Defination and Purpose of Pasteurization

Fresh milk produced from healthy milch animals generally contains minimum load of microorganisms. In the course of handling at the farm, milk is liable to be contaminated by various microorganisms mainly bacteria. Rapid chilling to below 4°C temperature slows down the growth of microorganisms in the milk. Milk must be treated by an established process so that all pathogenic microorganisms are killed before it is consumed as fluid milk. This is achieved by heat treatment.

Pasteurization is one of the most important heat treatment processes. The term as applied to market milk refers to the process of heating every particle of milk to a temperature of at least 63°C (145.4°F) for 30 minutes or 71.7°C (161°F) for 15 seconds (or to the temp-time combination which is equally efficient) in properly designed equipment. Milk is immediately cooled to 4°C and stored in cold storage maintained at 4°+1°C.

As per definition of International Dairy Federation (IDF) “Pasteurization is a process applied to a product with an objective of minimizing possible health hazard arising from pathogenic microorganisms associated with milk by heat treatment, which is consistent with minimal chemical, physical and organoleptic changes in the product” The heat treatments suggested by the IDF for the pasteurization of milk are 15 seconds at 71.7°C=161°F or 30 minutes at 62.8°C=145°F can be regarded as “universal” reference treatments. Three aspects emerging from the definition are:(i) level and degree of heat treatment, (ii) minimum chemical, physical and organoleptic changes, and (iii) minimum health hazards. These are elaborated here.

 

i. Time-Temperature Combination

The time-temperature combinations normally used for pasteurization of fluid milk are as follows:

•  63°C (145.4°F) and held at that temperature for at least 30 minutes
•  72°C (161.6°F) and held at that temperature for at least 15 seconds.

The milk is then immediately cooled to a temperature not greater than 4°C. The selected heat treatment shall be applied only once. This means pasteurization includes heating to a specific time-temperature combination followed by immediate cooling to 4°C.

 

ii. Purpose

Milk is pasteurized for two purposes:

•  To make safe for human consumption by destroying pathogenic microorganisms present in milk.
•  To improve its keeping quality.

The most heat resistant pathogenic organism at pasteurization temperature is the Mycobacterium tuberculosis and hence this has been made as an index organism to achieve complete safety of milk. Any heat treatment, which will destroy this organism, can be relied upon to destroy all other pathogenic organisms as well as other organisms involved in milk spoilage. Some bacteria, call thermodurics (heat resisting) may survive during pasteurization but immediately cooling slows down their growth and thus prevents them causing spoilage such as flavour taint or souring. Although, the main purpose of heat treatment is to destroy all microorganisms capable of causing disease in humans but pasteurization has two additional benefits,i.e. the destruction of a large number of spoilage microorganisms present in raw milk and deactivation of some natural enzymes like lipases, which can adversely affect the quality of manufactured products, i.e. lipolysis or breakdown of fat into glycerol and free fatty acid. However, we must be clear that pasteurization is not a substitute for cleanliness during milk production. The pasteurization process should only be applied to raw milk obtained from healthy cow, which is clean, sweet and has a low bacterial count.