Cohesion of Proteins
In a nutshell, the mechanism of making cheese by adding acid to milk is “protein cohesion due to pH change”. Let’s think about this in more detail.
Milk contains about 3.3% protein. Of that, about 80% is casein and about 20% is whey protein. Cottage cheese is made mainly by cohesion of casein.
Protein is made up of many amino acids. Amino acids have functional groups called carboxy groups and amino groups. A functional group is a part of a molecule that exhibits special properties. Amino acids and proteins made up of amino acids are soluble in water because of the function of these carboxy and amino groups. The state of the carboxy and amino groups changes depending on the pH of the solution.
pH Change and Protein
When the pH of the solution is quite low, that is, when the liquid is acidic, the entire amino acid is cationic. In this state, they dissolve well in water.
When the pH of the solution is high, that is, when the liquid is basic (alkaline), the entire amino acid is an anion. In this state, they also dissolve well in water.
In between the positively and negatively charged states, there is a state that is neither cationic nor anionic. The pH at which this state occurs is determined by the type of amino acid. This pH is then called the isoelectric point of the amino acid. The same can be said for proteins.
When the pH of a solution is near the isoelectric point, amino acids and proteins are neither cations nor anions. Therefore, they are quite insoluble in water.
In other words, when the pH of the solution is gradually raised to near the isoelectric point, the amino acids and proteins become less soluble. The same is true when the pH is lowered. If there are a lot of amino acids and proteins dissolved in the solution to begin with, they will not be able to cut through the solution and will become sediment.
When acid is added to milk to make cheese, it is because the proteins in the milk cannot be dissolved due to the change in pH and precipitate out.
Isn’t It Acid Denaturation of Proteins?
If you search for the mechanism of cottage cheese, you will often find pages that say it is acid denaturation of proteins. However, given the amount of acid added, I don’t think acid denaturation is the main reason for hardening.
The acid denaturation of proteins is when the acid changes the three-dimensional structure of the protein and prevents it from returning. Something similar is the thermal denaturation of proteins.
Of course, when a protein that is soluble in water is denatured, it becomes insoluble in water due to the change in its three-dimensional structure and can clump together. But the more accurate the page, the more it mentions pH or refers to it as “acid cohesion”.
Of course, acid denaturation can work also. In addition, there may be thermal denaturation, since it is heated up to about 60℃. In addition, salting out, which makes stable colloidal aggregation, may also play a role.
The interesting and profound thing about the science of cooking is that it is impossible to say exactly how it works.
Cottage Cheese is Different from Ricotta Cheese
Along with cottage cheese, ricotta cheese is another good choice for homemade. The process of making ricotta cheese is the same as that of cottage cheese. The difference is that cottage cheese is made from cow’s milk, while ricotta cheese is made from the whey after the cheese is made. In other words, cottage cheese contains mainly casein, while ricotta cheese contains mainly whey protein.
Can Cottage Cheese Become Leipäjuusto?
Cottage Cheese – Easy Homemade Cheese with Vinegar
- milk 1 L
- vinegar (5%) 4 Tbsp
- Measuring spoon
- silicone spatula
- Put the milk in a pot and heat it up to 60°C. You should see small bubbles around the edge of the pot.
- Add the vinegar and mix well. A lump of cheese will form.
- Place a colander over the pot. Line the colander with gauze.
- Pour the contents of the pot into the gauze little by little. The whey will separate and the cheese will remain on the gauze.
- Season with salt, etc. and serve.
- I tried my hand at making Leipäjuusto, Finnish bread cheese with a cottage cheese recipe.Can We Make Leipäjuusto Without Rennet?Leipäjuusto, bread cheese is famous food in Finland. We can make it at home easily. The only difficulty is to get rennet. Is rennet a must?
- Tofu, cheese, konjac, how do these foods solidify?The Science of Solidifying - How Cheese and Tofu SolidifyLlet's take a look at some of the foods that are solidified, such as cheese, tofu, and konjac. Do you know the mechanism how they harden?