What is protein coagulation. Changes in protein substances in food products during heat treatment

27.04.2020

The clotting system consists of enzymes coagulation, non-enzymatic protein cofactors and inhibitors coagulation. The purpose of this system is the formation of the enzyme thrombin, which is responsible for the conversion of fibrinogen to fibrin.

clotting factors

1. Enzymes, are serine proteases (except factor XIII):

factor II - prothrombin,
factor VII - proconvertin,
factor IX - antihemophilic globulin B or Christmas factor,
factor X is the Stuart-Prower factor,
factor XI - antihemophilic globulin C or Rosenthal factor,
factor XIII - fibrin-stabilizing factor or Lucky-Lorand factor.

2. Protein cofactors without proteolytic activity. The role of these proteins is to bind and fix enzymatic factors on the platelet membrane:

factor V - proaccelerin, is a cofactor of factor Xa,
factor VIII - antihemophilic globulin A, is a cofactor of factor IXa,
von Willebrand factor.
high molecular weight kininogen (HMC, Fitzgerald-Fluge factor) - cofactor f.XII and prekallikrein receptor. It must be borne in mind that, according to the new cellular theory, these proteins belong to the fibrinolysis system.

3. Structural protein of thrombus formation - factor I ( fibrinogen).

Thrombin (factor II)

Thrombin, a key enzyme in hemostasis, is serine protease. In the liver, with the participation of vitamin K, its inactive precursor is synthesized - prothrombin, which then circulates in the plasma. In blood plasma, the conversion of prothrombin to thrombin occurs directly under the action of factor Xa (together with Va).

Functions of thrombin in hemostasis

In the zone coagulation:

conversion of fibrinogen to fibrin- monomers,
activation fibrin-stabilizing factor(f.XIII, transglutaminase),
acceleration of coagulation through activation of factors V, VIII, IX, XI ( positive feedback),
activation platelets(secretion of granules),
in combination with thrombomodulin(v high concentrations) activates TAFI (thrombin-activated fibrinolysis inhibitor),
activation of smooth muscle cells,
stimulation of leukocyte chemotaxis,

Out of zone coagulation

in combination with thrombomodulin activates protein C,
stimulates secretion from endothelial cells prostacyclin and t-PA.

Fibrinogen (factor I)

fibrinogen(factor I) is a large multicomponent protein that consists of three pairs of polypeptide chains - Aα, Bβ, γγ, interconnected by disulfide bridges. The spatial structure of the fibrinogen molecule is a central E-domain and 2 peripheral D-domains, α- and β-chains at the N-terminus have globular structures - fibrinopeptides A and B(FP-A and FP-B), which cover complementary sites in fibrinogen and prevent this molecule from polymerizing.

The structure of fibrinogen

Fibrinogen synthesis does not depend on vitamin K and occurs in the liver and in RES cells. Some fibrinogen is synthesized in megakaryocytes and platelets. The conversion of fibrinogen to fibrin occurs under the influence of thrombin.

fibrin stabilizing factor

fibrin stabilizing factor(factor XIII) belongs to the family of transglutaminase enzymes. It is synthesized in the liver and in platelets, in blood plasma most of inactive factor XIII is associated with fibrinogen. Factor XIII is activated by thrombin limited proteolysis from an inactive predecessor.

Like most other enzymes, factor XIII performs several functions in hemostasis:

stabilizes fibrin clot by forming covalent bonds between γ-chains of fibrin monomers,
attaches fibrin clot to fibronectin extracellular matrix,
involved in binding α2-antiplasmin with fibrin, which helps prevent premature lysis of the fibrin clot,
required by platelets for the polymerization of actin, myosin and other cytoskeletal proteins used in retractions fibrin clot.

To isolate whey proteins, it is necessary to change the native structure of the protein. With this change (denaturation), its structure is disturbed. The protein globule expands during denaturation. The process is accompanied by a change in the configuration, hydration and aggregate state of the particles. The protein globule becomes less stable during denaturation.

The stability of whey protein globules is determined by particle conformation, charge, and the presence of a hydrate shell (solvate layer). To isolate proteins, it is necessary to disturb the balance of three or at least two of these resistance factors.

In fresh whey, protein particles are in their native state. When the native state of a protein changes (denaturation), its structure is first of all disturbed. The protein globule unfolds in the process of denaturation, for which it is necessary to break from 10 to 20% of the bonds involved in its formation. The process of denaturation is accompanied by a change in the configuration, hydration and aggregate state of the particles. The protein globule becomes less stable as a result of denaturation.

To overcome potential barriers to the stability of protein particles, various methods of denaturation can be used: heating, irradiation, mechanical action, the introduction of desolvating substances, oxidizing agents and detergents, and changing the reaction of the medium. The introduction of certain substances into solutions promotes thermal denaturation.

The classification of serum coagulation methods considered in this work is shown in the diagram (Fig. 3).

Rice. 3.

Ultimately, secondary phenomena after denaturation lead to the release of proteins, such as the association of unfolded globules and their chemical change. Here, the formation of intermolecular bonds and aggregation comes to the fore, in contrast to the intramolecular processes that occur during denaturation.

In general, the process of isolating whey proteins can be characterized as coagulation.

Taking into account the feasibility of extracting and using proteins, the coagulation of whey proteins must be fixed in order to avoid the process of renaturation (restoration of the native structure of proteins), as well as the maximum possible limitation of the breakdown of the resulting aggregates.

However, it should be taken into account that as a result of thermal denaturation, in addition to breaking the hydrogen bonds of the protein particle, they are also dehydrated, which facilitates the subsequent aggregation of the protein particles. Coagulant ions (calcium, zinc, etc.), being actively sorbed on the surface of a protein particle, provide coagulation, and at significant doses can lead to protein salting out.

Properly carried out heat treatment, as a rule, increases the nutritional value of food products by improving their palatability and digestibility. In addition, the thermal effect ensures the sanitary well-being of food.

To recommend the most appropriate method of heat treatment of a particular product and obtain a finished culinary product with desired properties, it is necessary to know what physical and chemical changes occur in the products.

However, since food products are complex compositions consisting of many substances (proteins, fats, carbohydrates, vitamins, etc.), it is advisable to first consider the changes in each of them separately.

Protein changes

During the heat treatment of products, the protein systems included in their composition undergo various changes.

Violation of the native secondary and tertiary structures of proteins is called "protein denaturation". Protein denaturation can occur due to heating, mechanical action (when whipping), an increase in the concentration of salts in the system (during freezing, salting, drying products) and some other factors.

The depth of violation of the structure of proteins depends on the intensity of the impact of various factors, the possibility of simultaneous action of several of them, the concentration of proteins in the system, the pH of the medium, and the influence of various additives.

Protein denaturation entails a change in their hydration properties - water-binding capacity, which determines the taste of finished products.

During denaturation of soluble proteins, their water-binding capacity decreases to varying degrees, which depends on the depth of denaturation changes. Proper regulation of the factors that determine the denaturation and hydration properties of proteins during the technological process makes it possible to obtain high-quality culinary products.

Thus, in practice, the dependence of denaturation and water-binding capacity of proteins on the pH of the medium is often used. Denaturation of muscle proteins in meat and fish at a pH close to the isoelectric point occurs at lower temperatures and is accompanied by a significant loss of water.

Therefore, by acidifying protein systems in some ways of processing fish and meat (pickling, etc.), conditions are created to reduce the depth of protein denaturation during heat treatment.

At the same time, the acidic environment promotes denaturation and disaggregation of the connective tissue protein collagen and the formation of products with increased water-holding capacity. As a result, the time of thermal processing of products is reduced, and finished products acquire juiciness and good taste.

During denaturation, the physical state of protein systems also changes, which is usually defined by the term "protein folding". The folding of various protein systems has its own specifics.

In some cases, coagulated proteins are released from the system in the form of flakes or clots (formation of foam during cooking broths, jams), in others, the protein system is compacted with part of the water squeezed out of it along with the substances dissolved in it (production of cottage cheese from curdled milk) or an increase in strength of the system without compaction and release of moisture (coagulation of egg whites).

Along with physical changes during heating of protein systems, complex chemical changes occur in the proteins themselves and in the substances interacting with them.

Vegetable and fruit proteins

The amount of proteins in fruits and vegetables does not exceed 2-2,5%. Proteins are new building blocks cytoplasm, its organelles and nuclei of plant cells.

During heat treatment, cytoplasmic proteins coagulate and form flakes; the cell membrane structure is destroyed. Its destruction contributes to the diffusion of substances dissolved in the cell sap into the broth or other liquid in which the vegetables were cooked or stored, and the penetration of substances dissolved in the broth or other liquid into them.

Proteins of grain flour products

Peas, beans, lentils contain about 20-23% proteins, soy - 30%. in cereals and X amount reaches 11% , and in wheat flour of the highest and first grades - 10 - 12%.

In grain flour products, proteins are in a dehydrated state, therefore, when soaking legumes, cooking cereals or kneading dough, they are able to absorb moisture and swell.

When heated to 50-70 ° C, swollen proteins coagulate and squeeze out part of the absorbed moisture, which is bound by gelatinizing starch.

Used in culinary practice, the browning of wheat flour with or without fat at a temperature of 120 ° C and above affects the proteins contained in it, which are denatured and lose their ability to swell and form gluten.

Chicken egg whites

Egg white contains 11-12% proteins, yolk-15-16%. At a temperature of 50-55 ° C, the egg white begins to coagulate, which manifests itself in the form of local opacities, which, with a further increase in temperature, spread to the entire volume; upon reaching 80 ° C, the coagulated protein retains its shape.

Further heating increases the strength of the protein system, and is especially noticeable in the temperature range from 80 to 85 ° C. Upon reaching 95-100 ° C, the strength of the protein changes slightly over time.

Egg yolk coagulates at higher temperatures. To increase its viscosity, the yolk must be heated to 70°C.

A mixture of protein with yolk manifests itself, similarly to the yolk. Coagulated protein, yolk or a mixture of them keeps moisture in a bound state and does not squeeze it out. The coagulation of egg whites does not change if they are diluted with a certain amount of water and the mixture is thoroughly mixed, however, the mechanical strength of the system decreases.

The ability of egg whites to bind moisture during clotting is used in culinary practice. The addition of eggs, water or milk to the proteins in the manufacture of omelettes makes it possible to reduce the mechanical strength of protein systems and obtain culinary products with a more delicate taste than products made from natural eggs.

The mechanical properties of curdled egg whites are also used for structuring (linking) some culinary products (vegetable cutlets, etc.).

Milk proteins

The main milk proteins are casein (2.3-3.0%), lactalbumin (0.5-1.0%) and lactoglobulin (0.1%).

When milk with normal acidity is heated, noticeable changes are observed only with albumin, which coagulates and precipitates in the form of flakes on the walls of the dish. The process starts at 60°C and ends at almost 85°C.

Heating milk does not actually affect the solubility of casein: only a small amount of it in an insoluble form is present in the foam formed on milk. In fermented milk, heating causes the casein to coagulate and separate the system into two fractions: curd (curdled casein) and whey.

Casein also coagulates when milk with high acidity is heated. Cottage cheese, when heated, releases some of the moisture. To connect it to culinary products from cottage cheese, cereals or flour are added.

Meat, poultry, fish proteins

Technological processing of these products is largely due to the morphological structure and composition of their protein systems.

Features of the structure and composition of muscle tissue . The bulk of the meat processed in culinary practice is skeletal muscles. Separate skeletal muscles consist of muscle fibers connected into a single whole by connective tissue layers.

A muscle fiber is a specialized contractile cell, the length of which can reach 12 cm and more, and thickness up to 120 mm. The content of the fiber consists of two parts: liquid (homogeneous) - sarcoplasm and gelatinous (in the form of gelatinous threads) - myofibrils. Outside, the fiber is covered with a sheath - sarcolemma (Fig. 3).

In the muscles, the fibers are collected in bundles: primary, consisting of muscle fibers; secondary, consisting of primary bundles; bundles higher order that make up the muscle.

The proteins that make up the muscle fibers of meat, poultry, and fish are called muscle proteins. Some of them are contained in the liquid state in the sarcoplasm, including the protein myoglobin, which stains the meat red, and some in the gelatinous state is part of the myofibrils. The content of proteins in some meat and fish products is given in table. 12.

Muscle proteins have a high biological value: the ratio of essential amino acids in them is close to optimal. The content of muscle proteins in the skeletal muscles of cattle of the 1st category averages 13.4% with fluctuations from 6.1 to 14.3% in various parts carcasses (Fig. 4).

Rice. 3. Scheme of the structure of the muscle fiber:
1 - myofibril; 2 - sarcoplasm; 3 - core
Rice. 4. The content of muscle proteins in various parts of the carcass of cattle

The connective tissue of the muscle is called the mysium. That part of it that connects the muscle fibers in the primary bundles is called the endomysium, which combines the bundles of muscle fibers with each other - from and to them, and the outer shell of the muscle - epimysium

Important components of connective tissue are fibrillar proteins - collagen and elastin.

Through X-ray diffraction analysis, it was found that the collagen molecule consists of three polypeptide chains (triplet) twisted together around a common axis.

The strength of the triple helix is mainly due to hydrogen bonds. Separate molecules of collagen and elastin form fibers. In turn, bundles of collagen and elastin fibers, together with the substance that unites them into a single whole and consists of a protein-polysaccharide complex, form films of endomysium and perimysium.

Rice. 5. Microscopic preparation of the pectoral muscle of cattle. Visible layers of endomysium between muscle fibers and layers of perimysium between their bundles

The structure of the endomysium practically does not depend on the contractility of the muscle and the nature of the work performed by it. The collagen included in its composition forms very thin and slightly wavy fibers. Elastin in the endomysium is poorly developed.

The structure of the perimysium is greatly influenced by the nature of the work performed by the muscles. In muscles that experienced little stress during the life of the animal, the structure of the perimysium is close to that of the endomysium.

The perimysium of the muscles that perform hard work has a more complex structure: the number of elastin fibers is increased, the collagen bundles are thicker, in the perimysium of some muscles the fibers are crossed and form a complex cellular weave. In the muscles, the percentage of connective tissue is increased.

Table 13.Approximate ratio (by tryptophan) of essential amino acids in muscle proteins of some products

Thus, the connective tissue of endomysium and perimysium forms a kind of skeleton or frame of muscle tissue, which includes muscle fibers. The nature of this skeleton determines the mechanical properties, or, as they say, the "stiffness" or "tenderness" of the meat.

On average, most of the muscles of cattle contain from 2 to 2.9% collagen, but its amount in different parts of the carcass is very different (Fig. 6).

Rice. 6. The content of collagen (c) and elastin (e) in various parts of the carcass of cattle

In small cattle, the difference in the structure of the perimysium in different parts of the carcass is much less pronounced than in cattle, and, in addition, the perimysium has a simpler structure. The features of the anatomical structure of the muscle tissue of the bird include a low content and lability of the connective tissue.

The muscle tissue of fish also consists of muscle fibers and connective tissue, but has its own characteristics. Her muscle fibers are united by perimysium into zigzag myokomas, which, with the help of connective tissue layers (septa), form the longitudinal muscles of the body. Septa are transverse and longitudinal (Fig. 7).

Like the muscle tissue of warm-blooded animals, the muscles of fish, which have increased load(muscles adjacent to the head and tail), contains a more developed connective tissue, however, due to its low strength, fish are divided into varieties and culinary purposes during cutting, as is customary for meat of slaughtered animals. The main protein in the connective tissue of fish is collagen (from 1.6 to 5.1%), there is very little elastin in it.

In addition to muscle tissue, collagen in significant quantities is part of organic matter cartilage, bones, skin and scales. So, its content in bones reaches 10-20%, in tendons -25-35%. The collagen found in bones is called ossein.

As a protein, collagen has a low biological value, since it is practically devoid of tryptophan and contains very little methionine; its composition is dominated by glycocol, proline and hydroxyproline.

The main components of the meat of different animal species are (in%): water - 48-80, proteins - 15-22, fats - 1-37, extractives - 1.5-2.8 and minerals - 0.7-1, 5.

The amount of water depends on the age of the animal and the fat content of the meat. The younger the animal and the less fat in its meat, the more moisture in the muscles.

Most of the moisture (about 70%) in the muscles is associated with myofibril proteins. A smaller amount of moisture with proteins dissolved in it, extractive and mineral substances contains the sarcoplasm of muscle fibers. A certain amount of moisture is contained in the intercellular cavities of muscle tissue.

Extractives are products of metabolism. They consist of amino acids, dipeptides, glucose, some organic acids and others. Extractive substances and products of their transformation are involved in creating the taste and aroma characteristic of meat.

Rice. 7. Scheme of the structure of the muscle tissue of fish: 1 - muscle fibers (their direction is shown by dashes); 2 - myocommas; 3 - transverse septa; 4 - longitudinal septa

So, solutions of glutamic acid and its salts have a meaty taste, so monosodium glutamate is used as one of the components of dry soups, sauces and other concentrates. Amino acids such as serine, alanine, glycine have a sweet taste, leucine is slightly bitter, etc.

When heated, extractives undergo various chemical changes- reactions of melanoidin formation, oxidation, hydrolytic cleavage, etc. The substances formed in this process are also considered to be extractive: their taste, smell and color affect the organoleptic characteristics of the finished product.

The extractive substances of fish muscle tissue differ significantly in composition from the extractive substances of meat. It has little glutamic acid and more histidine, phenylalanine, tryptophan, cystine and cysteine. It is generally accepted that the taste and smell of fish are mainly due to the nitrogenous bases of extractive substances, which are especially abundant in sea fish and are few or completely absent in the meat of land animals.

Among the mineral substances of the muscle tissue of terrestrial animals and fish, a significant proportion falls on sodium, potassium, calcium and magnesium salts.

Changes in proteins during heat treatment

During heat treatment, muscle and connective tissue proteins undergo significant changes.

The muscle proteins of meat and fish begin to denature and coagulate at a temperature of about 40 ° C. At the same time, the contents of the muscle fibers become denser, since moisture is released from them with mineral, extractive substances dissolved in it and soluble proteins that are not denatured at this temperature.

The release of moisture and compaction of muscle fibers increases their strength: they are more difficult to cut and chew.

If meat or fish is heated in water, then the proteins that have passed into it, upon reaching the appropriate temperatures, are denatured and coagulated in the form of flakes, forming the so-called foam.

About 90% of the soluble proteins of meat and fish are denatured at temperatures of 60-65 ° C. At these temperatures, the diameter of muscle fibers in beef is reduced by 12-16% of its original value. The subsequent increase in temperature entails additional moisture loss, compaction of muscle fibers and an increase in their strength.

When meat is cooked, the protein myoglobin is denatured, which determines the color of the meat. Denaturation of myoglobin is accompanied by a change in the color of muscle tissue, which makes it possible to indirectly judge the culinary readiness of meat.

The meat retains its red color at temperatures up to 60 ° C, at 60-70 ° C it turns pink, and at 70-80 ° C it becomes gray. The meat brought to culinary readiness retains a gray color or acquires a brown color.

Heating of the connective tissue causes disaggregation of the collagen contained in it and changes the structure of the tissue itself. initial stage This process is the denaturation of collagen and the violation of the fibrillar structure of the protein, which is defined by the term "collagen welding".

The temperature of denaturation or welding of collagen is higher, the more it contains proline and hydroxyproline. For meat, welding is observed at a temperature of about 65 ° C, for fish - about 40 ° C.

At these temperatures, a partial rupture of the cross-links between the polypeptide chains of the fibrillar protein molecules occurs. As a result, the chains contract and take on an energetically more favorable folded position.

In collagen fibers isolated from connective tissue, collagen welding occurs at certain temperatures and has a jump character. In perimysial films, collagen welding is extended in the temperature range. The process begins at the above temperatures and ends at higher temperatures, the higher the temperature, the more complex the structure of the connective tissue.

Changes at the molecular level entail changes in the structure of collagen fibers and connective tissue layers. Collagen fibers are deformed, bent, their length is reduced and they become more elastic and transparent-glassy.

The structure of the connective tissue layers themselves also changes: they also deform, increase in thickness, become more elastic and transparent glassy.

Collagen welding is accompanied by absorption of a certain amount of moisture and an increase in the volume of connective tissue layers. Compression of the connective tissue layers greatly contributes to the squeezing out of the muscle tissue of the fluid released during denaturation and coagulation of muscle proteins.

With further heating of the connective tissue, a partial or complete rupture of the cross-links between the polypeptide chains of denatured collagen occurs, while some of them pass into the broth, forming a gelatin solution; the structure of the connective tissue layers is largely disturbed, and their strength is reduced.

The weakening of the strength of the perimysium is one of the factors that determine the readiness of the meat. Meat that has reached readiness should not offer significant resistance to cutting or biting it along the muscle fibers.

Like the welding temperature, the rate of collagen disaggregation depends on the structure of the perimysium. Yes, for 20 min In the lumbar muscle, the perimysium of which is poorly developed, 12.9% collagen was disaggregated and passed into the broth, and in the pectoral muscle with a coarser perimysium, under the same conditions, only 3.3% collagen was disaggregated.

over 60 min cooking, these figures increased: for the lumbar muscle, up to 48.3%, for the pectoral muscle, only up to 17.1. The temperature at which the heat treatment process is carried out has an insignificant effect on the rate of collagen disaggregation and softening of the perimysium.

For example, when cooking a shoulder muscle at a temperature of 120°C (in an autoclave), the amount of disaggregated collagen is twice as high as its content in a similar muscle, which was cooked in the usual way at a temperature of 100°C.

However, it should be noted that with an increase in the cooking temperature, simultaneously with a reduction in the heat treatment period, excessive compaction of muscle proteins occurs, which negatively affects the consistency and taste of meat.

When used for frying muscles with a complex peri-life, the meat is treated with acids (pickling) or enzyme preparations. For pickling, citric or acetic acid is usually used. In marinated meat, the disaggregation of collagen and the weakening of the perimysium are noticeably accelerated.

fried products are juicy, with good taste.

As meat softeners, proteolytic enzymes of plant, animal and microbial origin are successfully used: ficin (from figs), papain (from melon tree), trypsin (of animal origin), etc.

Enzyme preparations are powders, pastes or solutions with which meat is treated in one way or another (moistened, smeared, injected). Often the meat is loosened before being processed with enzymes.

Heat treatment slightly reduces the strength of elastin fibers, therefore, muscle tissue with a high content of elastin (neck, flank) remains stiff after heat treatment and is used mainly for preparing cutlet mass.

Proteins in fresh blood are in a native, unchanged state.
During the technological processing of blood, in some cases it is necessary to prevent or reduce the denaturation of proteins, in other cases it is mandatory.
When obtaining dry plasma or blood, called albumin in technology, they strive to dry it in such a way as to denature blood proteins as little as possible, to preserve their ability to dissolve. For this purpose, the blood is dried in spray dryers. Carefully and quickly dried protein is not further denatured by high temperature. This fact was established back in 1857 by a brilliant Russian scientist. D. I. Mendeleev, who proved that dry protein does not change when heated to 100-110 °.
For dietary and technical albumin proteins, high solubility is a must. Glue is obtained from technical albumin; the more soluble proteins it contains, the higher its adhesive power.
In the manufacture of various preparations from blood, it should also be remembered that in the process of its processing, protein denaturation, accompanied by coagulation, should not be performed. For example, in the production of liquid hematogen, direct mixing of blood with alcohol is unacceptable, since the latter, in contact with protein, causes coagulation and reduces solubility; in this case, a precipitate forms, which prevents a clear hematogen preparation from being obtained. To prevent thermal coagulation of blood proteins, vials with hematogen are pasteurized at a temperature not exceeding 52-53 °.
In the manufacture of blood plasma substitutes, it is necessary to keep the denatured proteins in solution. To do this, formaldehyde and glucose are used as a stabilizer that prevents the precipitation of proteins during heating. The stabilizing effect of glucose, apparently, is due to the fact that it is adsorbed on globular protein molecules, in connection with which the latter become the center of a large soluble complex. Formaldehyde, blocking amino groups, does not allow salt groups to form inside the molecule and thereby prevents coagulation.
In the manufacture of coagulates, on the contrary, it is necessary to denature the proteins in order to achieve their coagulation and separation of most of the water from the protein clot. In this case, the denaturing factor is exposure to acid or heat.
Thermal coagulation of different blood proteins occurs at different temperatures. Fibrinogen solution in 10% NaCl solution coagulates at 52-53°. fibrin solution - about 56 °, albumin solution in pure water - at 50 °; when salts are added (5% NaCl solution), the coagulation temperature rises to 72-75°; a solution of globulin in a 10% NaCl solution coagulates at 75°. Defibrinated blood coagulates at 61°. Serum begins to become cloudy at 64°.

Coagulation of milk is nothing more than turning it into a gel (clot), that is, its coagulation.

It is a bound solid fraction of milk proteins with the presence of dissolved fats, which can then be easily separated from the liquid (whey).

Milk protein coagulation can be latent and true. With latent coagulation, micelles do not bind to each other over the entire surface, but only in some of its areas, forming a spatial fine-meshed structure, which is called a gel.

When all or most of the particles of the dispersed phase are destabilized, the gel covers the entire volume of the dispersed medium (initial milk).

Latent coagulation is simply referred to as coagulation, gelling, or coagulation.

True coagulation consists in the complete merging of colloidal particles and the precipitation of the dispersed phase in the sediment or floating up.

Coagulants are substances that perform several functions, but most importantly, they form a jelly-like clot - they separate dense fractions of milk from liquid ones.

For this purpose, only previously used, which is obtained from the stomachs of calves.

It is this enzyme in the stomachs of calves (chymosin) that helps them ferment their mother's milk for nourishment.

V modern world to form a clot (also called kalya) use:

Veal rennet (rennet), made from the stomachs of calves (milk-clotting enzyme - chymosin).
It comes in powder, paste and liquid forms. It is chymosin (from veal rennet or artificially grown chymosin) that is best suited for the production of hard and semi-soft cheeses.
Pepsins are extracts from the stomachs of other domestic animals. Mostly cow or pepsin is used, pork and chicken pepsins are also commercially available, but they are very sensitive to acidity and unstable. Their use is not recommended.
Bovine pepsin (especially mixed with chymosin) can be used for the production of pickled cheeses (brynza, suluguni). For the production of soft, semi-soft and hard cheeses, pepsins are not recommended.
Microbial rennin (microbial pepsin) - Some yeasts, molds and fungi naturally produce enzymes suitable for coagulation. The most widely used enzymes are derived from the microscopic fungus Rhizomucor meihei (formerly Mucor meihei). It is a vegetarian coagulant. An example of such a coagulant is.
Fermented chymosin (recombined chymosin) - the calf chymosin gene has been introduced into the genome of several host microorganisms (Kluyveromyces lactis, Aspergilleus niger, Escherichia), as a result of which they have become able to produce a protein completely identical to calf chymosin during fermentation.
This enzyme has proven itself in the manufacture of all types of cheeses, where veal rennet was usually used. It is a vegetarian coagulant.

For the preparation of fresh cheeses, cottage cheese, pickled cheeses, you can use any coagulant.

However, only chymosin (animal rennet or recombined chymosin) is suitable for semi-soft and hard cheeses, since it, together with lactic acid bacteria (sourdoughs), is involved in the formation of the cheese's texture, taste and ability to preserve for a long time.

During protein coagulation, milk fat and water with solutes (whey) are sufficiently firmly captured by the resulting gel; during protein precipitation, only a small amount of milk fat and the aqueous phase can be mechanically retained by the sediment.

The production and maturation of rennet cheeses is carried out at low temperatures and active acidity, called physiological, in order to enable the biological transformation of milk components with minimal loss of nutritional value.

When using the thermoacid method, the fatty phase of the milk is separated by separation, the skimmed milk proteins are precipitated and mixed with cream.

Precipitation consists in the rapid acidification of milk to a level lower than the isoelectric point by the addition of acid whey, sour milk, lemon juice, acetic acid and heating it to high temperatures (90-95 ° C).

Thus, with enzymatic coagulation, casein and milk fat are concentrated simultaneously, with thermal acid coagulation, as a result of two processes: centrifugal and sedimentation.

The acidic method consists in curdling milk at the isoelectric point of casein (pH 4.6) by slowly forming acids by microorganisms or by adding acids (usually hydrochloric) or acidogens (for example, glucolactone) to milk; it is used in the production of fresh cheeses or cheeses with short terms maturation.

The enzymes involved in the maturation of rennet cheeses are not active in acidic cheeses due to the low pH. The degree of transformation of milk proteins and lipids in fermented milk cheeses is lower, the flavor bouquet is narrower than in rennet cheeses.

The acid-enzymatic method is a variant of acid coagulation, with the addition of a small amount of milk-clotting enzymes to milk, insufficient for enzymatic coagulation at the pH of fresh milk.

In this case, milk coagulation occurs at pH 5.1-5.4 (in paracasein isopoint). The addition of milk-clotting enzymes favorably affects the coagulation rate, clot strength and whey release, however, at the pH of acid-rennet coagulation of milk, radical changes in casein micelles occur, which dramatically changes the structure of the clot and cheese compared to those during rennet coagulation.

The clot formed during the production of cheeses by the acid-enzymatic method is closer in its properties to the acid clot, the quality of the products is closer to sour-milk cheeses.

Concentration of milk by ultrafiltration has received a certain distribution in the production of brine and some other cheeses.