There are so many different types of cheese: brie, fresh cheese, mozzarella, gruyere, parmezan, Gouda, paneer, emmentaler and I could go on for a lot longer. What’s interesting though is how they just about all depend on one type of protein in milk: the casein proteins. Thanks to these proteins the milk can be made to curdle and form cheese.
Therefore, it’s about time we discuss these proteins in more detail, they are fascinating from a science perspective. In previous posts we’ve discussed the process of making cheese. We discussed the casein proteins slightly, however, now it’s time to dive deeper into the science of these casein proteins.
It all starts with milk
Cheese is made from milk. Let’s do a quick recap of what milk is (see also the infographic). Milk is mostly water, but also contains fat, proteins and sugars as well as some other minor components such as vitamins and minerals. When making cheese a lot of this water is removed. The fats, some of the sugars as well as some of the proteins will form the cheese.
Proteins play a central role
In order to make this cheese and separate the water and whey from the milk, a split has to happen. Essentially, making cheese is nothing more than splitting these two phases from one another. Here is where the casein proteins come in. These proteins can be made to make the milk curdle and thus make cheese.
Proteins are highly complex molecules in food (catch up on the basics here). They are very long strands of amino acids. Because of their size and length, it is not really useful to draw out their chemical structures and bonds, they’ll be too long. And anyway, it is nothing more than a long chain of amino acids. It is not the chain that is interesting. It is how all these different amino acids interact.
Each amino acid has a different side chain and it is these side chains that determine the functionality of a protein. These side groups can interact with one another, but also with other molecules.These long chains can curl themselves up in a lot of different, complex ways. The structures that result from this are essential for life. Proteins can do highly complex processes. An example of a group of proteins are enzymes. Enzymes are proteins that can catalyze chemical reactions.
Whey & casein proteins
In milk the proteins can be split into two main groups: whey & casein proteins. Within each of these groups there exist a lot of different proteins. We will zoom in on the casein proteins below, but let’s quickly see what happens to the whey proteins. Whey proteins do not end up in the cheese, instead, they end up in the liquid that is separated from the cheese. Whey proteins dissolve in the water and do not curdle at the low pH that causes casein to curdle. The proteins are used for a lot of other applications in the industry, e.g. processed cheeses.
Casein proteins & micelles
This long chain of amino acids of a protein can fold themselves up in a variety of very complex 3D structures. These structures are very important for determining the functionality of the proteins. In the case of casein, researchers are still trying to understand the exact structures of the proteins. It is not completely clear how they fold up and arrange themselves.
It is known that there are four main types of casein molecules in milk, it is not just one type. These are αs1 , αs2, β and κ. Despite all being casein molecules they have different lengths and a different composition of the amino acids. This gives them all slightly different abilities.
Casein molecules have hydrophobic and hydrophilic regions in the molecules. This means that some parts like to sit in water, whereas other parts don’t. This caseus casein molecules to organize themselves in micelles. They form globular round structures which float around in milk. The different casein types each take up a different place within the micelles. Some will sit more towards the outside (the κ-casein) whereas others sit more on the inside.
These micelles do not consist of solely casein molecules. Instead calcium phospohate plays and essential role in forming this stable micellar structure. Exactly how the casein micelles look like isn’t fully known yet it seems, there’s still discussion between several possible models (see sources at the bottom of this article for some interesting links to learn more).
Casein makes milk turn white
It is known that the micelles are small (50-500nm) and are able to reflect light. It is these micelles that contribute largely to the white colour of milk. During cheese making the casein micelles fall apart (more on that in the paragraph below) and casein molecules will aggregate in larger clusters than these small micelles. This causes the milk to lose its white colour and is a useful indication to determine whether the cheese making process is proceeding well.
Casein micelles falling apart
We know that casein micelles do not stay stable all the time. Casein proteins can handle heat very well (unlike most other proteins), but are sensitive to a change in pH (acidity). Also, the micelles depend on the casein proteins to stay intact. If some break down, this will instabilize the micelles. It’s these two properties that are used to make cheese. As discussed in other posts, there are roughly two ways to make cheese:
- Lower the pH (either by the addition of an acid or by the addition of micro organisms that make acids)
- Add an enzyme
Both methods will cause the stable micelles to lose stability and curdle into larger aggregates. The lower pH will destabilize the casein micelles and cause them to aggregate. They will form clumps which catch the fat and which will form the final cheese. Enzymes on the other hand come into play to change the size of the proteins.
Enzymes cut κ-casein
Within the micelles it is the κ-casein that plays a very important role in stabilizing the whole structure. It sits on the outside of the micelle and prevents the micelle from growing too large. The hydrophilic section that sits on the outside keeps it flowing around in the watery milk. When κ-casein is broken down somehow, it will lose its ability to stabilize these micelles.
When making cheese the enzyme chymosin can be used to cut the κ-casein in two parts. Let’s look at little deeper at this cut. As discussed before, the κ-casein protein is, just like any other protein, a long chain of amino acids. The κ-casein protein is 148 amino acids long.
As we discussed before as well, enzymes are proteins which are very good in catalyzing a specific reaction. When making cheese we use an enzyme called chymosin. This enzyme is very good in cutting up κ-casein at one specific spot, between the 105th and 106th amino acid. Thus that leaves us with two shorter chains, one of 105 amino acids and one of only 43. These shorter chains aren’t as good in stabilizing the casein micelles anymore, resulting in the proteins aggregating together.
And that leaves us with cheese. Without casein proteins we wouldn’t have brie, gruyere, Edam, Gouda, etc! It’s just one family of proteins, but has significantly contributed to diets around the world!
Review from 2005 on the theories of the different casein micelle structures. Another research article discussing the different possible structures of casein micelles (from 2006) as well as a whole book chapter.
Book: Cheesemaking practice, R.Scott, third edition, p.50