The Propionibacteria

An important bacterium in Swiss starter cultures is Propionibacter shermanii, the hole-maker. The propionibacteria consume the cheese’s lactic acid during ripening, and convert it to a combination of propionic and acetic acids and carbon dioxide gas. The acids’ aromatic sharpness, together with buttery diacetyl, contributes to the distinctive flavor of Emmental, and the carbon dioxide forms bubbles, or the characteristic “holes.” The propionibacteria grow slowly, and the cheesemaker must coddle them along by ripening the cheese at an unusually high temperature—around 75°F/24°C—for several weeks. This need for warmth may reflect the cheese propionibacteria’s original home, which was probably animal skin. (At least three other species of propioni-bacteria inhabit moist or oily areas of human skin, and P. acnes takes advantage of plugged oil glands.)