carbonel

Citation? I'm not saying I don't believe you, but I do have anecdata on my side, for what that's worth.

I've eaten an awful lot of hard-boiled eggs that have floated because of the air bubble at the top. I tend to turn eggs that have been in the fridge too damned long into hard-boiled eggs because that gives them somewhat of a lease on life.

The last batch of eggs I did that to had one where the shell didn't want to come off, and it smelled a little off. That one I tossed. But the others were all fine. I ate the final one tonight as part of a Cobb salad.

My theory is that my sense of smell is the best guide. Bad eggs (not counting salmonella; that's a different story) are not at all subtle.