Eggs do not float in water. That is, they do not float in water all of the time. If you put a fresh egg in a bowl of water, it will sink to the bottom.
If you want to get the egg to float, you will have to change the density of the water, such as by adding salt to the water. This floating egg experiment is often used as a science fair project to demonstrate the effect of changing densities on the egg’s flotation, since, as more and more salt is added to the water, the egg floats at a higher level. As well, plain tap water serves as a control, demonstrating its importance.
But, it is possible to see an egg float in plain old water. Eggs are normally too dense to float, even though they do have an air cell inside them. So what is happening to cause some eggs to float?
Eggs that float are old, bad eggs. As a general rule, if an egg floats at the top of a container of plain water, it is a bad egg. This does not necessarily mean that it is spoiled but it is definitely not at all fresh.
The shells of eggs are not impermeable. Although the seem as solid as anything, the shell of an egg actually has tiny pores all over the surface, as many as 7,000 to 17,000 of them.
When hens lay eggs, the shells are covered with a protective coating called a cuticle or bloom which helps protect the egg. If you lived on a farm and had egg-laying hens, you probably would not wash off your eggs until you were ready to use them. Regardless, all eggs start to slowly lose moisture through the shell, as soon are they are laid, and they begin to age and spoil.
With commercial eggs, the cuticle may be washed off. This means the eggs is more porous, allowing moisture to evaporate out of the egg and be replaced by air. At the same time, as the egg ages and the proteins break down, hydrogen sulfide gas is formed, responsible for the ‘rotten egg smell.’ With less moisture inside the shell, but more gas, including the air and hydrogen sulfide, the egg becomes light enough to float at the surface of water.
Mineral oil is sometimes used to replace the cuticle and partially seal the pores, causing less permeability to microorganisms, moisture retention, and other advantages.
All eggs have an air-cell, also known as an air sac or air pocket. This is basically an empty space formed at the large end of the egg. When an egg is first laid, there is neither no air cell or a very small one. Then, as the warm contents of the egg cool down, they contract, causing the inner membrane to pull away from the outer membrane, and the air cell enlarges. Then, as above, as the air ages, this air cell enlarges. Since it contains oxygen, it can introduce O2 to the liquid contents of the egg, which encourage microbial spoilage.
If you’ve ever heard the recommendation that you should store eggs with the blunt end up and “pointy” end down, this is because storing eggs this way makes it less likely that oxygen will leak into the egg contents.
The air cell of an egg is actually a part of USDA guidelines for assessing egg quality. It is also possible for an air cell to rupture, releasing air bubbles and resulting in a “bubbly air cell” that is also part of the egg quality guidelines. Sometimes, an air cell is not fixed at the blunt end but is able to freely move around in the egg. This is call a free or floating egg cell.
The reason I have explained all this is that there are ‘egg float tests’ that supposedly will tell the approximate egg age. According to most versions, an egg that sits, or just barely floats on its side at the bottom of a container of water is a nice, fresh egg. An egg that sits on its pointy end in the water with the blunt end floating up is two or three weeks old. It would stand to reason that an egg with the blunt end beginning to float upwards, so that the egg floats diagonally at the bottom, is one week to two weeks old. An egg that floats on its side somewhere in the middle of the water is, perhaps, older than three weeks, and an egg that floats on its side at the top of the water is a bad egg and should be discarded. Obviously, this is not an exact science.
Although this test might give you an approximate age since air cells can rupture or move around sometimes, they are not fool-proof and this is not necessarily the best way of testing eggs for freshness, although it is the easiest to check without actually breaking the eggs. Otherwise, you’d have to use a candling method (see egg quality link above) and be able to understand the results. Do not throw out a whole carton of eggs just because one floats, though! The other eggs may well be fine.
If an egg is absolutely rotten, something you’ll rarely encounter, its contents will smell bad and be blue-green in color. However, most of the time, the freshness of eggs does not effect their nutrition, but only their suitability for certain forms of cooking. For example, to make the best fried eggs, you want eggs that are as fresh as possible. On the other hand, older eggs will tend to be easier to peel when boiled. For scrambling, either fresh or a bit older eggs are generally fine, but if you have older eggs and are serving them to guest for breakfast, scrambled is probably the best choice, if not omelets.
As well, older eggs are better for baking than fresh eggs. It is also easier to separate the yolks from the white in older eggs (room temperature is best) and the egg whites will generally whip to a firm froth, as mentioned above. So, believe it or not, the average cook doesn’t need to be as concerned with eggs being absolutely fresh as you might think. The idea, then, that any egg that floats to any degree is a bad egg, is erroneous. For those eggs that bear a USDA seal, the carton will list a “best by” date, which is generally 30 days past the date of packing.