Cheese: acid and rennet coagulation

The Coagulation Process

In milk, caseins are found in the form of micelles, lipoprotein particles with a tendency to unite and coagulate; in standard conditions this does not happen for two reasons, the first lies in their electric charge, which at natural pH is negative (negatively charged particles repel each other); the second is related to the presence of the colloidal protector peptide in the C-terminal part of the K-casein. The lack of one or both factors determines the coalescence of these particles, therefore the protein coagulation.

Acid Coagulation

Microbial starters ferment lactose to lactic acid, resulting in a decrease in pH; when this reaches the value of 4.6 the micelles lose their negative charge and become neutral; at this point, having lost the electrical repulsion, they begin to coagulate. During this process, Ca2 + ions are expelled (which are 2/3 inside the casein micelles), which bind to the lactic acid, salifying and forming calcium lactate. Casein, which is found in micelles as calcium phosphocaseinate, loses calcium and becomes acid phosphocaseinate, characterized by a particular consistency that allows the formation of a curd with a gelatinous consistency.

The clot that forms following acidification is looser and softer than that obtained from treatment with enzymes; for this reason, acid coagulation is used only in the production of fresh and soft cheeses, which are characterized by their acid taste and must be consumed within a few days (like cottage cheese).

Presamic coagulation

For all cheeses, with the exception of fresh and soft ones, rennet coagulation is used. The rennet, or rennet, is added to the milk at a temperature of 30-37 ° C, respectively for soft or hard cheeses; it is a product rich in proteolytic enzymes, which determine the detachment of the C-terminal part of K-casein, giving the micelles the ability to coagulate with each other.

The rennet or rennet is obtained from the fourth stomach (abomasum) of unweaned ruminants (calves, lambs, kids); inside we find all the enzymes necessary for the digestion of milk and in particular CHIMOSINA (OR RENNINA) and PEPSIN, which act directly on the protein chains. The stomach of adult animals cannot be used precisely because they are devoid of lactase and other enzymes necessary for the digestion of "food."

Traditional rennet or rennet is obtained by macerating fragments of abomasum dried or preserved in brine at 10% NaCl (which facilitates the extraction of enzymes), with the addition of antiseptics for 10-12 hours at 20 ° C and pH 4. The juice, is then filtered, clarified and dried.
There are also rennet substitutes on the market, extracts of microbial origin, and above all recombinant chymosin. The commercial rennets are all standardized and brought to a fixed title: the rennet title is the quantity of coagulated milk from 1 cc of rennet in 40 minutes at 37 ° C; it is therefore an extremely important parameter for dairies.

During rennet coagulation, chymosin hydrolyzes K-casein in a very specific point, located between amino acids 105 (phenylalanine) and 106 (methionine). By cutting in this position, the colloidal protector peptide is lost, which represents the C-terminal part of the protein which, being glycosylated, increases the hydrophilicity of the micelle; losing this peptide reduces hydrophilicity and casein micelles acquire a greater tendency to " aggregation. After the detachment of the C-terminal peptide the casein is transformed into para-casein, which in the presence of the calcium ions contained in the micelle becomes dicalcium paracaseinate, forming a three-dimensional lattice (gel) more rigid than that seen for acid coagulation, therefore able to retain a greater quantity of lactose and mineral salts in the lipid globules. This gel is so strong that over time it tends to contract and expel the serum. While during acid coagulation a prompt expulsion of calcium occurs, in this phase of rennet coagulation calcium remains bound to caseins.

The formation of the curd depends on many factors, which must be carefully controlled to obtain the cheese:

- concentration and title of the rennet
- temperature: caseins do not precipitate below 10 ° C and above 65 ° C; in general their tendency to curdle is maximum between 20 and 40 ° C.
- pH: above pH 7 the milk does not coagulate, since the charges of the caseins are too strong to allow the approach of the casein micelles
- concentration of Ca ++ ions
- size of the micelles
- milk storage: if it is kept for more than 2 days at +4 ° C it does not coagulate.

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