How does mold turn milk into blue cheese?

Blue cheese begins as milk that has been thickened into curds by helpful bacteria. Cheesemakers mix in safe spores of Penicillium roqueforti, then pierce the cheese so air can reach inside. With oxygen, the mold grows through the curds and changes fats and proteins into strong flavors.

The blue marks are the mold's own color, spreading where air holes give it room. As it grows, it also softens the cheese, which is why blue cheese can taste salty, sharp, creamy, and a little earthy all at once.