Does the Egg Float Test Work? — What Floating Really Means

The float test is simple. You put an egg that is still in its shell into a bowl of water, and you watch what it does. People often use it to decide if an egg is still good.

That last part is where the test gets misused. The United States Department of Agriculture (USDA) is clear about what floating means, and it is not the same as “this egg is rotten.”

What each result means

• It sinks and lies on its side. The egg is fresh. Iowa State University Extension puts it this way: fresh eggs sink and lie on their side.
• It stands on one end at the bottom. The egg is of poor quality and is likely old.
• It floats. The egg is old. But the USDA says it may be perfectly safe to use. Floating alone is not a reason to throw the egg away without checking it first.

So the test works as a guide to age and quality. It does not work as a stand-alone test for whether an egg has spoiled.

Why older eggs float

An egg, as laid, normally has no air cell. As the egg cools, an air cell forms, usually in the large end of the egg, between the two shell membranes.

Over time, moisture and carbon dioxide in the white evaporate through the pores in the shell. More air gets in through the shell, and the air cell becomes larger.

An egg can float in water when its air cell has enlarged enough to keep it buoyant — buoyant means able to float. That is the whole reason an old egg rises: there is more air inside it.

As an egg ages, its quality also goes down. The white becomes thinner, and the yolk becomes flatter, larger, and more easily broken. Refrigeration slows this loss of quality.

The check that actually decides

For an old egg — floating or standing — the USDA gives a next step. Crack the egg into a bowl, and look at it and smell it before you decide to use it or discard it.

• A spoiled egg will have an unpleasant odor when you break open the shell. That is true whether the egg is raw or cooked.
• A pink or iridescent — shiny, rainbow-like — egg white is a sign of spoilage, and that egg is unsafe.
• A cloudy white is a sign the egg is very fresh. A clear white is a sign the egg is aging. Neither one, by itself, means the egg is spoiled.
• A small blood spot in the yolk does not mean the egg is unsafe. Blood spots are caused by a rupture of one or more small blood vessels in the yolk at the time the egg was formed.

If the smell or the look is off, discard the egg. That crack-and-check step, not the float test, is the USDA way to judge an old egg.

Why a sinking egg still needs care

The test can mislead in the other direction, too. Even unbroken, clean, fresh shell eggs may contain Salmonella bacteria. A fresh egg that sinks is not a promise that the egg needs no care.

To be safe, eggs must be safely handled, promptly refrigerated, and thoroughly cooked. Cook eggs until both the white and the yolk are firm. Scrambled eggs should not be runny. Casseroles and other dishes containing eggs should be cooked to 160 °F in the middle of the dish, checked with a food thermometer.

Storing eggs so they stay good

• Buy eggs from a refrigerated case. Choose eggs with clean, uncracked shells, and do not buy out-of-date eggs.
• Take eggs straight home and store them right away in a refrigerator set at 40 °F or below.
• Keep eggs in their carton, in the coldest part of the refrigerator — not in the door. Iowa State University Extension gives the same advice in a different way: store eggs in their original carton on an inside refrigerator shelf, and avoid the door, where the temperature goes up and down.
• Raw eggs in the shell keep 3 to 5 weeks in the refrigerator. Do not freeze eggs in the shell.
• Do not keep refrigerated eggs out of the refrigerator for more than 2 hours.
• Never buy cracked eggs, because bacteria can enter eggs through cracks in the shell. If an egg cracks on the way home, break it into a clean container, cover the container tightly, keep it refrigerated, and use it within 2 days. If an egg cracks during hard cooking, it is safe.

The dates on the carton

Processors print code dates on cartons to rotate stock and control inventory. Words like “EXP,” “Sell By,” and “Best if Used Before” are examples of these code dates.

Some cartons also carry a three-number pack date, from 001 to 365. It tells you the day of the year the eggs were packed.

One good use for an older egg

Older eggs are good candidates for hard cooking. The fresher the egg, the harder it is to peel after hard cooking. That is because the air cell grows the longer the raw egg is stored, and a larger air cell makes the shell easier to peel.