Bacon grease
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@George-K Somebody may have a more science-based answer, but here's mine.
How long is "a long time ago"?
How long is it good for? How long is a piece of string? I've kept bacon grease for a very long time and it was fine.
Barring any better advice, I'd heat up a teaspoon or so and fry an egg in it. Eat the egg, and if it tastes funny, there's your answer. Out goes the bacon grease.
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Unless it's highly filtered, I'm loathe to save any rendered animal fats for more than 8 weeks.
I'll put turkey fat in the freezer and keep that indefinitely (or max until the next thanksgiving).
But, in the fridge - I'll only keep it if I have a specific idea of what I'm going to use it for.
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If it's rancid, it'll smell bad.