Summa Cum Biologia - Volume 16, Issue 5

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Cheese Biology

Gordon Wilson

This will not be the musings or rather, random neuron firings of a gifted writer. Rather, it will be an actual attempt at data transfer of true phenomena from the real, empirical world of food microbiology from a marginal writer who hopes he may have some potential for the craft (being genetically related to two writers within these shiny pages).

For those of you who are cheese-o-philes you can thank God for creating the various and sundry species of bacteria and sometimes fungi essential to its formation. We are all usually revolted when milk goes sour, but often some of our greatest blessings are the result of harnessing certain critters—which if left alone often produce putrid things—and putting them to good use. Cheese production is just one example. I will try to simplify things, so don't be too offended if I insult your intelligence.

Milk has a particular protein called casein (pronounced case-seen). When it begins to sour, it is because certain species of Streptococcus (or other bacterial species) are living off of the milk sugars (lactose). One of their metabolic waste products is lactic acid (or other acids from other bacteria) which they excrete. This causes the pH to drop below neutral (acidic) and the casein protein can no longer remain dissolved in solution. When this happens, the proteins are said to precipitate—they settle at the bottom of the liquid as a solid.

When casein precipitates en masse, we say that the milk is curdling. Rennin is an enzyme which is used in the cheese industry that speeds up the curdling process. Do you want to know where we get rennin? You do? Good—from the lining of a calf's stomach.

When milk has been properly curdled, we basically have unripened cheese. At this point, it can be commercially marketed as cottage cheese. But if ripened cheese is the goal, the curds are washed, pressed (to squeeze out the whey), sometimes cooked, and then sliced up into the appropriate shape. Many times the curds are salted to the desired taste and allowed to ripen (bacterial fermentation) for a particular length of time, at a particularly warm temperature and high humidity. Salting obviously adds flavor, but it also controls moisture and prevents the unwanted growth of other microbes.

You may have wondered about the curious cavities in Swiss cheese. In short, two species of bacteria are grown within the curdling milk. Lactobacillus is used to produce the aforementioned lactic acid which curdles the casein. Another bacterial species called Propionibacterium produces propionic acid and carbon dioxide as metabolic waste products. The latter, being a gas, accumulates in nooks and crannies between the curds and exerts enough pressure on the surrounding cheese to form CO₂ bubbles that remain trapped in the stiffening cheese. These trapped bubbles are referred to as eyes or holes.

Both cheddar cheese and Swiss cheese are internally ripened by fermenting bacteria. However, many cheeses require more microbial action than just internal bacteria.

Certain fungi have earned a right to grow in and on certain cheeses because of the peculiar flavor they bring to the palate. Two species of mold, Penicillium roquefortii and Penicillium camemberti, are purposely added to produce Roquefort (or blue cheese) and Camembert cheese. The fungal threads grow in and on the cheese, feeding on this land of plenty and creating each cheese's distinctive texture, flavor, and color.

These molds produce structures (conidiophores) that produce millions of pigmented spores (conidia) resulting in the bluish veins we are familiar with. Legend has it that a shepherd boy eating his lunch of cheese curds by the mouth of a cave near Roquefort, France temporarily abandoned his lunch (to pursue a girl). When he returned (he must have been gone for quite some time for the mold to grow) he was famished and ate his moldy cheese. Apparently he wasn't turned off by the new powerful favor.

I can't vouch for the historicity of the legend, but I do know that Penicillium roquefortii grows in those caves. Moreover, to be officially dubbed Roquefort cheese, the mold used in the ripening process must be originally obtained from those caves. If the cheese is produced using the same species of mold (Penicillium roquefortii) yet traces its genealogy to another location other than those caves, it must be called by some other name, usually blue cheese.

I have attempted to give you an elementary introduction to the microbiology of cheese using just a few examples, but keep in mind that there are hundreds of kinds of cheese. Some are moldy and some are not. This is a good reminder that many more creatures great, small, and microscopic that may appear useless or malignant now, could be a rich future blessing from God. We may not see it yet because of our dulled curiosity, creativity, and imagination. But I am quite sure that there are many more creatures able to bless mankind in countless ways, utilitarian, aesthetic, or culinary.