Placing an egg in water often reveals its age. A fresh egg sinks immediately, but one that has been sitting in the refrigerator may bob or float. This common kitchen test, known as the float test, works because the egg’s internal composition and density change over time. This transformation involves the eggshell’s porous structure and the expansion of an internal air pocket.
The Science of Sinking
A newly laid egg sinks because its overall density is greater than water. A fresh egg, composed mainly of a dense yolk and thick albumen (egg white), has a density of approximately 1.03 g/mL, compared to water’s 1.0 g/mL. This difference means the egg is heavier than the volume of water it displaces, causing it to overcome the buoyant force and settle on the bottom. The compact contents ensure the egg’s mass is concentrated, keeping it grounded.
The shell of a fresh egg contains a tiny air cell that forms at the broad end as the egg cools. This initial air cell is minimal and does not provide enough buoyancy to counteract the weight of the liquid contents. The dense liquid mass of the fresh egg exerts a downward force greater than the upward buoyant force of the water. This results in the egg sinking and lying flat, representing its heaviest and most compact stage.
The Role of the Air Cell
The eggshell, despite its solid appearance, is covered in thousands of microscopic pores. These pores allow gases and moisture to pass through the shell membrane, a process that continues throughout the egg’s life. As an egg ages, the moisture inside, primarily from the albumen, slowly evaporates through these pores into the surrounding atmosphere. This loss of water mass is immediately replaced by air that is drawn in through the same porous shell.
This continuous exchange causes the small air cell located between the inner and outer shell membranes to gradually expand. The expansion of this air pocket decreases the egg’s overall density because the lost water, which is relatively heavy, is replaced by air, which is significantly lighter. The increasing volume of the low-density air cell begins to exert a noticeable upward buoyant force. When the air cell becomes large enough, the egg’s total density drops below that of the water, causing it to float.
Interpreting the Float Test
The degree to which an egg floats or sinks indicates its approximate age, not necessarily its safety. When an egg sinks quickly and lies flat on its side, it indicates minimal air cell expansion and is considered very fresh. This position confirms the egg has lost very little internal moisture and is typically less than one week old.
An egg that sinks but stands upright on its pointed end is moderately aged, usually between one and three weeks old. In this case, the air cell at the broad end has expanded enough to lift that portion of the egg, but the dense contents still keep the opposite end touching the bottom. These eggs are safe for consumption and are often preferred for hard-boiling, as the larger air cell makes the shell easier to peel.
An egg that fully floats on the surface has a large air cell, indicating significant moisture loss and age. While a floating egg is old, it is not automatically spoiled, though the risk of spoilage increases with age. The only definitive test for safety is the “sniff test” after cracking the egg, as a spoiled egg emits a distinct, foul, sulfurous odor due to bacterial activity.