i have a jar of white miso in the fridge that apparently expired april 2010 (i know, i know...) but there's no growth on it and it's a dark tan color (i can't remember what it was before). think it's safe to use or should i just toss it?
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That's a tough call because of the color change. Usually miso can last quite a long time in the 'fridge..3 years isn't that long for miso. But the color change could mean something evil could be going on in there.
Color change of miso occurs due to temperature , that means even if it isn't expired it could happen. So color change itself is not a problem, if you opened the miso today as the first time, it is OK, go ahead. But if the package has been opened 3 years ago, if I were you,won't try.
When in doubt, toss it out.
When I first learned to enjoy and use miso, it was sold unrefrigerated and undated. Has something changed?
Miso can last a very long time due to its high salt content and the fact that it has been fermented. It's a preserved food that has been used for hundreds of years, including before refrigeration -- it doesn't really "go bad" per se.
Miso has active bacterial cultures in it from the fermentation process. Expired miso is likely to have reduced microbial activity and perhaps a change in colour because of the maillard reaction where the amino acids in the soybeans reacts with the sugars. It happens far faster in a warm environment. It also might not taste as good as it used to, but it's in all likelihood safe to eat.