Moldy Onions: Eat or Toss?, and How to Prevent

David J. Allen

I spent the better part of 2 ½ years learning about mold and indoor air quality because my child's life depended on it. Now I help others avoid ever having to experience the same thing. The mold industry suffers from conflicting opinions and I do my best to distill loads of data into something practical and useful. I love hearing from and helping readers. • "A house desecrated by mildew, mold, or fungus would be a defiled place to live in, so drastic measures had to be taken." — Leviticus 14:45

If you can help it, it’s best not to eat moldy onions or any other fruit, food, or vegetable that is moldy.

But if you’re not personally too worried about it, the U.S. Department of Agriculture does say that since onions are firm and have a low relative moisture level, they are one of the many vegetables that can be eaten after the moldy part is cut off and discarded (thrown away).

“Small mold spots can be cut off FIRM fruits and vegetables with low moisture content. It’s difficult for mold to penetrate dense foods.”

U.S. Department of Agriculture, “Molds on Food: Are They Dangerous?” Last Updated: Aug 22, 2013

According to the USDA, you can simply rinse off mold on the outside of an onion under cool running tap water.

Unless you’re allergic to mold, in which case you’re best off not using even the non-moldy part of an onion.

Not sure whether you are allergic to mold or not? Then it’s best not to risk it and to throw away the onion.

Ingesting mold could result in certain adverse health effects.

That said, finding a bit of mold on the outside of your onions doesn’t necessarily mean you have to throw them out.

It really depends on what the mold looks like, how much of it there is, and your own personal preferences.

Mold on onions typically presents as a fine dry black powder, but it can progress to a wet black goo which indicates rot beyond salvage.

Black mold on onion is caused by aspergillus niger, a common fungus in soil. To discourage mold growth, store onions in the refrigerator up to two months. Rinse off small amounts of the black mold on the outer scales of the onion under cool, running tap water or cut off the affected layers. The unaffected part can be used. Persons known to be allergic to Aspergillus niger should not use onions with black mold.

U.S. Department of Agriculture, July 17, 2019, “Can you use onions with black mold?”

What Type of Mold Is It?

It’s important to understand that certain species of mold are much more dangerous to consume than others. While the most common type of mold found on onions tends to be black mold, you may come across the fungus in one of several other colors.

As a general rule, if you discover mold that’s shaded blue, green, pink, or any other non-black color, the safest course of action is to throw the affected onions away.

The species of mold that grow in these colors can make you sick if you accidentally ingest the spores, and it simply isn’t worth the risk.

However, if you find black mold on your onions, you should be safe to cut around the fungus and eat any parts of the onion that aren’t affected. If the affected area is small, you may even be able to rinse the mold off with cool running water.

Generally, if you successfully remove all of the visible black mold, the rest of the onion should taste fine and be completely safe to consume.

But again, if you’re at all concerned about it and you don’t know how your body would react, please just toss the onion(s) and grab another if you can. Because you can’t identify a mold type by its color alone.

“Basically you shouldn’t be eating food with mold on it,” Dr. Gourama warns. But there are some situations where mold can safely be cut off and the remaining food saved. “It depends on the texture,” Dr. Gourama says. Onions being hard textured, they fall into the salvageable camp. “If it’s only a few spores of the mold, you can remove the moldy layers, then wash the onion very well.”

Dr. Hassan Gourama, Associate Professor of Food Science at Penn State University, as told to Epicurious, April 30, 2015

5 More Ways to Tell if an Onion is Too Bad to Eat

Besides visible mold growth, there are other ways to tell if your onions have gone too far south to comfortably eat.

  1. Dark discoloration is usually a good sign that an onion is beyond salvagable.
  2. Horrid smells can come from some onions, and they’re as bad as they smell. Don’t use.
  3. If an onion’s actual flesh is dry—not the skin you’ll peel off—it’s a small sign it may be at or near the end of its useable lifespan, but isn’t the biggest concern if it’s the only problem you see.

Preventing Your Onions From Getting Moldy

  • Store onions in a dry area.
  • Store onions out of direct sunlight.
  • Store onions at fair temperatures; not too hot.
  • Store onions in a space with adequate air flow/ventilation.
  • Store uncut onions in a way that allows them to “breathe.”
  • Burlap sacks are great for storing uncut onions.
  • Don’t wet or wash onions before storing them.
  • Don’t wash onions before you plan to use them.
  • Store cut onions away from other food.
  • Store cut onions in a ZipLoc bag or a glass jar.

Whole onions can be safely stored for 2-4 weeks at room temperature, if in a dry and well-ventilated area out of direct sunlight. Diced onions can last 3-4 months in a fridge, if the are inside of a closed container such as a glass jar.

Once you’ve purchased your onions from the grocery store or market, there are a few things you can do to reduce the chances of mold developing.

For one thing, if you discover after purchasing them that one or more of the onions have noticeable black mold growth, get them away from the rest of the batch.

You can either store them separately, cut the affected layers away, or immediately throw them out. It will also help to avoid rinsing the onions off until you’re ready to use them.

If you wash onions before storing them, they’ll sit with all of that moisture for several days, which will make it much easier for the fungus to grow.

The other effective measure you can take is to be deliberate about the way you store the onions.

Ideally, you want to make sure they’re somewhere dry, properly ventilated, and not too warm.

For example, if you leave them on the kitchen counter, during the summer, in a sealed plastic bag, you’ll just be asking for mold to show up. Instead, consider storing them in a room with low humidity and a consistent temperature below 70 degrees.

And instead of sealing them in a bag, try going with an open basket or even a mesh bag.

If you can’t find any appropriately cool place in your home, you can store them in the fridge, but you should only do so if they’re peeled, sliced, or diced.

Otherwise, the whole onion’s papery skin will end up absorbing much of the moisture from inside your refrigerator.

If you take these measures and keep your onions stored wisely, you ought to have at least 2 weeks and up to 4 months or so before you need to worry about mold growth appearing.

So, if possible, try not to purchase your onions too far in advance of when you plan to prepare them.

That way, as long as you’re proactive, you should be good to go when it’s time to start cooking!