Dropping an egg into water reveals its age. A fresh egg typically sinks, while an older one might float. This phenomenon is not magic, but rather a demonstration of fundamental scientific principles at play within the egg as it ages.
The Principle of Buoyancy
An object’s ability to float or sink in a fluid, such as water, is determined by its density relative to the fluid. Density is a measure of mass per unit volume. If an object is denser than the fluid, it will sink; if it is less dense, it will float. Fresh eggs are denser than water, which is why they readily sink when placed in a bowl of water. This density is attributed to the combined mass of the yolk, egg white, and a very small air cell naturally present within the egg.
How Eggs Change Over Time
Eggshells are porous, containing thousands of tiny pores that allow for gas exchange. As an egg ages, moisture from its internal contents slowly evaporates through these pores. This loss of water causes the egg’s overall mass to decrease. Simultaneously, air enters the egg through these pores to replace the lost moisture, leading to the expansion of the air cell, which is typically located at the wider end.
In addition to moisture loss, natural decomposition processes within an aging egg contribute to gas production. Microorganisms can penetrate the shell and break down the egg’s contents, releasing gases. These gases accumulate within the egg, further enlarging the air cell. The increase in the volume of the air cell, filled with gas that is less dense than the egg’s liquid components, reduces the egg’s average density. This reduction in overall density causes an older egg to become buoyant and float.
The Egg Float Test
The egg float test is a simple method to assess an egg’s freshness. To perform the test, place an egg into a bowl of water deep enough to fully submerge it. Observing its position provides an indication of its age. If the egg sinks to the bottom and lies flat, it is very fresh, typically less than a week old.
An egg that sinks but stands upright is older, perhaps a few weeks old, but often remains suitable for consumption. This upright position indicates a larger air cell compared to a very fresh egg. If the egg floats completely, it is quite old, generally three weeks or more, and has a significantly expanded air cell. While floating suggests an egg is old, it does not always mean it is spoiled; it is a strong indicator of reduced quality and potential spoilage. A foul smell upon cracking the egg is the most reliable sign of spoilage.