Dropping an egg into water can produce a dramatic result: one lies flat, another stands on end, and another rises to the surface. The difference is mainly buoyancy caused by the egg’s growing air cell. USDA explicitly notes that a floating egg is of poorer quality but may still be perfectly safe to use. The test is therefore useful as a rough indicator of age and internal quality, but it cannot tell you whether Salmonella or another pathogen is present.
The air cell starts small and grows during storage
Soon after an egg is laid, its contents cool and contract, creating a small pocket of air between membranes at the large end. The shell is porous. Over time, moisture and carbon dioxide move out and air moves in, gradually enlarging that pocket. The total mass and density of the egg change as storage continues.
A relatively fresh egg usually sinks and lies flatter because it is denser than an older egg with a large air cell. As the pocket expands, the broad end becomes more buoyant. The egg may first tilt upward, then stand, and eventually float if the average density drops enough.
Floating means older, not automatically rotten
This distinction matters because many popular explanations turn the float test into a simple pass-or-fail safety test. USDA does not. Its guidance says an egg can float because its air cell is enlarged, meaning quality has declined, while the egg may still be safe. Age can affect cooking performance and texture without creating an obvious spoilage problem.
A floating egg may have a thinner white, flatter yolk and weaker internal structure. Those changes can matter for poached or fried eggs where appearance is important, while older eggs can actually peel more easily after hard cooking. Quality and safety overlap in some situations, but they are not the same measurement.
The water test cannot detect Salmonella
Pathogenic bacteria do not make an egg reliably sink or float. A contaminated egg may look, smell and behave normally in water. That is why food-safety practices still matter regardless of the test result: keep eggs refrigerated, avoid using badly cracked shells, and cook according to the needs of the person eating the food.
People at higher risk of severe foodborne illness may need extra caution with raw or undercooked egg dishes. Pasteurized eggs or egg products can be useful when a recipe will not fully cook the egg. The float test does not replace those precautions.
How to evaluate an egg that floats
USDA suggests cracking a floating egg into a separate bowl rather than directly into a recipe. Check for an unusual appearance and, especially, an off odor. A spoiled egg produces an unpleasant smell when opened, whether raw or cooked. Cracking separately also prevents one bad egg from ruining a bowl of other ingredients.
Do not taste a suspicious raw egg to test it. If the odor is clearly abnormal, the shell was badly damaged, or storage conditions are uncertain, discarding is the sensible choice. A normal smell still cannot certify freedom from pathogens; it only tells you that obvious spoilage is not present.
Refrigeration slows the quality changes
Cold storage slows moisture loss, gas exchange and other age-related changes. Keep eggs in the refrigerator rather than repeatedly warming and cooling them. The original carton also protects against breakage and makes it easier to track dates. Storage guidance varies by country and supply chain, so follow local labeling and refrigeration instructions.
The size of the air cell is one reason eggs of different ages behave differently even when they came from the same carton. It also explains the crater often seen at the broad end of a hard-cooked egg: that space corresponds to the air pocket that formed and enlarged during storage.
Use the float test for what it actually measures
If you enjoy the test, treat it as a rough freshness clue. A flat-sinking egg is generally younger; an upright or floating egg is generally older. Then make a separate decision about whether the egg has been stored correctly and shows any sign of spoilage.
Floating is a physics signal, not a microbiology certificate. It tells you that the air cell has become large enough to change buoyancy. That is useful information about age and quality, but safe handling, refrigeration and appropriate cooking remain the real food-safety controls.
Sources
- USDA FSIS — What does it mean when an egg floats in water? — Explains that buoyancy reflects an enlarged air cell and lower quality, not automatic spoilage.
- USDA FSIS — Shell Eggs from Farm to Table — Egg storage, air-cell growth and quality changes.