The value of heat for the preservation of food has been known for thousands of years, but it was not realized until the nineteenth century that a very mild heat treatment far below the boiling point, made liquid foods such as milk keep much longer. The discovery followed the work of the French scientist Louis Pasteur on wine and beer.
The process, called after him ‘pasteurization’, is a carefully controlled mild heat treatment. It was found that the process served two purposes; it prevented the souring of milk, and it destroyed the dangerous disease germs which sometimes occur in this product. These germs include the bacteria which cause tuberculosis, undulant fever, typhoid and paratyphoid fevers, dysentery, diphtheria, scarlet fever and septic sore throat.
It has long been know to bacteria experts that the tubercle bacillus is the germ in milk which most strongly resists heat treatment. To destroy this organism it is necessary to heat milk to about 60oC. for 15 minutes, and its destruction has always been taken as a way of testing the efficiency of pasteurization. A heat treatment of this kind destroys about 99% of the common bacteria in milk, including nearly all those which cause milk to turn sour.
To ensure the certain destruction of tuberculosis and other disease germs in milk, it must be held at a fixed temperature for a fixed time. In Britain, for example, these conditions were defined by law in 1923 as 63-66oC. for 30 minutes. This became known as the ‘holder’ process, since the raw milk had to be pumped into a large tank, heated to just over 63oC., held in the tank for half an hour and then pumped out and cooled. This was a slow process and required a very cumbersome plant, so scientists worked for many years to produce a simpler, more convenient method, with less bulky equipment.
The latest method, officially approved in Britain in 1949, is known as the high-temperature-short-time, or H.T.S.T. method. It has now almost entirely replaced the ‘holder’ process. In the H.T.S.T. system, the milk flows continuously through many sections of thin stainless steel pipes. During the process, the milk is held at 72oC. for at least 15 seconds, then, as it cools, the heat it loses is used, in part, to raise the temperature of the incoming milk in a device called a ‘heat-exchanger’.
Efficient pasteurization may reduce the bacteria in raw milk from, say one million to only a few thousand per cubic centimeter. The bacteria left are chemically mostly of the inert type, that is, they either do not sour milk at all, or sour it only slowly. Very strict cleanliness is, however, essential and all pipes, containers and bottling machines in a pasteurizing plant must be cleaned and sterilized daily. If the slightest trace of dirt remains all the benefits of pasteurization are wasted.