On the other hand, if the butter has already turned dark yellow and smells or tastes rancid or sour, then you have no choice but to throw it away. Because then it is no longer suitable for consumption and may even contain harmful degradation products.
However, the dark yellow coating alone is not an indication that the butter has already spoiled, but rather the start of the process. Staining is caused by the meeting of fats and oxygen. That is: If you wish, you can remove the discolored areas and continue to consume the butter. Butter turns rancid when its fats are broken down into their oxygen-interacting components – including the foul-smelling butyric acid.
Due to its high fat content, whole butter cannot mold. However, if it comes in contact with a dirty knife, mold can still form. Therefore: Always use a clean knife to avoid contamination.