Brie. Goat cheese. Parmesan. Each so very different from one another, and yet, all so tasty. If you’re a cheese lover like me, you’re in the right place. Today’s blog article is all about cheese and the microbes behind it.
Here’s the breakdown:
- Steps of Cheesemaking
- Microbes Involved in Making Cheese
- Why Study Cheese Microbiology
- Cheese Microbiology Resources
Steps of cheesemaking
While there are many types of cheese and many types of microbes involved, cheesemaking typically follows the same general steps. This process actually has many parallels to how cheese was first made. As with many fermented foods (kefir, sourdough bread, etc.), cheese was probably first made by accident. Many sources say that cheese was first discovered around 8000-7000 BC, around the time when sheep became domesticated. At the time, milk was stored and transported in “bladders” made of ruminant stomachs which contain a set of enzymes called rennet. Along with the warm summer temperatures, rennet causes the milk to curdle, separating curds from the whey. These curds were then preserved with pressing and salting. Over the centuries, these steps were refined and tweaked into the steps described below.
Acidification
The microbes added in this stage are lactic acid bacteria and are typically called the “starter culture.” These microbes, which include Lactobacillus and Streptococcus species, convert lactose to lactic acid, lowering the pH of the cheese. This lowered pH prevents other microbes from growing in the milk that can cause spoilage. In many cheeses, lactic acid bacteria may die off during the cheesemaking process but in other cheeses, they may survive and contribute to the cheese’s flavor. In addition to the starter culture, some lactic acid bacteria originate from the milk itself.
Coagulation
To separate curds (casein and milk fat) from the whey, the milk protein casein needs to coagulate. The lactic acid bacteria added in the previous step slowly causes casein to coagulate. However, enzymes such as those in rennet and/or heat helps speed up the coagulation process.
Cutting curds
Cutting the curds removes even more moisture from them. It increases the surface area so the whey have more space to leave the curds. The smaller the pieces, the harder the cheese eventually becomes while soft cheese is barely cut at all.
Cooking, Draining, and Cheddaring
For some cheese, the mixture is then cooked at a low temperature. The heating further encourages liquid evaporation. Then the whey is drained, leaving behind the curds. In some cases, the curds are pressed to further force the whey out.
Some cheeses also undergo a process called cheddaring. And if you were thinking about cheddar cheese, you thought correctly! Cheddaring is a requirement to make cheddar cheese. Here, slabs of curds are stacked on top of one another. The weight from the slabs presses the curds below, pushing out even more whey.
Salting
Salt acts as flavoring and as a preservative. It can be added dry directly to the curds or it could be washed over the cheese with a damp cloth soaked in brine. Cheese such as mozzarella are immersed in a pool of brine. This is the point where you can add spices and herbs.
Shaping
The curds are then shaped using weights or machines to the desired shape. This can be done with molds (the shaping container, not the microbial kind) and presses. Here, even more liquid is expelled. At this stage the cheese can be eaten fresh or undergo further fermentation.
Ripening
The best part! At this stage, the cheese develops its flavor, aroma, and textures and there are many different things cheesemakers can manipulate to give the cheese its unique characteristics: temperature, humidity, ripening time, and the species of microbes used.
Any microbes already added to the cheese (ex: lactic acid bacteria from the acidification stage) can further grow within the cheese and the surfaces become colonized by bacteria and fungi to form the cheese rind. Ripening can occur internal to the cheese (for example, Swiss cheese or blue cheese) or on the surface (such as Brie or Limburger). In some cheeses, the surface is brushed with oil or brine, further favoring the growth of some microbes over others.
Ripening can also involve “aerating” the cheese to give the molds interior to the cheese more oxygen (as needed for blue cheese, described below) or molds can be sprayed onto the surface.
The biggest difference between fresh cheese and aged cheese is that aged cheese goes through the ripening process. Fresh cheese, such as cottage cheese, paneer, goat cheese, and feta, still undergoes acidification using lactic acid bacteria. However, microbes can be skipped all together by using an acidifying agents such as lemon juice or vinegar.
Whether a cheese is considered a hard cheese or a soft cheese depends on the moisture level, pressure when pressed, and aging time. Soft cheese like Brie matures for one month or less. In contrast, hard cheese such as Parmesan or Pecorino are aged for months to years.
Microbes Involved in Making Cheese
Microbes in cheese can come from many different sources: the milk itself, microbes floating around in the air or on surfaces, or microbes that are intentionally introduced by the cheesemaker.
Cheeses are traditionally made with cultures passed on from the previous batch using whey that can contain dozens of different microbial species. However, many modern cheeses are made with a selected few types of microbes. We’ve already covered lactic acid bacteria above, so let’s take a look at some other cheese microbes. There are many, many more microbes involved in cheesemaking than what I could cover here, but here are some of the common ones.
Molds: Blue Cheese and Soft Cheese
There are two main types of molds used in cheese: blue mold and white mold.
As you can probably guess, blue molds help produce blue cheese. These molds include Penicillium roqueforti and Penicillium glaucum. Blue molds are added to the milk to distribute it throughout the cheese. Then piercing the cheese during the aging process introduces oxygen throughout the cheese and helps encourage mold growth.
In contrast to blue molds, white molds, such as Penicillium camemberti are added to the surfaces of cheese such as Camembert and Brie. As the mold grows, it forms a thick “crust” of hyphae. At the same time, the mold secretes enzymes into the cheese which creates a soft cheese inside.
Brevibacterium linens
Brevibacterium linens is used to ferment washed-rind and smeared cheese, such as Munster and Limburger. It’s also closely related to Brevibacterium epidermidis, the bacterium behind body odor. This bacterium can’t live in acidic environments or without oxygen and therefore can’t thrive in the interior of cheese. Because it prefers a salty, moist environment, cheesemakers encourage the growth of the organism by continually washing or wiping the surface of the cheese with a brine solution. This results in a red “smear” surface. B. linens metabolizes casein to produce amines and sulfur compounds that contribute to the cheese aroma.
Eye Forming Bacteria
The classic Swiss cheese. Bacteria in cheese such as Swiss and Gouda turn lactic acid into carbon dioxide and other acids. For Swiss cheese, this bacterium is Propionibacterium freudenreichii spp. Shermanii and it converts lactic acid into carbon dioxide, propionic acid, and acetic acid. Gouda on the other hand is fermented with Leuconostoc mesenteroides and Lactococcus lactis ssp lactis biovar diacetylactis. It converts citric acid into carbon dioxide and diacetyl, which gives the cheese a buttery flavor. Since the cheese is kept at a warm temperature during the process, the cheese is soft so that as the bacteria grows, the gasses create round openings. When the cheese cools, the bubbles stay in place. Hello eyes!
What Cheese Microbes Can Tell Us About Microbial Communities
Most microbes in nature don’t live in an isolated single culture like they often do in the lab. Instead, microbes form communities where interactions occur between species or within the same species. Microbes can either help each other out or compete with one another. All of these interactions take place in cheese. Using cheese to study microbial interactions can help microbiologists learn about other microbial communities that inhabit our gut, skin, and other environments.
Cheese microbiologists, like Rachel Dutton at UCSD and Benjamin Wolfe at Tufts University, collects cheeses from around the world, identifies the exact microbial composition of each one, and then reconstructs them in the lab to study how microbes interact (Wolfe et al., 2014). The great thing about the cheese microbiome is that the community is relatively simple, easily culturable, and reproducible.
Here are some of the cool findings over the last few years:
- Fungal species in cheese can control how much iron and biotin are available to bacteria (Pierce et al., 2021).
- Airborne chemicals emanating fungi in cheese rind could control the make-up of the microbiome (Cosetta et al., 2020).
- When given the chance to evolve in the cheese environment, Penicillium molds quickly evolve to produce new aromas, for example, shifting from producing earthy or musty volatiles to fatty and cheesy volatiles (Bodinaku et al., 2019).
- Microbes exchange a lot of genes with one another in the cheese environment. Most of these genes transferred are involved in iron transport (Bonham et al., 2017).
Cheese Microbiology Resources
These websites and publications I read to help me put this article together. They’re all great places to learn even more about cheese.
- Cheese Science Toolkit: great graphics and explainers about cheese
- MicrobialFoods.org: managed by Bronwen Percival and Benjamin Wolfe
- The Spruce Eats: contains a collection of cheese related articles
- History of Cheese on Wikipedia
- Cheese Microbes in Current Biology: Q&A style article covering questions about cheese
Featured image from Vane Monte.
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