By Harold McGee
Penicillium roqueforti, as its name suggests, is what gives sheep’s milk Roquefort cheese its veins of blue. It and its cousin P. glaucum also color the interior of Stilton and Gorgonzola and the surface of many aged goat cheeses with the complex pigment produced in their fruiting structures. The blue penicillia are apparently unique in their ability to grow in the low-oxygen (5%, compared to 21% in the air) conditions in small fissures and cavities within cheese, a habitat that echoes the place that gave Roquefort its mold in the first place: the fissured limestone caves of the Larzac. The typical flavor of blue cheese comes from the mold’s metabolism of milk fat, of which P. roqueforti breaks up 10 to 25%, liberating short-chain fatty acids that give the peppery feel to sheep’s milk and goat milk blues, and breaking the longer chains and converting them into substances (methyl ketones and alcohols) that give the characteristic blue aroma.