Drop a questionable egg into a cup of cold water and watch for one of three behaviors that reveal its age within seconds. Fresh eggs sink and rest flat on their side, while old eggs tilt upward or float because the air pocket inside has grown large enough to lift the shell. Add a sniff after cracking and a quick look at the white and yolk, and you can sort good eggs from risky ones in under a minute. That moment in the kitchen usually starts the same way: a quiet fridge, a half-forgotten carton pushed to the back, and one egg that feels like a gamble.
The five tests below cover what you actually need at the counter, what each sign means, and how to keep your next dozen fresher for longer.
Why Eggs Spoil and What Goes Wrong Inside the Shell
Spoilage begins before anything is visible, deep in the chemistry of the egg itself. A fresh egg holds protective layers (the cuticle, shell, and inner membranes) that slow moisture loss and block outside bacteria. Over time those defenses weaken, and the contents shift in ways your nose and eyes can eventually detect.
Two changes happen in parallel. Moisture and carbon dioxide escape through thousands of tiny pores in the shell, while a small amount of air seeps in and enlarges the air cell at the wide end. The albumen proteins loosen and the yolk membrane thins at the same time, so the white turns watery and the yolk flattens when cracked. These shifts signal age and quality loss, not necessarily danger on their own.
The Two Risks Worth Distinguishing
The first risk is bacterial contamination, chiefly Salmonella, which can sit inside an egg that looks and smells perfectly normal. Because no home test can find it, the only real safeguard is cooking eggs to 160°F, the temperature that kills the bacteria. Soft yolks and homemade mayonnaise remain a calculated choice.
The second risk is spoilage from age, moisture, or surface contamination. Cracks, condensation, manure, or a wet shell give bacteria an entry point and shorten the safe storage window. A damaged or dirty egg belongs at the front of the line, used the same day, or discarded without hesitation.
The Float Test and the Science Behind Why It Works
Among all the kitchen shortcuts for sorting fresh from spoiled, the float check stands out for its speed and accuracy. As moisture leaves and gas enters, the egg’s average density drops, and buoyancy in cold water changes in three recognizable stages.
How to Read the Three Positions
Lower an egg gently into cold water and watch how it settles. Fresh eggs sink and lie flat on their side; a few days older, they sink while angling upward; very old eggs stand on the small tip or float entirely. A floating egg has lost enough internal structure to be a write-off every time.
Treat that result as a quick read, not a final verdict. A sinking egg is very likely still good, but the test does not screen for bacterial contamination unrelated to aging, so pair it with a sniff after cracking, a check for cracks in the shell, and a sense of how long the egg has already been in your fridge.
Visual and Smell Clues That Signal a Spoiled Egg
Once an egg is open, your senses do the rest of the work. Crack it into a small bowl rather than directly into a pan, because that buffer gives you a chance to inspect before committing, and it keeps the rest of your dish safe if something looks wrong.
What to Look For in the White and Yolk
Crack a fresh egg onto a plate and the white appears slightly cloudy, hugging the yolk in a tight, compact mound. A yolk that holds a tall dome is a good sign, while a flat yolk that breaks on its own suggests the egg is past its prime for presentation, though it may still be safe when cooked thoroughly. Pink, green, or iridescent discoloration in either part is the clearest signal that bacterial growth has begun, and that egg must be discarded without tasting.
When the Smell Tells the Whole Story
A fresh egg has almost no scent, with only a faint sulfur note at most. A spoiled egg announces itself with a sharp sulfurous, rotten, or pungent odor that fills the kitchen the moment the shell breaks. Even a slightly off smell is enough reason to dump the egg and wash the bowl, because the odor compounds intensify fast once the egg is exposed to air.
Reading Carton Dates and Deciding Whether to Use or Toss
Carton dates trip up more home cooks than any other part of egg handling. Sell-by, use-by, best-by, and pack dates are quality indicators set by the producer, not hard expiration deadlines stamped by regulators. The USDA treats eggs as safe for 3 to 5 weeks past the sell-by date when kept continuously refrigerated at 40°F or below.
A Simple Use-or-Toss Decision Tree
Run through this three-step check before cooking any egg you are unsure about:
- Shell check: clean, uncracked, and dry means you can proceed; visible cracks, dirt, or moisture means cook immediately or discard.
- Float check: sinks and lies flat means likely fresh; tilts upward means use soon and cook well; floats means discard without cracking.
- Crack-and-sniff check: cloudy tight white, domed yolk, and no odor means cook confidently; watery white or faint sulfur means cook thoroughly and taste cautiously; pink color, strong sulfur, or any sour smell means throw it out.
Any egg refrigerated for more than 5 weeks should be discarded to stay within USDA guidance, regardless of date or appearance. When the result is uncertain, the cost of one egg is never worth the cost of a bout of food poisoning, so the safest move is always the trash can.
Hard-Boiled Eggs and Spoilage Signs Most Guides Skip
Hard-boiled eggs spoil faster than raw ones, and the cues differ. Cooking removes the protective cuticle and allows faster moisture and gas exchange through the now-porous shell, which shortens the safe window to about one week in the refrigerator.
What a Spoiled Hard-Boiled Egg Looks and Feels Like
A cooked egg past its prime often shows a greenish-gray ring around the yolk from sulfur and iron reacting during a long cook, a rubbery or excessively dry texture, or a faint off smell once peeled. The ring itself is harmless and forms during extended cooking, but combined with odor or surface changes it points to a real problem. Discard any hard-boiled egg that feels slimy, smells sour, or shows black, pink, or iridescent discoloration, and keep refrigerated cooked eggs tightly covered for no more than seven days.
Why Salmonella Still Matters for Fresh-Looking Eggs
Raw eggs may carry Salmonella without any sensory warning at all, which is why cooking temperature remains the only real safety guarantee. Cook casseroles, custards, sauces, and homemade mayonnaise to 160°F, and avoid runny yolks for young children, pregnant individuals, older adults, and anyone with a compromised immune system. The sniff-and-look routine catches spoilage from age, while proper cooking handles the contamination you cannot see or smell.
Storage Habits That Keep Eggs Fresh Longer
Most eggs go bad in the fridge before their time because of where they sit and how they are handled. A few small changes, based on USDA and American Egg Board guidance, can easily add two extra weeks of quality to a carton.
Where and How to Place the Carton
Store eggs at 40°F or below in the main body of the refrigerator rather than the door, because door temperatures swing with every opening and accelerate moisture loss. Keep eggs in the original carton to block odor absorption and protect the shell from drying out, and place the carton with the pointed end down to help center the yolk and extend freshness. Separate raw eggs from raw meat, poultry, and seafood to prevent cross-contamination from drips or shared surfaces.
Washing Rules for Farm-Fresh and Store-Bought Eggs
For commercially washed eggs, do not rewash before refrigerating, since extra moisture on the shell can invite bacterial growth once chilled. For farm-fresh eggs with the natural bloom still intact, leave them unwashed until just before use, then rinse with cool water and pat dry. Either approach keeps the shell’s protective layer intact and gives your eggs the longest possible run at peak quality.
The Bottom Line
The single most useful habit is a 30-second routine: float-check before cracking, sniff the moment the shell breaks, and look at the white and yolk before they hit the pan. Most bad eggs announce themselves clearly once you crack them, and the float test catches the few that should never be cracked at all. Pair that routine with proper cold storage, and a stray carton in the back of the fridge stops being a gamble.
FAQ
How can you tell if an egg is still good?
Drop it in cold water. Fresh eggs sink and lie flat, older eggs tilt upward, and bad eggs float. Confirm with a sniff after cracking and a look at the white and yolk before cooking.
Do eggs go bad in the refrigerator?
Yes, slowly. The USDA considers refrigerated eggs safe for 3 to 5 weeks past the sell-by date, and any egg stored longer than 5 weeks should be discarded regardless of appearance or smell.
What does a bad egg look like?
A bad raw egg has a clear watery white, a flat yolk that breaks on its own, or pink, green, or iridescent discoloration in either part. A bad hard-boiled egg feels slimy or shows black or pink spots once peeled.
Is the egg float test accurate?
It is reliable as a freshness screen because the air cell grows predictably with age. It cannot detect Salmonella, so always pair the float result with proper cooking to 160°F.
How long are eggs safe to eat past the sell-by date?
Cartons stamped with a sell-by date 30 days after packing are legal under FDA rules, and USDA guidelines extend that window to 3–5 weeks for continuously refrigerated eggs held at or below 40°F.
Can you eat an egg that smells a little?
No. Any noticeable sulfurous, sour, or rotten odor means the egg should be discarded. A fresh egg has almost no scent, and the smell compounds only get stronger as the egg sits exposed to air.
