Are Leftover Onions Poisonous?

By Conan Milner, Epoch Times
March 25, 2016 10:40 am Last Updated: March 27, 2016 4:07 pm

Onions are a kitchen staple. To many cooks, they are as essential as salt and pepper. But does this common vegetable have a dark side? Do leftover onions act as a sponge for illness and bacteria?

The Internet seems to think so. Email chain messages and social media memes have been warning for years that once you cut open an onion it becomes dangerous virtually overnight. One widely circulated message claims that cut onions become “highly poisonous” because they “create toxic bacteria, which may cause adverse stomach infections because of excess bile secretions and even food poisoning.”

Another prevalent piece of Internet onion advice involves a charming anecdote from the 1919 influenza epidemic: In a community where everyone else has fallen ill, a doctor finds a family who has managed to avoid getting sick by leaving cut onions around the house. When the doctor examines the family’s onions under a microscope, he finds a trace of the flu virus—proof that onion’s illness-absorbing power saved the day.

The author’s source for this tale is an unidentified hairdresser, but the basic idea has been around a long time.

I tell consumers that it is all a hoax.
— Marianne H. Gravely, U.S. Department of Agriculture's Meat and Poultry Hotline

According to 17th century herbalist Nicholas Culpepper, onions have an ability “to draw any corruption to them, for if you peel one, and lay it upon a dunghill, you shall find it rotten in half a day, by drawing putrefaction to it.” Similar beliefs even advise burying onions used to extract disease, because their concentrated poison can become so potent.

Onion has been an important food and folk medicine for thousands of years, and has been used for a wide variety of ailments, such as vision problems, dog bites, and toothaches. Modern research has shown that onions can strengthen the immune system, act as a natural blood thinner, prevent cancer, and treat respiratory infections.

But do onions really have a special ability to suck up infection? Let’s peel away the layers to find out.

Onion illustration by Otto Thome, 1885. (Public Domain)

Credible Sources, False Quotes

Stories that support the poisonous onion idea often point to reliable sources for credibility. One such message states: “The U.S. Food and Drug Administration has come to the conclusion, after research, that onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion. It is not even safe if you put it in a zip-lock bag and put it in your refrigerator. It is already contaminated enough just by being cut open and out for a bit, that it can be a danger to you.”

Sounds convincing, but it’s not true. In an email, FDA spokesperson Lauren E. Sucher said the agency “does not have specific recommendations for cut onions.” She offered FDA links on general food safety advice: Are You Storing Food Safely?, Safe Food Handling: What You Need to Know, and 7 Tips for Cleaning Fruits, Vegetables.

It’s not in her jurisdiction, but Marianne H. Gravely, webmaster for the U.S. Department of Agriculture’s Meat and Poultry Hotline, says she gets calls from people with concerns about poisonous leftover onions. “I tell consumers that it is all a hoax,” she wrote in an email. 

Barry E. Parsons, a food safety expert for Robson Forensic, confirms that when it comes to contamination, onions do not have “a greater propensity to absorb bacteria.”

Handled properly, whole or cut (for a day or two), they are just as safe as other foods.

“Handled properly, whole or cut (for a day or two), they are just as safe as other foods,” Parsons wrote in an email.

Snopes links much of the spread of the infected onion idea to an article which claims that cut onions “are responsible for more food poisoning than spoiled mayonnaise.”

According to Snopes, the article was written in 2008 by cookbook author and food writer Sarah McCann under the pen name Zola Gorgon. Gorgon said she got her onion information while touring the Mullins Food Products facility. She identified her tour guide as chemist Ed Mullins.

“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,” Gorgon writes. “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!)”

The Epoch Times contacted the Mullins company to speak to the source of the quote. According to an email from Bill Mullins, “Neither Ed Mullins nor Mullins Food Products participated in the preparation of the article.”

Onion illustration from the Icones Plantarum by F.B. Vietz, 1806. (Public Domain)

How Onions Heal

For generations, people have acknowledged onion’s healing powers; however, there seems to be long-held confusion with the way it works.

For example, folk medicine techniques such as spreading chopped onions on the chest to relieve congestion, or sticking an onion piece in the ear to resolve an infection, are said to work because onions draw out illness. But think about it: If leftover onions are sponges for harmful bacteria, then why aren’t there more problems on salad bars and prep stations in restaurants, or in the many kitchens across America where onion portions are often stored for later use?

Considering onion’s pungent nature, it’s more likely that its chemicals are invading you, rather than the other way around.

One undeniable feature of onion is its ability to encourage tears. When an onion is cut, cell walls are broken, releasing enzymes that were previously kept separate by the onion layers. When these enzymes combine, it sets off a chemical reaction creating a sulfuric vapor that can make your eyes water and your nose run. Onion chemicals have been shown to have anti-inflammatory, antibiotic, and antiviral properties. Onion also thins mucus, making the symptoms of infection easier to remove.

Onions have a lot of other verifiable benefits. They are rich in vitamin C, iron, and quercitin, an antioxidant that helps the cardiovascular system and may even stop tumor growth. Onions are also a good source of inulin—a prebiotic fiber that promotes healthy bacterial ecology.  

While many cultures have embraced onions, others limited or even shunned them. According to traditional Ayurvedic medicine, for example, onions,  garlic, and other pungent foods are restricted because too much is believed to stimulate desires. Onions and garlic are forbidden in ancient Buddhist traditions because the eater takes on a strong, offensive odor. Even today, people suffering from irritable bowel syndrome are recommended to avoid onions and garlic because these pungent vegetables can further irritate their condition.

There may be good reasons not to eat onions, but its questionable reputation as a disease sponge is not one of them.