How to Tell If an Onion Has Gone Bad: 7 Clear Signs

An onion has gone bad when it shows multiple signs including soft or mushy spots, dark patches or mold, a slimy texture, unpleasant odor, or excessive sprouting. Onions with significant discoloration (black, brown, or green mold) or those that feel soft and wet should be discarded immediately to avoid potential foodborne illness.

Discover exactly what to look for when checking your onions for spoilage. This guide gives you seven definitive signs that your onion has gone bad, plus practical storage tips to extend freshness and prevent waste. Learn how to distinguish between harmless sprouting and dangerous mold, understand proper storage conditions, and make safe decisions about using questionable onions.

Visual Inspection: What to Look For

Start your onion assessment with a thorough visual check. Healthy onions should have dry, papery outer skins that feel crisp to the touch. Any visible changes to this protective layer signal potential problems.

According to the U.S. Food and Drug Administration, proper visual inspection is the first critical step in determining vegetable safety. The FDA recommends discarding produce with visible mold growth as certain molds can produce mycotoxins that may cause health issues.

Texture Tells the Truth

Gently squeeze your onion to assess its firmness. A fresh onion should feel solid and heavy for its size. If you detect any soft spots, mushiness, or areas that give way under light pressure, these indicate internal decay.

Research from the University of California Agriculture and Natural Resources shows that moisture accumulation inside the onion layers creates ideal conditions for bacterial growth. Once an onion becomes soft or slimy, it's entered an advanced stage of decomposition that cannot be reversed.

Smell Test: Your Nose Knows

Fresh onions have a characteristic sharp, slightly sweet aroma. As onions spoil, they develop unpleasant odors that range from sour to musty to distinctly foul. If your onion emits any odor that makes you hesitate, trust your instincts and discard it.

Food science studies at Cornell University confirm that spoiled onions produce volatile organic compounds that create these off-odors. These compounds indicate microbial activity that compromises both safety and quality.

Understanding Sprouting: When It's Still Safe

Sprouting doesn't automatically mean your onion has gone bad. Many onions will begin sprouting as they age, which is simply the onion attempting to grow new plants. While sprouted onions have reduced flavor and texture quality, they're generally safe to eat if other signs of spoilage are absent.

Remove the green sprouts and any soft layers, then use the remaining firm portions promptly. The USDA confirms that sprouted onions remain safe for consumption as long as no mold or significant softening has occurred.

Storage Mistakes That Accelerate Spoilage

Improper storage dramatically shortens onion shelf life. Common mistakes include:

• Storing onions in plastic bags (traps moisture)
• Keeping onions near potatoes (emits gases that accelerate sprouting)
• Refrigerating whole onions (causes moisture buildup)
• Storing in humid environments

Good Onion Characteristics	Bad Onion Characteristics
Dry, papery outer skin	Mold growth (white, green, or black)
Firm texture throughout	Soft, mushy, or slimy spots
Distinct onion aroma	Sour, musty, or foul odor
Heavy for its size	Lightweight or hollow feeling
No visible dark spots	Dark patches beneath skin

Proper Storage for Maximum Freshness

Extend your onions' shelf life with these science-backed storage techniques:

• Store in a cool, dark, well-ventilated space (55-60°F is ideal)
• Use mesh bags or baskets instead of plastic containers
• Keep away from potatoes and other ethylene-producing produce
• Never wash onions before storage (adds moisture)
• Check stored onions weekly and remove any showing early spoilage signs

When stored properly, yellow onions can last 2-3 months, red onions about 1-2 months, and white onions approximately 1 month. Sweet onions have the shortest shelf life at 3-4 weeks due to higher water content.

When to Definitely Throw Out an Onion

Discard onions immediately if you notice:

• Extensive mold growth (even if confined to outer layers)
• Black or brown centers when cut open
• Significant softening or sliminess
• Foul or ammonia-like odors
• Visible insect damage or infestation

Remember that cutting away moldy portions isn't safe with onions. Unlike hard cheeses or firm vegetables, onions' layered structure allows mold to penetrate deep into the tissue even when not visible on the surface.

Food Safety Considerations

Consuming spoiled onions can cause foodborne illness with symptoms including nausea, vomiting, and diarrhea. The Centers for Disease Control and Prevention reports that improperly stored vegetables can harbor pathogens like Salmonella and E. coli.

When in doubt about an onion's safety, follow the food safety maxim: "When uncertain, throw it out." The minor cost of replacing a questionable onion pales in comparison to potential medical expenses from foodborne illness.

Can you eat an onion with black spots inside?

No, black spots inside an onion indicate advanced spoilage and potential mold penetration. Discard onions with black spots as they may contain harmful mycotoxins that can cause foodborne illness.

How long do onions last in the refrigerator?

Whole onions should not be refrigerated as cold temperatures cause moisture buildup. However, once cut, store onions in a sealed container in the refrigerator for 7-10 days. Sweet onions can be refrigerated whole for 1-2 months due to their higher water content.

Is it safe to eat sprouted onions?

Yes, sprouted onions are generally safe to eat if no other signs of spoilage are present. Simply remove the green sprouts and any soft layers, then use the remaining firm portions promptly. The sprouts themselves are edible but have a bitter taste.

What causes onions to become slimy?

Onions become slimy when moisture accumulates between layers, creating ideal conditions for bacterial growth. This typically happens when onions are stored in plastic bags, in humid environments, or near moisture-producing foods. Slimy texture indicates advanced spoilage and the onion should be discarded.

Can you freeze onions to extend their shelf life?

Yes, freezing is an excellent way to preserve onions. Chop or slice onions, spread them on a baking sheet to freeze individually, then transfer to airtight containers. Frozen onions maintain quality for 6-8 months and can be used directly in cooked dishes without thawing.