While the article makes a valid point about the relative safety of commercially-produced mayonnaise versus the other ingredients typically found in homemade potato salad (e.g. onions and potatoes), it exaggerates the danger of keeping and using leftover raw onions.

It's Not the Onions; It's How You Handle Them

According to science writer Joe Schwarcz, onions are in no sense a "magnet for bacteria." In fact, Schwarcz writes, cut onions contain enzymes that produce sulphuric acid, which inhibits the growth of germs. Onions can become contaminated during handling, but there's nothing about them that makes them intrinsically more susceptible to bacterial growth or spoilage than any other raw vegetable.

"So unless you have sliced your onions on a contaminated cutting board, or handled them with dirty hands," Schwarcz explains, "you can safely put them in a plastic bag and store them and there will not be any bacterial contamination."

Food Folklore: Onions 'Attract' or 'Collect' Infectious Bacteria

The notion that onions are a "bacteria magnet" may stem from an old wives' tale dating at least as far back as the 1500s, when it was believed that distributing raw onions around a residence guarded against the bubonic plague and other diseases by "absorbing the elements of infection."

Though it has no scientific basis whatsoever, some people still believe this today.


Is It True that Onions Are 'Magnets for Bacteria'?
By Dr. Joe Schwarcz, McGill University

Onions as Bacteria Magnets
The Chemist's Kitchen, 6 April 2009

Food Safety Facts: Mayonnaise and Dressings
Association for Dressings and Sauces

Onions and Flu
Urban Legends, October 23, 2009

Refrigerate Cut Onions for Best Storage
Charlotte Observer, January 2, 2008