Why not all floating eggs are rotten
Please read the following with an open and learning mind, so that the understanding is thorough and useful.
It is long understood that a floating egg is a rotten egg. Research has proved that the idea is only partially right. The below four paragraphs are essentially the reasons substantiating the floating egg idea. It is also the common understanding with no scientific explanation.
“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!”
The paragraph below describes the same phenomenon; scientifically.
The above explanation is only partially correct. 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. Please note that if the yolk and the 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 remain the same and the egg would still sink. The reason why an egg floats is because as it ages it dehydrates, water vapours are released through the porous shell. This is what causes the yolk and whites to shrink which in turn reduces the overall mass of the egg. More mass leaving = less weight = more floating!
Why not all floating eggs are rotten.
Our eggs are laid in the nests inside an open ended shed which are scattered through paddocks. As soon as we collect the eggs we sanitise, pack and refrigerate them in 10 degrees Celsius. On all occasions the eggs temperature is equal to the air temperature. On hot days when the temperature exceeds 35 degrees or above the egg starts dehydrating as soon as it is laid. Even though we cool them within three hours of lying a degree of dehydration has occurred (the eggs’ mass relative to its volume has been decreased). Once inside the cool room and within an hour the eggs temperature drops from 35 to 10 degrees causing the yolk and the white to shrink (due to sudden cooling); which in turn creates a mild vacuum inside the shell and since the shells are porous extra air molecules enters the egg. The air however does not replace the mass of the water vapour which the egg lost while hot. And since the volume of the egg remains the same with slightly less mass and slightly more air the egg which is less than 4 hours old will float.