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How to tell if eggs are bad — according to science

Every once in a while, you come across an egg and you’re not exactly sure if it’s still good anymore. Maybe it smells a bit funny, maybe it’s past its due date, or it’s just been in the fridge for a few days and you wanna be sure. But due dates can be misleading, and smell alone is not a reliable indicator. So how can you tell if the eggs are still good?

We’ve looked at some of the methods and found what really works.

Why it matters

Every year, the average person on the globe consumes 197 eggs. In many countries (like the US) that figure is much higher, at almost 300 per year. But many eggs are also thrown away. In the UK alone, 720 million eggs are wasted every year and worldwide, and while global estimates are scarce, wasted eggs probably number in the billions every year.

Granted, some of this waste happens due to restaurants or producers, but consumers can also play their part and not throw away eggs unless they have gone bad. At the same time, you really don’t want to consume bad eggs, as this would increase the risk of Salmonella or E. coli infection — which can cause diarrhea, fever, and vomiting.

To reduce the risk of bacterial infection from eggs, you can keep eggs refrigerated (which keeps them fresh for a longer time), and cook them thoroughly. A 2011 research project has found that keeping eggs at steady, low temperatures can help their natural defenses against bacteria.

Generally speaking, you shouldn’t eat eggs past their expiration date. However, some eggs have sell-by dates, others have eat-by (or expiration dates), which can get confusing. Also, those dates aren’t absolute. Most health and food organizations note that eggs are usually good for several weeks past the stamped date, but they can also get bad quicker, if stored improperly.

This is why it’s important to have a reliable method to check if your eggs are still good.

How to tell if eggs are bad: the floating test

The most common (and probably most reliable) test to check if an egg is bad is the floating test. You take a glass (or a pot, or any container really), and fill it with room temperature water. Place your egg (or eggs, just one at a time) in the water. If the egg floats, it’s not good anymore — simple as that.

Good eggs are heavier than water, which is why they sink. But when an egg starts to decompose, it becomes lighter by giving off gases. This creates pockets of air, especially at the bottom of the egg. But if the egg was a perfectly isolated system, it wouldn’t float. After all, even when solid or liquid mass changes into gas, it has the same mass.

This common method is not a myth, it actually works, and there’s some interesting science as to why it does.

Why it works

There’s a common misconception about the egg float test. The reason why bad eggs float has to do with pockets of air forming, but that’s only half of the story.

Good eggs are heavier than water, which is why they sink. But when an egg starts to decompose, it becomes lighter. When an egg starts to decompose, it gives off gases. This creates pockets of air, especially at the bottom of the egg. But if the egg was a perfectly isolated system, it wouldn’t float. After all, even when solid or liquid mass changes into gas, it has the same mass.

However, eggs aren’t perfectly isolated, they have pores and gases can escape. These gases, light as they may be, still have mass, and when they escape, they make the egg lighter. At some point, when the egg becomes lighter than water, it floats — and it’s not good for consumption anymore. This is probably the best test to employ to see if eggs are still good.

The shake test

A less reliable but still useful test is to take an egg and shake it gently by your ear. Listen carefully; is there a sloshing sound and feel? If not, you’ve probably got a fresh egg. If you do hear it, you may be dealing with an egg that has gone bad.

Keep in mind that if you shake them hard enough, even fresh eggs can make a sloshing sound, so shake them gently.

Why it works

As an egg gets older, the yolk becomes more alkaline and runny. It’s hard to say exactly where exactly the point of no return is, but as a rule of thumb, if the yolk seems too runny, it’s a bad egg.

There’s a bit of art to this test, and it’s best to complement it with another.

The good old smell test

A good cracked egg.

We’ve mentioned before that smell alone is not a reliable indicator — and it’s not. But if you crack an egg and it just smells bad, you should throw it away (they don’t call them rotten eggs for nothing). There’s a good chance the egg may actually be bad, but even if it’s not, you probably won’t be able to enjoy it, so better not to take any risks.

Why it works

The smell from bad eggs is a mixture of things, but a key component is hydrogen sulfide (H2S) — a heavy and pungent gas. If you feel any sulfur-type smell coming from the eggs, that’s a sign of decomposition.

Fresh eggs don’t emit a smell, but keep in mind that eggs can “suck” up smells from your fridge (which is why you should keep them covered and in a carton that can absorb any unwanted smells).

The visual test

If you crack open an egg and you see a discolored yolk, it’s likely bad. The same thing goes for eggs with white parts that are cloudy. But if you’ve reached that point, the odds are the egg is stinky already.

Why it works

It’s not just decomposition and bacteria, there’s also some chemistry that changes the color (and smell) of the eggs. Eggs contain carbonic acid — an acid that forms when carbon dioxide reacts with water. Carbonic acid slowly turns into CO2 (and other gases), and leaves the egg; this is why the floating egg test works. But at the same time, this makes the remaining egg more alkaline, and more chemically capable of interacting with hydrogen.

This changing chemistry is a big part of why the inside of the egg looks different, and also a big part of why it smells differently.

The “not sure” test

Are you unsure if an egg is safe to eat? Just don’t eat it — that’s the “not sure” test.

We all want to play our part and fight food waste, and that’s a very noble goal. But if you’ve done the test and still have doubts about it, it’s best to just play it safe and not take any risks.

Egg tips

Boiling eggs and then storing them in the fridge for a few days can be useful for salads, sandwiches, etc.

Always cook eggs properly. Cooking isn’t just something we do to make food edible or tastier, it’s also something we do to kill off pathogens.

If you’ve got eggs and want to cook them but not consume them right away, the best thing to do is boil them. Boiled eggs don’t last as long as fresh eggs in the fridge, but hard-boiling eggs is a good way of giving them a couple of extra days. Boiled eggs can last up to a week when stored in the fridge, so if you’ve got a bunch of eggs you need to eat in a few days, you can use this for dressing or sandwiches or whatever else you like.

To give your eggs the most fridge life, store them in the coldest part of the fridge where they won’t freeze. It’s common to store eggs on the door, but that’s actually the least cold part of the fridge. Go deep and put them where it’s cold.

If you take eggs out, either put them back in quickly or cook them. When you take cold eggs out of the fridge, they “sweat” as the water condenses, creating an environment well-suited for bacterial growth. Avoid leaving eggs out for more than an hour, and if you do, it’s safest to cook them.

You can also freeze eggs (after cracking them), but if you don’t know what you’re doing, it’s best to avoid this.

As mentioned, leave eggs in their original carton. If you don’t have one, store them in something covered. Eggs can absorb smells and pick up unpleasant odors from your fridge.

Some countries (most notably in Western Europe) don’t store supermarket eggs in the fridge — but the fridge is still the best place to store them at home.