Science: The Truth About... pasteurisation

WE CAN thank Louis Pasteur (pictured) for inventing the idea of heating something in order to eliminate the harmful bacteria it may harbour. If there was a single moment in the career of the great 19th century microbiologist when he became convinced of the importance of heat treatment, it was when he decided to climb a glacier on Mont Blanc leading a mule carrying 20 glass flasks.

Each sealed flask contained a different ``putrescible'' liquid, such as blood, urine, wine or milk. He climbed to 15,000 feet and exposed each briefly to the mountain air, which he deduced would be relatively free of germs. Only one of the liquids subsequently went off - the one that had not previously been heated in its flask.

Pasteur had proven his germ theory of disease and the benefits of heating to get rid of microbes. Today, about 90 per cent of milk is pasteurised, or heated, to eradicate the possibility of its containing dangerous bacteria. However, the Government cast doubt over this with its nationwide survey to investigate the microbiological quality of pasteurised cows' milk, which it announced this week. It intends to investigate the possibility that a type of microbe, Mycobacterium paratuberculosis, may survive pasteurisation, although government scientists emphasised that the preliminary findings are still unconfirmed.

``In spite of its name, Mycobacterium paratuberculosis is not the cause of tuberculosis in man or animals,'' a government spokesman said. ``However, the bacterium is the cause of Johne's disease, a disorder in cattle across the world, and is one of several factors that have been suggested as a possible cause of Crohn's disease, a chronic inflammation affecting the intestine in humans.''

One of the most deadly micro-organisms that can live in milk is bovine tuberculosis, which killed thousands of children before pasteurisation became widespread in Britain following the 1922 Milk and Dairies Act. In the last century, milk used to be delivered in uncovered pails through dusty streets. It was not uncommon for people to test its freshness by dipping their fingers into the pail to see whether it was warm and therefore ``straight from the cow''.

Pasteurised milk was in fact introduced into the US in 1893, when the city of New York built its first pasteurisation plant, which was established by Nathan Strauss, an industrialist whose daughter had died of bovine TB, and who became a passionate advocate of pasteurisation at the many meetings he attended on the subject in Britain.

There are essentially two way of pasteurising milk. The first is to heat it in batches for 30 minutes at between 62.8C and 65.6C. The second is a continuous process known as high-temperature, short-time (HTST), where milk is passed over heat exchangers that can raise its temperature to 71.7C for about 15 seconds.

Although heating can kill harmful bacteria, it also carries the disadvantage of destroying the colour, taste and nutritional value of the milk. So- called "sterilised milk" (it is not technically sterile), which is heated to about 115C for 20 minutes, loses many of its vitamins. Ultra-heat- treated (UHT) milk is flash-heated to 135C for a second and is about as nutritionally valuable as pasteurised milk, but despite this many people find it less palatable than pasteurised milk.

STEVE CONNOR