I recently read that the sink-or-float test to see if eggs are still good is not a very reliable judge; eggs float because as they age the eggs lose moisture and air replaces it. That’s what I read. If that’s true, is there a reliable way to test the viability of eggs, aside from breaking them open and smelling them?
Eggs are a vehicle for a growing embryo. When that embryo is developing, it needs nutrients and fluids, and the egg yolk and white are those reserves, The fluid requirements decrease as the embryo develops, while the space requirements increase, and nature devise the mechanism of making the shell of an egg porous so that over time, while the embryo develops, some of the fluid evaporates. If the egg is not incubated, it is still porous, the the fluid still evaporates, but rather than the space being used by an embryo, it is i filled with air as you read.
Thus, in general, if an egg is put in water, and it floats, it is old, or worse, it may be flat out rotten and have a build-up of gases from spoilage and bacterial contamination. If it goes upright in the water, but stays submerged it is likely getting old as the testing method claims, while if it stays sunk it is presumed fresh. These tests are, and should always be stated as almost completely unreliable and quick tests only. Refrigeration slows down the evaporation. Oil coatings can slow it more and can many other things which would make an egg look fresher than it is.
The only real reliable thing you can learn from such a test is if it floats, assume it is old, at the least probably very low quality and potentially bad, and not something you want to eat. If it does not float, it may be good. A reasonable use for the test is you have two container of eggs, one old, one new and you do not know which is which. Put one set in the water and some or all float or at least go upright, then try the other and they stay sunk, and you know which is the old one. Nothing about this test assures you the eggs are good, only which is more likely to be good.