Definitely a good rule of thumb, though there are exceptions. I gather that surströmming, a type of canned fermented herring popular in Sweden, is considered t

Definitely a good rule of thumb, though there are exceptions. I gather that surströmming, a type of canned fermented herring popular in Sweden, is considered to be at its best when the can starts to bulge (for some value of "best"). Apparently the key difference is that it has enough salt in it that botulism and other nasty bacteria can't survive. Opening a can of this stuff is memorable. Even the Swedes generally open it outdoors. https://en.wikipedia.org/wiki/Surstr%C3%B6mming https://en.wikipedia.org/wiki/Surstr%C3%B6mming