We’ve all come home from the supermarket and cut into an onion, only to find that it has a mushy brown ring on the inside. And then there are the times when you pull an onion from the pantry and notice signs of mold on the skin. What do you do in those cases? Can you simply remove the blighted layers, or do you need to discard the whole bulb? And is there a way to tell if an onion is bad before you buy it or try to cook with it?
Cooking Tips
How to Tell If an Onion Is Bad
A plant pathologist and an agricultural educator peel back the layers on how to identify rot—and how to prevent it in the first place.
By Published June 21, 2023.
Sign up for the Cook's Insider newsletter
The latest recipes, tips, and tricks, plus behind-the-scenes stories from the Cook's Illustrated team.
America's Test Kitchen will not sell, rent, or disclose your email address to third parties unless otherwise notified. Your email address is required to identify you for free access to content on the site. You will also receive free newsletters and notification of America's Test Kitchen specials.
To answer these questions and more, I contacted two experts: Dr. Bhabesh Dutta, associate professor and extension vegetable disease specialist at the University of Georgia, and Nick Frillman, local food systems & small farms educator—Livingston, McLean and Woodford Counties—University of Illinois Extension.
What Causes a Brown, Mushy Ring Inside an Onion?
A soft, brown layer can have many origins, said Dutta, though it’s likely due to an infection caused by pathogenic bacteria. These include Pantoea spp., which leads to so-called center rot, and Burkholderia spp., which brings about an onion disease dubbed sour skin. Rot can also develop from saprophytic bacteria, which feed on dead and decaying organic matter.
You may have also noticed a dry, papery brown layer inside an onion instead of a pulpy one. This, he explained, is due to Pantoea bacteria that enter the bulb during maturation. After a while, it will also turn to mush, just like the other types of rot.
Regardless of the cause, an onion with a brown ring on the inside, whether mushy or dry, should be discarded.
Is It Safe to Eat Moldy Onions?
Molds proliferate in moist environments and tend to rot the bulb from the outside in. There are many types and colors of mold that can develop on onions—mostly on the outer scales or just beneath the papery layer—said Dutta, and black and gray molds are quite common.
Per the USDA, it’s OK to use onions that have a little black mold (Aspergillus niger) on the outside after rinsing them under cool running water or cutting off the moldy area to remove as many spores as possible.
Onions that are heavily molded or that contain other colors of mold (including gray, white, or blue) should be discarded as they are likely contaminated below the surface.
Book
Vegetables Illustrated
This is the book for anyone looking for fresh, modern ways to add more vegetables to their diet—which is just about all of us. And this exciting guide could have come from only Cook’s Illustrated.Buy Now
How to Tell If an Onion Is Bad on the Inside
There’s no perfect way to determine if an onion is rotten before you purchase it or cut into it, agreed Dutta and Frillman, but they did offer a few shopping tips to greatly increase your chances of avoiding rot:
- Buy loose onions, not bagged so that you can inspect each one before purchase.
- Use your thumb to press on the “shoulder” of the onion (the area near the stem), to make sure that it’s firm. If it is soft or mushy, the onion will likely contain some internal rot.
- Employ the sniff test: An onion that is decomposing may smell like rotting food.
- Avoid onions that have specks of what may look like black dirt on the skin. This is actually mold.
- Avoid onions with cuts or breaks in the skin, as these channels offer an opportunity for bacteria or mold spores to enter.
Can One Bad Onion Spoil the Others Near It?
A single rotted or moldy onion will certainly spoil others that are stored alongside it, so if you spot a bad one in the mix, be sure to trash it right away.
In addition to visually inspecting your onion stash once a week for decay and deformities such as brown spots, sunken lesions, and molds, Frillman said to be aware of unpleasant odors: “If anything smells bad, you’ve got a bad onion in there and you better find it.”
Is It OK to Eat a Sprouted Onion?
Sprouting is a physiological change in the onion, not a pathogen or disease, explained Dutta, so sprouted onions are safe to eat. That said, the sprouts don’t taste very good.
In the test kitchen, we have found sprouted alliums to be more fibrous and less sweet and flavorful than unsprouted ones. And although the sprouts themselves bear a physical resemblance to chives and scallions, they taste unpleasantly bitter rather than grassy and pungent.
That’s because sprouts use the sugar stored in the parent onion as an energy source, so as the sprouts grow, the onion flesh loses sweetness. Sprouting also causes the onion to lose moisture, making it seem tougher and stringier.
You may want to consider tossing a sprouted onion, but if you do want to cook it, do so as soon as possible after the sprouts appear, since the flavor and texture of the onion will only continue to deteriorate. And the sprout itself? Get rid of it.
How to Store Whole Onions
Most pathogens, said Dutta, like “warm, cozy, moist conditions,” so it’s best to store onions in a cool, dry, well-ventilated place. (The humidity inside a refrigerator may encourage spoilage.) Kept this way, everyday supermarket onions should last at least 2 weeks, though some top-notch storage onions such as Copra or Red Zeppelin boast a shelf life of five to eight months.
It’s also best to keep onions away from light, which can give them a germination cue and cause them to sprout. We have found that a colander in a pantry is ideal for storing onions.
