The most widely used valve homogenizers, the main components of which are a high-pressure pump and a homogenizing head.
On fig. a two-stage homogenizing head is shown, consisting of a body 3 and a valve device, the main parts of which are the valve seat 1 and valve 2. The valve is connected to the stem, on the protrusion of which the spring 6 presses. with a spring, a rod 7 and a glass 8 forms a pressure device 4.
Rice. Two-stage homogenizing head:
I - the first stage; II - second stage
The liquid injected by the pump under the valve disc presses on the disc and moves the valve away from the seat, overcoming the resistance of the spring. In the gap formed between the valve and the seat with a height of 0.05 to 2.5 mm, the liquid passes at high speed and is thus homogenized. At the next step, the process is repeated.
According to the type of homogenizing head, homogenizers can be divided into one-, two- and multi-stage. In practice, only one- and two-stage ones are used, since multi-stage ones do not justify themselves, since they lead to cumbersome design, inconvenience in operation and a slight improvement in the homogenization effect compared to two-stage ones.
The main performance indicators of homogenizers are the universal operating and cavitation characteristics. The universal characteristic of a homogenizer is the relationship between its performance, power consumption and efficiency. It gives an idea of the level of perfection of the design of the homogenizer and its technical condition.
The removal of the cavitation characteristic requires the installation of a pressure and vacuum gauge on the suction side of the homogenizer. The beginning of cavitation is determined by the beginning of the supply decrease by more than 2%.
The cavitation curve shows the features of the work of the homogenizer on its suction side and allows you to decide on the improvement of working conditions in a particular case.
Homogenizer A1-OGM(Fig.), designed to obtain a finely divided homogeneous product, consists of an electric motor 1, a frame 2, a crank mechanism 3 with lubrication systems 7 and cooling, a plunger block 4 with a homogenizing 6 and a manometric 5 heads and a safety valve.
Rice. Homogenizer A1-OGM
The principle of operation of the homogenizer is to force the product through a narrow gap between the seat and valve of the homogenizing head. The pressure of the product before the valve is 20...25 MPa, after the valve it is close to atmospheric pressure. With such a sharp pressure drop, along with a significant increase in speed, the product is crushed.
The homogenizer is a three-plunger pump. Each of the three plungers, making a reciprocating motion, sucks liquid from the intake channel closed by the suction valve, and pumps it through the discharge valve into the homogenizing head at a pressure of 20...25 MPa.
The homogenizing head is the most important and specific part of the homogenizer. It is a steel body containing a cylindrical centered valve. Under the pressure of the liquid, the valve rises, forming an annular gap through which the liquid passes at high speed and then is discharged through the fitting from the homogenizer.
A plate is hinged inside the frame, the position of which is regulated by screws. An electric motor 1 is installed on the plate, which drives the crank mechanism 3 through a V-belt drive. In the case 2, which is a tank with an inclined bottom, there is a crank mechanism 3, a cooling system and an oil strainer. The cooling system is designed to supply cold water to the plungers. It includes a coil laid on the bottom of the housing 2, a perforated tube above the plungers and branch pipes for water inlet and outlet. The lubrication system is used to supply oil to the crankshaft journals to reduce friction.
The technical characteristics of the homogenizer A1 -OGM are given in table.
Homogenizer K5-OGA-U(fig.) is designed for crushing and uniform distribution of fat globules in milk and liquid dairy products, as well as in ice cream mixtures.
Rice. Homogenizer K5-OGA-U
It is a five-plunger high-pressure pump with a homogenizing head. It consists of a frame 1 with a drive, a crank mechanism 5 with lubrication and cooling systems, a plunger block 14 with a homogenizing 13 and a gauge 12 heads and a safety valve. Inside the plunger block 14 there is a plunger 15 connected to the slider 11. The drive of the homogenizer is carried out from the electric motor 17 through the driving 20 and driven 21 pulleys and V-belt transmission. Inside the frame 1, a plate 18 is hinged, the position of which is regulated by screws 2. The frame is mounted on six supports 19 that vary in height.
The crank mechanism 5 consists of a cast iron housing, a crankshaft 7 mounted on two roller bearings, connecting rods 8 with covers 6 and liners 9, sliders 11 pivotally connected to the connecting rods 8 by means of pins 10, cups and seals. The inner cavity of the body of the crank mechanism is an oil bath. An oil level indicator 4 and a drain plug 3 are mounted in the rear wall of the housing. A crank mechanism 5, a cooling system, an oil strainer and an oil pump 22 are located in the housing, which is a tank with an inclined bottom.
The homogenizer has a forced lubrication system for the most loaded rubbing pairs, which is used in combination with oil splashing inside the housing. The oil is cooled with tap water by means of a coil 16 of the cooling device laid on the bottom of the housing, and the plungers are cooled with tap water that enters them through the holes in the pipe. A flow switch is installed in the cooling system, designed to control the flow of water.
By adjusting the pressure of the spring on the valve, the optimal homogenization mode for various products is achieved.
The technical characteristics of the K5-OGA-10 homogenizer are given in Table.
Table. Technical characteristics of homogenizers
| Indicator | |||
| Productivity, l/h | |||
| Working pressure, MPa | |||
| The temperature of the product entering the | |||
| homogenization, °С |
|||
| Electric motor: | |||
| power, kWt | |||
| speed, min" | |||
| Frequency of rotation of a cranked shaft, min | |||
| Number of plungers | |||
| Plunger stroke, mm | |||
| Number of homogenization stages | |||
| Overall dimensions, mm | |||
| Weight, kg |
Homogenizer A1-OG2-C(fig.) is intended for mechanical processing of viscous dairy products such as creamy, processed and plastic cheeses to impart uniformity to the product in order to improve its quality.
Rice. Homogenizer A1-OG2-C
The homogenizer is a horizontally located three-plunger high-pressure pump with a homogenizing device 8.
The pump is driven by an electric motor 4 using a V-belt drive, driven 15 and driving 16 pulleys. The homogenizer consists of the following main units: a crank mechanism 1, a drive, a plunger block 9, a homogenizing device 8, a safety valve 7, a hopper, a casing, a frame 13.
The crank mechanism 1 includes a cast iron housing, a crankshaft 14 mounted on two roller bearings, connecting rods 12 with covers 2 and liners, sliders 10 pivotally connected to the connecting rods 12 by fingers 11, cups and a seal. The inner cavity of the body of the crank mechanism is an oil bath.
An oil level indicator and a drain plug are installed in the rear wall of the housing. Lubrication of rubbing parts is carried out by spraying oil. The body of the crank mechanism is closed with a cover, which has a neck with a filter mesh for filling oil. The drive of the homogenizer is carried out from the electric motor 4, which is installed on the rocking under-engine plate 3, mounted on the body of the crank mechanism 1. The tension of the V-belts is provided by tension screws 5.
The crank mechanism is attached with pins to the frame 13, which is a welded structure lined with sheet steel. The frame has a removable cover 17, designed to protect rotating and moving mechanisms. A terminal box 18 is installed at the bottom of the frame 13.
The bed is mounted on four height-adjustable supports 19. A plunger block 9 is attached to the body of the crank mechanism using two pins, which is designed to suck the product from the hopper and pump it under high pressure into the homogenizing device 8. The plunger block 9 consists of a block, plungers 6, hollow cylindrical cups with holes in the walls. There are no suction valves and seals; the product is sucked into the working chambers of the plunger block directly from the hopper through hollow cylindrical cups.
The sealing of the plungers, taking into account the low fluidity of the molten cheese mass, is achieved by precise manufacturing with small tolerances of the mating surfaces of the plungers and the holes of the glasses.
A homogenizing device is attached to the plunger block using pins, designed to homogenize the product by passing it at high speed under high pressure through the gap between the valve and the seat.
The homogenizing device 8 consists of a body, gaskets, delivery valves, valve seats, springs, a homogenizing valve with a seat, a cup, and a handle.
To control the homogenization pressure, a manometer is used, which is attached to the end face of the body of the homogenizing device. On top of the homogenizing device is a safety valve 7, designed to limit the increase in pressure above the set value. It consists of a cup, flange, valve, valve seat, spring, pressure screw and cap. The safety valve is adjusted to the working pressure of homogenization by means of a screw.
The product to be homogenized is fed into the homogenizer hopper, which is a welded stainless steel tank.
When the plungers reciprocate in the working cavity of the plunger block, a vacuum is created and the product from the hopper is sucked into the working cavity, and then the plungers push the product into the homogenizing device, where it, under a pressure of 20 MPa, passes at high speed through the annular gap formed between the ground surfaces of the homogenizing valve and its seat. In this case, the product becomes more homogeneous. From the homogenizing device through the nozzle it is sent through the pipeline for further processing. An ammeter is installed on the homogenizer, with the help of which the pressure gauge readings are controlled.
The technical characteristics of the A1-OG2-C homogenizer are given in Table.
Homogenization is the crushing (dispersion) of fat globules by exposing milk or cream to significant external forces. During processing, the size of the fat globules and the speed of ascent are reduced. There is a redistribution of the shell substance of the fat globule, the fat emulsion is stabilized, and the homogenized milk is not settled.
Valve-type homogenizers are used to process milk and cream in order to prevent their separation during storage.
Homogenizers-plasticizers of the rotary type are used to change the consistency of dairy products such as processed cheeses and butter. In the butter processed with their help, the aqueous phase is dispersed, as a result of which the product is better stored.
The principle of operation of valve-type homogenizers, which are most widely used, is as follows. In the cylinder of the homogenizer, milk is subjected to mechanical action at a pressure of 15...20 MPa. When the valve is lifted, slightly opening a narrow gap, the milk comes out of the cylinder. This is possible when the operating pressure in the cylinder is reached. When passing through a narrow circular gap between the seat and the valve, the speed of milk increases from zero to a value exceeding 100 m/s. The pressure in the flow drops sharply, and a drop of fat that has fallen into such a flow is drawn out, and then, as a result of the action of surface tension forces, it is crushed into small droplets-particles.
During the operation of the homogenizer at the outlet of the valve gap, adhesion of fragmented particles and the formation of "clusters" are often observed, which reduce the efficiency of homogenization. To avoid this, two-stage homogenization is used. At the first stage, a pressure equal to 75% of the working one is created, at the second stage, the working pressure is set.
Rice. 2.22. Homogenizing head
For homogenization, the temperature of raw milk should be 60...65 °C. Lower temperatures increase fat settling, while higher temperatures can precipitate whey proteins.
A homogenizer with a two-stage homogenizing head consists of a frame, a body, a plunger block, a homogenizing head, a drive and a crank mechanism.
Fig.2.23. Homogenizer A1-OGM-5:
1 - electric motor; 2 - bed with a drive; 3 - crank mechanism with lubrication and cooling systems; 4 - plunger block with homogenizing and manometric heads and safety valve; 5 - manometric head; 6- homogenizing head; 7- V-belt transmission
In the case when during homogenization it is necessary to exclude the access of microorganisms to the processed product, special aseptic homogenizing heads are used. In such heads, the space limited by two sealing elements is supplied with hot steam at a pressure of 30...60 kPa. This high temperature zone acts as a barrier to prevent bacteria from entering the homogenizer barrel.
Homogenizers-plasticizers differ from valve-type homogenizers in terms of the principle of operation and device. The working body in them is the rotor, which can have a different number of blades - 12, 16 or 24.
The homogenizer-plasticizer consists of a frame, a body with screws, a receiving hopper and a drive.
Fig.2.24. Homogenizer Homogenizing Tool Kit:
1-fixed ring; 2- movable ring; 3 - paddle wheel; 4- bunker; 5 - movable knife; 6- body; 7- fixed knife; 8- unloading rotor; 9- homogenizer shaft
The drive allows you to adjust the frequency of rotation of the feed screws (with the help of a variator) within 0.2 ... 0.387 s 1 . The speed of the rotor with the blades is not adjustable and is 11.86 s".
The principle of operation of the machine is as follows. Butter is fed into the bunker, from where, with the help of two screws rotating in opposite directions, it is forced through the rotor and from the nozzle with the diaphragm goes into the bunker of the filling machine. To prevent sticking of oil, the working parts of the homogenizer are lubricated with a special hot solution before starting work. The performance of the homogenizer depends on the frequency of rotation of the feed screws and is 0.76 ... 1.52 m 3 / h. The drive power of the machine is 18.3 kW.
Homogenizer YAZ-OGZ is designed for processing melted cheese mass in the production of processed cheese and consists of the following parts: base, body, set of homogenizing tools, hopper, unloader and drive.
The base serves to attach the components of the homogenizer to it. The housing contains working units and sealing devices.
The homogenizing tool for feeding, grinding and mixing the molten cheese mass is made in the form of movable and fixed knives separated by spacer rings, as well as a loading impeller and an unloading rotor. Movable knives have special grooves made at a certain angle to the end surface, which facilitates the movement of the crushed product to the unloading device. The shaft of the homogenizing tool rotates with a frequency of 49s 1 .
The bunker for receiving and accumulating cheese mass has a heat-insulating jacket.
The unloading device in the form of two pipes connected to each other by means of a crane serves to discharge the homogenized mass into the batcher of the filling machine.
The drive consists of an 11 kW motor designed to transmit rotation from the shaft to the moving part of the homogenizing tool.
Processing of the product on the homogenizer YAZ-OGZ is carried out as follows. The melted cheese mass is periodically or continuously fed into the homogenizer hopper. Under the action of the vacuum created by the loading impeller, the product enters the homogenizing tool, in which, passing sequentially through the movable and fixed knives, it is homogenized and fed to the unloading device.
The use of a homogenizer makes it possible to abandon the technological operation of filtering the cheese mass in order to remove its unmelted particles.
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, by means of lip seals, enter the working chambers of the plunger block and, 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 K5 - OG2A - 1.25 homogenizer makes it possible 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 an opening 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 suction channel of the block. 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 an increase in the homogenization pressure 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-agglomeration 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.
Homogenization has become a standard manufacturing process widely practiced as a means of keeping the fat emulsion from separating due to gravity. Gaulin, who developed this process in 1899, gave it the following definition in French: "Fixer la composition des liquides".
First, homogenization leads to the splitting of fat globules into much smaller ones (see Fig. 1). As a result, creaming is reduced and the tendency of the balls to stick together or form large agglomerates can also be reduced. Basically, homogenized milk is produced mechanically. It is driven at high speed through a narrow channel.
The destruction of fat globules is achieved by a combination of factors such as turbulence and cavitation. As a result, the diameter of the balls decreases to 1 micron, and this is accompanied by a four to six-fold increase in the area of the intermediate surface between fat and plasma. As a result of the redistribution of the shell substance, which completely covered the fat globules before their destruction, the newly formed globules have insufficiently strong and thick shells. These membranes also include adsorbed milk plasma proteins.
Fox, along with his colleagues, investigated the fat-protein complex obtained by homogenizing milk. He proved that casein is the protein component of the complex and that it is possibly associated with the fat fraction through polar attractive forces. He also found that casein micelles are activated as they pass through the valve of the homogenizer, predisposing them to interact with the fat phase.
Process Requirements
The physical condition and concentration of the fat fraction during homogenization affect the size of the fat globules. Homogenization of cold milk, in which fat is mainly present in a solidified state, is practically not feasible. Processing milk at a temperature of 30-35°C leads to incomplete dispersion of the fat fraction. Homogenization is truly effective when the entire fat phase is in a liquid state, and at concentrations that are normal for milk. Foods with a high fat content tend to form large fat globules, especially at low whey protein concentrations with a high fat content. Cream with a fat content of more than 12% cannot be successfully homogenized at standard elevated pressure, because due to the lack of membrane material (casein), fat globules stick together into clusters. For sufficiently effective homogenization, one gram of fat should have 0.2 grams of casein.
High pressure homogenization processes result in the formation of small fat globules. With an increase in the homogenization temperature, the dispersity of the fat phase increases - in proportion to the decrease in the viscosity of milk at elevated temperatures.
Usually homogenization is carried out at a temperature of 55 to 80°C, under a pressure of 10 to 25 MPa (100-250 bar), depending on the type of product being processed.
Flow Characteristics
When the flow passes through a narrow channel, its speed increases (see Fig. 2). The speed will increase until the static pressure decreases to a level at which the liquid boils. The maximum speed mainly depends on the inlet pressure. As the fluid leaves the slot, the velocity decreases and the pressure begins to rise. The boiling of the liquid stops and the vapor bubbles explode.
Homogenization theories
Over the years of application of the homogenization process, many theories have arisen that explain the mechanism of homogenization at high
pressure. Two theories explaining the dispersed oil-water system by analogy with milk, where the diameter of most droplets is less than 1 micron, have not become obsolete to date.
They provide an explanation of the influence of various parameters on the efficiency of homogenization.
The theory of the destruction of balls by turbulent whirlpools (“microvortices”) is based on the fact that a large number of turbulent microflows arise in a fluid moving at high speed.
If a turbulent microflow collides with a drop comparable to it, the latter is destroyed. This theory makes it possible to predict changes in the results of homogenization with changes in the applied pressure. This link has been found in many studies.
On the other hand, the theory of cavitation states that fat droplets are destroyed by shock waves that occur when steam bubbles explode. According to this theory, homogenization occurs when the liquid leaves the gap. Thus, the back pressure required for cavitation is of great importance in this case. This has been confirmed in practice. However, homogenization is possible without cavitation, but in this case it is less effective.
Fig.3 Destruction of fat globules at the first and second stages of homogenization.
1 After the first stage
2 After the second stage
One-stage and two-stage homogenization
Homogenizers can be equipped with one homogenizing head or two connected in series. Hence the name: single-stage homogenization and two-stage homogenization. Both systems are shown in figures 5 and 6. In single-stage homogenization, the entire pressure drop is used
in a single step. With two-stage homogenization, the total
the pressure is measured before the first stage P 1 and before the second stage P 2 .
For optimum homogenization efficiency, a two-stage variant is usually used. But the desired results can be obtained if the ratio P 2: P 1 is approximately 0.2. One-stage version is used for homogenization
- low fat products
- products requiring high viscosity (formation of certain agglomerates).
- in products requiring low viscosity
- to achieve maximum efficiency of homogenization (micronization).
Figure 3 shows the formation and destruction of accumulations of fat globules in the second stage of homogenization.
Influence of homogenization on the structure and properties of milk
The effect of homogenization has a positive effect on the physical structure
and properties of milk and is manifested in the following:
- Reducing the size of the fat globules, which prevents the cream from settling
- Whiter and appetizing color
- Increased resistance to fat oxidation
- Improved aroma and taste
- Increased safety of fermented milk products made from homogenized milk.
However, homogenization also has certain disadvantages. Among them:
- Impossibility of separation of homogenized milk
- Slightly increased sensitivity to light, both from sunlight and from fluorescent lamps, can lead to a so-called sunny taste.
- Reduced heat resistance - especially pronounced when testing the first stage of homogenization, homogenization of skim milk and in other cases that contribute to the formation of accumulations of fat globules
- The unsuitability of milk for the production of semi-hard and hard cheeses, since the clot will not separate whey well.
Homogenizer
High pressure homogenizers are usually required to ensure maximum homogenization efficiency.
The product enters the pumping unit, where it is pressurized by a piston pump. The level of pressure generated depends on the back pressure, determined by the distance between the piston and the seat in the homogenizing head. The pressure P 1 always means the pressure of homogenization. P 2 is the back pressure of the first stage of homogenization or the pressure at the inlet to the second stage.
Fig.4 The homogenizer is a large high pressure pump with a back pressure device.
1 Main drive motor
2 V-belt drive
3 Pressure gauge
4 Crank mechanism
5 piston
6 Piston seal
7 Cast stainless steel pump block
8 Valves
9 Homogenizing head
10 Hydraulic system
Fig.5 Single-stage homogenization. Schematic of the homogenizing head:
1 valve
2 Impact ring
3 Saddle
4 Hydraulic drive
High pressure pump
The piston pump is driven by a powerful electric motor (pos. 1 in Fig. 4) through the crankshaft and connecting rods - this transmission converts the rotation of the engine into reciprocating motion of the pump pistons.
The pistons (pos. 5) move in the high pressure cylinder block.
They are made from high strength material. The pistons are equipped with double seals. Water is supplied to the space between the seals to cool the pistons. Hot condensate can also be supplied there to prevent re-contamination of the product with microorganisms during the operation of the homogenizer. It is also possible to use hot condensate to maintain the conditions of aseptic production of the product during the operation of the homogenizer.
Homogenizing head
Figures 5 and 6 show the homogenizing head and its hydraulic system. The piston pump raises the milk pressure from 300 kPa (3 bar) at the inlet to a homogenization pressure of 10-15 MPa (100-240 bar), depending on the type of product. The pressure at the inlet to the first stage before the mechanism (homogenization pressure) is automatically kept constant. The oil pressure on the hydraulic piston and the homogenizing pressure on the valve balance each other. The homogenizer is equipped with one common oil tank, regardless of whether it is a single-stage or two-stage version. However, in a two-stage homogenizer, there are two hydraulic systems, each with its own pump. The new homogenization pressure is set by changing the oil pressure. The homogenizing pressure is indicated on the high pressure gauge.
The homogenization process takes place in the first stage. The second mainly serves two purposes:
Creating a constant and controlled back pressure towards the first stage, thus ensuring optimal homogenization conditions
Destruction of sticky clusters of fat globules that form immediately after homogenization (see Fig. 3).
Note that the homogenization pressure is the pressure before the first stage, not the differential pressure.
The parts of the homogenizing head are machined on a precision grinding machine. The shock ring is seated in its place in such a way that its inner surface is perpendicular to the slot exit. The seat is beveled at a 5 degree angle to give the product a controlled acceleration, thus preventing the accelerated wear that would otherwise be inevitable.
Milk under high pressure penetrates between the seat and the valve. The gap width is approximately 0.1 mm, which is 100 times the diameter of the fatty pressure produced by the piston pump, converted into kinetic energy. Part of this energy after passing through the mechanism is again converted into pressure. The other part is released as heat; every 40 bar pressure drop after passing through the mechanism raises the temperature by 1°C. Homogenization consumes less than 1% of all this energy, yet high pressure homogenization is still the most efficient method available today.
Fig.6
two-stage homogenization.
1 First stage
2 Second stage
Homogenizing efficiency
The purpose of homogenization depends on how it is applied. Accordingly, the methods of evaluating effectiveness are also changing.
In accordance with the Stokes law, the growing speed of a particle is determined by the following formula, where: v is the speed
q is the acceleration due to gravity p is the size of the particle η hp is the density of the liquid η ip is the density of the particle t is the viscosity
Or v = constant x p 2
It follows from the formula that reducing the particle size is an effective way to reduce the increase in velocity. Therefore, a decrease in the particle size in milk leads to a slowdown in the rate of cream settling.
Analytical Methods
Analytical methods for determining the efficiency of homogenization can be
divided into two groups:
I. Determination of the cream settling rate
The oldest way to determine cream settling time is to take a sample, let it sit for a certain amount of time, and then analyze the fat content of the different layers. The USPH method is based on this principle. For example, a one liter sample is aged for 48 hours, after which the fat content in the top layer (100 ml) is determined, as well as in the rest of the milk. Homogenization is considered satisfactory if the mass fraction of fat in the lower layer is 0.9 times less than in the upper layer.
The NIZO method is based on the same principle. According to this method, a sample with a volume of, say, 25 ml is subjected to centrifugation for 30 minutes at a speed of 1000 rpm at a temperature of 40°C and a radius of 250 mm. The fat content of the 20 ml bottom layer is then divided by the fat content of the entire sample and the result is multiplied by 100. This ratio is called the NIZO value. For pasteurized milk, it is usually 50-80%.
II. Fractional Analysis
The size distribution of particles or droplets in a sample can be determined by a well-established method using a laser diffraction setup (see Fig. 7), which sends a laser beam into the sample in the cuvette. The degree of light scattering will depend on the size and number of particles contained in the milk under study.
The result is shown in the form of particle size distribution graphs. Fat percentage by mass is presented as a function of particle size (fat globule size). Figure 8 shows three typical fat globule size distribution graphs. Note that as the homogenization pressure increases, the graph shifts to the left.
Energy consumption and its effect on temperature
The electrical power input required for homogenization is expressed by the following formula:
Homogenizer in the production line
Usually the homogenizer is installed at the beginning of the line, i.e. before the final heating section in the heat exchanger. In most pasteurization plants for the production of drinking milk for the consumer market, the homogenizer is located after the first regenerative section.
In the production of sterilized milk, the homogenizer is usually placed at the beginning of the high-temperature treatment process in an indirectly heated system and always at the end of the process in a direct product heating system, i.e. in the aseptic part of the plant after the product sterilization section. In this case, an aseptic version of the homogenizer is used, equipped with special piston seals, gaskets, a sterile condenser and special aseptic dampers.
The aseptic homogenizer is installed after the sterilization section of plants with direct heating of the product in cases of production of dairy products with a fat mass fraction of more than 6-10% and/or with a high protein content. The fact is that at very high processing temperatures in milk with a high content of fat and / or proteins, accumulations of fat globules and casein micelles are formed. An aseptic homogenizer located after the sterilization section breaks down these agglomerated particles.
Complete homogenization
Full homogenization is the most common method for homogenizing drinking milk and milk intended for the production of fermented milk products. The fat content of milk, and sometimes the content
dry fat-free residue (in the production of yogurt, for example) are normalized before homogenization.
Separate homogenization
Separate homogenization means that the bulk of the skimmed milk is not homogenized. Cream and a small amount of skimmed milk are homogenized. This homogenization method is usually used for pasteurized drinking milk. The main advantage of separate homogenization is its relative economy. The total energy consumption is reduced by up to 65% due to less milk passing through the homogenizer.
Since the highest homogenization efficiency can be achieved if the milk contains at least 0.2 g of casein per 1 g of fat, the recommended maximum fat content is 12%. The hourly output of a plant in which separate homogenization is carried out can be determined by the following formula.
The production of pasteurized normalized milk (Q sm) per hour will be approximately 9690 liters. If we substitute this figure into formula 2, we get,
that the hourly output of the homogenizer is approximately 2900 liters,
that is, about a third of its total performance.
The scheme of flows in the installation for partially homogenized milk is shown in Fig.10.
The effect of homogenized dairy products on the human body
In the early 1970s, the American scientist K. Oster (K. Oster) came up with a hypothesis that the homogenization of milk allows the enzyme xanthine oxidase to penetrate through the intestines into the circulatory system. (Oxidase is an enzyme that catalyzes the addition of oxygen to or the removal of hydrogen from a substrate.) According to Oster, xanthine oxidase contributes to the process of damage to blood vessels and leads to atherosclerosis.
This hypothesis was rejected by scientists on the grounds that the human body itself produces thousands of times more of this enzyme than homogenized milk could theoretically bring into it.
So, there can be no harm from homogenizing milk. From a nutritional point of view, homogenization does not bring any special changes, except, perhaps, that fat and protein are broken down faster and easier in homogenized products.
However, Oster is right that oxidation processes can be harmful to the human body and that diet is important for health.