Pasteurised Milk

May 11, 2012 by

One thousand bacteria laid end to end would cross the head of a pin

Microbes are everywhere. Tiny living organisms, feeding, multiplying, in the air, the soil, on our hands, in food, in our intestines. They maintain the cycle of life, growth and decay. Microbes are the microscopic creatures for which we have many names: we call them germs, moulds, yeasts, bacteria, viruses. One thousand bacteria laid end to end would cross the head of a pin. In every gram of fertile soil there exist about one hundred million living bacteria. Most of these teeming microbe populations are harmless. Many are very useful, but some can cause disease in men, animals and plants.

Anton van Leeuwenhoek, a Dutchman, was the first to learn of the existence of microbes; he saw some squirming and multiplying under the newly-invented microscope in 1674. Leeuwenhoek spat on a glass plate and looked in astonishment at this new world of living creatures that inhabited his mouth. But until the middle of the last century men knew almost nothing about microbes or what they did. The fact that we cat, sleep, live and breathe microbes has only been realized during the last hundred years or so. Most of modern medicine and hygiene depends on understanding the existence and role of microbes. The man who did most towards this understanding was the great French chemist Louis Pasteur.

“How explain the disintegration of a dead body or a fallen plant?2 asked Pasteur. “How account for the working of the vintage in the vat? Of dough left to rise and then souring? Of curdling milk? Of straw ripening on the dungheap? Of dead leaves and plants buried in the soil and turning to humus?” Accepted theories were most confused. Some leading scientists thought these changes were chemical, that they were caused perhaps by the oxygen in the air. Many scientists believed in something called spontaneous combustion: that some things just sprang into life, that maggots, for example, were ‘made’ by putrid meat. Pasteur’s research and thinking and experiments made him decide that all living things can come only from other living things. And that changes like decay and fermentation (which he investigated carefully in the making of beer and wine) were caused by living things – microbes – and not by chemical reactions, as other scientists thought Pasteur was only thirty-four in 1857 when he published an important paper putting forward these theories.

Pasteur realized that some microbes perform useful tasks which could be interrupted by other harmful microbes. From this, he and other scientists began to understand how microbes can cause different infections and diseases, and therefore how to begin to combat them. Antiseptics and disinfectants began to be used in medicine with real understanding to control infection. A few doctors – and this was only a hundred years ago – had already realized that dirty hands and dirty clothes spread infection, but had not known why. They were laughed at and criticized for their stupid ideas. How could a dirty shirt-sleeve have an invisible, living, disease-carrying agent on it? By the mid-nineteenth century, some people knew that water polluted by sewage and the muck of towns could carry diseases like the cholera that swept England in four epidemics after 1830, killing about one hundred thousand people. But they didn’t know what it was in the water that did this. In 1861 Queen Victoria’s beloved husband, Prince Albert, fell sick of typhoid fever, probably contracted from contaminated water. All that the best doctors could do was diagnose a ‘slow fever’, feel his brow, advise him to rest, and wait while the illness ‘took its course’… and the Prince Consort died, aged forty-four.

Milk, that great all-in-one food for humans, turned out to be an excellent food for bacteria. Disease-carrying bacteria could get in any time during handling of the milk from cow to customer. Some bacteria which could infect humans came straight from diseases in the cow itself, such as the dangerous and common bovine tuberculosis. Pasteur’s researches showed that heating a liquid food to a certain temperature kills most of the harmful bacteria present. The process came to be called pasteurization. But it took a long time to be applied to milk.

Getting milk from the cow to the customer before it spoils has always been a problem. Milk lasts only if it is turned into butter or cheese. As long as towns stayed fairly small, cows could be kept close enough to people’s houses for the milk to be delivered while it was still fresh. Sometimes (and this still happens in parts of the world) the cow was walked from house to house and milked straight into a container provided by the customer. Or open carts carried milk through the streets, and people bought what they needed. There were plenty of chances for dirty handling and bacterial infection. Cows kept in New York City were fed on such poor food they gave no cream. When the first trains brought milk in from the country in 1842, people complained. What was this yellow scum on top of their milk?

Towards the end of the nineteenth century, German researchers gathered evidence about the bacteria in milk which were harmful to humans, and the German Emperor insisted that commercial dairies pasteurize their milk. It was already being done in Denmark. But the idea of pasteurization did not really catch on. People did not like their milk being messed around with, or drinking what they called ‘dead bodies’ in it. As early as 1886 a German chemist had advised mothers to feed babies with sterilized milk in sterilized bottles, but that was not popular either.

In the 1890s a German immigrant to New York who had made lots of money went home to the old country, where he heard about pasteurization. He came back to New York and opened a milk kitchen in a poor area of the city where he had once lived, giving free pasteurized milk to anyone who came. Hardly anyone did. But the man would not give up. With the help of a biologist from the university, he decided to conduct an experiment. Everyone in one row of streets was persuaded to get their milk from his free kitchen. Everyone in the next row bought theirs as usual from milk sellers in the streets. A careful count of the cases of scarlet fever and diphtheria in the two areas was kept. The results were such startling proof of the value of pasteurization in cutting down disease that New York City soon passed a law requiring pasteurization in all commercial dairies.

But arguments and prejudice still continued. Some people were convinced that pasteurization was harmful, or wasn’t necessary. Why interfere with cows’ milk? Careful research and scientific proof made no difference, once people had made up their minds. Certainly, improved methods of dairying and milk handling greatly reduced the risk of bacterial infection. Pasteurization became much more accepted and widespread in England during the Second World War, and today almost all milk there is treated.

The exact temperature for pasteurization varies from country to country. Generally it means heating milk to between 61-63 C for thirty minutes, or 71-72 C for fifteen seconds. The taste does not change, no nutritional value is lost, and the milk is now quite safe to drink. It also lasts much longer because many of the bacteria which turn milk sour are killed. Pasteurization is not a complicated process. But it must be carefully controlled, and of course it is no use if bacteria are allowed to infect the milk again afterwards through careless handling.

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