How can you tell cooked black beans are spoiled?

If the beans are bubbly, smell yeasty or of rotten eggs, they are likely bad. If they have been mishandled and left unrefrigerated for some time, you might not be able to tell if they are safe to eat since some pathogenic microorganisms can grow without leaving telltale signs.

Cooked pinto beans are a potentially hazardous food and should be treated as such. In other Words, after cooking, they should be refrigerated within 2 hours or less. Use them up within a few days.

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If there is any doubt, there's NO doubt; throw it away. There is no point in taking chances with spoiled food.