When bugs have you on the run
This topic is sponsored by the Cooperative Research Centre for International Food Manufacture and Packaging Science.
Consumers are demanding food with fewer chemical preservatives and additives this means that good hygiene and safe storage conditions will have to play an even more important role in preventing food poisoning.
The chances of getting food-poisoning are really very low when the number of incidents are compared with the number of meals consumed. In Australia, where the population is about 21.3 million and most people eat at least three meals a day, the total number of 'eating opportunities' for food-poisoning to occur is over 23,323,500,000 per year!
Counting the sick
Statistics on food-poisoning are hard to collect because many cases are mild and not reported. Nevertheless, it is a common complaint. In 2003, the Food Safety Information Council estimated that there are approximately 5 million Australians affected by food-poisoning every year. Although this is a very large number and we all must work toward reducing it, it indicates that only 0.02 per cent of meals caused some form of food-poisoning.
What is worrying is that food-poisoning can be fatal. Two elderly people died after eating contaminated ham and corned beef in Victoria in February 1997; another Victorian died from food-poisoning in a nursing home in 1997; a child died after eating contaminated mettwurst in Adelaide in 1995. This triggered the development of new national food safety standards. A 2005 report found that approximately 120 people die from foodborne illnesses in Australia every year.
What causes food-poisoning?
Microorganisms are the most common cause of food-poisoning. Disease-causing microorganisms occur commonly in the environment and are found in soil, water, air and on animals (including humans). For this reason it is likely that some microorganisms will be present on uncooked food. It is important to handle food in such a way that the microorganisms present do not have a chance to multiply and to prevent food from becoming contaminated with other microorganisms.
Many of these microorganisms are bacteria single-celled organisms that can only be seen under a microscope. They include Salmonella, Listeria and Escherichia (Box 1: Common microbial agents of food-poisoning). Fungi, such as the moulds commonly seen on stale bread, can also cause illness, while viruses such as hepatitis A may also be food-borne.
Food-poisoning symptoms range from stomach cramps to diarrhoea, vomiting, headaches and fever. They vary depending on the pathogen causing them. Often the most dangerous effect of food-poisoning is dehydration, which can lead to death if untreated.
Who gets food-poisoning?
Healthy adults are usually less seriously affected by most forms of food-poisoning: the elderly, the young, and people already suffering from other diseases are most likely to succumb. In Australia, the annual death rate from intestinal infectious diseases many caused by food-borne pathogens is 1.4 per 100,000 amongst children less than a year old, 4.5 per 100,000 amongst people older than 75, and almost zero for people in the age range of 5-54 years.
The safe food tightrope
Every time we eat we have to rely on the many links in the food supply chain. The chain starts in the paddock and goes through various stages: washing, preparation, packaging, distribution and presentation. Everyone involved must take responsibility for their part. Most food-poisonings are the result of 'food abuse' during transport and final preparation, particularly in the home. When food-poisoning does result from food prepared in the home, it is rarely recognised or reported we usually try to find a restaurant or a manufacturer to blame. We also blame the last meal we ate before the onset of symptoms, even though the incubation period can be as long as 10 days.
Food abuse can take many forms. The three most common occurrences that can disrupt the safe-food chain are temperature abuse, cross-contamination and inappropriate processing methods.
Food poisoning bacteria grow best at 35-37°C (our body temperature). For example, E. coli bacteria take 7-10 hours to double in number at a temperature of 10°C, but only 1½ hours at 20°C and just 15 minutes at 37°C. When the temperature is increased, such as during cooking, most pathogenic bacteria are killed. When the temperature is decreased, such as in refrigeration, the growth of the bacteria slows. Refrigeration is used extensively to extend the shelf life of food.
Temperature abuse usually takes one of two forms:
- A failure in the 'cold chain' can lead to conditions in which bacteria thrive, such as when a refrigerator in a shop breaks down or when a shopping bag full of fresh food is left in the car for an hour or two.
- The second form of temperature abuse can occur during food cooking and storage. Microorganisms can grow when food is cooked at temperatures too low to kill the bacteria and then stored above 4°C.
A food-poisoning outbreak at a youth camp near Dubbo in 1994 was caused by the bacterium Clostridium perfringens. Cold chicken served at lunch had been cooked the day before and allowed to cool at room temperature for more than 2 hours before it was refrigerated. Result: 230 camp participants sick with gastroenteritis, of whom 118 were treated in emergency departments and 13 admitted to hospital.
Temperature abuse was also the likely cause of an outbreak of E. coli infection among Japanese school children in 1996. As a result, 6000 children fell ill and two died.
Another form of food abuse, cross-contamination, can occur when bacteria from contaminated food or utensils are transferred onto other food. This is particularly perilous if the newly contaminated food is not cooked before eating. For example, a knife used to cut contaminated meat might then be used to chop lettuce. The meat is then cooked and the microorganisms killed, but because the lettuce is not cooked it can cause food-poisoning.
A spectacular example of the dangers of cross-contamination - and, coincidentally, of mass-distribution occurred in the United States in 1994. Health investigators there found that cross-contamination of icecream pre-mix occurred because it was transported in tanker trailers that had previously been used to haul liquid eggs contaminated with Salmonella enteritidis. The contamination was not detected until the icecream had been distributed across the nation. Researchers estimated that 224,000 people in many different states contracted gastroenteritis as a result of eating the contaminated icecream.
Inappropriate processing methods and processing failure
Food-poisoning incidents may also occur when inappropriate processing methods are used or when food-processing systems fail. For example, mettwurst is fermented uncooked meat: this can hang for weeks in a butcher's window before being purchased and consumed. What stops it from going off?
The safe manufacture of sausages such as mettwurst relies on four factors:
- the addition of a 'starter culture' containing good bacteria that can out-compete spoilage organisms;
which lowers the pH to a level of acidity at which pathogens
die or stop growing;
- the addition of salt, which effectively removes moisture, an essential element for bacterial growth;
The system is dynamic and can break down. If, for example, the original meat supply was grossly contaminated, the fermentation process may not be capable of killing all microbes present. In addition, if controls on such factors as pH, humidity and temperature during fermentation are inadequate, further growth of pathogens may occur.
The contamination of mettwurst in Adelaide in 1995, which led to the death of a child, was caused by a breakdown in the processing system, although the precise nature of the breakdown was not identified.
Other reasons for the increase in food-poisoning have been proposed by health experts. The globalisation of the food supply means that harmful bugs common in one region may spread to other regions. Australia has so far remained free of some new and nasty bugs such as the strain of E. coli responsible for the school lunch contamination in Japan and Salmonella enteritidis, the bug responsible for the icecream fiasco in the United States. Nevertheless, experts consider it just a matter of time before such pathogens slip into the food supply chain here. The organism could be introduced into Australia either by infected travellers or by imported food.
Many consumers are expressing a preference for foods that are unprocessed or minimally processed and have fewer additives (eg, unpasteurised apple juice and preservative-free sausages). A reduction in the processing of some foods could be a factor in the increase in food poisoning.
Manufacturing processes such as canning, salting and acidification are designed to prolong shelf-life (Box 2: Food preservation). Refrigeration is increasingly used to prolong shelf-life in the face of increasing consumer preference for minimally processed foods. Maintenance of the cold chain has become even more critical.
Chemicals in food
Food poisoning from microorganisms is not the only food safety issue that concerns health professionals and thoughtful consumers. Some chemicals (eg, pesticide residues, food additives and antibiotics) may be added to food either intentionally or unintentionally (Box 3: Chemicals in food).
Industries in Australia and elsewhere are improving food safety through the introduction of what are known as hazard-analysis-critical-control-point (HACCP) systems. In these systems, the traditional reliance on testing the end-product for contamination is replaced by monitoring at key points along the chain. Scientists are also working to develop packaging and processing techniques that help keep food fresh and free from contamination.
Despite the advances of science, the basic rules for avoiding food contamination have been known for thousands of years, as people learned by trial and error to distinguish between safe and unsafe practice (Box 4: Safe cooking). If these rules are applied today at all points in the food supply chain, including how we use food in the home, our food should remain good enough to eat.
Page updated May 2008.