Sunday, December 10, 2006

To pasteurize or not?

Over the past few weeks the issue of milk pasteurization has exercised the people and government of Ontario.

The argument is not a new one: some people believe that the practice of milk pasteurization destroys the nutritional characteristics of the milk and reduces its value as a food. The official line is that the sale (or even giving) of raw milk to the public is illegal, though it is permitted with varying restrictions in 28 states in the USA. Pasteurization of milk has been mandatory in Ontario since 1938. Earlier, around 1900, over 10% of all childhood tuberculosis was thought to be caused by unpasteurized milk. The rate of tuberculosis infection and many other milk-borne diseases in children fell dramatically after the law enactment and was hailed as a major achievement.

To understand the reasons given for pasteurization, we need to go back to the source of milk. Most of our milk supply comes from cows, though goats and sheep are also milked commercially. All of these animals carry bacteria in their intestines and on their skin and feet. They may also carry viruses internally. Some of these micro-organisms can infect humans, occasionally with very serious consequences. Examples are Listeria monocytogenes, Salmonella, Campylobacter, Mycobacterium tuberculosis, Eschericia coli, Brucella and Staphylococcus aureus.

Inside the udder of a healthy animal, the milk is essentially sterile. However, even in a well-run milking shed, it is very difficult to prevent some contamination of the milk from the exterior of the udder. A quick check of the filters will often show a small amount of dirt or fibrous material that has originated either in the gut or on the hide of the cow. Bacteria are too small to be seen with the naked eye, but will be present in the material in the filter and the milk stream. We have to accept that raw milk will contain bacteria and possibly faecal viruses.

All of these micro-oganisms are destroyed by heating the milk to 72C for 15 seconds.

Proponents of raw milk consumption (and the manufacture of cheeses etc. from unpasteurized milk) claim that pasteurization also destroys enzymes in the milk that aid in digestion of the product; the cheeses have less flavour and are inferior compared with their unpasteurized counterparts. I don't intend to get into a debate in this post about nutrition - I am not a nutritionist. So, I will list the effects of just some of these micro-organisms on the human body and leave you, the reader, to do your own nutritional research.

Listeria monocytogenes rarely affects healthy adults; those at high risk include pregnant women, newborn infants, the elderly, and those with weak immune systems. The Centers for Disease Control and Prevention (CDC) estimate that 2,500 people become seriously ill each year in the United States and among these, 500 will die. According to research, pregnant women account for 27% of these cases. CDC claims that pregnant women are 20 times more likely to become infected than non-pregnant healthy adults. Babies may be stillborn or infected at birth. Refrigeration will not stop this micro-oganism from growing, though its growth rate will slow.

Salmonella causes abdominal cramps, diarrhoea, vomiting, headache and elevated temperature when the bacterial cells invade the mucosal cells of the gut. The acute symptoms usually last for only one to two days, though some patients show a much longer illness. Chronic consequences may include arthritic symptoms developing some 3-4 weeks later. Obviously, during the diarrhoeal phase, contamination of the environment and thus other family members can occur.

Campylobacter jejuni is now regarded as the leading cause of gastrointestinal illness in many countries. See earlier posting "So what is this Campylobacter thing?"

Mycobacterium tuberculosis is probably a human-specialised form of M. bovis, which was probably passed to humans from cattle when milk consumption began. There is archaeological evidence that this occurred between 4000 and 8000 years ago and then spread through the migrations of Indo-Europeans throughout the world ( www-micro.msb.le.ac.uk). The normal transfer route is through droplets released during coughing, but tuberculosis can also be transmitted in foods, including unpasteurized milk. If M. tuberculosis reaches the lungs and sets up infection, lung damage results in the longer term if an inflamatory response occurs.

Escherichia coli are found in the gut of man and animals. Microbiologists have always known that some strains of these micro organisms can cause diseases, such as "Travellers' diarrhoea". However, we now know that many strains can cause disease, some life-threatening. There also seems to be an increase in the distribution of these pathogenic strains in farmed animals, as we see from the number of recently reported cases of E.coli O157:H7 in 5 states in USA.

E. coli O157:H7 infection often causes severe bloody diarrhea and abdominal cramps that may last for 5 to 10 days. In some people, particularly young children and the elderly, the infection can also cause hemolytic uremic syndrome (HUS), in which the red blood cells are destroyed and the debris causes kidneys failure. About 8% of patients may develop HUS. Sometimes the patients recover after a long illness, but a significant number of children have required kidney transplants.

Brucella species are passed between animals in many countries, but can also affect humans. Unpasteurized cheeses, often called "Village Cheeses" sold in the Mediterranean Basin, Central America, Eastern Europe, Asia, Africa, the Caribbean and the Middle East may be the source of infection for travellers. The disease may produce flu-like symptoms - fever, sweating, headache, back pain, and physical weakness. However, severe infections of the central nervous systems or heart lining may occur. Brucellosis may also manifest as "undulant fever", a recurrent fever accompanied by joint pain and fatigue.

Staphylococcus aureus produces a heat-stable toxin when it grows in foods. The toxin usually causes vomiting and the effect may become apparent within 30 minutes of consuming the affected food. Diarrhoea may also be present. The symptoms do not usually last for more than 1-3 days and are not life threatening (though those suffering the symptoms may wish that it could be all over!)

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