by Ann Campbell, CCP
In the wide world of microbes responsible for distinction in cheese, Penicillium roqueforti is undoubtedly the most easily identified. Named for Roquefort, the French sheep’s milk blue that has enjoyed centuries of popularity and protection, P. roqueforti, the blue cheese mold, is recognized by the blue-green veins formed as it grows and extends through its substrate cheese, and it’s equally recognizable flavor, which is notably divisive amongst eaters. Whether you embrace or refuse the blue, the distinct qualities of this cheese-family are the direct result of P. roqueforti’s unique biochemical abilities, and the range of blue cheeses reflects an interesting genetic diversity within the species.
Cheesemakers exploit the accidental skills of the blue cheese mold when they stir spores into their curds. Subsequently, the cheeses, of varying ages depending on the style and desired outcome, are pierced. This not only permits oxygen, necessary for P. roqueforti to start metabolism of cheese nutrients, blue cheese mold.
Molds are pretty familiar organisms, and they really aren’t much different from us in some pretty fundamental areas. Take, for example, how we get energy. Fungi and animals both rely on gleaning a carbon source (that’s sugar, primarily) from their environment and using oxygen to turn it into energy. This is unlike other enormous and diverse groups of organisms in our world (like plants, which make their own carbon source, or many bacteria, that can metabolize without oxygen). At the end of the day, we are not much unlike a mold; we need to eat and breathe.
What else, dear reader, do we have on common with a mold like P. roqueforti? We both love cheese. The fundamental difference between a fungus and an animal is how we acquire our carbon source. We, humans and other animals eat and love to do it. It’s likely your favorite thing to do (don’t worry; we’ll talk about sex soon…), the very thing that led you to this article. Fungi go about it a little differently, though. Cheese feeds P. roqueforti, too, but instead of eating, the blue cheese mold, like its brethren fungi, absorbs its food. The “body” of the blue cheese mold, that part which we can see macroscopically, those strands of fluffy blue stuff, are made of hyphae. Hyphae are long, thin filaments, that grow, extend, and branch, over and over, producing the characteristic veins of blue cheese. While we have digestive enzymes in our bodies, ready to break down our food once it’s inside us, molds secrete such enzymes, breaking down nutrients outside of the organism. These nutrients are now easily absorbed by the hyphae, the ends of which may be as thin as one cell layer. When those nutrients are cheese curds, we get to feed on the spoils. Clever beasts we are, we have actually fooled the blue cheese mold into metabolizing milk for us, no easy task for the digestive system of an adult mammal, and learned to love, even select for, the resulting flavor and texture.
As P. roqueforti expands through the curds, the mold is eating cheese and spitting out flavor. Distinctively, cells of the blue cheese mold are loaded with lipases, enzymes that rapidly digest fats. Consider the basic composition of a fat: long, streaming chains of carbon atoms, anchored at one end by a common molecule of glycerol. The relative length, in carbon atoms, of each of these fatty acid chains is what makes one fat different from the next. As fats are digested, those chains are broken down into shorter and shorter chains, which also, of course, vary in length. The shorter the chains, the sharper the flavor that fatty acid provides. P. roqueforti’s rapid lipolysis results in primarily short-chain fatty acids, creating the dominant, pungent tingle of blue cheese. These fatty-acids are further converted to methyl ketones, another result of the species unique complement of enzymes. This results in the effervescent, almost minty or spicy quality that we distinctly associate with blue cheeses.
Fats aren’t the only thing that this mold eats up. P. roqueforti is also highly proteolytic, breaking down milk proteins into shorter peptides and amino acids. Although we certainly see a range of texture within blue cheeses, overall they remain soft. This is due to the mold’s quick digestion of the casein matrix of the curd. That silty grit we sometimes feel in blue cheese comes from the mold’s proteolytic action, as well. As the casein matrix breaks down, the calcium phosphate, previously trapped in the protein complex, is released into the residual whey of the cheese, where is mineralizes and gives us some texture.
The blue cheese mold shares famous company with other species that comprise the genus Penicillium, including its cousin Pencillium camemberti, the fluffy white mold that typifies the bloomy-rind cheese style. Though not as ubiquitous, Peniciliium glaucum makes frequent, fuzzy gray appearances on hard cheese rinds. That’s a lot of cheese glory for on genus of molds, hinting at this group’s wide range of biochemical actions. Another noteworthy species, Penicillium notatum, plays the starring role in one of the most significant discoveries in the history of medicine: penicillin, the first antibiotic synthesized and now one of the most widely administered and effective. P. roqueforti is known to have some antimicrobial effects, as well, which is why it’s so good at ripening cheese (blue cheese mold can out-compete other microbes that want at that nutritious curd.) However, P. roqueforti does not produce penicillin, which is good news for cheese lovers that suffer from penicillin allergies (if you experience an allergic reaction when eating blue cheese it is important that you remember this: I am not a doctor, and you should seek real medical attention).
Although P. roqueforti is known in the wild, where it lives in soil, on bark, and, generally, in decaying matter (what else, after all, is cheese?), the overwhelming majority collected and identified P. roqueforti specimens come from cheese, and cheesemaking or aging facilities. In fact, pretty much everything we know about this organism is from studies of cheese, but these studies have shed a lot of light on the natural history of this mold.
Molds are hard to classify. Many of them look too much like each other, even to a skilled mycologist. Historically, fungi have been classified based on their means of sexual reproduction. A significant portion of fungal species, however, are known only from the asexual stages. This poses a major problem for mycological taxonomists. Should all asexually reproducing fungus be lumped together, or have we simply not discovered the sexual state of all the fungi? The answer is a little column A and a whole lot of column B situation. Molecular techniques to investigate the genetic make-up of particular fungi, not merely its structure and appearance, have started to clarify some relationships and sort out classification, and the application of such techniques to the diversity of P. roqueforti has given us some fascinating insight, with a lot of potential to unravel the mysteries of blue cheese and its key microbe.
Now, I know you dying to ask: does the blue cheese mold have sex? We’re still not sure; we’ve never actually seen it happen. Previously, we determined this is because it simply doesn’t happen. However, scientists have found regions of P. roqueforti’s genome that contain “sex footprints”, which are gene sequences associated with sexual reproduction in other species. This strongly suggests that this species may, in fact, reproduce sexually, and if it doesn’t, it used to, switching to a purely asexual lifecycle relatively recently, evolutionarily speaking.
The new insight into P. roqueforti’s genome doesn’t end there. As said above, we know the blue cheese mold only exclusively from cheese, but with good reason. Like dogs or wheat or horses or potatoes, P. roqueforti is domesticated, whether we planned it or not. The domestic strains of P. roqueforti are found to contain large chunks of DNA not inherited from its wild ancestors, but rather horizontally transferred from other organisms. This doesn’t happen often but does happen in nature (it happens in laboratory settings a lot, where we use a horizontal transfer of genetic material to synthesize proteins for medical or industrial use.) It is yet unknown from what organism these HTR’s (horizontally transferred regions) came, but this is another very recent change in the genome of the blue cheese mold. Furthermore, these HTRs contain genes coding for the enzymes that metabolize lactose, suggesting that, like humans, P. roqueforti only recently adapted to a life of eating milk; we really do have a lot in common with this mold.
These genomic studies have researchers unraveling the relationships amongst blue cheese mold strains. We can be confident that all strains of blue-cheesemaking molds are more closely related to each other and any are to the wildtype. Although this is exciting news for our studies of natural history, relationships amongst these domestic strains could shed fascinating light on the cultural history that is cheesemaking, as well. Geneticists have identified three major “population clusters” amongst the blue cheese molds. Gorgonzolas cluster together, sharing less in common with the other European greats. One cluster contains strains used for Roquefort and its geographic neighbor, Bleu d’Auvergne. The third cluster includes Forme d’Ambert, which, despite sharing geographic, and presumably cultural, ties to the previous two cheeses, uses a strain more closely related to Britain’s king-blue Stilton and the standout electrical blue of Spain, Cabrales, three cheese that are about as different as can be when it comes to flavor and style, not to mention geographically disparate! The relationship between theses cheese’s respective strains of P. roqueforti, however, shows a tremendously interesting pattern. When we lay these clusters over a map, the historical questions start asking themselves. Was blue cheese invented once, or multiple times? Who taught whom how to make blue cheese, and how did these strains travel? Culture, after all, is what we call our own species’ story as well as our microscopic cheesemakers. Since humans aren’t the only organisms making our cheese, we certainly aren’t the only ones who can tell a story of our cheese’s history.
I invite champions and dissenters of blue cheese alike to celebrate the diversity of P. roqueforti. If you’re a lover of blue, branch out. Are you a fan of the intense zing of Spanish blues? See if you can taste its relationship to a fudgy, noble Stilton, different as can be in every other way. Do you just adore Gorgonzolas? Maybe something from another one of these “population clusters” will give you a new flavor to befriend. If you’re still timid about blue cheese, give it a shot. You’ve already made it to the end of this article, which would have me believing you are a little more than curious. We recommend the ice-creamy sweet Chiriboga to get you over the first hurdle, then you are likely to develop an inordinate fondness for blue cheese. I can’t tell you into which cluster this Bavarian blue falls, but we have consistent success with its ability to convert. If the recent research into P. roqueforti has told us anything, it is that the blue cheese mold does a lot more than we give it credit for.
