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Approximately 99% of milk sold in the UK is heat-treated, to kill harmful bacteria and to improve its shelf life.
Pasteurisation is the most popular method of heat treatment. It is a relatively mild form of treatment, which kills harmful bacteria without significantly affecting the nutritional value or taste of the milk.
The basic process for whole milk involves heating the milk to a temperature of no less than 71.7ºC for a minimum of 15 seconds (max 25 seconds). This process is known as High Temperature Short Time (HTST).
Following recommendations by The Advisory Committee on the Microbiological Safety of Food (ACMSF), which advises the UK Government Food Standards Agency, the UK dairy industry has extended the holding time for heat treatment of liquid milk to 25 seconds at 72°C. The purpose of this recommendation was to eliminate M. paratuberculosis (MAP) from milk.
After this the cold milk that enters the heat exchanger is heated by the hot milk leaving it, which in turn is partly cooled. Following heating, the milk is cooled rapidly to below 6ºC using chilled water on the opposite side of the plate. This process also extends the keeping quality of the milk.
Sterilised milk is available in whole, semi skimmed and skimmed varieties. It goes through a more severe form of heat treatment, which destroys nearly all the bacteria in it.
Firstly the milk is pre-heated to around 50oC, then homogenised (see below for a brief outline of homogenisation), after which it is poured into glass bottles which are closed with an airtight seal.
There is no legally defined process for sterilising milk but, commonly, filled bottles are carried on a conveyor belt through a steam chamber where they are heated to a temperature of between 110-130ºC for approximately 10-30 minutes. Then they are cooled using a cold water tank, sprays or, in some cases, atmospheric air and then crated.
The sterilisation process results in a change of taste and colour and also slightly reduces the nutritional value of the milk, particularly the B group vitamins and vitamin C.
Unopened bottles or cartons of sterilised milk keep for approximately 6 months without the need for refrigeration. Once opened it must be treated as fresh milk and used within 5 days.
UHT or ultra heat treated milk is a form of milk that has been heated to a temperature of at least 135ºC in order to kill off any harmful micro-organisms (e.g. harmful bacteria) which may be present in the milk. The milk is then packaged into sterile containers.
All milk that is available for sale to consumers through supermarkets and milkmen must be pasteurised i.e. heated to 71.7ºC in order to make it safe for consumers and improve its shelf life. However UHT milks have a longer shelf life as a result of the higher temperatures to which they are heated and the packaging used to store them.
UHT milk is available in whole, semi skimmed and skimmed varieties.
Evaporated milk is a concentrated, sterilised milk product. It has a concentration twice that of standard milk.
The process of producing evaporated milk involves standardising, heat treating and evaporating the milk under reduced pressure, at temperatures between 60ºC and 65ºC.
The evaporated milk is then homogenised to prevent it separating under storage and then it is cooled.
The evaporated milk is poured into cans, which are then sealed. At this point the cans are moved to a steriliser where they are held for 10 minutes.
A cooling stage follows and the cans are then labelled and packed.
As a result of processing, evaporated milk possesses a characteristic cooked flavour as well as a characteristic colour.
The shelf-life of canned evaporated milk is commonly stated as one year stored at ambient temperatures, though in practice the product will keep for longer.
Condensed milk is concentrated in the same way as evaporated milk, but with the addition of sugar.
This product is not sterlised but is preserved by the high concentration of sugar. It can be made from whole milk, semi skimmed or skimmed milk.
The heat treatment used consists of holding standardised milk at a temperature of 110-115ºC for one to two minutes.
The milk is then homogenised, the sugar added and the sweetened milk is then evaporated at low temperatures (between 55-60ºC). The concentration of the condensed milk is now up to 3 times that of the original milk.
The milk is then cooled rapidly to 30ºC and packaged.
Sweetened condensed milk is commonly used in the sugar confectionary industry for the production of toffee, caramel and fudge. It is also an alternative to liquid milk which was once traditionally used in these products.
· 4 years ago