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!)

Campylobacter

So what is this Campylobacter thing?

Campylobacters are spiral-shaped bacteria (see picture above). They can infect both humans and animals, including birds. About 99% of the Campylobacter infections of humans are caused by just one species - Campylobacter jejuni. The micro-organisms appear to grow well in birds, which have a slightly higher body temperature than humans. Though chickens are often carriers, they show no symptoms of infection, so it's not possible to tell that the bird is infected without doing microbiological testing. C. jejuni does not grow below about 30C, so the presence of high numbers on chicken are unlikely to be the result of temperature abuse.

In humans the disease usually takes the form of abdominal pain and diarrhoea, which may contain blood, fever (typical of the body response to an infection), and vomitting may also occur. The symptoms usually last for about a week and usually no special medical intervention is required, but sometimes the patient requires hospitalization. There are some potential long-term consequences of infection - arthritis or Guillain-Barré Syndrome may follow infection. This is an auto-immune disease caused by the body's own defence system being triggered to attack the nerves. The incidence of Guillain-Barré Syndrome is estimated to be about 1 per 1000 cases.

Human infection is probably caused mainly by handling raw poultry, though the organisms may also be carried by farm animals and can be found in water supplies that have been contaminated by farm run-off. Though consumption of undercooked poultry has been suggested as a cause, in my opinion cross contamination in the kitchen (see "Should I eat the chicken?") is more likely.

The bacterium is easy to destroy by cooking. It cannot tolerate drying and can be killed by oxygen, so microbiologists have to take special precautions to handle and culture it - we grow Campylobacter in special containers where we can reduce the amount of oxygen present, replacing oxygen with carbon dioxide.

Should I eat the chicken?

A recent publication in New Zealand caused a lot of interest by claiming that sale of raw chicken should be banned, in favour of frozen chicken. The basis for the claim was that chicken meat is routinely contaminated with Campylobacter. The freezing process would kill the Campylobacter and render the meat safe.

Was this claim justified?

First we need to understand something about chicken farming, chicken processing and, not least, Campylobacter.

Let's start with Campylobacter. Campylobacter is a bacterium commonly found in animals and the environment. It can get into food via many different contamination vectors (carriers), including untreated water, birds, insects and animals. It is often found in the farm environment, which may be contaminated by animals.

The number of Campylobacter we need to eat to become infected is very low; various estimates put the "infecting dose" at between 6 and 800 cells. This means that even low levels of contamination in food can cause a disease if the food is consumed. Even contact with infected animals, for example in petting zoos, can put sufficient bacteria onto the fingers to give an infecting dose if they are put into the mouth (a problem with little children) or used to prepare food without first washing them carefully.

"But surely" you are asking "we cook chicken before we eat it". Well, yes we do. Assuming that we cook it properly, the Campylobacter cells will be killed and the chicken is safe to eat. But supposing that we handle the raw chicken and then prepare a salad; or we use the same chopping board to prepare the raw meat and then cut the salad on the same board, using the same knife? If we don't wash and sanitize the knife and board, we can "cross contaminate" the food from the chicken, via the knife and board.

The suggestion that raw chicken should be banned, and only frozen chicken sold now has some attraction. "Freezing kills Campylobacter, so that will make the chicken safe and we won't get cross contamination, right?" Well, it goes some way to reducing the contamination. But, as with most things to do with bacteria, it's not black and white. Again, estimates vary; freezing may destroy tens of millions of Campylobacter, but equally may have little effect. FAO/WHO data suggest a hundred-fold reduction if the product is frozen and held for a minimum of 3 weeks. A reduction of a hundred-fold sounds a lot, but in microbiological terms this might still leave millions of bacteria on the product. And as we have seen, this is plenty to give an infecting dose.

So, to go back to the title of this posting, should we eat the chicken? My personal opinion is a qualified "Yes". The caveat is that we must make sure to handle the raw chicken safely, preventing cross contamination of utensils, surfaces and other foods in the kitchen. Cook the meat properly and prevent recontamination.

The New Zealand Food Safety Authority (NZFSA) says that two of the most important ways of keeping food safe are to follow the 4Cs rule – Clean, Cook, Cover and Chill the food. Also follow the 20+20 hand wash rule before and after handling poultry and preparing food - wash your hands, using plenty of soap and warm water for at least 20 seconds, paying careful attention to finger nails and the webs between fingers. Rinse well and dry them for a further 20 seconds, using a clean dry hand towel or disposable paper towel. It has been shown that wet hands can transfer microbial contamination to utensils, refrigerator door handles, taps and other potential cross contamination points, so the drying is just as important as the washing.

Odd Spot: On 3 December 1988, Edwina Currie, the outspoken junior health minister in the UK Thatcher government was quoted as telling ITN, "Most of the egg production in this country sadly is now infected with salmonella." Sales of eggs plummeted 60 per cent overnight. The loss of revenue forced farmers to slaughter four million hens and 400 million unsaleable eggs had to be destroyed. The government had to provide a multi million pound rescue package for the poultry industry and the unfortunate Ms. Currie resigned on the 17th December. Undoubtedly, there was some substance to her claims. Vaccination of hens to protect against salmonella has now greatly reduced the levels of egg contamination to the present 0.3% reported by the latest FSA survey and since 1997 the incidence of salmonellosis in UK has fallen by a factor of about 4x. However, there is still a concern that eggs on sale in UK may contain Salmonella Enteritidis organisms, perhaps as a result of importation of eggs produced by unvaccinated hens.

Perhaps Edwina was right. Only time will tell if the advocates of frozen chicken are right too.

Bacteria and food

By far the majority of food poisoning is caused by allowing bacteria to grow in foods to the point where they either make the food toxic (poisonous) by virtue of the by-products they secrete into the food, or because they grow to a population large enough to cause an infection in the consumer when the food is eaten.

In either case, the food has normally been "abused" in some way, perhaps leaving it in a car parked in the sun, or leaving food out on the kitchen table instead of putting it in the refrigerator.

Where do these bacteria come from? Well, bacteria are all around us - they help break down wastes, such as dead plant material or dead animals. If they didn't, life would soon become impossible on Earth, as all the organic matter would be locked up in these dead organisms. Bacteria are normal inhabitants of soil, the intestinal tract of man and animals and on the hides and hair.

Only a very small proportion of all the bacterial types on Earth are pathogens (organisms that can cause disease) and even fewer of those are transmitted in foods. In fact, the most important of these micro-organisms are members of just a few genera: Salmonella, Clostridium, Listeria, Escherichia, Campylobacter and Staphylococcus. There are others, but these will do for a start. Over time, this blog will contain lots of examples of these bacteria.

At first sight, the names of these groups are enough to put anyone off reading any further. But once you have seen them a few times, you'll recognize them (though only a microbiologist would think of calling them "Old friends"!)

I didn't really answer the question about where these things come from. Well, if they are in the environment, they'll get into food by many routes. They might get onto meat as the animal is slaughtered and butchered; careless food handlers might go to the toilet at break time and not wash their hands on their return - this could transfer bacteria from their own gut to the food when they handle it; flies and other insects may act as carriers or vectors to transfer bacteria from dirty places to food; vermin, such as rats and mice may also transfer bacteria around. Even birds may get into food premises and deposit droppings on surfaces, equipment or directly onto food. Sometimes the entry route is much less obvious, such as bird droppings on the roof getting washed by rain into a food premises through cracks in the roof.

All this comes down to keeping food clean, keeping it cold and not allowing it to be exposed to the environment for any longer than necessary. There is a lot to be said for cooking food and serving it straight from the pot.

Introduction

I am a professional food microbiologist and I'm creating this blog to help ordinary folk and food manufacturers to understand food safety so that we can reduce the incidence of food mediated illness. I would like it to become a resource for teachers in schools.

I also want to dispell some of the myths about food safety. If readers send comments or questions, I'll try to incorporate them into future postings.

Obviously, some of the posts will be my own opinions, but I'll try to keep these comments based firmly in science.

I hope you find the site useful and perhaps even thought-provoking.

Regards
John D Brooks
November 2006