Heat treated milks
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 71.7ēC for a minimum of 15 seconds (max 25 seconds). This process is known as High Temperature Short Time (HTST).
The cold milk that enters the heat exchanger is heated by the hot milk leaving it, which in turn is partly cooled. After heating, the milk is cooled rapidly to below 6ēC 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.
First the milk is pre-heated, sterilised, then homogenised (see below) and poured into glass bottles or plastic cartons, which are closed with an airtight seal.
The bottles are put on a conveyor belt and pass through a steam chamber where they are heated to a temperature of between 113-130ēC for approximately 10-30 minutes. Then they are cooled and 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 several 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 135ēC 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 71.7ēC 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 60ēC and 65ēC.
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 sterilised 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 110-115ēC for one to two minutes.
The milk is then homogenised, the sugar added and the sweetened milk is then evaporated at low temperatures (between 55-60ēC). The concentration of the condensed milk is now up to 3 times that of the original milk.
The milk is then cooled rapidly to 30ēC 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.
Untreated (raw) milk
All milk sold via the supermarkets and milkmen has to be heat-treated (pasteurised) to kill harmful bacteria. However, untreated milk can be bought direct form a limited number of farm distributors in England and Wales.
The farmer must hold a licence from DEFRA, the Department for Environment, Food and Rural affairs (formerly known as MAFF) to be able to sell this milk.
In 1999 the government introduced tighter controls on the production of milk when sold untreated, including more prominent and comprehensive labelling. Labels must indicate that the milk has not been heated above 40ēC. It must also carry the following warning: This milk has not been heat treated and may therefore contain organisms harmful to health.
The government prohibited sales of untreated milk through shops, hotels and other catering establishments in 1989. The sale of untreated milk in Scotland was banned in 1983.
Untreated milk represents less than 1% of the household milk market.
Filtered milk goes through an extra, fine filtration system, which prevents souring bacteria from passing through.
The nutritional content of the milk is unaffected but the shelf life is increased.
The processes involved include, microfiltration, ultrafiltration and nanofiltration.
Microfiltration is the most commonly used process and is a pressure-activated separation process which uses a membrane that is permeable to substances with a low molecular weight but rejects material with a high molecular weight.
In the process of microfiltration of skimmed milk, bacteria are removed using ceramic filters with 1.4 micrometer holes to separate the milk from the bacteria. After this process, virtually all the bacteria present in the milk are removed.
The milk is then homogenised to standardise and evenly distribute the fat molecules, where it then undergoes the pasteurisation process before being chilled down quickly to 5ēC or less.
Microfiltration adds an extra level of cleanness which can extend shelf life up to 45 days when stored at temperatures of up to 7ēC and an average 7 days once opened.
Filtered milk is available in whole, semi skimmed or skimmed milk varieties.
Dried milk powder
Milk powder is produced by evaporating the water from the milk using heat. The milk is homogenised, heat treated and pre-concentrated before drying.
There are a number of ways to produce dried milk powder including spray drying and roller drying.
In the most commonly used spray drying process, the concentrated milk is introduced into a chamber (usually as a fine mist) through which hot air is circulating. The droplets of milk soon lose their water and fall to the floor as fine powder.
Skimmed milk powder can be mixed easily with water; however whole milk isnt easily reconstituted due to its higher fat content.
Roller drying is an old process of producing milk powder-this involves spreading the concentrated milk onto heated rollers. The water evaporates quickly and leaves a thin film of powder, which is scraped off the rollers. This powder has a cooked flavour and trends to form lumps when mixed with water.
Whole milk powder contains all the nutrients of whole milk in a concentrated form with the exception of vitamin C, thiamin and vitamin B12. Skimmed milk powder contains hardly any fat and therefore no fat soluble vitamins. However, the protein, calcium and riboflavin content remain unaffected.
If stored correctly, skimmed milk powders can be kept for up to one year. Once they are reconstituted, they must be treated as fresh milk.
Homogenisation of milk involves forcing the milk at high pressure through small holes. This breaks up the fat globules in order to spread them evenly throughout the milk and prevent separation of a cream layer.
This process basically results in milk of uniform composition or consistency and palatability without removing or adding any constituents. Homogenisation increases the whiteness of milk because the greater numbers of fat globules scatter the light more effectively.
Most milk available on the market is homogenised at present.