Curdled yogurt is completely fine to eat, as long as it does not have a bad smell or noticeably bad texture. Yogurt can curdle by sitting in the fridge, or by being heated up too much while cooking.

A change in texture may also occur in spoiled yogurt. If you stir the yogurt with a spoon and notice that its texture looks grainy, unusually thick or curdled, it should be tossed. Spoiled yogurt may also have a sour smell or even visible mold of any color, which are both obvious indications that it shouldn’t be eaten.