Microbes get a bad rap. Like many things in life, we focus on their negative aspects—ignoring the positives. While some microbes can make us sick, most of them do not, and many actually help us! One tasty example of humans and microbes working together is the production of cheese.
Fig. 1: A tiny subset of the varieties of cheese! Credit: dairy.org
What better measure of humanity’s collective ingenuity and experimentation is there than the wide variety of cheeses that bless our palettes? While no one knows when the first cheese came into existence, the earliest known traces are cheese-specific compounds isolated from 7,500 year old pottery found in Northern Europe.
Given the sheer diversity of cheeses, and the fact they are made from only two core ingredients (milk and salt), what makes them so different from each other? Well, it comes down to something not even known about 7,500 years ago: bacteria. These microbes assist us in undoing some of nature’s clever chemical structures, as well as making scrumptious new ones. It all begins with the milk…
Fig. 2: Both of these glasses contain water and fat. However, nature brings these together efficiently in milk. *Bonus Fact: milk appears white because the tiny spheres of fat scatter light – analogous to clouds* Adapted from: Carpinteria Valley Association and S. Kühn
To deliver adequate nutrition to our fast-growing new-borns, mammals pack energy rich fats into water, forming the ultimate calorific vector—milk. To accomplish this, the fats are coated in proteins that prevent them clumping together and separating from the water. To make cheese, we need to reverse this remarkable feat of biological engineering.
This reversal process is called curdling. Technically, it is defined as the separation of curds (fats/proteins) from whey (water). There are two, complementary steps to curdle milk. Firstly, we use an enzyme called rennet. Rennet’s task in cheesemaking is to cut the protein coatings that prevent the fat in milk from joining together (Fig. 3). Rennet was initially discovered in the stomachs of calves (not surprisingly!), but nowadays we engineer bacteria to make it for us! Secondly, acidity enhances curdling, this acidity can be brought about by introducing specific bacteria that use lactose, a sugar found in milk, to produce acid (this is also what makes yoghurt thick!). Through the combined efforts of rennet and bacteria, whey is separated from curds.
Fig.3: (A) Two fat spheres (yellow) coated with proteins (blue) that prevent them from joining. (B) The enzyme, rennet (red) cuts the proteins surrounding the spheres. (C) Fat spheres combine without the protein coating. Credit: R. Eisenhofer
Now the curds are cut to the desired size, salted to the desired saltiness, and pressed to remove more whey. These steps vary slightly depending on the desired cheese. However, if you were to taste the cheeses at this stage, you would find them almost identical! What separates a Gruyère from a Gouda? A Cheddar from a Camembert? The answer is more microbes! In a final step, which can take months or years to complete, these micro-magicians masterfully convert bland curds into a staggering variety of compounds that contribute to alluring aroma, tantalising taste, and tempting texture.
This last ripening step has a fancy French name—Affinage. In this essential step, the fromagier (cheesemaker) carefully crafts the curds into the right environment for the microbes, further adjusting saltiness, curd size (which influences oxygen availability), and moisture content. This is followed by the inoculation of the curds with specific microbes. While the microbes are busy converting basic molecules into complex new ones, the fromagier controls the environment for them—adjusting the temperature and humidity at specific stages of their maturation. Thus by working together, the fromagier and the microbes turn tasteless curds into flavourful cheese!
Fig. 4: Some of the bacteria used in cheesemaking – (A) L. heleveticus, (B) L. delbrueckii bulgaricus, (C) L. lactis, (D) L. casei, (E) P. pentosaceus, and (F) L. brevis. Credit: Broadbent and Steele
While this sounds complex, people learned how to make their own unique cheeses even before we knew microbes existed. Some recipes have been around for hundreds of years, and are made without the use of fancy environment controlled aging rooms.
Fig. 5: Visible as blue veins, Roquefort uses Penicillium roqueforti to produce its unique cheese. Adapted from: buzzle.com and C. Finot.
Roquefort, the famous blue cheese, is still aged in caves near the town that is its namesake in southern France. This cheese has a ripe history, potentially going as far back as 79 AD! To protect its historical integrity, European Union law dictates that only cheese made in these natural caves can be named Roquefort.
Next time you feast on your favourite fromage, praise the fromagier and the microbes that are responsible for giving it its distinctive aroma, texture, and flavour. Cheese is the product of a symbiotic relationship between us and microbes. Without them, we would not have the staggering diversity of cheeses that bring pleasure to our lives, just bland curds.
Australian Centre for Ancient DNA (ACAD) 