An egg floating in water often sparks curiosity about its freshness. This phenomenon relates directly to the egg’s physical properties, providing a simple indicator of its age and quality.
The Science of Egg Buoyancy
An egg’s ability to float or sink in water is determined by its density relative to the water. Denser objects sink, while less dense objects float. A fresh egg is typically denser than water, causing it to sink to the bottom of a container.
The internal structure of an egg includes a small air cell. When an egg is newly laid, this air cell is small. Eggshells are naturally porous, containing pores that allow for the exchange of gases and moisture.
As an egg ages, moisture from its contents gradually evaporates through these pores in the shell. As moisture escapes, the internal volume of the egg slightly decreases. Air from the external environment then seeps in through the porous shell to fill this vacated space, causing the air cell to expand.
This increase in the air cell’s size, adding more air, reduces the egg’s overall density. When the egg’s density becomes less than that of the surrounding water, it floats.
Testing Egg Freshness
A simple method to gauge an egg’s age is the float test. To perform this test, fill a container with enough cold tap water to fully submerge an egg. Gently place the egg into the water.
Observe the egg’s position in the water. A very fresh egg will sink to the bottom and lie flat on its side. If the egg sinks but stands upright on one end, it indicates that it is older but still generally acceptable.
An egg that floats completely to the surface, remaining entirely submerged or partially breaking the surface, signals that it has considerable age. This visual assessment provides a quick indication of the egg’s approximate age without needing to crack it open.
What a Floating Egg Means
The position an egg takes in the float test reveals its approximate age. An egg that sinks flat to the bottom is considered very fresh, indicating a minimal air cell size. If an egg sinks but stands upright on its narrower end, it suggests the egg is a few weeks old; its air cell has expanded, but it remains suitable for use. An egg that floats to the surface is the oldest of the three categories, possessing a significantly enlarged air cell.
It is a common misconception that a floating egg is always spoiled and unsafe to eat. While a floating egg is indeed older, its buoyancy primarily indicates age, not necessarily spoilage. The United States Department of Agriculture (USDA) indicates that an egg with an enlarged air cell that floats may still be perfectly safe for consumption if it has been stored properly. The definitive indicator of a spoiled egg is a foul, sulfurous odor upon cracking it open. If an egg floats, it should be cracked into a separate bowl and checked for any unpleasant smell or unusual appearance before deciding whether to use it.