What you see: Sliced onion in the fridge that reminds you of that scary, improbable thing you read on the Internet that one time. Even though it didn’t really seem true, you’re not sure you want to take the chance...
What it is: Perfectly fine sliced onion
Eat or toss? Eat!
Once upon a time, in 2008, someone wrote somewhere in a paranoid corner of the web that sliced onions are bacteria magnets. So much so, the writer proclaimed, that you should never save them for later. Soon thereafter, a great battle ensued, with fact and science facing off against rumor and fear. Ultimately, however, our heroes, fact and science, emerged victorious.
At least, this is how I like to think the story ended. I’m not sure that the battle is really over yet, but I’d like to use this space to cast one more vote for fact and science. The answer is no. There is nothing unsafe about sliced onion that’s been stored in the fridge.
The original claim said that sliced onions were a magnet for bacteria. The National Onion Association begs to differ, pointing out that sliced onions can be stored safely in the fridge for up to seven days. In fact, cutting onions releases sulfur compounds that can kill or inhibit some microorganisms that can harm people. Those are the very same compounds make your eyes water when you cut an onion.
Like anything else, to keep away harmful bacteria, you need to practice good kitchen hygiene yourself. Make sure your hands and work surfaces are clean and rinse off the onion before you cut it. And steer clear of ridiculous rumors on the Internet—avoiding them is good for your health too.
National Onion Association - Cut Onion Statement
National Onion Association - Myths About Onion
Snopes.com - Cut Onions Contamination
McGill Office for Science and Society - Is it true that onions can absorb bacteria?
Hints from Heloise - Onion Story Tangled in Net