Virtually all the milk and cheese you buy in grocery stores today has undergone a sterilization process called pasteurization, which kills germs and makes dairy products safe to eat. This process was developed by the French microbiologist Louis Pasteur (1822–1895), who was instrumental in proving where germs come from and what they do.

Trained as a chemist, Pasteur went to work in 1854 researching solutions for the production problems of French manufacturers of alcoholic drinks. He invented pasteurization— the process of heating and then cooling liquids to kill bacteria—as a way of preventing wine and beer from going sour, an invaluable discovery for France’s brewers and vintners. Pasteur also did much to disprove the ancient theory of spontaneous generation. In a famous experiment, he monitored soup in three groups of flasks: one that was open to the air, another that was sealed tightly with cotton, and a third that was fitted with a swan-necked spout that kept air out. Only the soup in the swan-necked flask was unexposed to air and did not grow bacteria, proving that microbes must be introduced from the environment.

Applying pasteurization to dairy products sharply reduced foodborne illnesses in France. Before the advent of pasteurization, many diseases were transmitted through raw milk. Straight from the cow may seem like the best way to drink milk, but in fact bacteria from within an animal’s body can make people fatally ill. Animals also collect foreign organisms on and around their udders, and these organisms end up in raw milk as well.

Traditionally, the process of pasteurization involved heating milk to a temperature just short of a boil and keeping it there long enough to ensure that the bacteria were dead. Commercial cartons of milk today, however, are usually heated to 285°F for 1 to 2 seconds in a process called ultrahigh-temperature pasteurization. This method of treating dairy products has the added benefit of extending the amount of time before they will spoil in the fridge.

Some people today claim that unpasteurized milk is healthier and better tasting, but scientists warn against the danger of foodborne illnesses caused by pathogens such as Escherichia coli, Listeria, and Salmonella. It is illegal to sell raw milk across state lines and in more than 20 states.

ADDITIONAL FACTS

  1. Pasteur saved a troubled silk industry in southern France—where silkworms were dying at an alarming rate—by identifying parasitic infections among the silkworms and advocating that only disease-free eggs should be selected.
  2. Pasteur also enhanced the practice of vaccination by helping to explain the causes of anthrax, cholera, smallpox, and tuberculosis and developing a vaccine for rabies.
  3. Milk today goes through another process called homogenization, which prevents it from separating into fat-free milk and full-fat cream. The procedure breaks up molecules of fat into such small particles that they remain suspended throughout the liquid and create a smooth, uniform texture.