Propionibacteria
Bacteria that make the round holes and sweet, nutty flavour of Emmental and other Swiss-type cheeses.
Also: Propionibacterium freudenreichii, propionic bacteria, PAB
Propionibacteria are bacteria that eat the lactic acid left in a young cheese and turn it into propionic acid, acetic acid and carbon dioxide. In Emmental, Jarlsberg, Leerdammer-style cheeses and some Comté and Gruyère, the cheese is moved to a warm room after its first weeks, around twenty degrees, where these bacteria wake up. The gas collects in the elastic paste and forms round eyes, sometimes the size of walnuts, while propionic acid gives the sweet, nutty taste of Swiss cheese. The cheese is then moved back to a cool cellar to slow them down. Too much activity can split the wheel, a fault cheesemakers watch for.