You’re standing in the kitchen, carton in hand, staring at that one stray egg that’s been sitting in the back of the fridge for... well, you aren’t quite sure how long. Is it three weeks? Two months? The date on the carton is smudged, and honestly, you don't want to risk a case of salmonella just for a quick omelet. But you also hate wasting food. Eggs are expensive these days.
So, how do I know if an egg is rotten without actually cracking it open and ruining your morning with that horrific sulfur smell?
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It's a common dilemma. Most people assume the "best by" date is a hard deadline. It isn't. In fact, eggs are surprisingly hardy little packages of protein, designed by nature to stay fresh way longer than most of the stuff in your crisper drawer. But when they go bad, they go bad.
The Float Test: Science or Old Wives' Tale?
If you've ever Googled this before, you've seen the float test. It's the gold standard of "is this egg okay" internet advice.
Here is how it works: you fill a bowl with cold water and gently drop the egg in. If it sinks to the bottom and lays flat on its side, it’s very fresh. If it stands upright on the bottom but stays submerged, it’s getting older but is still perfectly fine to eat. If it floats to the very top? Most people say toss it.
But why does this happen? Eggshells are porous. Over time, the liquid inside the egg evaporates through those tiny pores and is replaced by air. This creates a larger air cell at the blunt end of the egg. The more air inside, the more buoyant the egg becomes.
However, a floating egg isn't always a rotten egg.
This is where the nuance comes in. A floater is definitely an old egg, but "old" and "rotten" are two different things. An egg can be old, dry, and lose its leavening power for baking, yet still be safe to consume. Conversely, an egg could technically sink but have a hairline crack that allowed bacteria like Pseudomonas to enter, making it unsafe.
The Sniff Test Never Lies
Honestly, your nose is a much better tool than a bowl of water.
If you are questioning an egg, crack it into a separate small bowl—never directly into your pan with the other ingredients. If the egg has gone bad, you will know the second the shell breaks. The smell of a truly rotten egg is unmistakable. It’s a pungent, sharp, sulfurous odor that can literally fill a room.
It’s a biological warning signal. Evolution has wired our brains to find that smell absolutely revolting so we don't eat it and get sick.
If there’s no smell, the egg is likely fine. If you have to put your nose right up to it and "search" for a scent, it’s probably just a normal egg. Rotten eggs don't play hard to get; they announce themselves.
What about the "Best By" date?
Don't let the date on the carton scare you into throwing away perfectly good food. In the United States, the USDA requires a "pack date" on all graded eggs. This is a three-digit Julian date representing the day of the year the eggs were washed and packed. For example, 001 is January 1st, and 365 is December 31st.
Eggs are usually good for 3 to 5 weeks after you bring them home, regardless of the "sell by" date. Many experts, including those at the USDA Food Safety and Inspection Service, note that eggs can stay safe and high-quality for weeks beyond that stamped date if they have been kept refrigerated at $40°F$ ($4°C$) or slightly below.
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Look for Visual Red Flags
Before you even crack the egg, look at the shell.
Is it slimy? Does it have a powdery residue? Sliminess can indicate bacterial growth, while a powdery look often means mold. If you see cracks, even tiny ones, the structural integrity is compromised. Toss it. Bacteria love a shortcut into a nutrient-dense environment.
Once you crack it, look at the whites (the albumen) and the yolk.
- The Spread: A fresh egg has a yolk that sits high and round. The white is thick and stays close to the yolk. An old egg will have a flat yolk and thin, watery whites that spread across the entire pan. This doesn't mean it's rotten—it just means the proteins have broken down. It's fine for hard-boiling, but terrible for poaching.
- Color Shifts: Have you ever seen an iridescent or pearly sheen on the egg white? This is a sign of Pseudomonas bacteria. If the white is pink, green, or blood-red (beyond a simple, harmless meat spot), get rid of it.
- The Yolk Membrane: If the yolk breaks the second it touches the pan, the membrane has weakened with age. Again, not necessarily rotten, but a sign of a senior-citizen egg.
Blood Spots vs. Rot
Finding a tiny red spot on the yolk can be jarring. You might think it's the start of a chick or a sign of disease. It’s actually just a ruptured blood vessel in the hen's ovary during the laying process. It is perfectly safe to eat. You can scoop it out with the tip of a knife if it grosses you out, but it’s not a sign of spoilage.
Storage Secrets to Prevent Rotting
The way you store eggs matters more than you think.
Stop putting them in the little egg trays built into your refrigerator door. Every time you open the fridge, the door is hit with a blast of warm air. This causes the temperature of the eggs to fluctuate, which can lead to condensation. Moisture on the shell is a playground for bacteria.
Keep them in their original carton. Keep them on the middle or bottom shelf where the temperature is most consistent. The carton also protects the eggs from absorbing odors from other foods like onions or leftover fish—remember those porous shells?
The Candling Method
If you want to feel like a 19th-century farmer, you can try "candling."
Back in the day, people held eggs up to a candle to see what was inside. You can do the same with a high-powered flashlight in a dark room. Hold the light against the large end of the egg. You’ll be able to see the size of the air cell. In a fresh egg, the air cell is thinner than 1/8 inch. If it’s a huge, dark void, the egg is old. You might also see shadows of the yolk; in a fresh egg, the yolk is centered and doesn't move much. In an old egg, it sloshes around because the whites are thin.
When in Doubt, Throw it Out
Foodborne illness is no joke. Salmonella Enteritidis can be inside an egg that looks and smells totally normal, though this is rare in commercially produced eggs due to strict washing and cooling protocols. However, if an egg looks "off," smells "funky," or the shell is compromised, it isn't worth the risk.
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The cost of one egg is about 25 cents. The cost of a day in the bathroom (or the hospital) is significantly higher.
Quick Summary of Egg Freshness
- Sinks and stays flat: Fresh.
- Sinks but stands up: Eat soon.
- Floats: Old. Use for baking or hard-boiling after a sniff test.
- Smells like sulfur: Danger. Throw away immediately.
- Pink or iridescent whites: Bacterial contamination. Do not eat.
Actionable Next Steps
To make sure you never have to play "egg roulette" again, try these three things:
- Check the Julian Date: Next time you're at the store, look for the three-digit number next to the "best by" date. Choose the highest number available to get the freshest batch from the farm.
- The Sharpie Method: If you take eggs out of the carton and put them in a bowl, write the date you bought them on the shell with a pencil or food-safe marker.
- Cook Thoroughly: If you're using eggs that are nearing their limit, avoid eating them over-easy or raw in cookie dough. Cooking eggs until both the white and yolk are firm kills most bacteria, including Salmonella.
Store your eggs in the coldest part of the fridge, trust your nose above all else, and remember that a "floater" is just an egg that's lost its youth, not necessarily its safety.