Coagulation – Curds Getting Comfy
If you wanted to break down the steps of cheesemaking to the very basics, you could say that it involves the following:
1) Acidification of milk
I think it goes without saying that the aging process is by far the most varied and technically challenging aspects of making cheese, but the first two steps play such an important role in the process that they are the primary means by which I (and most other cheesemakers) classify all the different types of cheese.
There are several methods that one can use to get curds out of milk: temperature-mediated coagulation, acid-mediated coagulation, and enzyme-mediated coagulation. In order to understand how these processes work, though, we have to talk about the structure and composition of the delicious, delicious globs of fat in milk that give rise to cheese.
To a cheesemaker, the most important parts of the milk are the micelles. What is a micelle, you say? It’s a clump of molecules that hang around together in a liquid, and they are usually in a spherical form.
Image from University of Calgary Department of Chemistry
They hang out in a sphere because these molecules, such as the fatty acids found in milk, contain both a polar and a non-polar portion. Polar molecules, like water, do not like to mix with non-polar molecules, like oils or fats. At the molecular level, polar and non-polar molecules will repel each other – which makes them thermodynamically unstable. As a result, the molecules will begin to align such that there is minimal contact between the polar and non-polar molecules. This is the precise reason why oil and water mixed together will eventually separate in to two layers. Prior to forming a full-on layer, though, the oil will form micelles. You can see the spherical oil micelles form if you shake up a bottle of oil and water.
Milk is considered an emulsion of fat, protein, and lactose in water. In all milk types, the fat is present in micelles that are composed of both saturated and unsaturated fats. Cow milk has very large fat globules compared to other mammals. Since these can be hard to digest, most dairies will homogenize the milk. This results in much smaller, emulsified fat globules that don’t float to the top of the container to form a cream layer. In addition to the fat micelles, there are also casein micelles. These are much smaller globs that contain calcium phosphate (CaP) inside. In an animal, these micelles are meant to carry the CaP to the infant’s stomach.
Image from University of Guelph Dairy Science
Up to 80% of all the protein in cow’s milk is associated with the casein micelles. The molecules of casein are negatively charged, which keeps the very hydrophobic micelles happy and floating around in the water.
Image from University of Guelph Dairy Science
Our goal is to get the micelles interacting with each other so that they will coagulate and form a gel (the curds). We can do this using one of the following methods.
This process involves the use of rennet, which is used to make most cheeses. Rennet can be composed of one or more proteases, including pepsin, chymosin, and proteses derived from microbes such as Mucor miehei. When they are added to the milk, they begin to break up the portions of the casein protein that sticks out of the micelle surface. These portions of the protein are what keeps the the hydrophobic micelles floating around in the milk. When they are removed, the micelles become incredibly unstable. The only way for them to stabilize is to start interacting with each other, thereby reducing the amount of the micelle that is exposed to the rest of the milk. This forms a gel matrix “net” which traps all the very large fat globules floating around. If properly done, the whey will be almost perfectly clear and all the curds will form a mass that floats at the top of the cheese vat.
In this process, the pH of the milk drops to the acidic range (pH < 4.6). This alters the interaction of the calcium phosphate molecules with the micelles, and they begin to leak out of the globs. Once this happens, the micelles become destabilized and begin to interact with each other, forming a gel matrix. The source of the acid can be exogenous (directly adding acid such as citric acid or vinegar to the milk) or endogenous (from the lactic acid produced by bacteria).
Saying that temperature is the primary mechanism involved in this process is kind of a misnomer, since it also involves acid. If you wanted to, you could simply heat up a vat of milk to boiling and get precipitated curds. However, I would not recommend doing that because the milk ends up tasting burnt and you have a vat of burnt cheese. Gross. So we add acid to the milk (making a pH of about 5) to help the process along and lower the temperature at which the curds begin to coagulate.
So there you have it – a crash course in the science of milk coagulation. The method that you choose determines what type of cheese you will be making. For instance, using heat to coagulate will kill off all the microorganisms in your milk. This is bad news for cheeses such as swiss and camembert, which rely on additional microorganisms munching away at the cheese during the ripening process. However, temperature-coagulated cheeses are ready in under a few hours whereas the rennet-coagulated cheeses usually require months of aging time.
No matter the method, though, coagulation of milk micelles is an excellent example of how pH, temperature, and proteases can affect protein-protein interactions.
Cheese: Chemistry, Physics, and Microbiology, Volume 1. Edited by P.F. Fox.