Why Do Some Eggs Float? – Fresh Eggs vs. Old Egg

QUESTION:

If you have kept eggs past the use before date, try to boil them, and if most sink to the bottom of the pan and a few float on the water – should you throw out the floating eggs? It has been a while since I was taught much about eggs and I do not remember any of my eggs ever floating. If the floating eggs are not bad then why do they float?

QUESTION:

I sometimes notice that in a batch of six (6) or so chicken eggs that have finished hard-boiling, several will barely float to the surface. All six are known to be average freshness for supermarket supplied eggs. Upon peeling, careful inspection shows no abnormalities. When eaten they taste fine. Any idea what’s going on? A pretty thorough search of the WEB didn’t turn up any answer, so the information isn’t commonly known.

ANSWER: Learn All About Eggs and How To Cook Them. Egg shells may seem pretty solid, but they are in fact slightly porous. Old eggs float in fresh cold water because of a large air cell that forms as the egg cools after being laid. As the egg ages, air enters the egg and the air cell becomes larger and this acts as a buoyancy aid. Generally, fresh eggs will lie on the bottom of the bowl of water. Eggs that tilt so that the large end is up are older. The tilting is caused by air pockets in the eggs that increase in size over time as fluid evaporates through the porous shell and oxygen and gases filter in. The older an egg gets the more gas builds up inside it. More gas = more floating! Eggs that float could be bad. You might want to break it open and check it for a foul smell, a sure sign that it has rotted.

Carefully lower your eggs into fresh cold water (do not use salted water) using a spoon:

If the egg stay at the bottom – it is fresh.

If the egg is at an angle on the bottom – it is still fresh and good to eat.

If the egg stands on its pointed end at the bottom – it is still safe to eat but best used for baking and making hard-cooked eggs.

If the egg float – they’re stale and best discarded.

The final test:

To make sure the egg is not spoiled, break it into a clean bowl and check to make sure it doesn’t have a bad odor or appearance.

Comments from Readers:

Recently I was informed the way to tell if an egg was bad was to place it in water and see if it floats. Because as the egg aged, gasses were created in the egg causing it to float. Having more air in the egg makes the egg more buoyant. I disagreed with this explanation but was pointed to your website which supported this explanation. On your website under the question: “Why do some eggs float?” you state:

Egg shells may seem pretty solid, but they are in fact slightly porous. Old eggs float in fresh cold water because of a large air cell that forms as the egg cools after being laid. As the egg ages, air enters the egg and the air cell becomes larger and this acts as a buoyancy aid.

Generally, fresh eggs will lie on the bottom of the bowl of water. Eggs that tilt so that the large end is up are older, and eggs that float are rotten. The tilting is caused by air pockets in the eggs that increase in size over time as fluid evaporates through the porous shell and oxygen and gases filter in. The older an egg gets the more gas builds up inside it. More gas = more floating!

I am glad to now know how to determine if an egg is bad or not, but I did want to set you (and your readers) straight on why egg floats. You are partially correct, however, it is not the fact that air is getting into the egg that makes it float. It is the fact that mass is leaving the egg. Since the eggshell does not expand or contract, the density of the egg is only dependent on the mass of the egg (density equals mass divided by volume and the volume of the egg is constant). If the egg weighs less than the amount of water displaced by the egg it will float. If it weighs more, it will sink. It does not make a difference if there is more or less air in the egg. For example, if the inside matter (yolk and whites) were replaced with a steel ball of equal weight, there would be a lot of air in the egg. But the overall weight of the egg would be the same (actually it would be slightly heavier since air does have a weight associated with it and now you would have more air in the egg) so the egg would still sink. The reason why an egg floats is because as it decomposes, water vapor and gases are released through the porous shell. This is what causes the yolk and whites to shrink which in turn makes the air cell larger. Some ambient air enters the shell as the water vapor and decomposition gasses leave, but overall the mass of the egg is reduced. More mass is going out of the shell than coming in through the shell. More mass leaving = less weight = more floating!

Thank you – Rob Dyer