The blue veins in cheese are created by a mold closely related to the source of penicillin.

The distinct blue-green veins in cheeses such as Roquefort, Gorgonzola, and Stilton are primarily caused by the fungi Penicillium roqueforti or Penicillium glaucum. These species belong to the same genus as Penicillium chrysogenum, the mold famously discovered by Alexander Fleming in 1928 at St. Mary's Hospital in London. While they share a common ancestor, the culinary strains do not produce the antibiotic penicillin in amounts that would affect human health or bacteria.The process of creating blue cheese is known as 'needling.' Because Penicillium molds are aerobic, they require oxygen to survive and grow. Cheesemakers use stainless steel needles to poke hundreds of holes into the cheese wheels. This allows air to penetrate the dense interior, providing the necessary environment for the mold to bloom along the oxygen-rich channels.The characteristic sharp flavor and creamy texture are the results of lipolysis and proteolysis. During the aging process, the mold enzymes break down fats and proteins into fatty acids and ketones. This chemical transformation creates the pungent aroma and complex taste profile that blue cheese is known for. The specific strain Penicillium roqueforti was first scientifically described by American mycologist Charles Thom in 1906, though the cheese-making tradition dates back to at least the 7th century in France.