Leftover Onions are Poisonous Because They’re Magnets for Bacteria-Mostly Fiction!
Summary of eRumor:
Leftover onions become poisonous after just a day because they’re magnets for bacteria.
Claims that leftover onions are magnets for bacteria and should never be eaten are mostly fictional.
That rumor first surfaced in a 2008 blog post that appeared at the website DinnerWithZola.com. The blog, headlined “Watch out for those spoiled onions,” reported that cut or leftover onions are magnets for bacteria and are among the most common causes of food poisoning:
…Onions are a huge magnet for bacteria, especially uncooked onions. You should never plan to keep a portion of a sliced onion. (A food expert) 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 National Onion Association took exception to that claim. The trade group argued that the opposite is true: juices released from cut onions kill and inhibit the growth of some microorganisms that can cause illness:
When cut, onions release compounds that do not promote pathogen growth. This is the same compound that causes our eyes to produce tears. When handled properly, cut onions can be stored in the refrigerator in a sealed container for up to 7 days.
Uncut onions, meanwhile, should be stored in a dark, dry and well-ventilated place. It’s always important to make sure that you store onions and potatoes separately, too, because they can make each other spoil faster, according to the Penn State Extension:
Avoid storing onions near potatoes; onions will absorb the potato’s moisture and spoil from exposure to the potato’s gas. Also avoid storing onions where they will be exposed to moisture.
So, it’s true when not properly stored after they’re cut, onions can become magnets for bacteria. The claims that all onions are poisonous and should not be eaten within a day or two of being cut is false.