Sour smell that is unusually strong, rotten, or unpleasant
Mold growth on the surface or around the lid
Pink, green, blue, or black discoloration
Chunky, slimy, or stringy texture
Separation that does not mix back together after shaking
Bitter, rancid, or off taste
Bulging container or leaking package
Expired date far past the use-by date
Curdling that appears abnormal for the product
Any sign of spoilage after being left unrefrigerated for too long