Pasteurization of milk

PASTEURIZATION

Heat treatment of milk

The term pasteurization has been coined after the name of Louis Pasteur of France, who in 1860-64 demonstrated that heating wine at a temperature between 122 to 140 ° F killed the spoilage organisms and helped in its preservation. The application of this process resulted in coining a new term ‘pasteurization’, which soon became current in technical language. Although Louis Pasteur pioneered studies on heat treatment for preservation of the wine, pasteurization of milk first was attributed to Dr Soxhlet of Germany in1886.

The term pasteurization as applied to market milk today, refers to the process of heating every particle of milk to at least 63 ° C for 30 minutes, or 72 ° C for 15 seconds or to any temperature time, which is equally efficient, in an approved and properly operated equipment. After pasteurization the milk is immediately cooled to 5 ° C or below.

The objectives of pasteurization are,

  • To render the milk safe for human consumption by destruction of cent percent pathogenic microorganism.
  • To improve the keeping quality of milk by destruction of almost all spoilage organisms. (85-99 per cent.)

The standards for Pasteurization were such as to ensure:

  • Complete destruction of pathogens.
  • Negativate phosphatase test and
  • Least damage to the cream layer.

As Coxiella burnetti are destroyed by a heat treatment slightly lower than that for phosphatase inactivation, pasteurization. is carried out at a heat treatment temperature above that for phosphatase inactivation and below that for cream line reduction.

DIFFERENT METHODS OF PASTEURIZATION

In- bottle pasteurization

Bottle filled with raw milk and tightly sealed with special caps is held at 63-66 ° c for 30 minutes. Then the bottles pass through water sprays for decreasing temperature, which cools both the product and the bottle.

Advantage

  • Prevents the possibility of post pasteurization contamination.

Disadvantage

  • The transfer of heat is very slow and there is greater risk of bottle breakage.
  • This method at present is out dated, although in bottle sterilization is widely prevalent.

Batch or holding pasteurization ( LTLT)

  • This is also called the low-temperature –long time –method. The milk is heated to 63 ° C for 30 minutes and promptly cooled to 5 ° C. The pasteurizers may be of three types

Water-jacketed vat

  • This is double walled around the sides and bottom in which hot water or steam under partial vacuum circulates for heating and cold water for cooling. The outer wall is usually insulated to reduce heat loss. The heat exchange takes place through the wall of the inner lining. The milk is agitated by slowly moving paddles or propellers. When heating, the vat cover is left open for escape of off-flavours, and when holding, the cover is closed.
  • Advantage: Flexibility in use.

Water spray type

  • A film of water is sprayed from a perforated pipe over the surface of the tank holding the product.

Coil vat type

  • The heating /cooling medium is pumped through a coil placed in either a horizontal or vertical position, while the coil is turned through the product. The turning coil agitates the product.

High Temperature Short Time (HTST) pasteurization

  • It is the modern method of pasteurizing milk and is invariably used where large volume of milk is handled. The HTST pasteurizer gives a continuous flow of milk, which is heated to 72 ° C for 15 sec. and cooled promptly to 5 ° C or below.
  • The following steps or stages are involved as milk passes through the HTST pasteurization system: float controlled balance tank; pump; regenerative heating; heating; holding; regenerative cooling; and cooling by chill water or brine. An arrangement for incorporation of the filter / clarifier, homogenizer, etc., in the circuit is also made when desired.

Float controlled balance tank (FCBT)

  • Maintains a constant head of milk for feeding the raw milk pump; also receives any sub-temperature milk diverted by FDV.

Pump

  • Either a positive pump between the regenerative heating section and heater or a centrifugal pump with a flow control device to ensure constant flow, after FCBT is used.

Plates

  • The plate heat exchanger, also called Para flow is a compact, simple, easily cleaned and inspected unit. Its plate may be used for heating, cooling, regeneration, and holding. A space of approximately 3-mm is maintained between plates by a non-absorbent rubber gasket. These plates are designed to provide a uniform turbulent flow of product with rapid heat transfer. Corrugations on the plate in the form of knobs, diamonds, and channels, help to provide the turbulent action required.

Regeneration (heating)

The raw incoming milk is partially and indirectly heated by the hot outgoing milk. This adds to the economy of HTST process. For example,

  • Milk entering at 4ºC
  • Heated in regenerator to 34ºC
  • Heated in heating section to 74ºC
  • Cooled in regenerator to 44ºC
  • Cooled in cooling section to 4ºC

Here, the increase from 4ºC to 34ºC is a change of 30ºC, and the decrease from 74ºC to 44ºC is also a change of 30ºC. Without regeneration, the milk would need to be heated by hot water or steam from 4ºC to 74ºC, a difference of 70ºC. With regenerative heating, however, hot water or steam need not be used for the temperature change between 4ºC and 34ºC. This temperature change is brought about by use of the outgoing hot milk. The saving of heat due to regeneration here is thus 43%. On the other hand, without regeneration the milk would need to be cooled by chilled water from 74ºC to 4ºC, a difference of 70ºC. With regenerative cooling, however, chilled water need not be used for the temperature change from 74ºC to 44ºC, a difference of 30ºC. This temperature change is brought about by use of cold incoming milk. The savings of refrigeration due to regeneration are thus 43%. Currently, as much as 90% efficiency has been achieved by the use of counter-current flow.

Filter

  • Variously shaped filter units to connect directly to the HTST system are placed after the regenerative heating section.

Holding

  • The holding tubes or plates ensure that the milk is held for a specified time not less than 15 sec at the pasteurization temperature of 72 ° C or more.

Flow diversion valve (FDV)

  • This valve diverts the unpasteurized milk automatically back to the FCBT for reprocessing.

Regeneration (cooling)

  • The pasteurized hot outgoing milk is partially and indirectly cooled by the incoming cold milk. This again adds to the economy of the HTST process.
  • Steam or hot water is used for the heat treatment of milk. For the cooling, chilled water is used. The milk to be heated flows across one side of the plate and heating or cooling medium flows across the other side in the opposite direction. Inserting more plates can increase the capacity of this heater.
  • Plate Heat Exchanger and corrugated plates with gaskets controlling the flow of milk

Advantages

  • Capacity of the equipment to heat treat the milk can be done quickly and effectively, while maintaining the quality control over both the raw and finished product.
  • Less floor space is required
  • Lower initial cost
  • Milk packaging can be started as soon as pasteurization begins thus permitting more efficient utilization of labour for packaging and distribution.
  • Easily cleaned and sanitized, this system adopts itself well to CIP-cleaning.
  • Lower operating cost. Reduced milk losses.
  • Development of thermophiles is not a problem.

Disadvantages

  • This system is not well adopted for small quantities of several liquid milk products.
  • Gaskets require constant attention for possible damage and lack of sanitation.
  • Complete drainage is not possible.
  • Margins of safety in products sanitary control are so narrow that automatic control precision instruments are required in its operation.
  • Pasteurization efficiency of high thermoduric count raw milk is not as great as it is when the holder system is used.
  • Greater accumulation of milk stone in the heating section.

Electric pasteurization

Electric pasteurization of milk or Electro pure process:

  • This method employs electricity, as the heating agent and is fairly popular in America. Milk is heated in a small specially constructed chamber. This pasteurizer is a rectangular, vertical chamber of 2 feet height, and about 2 inches in cross section, with two sides made up of carbon electrode separated by intervening walls of plate glass.
  • The cold milk passes through the regenerative section on which it is preheated to about 120 ° F by the outgoing hot milk and then pass through the electric heating chamber, here it is heated to a temperature of 161 ° F to 163 ° F by the resistance offered by the milk to the passage of a 110-volt alternating current. The milk is exposed at this temperature for 15-20 sec. after which it is cooled.

Vacuum pasteurization (vacreation)

  • This refers to pasteurization of milk /cream under reduced pressure by direct steam. This process removes feed and other volatile flavors from cream, and to pasteurize it for butter making. The equipment used in this process is called “vacreator” and the process is called “vacreation.”
  • The vacreator consist of three stainless steel chambers connected to one another for steam heating and vacuum treatment with continuous product flow. The product in the form of droplet enters the first chamber of the vacreator where pasteurization occurs. The chamber is operated under a vacuum of 5 inches Hg, which maintains a temperature of 90-95 ° c. While steam is fed from the top and falls by gravity to the bottom of the chamber, then the product and free steam is removed from the bottom of the first chamber to the top of the 2nd chamber.
  • The temperature of 2nd chamber is maintained at 71-82 ° c under a vacuum of 15 – 20 inch Hg. A portion of the steam previously added is removed and the product moves down through the chamber. Some of the tainting substances and off-flavours are removed by heat and vacuum treatment.
  • The product then moves on to the 3rd chamber at 43 ° c by maintaining a vacuum of 26- 28 inches of Hg. And here more water and off flavours are removed. A multistage centrifugal pump removes the product from the 3rd chamber. This process takes about 10 seconds to pass through the three chambers.

Stassanization

  • Henri Stassano invented it in France. This method of pasteurization is carried out in a tubular heat exchanger consisting of three concentric tubes. The principle of its operation is the heating of milk to the desired temperature by passing it between two water-heated pipes through the narrow space of 0.6 to 0.8 mm. The milk is heated to about 74 ° c for 7 sec. and then promptly cooled.

Ultra high temperature pasteurization

  • Ultra high temperature pasteurization was developed in the 1950s; this usually encompasses temperature time combination of 135 ° C to 150 ° C for no hold. The success of UHT heat treatment of milk depends on immediate aseptic packaging.

Uperization

  • This is other wise called as “ultra – pasteurization” and it was developed in Switzerland. In this process, milk is heated with direct steam up to 150 ° c for a fraction of a second. This is a continuous process.
  • The first step in the uperization process is fore warming of milk to 50 ° C and intended to remove most of the dissolved oxygen and volatile off-flavours by vacuum treatment. In the second part, the milk is first preheated to about 80-90 ° C and then heated on the uperization chamber with high-pressure steam to around 150 ° C for ½ to ¾ th of a second. After this heating, the product moves into an expansion chamber at near atmospheric pressure, thereby forcing some evaporation of moisture. The product is then moved to a cooler and then into storage chamber.

The advantages of uperization are

  • Long keeping quality.
  • Removal of feed and other volatile off-flavours:
  • Appreciable homogenization effect.
  • Reduction in acidity.
  • Efficient destruction of microorganism.
  • Effect of uperization on nutritive value and flavour are not greater than that of
  • pasteurization.

Boiling of milk

  • The boiling of milk brings about important changes in milk. Milk boils at a temperature of 212.3 ° F at sea level. At this temperature the milk sugar is burnt causing a condition called caramelisation in which the milk is brown in colour. The casein and albumin are somewhat hardened. The calcium, magnesium, and phosphoric salts are partially precipitated, all of which renders milk less digestible. The enzymes are destroyed. Prolonged heating at high temperature causes a destruction of vitamin C, A, and D in that order, but boiling under ordinary conditions does not destroy vitamin A. When milk is brought to the boiling point in the air, a thin film forms over the surface due to the coagulation of small amount of casein, albumin, small amount of calcium salts, and fat.

Sterilization

  • The term sterilization when used in association with milk means heating continuously to a temperature of 115 ° C for 15 min. or 145 ° C for 3 sec. or equivalent approved temperature time combination to ensure preservation of milk at room temperature for a period of not less than 15 days from the date of manufacture. Sterilized milk shall show absence of albumin by a negative turbidity test. Sterilized milk shall be sold only in the container in which the milk was sterilized.
Last modified: Thursday, 12 April 2012, 8:39 AM