Onions are a versatile vegetable that can last a long time if stored properly. However, onions will eventually spoil and go bad. Determining if an onion is still usable or has gone bad can be challenging. This article will cover the signs of fresh onions, what causes them to spoil, how to tell if they are bad, proper storage methods, and how long onions last in various conditions. Read on to learn how to identify bad onions and keep your onions fresh for as long as possible.
Table of Contents
Signs of a Fresh Onion
When selecting fresh onions at the store, look for a few key signs that indicate quality:
- Firm, dry outer skin – Avoid onions with soft, moist, or slimy skin
- No scent – Onions should not smell pungent or unpleasant
- Heavy for size – Heavier onions are denser and fresher
- Tight, dry necks – The neck area should not show signs of sprouting
- No visible mold – Mold appears fuzzy or web-like on the skin
A fresh, high-quality onion will feel firm all over when gently squeezed. The outer skin should be only slightly slippery, not wet or greasy. Internally, the flesh should look moist and fresh, with no dark or mushy spots.
What Causes Onions to Go Bad?
Onions spoil due to a combination of factors:
- Natural moisture loss – Onions lose moisture over time, causing them to shrivel and go limp
- Sprouting – Improper storage can cause early sprouting and deterioration
- Mold – Excess moisture promotes mold growth on onions
- Bacterial soft rot – Bacteria causes onions to liquefy, creating a foul odor
- Physical damage – Cuts, bruises, and crushing can accelerate spoilage
Exposure to heat, light, and excess air circulation also speeds up moisture evaporation and sprouting in onions. Storing onions in the proper conditions is key to extending their shelf life.
How to Tell if an Onion Has Gone Bad
Look for these key signs of spoiled onions:
Mold Growth
- Fuzzy gray, green, or black mold on the onion’s outer skin or inner flesh
- Webbing or spotting in mold patterns
- Discard at first sign of mold – it can penetrate deep into the onion
Unpleasant Odor
- Foul, rotting, fermented or sulfurous smell
- Strong onion smell is normal, but it turns acrid and unpleasant when bad
Discolored Outer Skin
- Dark or black spots on dry skin
- Yellow or brown color instead of bright white/red
- Soft or shiny skin instead of crisp and dry
| Good Onion | Bad Onion |
|---|---|
| Firm, dry skin | Soft, moist, or slimy skin |
| No scent | Pungent, unpleasant smell |
| Tight, papery skin | Loose, yellowed skin |
Interior Rot
- Brown, gray, black, or mushy spots in the flesh
- Liquefied or slimy texture
- Concentrated at the top near onion neck
Presence of Mold Spores
- Visible mold growth anywhere on exterior or interior
- Possible musty smell
- Allergies or asthma triggered by spores
Sprouting
- Small green shoots emerging from center of onion
- Indicates onion is past prime and aging
Even minor bruising or damage can allow rot to take hold, so inspect the entire onion closely before using.
How Long Do Whole Onions Last?
The shelf life of whole, unpeeled onions depends on the storage method:
| Storage Method | Shelf Life |
| Room temperature | 2-4 weeks |
| Refrigerator | 2-3 months |
| Freezer | 6-8 months |
| Cold root cellar | 4-6 months |
Proper storage with cool, dry conditions extends shelf life by slowing sprout growth and moisture loss. But eventually all whole onions will deteriorate. Signs of old onions include wrinkled skin, rubbery texture, and diminished flavor when cut open.
The Best Way to Store Onions
To maximize freshness, store whole, unpeeled onions in a cool, dark space with plenty of air circulation:
- Pantry or cellar – The ideal storage temperature is 35-40° F.
- Mesh bag – Allows airflow while containing onions.
- Away from light – Light accelerates sprouting.
- Not in seal plastic – Traps moisture and causes rot.
- Not with potatoes – Potatoes release moisture and gases that hasten sprouting.
Loose storage in a wire-mesh basket or pan is optimal for air circulation. Keep onions far from heat sources like appliances or pipes. Check frequently and remove any spoiled onions to prevent rot from spreading.
Can You Get Sick From Eating A Bad Onion?
It’s possible to get sick from eating onions that are moldy, slimy, or rotten. Potential risks include:
- Foodborne illness – Bacteria like salmonella, E. coli, and listeria can grow on spoiled produce.
- Allergic reaction – Mold spores may trigger coughing, skin rash, or anaphylaxis in those allergic.
- Gastrointestinal issues – Onions harboring toxins may cause nausea, vomiting, or diarrhea if ingested.
Always inspect onions before using and discard any that show even minor bruising, skin damage, or odor. Peel off outer layers of good onions if appearance is questionable. When in doubt, throw it out!
Is It Safe to Eat Moldy Onions?
It’s not recommended to eat onions with any mold growth. Surface mold can send threads and spores deep into produce. Invisible mold below the surface can also contaminate the entire onion. Cooking doesn’t destroy mold or its toxic byproducts. Even a small amount of mold is unsafe to eat. Always discard moldy onions.
Frequently Asked Questions
Can you freeze onions whole?
No, whole onions do not freeze well. The low freezing temperature causes cell walls to rupture. Thawing results in mushy, waterlogged onions. Only chop onions before freezing to stop this enzymatic action.
How can you keep cut onions fresh?
Wrap cut onion halves or slices tightly in plastic wrap or store in an airtight container. Refrigerate for 3-5 days maximum. The pieces will last a bit longer than if left exposed to air.
Do onions need to be refrigerated?
Whole, unpeeled onions can be left at room temperature for 2-4 weeks. For long-term storage of more than 1 month, refrigerate whole dry onions. Keep yellow, white, and red onions loosely wrapped in paper towels inside the vegetable crisper.
Can sprouted onions make you sick?
It’s best to discard onions that are sprouting. They tend to be past their prime and more likely to harbor bacteria. Eating mildly sprouted onions may just cause an upset stomach. But fully sprouted onions can potentially cause food poisoning.
How long after the “best by” date are onions good?
The best by date is typically 1 year from harvest. Storage conditions are more important than this date. Tightly wrapped whole onions stay fresh for 2-3 months after the printed date if kept in a cool, dry pantry. Refrigerate to get up to 4 months more.
Final Takeaway
Checking for signs of spoilage and storing onions properly allows you to keep them fresh as long as possible. Discard any onions that appear moldy, slimy, discolored, or have an off smell. Optimal storage conditions are cool, dry, and with plenty of air circulation. Follow the recommended storage times for maximum freshness. With proper handling, onions can stay good for use in recipes long after purchase.