just looked it up on google and here it is copied and pasted:
Heat Treatment of Milk
Heat treatment is applied to milk to destroy the bacteria which makes milk go sour. There are a number of different types of heat treatment.
Pasteurisation
Louis Pasteur demonstrated that it was the multiplication of bacteria that caused wine, beer and milk to go sour and that by heating the liquid, the bacteria would be destroyed. The heat treatment process known as pasteurisation derives its name from the French scientist Louis Pasteur.
Although Pasteur's objective was to delay natural souring, the process had another, more important benefit, that it would also kill off the bacteria in milk that could cause tuberculosis and other diseases.
Pasteurisation also encouraged greater use of sealed bottles since such care had been taken to ensure the safety of milk by heat treatment; it was necessary to maintain it's hygienic quality until it reached the customer by means of a container that would minimise bacterial contamination.
Pasteurisation is one of the oldest forms of heat treatment applied to liquid milk and accounts for approximately 89.5% of all processed drinking milk in Great Britain. The objectives of milk pasteurisation are to destroy all vegetative pathogenic micro-organisms, reduce the number of potential food spoilage micro-organisms to acceptable levels and inactivate specific enzymes contained in raw milk. Traditionally, milk was pasteurised using the 'batch' or 'holder' method. The legal definition of this method was set out in the Milk and Dairies (Amendment) Act 1922. This required milk to be pasteurised by heating to between 62.8C (145F) and 65.6C (150F) for at least 30 minutes and to be immediately cooled to 12.8C (55F), with the object of destroying disease-causing bacteria.
With the development of plate and tubular heat exchangers the process was shortened to a minimum of 15 seconds at a minimum temperature requirement of 71.7C. This continuous process is known as HTST, high temperature short time method. The legislation allows any combination of time and temperature (within prescribed ranges) which achieves an equivalent effect, that is, the destruction of all vegetative pathogens and the inactivation of the enzyme phosphatase which is used as a key to the destruction of Mycobacterium Bovis, the organism responsible for tuberculosis in cattle.
Milk can be passed through a separator and standardisation unit where the fat content may be adjusted to produce standardised milk (3.5% fat), semi-skimmed milk (1.7% fat) or skimmed milk (0.1% fat).
Where the milk has been separated it is normal practise to homogenise it. Homogenisation is a process which breaks up the fat globules in the milk by forcing them through a fine aperture, allowing the cream to be mixed evenly throughout the milk.
Thermisation
This is a relatively new heat treatment process, rarely used in this country, which is applied to raw milk in order to inhibit spoilage and extend storage at refrigeration temperatures prior to final heat processing and packing.
Under the Dairy Products (Hygiene) Regulations (1995) 'thermised' milk is required to be heated to between 57C and 68C for at least 15 seconds. After heat treatment the milk must show a positive reaction to the phosphatase test which should show that the milk has not been pasteurised. In addition, if the milk is to be used to produce pasteurised, sterilised or UHT milk it must have a plate count (total microbial count) at 30C of not more than 100,000 bacteria.
Sterilisation
This does not mean sterile in the strict microbiological sense, but the level of micro spoilage is reduced by increasing the severity of the heat treatment process. Milk is pre-heated to 50C, separated and standardised to produce whole, semi-skimmed or skimmed milk. The modern process is a continuous one; filled bottles are carried on a conveyor belt through a steam pressure chamber at temperatures between 110C and 130C, and emerge after 10 to 30 minutes to be cooled in a cold-water tank.
Ultra Heat Treatment
The legally defined minimum heat treatment for UHT milk is treatment at 135C for one second. This causes less chemical change to milk than traditional sterilisation. Milk treated by this method is virtually sterile.