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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?

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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.

  • Pretty sure that’s what I thought, too, but it’s reassuring to have it verified! Thank you! Now I’m off to the store for new eggs! – Just Joel May 18 '18 at 4:26
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    another way to say it is that sinking is necessary but not sufficient for being good. If it floats, toss it. If it sinks, it might be ok, but unless you know that actual age of the egg, you're going to want to crack them one at a time and smell them before adding them to anything. – Kate Gregory May 18 '18 at 16:21
  • @KateGregory Yes, I have raised birds for egg sales and this is a common question, so I like to give the actual explanation of how the float test came about and why it was not just a gimmick idea, but is also is just a piece of evidence, not something that is sufficient to indicate safety, but a strong indicator of unsafe. Its basis is in a natural process, but too many other things can go on with the egg to use it to indicate the egg is good. – dlb May 19 '18 at 14:10

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