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For most people today, a glass of milk is a safe and ordinary part of daily life. However, this has not always been the case. In the past, milk was one of the most dangerous foods a family could buy. It spoiled quickly, and it often carried bacteria that caused serious illness. The change came from a simple idea: heating milk to a carefully chosen temperature for a set amount of time. This process is called pasteurization. It is now used in dairies all over the world, and it has become one of the most important developments in the history of food safety.

The process takes its name from Louis Pasteur, a French scientist who worked in the middle of the nineteenth century. Interestingly, Pasteur did not begin by studying milk at all. In the 1860s, he was asked to help winemakers whose products were turning sour. Through his microscope, he observed that tiny living organisms were responsible for the spoilage. He then discovered that gently warming the wine to around 55 degrees Celsius killed many of these organisms without ruining the flavour. This finding supported his wider belief that microbes, rather than bad air or chance, caused food to go bad and many diseases to spread.

At first, the method was used mainly for wine and beer. Milk was a different problem, and it became an urgent one as cities grew. In the late 1800s, large numbers of people moved from farms to industrial towns. Milk now had to travel long distances, often in open containers and without any cooling. Some city dairies kept cows in crowded, dirty sheds, and the animals were fed poor-quality waste from breweries. As a result, the milk sold to urban families was frequently contaminated. It was linked to outbreaks of typhoid, scarlet fever and diphtheria, and it was also a major route for tuberculosis to pass from cattle to humans. Young children, who drank the most milk, suffered the highest death rates.

Doctors and public health campaigners soon realised that heat treatment could solve much of this problem. In the 1880s and 1890s, scientists in Germany and elsewhere recommended heating milk for infants, and some charities opened milk stations that supplied treated milk to poor mothers. Commercial pasteurization machines appeared towards the end of the century. However, progress was slow. Many farmers worried about the cost of new equipment, and some consumers believed that heated milk tasted worse or was less natural. It was only in the early decades of the twentieth century that many cities began to require pasteurization by law. Where these laws were introduced, deaths from milk-borne diseases fell sharply.

The science behind pasteurization is quite straightforward. Most harmful bacteria cannot survive high temperatures for long. Heat damages their proteins and cell walls, and they stop working. The key is to find a balance. If milk is heated too much, its taste and colour change and some nutrients are lost. If it is not heated enough, dangerous microbes may survive. For this reason, pasteurization is always defined by two factors together: temperature and time. A lower temperature can be effective if it is held for longer, while a higher temperature needs only a very short period.

Over the years, three main methods have developed. The oldest is the 'batch' or low-temperature method, in which milk is held at about 63 degrees Celsius for 30 minutes. This is still used by some small producers. The second, and today the most common, is high-temperature short-time pasteurization. In this method, milk flows continuously between thin metal plates and is heated to about 72 degrees for just 15 seconds before being cooled quickly. In addition, there is ultra-high-temperature treatment, often called UHT, which heats milk to around 135 degrees for two to five seconds. UHT milk is packed in sealed cartons and can be stored unopened at room temperature for several months. Modern plants also save energy by using hot, freshly treated milk to warm the cold milk that is coming in.

It is important to understand what pasteurization does not do. Standard pasteurization is not the same as sterilization. It kills disease-causing bacteria, but it does not remove every microbe. This is why ordinary pasteurized milk must still be kept in a refrigerator and used within a week or two. Furthermore, the process cannot make poor-quality milk good; clean farms and careful handling are still essential. As for nutrition, research shows that the main nutrients in milk, such as protein and calcium, are largely unchanged. There are small losses of some vitamins, including vitamin C and certain B vitamins, but milk is not an important source of these vitamins in most diets.

Even today, some people prefer to drink raw milk. They often claim that it has a richer flavour and more health benefits. However, most food safety authorities disagree. They point out that raw milk can still carry harmful bacteria, such as certain types of E. coli and Listeria, and that the risks are greatest for children, pregnant women and older people. To check that milk has been treated correctly, laboratories test for an enzyme called alkaline phosphatase, which is naturally present in raw milk but is destroyed by proper heating. In this way, a discovery first made in a nineteenth-century wine cellar continues to protect millions of people every day.

The science and history of pasteurization in dairy products

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