<P> The main structure of the blue cheese comes from the aggregation of the casein . In milk, casein does not aggregate because of the outer layer of the particle, called the "hairy layer ." The hairy layer consists of κ - casein, which are strings of polypeptides that extend outward from the center of the casein micelle . The entanglement of the hairy layer between casein micelles decreases the entropy of the system because it constrains the micelles, preventing them from spreading out . Curds form, however, due to the function that the enzyme, rennet, plays in removing the hairy layer in the casein micelle . Rennet is an enzyme that cleaves the κ - casein off the casein micelle, thus removing the strain that occurs when the hairy layer entangles . The casein micelles are then able to aggregate together when they collide with each other, forming the curds that can then be made into blue cheese . </P> <P> Penicillium roqueforti and Penicillium glaucum are both molds that require the presence of oxygen to grow . Therefore, initial fermentation of the cheese is done by lactic acid bacteria . The lactic acid bacteria, however, are killed by the low pH and the secondary fermenters, Penicillium roqueforti, take over and break the lactic acid down, maintaining a pH in the aged cheese above 6.0 . As the pH rises again from the loss of lactic acid, the enzymes in the molds responsible for lipolysis and proteolysis are more active and can continue to ferment the cheese because they are optimal at a pH of 6.0 . </P> <P> Penicillium roqueforti creates the characteristic blue veins in blue cheese after the aged curds have been pierced, forming air tunnels in the cheese . When given oxygen, the mold is able to grow along the surface of the curd - air interface . In pressing the cheese, the curds are not tightly packed in order to allow for air gaps between them . After piercing, the mold can also grow in between the curds . </P>

What type of milk is blue cheese made from