How does mold turn milk into blue cheese?

Blue cheese begins as milk turned into curds, then cheesemakers add safe mold spores. When air holes are made through the cheese, the mold grows inside and changes fat and protein into tangy flavors and softer texture. The blue streaks are Penicillium roqueforti doing its work.

That mold does not spread evenly like food coloring. It follows the air passages, which is why the color appears in lines and patches. A cheese wheel is quietly breathing while it ages, and the flavor grows stronger as the mold keeps changing the cheese.