How to Tell If an Onion Is Bad (5 Warning Signs)

Wondering how to store onions and how long they last? We’ve put together a guide that covers everything you need to know about onions and their storage.

Onions have long been a kitchen staple—adding depth of flavor and aroma to countless dishes. Whether you’re caramelizing them for a savory French onion soup or dicing them up for a fresh salsa, onions can be found in virtually every meal.

Like any perishable food, onions can go bad over time. While celebrity chefs and cookbook authors seldom go into the technicalities of how to tell if an onion is bad, this is an important skill to have if you want to get the most out of your alliums in your home cooking.

If you’re wondering how to tell if an onion has gone bad, we’ve got you covered. In this guide, we’ll provide you with helpful tips and information to answer this question, and more.

The Signs That an Onion Has Gone Bad

Sprouting: If you notice green sprouts emerging from the onion’s top, it’s an indication that the onion is past its prime.

While you can still use the onion if the sprouts are small and it shows none of the other signs of spoilage, they can affect the onion’s taste and texture and make it harder to peel.

If you’re still undecided, examine the onion and look for any of the signs below.

Dark spots: Dark spots on the onion’s outer layers are typically a sign of decay. These spots may be mucilaginous and slimy, and they can spread quickly to the rest of the onion.

The color of the dark spots on your onions may vary depending on the variety you have at hand. For instance, spring onions and white onions may develop light brown spots, while red onions and brown onions may develop dark purple or dark brown spots, respectively.

Mold growth: Check the onion for any signs of mold growth, which is typically found near the root end. If you see mold, it’s a sure-fire sign that the onion has gone bad.

Moldy onions are not safe to eat. Mold can leave spores deep inside the onion, so it’s best to discard any moldy onions and not consume them.

Mushy, slimy texture: A good onion should be firm and dry to the touch. Although green onions and younger, less dry yellow onions and white onions feel tender and ooze out a milky substance when cut, they should never feel mushy and slimy.

Hold the onion in your hand and give it a gentle squeeze. If you notice any overly soft or mushy spots, it’s a sign that the onion has started to rot from the inside out, and that it may be best to discard it.

Bad smell: A fresh onion has a distinct but mild aroma that’s not overpowering. However, a bad onion may emit a strong, unpleasant odor that smells rotten or sulfurous. This “bad onion smell” is caused by bacteria that thrive in the moist environment of a decaying onion.

If an onion smells bad, don’t eat it; it may already be overgrown with bacteria that can make you sick.

How to Store Onions: Onion Storage Secrets

To make a long story short, dry whole onions should be kept in a cool, dry, dark, and well-ventilated place. Green onions, cut onions, and cooked onions, on the other hand, should be refrigerated. Read on below for the details.

Tips for Storing Whole Onions

Store your onions in a cool and dry place, away from direct sunlight and sources of heat in your house. Consider using a mesh bag or a crate, mesh bag, wire basket, or perforated plastic bag to store your onions, which allows for air circulation and prevents moisture buildup.

Opt for a dark place like the pantry, a cupboard, or a kitchen cabinet, and avoid the countertop, windowsill, and the top of your fridge. Other potentially good places for storing onions are the basement, a cellar, and even your garage.

In an article discussing onion harvesting and preservation, Richard Jauron, an Extension Horticulturist at Iowa State University, recommends storing onions at temperatures between 32 and 40°F and humidity levels between 65 and 70% for optimal storage conditions.

Storing onions in temperatures that are too warm can cause sprouting, Jauron writes, while storing them in overly humid conditions can promote rotting.

The tips mentioned above apply to all varieties of dry, whole onions, including brown, red, yellow, and white onions.

The exception to the rule are green onions, also known as spring onions, which should be stored in the fridge from the moment you bring them back home. As Trevor Suslow and Marita Cantwell of the University of California’s Post Harvest Center explain in an article on the subject, higher temperatures greatly hasten the yellowing and decay of the leaves.

To keep spring onions fresh, cut off the roots, then cut them in halves and refrigerate them in a sealed container.

Tips for Storing Cut Onions

Once you cut into an onion, it’s more susceptible to moisture and bacteria, which can make it spoil faster. If you’ve peeled and sliced your onions, you have to store them in the fridge, in a zipper bag or food storage container to prevent them from tainting other foods.

When refrigerating sliced onions, avoid storing them together with other vegetables, especially potatoes, which can release moisture and ripening gases that accelerate decay.

Can You Refrigerate Onions?

Although onions can be stored in the fridge, doing so can cause them to become soft and soggy since the starches in the onion can convert into sugar at low temperatures. Additionally, the moisture level in the fridge can contribute to molding and rotting.

Store whole onions in a cool, dry, and dark place, keep unpeeled onions whole until you’re ready to use them, and remember that sliced onions should be refrigerated immediately after cutting.

How Long Onions Last: Onion Shelf Life 101

Green onions keep for at least 5 days in the refrigerator.¹

Dry, whole onions can be stored for up to 1 month in the pantry and up to 2 months in the fridge.² Cut onions should be refrigerated and must be used up within 7 days, and cooked onions should be used up within 4 days.³⁴

Onions can be stored in the freezer at 0°F (-18°C). However, they don’t freeze well and their texture becomes mushy after thawing. Since freezing halts bacterial growth, frozen onions will always be safe to eat.⁵ However, it’s recommended to use the onions up within 10 to 12 months from the date of freezing for best quality.

Bottom Line

If you notice soft spots or mold on your onion, it’s time to toss it out. In addition, if an onion sprouts, has a rotten odor, dark or brown spots, and feels mushy or slimy, it’s almost certain to be bad and should be discarded.