Why Do Eggs Float?
Eggs float when they're old because the shell is porous and allows air to seep in over time. Fresh eggs are denser than water and sink. As eggs age, moisture evaporates and air replaces it, creating a larger air cell that provides buoyancy. Floating eggs are older but not necessarily spoiled - the float test indicates age, not safety.
Key Takeaways
- Eggshells have thousands of tiny pores that allow air and moisture to pass through.
- Hard-boiled eggs float more easily than raw eggs of the same age due to trapped air between shell and white.
- Eggs can sink and still be bad if contaminated with bacteria before significant air entered.
Explanation
Eggshells have thousands of tiny pores that allow air and moisture to pass through. When eggs are fresh, the contents are dense and the air cell (the bubble at the wide end) is small. Over time, moisture evaporates through the pores and air enters, expanding the air cell. This makes the egg less dense than water.
The float test works in stages: very fresh eggs lie flat on the bottom; slightly older eggs stand on one end (the air cell end rises); old eggs float. An egg that floats has a large air cell but may still be edible - crack it into a bowl and smell it. A rotten egg has an unmistakable sulfur smell.
Temperature and humidity affect how quickly eggs age. Refrigeration slows moisture loss significantly. Farm-fresh unwashed eggs have a protective 'bloom' coating that seals pores and extends freshness. Commercial eggs are washed (removing the bloom) and must be refrigerated.
An eggshell contains approximately 7,000-17,000 tiny pores, with the majority concentrated at the larger, rounded end where the air cell forms. These pores are only 10-30 micrometers in diameter, small enough to slow moisture loss but large enough for gas exchange. At room temperature (68-72°F), an egg loses about 4-6 milligrams of water per day through these pores. Under refrigeration at 35-40°F, that rate drops to roughly 1-2 milligrams per day, which is why refrigerated eggs maintain quality for 4-5 weeks while room-temperature eggs degrade within 1-2 weeks.
The density difference between fresh and old eggs is measurable. A fresh egg has a specific gravity of about 1.08-1.09 (slightly denser than water at 1.0), while an egg old enough to float has dropped below 1.0. The air cell in a fresh egg measures about 3 millimeters deep. After 3 weeks at room temperature, it can expand to 9-10 millimeters. USDA egg grading uses air cell depth as a primary quality indicator: Grade AA eggs have air cells under 3.2 mm, Grade A under 6.4 mm, and Grade B under 9.5 mm.
Things to Know
- Hard-boiled eggs float more easily than raw eggs of the same age due to trapped air between shell and white.
- Eggs can sink and still be bad if contaminated with bacteria before significant air entered.
- Very fresh eggs (less than a week old) are harder to peel when boiled - slightly older eggs peel easier.
- Adding salt to water makes it denser, so eggs float more easily in salted water - not a freshness indicator.