Microbes and food 1. Menu - you are what you eat
Milking a cow photo
Picture 2.2a Harmful bacteria can contaminate milk in the milking parlour.
E. coli bacteria photomicrograph
Picture 2.2b Tiny E. coli bacteria.
1.12 Milk
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What is it?
Milk is an important part of many people’s diet. It is a complete food containing water, fats, proteins, vitamins and minerals. However, this mix of nutrients and a pH of around 7 also make it an ideal growth material for many microbes.

In its raw state ‘straight from the cow’ milk contains bacteria from several sources:

  • the inside and the outside of the cow’s teats
  • the udder
  • the hide of the animal.

More microbes can be picked up from the milking equipment and the people handling it. If these microbes grow, the milk soon "goes off".

There is also a strong likelihood of faecal contamination and so raw milk may contain harmful bacteria, like Salmonella and E. coli O157, which cause serious diseases. It can also carry certain pathogens directly from the animal, such as Mycobacterium bovis which caused outbreaks of TB in the past.

Most milk is drunk fresh. Therefore, to make it safe from pathogens and to extend its storage life, it has to be heat-treated. Various methods are used.

In the UK over 90% of milk is pasteurised; nearly all the rest is UHT or sterilized. Hardly any milk is drunk untreated; because of the health hazards the sale of raw milk is prohibited in many countries. Most milk products such as yoghurt and cheese are also made from milk that has been pasteurised.

72 105 132
15 15 2
some all all
none yes some
Table 1. Milk treatments.
How is it produced?
Milk is flash pasteurised by heating it to 72°C for about 15 seconds. Most of the bacteria present are killed, including the pathogens. However some bacteria such as lactobacilli survive and can cause spoilage eventually. Bacterial spores also survive pasteurisation.

Milk is sterilised by heating it to 105°C for 15 seconds. This process kills all the microbes but it changes the flavour of the milk – it tastes "cooked".

UHT milk
UHT milk is heated to 130°C for 2 seconds. This kills all the microbes but has less effect on the taste of the milk.

Milk collection
Picture 1.12c Milk is collected each day from the farm.

How does it spoil?
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Milk is cooled in the dairy and transported in refrigerated tankers for processing. Unchilled milk would soon go sour due to acid production by lactobacilli. Even if kept refrigerated, raw milk goes off rapidly due to the action of psychrophilic (cold-tolerant) bacteria. These produce proteinases and lipases which break down both the protein and the fat in milk, causing rancid and bitter tastes and clotting.

One unusual type of bacteria which can grow is called Alcaligenes viscolactis – this produces sticky threads of exopolysaccharide in the milk called ropiness.

Pasteurized milk produced in modern, hygienic plants will keep for at least 10 days in the fridge. Spore-formers or heat tolerant bacteria may survive the heat treatment and the milk can also be contaminated after heating by psychrophiles in the factory environment. The spoilage effects are the same as for raw milk, but take much longer to develop as the initial number of microbes is small. Sometimes taints are due to microbial enzymes that get through the heating. One form of spoilage often seen in the summer is ‘bitty cream’ where enzymes produced by the spore-former Bacillus cereus cause the formation of proteinaceous fat particles.

Few people drink full-cream milk these days – they prefer the lower fat options of skimmed or semi-skimmed milk. Interestingly, these spoil quicker than whole milk because there are extra stages in the manufacturing process where the fat is taken out, and in the case of semi-skimmed, some put back again, before the product is packaged. This provides more opportunities for microbial contamination.

UHT and sterilised milk do not need to be kept in the fridge as long as their containers are kept sealed. Once the container has been opened, microbes can get into the milk and it must be stored chilled to slow their growth.

Can it be harmful?
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Bitty milk photo
Picture 1.12d Bitty milk. Not very appealing!

Raw milk can be very harmful indeed. In the past drinking it caused much disease. Before pasteurisation was introduced in the 1930s, milk often carried typhoid fever, diphtheria, and brucellosis as well as TB. Cattle tend to act as reservoirs for the bacteria concerned. Regular testing and culling of animals have helped to eradicate some of these diseases in the UK, although this is not true for all parts of the world. Even with these measures and the highest attention paid to hygienic production and storage, drinking raw milk is still considered to be a big hazard to human health.

Pasteurized milk should be safe unless the heating process has failed or the product has been contaminated with pathogens after treatment. In the UK milk on the doorstep has occasionally been contaminated with Campylobacter on the beaks of birds pecking through the foil bottle tops, causing human illness.

UHT and sterilised milk are rarely implicated in cases of disease, unless they have become contaminated after opening.