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Even though you can make a lot of different types of cheese from cow’s milk, at their core, they all work the same way. Whether it’s brie, a fresh herby cheese, mozzarella, Gruyere, Parmesan, Gouda, paneer, or Emmentaler, they all rely on a crucial component: casein proteins.
Casein proteins are special. In milk, they just float around, invisible to the eye. But, the addition of acid or rennet, under the right conditions, causes them to clump together, forming curds. The key to making cheese.
- What are casein proteins?
- To make cheese, casein curdles
- Casein during a cheese's shelf life
What are casein proteins?
Casein proteins are a group of proteins that are naturally present in animal milk, including cow, human, goat, and buffalo milk. Despite the name, casein proteins aren’t just one type of protein. Instead, casein proteins consist of four different types:
They’re all proteins, meaning they are all made up of a chain of amino acids. However, they all have a slightly different structure and thus behavior. None of them seem to have a very well-defined 3D structure though, which is quite unusual for proteins.
Casein proteins make up about 70-80% of the protein content of milk. The remainder is made up of whey proteins.
Caseins form micelles
Casein proteins don’t like to sit in water. Instead, they prefer to cluster together. As such, in milk, casein proteins don’t float around individually. Instead, they cluster together in so-called micelles. Each micelle is made up of all 4 types of caseins, though not in equal amounts. Over 90% of a micelle is made up of these proteins, the rest is made up of calcium phosphate. It’s where the calcium present in milk ‘hangs out’.
These micelles are complex in structure and scientists still don’t fully agree on their structure. What is known is that κ-casein can be found mostly on the outer layer of the micelle, whereas the others make up the center of the micelle. As a matter of fact, it’s the κ-casein that prevents all casein proteins from clustering together. Instead, it ensures the formation of the smaller micelles.
Casein makes milk turn white
Interestingly, without these casein micelles, milk wouldn’t be white, it would be translucent. The micelles are small (50-500 nm), however, they’re large compared to the size of molecules. Their size allows them to reflect light. This scattering reflection causes us to perceive milk as being white. It’s also why, when making cheese, the remaining liquid is no longer white, it’s translucent. All the casein micelles have left this liquid to sit in the cheese.
To make cheese, casein curdles
In milk, casein proteins just hang out as micelles. However, humans have found out a long time ago that we can destabilize these micelles. Of course, at the time, people didn’t know they were casein micelles, but, they did know that the milk could be curdled and made into cheese. This curdling is the destabilization of casein micelles.
Making cheese 101
To make cheese from animal milk, you add either rennet or acid (this is how to make paneer for instance) to warm milk. The addition of either ingredient causes the milk to curdle. During this process thick ‘curds’ form within the milk. These curds are then removed from the remaining liquid (the whey) and pressed into cheese.
The whey that’s left over after making cheese contain a lot of whey proteins, the other major protein in milk. Whey proteins do not curdle during cheese making.
Casein micelles destabilize due to acid
Casein proteins and the micelles they form, can handle heat very well, unlike egg proteins for instance. It’s why you can cook milk, without it greatly affecting the appearance and taste.
Even though they can handle heat, they can’t withstand acid. Once the pH value of milk drops to a value of 4.6, the micellar structure becomes unstable. The structures are no longer stable due to a change in charge of the overall system. Instead, the casein proteins will form larger, more complex structures, referred to as curd. These no longer just contain calcium, they also ‘capture’ most of the fat that’s present in the milk.
Or enzymes cut κ-casein
Recall how κ-casein can be found on the exterior of the casein micelles? Here it serves an important function to stabilize the micelle as a whole. If you’re making cheese using rennet, you’re attacking these κ-casein proteins. The enzymes in the rennet, of which chymosin is one, break down κ-casein, but don’t affect the other types. The enzyme cuts the protein into two pieces, always at the exact same spot, resulting in two smaller proteins. These proteins can no longer stabilize the micelles.
Again, this causes the casein proteins to curdle and form large curds structures, suitable for making into cheese.
Extra calcium helps the cheese
Calcium salts are part of the casein micelles, but they’re also a crucial part of cheese curds. To help form more of these stable cheese curds, cheese makers may actually add additional calcium in the form of CaCl2.
Curd’s texture depends on the conditions
Using acid vs. rennet has a profound impact on how the casein proteins cluster and form curds. If you’d zoom in, you’d notice they’d be organized differently. But, other aspects can play a role as well. The above mentioned impact of calcium concentration is one, but so is the impact of salt and the composition of the milk itself. All contribute to nuanced differences between cheeses that are otherwise made the same way.
Casein during a cheese’s shelf life
Once cheese has been made, the main role of casein is over. It’s done its job. However, it will continue the behavior of the final cheese. For instance, many cheeses are ripened. Whether it is and for how long depends on the style of cheese. During this time changes in the casein may again occur. Some types of casein proteins may be broken down over time and some reorganization of the proteins may occur. This continues to impact the consistency and even flavor of the cheese over time.
Stringiness is caused by casein
Some cheeses become stringy when they’re melted. Since casein proteins play such a crucial role in forming a cheese’s structure it probably won’t surprise you they’re involved here as well. The stringiness is caused by casein proteins linking together into long fibers. If the casein is broken down during ripening, the casein proteins become too small to form these strings. As such, they will be less stringy when melted.
Scientists have been stumped by the structure of casein micelles and so there is a lot of scientific literature on the topic. Unfortunately, less has been written about what happens to casein proteins during the cheese-making process!
Farrell, Jr, Harold & Malin, E.L. & Brown, Eleanor & Qi, Phoebe. (2006). Casein micelle structure: What can be learned from milk synthesis and structural biology?. Current Opinion in Colloid & Interface Science. 11. 135-147. 10.1016/j.cocis.2005.11.005. link
C.G. de Kruif, Advanced Dairy Chemistry – 1: Dairy proteins, Jan-2003, link
De Kruif, C.G. (Kees) & Huppertz, Thom & Urban, Volker & Petukhov, Andrei. (2012). Casein micelles and their internal structure. Advances in colloid and interface science. 171-172. 36-52. 10.1016/j.cis.2012.01.002. link
Harold McGee, On Food and Cooking, p. 64-65, see review
R. Scott, Cheesemaking practice, third edition, p.50
Patrick F. Fox, Paul L.H. McSweeney, Timothy M. Cogan, Timothy P. Guinee, Cheese: Chemistry, Physics and Microbiology, Volume 2: Major Cheese Groups, Elsevier, 2004, link