Curd is the first step in making cheese and is formed because the structure of casein (milk protein) coagulates with acidity and when it coagulates it separates from the liquid part of milk. Here we explain the different ways to acidify milk and what happens in each process. 

Casein coagulates in several ways:

• With various acids, since milk coagulates at an acidic pH.
• Using the enzymes chymosin and pepsin that coagulate milk.
• From the combination of both factors.

The  enzymes  are the "cutting and pasting" of nature; a kind of bricklayers who transform the elements to make all kinds of changes in the molecular structures.

Ways to curdle milk

On a practical level, the  most common methods for curdling milk  are:

• With  rennet from the animal, which is traditionally removed from the stomach of the suckling kid or calf and contains microorganisms that ferment and enzymes that coagulate milk.
• With rennet from the pharmacy. There are two types:  liquid rennet , made from microorganisms and enzymes, and  powdered rennet , made from enzymes that the chemical industry produces, isolates and markets.
• Adding  vegetable rennet,  extracted from thistle flower, fig tree sap, pineapple, papaya or nettles ... The milk curdles as these vegetables contain different coagulating enzymes. It is a rennet suitable for lacto-vegetarians and Halal, since no animal is sacrificed.
• Leaving the  milk at room temperature  at the mercy of its microorganisms. Fermentation produces an acid that lowers the pH of the milk and makes it curdle, but in this case the fermentation is wild and therefore dangerous, since there is no control over the fermentations that take place and their by-products.
• Adding  Bulgarian milk kefir , a natural, healthy and controlled culture of microorganisms that ferments lactose producing lactic acid. With the kefir, some delicious fresh spreadable cheeses are made, since the curd, being only due to acidification, is weaker than that made by enzymatic action.
• Adding  lemon juice or vinegar , since they contain citric acid and acetic acid respectively and will curdle the milk due to its acidic pH. In this case, fresh cheeses are also made, since the enzymatic action is also lacking, but in addition to this there is no fermentation so the cheese will contain lactose. With this method you can make a lactose-free cheese using lactose-free milk.

Experience in the Campichuelo

The  liquid pharmacy rennet  has given us better results than the powdered pharmacy rennet and  kefir  is highly recommended since it transforms milk into two products: into a drink and into cheese. You can flavor the cheese as you want and with the kefried milk you can drink it, make flavored shakes and use it in your desserts as if it were yogurt.

Vegetable rennet, lemon and vinegar have not yet been tried.

Curd chemistry

We already understand that milk curdles when the structure of its protein changes, either by fermentation that produces lactic acid, acidification with other acids or enzymatic action.

There now differentiate the three methods that curdle milk: the  acid coagulation , the  enzymatic coagulation  and  mixed coagulation , in which both occur: acidic and enzymatic clotting.

Mixed coagulation

We have started by explaining lactic fermentation, which occurs spontaneously or induced in milk when it is subjected to temperature and time. In the process of  lactic acid fermentation  the  lactic acid bacteria present in milk and also called  lactobacillus,  feeding the milk sugar, called lactose, and the feed convert lactose into lactic acid, so that during the increasing fermentation concentration acid in milk and decreases sugar.

Cheese is less sweet and more digestive than milk, since during lactic fermentation it loses lactose (milk sugar).

The  rennet from the animal and the liquid rennet from the pharmacy  also contain those microorganisms that acidify the milk through lactic acid fermentation, and the milk protein, called casein, coagulates when the pH of the milk reaches a pH of 4 , 6, therefore at an acidic pH.

In addition to this  acid coagulation , natural rennet and pharmacy liquid contain enzymes that, as we will see, also make casein coagulate by  enzymatic action .

The  mixed coagulation  is just that, when the curd is formed by both processes at once: on the one hand by the acidification of milk, in this case produced as a result of fermentation generated naturally by acidolácticos microorganisms, and secondly by the action of the enzymes naturally present in the rumen.

When casein (milk protein) coagulates, either by the action of acid or by enzymes, most of the solid substances in milk bind together with it and thus the curd is formed.

Acid coagulation

When casein coagulation is carried out solely by the action of acids,  a brittle and demineralized curd is formed , since in the case of acid coagulation, calcium is carried away by the serum. This process is called acid coagulation because the structure of the milk protein changes only because of the acidity of its environment: the milk.

Acidity also acts as a preservative, promoting the growth of healthy microorganisms and inhibiting the growth of pathogens.

When we  curdle the milk by adding lemon juice (citric acid) or vinegar (acetic acid)  we are producing an acid coagulation: By adding the acid to the milk, it becomes acidic and the casein coagulates due to being in an acidic environment, without the need for any fermentation, so the curd still has the lactose intact.

Cheeses curdled by acid coagulation generate a brittle curd, without rigidity or compactness and which breaks easily, so they will be eaten fresh. In acid coagulation the minerals move towards the serum, so the cheeses will be demineralized. In the case of curdling them by adding external acids (citric or acetic) instead of producing the acids by fermentation of milk (lactic), the curds will keep the lactose intact, since there will not have been a fermentation process that converts the lactose into lactic acid.

If we want to make  cheese with lactose-free milk , we can use lemon or vinegar, since its acid will coagulate the milk protein without the need to ferment the milk sugar (lactose).

Enzymatic coagulation

On the other hand, natural rennet contains enzymes that coagulate casein in another way and with different results. The  chymosin and pepsin  are specific enzymes that curdle milk and found in the rumen and stomach of animals, with the function of forming cheese milk entering our stomach and make it more digestible as well.

In enzymatic coagulation, enzymes first degrade casein into  para-K-casein  and a macropeptide, and then rejoin the  para-K-casein s that had been separated from the macropeptide.

Thus, globes of fat are  formed that contain most of the solid substances in milk and that bind together through calcium bridges . Being fatty (lipid), they separate more intensely from the water-soluble parts of milk, like a drop of oil in a glass of water.

Enzymatic coagulation occurs when using the rennet powder from the pharmacy or the various vegetable rennet that contain enzymes that act on casein, in this case they do not acidify the medium and act without fermentation.

The  plant curds  contain other various enzymes chymosin and pepsin, but also have the ability to coagulate milk through enzymatic coagulation.

At this point, the acidity of the medium also helps, since it gradually contracts this network of fatty solids that has been formed by enzymatic action and when it contracts it expels more and more of the whey, which is why  mixed coagulation , caused Due to enzymatic factors and acidification, it  forms solid and strong curds that drain more easily .

Therefore, the acidity produced in the lactic fermentation of the microorganisms in the milk, together with the activity of the enzymes naturally contained in the rennet, are the factors that  "make the cheese" , completely transforming the structure of the protein. of milk to divide the solid part from the liquid part that it contains contains most of its water-soluble compounds.

Conclusion:

As we have already seen, we are going to review the 3 techniques to curdle milk:

1. Due to the  action of enzymes , industrially produced by GMOs and marketed in the liquid and powder version of the pharmacy rennet. They are found naturally in the rumen of a nursing ruminant, fig milk and thistle flower. They create strong and mineralized curds, especially when combined with acidification, with which the cheese can be aged.
2. Due to the action of  lactic acid ferments  , which produce acidification as a by-product of fermentation. We find these bacteria in the rumen of a lactating ruminant, in the liquid version of pharmacy rennet, and in milk kefir. They create loose and demineralized curds, suitable for fresh cheeses and spreads.
3. Adding acid  to milk to acidify it, so that the milk is acidified by an external acid, normally it is the  citric acid  from the lemon juice or the  acetic acid  from the vinegar. They create loose and demineralized curds, in addition to keeping the lactose, although it is a good way to make fresh cheeses with lactose-free milk.

The curd made with natural rennet or with the liquid version of the pharmacy rennet curdles the milk by mixed coagulation, so the curd is stronger, contains the minerals of milk and has a greater ability to become a mature cheese.