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Listeria monocytogenes and the smallgoods industry

Listeria monocytogenes can cause a foodborne disease called listeriosis. The disease is rare in Australia and usually only affects people with suppressed immune systems such as very young babies, the elderly, pregnant women and those who are immune compromised (for example, chemotherapy patients and AIDS sufferers). However, listeriosis is a very serious systemic disease that causes death in 20 to 30% of victims. About 60 cases are reported per year in Australia and it is thought that at least as many go undetected. Most Australian listeriosis cases reported are in the elderly (http://www9.health.gov.au/cda/Source/CDA-index.cfm).

L. monocytogenes is present in many natural environments. It prefers cool, moist conditions with decaying organic matter and is a common contaminant in food processing plants. It can grow, albeit slowly, at refrigeration temperatures and in the absence of oxygen. For these reasons it is a problem in some 'ready-to-eat' foods (RTEs) that may be eaten without further cooking, particularly those that have long refrigerated shelf lives. Foods in this category include some cheeses, some vacuum or modified atmosphere packaged processed meats, vacuum packed cold-smoked fish and some prepared salad vegetables. Outbreaks have been linked to pate, fresh and soft cheeses, turkey frankfurters and coleslaw.

Due to the severity of listeriosis, Australian food legislators have adopted a 'zero tolerance' (not detected in 25g of product) approach for L. monocytogenes in RTEs that are able to support its growth. For other RTEs, which are not expected to support growth of the microorganism, a tolerance of 100 cells/g at the point of sale has been set. If these levels are exceeded, a recall may be initiated by authorities to protect public health.

Certain categories of processed meats are not expected to support growth of L. monocytogenes, due to the presence of antimicrobial ingredients used in traditional recipes. For example, prosciutto (a salted leg ham) would normally have salt levels exceeding those required to control L. monocytogenes growth (~14% in aqueous phase) and the combined effects of salt, pH, nitrite and lactic acid used in fermented meats may also prevent growth of the microorganism.

However, it is sometimes unclear whether particular combinations of inhibitory factors will effectively prevent the growth of L. monocytogenes. In these cases, the recall guidelines state that: '… the authorities may take a conservative approach and regard growth as possible, unless there is documented evidence provided by the manufacturer that the product does not support growth of L. monocytogenes'. Therefore, it would be invaluable to determine from a product's formulation whether it would support the growth of L. monocytogenes or not, especially since levels of contamination are usually below 100 cells/g.

Meat & Livestock Australia (MLA) has sponsored a series of research projects aimed to help smallgoods producers improve their management of L. monocytogenes, including understanding which combinations of inhibitory factors are needed to prevent its growth in processed meats.

Results from these projects, in addition to L. monocytogenes risk management guidance, is available in the Listeria monocytogenes in smallgoods: risks and controls booklet published by MLA in 2006. There is also now a DVD available which includes an electronic copy of the booklet and other useful food safety information for smallgoods producers. For details, contact Fiona Sparke at MLA on 02 9463 9223.