The float test is a common, non-invasive way to check an egg’s freshness before cracking it open. This method relies on the physics of density and buoyancy to determine the egg’s age. Observing how the egg interacts with water indicates if it is safe for consumption or if spoilage has begun. This provides a quick determination of whether the egg is suitable for use.
The Definite Answer
A truly rotten egg, which has undergone significant decomposition, will float completely on the surface of the water. This full flotation indicates the egg is very old and likely spoiled past the point of safe use. The presence of a large internal air cell and the loss of mass change the egg’s density enough to overcome buoyancy. If an egg rises to the surface and floats, it should be promptly discarded to avoid food safety risks.
The Science of Egg Density
The ability of an egg to float or sink is a direct result of its density compared to the density of water. A fresh egg has a high density because the liquid contents of the yolk and white have a greater mass per unit of volume than water. The eggshell is porous, containing thousands of microscopic holes that allow gases and moisture to pass through it over time.
As the egg ages, water inside the shell slowly evaporates through these pores, and air moves in to replace that lost volume. This process causes the small air cell, initially found at the wide end of the egg, to gradually increase in size. The loss of water mass, coupled with the introduction of lighter air, lowers the egg’s overall density.
When true spoilage occurs, bacterial decomposition accelerates this process by generating gases, such as hydrogen sulfide, which have a distinctive odor. While this decomposition converts some internal mass into gas, the primary reason for flotation is the escape of mass through the porous shell. Once enough mass is lost, the egg’s density drops below that of the water, causing it to float.
Interpreting the Float Test Results
The float test provides a spectrum of freshness, not just a simple sink-or-float result. An egg that is very fresh will sink immediately to the bottom of the container and lie flat on its side. This position indicates the air cell is minimal, meaning the egg has lost very little moisture and is ideal for all culinary uses.
An egg that sinks but stands upright on its pointed end, or tilts slightly off the bottom, has an air cell that has grown larger. This egg is older, possibly one to three weeks old, but it is still perfectly safe to eat. Older eggs with a larger air cell are often preferred for boiling because the space makes them easier to peel.
Complete flotation on the surface means the air cell is extensive and the egg has lost a significant amount of mass. While some eggs that float may technically not be “rotten,” the floating position is the most reliable indicator of substantial age and potential spoilage. When an egg floats, the safest course of action is to assume it is spoiled and avoid consumption.