Pasteurization of Milk : Temperature, Types, Advantages and Disadvantages

 Pasteurization process developed in 19th century in France by Louis Pasteur, for the preservation of wine. Its application resulted in coining a new term 'Pasteurization'.
  Louis Pasteur was pioneer in its use for the preservation of wine Dr. Soxhlet of Germany introduced this process for preservation of milk in 1886.

Definition of Pasteurization

Pasteurization refers to - The process of heating of each and every particle of milk to at least 62.8° C (145°F) for 30 min. or 71.7° C (161ºF) for 15 seconds or to any temperature-time combination, which is equally efficient, in an approved and properly operated equipment.
  After pasteurization the milk is immediately cooled to 5°C or below. The term pasteurization is mainly applied to the market milk today.

Objectives of Pasteurization :

  • To render the milk safe for human consumption by destruction of all pathogenic microorganism.
  • To improve the keeping quality of milk by destruction of almost all spoilage organisms.
  • Thus, the pasteurization does not kill all micro-organisms in milk and therefore, it is not a method of milk sterilization but it is a process of disinfection.
  • Pasteurization is also intended to inactivate some enzymes in the raw milk.

Relationship Between Time & Temperature :

  • The efficiency of pasteurization depends upon time-temperature relationship.
  • Initially the pasteurization temperature was set to kill Mycobacterium tuberculosis, which was supposed to be the most heat resistant organism in milk.
  • This organism gets destroyed at a temperature of 140°F in 10 minutes.
  • Thus, the initial temperature of pasteurization was set to 143°F (61.6°C) for 30 min.
  • Later on, it was discovered that another pathogen namely Coxiella burnetii (causative agent of Q fever), also occurs in the milk and it is able to survive at the temperature of 143ºF.
  • Thus, to ensure the destruction of Coxiella, temperature of pasteurization was raised to 145°F (62.8°C).

Methods of Pasteurization

There are three methods of Pasteurization in common practice.
  1. Low temperature holding (LTH
  2. The high temperature short time (HTST)
  3. Ultra high temperature (UTH).

1. Low Temperature Holding  (LTH) :

  • This is also called as low temperature long time (LTLT) method or batch or vat pasteurization method.
  • In this process, the milk is heated to 62.8° C(145°F) for 30 minutes and promptly cooled to 5° C or below.
  • It is a batch method and thus it takes a little more time. The heating of milk in this method is carried out by three different types of pasteurizers -

i]. Water-Jacketed Vat -

  • This is double walled tank 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.

ii]. Coil Vat Type -

  • The heating or cooling medium is pumped through a coil placed in either a horizontal or vertical position.
  • Such coil is turned through the milk.
  • The turning of coil agitates the milk.

iii]. Water Spray Type -

  • A film of hot water is sprayed over the surface of the tank holding the milk.

2. High Temperature Short Time Method (HTST) :

  • It is the modern method of pasteurization of milk
  • It is used for large volume of milk.
  • In this method, the milk is heated to 71.7°C(161ºF)for 15 second and cooled promptly to 5°C or below. This method gives the continuous flow of milk.
  • Milk is heated either in tubes or in thin metal plates or in heat exchangers by electricity or hot water.
  • Raw cold milk is allowed to enter in heat exchanger and hot, pasteurized milk is taken out.
  • It is comparatively faster method. There are different designs of HTST pasteurization system.

Essential Components of a Standard HTST System

1. Raw milk tank : It is a tank where chilled raw milk is stored.
2. Balance tank : It maintains a constant head for the incoming milk.
3. Milk feed pump : It creates suitable pressure that is necessary for efficient flow.
4. Flow control system : It controls the flow rate of milk.
5. Filters and clarifiers : They remove the dirt (if any) from the milk.
6. Homogenizer : It divides fat globules into micro globules to avoid fat separation in standing milk.
7. Plate Heat Exchanger (PHE) with regeneration section, heating section, holding section and cooling sections : It facilitates an efficient pasteurization.
8. Flow diversion valves :
  • They ensures that all the conditions for pasteurization have been met.
  • It diverts the milk to head tank if it is not properly heated to pasteurization temperature for reprocessing.
  • If milk is properly pasteurized milk, it passes forward through the flow diversion valves into the regeneration where it is cooled.

Plate Heat Exchanger

  • The plate heat exchanger is a compact and simple unit.
  • Its plate may be used for regeneration, heating, holding and cooling.
  • 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 milk with rapid heat transfer.

Regeneration (heating) -

  • The raw incoming milk is partially and indirectly heated by the hot outgoing milk.
  • This reduces the cost of HTST process.
For example -
  • Milk entering at 4°C
  • Heated in regenerator to 34°C
  • Heated in heating section to 74°C
  • Cooled in regenerator to 44°C
  • Cooled in cooling section to 4°C
  • Here, the increase from 4 °C to 34°C is a change of 30°C and the decrease from 74°C to 44°C is also a change of 30°C.
  • Without regeneration, the milk would need to be heated by hot water/steam from 4°C to 74°C, a difference of 70°C
  • With regenerative heating, however, hot water or steam need not be used for the temperature change between 4°C and 34°C.
  • This temperature change is brought about by use of the outgoing hot milk.
  • Thus, here regeneration saves the heat.
  • On the other hand, without regeneration the milk would need to be cooled by chilled water from 74°C to 4°C, a difference of 70°C.
  • With regenerative cooling, however, chilled water need not be used for the temperature change from 74°C to 44°C, a difference of 30°C.
  • This temperature change is brought about by use of cold incoming milk.
  • Here also the savings due to regeneration takes place.
  • 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.

Advantages of Regenerative Heating :

  • Utilization of the incoming chilled milk to cool the outgoing hot pasteurized milk increases the efficiency of the PHE.
  • Smaller amount of energy is required to heat the milk to pasteurization temperatures since the heating does not start from 4°C of the chilled milk.
  • Reduces the amount of time required to pasteurize milk.

Advantages of HTST :

  • The milk can be pasteurized quickly and effectively
  • Initial cost of the method is less.
  • Milk packaging can be started as soon as pasteurization begins.
  • This permits more efficient utilization of labour for packaging and distribution.
  • The system can be easily cleaned and sanitized Lower operating cost.
  • Reduced milk losses. Development of thermophiles is not a problem.

Disadvantages of HTST :

  • This system is not well adopted for small quantities milk or milk products.
  • Gaskets require constant attention for possible damage and lack of sanitation.
  • Complete drainage is not possible.
  • Raw milk with high thermoduric bacterial count is not efficiently pasteurized as compared to LTH method.

3. Ultra High Temperature Method (UHT) :

  • Milk is heated at 135ºC to 150°C (average 141°C) for just two seconds or no hold time.
  • It can be carried out by the direct or indirect means of heat treatment
  • Direct treatment is either injection of steam into milk or infusion of milk into a steam chamber
  • Indirect treatment includes the use of heat exchangers.
  • The success of method depends on immediate aseptic packaging. 
  • The temperature and time combination is much more lethal to bacteria and kills all bacteria.

UHT Pasteurization by Steam Injection Technique :

  • In this technique, the steam is injected into the milk.
  • The steam at a pressure higher than that of the pressure of milk flow is injected into the milk stream through a suitable nozzle.
  • There are many different injector designs.
  • In all of these, the steam and milk are kept in thermally separate zone until they reach the mixing zone.

UHT Pasteurization by Steam Infusion Technique :

  • It is very similar to injection system, except for the method of mixing of milk with steam.
  • The infusion is done by dropping heated milk into a steam pressure vessel with a conical base.
  • The heating is instant.
  • The milk is then cooled rapidly by evaporative cooling with exposure to a slight vacuum.
  • This removes the water that is added to the milk due to the condensation of the steam.
  • Steam infusion can be used to pasteurize a variety of dairy products like cream and special dairy products but now it is popular for milk.

Pasteurized Milk

  • Packaging of ultra-pasteurized milk is also carried out under sterile conditions to prevent the recontamination with spoilage bacteria.
  • Ultra-pasteurization makes milk free of spoilage and harmful bacteria, but it is not considered sterile and, thus, it requires refrigeration.
  • Ultra-pasteurized milks have more "cooked" flavor as compared to conventionally pasteurized milks.
  • Average shelf-life of ultra-pasteurized milk products is 30-90 days if held under refrigeration and in packed condition
  • Once an the product is opened, it may become contaminated with spoilage bacteria. Thus, after opening, ultra-pasteurized milk should be kept well refrigerated and consumed within 7-10 days for best quality and taste.

Advantages of Pasteurization :

  1. Pasteurized milk is safe for consumption.
  2. Pasteurization is designed to decrease microbial count.
  3. Pasteurization has little effect on the nutritive value of milk.
  4. There is some loss of vitamin C and B group vitamins, but this is insignificant.
  5. With Pasteurization, keeping quality of milk remains unaltered. 6. Pasteurization does not reduce the fat content of milk.

Disadvantages of Pasteurization :

  1. Cooked taste may be developed, for which consumer may complain.
  2. Pasteurization reduces cream layer of the milk.
  3. Vitamin content may get changed.