Varieties of milk
There are many different varieties of milk available for consumption within the UK.
The different milks tend to vary according to the way they are produced, and their fat content. The fat content of milk varies depending on the product e.g. whole milk has a fat content of about 4% fat, whole standardised milk, that which is widely available for sale, has a minimum fat content of 3.5% fat, semi skimmed milk contains 1.7% fat, skimmed milk contains about 0.1% fat, and in addition there is 1% fat milk.
Which milks are available?
Natural whole milk is milk with nothing added or removed.
Whole standardised milk is whole milk standardised to a minimum fat content of 3.5%.
Whole homogenised milk is identical in fat and nutrient content to whole standardised milk however it has undergone a specific process known as “homogenisation” which breaks up the fat globules in the milk. This spreads the fat evenly throughout the milk and prevents a creamy layer forming at the top.
How is it produced?
Natural whole milk is collected from the dairy herd and undergoes various processing techniques before it reaches the shelf for consumption by the general public.
Most of the milk consumed in Europe, Scandanavia, the USA, Australia and New Zealand is pasteurised.
Pasteurisation is the process whereby milk is heated with the purpose of killing potentially harmful micro-organisms such as certain pathogenic bacteria, yeasts and moulds which may be present in the milk after initial collection. This helps to protect against any food born illness that can occur through consumption of raw (unpasteurised) milk.
Following pasteurisation, the milk is rapidly cooled and is then stored in a refrigerator in order to preserve its shelf life.
Various pasturisation and heat treatment techniques can be used in the production of milk which can affect storage capacity-detailed in later sections.
Much of the milk in the market is now homogenised as well as pasteurised. Homogenisation offers a way to reduce the fatty sensation of whole milk and prevent the formation of a cream plug.
Semi skimmed milk
Semi skimmed milk is the most popular type of milk in the UK with a fat content of 1.7%, compared to a minimum of 3.5% in whole standardised milk and 0.1% in skimmed milk -see nutritional composition of milks
Skimmed milk has a fat content of between 0-0.5% and an average fat content of 0.1%. Skimmed milk therefore has nearly all the fat removed.
It contains slightly more calcium than whole milk and lower levels of fat soluble vitamins, particularly vitamin A, as this is lost when the fat is removed-see nutritional composition of milks
The lower level of fat in skimmed milk reduces its calorie (energy) content. For this reason it is not recommended for children under the age of 5 years as they need the extra energy for growth. However it is ideal for adults who wish to limit their fat or calorie intake.
Skimmed milk has a slightly more watery appearance than other types of milk and has a less creamy taste due to the removal of fat.
1% fat milk
The EU regulations for milk classification previously divided milk into three categories defined by the fat content; whole, semi-skimmed or skimmed. Prior to 2008, any milk that contained a different fat content was defined as a ‘milk drink’.
On the 1st of January 2008 new regulations came into force to facilitate consumer choice. Now any milk with a fat content other than those laid out can also be considered as ‘milk’, provided that its fat content is clearly indicated on the packaging in the form of ‘….% fat’. However, these milks cannot be described as whole, semi-skimmed or skimmed.
Following this change in regulation 1% fat milk is now offered to consumers who like the taste of semi-skimmed, but want to enjoy milk with a lower fat content.
The nutritional differences between semi-skimmed and 1% fat milk are small and dependent mainly on the difference in fat content. 1% fat milk contains 40% less total and saturated fat than standard semi-skimmed milk. In addition, it has a lower energy content than semi-skimmed, and slightly lower levels of vitamins A and E, but has a higher calcium content. See nutritional composition of 1% fat milk.
Organic milk comes from cows that have been grazed on pasture that has no chemical fertilisers, pesticides or agrochemicals used on it.
The producers must register with an approved organic body and are subject to regular inspection.
Once the cows have been milked, the milk is treated in exactly the same way as regular pasteurised milk.
There is no evidence to suggest that organic milk is any more nutritious than conventionally produced milk. Although there have been studies to show that organically produced milk contains higher levels of omega-3 polyunsaturated fatty acids, these are plant derived, short-chain fatty acids which appear to be of limited health benefit compared to the longer chain omega-3 fatty acids found in oily fish.
Jersey and Guernsey milk
Channel Island milk is produced from Jersey or Guernsey breeds of cow and has a particularly rich and creamy taste.
It tends to be slightly higher in calories and fat than regular whole milk and also has a higher content of fat soluble vitamins -particularly vitamin A which is important for the promotion and maintenance of healthy growth and development -see nutritional composition of milks
Jersey and Guernsey milks tend to have a visible cream line and are commonly found in supermarkets as “breakfast milk”.
The flavoured milk market is one of the fastest growing dairy sectors.
There are a wide variety of flavours and consistencies to cater for all ages and tastes with a choice of long-life (i.e. Ultra Heat Treated or sterilised) or fresh flavoured milk.
Most flavoured milk products are produced using reduced fat milk varieties and usually have a fat content of around 1%.
The most popular flavours are chocolate, strawberry and banana however more sophisticated flavours such as peach, mocha or products made with real Belgian and Swiss chocolate have been developed for the more adult market.
In comparison with plain milks, flavoured milks tend to have slightly higher sugar content, however studies have suggested that they are still a favourable option for children and teenagers as they provide a wide range of beneficial nutrients.
One study has shown that children consuming flavoured milk are not actually likely to have higher sugar or energy intakes as children consuming flavoured milk would likely, consume fewer less healthy sweetened drinks.
Flavoured milk is also less likely to cause damage to teeth than sugary foods and drinks.
Interestingly recent studies have suggested that chocolate flavoured milk can be used as an effective recovery aid after intense bouts of exercise.
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 71.7ºC 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.
Untreated (raw) milk
All milk sold from the supermarkets has to be heat-treated (pasteurised) to kill harmful bacteria.
However, untreated milk can be bought direct from registered milk production holdings (at the farm gate or in a farmhouse catering operation) or through a limited number of milkmen in England and Wales.
Sales through other outlets have been banned since 1985 (although sales by the farmer at farmers markets are allowed). In addition, the milk must bear the appropriate health warning.
A ban for the sale of raw milk in Scotland has been in place since 1983.
Raw milk represents only a very small fraction of total milk consumption. Animal Health Dairy Hygiene has estimated this to be 0.01% of total cows' milk consumption.
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 5ºC 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 7ºC 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 isn’t 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 tends 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.