What does this…

…have to do with this?

The answer lies in these tiny organisms:

When we think of milk, we usually think of cows, but by definition, all mammals produce milk. Mammals lactate, or secrete milk, that is unique to each species and contains a mixture of fats, proteins, and some carbohydrates. Milk from several of those mammals, including cows, buffalo and goats, is used in a wide variety of foods: it’s steamed for your pumpkin spice latte, churned to make butter, and whipped and frozen to make ice cream. But by far one of milk’s most common and versatile products is cheese.

Cheese is made out of milk’s two major components. Little Miss Muffet knew all about them – curds and whey. These components form when milk curdles, which happens when someone adds an acid (like from lemons or vinegar) to it. The solids (mostly fat and a protein called casein) turn into lumps called curds , while the remaining liquid (a mixture of different proteins suspended in water) is called the whey . While whey can be used to make certain cheeses such as ricotta, it is the curds that cheesemakers prize as the ultimate base for cheese.

There are several wheys – oops, I mean ways – to make the milk acidic so that it will curdle. Someone can manually add acid, such as vinegar or lemon juice, to it, as is done to produce queso fresco and paneer. More often, however, cheesemakers go a more scientific route: they use bacteria.

Most of the time, cheesemakers use a natural bacteria in milk called lactobacillus to eat milk sugars such as lactose and secrete lactic acid. Lactic acid builds up, causing the milk to curdle. (This process happens in your refrigerator, too – have you ever left a gallon of milk in the back of your refrigerator for a bit too long and found a lumpy, gooey mess? Blame lactobacillus.)

Other bacteria in milk cause lactic acid to build up as well, including lactococcus and streptococcus bacteria. Swiss cheese uses a bacteria species called propionibacter, which exhales carbon dioxide (just like us!) and leaves behind holes in the cheese. Each type of bacteria has unique by-products that affect the final texture and taste of the cheese.

While cheesemakers use tiny bacteria to make lactic acid from milk sugars, they also make curds using a compound found in cows. Cows’ stomachs naturally produce a mixture of enzymes called rennet . Cheesemakers use rennet to process the milk protein casein . The enzyme mix chews up the casein, causing it to lump up and curdle into the curds.

Once you have curds, you can make all different kinds of cheeses. Curds initially feel like a soft gel, which is perfect for making soft cheeses. To retain this texture, soft cheeses are almost immediately drained and salted, then packaged and sold. Cheesemakers make harder cheeses by heating curds. Heat not only evaporates the whey and makes the cheese less gel-like, it also affects the cheese’s taste by changing the chemistry of the bacteria culture in the milk. Cheesemakers typically add salt to preserve the cheese, but this salt also has a second function – it draws whey out. A cheesemaker can also change a cheese’s texture by stretching it (for example, mozzarella) or by washing it to make the flavor milder (for example, gouda and Colby).

Finally, once the cheese is made, cheesemakers use bacteria to ripen it. Like wine and scotch, cheeses can be aged for years! The remaining microbes in the curds break down the proteins, fats, and carbohydrates in the curd very slowly, which affects the cheese’s texture and taste. Cheesemakers let these bacteria do its work in carefully controlled environments so the cheese doesn’t spoil, and after months or years, the final product yields rich and varied tastes.

What’s in store for the future of cheesemaking? As you might have guessed, research scientists can use their knowledge of microbiology to make new and exciting cheeses for us to try. This is already happening in the underground bunker at Jasper Hill Farms. There, the Kehler brothers have started a microbiology lab dedicated to finding the perfect blend of bacterial species for their cheeses. You can read more about them here: https://www.nytimes.com/2017/02/06/dining/jasper-hill-farm-cheese-science.html.

The next time you next sink your teeth into some deep dish, think about all the bacteria that have worked so hard to bring that tasty cheese to your tongue.

Stefanie Kall is a Ph.D. candidate in biochemistry at the University of Illinois, Chicago.