The Two-Hour Rule

• The average shelf life of a whole, unpeeled onion is around four to six weeks when placed in a cool, dark location; whole onions don't require refrigeration. If you have leftover whole onions, simply store them at room temperature. Once you peel or chop your onions, they need to be refrigerated at 40 degrees Fahrenheit or less. You'll need to place the onions in the fridge within two hours to prevent harmful bacteria and other spoilage microorganisms from turning them toxic, recommends the Ontario Ministry of Health and Long-Term Care. Store your chopped leftover onions in a resealable plastic bag or other tightly sealed container for up to one week.

When Good Leftovers Go Bad

• Many types of spoilage microorganisms, including bacteria and yeast, can begin multiplying within your chopped onions if they aren't refrigerated after two hours, making them potentially dangerous to eat. Unfortunately, while the onions may be spoiled, you might not be able to tell that they are by simply looking at them or smelling them. For safety's sake, discard your leftover chopped onions if they haven't been refrigerated overnight. This is because spoilage bacteria can flourish in room temperatures between 41 and 135 F. Note that refrigeration is also necessary for cooked leftover onions, which have a shelf life of around three to five days, according to the Still Tasty website.

Bad Bugs

• When storing any leftover chopped onions, keep them away from raw meat and poultry to prevent them from becoming contaminated with bacteria, even in the refrigerator. If the chopped onions have come into contact with meats or unclean surfaces, they could immediately become contaminated with harmful bacteria such as E. coli or Salmonella, which can survive refrigeration, making them dangerous to eat. While cooking contaminated onions to temperatures greater than 135 F can kill off these pathogens, molds that form on unrefrigerated chopped produce such as onions can produce harmful substances known as mycotoxins. Cooking won't eliminate these toxins and they can cause food-borne illness if ingested.

When in Doubt

• While it might seem hard to believe, just one cell of harmful spoilage bacteria can multiply into more than one million cells at temperatures between 41 and 135 F within five hours, less than an entire night. Spoilage bacteria can make you sick within 12 to 72 hours of ingesting it, warns the Texas Cooperative Extension. You might experience symptoms of food poisoning, including vomiting, headaches, fever and diarrhea, if you eat leftover onions that haven't been properly refrigerated or frozen to keep these pathogens at bay. When in doubt, throw those potentially tainted onions out unless you know that they've been refrigerated overnight.