Oiling off
Beads of fat seeping from a cheese that has grown warm, or pools of oil on top of melted cheese.
Also: sweating, fat separation
Oiling off is when fat separates from cheese. On a cheese board, beads of fat appear on the cut face of a hard or semi-hard cheese that has been left in a warm room too long; cheesemongers call it sweating. It is harmless, though the cheese may taste flat and greasy, and blotting and cooling help. In cooking, oiling off is the yellow oil pooling on top of a pizza or gratin, which happens more with older, drier cheeses and with very high heat. Young, moist melting cheeses such as mozzarella oil off less. Buffalo milk cheeses, with more fat, tend to oil off more than cow milk ones.