The main goal of pasteurization is to kill lactic acid bacteria (which make milk go sour) so as to extend milk shelf life. However, raw (unpasteurized milk) may contain also pathogenic bacteria, specifically Mycobacterium tuberculosis, and these are also killed by the process. This is the main reason why there is a general recommendation to avoid drinking raw milk, and an embargo on selling it in many countries.

Interestingly, M tuberculusis was first identified by Koch, who was the main rival of Pasteur, the inventor of pasteurization.

Pasteurization is a relatively mild heat treatment, (usually performed below 100°C) which is used to extend the shelf-life of milk for several days. It preserves the milk by the inactivation of enzymes and destruction of heat-sensitive micro-organisms, but causes minimal changes to the nutritive value or sensory characteristics of a food. Some heat-resistant bacteria survive to spoil the milk after a few days, but these bacteria do not cause food poisoning.

For pasteurized milk, it is possible to ensure that pasteurization has been adequately achieved by testing for the presence of the enzyme phosphatase. The destruction of phosphatase is regarded as a reliable test to show that the milk has been sufficiently heat-processed, because this enzyme (present in raw milk) is destroyed by pasteurization conditions.

Pasteurization of milk was inhibition 99% from flora in milk . The target organism of pasteurization was Coxiella burnetii. It's kill in 63 C at 30 min.

The micro-organism target is Coxiella burnetti. The C. burnetti is inactivated before alkaline phosphatase enzyme, inactivated by the heat treatment by 72ºC/15 seconds. So the enzyme phosphatase can be used to verify the heat treatment efficiency.

Mycobacterium tuberculosis

Coxiella burnetti

The target of pasteurization of milk is to remove pathogens and spoilage organisms. In pasteurization, there is no complete removal of microorganisms. Around 85-99% microbial reval takes place. 

Pasteurization:

Definition: Heat treatment of a dairy product to such an extent that all pathogens are destroyed, and the result of the phosphatase test is negative in accordance with the current relevant national legislation.

The most important diseases that might be transmitted via milk include:

Disease Main sources

Brucellosis cow

Tuberculosis cow; man

Staphyloccocus aureus enterotoxicosis man (nasopharynx, skin); cow

Streptococcal infections man; cow

Listeriosis environment (silage); cow

Q fever (Coxiella burnetti) cow

There are many other diseases that can be spread via milk, including:

• anthrax,

• Escherichia coli,

• leptospirosis,

• paratyphoid fever (Salmonella),

• shigellosis,

• foot-and-mouth disease (not to humans)

• toxoplasmosis.

What is pasteurization?

Pasteurization is the heat-treatment of milk to kill pathogenic (disease-causing) micro-organisms. It is done in such a way as to change the nature of the milk as little as possible. It is also used for products other than milk to achieve the same objective. The technique was developed by Louis Pasteur in the 19th Century, and has been used widely in many countries of the world, particularly since the 1950’s, with a dramatic benefit in terms of improved health.

The technique involves heating the milk up to a temperature high enough to kill the disease-causing bacteria, but with the objective of altering the chemical structure or nutritional value as little as possible.

Two main techniques are used nowadays:

1) The “HTST” method: The milk is heated to 72°C for 15 seconds and then cooled immediately. “HTST” stands for “High Temperature Short Time”, but it is obvious that the temperature is not really very high, and is a long way from boiling.

2) The “Batch” method: The milk is heated to 63°C and held at that temperature for 30 minutes. This method is favoured by cheese manufacturers who find that the high temperatures have an effect on the way in which the curd forms during cheese making, which therefore has an effect on the texture of the cheese. They must pasteurize the milk before processing in order to kill any “wild” bacteria or yeasts that would spoil the cheese or be a disease hazard. After pasteurization, they add a culture of specially selected bacteria to produce the particular cheese desired.

Other heat-treatment processes used to preserve milk are Ultra High Temperature (UHT) treatment and sterilization. These are much more severe than pasteurization.

Why is pasteurization necessary?

Heat treatment (such as pasteurization) of milk is necessary to prevent the transmission of diseases through milk. The most commonly considered pathogens are those causing tuberculosis and undulant fever (Brucellosis).

* Tuberculosis: Cows carrying tuberculosis can be identified by a veterinarian using skin tests. Any cow identified as a carrier should be slaughtered. In this way it is possible to establish a dairy herd that is free of tuberculosis. It is also essential to test the people working with the cows to make sure they are not infecting the cows or contaminating the milk.

*Brucellosis: Brucella abortus causes abortions in cattle and undulant fever in people. It can be transmitted through milk. It can be detected in milk by using the “ring test”. Prevention is easily achieved by vaccinating all heifers at about six months of age. Although it is easily prevented, many herds are not free of the disease. In goats a similar disease is caused by Brucella melitensis, which causes Malta fever in people.

A question often asked is: “If a herd is free of Tuberculosis and Brucellosis, why then should pasteurization be necessary?”

The answer to this question is that there are many other diseases that can also be transmitted through milk. Pasteurization will eliminate the pathogens, and make the milk safe.

Disease-producing organisms that have been identified in unpasteurized milk include:

Campylobacter, Clostridium perfringens, Corynebacterium diphtheriae, Escherichia coli, Leptospira, Listeria monocytogenes, Pasteurella, Pseudomonas, Salmonella, Shigella, Staphylococcus, Streptococcus, Vibrio (cholera) and Yersinia.

Illnesses arising from contaminated milk include throat infections and diarrhoea, but some diseases are potentially very serious. The source of the infectious organisms may be from the milk itself, or from post-milking contamination.

“Adequate heat treatment, as currently practised in commercial pasteurization of milk, is satisfactory in preventing any notable outbreaks of communicable diseases, provided post-pasteurization contamination has not occurred.” (Heeschen, 1987). A number of viruses have also been shown to be eliminated by pasteurization (Lawrence, 1994).

How is milk affected nutritionally when it is pasteurized?

Boiled milk, UHT milk or sterilized milk is noticeably altered in its physical characteristics, so that it tastes significantly different. Milk that has been pasteurized correctly is not significantly altered. However, concurrent procedures such as homogenization (breaking down the fat globule structure so that the cream does not separate out) may be carried out at the same time, and may be confused with pasteurization.

Nutritionally, pasteurization has negligible effects. The major nutrients in milk are protein, milkfat, and lactose, and together with the important minerals such as calcium and phosphorus, these nutrients are not effected by pasteurization.

Heat-treatment has been shown to affect some minor nutrients and other components of human milk, but results have been variable. Vitamins A, D, E, B2, B6 choline, niacin and pantothenic acid have been shown to be reduced between 10% and 50%. Lactoferrin, lysozyme and immunoglobulins were not significantly affected (Lawrence, 1994). These data were derived from the analysis of human milk, where paediatricians were assessing methods of storing mothers’ milk for use for babies in hospital. The principles will not be different for other milks.

How is pasteurization done?

Pasteurization is done commercially using expensive equipment that automatically ensures the process is precisely carried out. Small quantities of milk may be heat-treated effectively in the home by heating milk to the required temperature using a water-bath on the stove or a steamer pot. If the milk is heated in the microwave, it should be stirred periodically to ensure that the heat is distributed evenly throughout. If a thermometer is not available, bringing the milk nearly “to the boil” should suffice.

Some relevant references:

1. Is alkaline phosphatase the best measure of the effectiveness of pasteurization?

Williams , D.J. & Nottingham, S.M. (1990).

Suitability of a modification to the Aschaffenburg and Mullen alkaline phosphatase test for goat's milk: collaborative study.

Food Research and Technology Branch, Queensland Department of Primary Industries, 19 Hercules Street, Hamilton, Queensland 4007, Australia.

Australian Journal of Dairy Technology 45: (1) 21-23.

The low alkaline phosphatase (AP) activity of goat milk makes the standard Aschaffenburg and Mullen (AM) test unsuitable for assessing pasteurization of goat milk. A modification of the AM test, in which 3ml milk was used instead of 1ml, and a 4h incubation instead of 2h, increased sensitivity of the test. Goat and cow milk samples were heated to 95°C for 1min and 0.2, 0.4, 0.6 or 0.8% of the corresponding raw milks were added. Samples were tested for AP by 23 Australian laboratories using both the standard and modified AM. For goat milk the modified AM detected a significantly lower AP concentration than the standard AM, av. 0.24% and 0.54% respectively (p

Formerly the target microorganism in milk pasterization was Micobacterium tuberculossis. But it was later discovered that Coxiella burnetii is more heat resistant than the former. 

Thanks a lot. Very useful information.

Earlier knowledge was that Mycobacterium tuberculosis. But it was later found out that Cooxiella burnettii is the most heat resistant bacteria in milk and we taget to kill it in milk pasteurization, then all the others are killed as they are less heat resistant.

yes it is the Cooxiella burnettii  organism which is heat resistant but killed at 63 for 30 minutes