![]() Tubercle bacilli are more resistant to heat treatment than coliform bacteria. These two combinations, 70 ☌/1 s and 65 ☌/10 s, consequently have the same lethal effect. At a temperature of 65 ☌ it takes a holding time of 10 seconds to kill coliform bacteria. According to these curves, coliform bacteria are killed if the milk is heated to 70 ☌ and held at that temperature for about one second. Figure 6.1.1 shows lethal effect curves for Coliform bacteria, Typhus bacteria and Tubercle bacilli. The combination of temperature and holding time is very important, as it determines the intensity of the heat treatment. To overcome these problems, heat treatment must be applied as quickly as possible after the milk has arrived at the dairy. In addition, the constituents of the milk are degraded, the pH drops, etc. Longer intervals between deliveries mean that, despite modern cooling techniques, microorganisms have more time to multiply and to develop enzymatic systems. This secondary purpose of heat treatment has become more and more important as dairies have become larger and less numerous. This requires more intense heat treatment than is needed to kill the pathogens. Hence, a secondary purpose of heat treatment is used to destroy as many as possible of these other organisms and enzymatic systems. can be relied upon to destroy all other pathogens in milk.Īpart from pathogenic microorganisms, milk also contains other substances and microorganisms that may spoil the taste and shorten the shelf life of various dairy products. is therefore regarded as the index organism for pasteurization: any heat treatment that destroys T.B. Complete safety can be assured by heating milk to 63 ☌ for 30 minutes. ![]() ![]() The most resistant organism is the tubercle bacillus (T.B.), which is considered to be killed by heating milk to 63 ☌ for 10 minutes. The absence of phosphatase indicates that the milk has been adequately heated.įortunately, all common pathogenic organisms likely to occur in milk are killed by relatively mild heat treatment, which has only a very slight effect on the physical and chemical properties of milk. In addition, its presence or absence is easily confirmed (Phosphatase test). This enzyme is always present in raw milk and is destroyed by the temperature/time combination necessary for efficient pasteurization. In the middle of the 1930s (JDR:6/191), Kay and Graham announced the detection of the phosphatase enzyme. Milk was frequently either overheated or underheated, so that it either had a cooked flavour or was found to contain viable T.B. In considering the history of pasteurization it is worth mentioning that although scientists everywhere agreed fairly closely on the necessary degree of heat treatment, the process was very loosely controlled in commercial practice for a long time. The pasteurization of milk is a special type of heat treatment, which can be defined as “any heat treatment of milk that secures the certain destruction of tubercle bacillus (T.B.) without markedly affecting the physical and chemical properties of the milk”. The term “pasteurization” commemorates Louis Pasteur, who in the middle of the 19th century made his fundamental studies of the lethal effect of heat on microorganisms and the use of heat treatment as a preservative technique. Diseases such as tuberculosis and typhus were sometimes spread by milk. By the end of the 19th century, heat treatment of milk had become so commonplace that most dairies used the process for some purpose or another, such as for milk intended for cheese and butter production.īefore heat treatment was introduced, milk was a source of infection, as it is a perfect growth medium for microorganisms.
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