Homogenizers for the dairy industry. Homogenizer: principle of operation, design and application in the dairy industry. The mechanism of the phase dispersion process in a valve-type apparatus
Milk homogenization- the process of crushing fat globules by exposing milk to significant external forces. The homogenization process is used in the production of pasteurized milk to increase homogeneity and improve its shelf life. The purpose of homogenization is to prevent spontaneous settling of fat in the production and storage of dairy products, to maintain a homogeneous consistency of the product without delamination.
The number and size of fat globules in milk are variable and depend on the breed, feeding and keeping conditions, lactation stage, age of the animal and a number of other factors. On average, 1 cm 3 of whole milk contains about 3 billion fat globules. The size of the fat globules varies widely - from 0.1 to 20 microns.
In the process of crushing fat globules during homogenization, a redistribution of the shell substance occurs. Plasma proteins are consumed to build the shells of the resulting small fat globules, which leads to the stabilization of a highly dispersed fat emulsion of homogenized milk.
In medium-fat milk, free fat is practically not formed, i.e. there are no accumulations of small fat globules. With an increase in the mass fraction of fat in milk, accumulations of fat globules may occur. That. Properly carried out homogenization eliminates the possibility of the appearance of free fat, thereby increasing the shelf life of dairy products: regulates the structural and mechanical properties of milk protein clots; improves the taste of products.
Undesirable consequences include reduced thermal stability of homogenized milk; the occurrence of hypersensitivity to light and as a result of a "sunny" taste; impossibility of separation of homogenized milk.
Conditions for effective homogenization:
- 1) Milk fat must be in a liquid state;
- 2) Crushing of fat globules is possible only with external influence;
- 3) It is necessary to form a new protective layer of each fat globule.
During the production of pasteurized milk, milk fat basically retains its original composition and properties. Thermal and mechanical effects do not cause significant changes in the fat phase of milk.
Currently, the following types of homogenization are used:
- 1) single-stage - the formation of small fat globules occurs;
- 2) two-stage - there is a destruction of these aggregates and further dispersion of fat globules;
- 3) separate - not all milk is processed, but only its fat part (cream) 16-20% fat.
With a single-stage homogenization, fat globules are crushed to a size of about 1 μm, i.e. there is a homogeneous dispersion of the fatty phase, not capable of settling. It is used to produce low-fat dairy products (drinking milk, etc.).
Two-stage homogenization is carried out in the production of high-fat products (cream, ice cream mixes, etc.). It allows you to break up the resulting accumulations of fat globules.
Homogenization of normalized milk is carried out separately as follows. To do this, normalized milk, heated to a temperature of 55-65 ° C, is separated. The resulting cream with a mass fraction of fat of 16-20% is homogenized in a two-stage homogenizer at a pressure of 8-10 MPa in the first stage and 2-2.5 MPa in the second. Homogenized cream is mixed in the flow with skimmed milk leaving the cream separator and sent to the pasteurization section of the pasteurization-cooling plant. Cream can also be homogenized before being mixed with skimmed milk to formulate normalized milk. Separating homogenization can significantly reduce energy costs.
In the production of various dairy products, a homogenization pressure of 5-25 MPa and a temperature of 55-70 ° C are usually used. The pressure and temperature of homogenization determine its mode. The pressure and temperature during homogenization are selected depending on the mass fraction of fat in the mixture. The higher the fat content of the mixture, the lower the pressure should be. Homogenization should be carried out at a temperature not lower than 50-60 0 С. For example, when homogenizing milk and low-fat cream (10-12%) at a temperature not lower than 70 0 С, a pressure of 10-15 MPa is used, when producing sour cream 25-30% fat content - 9-10 MPa.
During the homogenization process, free fat may be released, as mentioned above. In milk, with increasing homogenization pressure, the amount of free fat decreases, and in cream it increases. An increase in the amount of free fat is associated with a lack of protein necessary for the formation of a shell of newly formed fat globules. One of the conditions for the formation of a protective shell is the ratio of skimmed milk powder to fat; in a homogenized product, it should not be lower than 0.6-0.8.
The efficiency of homogenization is determined by the settling of fat, by centrifugation, by the change in optical density and the average size of fat globules. In homogenized milk, the diameter of fat globules should not exceed 2 microns.
Increasing the dispersion of milk fat leads to a more homogeneous, homogeneous and stable system. Increasing the stability of the system without cream sludge is necessary in the production of many dairy products. In addition, homogenization increases the viscosity of milk, cream and milk mixtures, which positively affects the consistency of finished products and expands the use of homogenization in dairy production.
Valve-type homogenizers, which are high-pressure multi-plunger pumps with a homogenizing head, have received the greatest application in the dairy industry. During the course of the plunger, high pressure is created, as a result, milk (or mixture) is forced through the gap of the homogenizer at great speed. At the entrance to the valve gap, the flow rate of milk increases sharply. A large fat globule, passing through the gap at great speed, is pulled into a cylinder, which is crushed into small fat drops, which are immediately covered with a protein shell of plasma proteins. With a large difference in velocities, the crushing of the balls can occur by successive detachment of particles without intermediate stretching into a cylinder. That. the fat of normalized milk, when it is forced through the annular valve gap of the homogenizing head, is dispersed. The required pressure is generated by a pump. In the production of whole milk, the size of fat globules decreases from 3-4 microns to 0.7-0.8 microns.
In addition to valve-type homogenizers, centrifugal homogenizers-clarifixators are used, which have a special chamber with a fixed homogenizing disk. The design of the disc itself provides an active mechanical effect on the milk particles.
Determination of the effectiveness of homogenization.
The stability of the fat emulsion of milk or cream is of great importance in the production of dairy products. In the production of some products, it is desirable to keep the fat emulsion stable as long as possible (pasteurized and sterilized milk and cream, fermented milk products, canned milk and ice cream). In the production of other products (for example, cow butter), it is desirable to break up the fat emulsion as completely as possible in order to aggregate the fat globules.
At rest, in fresh milk, 20-30 minutes after milking, a layer of settled cream appears, which is associated with the difference in density between milk fat (994-1025 kg/m3) and milk plasma (1034-1040 kg/m3). The rate of ascent of the fat globule under conditions of natural sludge is expressed by the equation
n \u003d 2 * g * r 2 * (s P -With and )/(9*µ) ,
n - the speed of the ascent of the fat ball, m / s;
g - free fall acceleration, m/s 2 ;
r is the radius of the fat globule, m;
с n is the milk plasma density, kg/m3;
cg - fat globule density, kg/m3;
µ - milk plasma viscosity, Pa s.
The dependence of the separation rate on the radius of the fat globule squared indicates the possibility of preventing settling by reducing its radius, which is achieved by homogenization.
The efficiency of homogenization is determined by the optical method, the fat settling method, the centrifugation method and the average size of fat globules, the fat content is determined by the Gerber acid method with triple centrifugation for 5 minutes for homogenized milk.
Optical method
The optical method for determining the efficiency of homogenization applies to milk and cream with a mass fraction of fat from 2 to 6%. The essence of the method is to measure the optical density (turbidity) of the sample at two wavelengths - 400 and 1000 nm. The value of the ratio of optical densities at different wavelengths (D400/D1000) characterizes the degree of dispersion of the fat phase of milk or cream.
The efficiency of homogenization (EG) is determined by the ratio of optical densities (D400 and D1000). Calculation of the average diameter of milk fat globules is carried out according to the formula:
d Wed \u003d 2.82 - 2.58 lg D 400 /D 1000 ,
d cf - average diameter of fat globules, microns;
D 400 and D 1000 are the optical densities of the sample at wavelengths of 400 and 1000 nm.
Determining the efficiency of homogenization
fat retention method.
To determine the efficiency of homogenization by fat settling, milk is kept for 48 hours at a temperature of 8 °C without stirring in a 250 ml measuring cylinder. The top 100 ml of milk is then taken and the fat content of the milk remaining in the cylinder is determined. Settling of fat is calculated by the formula:
ABOUT and \u003d 100 * (W m -AND n )/AND m -K* F n ,
About w - settling of fat,%;
F m, F n - mass fractions of fat in the original milk and the lower layer of milk remaining in the cylinder,%;
K is the ratio of the volume of the lower layer of milk in the cylinder to the total volume of milk (when sampling 100 ml of the upper layer, K = 0.6).
VNIMI centrifugation method
The efficiency of homogenization by centrifugation is determined at a certain mode of milk centrifugation in a special pipette (see Fig. 6.1).
milk processing homogenization centrifugation milk
Rice. 6.1.
Centrifugation is carried out for 30 minutes. After centrifugation, the pipettes are removed and placed vertically on the cork. Then, carefully, without turning over or shaking, the lower part of the product is poured from the pipette to the II mark into a glass, for which the upper hole of the pipette is closed with the finger of the left hand, and the rubber stopper is removed from the lower end of the pipette with the right hand. The fat content of the drained product is determined. The degree of homogenization is calculated by the formula:
r = 100*W n /AND m ,
r - degree of homogenization, % (for homogenized milk r=75-80%);
W n - mass fraction of fat in the lower layer of the product, drained from the pipette;
F m - mass fraction of fat in the original milk,%.
Microscopic method
When determining the efficiency of homogenization by a microscopic method, the average size of the fat globules of homogenized milk (d cf) is determined. To determine the size of fat globules, milk and cream are diluted with water. Using an eyepiece micrometer, the size of fat globules is determined at a magnification of 1350 times (objective 90, eyepiece 15 with immersion).
Fat globules are divided into fractions (groups) according to the size of the diameters, depending on the magnification of the microscope and the set value of the division of the eyepiece micrometer. The accuracy of the limits of these fractions is one or half the division of the eyepiece micrometer. In one sample of milk, the size of 600 to 1000 fat globules is determined and distributed into fractions. The size of the fat globules of each fraction is expressed by the average diameter. For example, for fraction III, the average diameter will be (2+3)/2 = 2.5 µm.
Homogenization is the mechanical processing of raw materials that have passed through milk filters, as a result of which fat balls are dispersed (crushed) under the action of an external force - pressure, high-frequency current, ultrasound, etc.
Why is homogenization necessary?
When storing poured into milk cans product, fat floats to the surface due to the fact that it is lighter than plasma (reverse). The raw material is defended. A large lump of fat, rising to the upper layers, collides with others like it. Under the influence of immunoglobulins, agglutination occurs (gluing of individual elements and their precipitation from a homogeneous mixture). As a result, the consistency changes and the quality decreases, which is not desirable. If the fat globules are broken into small pieces, they will not stick together into a film on the surface.
The speed of the ascent of the fat ball depends on its size - the larger, the faster. According to the Stokes formula, it is directly proportional to the square of the lump radius. The size of fat globules ranges from 0.5 to 18 microns. After homogenization, it is reduced by a factor of approximately 10 (average outlet size 0.85 µm). This means that they will float 100 times slower. In addition, in small lumps, less than 1 micron in size, the forces of mutual repulsion are greater than the forces of attraction.
During the crushing of fat, the substance of its shell is redistributed. Part of the phosphatides passes into the plasma, and plasma proteins go to the outer cover of small globules. Thanks to these factors, the fat emulsion is stabilized in milk. With a high degree of dispersion, the settling process is not observed, fat does not float, milk flasks are filled with a higher quality product. Cream, cottage cheese, butter, etc., made from homogenized (homogeneous) raw materials, have better organoleptic characteristics and consistency, nutrients are absorbed faster and more completely by the body.
Homogenization helps to:
- Pasteurized milk or cream, poured into stainless steel containers acquired uniform fat content, color and taste.
- Sterilized milk and cream are better stored.
- No fatty film was formed on fermented milk products, and protein clots were stronger and with better consistency.
- In condensed milk canned food, during long-term storage, the fat phase did not stand out.
- In whole milk powder, there was less free fat, without shells of protein - this leads to oxidation.
- Reconstituted fermented milk drinks, cream and milk did not develop a watery aftertaste, and the taste of the product became more intense.
- Milk with a filler (for example, cocoa) turned out to be more viscous, without sediment, with a better taste.
Homogenization mechanism
Homogenization is recommended to be done after the milk has passed through long-term pasteurization bath.
For this, different types of devices are used. The most common are valve-type units. At their core, they are high pressure plunger pumps. Fluid is passed through very small holes. At the same time, the flow rate increases sharply. Fat globules are crushed, the resulting small lumps are immediately covered with a protein shell. Why this happens will be discussed in the second part of the article.
Homogenizers are designed to crush fat globules in milk, liquid dairy products and ice cream mixtures. They are used in various technological lines for milk and dairy products. Other equipment (emulsifiers, emulsifiers, vibrators, etc.) is also known for milk homogenization, but it is less efficient.
Valve-type homogenizers K5 - OG2A - 1.25 received the greatest application in the dairy industry; A1 - OGM 2.5 and A1 - OGM are high-pressure multi-plunger pumps with a homogenizing head. Homogenizers consist of the following main units: a crank mechanism with a lubrication and cooling system, a plunger block with homogenizing and manometric heads and a safety valve, a frame. The drive is carried out from the electric motor with the help of a V-belt transmission. The crank mechanism converts the rotational motion transmitted by the V-belt transmission from the electric motor into the reciprocating motion of the plungers. The latter, through lip seals, enter the working chambers of the plunger block and, by making suction and discharge strokes, create the necessary pressure of the homogenized liquid. The crank mechanism of the described homogenizers consists of a crankshaft mounted on two tapered roller bearings; bearing caps; connecting rods with covers and liners; sliders pivotally connected to the connecting rods with fingers; glasses; seals; housing cover and driven pulley cantilevered at the end of the crankshaft. The internal cavity of the crank mechanism is an oil bath. An oil indicator and a drain plug are mounted on the rear wall of the housing. In the homogenizer K5 - OG2A - 1.25, the rubbing parts of the crank mechanism are lubricated by spraying oil with a rotating crankshaft. The design of the housing and the relatively small load on the crank mechanism of the homogenizer K5 - OG2A - 1.25 allows you to cool the oil placed inside the housing due to heat transfer from the surface to the environment. Only the plungers are cooled by tap water. In homogenizers A1 - OGM - 2.5 and A1 - OGM, in combination with oil splashing in the inside of the body, a forced lubrication system is used for the most loaded rubbing pairs, which increases heat transfer. The oil in these homogenizers is cooled by heat-conducting water, which enters the coil of the cooling device, laid at the bottom of the housing, and the plungers are cooled by tap water, which is supplied to them through a hole in the pipe. A flow switch is installed in the system to control the flow of water. A plunger block is attached to the KShM body with the help of two pins, designed to suck the product from the supply line and pump it under high pressure into the homogenizing head. The plunger block includes a body, plungers, lip seals, bottom, top and front covers, suction and discharge valves, valve seats, gaskets, bushings, springs, flange, fitting, filter in the block suction channel. On the end plane of the plunger block there is a homogenizing head designed to perform a two-stage homogenization of the product due to its passage under high pressure through the gap between the valve and valve seat in each stage system. A manometric head is fixed on the upper plane of the plunger block to control the homogenization pressure. The gauge head has a throttling device that makes it possible to effectively reduce the amplitude of the oscillation of the gauge pointer. The gauge head consists of a body, a needle, a seal, a tightening nut, a washer and a pressure gauge with a diaphragm seal. In the end plane of the plunger block on the side opposite to the mounting of the homogenizing head, there is a safety valve that prevents the homogenization pressure from increasing compared to the nominal one. The safety valve includes screw, locknut, heel, spring, valve and valve seat. The safety valve is adjusted to the maximum homogenization pressure by turning the clamping screw, which acts on the valve through a spring. The frame of the homogenizer is a cast or welded structure of channels covered with sheet steel. A KShM is installed on the upper plane of the frame. Inside, on two brackets, a plate with an el. engine. In addition, the plate is supported by screws adjusting V-belts. The bed has four supports regulated on height. The side windows of the bed are closed with removable covers. Milk or dairy product is pumped into the suction channel of the plunger block by means of a pump. From the working cavity of the block, the product under pressure enters through the discharge valve, the homogenizing head passes at high speed through the front gap formed between the ground surfaces of the homogenizing valve and its seat. When this occurs, the dispersion of the liquid phase of the product. From the homogenizer, the product is sent through the milk pipeline for further processing or preliminary storage.
Homogenizing heads were subjected to one or another little existing changes, however, the principle of their arrangement is still unchanged. The shape of the valve face is usually flat, poppet or conical with a small taper angle. A homogenizer with flat valves with concentric grooves has the same grooves on the seat surface. Consequently, the shape of the milk passage in the radial direction changes, which should contribute to better homogenization. Liquid product can be pumped into the head by any pump that has a uniform flow and is capable of creating high pressure. For this purpose, multi-plunger, rotary and screw pumps are applicable. The most widely used high-pressure homogenizers with three-plunger pumps.
The scheme of the valve-type plunger homogenizer is shown in fig. 3
When the plunger moves to the left, milk passes through the suction valve 3 into the cylinder, and when the plunger moves to the right, it is pushed through the valve 4 into the discharge chamber, on which a pressure gauge 10 is installed to control the pressure. Further, the milk through the channel into the head 5, in which the valve 7 presses, pressed against the seat 6 by the spring 8. The tension of the spring is regulated by the screw 11. The valve and the seat are lapped to each other. In the non-working position, the valve is tightly pressed against the seat by the spring 8, which has become the adjusting screw 11, and in the working position, when the liquid is pumped, the valve is lifted by the liquid pressure and is in a "floating" state. A characteristic indicator of the homogenization mode, which plays an important role in the adjustment of the machine, is the homogenization pressure. The higher it is, the more efficient the dispersion process. The pressure is regulated by the screw 11, guided by the readings of the pressure gauge 10. When the screw is screwed in, the pressure of the spring on the valve increases, therefore, the height of the valve gap increases. This leads to an increase in hydraulic resistance when fluid moves through the valve, i.e., to an increase in the pressure required to push a given amount of fluid.
The ability of a plunger pump to generate high pressure compromises the integrity of parts in the event that the port becomes clogged in the valve seat. Therefore, the homogenizer is equipped with a safety valve 9, through which the liquid comes out when the pressure in the machine is higher than the set one. The pressure at which the safety valve opens is adjusted by tightening the spring with a screw.
On fig. Figure 4 shows a double throttling homogenizer in which liquid passes through two working heads in series. In each head, the pressure of the spring on the valve is regulated separately, with its own screw. In such heads, homogenization occurs in two stages.
The working pressure in the discharge chamber is equal to the sum of both differences. The use of two-stage homogenization is mainly due to the fact that in many emulsions, after homogenization in the first stage, back-aggregation of dispersed particles and the formation of “clusters” are observed at the outlet, which worsen the dispersion effect.
The task of the second stage is to break up and disperse such relatively unstable formations.
This requires no such significant mechanical action, so the pressure drop in the second auxiliary stage of the homogenizer is much less than in the first, on the operation of which the degree of homogenization mainly depends.
Figure 4 - Scheme of two-stage homogenization
In the general design of modern homogenizers, the basic principles and provisions of technical aesthetics, sanitation and hygiene are applied. Following new trends in the development of dairy equipment, new designs of homogenizers are streamlined, lined and covered with stainless steel casings with a polished surface.
Based on the performance of the homogenizer and design considerations, for the prototype we choose the homogenizer brand A1 - OGM - 2.5.
A homogenizer is a device for obtaining homogeneous (homogeneous) dispersed systems. Systems can be single or multi-phase, i.e. in a dispersed medium, which is usually a liquid, there are particles (usually insoluble) of one or more solid or liquid substances, which are called dispersed phases. The term "homogeneous" means that the phases are distributed evenly, with the same concentration in any arbitrarily taken unit volume of the medium. The resulting system should be relatively stable. To do this, during homogenization, in the vast majority of cases, dispersion is carried out, that is, grinding of phase particles.
The use of homogenizers in the dairy industry
The milk homogenizer crushes fat globules. The speed at which they float to the surface depends on the square of their radius. Thus, after decreasing by 10 times, the speed drops by 100 times. Due to this, the product does not settle, does not separate into cream and skim. Its shelf life increases significantly.
In addition, after homogenization:
- In the manufacture of margarine or butter, water and other components are evenly distributed in the fatty medium. And in mayonnaise and salad dressings - fats in the aquatic environment.
- Cream and pasteurized milk are made uniform in color, taste and fat content.
- In condensed milk canned food, during long-term storage, the fat phase is not released.
- Kefir, sour cream and other fermented milk products are stabilizing. The consistency of protein clots improves. No grease plug forms on the surface.
- In powdered whole milk, the amount of free fat not protected by a protein shell is reduced. Due to this, its rapid oxidation under the influence of atmospheric air is excluded.
- Milk with cocoa or other filler improves the taste, it becomes more viscous. Reduces the likelihood of precipitation.
- Reconstituted fermented milk drinks, cream and milk do not have a watery taste. The natural taste becomes more intense.
Physical methods of the process and the main types of homogenizers
- Pushing through a narrow gap. Valve-type units are used, with high-pressure plunger pumps. Such devices in the dairy industry are the most common.
- mechanical mixing. Mixers with knives or spatula whisks are used, including high-speed mixers. The simplest example is a coffee grinder or a meat grinder with an electric drive. This also includes rotary pulsation apparatus (RPA). Although the effect on phase lumps is more complex in them, it is not limited to shock and abrasion loads.
- Impact of ultrasound. Ultrasonic installations operate here, which excite cavitation in a dispersed medium, due to which the phase is crushed.
Plunger homogenizer
Device
The homogenizer device is shown in fig. 1. The plunger cylinder 1 is connected to the inlet pipe through the suction valve 3, and to the high-pressure chamber through the delivery valve 4. From the chamber there is a channel to the homogenizing head 5, which has a seat 6, a valve 7, a spring 8 and an adjusting screw 11. For pressure control, a pressure gauge 10 is connected to the chamber. The channel has a branch to the safety valve 9. The plunger is driven by the pump 2.
An enlarged view of the homogenizing head is shown in Fig.2. It has a calibrated hole (channel) 1 in the seat 5, a spring 2, a valve 4 with a stem 3 and an adjusting screw 6. The seat and valve are ground to each other. 
The valve has a flat, conical with a small angle or poppet shape of the working surface. In the first case, there may be riffles (grooves) on it. If they are, then the same ones are made on the saddle. This increases the degree of phase splitting.
There are models in which the valve and seat are located in bearings mounted in a fixed housing. In this case, they, under the pressure of the product jet, rotate in different directions.
Since the liquid passing at high speed acts on the valve and seat strongly enough, they wear out quickly. Therefore, these elements are made of especially strong steels. In addition, their shape is symmetrical. With noticeable wear, it is enough to turn them over to the other side, thereby doubling the service life.
The pump is not necessarily used plunger, you can choose screw or rotary. The main thing is that it creates high pressure. Since the plunger mechanism does not provide a uniform supply, several of them are placed in homogenizers, with the start of cycles spaced apart in time. The most popular are three-plunger units. In them, on the shaft, the knees are rotated 120 degrees so that the cylinders work alternately. In this case, the feed unevenness coefficient, that is, the ratio of its maximum value to the average, is 1.047.
An indicator close to one means that the flow through the homogenizing head can be considered stable with a small error. Thus, during the homogenization process, the valve is always in the weighted (open) position. Between it and the seat there is a gap for the passage of fluid. Its size can also be taken constant, not taking into account a slight deviation from the average level. In many modern devices, the flow from each plunger goes to “its own” head. After phase splitting, they are connected in the output manifold.
The pressure gauge is equipped with a throttling device. This reduces the oscillation of the instrument pointer.
Operating principle
The principle of operation of the homogenizer is as follows. When the plunger works for suction (in the figure it moves to the left), milk enters cylinder 1 through valve 3. Then the plunger works for pumping (moves to the right) and pushes the product into the chamber through valve 4. After that, the liquid enters through the channel from the chamber to the homogenizing head 5.
When the valve is in the non-operating position, spring 8 presses it firmly against the seat. Pressurized milk lifts the valve so that a small gap forms between it and the seat. Passing through it, the fat globules are crushed, the product is homogenized, and then goes into the outlet pipe.
The gap usually has a size of not more than 0.1 mm. Milk particles move in this zone at a speed of about 200 m/s (only 9 m/s in the injection chamber). The size of fat lumps decreases from 3.5-4.0 microns to 0.7-0.8 microns.
The pressure generated by the plunger pump is very high. Therefore, clogging of the channel in the seat can lead to the destruction of parts. To avoid breakage, a safety valve 9 is installed.
The unit is adjusted with screw 11. One of the main characteristics of homogenization is pressure. When the screw is tightened, the spring presses the valve harder against the seat. Because of this, the size of the gap decreases, as the hydraulic resistance increases. The setting of the device is carried out according to the readings of the manometer 10.
According to the instructions for the homogenizer, the temperature of milk should be in the range from 50 to 65 degrees C. If it is below this range, the process of settling fat lumps will accelerate. If higher, whey proteins will begin to precipitate.
Increasing the acidity of the product negatively affects the efficiency of the process, since in this case the stability of the proteins decreases. Agglomerates are formed, the crushing of fat lumps is difficult.
At the time of passage of the liquid through the valve gap, due to a sharp narrowing of the cross section of the channel, there is a throttling effect. The flow rate increases many times, and the pressure drops due to the fact that potential energy is converted into kinetic energy.
After the passage of milk through the head, some of the crushed particles again stick together into larger conglomerates. The efficiency of the process drops. To combat this phenomenon, two-stage homogenization is used. The device is shown in fig. 3. The fundamental difference from the single-stage one is the presence of two pairs of working bodies, the first stage 4 and the second - 12. Each has its own pressure spring with a control valve 6.
The second stage, auxiliary, further increases the degree of phase crushing. It is designed to create a controlled and constant back pressure in the head of the first stage, which is the main one. This optimizes the process conditions. And also for the destruction of relatively unstable formations. The pressure in it is set lower than in the first.
Single-stage homogenization is designed for products with low fat content or high viscosity. Two-stage - with a high content of fat or solids and low viscosity. And also in those cases when it is necessary to ensure the maximum possible phase splitting.
Separate technology
In the dairy industry, homogenization can be complete or separate. In the first case, all available raw materials are passed through the unit. In the second, it is first separated. The resulting cream 16-20% fat homogenized, and then mixed with skim milk. And sent to the next stage of processing. This method provides significant energy savings.
The mechanism of the phase dispersion process in a valve-type apparatus
According to N.V. Baranovsky, based on the study of hydraulic factors affecting the crushing of fat lumps during milk homogenization in a valve-type apparatus, the following process scheme was proposed (Fig. 4).
At the point of flow transition from the seat channel to the gap, between the seat and the valve, the flow cross-sectional area sharply decreases. So, according to one of the basic laws of hydraulics, the speed of its movement U also quickly increases. More specifically, U0 on the way is several meters per second. And U1 at the entrance to the slot is 2 orders of magnitude higher, several hundred m/s.
A fat drop does not move from the zone of low to the zone of high speeds at once "all at once". The front part of the ball first enters the stream moving in the cracks with great speed. Under the action of the fast-flowing liquid, it is pulled out (the back is still moving slowly) and torn off. The remaining lump continues to move slowly (of course, the concept of "slowly" in this case is relative, since the entire cycle of the drop passing through the slot takes 50 microseconds) to move towards the velocity interface, and the part, now turned out to be in front, is extended in the same way as the previous one, and also comes off. Thus, the entire fat drop is gradually torn into pieces, passing through the boundary section. This occurs at a sufficiently large difference between the velocities U0 and U1.
If this difference turns out to be less than a certain threshold, then, before the detachment of particles, an intermediate stage takes place - the drop is first stretched into a filament. If the difference is even smaller, then the fat lump will pass through the speed boundary without destruction. But the impact of a high flow rate will still lead it to an unstable state, due to the formation of internal deformations. Therefore, due to the forces of surface tension and mechanical shocks of the jets of flow, the ball will still fall apart into smaller fractions.
Oil homogenizer
To obtain a homogeneous consistency of butter or processed cheeses, a plasticizer homogenizer is used. During processing, the aqueous phase is dispersed and evenly distributed throughout the volume. As a result, the product is stored longer, its taste improves. In addition, the time spent on defrosting is reduced, and water loss is reduced during packaging.
The design of the device can be considered on the example of one of the most popular models M6-OGA (Fig. 5). It consists of a housing and a frame (Fig. 6), a receiving hopper, under which the feed screws are located, and a rotor with 12, 16 or 24 blades. An electric motor is used as a drive. The screw speed is controlled by a variator. The angular velocity of the rotor is constant.
The operation of the homogenizer is as follows. Butter is placed in large chunks in the hopper. The augers rotate in different directions, when viewed from above - one towards the other. With their help, the oil is forced through the rotor, after which, through a rectangular nozzle, it goes into the receiving hopper (not shown in the figure). So that the oil does not stick to the working bodies, they are lubricated with a hot solution. Rotary-pulsating apparatus 
Recently, rotary-pulsation apparatuses (RPA) are increasingly used for milk processing. Such a homogenizer is similar in design and principle of operation to a centrifugal pump. The main difference is in the working bodies.
The RPA is organized as follows. An electric motor serves as a drive. Rotor in the form of a perforated cylinder is rigidly fixed on its elongated shaft. From the end of the cylinder, from the side of the cover, there can be an impeller. Perforation is not required. Inside the cover there is a similar cylinder, fixed, it plays the role of a stator.
Milk is fed through the axial nozzle on the lid and enters the impeller. This part produces primary phase crushing and accelerates the working mixture. The latter then passes through the perforation of the movable cylinder, is again partially dispersed under the action of shear and abrasive loads, and ends up in a homogenizing cavity between the rotor and the stator. 
Here, in addition to shock, other forces act on fat globules.
In a turbulent flow moving at a high speed (this is exactly what is observed in the RPA working zone), microvortex flows arise. If a small spherical whirlpool collides with a drop of fat, it destroys it. There is also a hydroacoustic effect. Intense cavitation, which leads to the collapse of air bubbles, generates shock waves against which the lumps of the phase cannot resist either.
The maximum impact of the apparatus on the particles is achieved at the moment when resonant vibrations occur between the rotor and the stator. To ensure this effect, it is necessary to calculate the diameter of the movable cylinder, the speed of its rotation, as well as the gap between it and the stator.
After the working area, the milk passes through the holes in the stator and, already homogenized, is discharged through a tangential outlet, usually directed upwards, so that it is easier to connect pipelines for reloading the hopper in the recirculation system. 
To increase the degree of crushing, the apparatus may have several pairs of "rotor-stator". After installing the cover, they are arranged alternately. There are models in which, instead of an impeller, a perforated disk is placed. RPA homogenizers can also be submersible. Optionally, the unit is equipped with the following devices:
- Dry run protection.
- Explosion-proof motor.
- Casing with heating/cooling jacket.
- The regulator of smooth change of frequency of rotation of the motor.
- Loading device (screw feeder), for viscous, poorly soluble, inhomogeneous emulsions and suspensions or free-flowing components.
- Unloading unit, for draining into a third-party container when working according to the circulation scheme.
- Mechanical bellows shaft seal made of silicon carbide ceramics - increases the service life of the unit, even when working with aggressive liquids or those containing abrasive inclusions.
RPA are single- or three-phase. All parts that come into contact with food are made of stainless steel AISI 304, AISI 316 or their domestic counterparts. Since the dispersed liquid leaves the apparatus under pressure, the RPA homogenizer simultaneously works as a centrifugal pump.
Ultrasonic homogenizers
Device (using BANDELIN as an example). The ultrasonic homogenizer consists of (from top to bottom in Fig. 15) an RF generator, an ultrasonic transducer, “horns” and probes (waveguides). The RF generator is connected to a household network with a current frequency of 50 or 60 Hz. It amplifies this parameter up to 20 kHz. The ultrasonic transducer, equipped with an oscillating circuit with a measuring piezoelectric element, transforms the current energy generated by the generator into oscillations of ultrasonic waves of the same frequency. The generated amplitude remains constant. Ultrasonic - increases due to the use of "horns" of a special shape. Probes are inserted into them, transmitting vibrations to a vessel with liquid. Depending on the volume of the working medium, they can be flat, in the form of cones or "micro", with a diameter of 2 to 25 mm. 
Domestic industry also produces ultrasonic homogenizers. Of the latest models, the development of 2015 I100-6 / 840 can be noted (Fig. 16). The device has digital control, pulse mode, amplitude control and a set of probes.
Operating principle. When ultrasonic waves pass through a liquid, they alternately, 20,000 times per second, create high and low pressure in it. The latter is practically equal to the internal vapor pressure of the liquid, as a result of which bubbles filled with vapor appear in it, the liquid boils. When the voids collapse, a pressure drop occurs, fast-flowing turbulent microflows are formed that destroy fat droplets.
Some experts believe that, under ultrasonic exposure, lumps disperse not from cavitation, but due to the fact that the wave, passing through the fat drop at different points, causes accelerations of different magnitude and direction. As a result, multidirectional forces arise, trying to break the ball.
Homogenization is an important step in the processing of milk and other products. With its help, the structure improves and the shelf life increases, and the taste becomes more saturated.
This method of mechanical processing of milk and liquid dairy products serves to increase the dispersion of the fat phase in them, which makes it possible to exclude the settling of fat during storage of milk, the development of oxidative processes, destabilization and churning during intensive mixing and transportation.
Homogenization of raw materials contributes to:
in the production of pasteurized milk and cream - the acquisition of uniformity (taste, color, fat content);
sterilized milk and cream - increasing storage stability;
fermented milk products (sour cream, kefir, yogurt, etc.) - increasing the strength and improving the consistency of protein clots and eliminating the formation of a fatty plug on the surface of the product;
condensed milk canned food - preventing the release of the fatty phase during long-term storage;
whole milk powder - reducing the amount of free milk fat, not protected by protein shells, which leads to its rapid oxidation under the action of atmospheric oxygen;
reconstituted milk, cream and fermented milk drinks - to create a full taste of the product and prevent the appearance of a watery aftertaste;
milk with fillers (cocoa etc.) - improving taste, increasing viscosity and reducing the likelihood of sediment formation.
Dispersion of fat globules, i.e., reduction in their size and uniform distribution in milk, is achieved by exposing milk to a significant external force (pressure, ultrasound, high-frequency electrical processing, etc.) in special machines - homogenizers.
The most widespread in the dairy industry is the homogenization of milk by forcing it through the annular valve slot of the homogenizing head of the machine. Fat globules, passing through this gap, are dispersed. The required pressure is generated by a pump. In the production of whole milk, the size of fat globules decreases from 3-4 microns to 0.7-0.8 microns.
The main unit of modern valve-type homogenizers is the homogenizing head. It can be one or two stages. The second stage usually operates at a lower pressure than the first.
The use of one- or two-stage homogenization depends on the type of dairy products being produced.
Two-stage homogenization with a large pressure drop on both stages is used in the production of high-fat dairy products (cream, ice cream mixtures, etc.).
It allows you to disperse (break) the resulting accumulations of fat globules. For the production of other types of dairy products, including drinking milk, one-stage homogenization can be used.
Heat treatment of milk
Heat treatment is one of the main and necessary technological operations of milk processing, carried out for the purpose of disinfection. The efficiency of heat treatment is related to the heat resistance of milk, which is determined by its protein, salt composition and acidity, which, in turn, depend on the season, lactation period, physical condition and breed of animals, feeding regimes and diet, etc.
During heat treatment, milk and dairy products undergo complex changes in biochemical and physico-chemical properties, as well as modifications of the components of milk. The purpose of heat treatment is diverse, namely: reducing the total number of microorganisms and destroying pathogenic forms, inactivating (destroying) milk enzymes to increase stability during long-term storage, providing specific taste, smell, color and texture, creating favorable temperature conditions for fermentation, evaporation, storage, as well as mechanical processing processes, etc.
Heat treatment of milk is a combination of temperature exposure modes (heating or cooling) and exposure time at this temperature. Moreover, the duration of exposure at a given temperature should be such that the desired effect is obtained. In the dairy industry, heat treatment is carried out at temperatures up to 100 and over 100 °C.
When heated to 100 °C, only vegetative forms die in milk, and at temperatures above 100 °C, vegetative and spore forms die. The main processes of heat treatment of milk, causing the suppression of the vital activity of microorganisms, are pasteurization and sterilization. Hot water and saturated water steam are used as a heat carrier for pasteurization, and saturated water steam is used for sterilization.
In addition, during heat treatment, milk is subjected to cooling, heating (heating), thermal vacuum treatment.
The mode of heat treatment of milk for the production of each type of product is determined by the technological instruction. In this case, the milk is heated to the pasteurization temperature, and then kept and quickly cooled to the required temperature. The combination of heating and cooling operations is dictated by technological and sanitary requirements, as well as the possibility of using the heat of a hot product.
To do this, the hot product is sent to a special section of the apparatus (plate or tubular) for preheating the cold product entering the pasteurization. This operation is called heat regeneration, and the apparatus or parts thereof are called regenerators or regeneration sections. The use of this operation allows you to get some savings in thermal energy spent on pasteurization.
The efficiency of the regenerator is characterized by the regeneration coefficient. It represents the ratio of the amount of heat returned by the regenerator to the amount of heat required to heat the product from the initial to the final temperature, i.e., at which the product begins to reverse movement through the regenerator.
Cooling and heating
Dairy raw materials are cooled at the enterprises in order to preserve its quality and limit the growth of the number of microorganisms before processing. In table. 4.1 shows data showing the growth of the number of microorganisms in milk depending on the temperature of cooling and the duration of storage.
Storage of milk at temperatures above 4.5 °C leads to an increase in the number of microorganisms. In practice, milk for short-term storage is cooled to 6--8 °C. For long-term storage (10-14 hours), milk is pasteurized and then cooled. In order to increase the shelf life of dairy products, they are cooled during the manufacturing process.
Heating (heating) does not play the main role, but most often performs an auxiliary (preparatory) function in the process of milk processing. Heating of milk is used before separation, homogenization, as well as in the production of various dairy products. During separation, heating milk reduces its viscosity properties, which has a positive effect on the separation of fat globules from the milk plasma and the formation of cream.
