Why Do Bad Eggs Float?

By Anupum Pant

Instead of cracking up an egg which has been stored for a long time, to end up disgusted by the ‘rotten egg smell’, or the smell of a gas called Hydrogen Sulfide, a simple and a fairly well known way to check if an egg has gone bad, is to drop it in a glass of water and see if it floats. I found out about this first, from an article written at Frugal Living, and spent some time to confirm its claims – Bad eggs float.
If you take my word for it (you should!), it really does work. The article describes this three-point test to find out if an egg is good to eat.

1. If the egg sinks and lies on its side, it is a fresh one. It is good enough to be eaten.
2. If it sinks and stands up on a point, or is at an angle, it is good enough. You can still use it up for making hard-cooked eggs or bake it.
3. But, if an egg floats, it needs to be discarded.

Why does this happen?

To understand, you’ll have to think of a chick – a young chicken.

Poor Chicks: Before chickens come out of the egg, they develop lungs and need oxygen to breath. Sitting inside a sealed egg, with no cords attached, for the chick to survive, oxygen has to come in from somewhere. For that, let us look inside.

Egg Science: The outer shell of an egg has two membranes under it. When an egg is laid, it is warm and starts cooling which contracts the inner part of the egg more than the shell and pulls the two membranes apart. As a result, air gets trapped in between the membranes (not enough air initially for it to float).

How does the air come in? The shell of an egg isn’t as simple as it looks. It has about 7000 tiny pores in that shell which let the air pass in and let the carbon dioxide pass out of it. This is how the chick breathes. And the reason, eggs boiled in colored water during Easter, get colored from the inside.

So, as there are pores present in the shell, bacteria enter the egg and start decaying the biological matter inside. This produces a smelly gas (and other gases too), Hydrogen Sulfide (also present in smelly farts). The gases from this decomposition, and the air from outside, keep increasing in volume as time passes.

Corollary: This is exactly what explains these floating bad eggs – Greater the amount of gas inside, older is the egg and the better it floats in water.

Side note: Egg shells and the two membranes inside have the ability to stop the invasion of micro-organisms and bacteria, but over time bacteria manage to enter.