MARKET MILK

Module 6. Common dairy operations

Lesson 24
HOMOGENIZATION - TYPES AND OPERATION OF HOMOGENIZERS

24.1 Types of Homogenizer

24.1.1 High pressure homogenizer

This type of homogenizer consists of single acting triplex pump with each cylinder having suction and discharges valves. The discharge valve of each pump empties into a common discharge pipe, in which a special valve is placed. The pressure ranges between 35 – 350 bar (500-5000 psi) depending on the type of construction of the valve.

24.1.2 Low pressure-rotary type homogenizers

Usual operating pressures are below 35 bar (500 psi). The construction is so designed that milk is subjected to grinding and shearing action.

24.1.3 Sonic vibrators

The milk is subjected to high frequency vibration in a device called sonic vibrator or oscillator. The machine consists of a flat disc actuated by an electric magnet located over an anvil containing a hole, through which milk enters. The milk passes through the space between the disk and an anvil, and the vibrating action of the disk against the film of milk hammers the fluid at high frequency.

24.2 Details of Homogenization

Homogenizers can be single-stage, double-stage or even multi-stage type. Single-stage homogenizers are equipped with only one homogenizing valve. A homogenizing valve usually consists of a valve, valve seat and an impact ring. These are held within a valve body. On the other hand, double-stage homogenizers are equipped with an additional homogenizing valve (Fig. 24.1).

Commercial milk homogenizers are high pressure reciprocating pumps, each having a sanitary head upon which is mounted a homogenizing valve or valves. Homogenizers have 3, 5 or 7 pistons driven by an eccentric shaft through connecting rods and cross heads. The pump head contains a section manifold with passages connecting to the individual cylinders. Each cylinder has suction and a discharge valves, either poppet or ball type. The valves are spring loaded in some models.

Fig. 24.1 Diagram of two-stage homogenizer valve

The homogenizing valve accomplishes its intended function by restructuring the product flow area which results in the pressure commonly known as "the homogenizing pressure." The degree of homogenizing effect can be controlled by regulating this pressure, or corrected by the restriction to flow. A pressure gauge is used to indicate the pressure. Generally, double-stage homogenizers are used and pressure of 140 bar (2000 psi) and 35 bar (500 psi) in the first and second stages respectively are maintained.

24.3 Factors Affecting Homogenization

24.3.1 Homogenization pressure

In a single-stage homogenizer, usually 140 – 175 bar (2000-2500 psi) pressure is sufficient for milk having up to 6.0% fat. Higher pressure may increase the tendency of the milk to curdle when cooked, due to the increased destabilizing effect on milk proteins. For milk on milk products with > 6 % milk fat, two-stage homogenization is needed to prevent fat clustering pressure of 140 bar (2000 psi) and 35 bar (500 psi) in the first and second stage respectively are applied in two-stage homogenization.

Table 24.1 Effect of homogenization pressure

24.4 Stage of Homogenization

For milk with more than 6% fat, two stage homogenization is better. If the broken up fat globules have a tendency to agglomerate after the first homogenizing stage (150-200 bar), they can be re-dispersed employing 20-40 bar in the second stage.

24.4.1 Temperature of homogenization

The enzyme lipase should be inactivated prior to homogenization by pre-heating milk to a temperature of 60°C or above. At this temperature fat is already in molten condition. In routine practice, the milk is heated to 65-70°C (149-158°F) for homogenization.

24.4.2 Fat content

Homogenization becomes less effective with increasing fat content. When the fat content is high, raising the temperature improves homogenization efficiency. When the fat content is high, the newly created total fat globule surface is so large that the material (plasma protein) becomes insufficient to form new membranes on fat.

24.5 Efficiency of Homogenization

24.5.1 Degree of homogenization

The degree of homogenization is defined as the ratio of the volume of fat with fat globules diameter of < 0.7 µ to the total fat content of milk or cream. The value of 0.7 µ was based on the fact that fat globules with smaller diameter do not form clumps because they break up again due to Brownian movements.

24.5.2 Farrall index

It is a widely accepted microscopic method for determining the homogenization efficiency. This index may be defined as the number of fat globules having < 2 µ in diameter. The efficiency of homogenization is apparently based on the number of fat globules larger than 2 µ (if any), as measured under specified conditions.

24.6 Operation of the Homogenizer

24.6.1 Before starting the homogenizer, the following points should be checked

• Water is turned on to lubricate and cool the pistons.

• Pressure controls are checked to see that they are in idling position.

• Check availability of the product to the machine.

• Check during starting of the machine whether the oil pressure records > 1 kg/cm2; otherwise the starter will trip.

24.6.2 Starting of homogenizer machine

• The motor is started.

• The homogenizer is run on water for about 5 min., then it is stopped and the water drained off by slackening the inlet union, which is tightened subsequently.

• The machine is checked for any leaks.

• The homogenizer is provided with milk supply by adjusting the 3-¬way valve accordingly.

• As soon as the machine starts pumping at full capacity, the pressure adjusting handle of the second stage valve is adjusted to the desired pressure, followed by adjusting the first stage pressure. This can be observed in the single pressure gauge provided.

• The product discharge from the machine is diverted back till the desired homogenizing pressure is obtained.

• When normal operation is attained, the bypass valve is turned to direct the product flow into the processing system.

Note: In some homogenizers, there might be provision to release the air from the machine.

24.6.3 Closing down the homogenizing operation

• At the end of the run, the product flow is diverted.

• As soon as the product to be homogenized is emptying out, the water is filled into the hopper for flushing.

• The first stage valve pressure and subsequently the second stage valve pressure are released in that sequence.

• The homogenizer can now be cleaned by switching over to the cleaning sequence.

• After the cleaning is over, the homogenizer is stopped.

24.6.4 Precautionary measures

• The homogenizer should never be run dry. Adequate feed must be maintained at all times to prevent starving of the machine.

• Before the homogenizer is put to use again, the strainer from the inlet chamber should be removed, cleaned and refitted.

• The homogenizer should always be started with both homogenizing valve handles in released position.

• The pressure should be built up gradually.

• While the homogenizer is running, the 3-way valve on the product delivery line should never be closed; otherwise it may result in severe damage to the machine.

24.7 Maintenance of Homogenizers

• The oil level should be inspected daily for maintaining desired level.

• Periodic check-up of the lubricating system must be done. Change the lubricating oil before 500 operating hours or 6 months or sooner, when it becomes emulsified.

• Inspect oil seals for preventing any chance of mixing of water with oil.

• Leakage can be prevented by tightening the nuts or couplings, inserting or replacing gaskets or replacing the pistons.

• The homogenizer gauges should be checked periodically for accuracy against a standard gauge.

• The homogenizer valves must be regularly inspected and if required, they should be lapped with a fine abrasive for perfect sealing.

• While dismantling the valves, care must be taken to avoid scratches. They must be kept on a rubber mat.