A bulletin for the Australian Food Industry November 2002
Contents: Listeria in ready-to-eat foods | Effect of common sanitisers on Listeria monocytogenes | National Risk Validation Project | Validation of food safety control measures
Listeria in ready-to-eat foods
The recent foodborne listeriosis outbreak in north eastern states of the US in which at least 44 people in eight states have become ill from the same strain of the organism serves as a reminder of how serious this disease can be to specific groups within the population. The outbreak has claimed at least seven lives and three pregnant women have had miscarriages or stillbirths. Of the seven patients who died, six had immunocompromising conditions, three were aged 65 or over and one was a neonate (ProMED- mail, 15 October 2002).
At the time of writing, sliced turkey deli meat is the suspected vehicle of infection and a massive recall of one company's ready-to-eat turkey and chicken products has taken place. The Listeria strain so far detected in the company's turkey deli meats is, however, different from that involved in the outbreak. However the strain responsible for the outbreak has been found in at least one environmental sample from the implicated plant (Food Chemical News, October 21 2002).
In a further development, a second US poultry processing company is withdrawing its ready-to-eat chicken and turkey products which have been sold nationwide. Tests have shown that some of this company's products do contain the same strain of Listeria monocytogenes linked to the outbreak (ProMED-mail, 5 November 2002).
Closer to home in October an Australian smallgoods producer conducted a voluntary recall of a variety of sliced, cooked meat products. The company had detected Listeria monocytogenes in its products during routine testing. The current microbiological specification for many ready-to-eat foods in the Food Standards Code requires that L. monocytogenes be absent in 25 g of product. Packaged, cooked, cured meat is one such product. No illnesses associated with this recall have been reported.
When the US Food and Drug Administration and the United States Department of Agriculture jointly released a listeria risk ranking for ready-to-eat foods in the US, deli meats ranked at the highest level on a consumption basis (Food Safety & Hygiene, May 2001).
Foodborne listeriosis is a worldwide concern and in 2001 was the subject of a risk characterisation study in some ready-to-eat foods by a Joint FAO/WHO Expert Consultation.
The Codex Committee on Food Hygiene will consider at its next meeting in 2003 proposed draft guidelines for the control of L. monocytogenes in foods. This draft has been prepared by Germany with assistance from a number of other countries and the International Commission on Microbiological Specifications for Foods. The authors of the paper note in their introduction that, during recent years, the incidence of listeriosis in most countries has not increased and in a number of countries appears to have decreased. They note that this apparent reduction in baseline levels of listeriosis likely reflects the efforts of industry and governments to reduce the frequency and extent of Listeria in industrially processed foods.
The draft guidelines address issues throughout the food chain from primary production to the consumer. However they concentrate on the management of L. monocytogenes in ready-to-eat foods as these are the products most associated with illness. The guidelines are at an early stage of development and considerable time is likely to elapse before a final recommended guideline is published.
In the interim, however, Bruce Tompkin, formerly Vice President of Product Safety at Conagra Refrigerated Prepared Foods has published a detailed review paper in Journal of Food Protection 65 2002 709. The purpose of the paper is described as being to provide guidance to food processors in controlling L. monocytogenes in food processing environments.
The author makes a number of important points with particular reference to cooked, cured meats but many of general application.
- Experience over the past 15 years points to recontamination as the primary source of L. monocytogenes in many commercially prepared ready-to- eat foods. This realisation has led to significant changes in how the post-processing environment is managed.
- It should be possible in most food processes that include a validated listericidal step (e.g. cooking) to keep the prevalence of product contamination at less than 0.5 per cent.
- At a product contamination level of 0.5 per cent, there is a 61 per cent probability that a production lot would be accepted even if 100 samples were tested.
- Thus, end product testing becomes of little value for assessing and verifying control.
- Environmental testing is a better, more cost effective measure for assessing control particularly when approached from a process control viewpoint.
- Prevalence rates of 1 to 10 per cent and higher are typical throughout the world for a wide variety of foods yet symptomatic listeriosis remains a rare illness.
- Certain strains of L. monocytogenes are more virulent and much more likely to be involved in foodborne illness than others.
- Investigations of cooked meat and poultry operations indicate that a niche source is commonly involved. A niche is a site within the manufacturing environment in which L. monocytogenes becomes established and multiplies. These sites may be impossible to reach and clean by normal cleaning and sanitizing procedures.
- Growth of L. monocytogenes occurs before the contaminated food is eaten in those situations where clinical symptoms develop (see also Food Safety & Hygiene, November 1998).
- A rather common misconception is that air is a notable source of contamination. Throughout 14 years of investigation (by the author), the air in a room has never been found to be a chronic source of contamination of product contact surfaces.
- Environmental sampling programs should reflect previous experience in a facility. Sites sampled should include equipment surfaces to which the food is exposed, for example, between cooking and final packaging.
- Experience shows that Listeria can be controlled but not eliminated from the cooked product/processing environment (meat and poultry).
- Of the greatest concern are lines with repeated positives over a prolonged period. Some of these events reflect difficulty in finding the source.
- In general, equipment that is the source of a positive sample should be dismantled, cleaned and sanitized. This procedure is normally adequate.
The author also makes a number of comments about regulatory policy in the US which, while not directly relevant to the Australian situation, make interesting reading. He maintains that regulatory policy can have a profound influence on industry's willingness to test for L. monocytogenes per se and to generate the information needed to better understand the ecology of this potential pathogen in food processing operations.
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