|MadSci Network: Microbiology|
First a little background: Fungi in cheese are of two types – mold and yeast.
Molds are deliberately used in some cheeses to give a characteristic appearance and flavor. Most (but not all) molds used in cheesemaking are in the genus Penicillium which is the same genus that produces the antibiotic penicillin.
Examples of cheeses which contain deliberately molds are: Camembert, Brie, Roquefort and Blue cheese.
Few (if any) cheeses are made only by the action of yeast, but yeast serve an important first step in mold ripening cheeses by metabolizing (i.e. eating) the lactic acid in cheese. The lactic acid was produced by bacteria when the lactose in milk was fermented.
As the yeast eat the lactic acid, this raises the pH of the cheese surface, making it easier for molds to grow.
For molds to grow, oxygen must be present, so one of the things that fungi “eat” in a sense is oxygen. The molds also eat amino acids. The amino acids are produced from milk proteins. Molds excrete proteases (enzymes) into the cheese that convert protein to amino acids. This enzymatic action gives cheeses their characteristic flavor. Molds also excrete enzyme that break down fats (lipases) which produces flavor (for us) and nutrients for the mold.
Sometimes cheese we keep in the refrigerator also turns moldy. Those contaminating molds eat the same things the useful molds eat, but because they were not deliberately added, and their enzymes might be slightly different they may change the flavor in ways that we don’t like.
The mold that contaminates cheese may also make toxins (called mycotoxins) that can harm us, so you should never eat moldy cheese… unless the mold was put there on purpose!
Try the links in the MadSci Library for more information on Microbiology.