6. Prevention and control of L. monocytogenes in fishery products


6.1 Introduction
6.2 Control measures to reduce or eliminate L. monocytogenes from the processing environment

6.2.1 Risk factors and hot spots of contamination
6.2.2 The effects of various processing parameters on the survival of L. monocytogenes

6.3 Control strategies

6.3.1 Raw materials
6.3.2 Processing environment
6.3.3 Retail trade


6.1 Introduction

Providing effective control of L. monocytogenes is challenging. The effective control of L. monocytogenes will be product, process and plant specific, therefore, these recommendations are presented as guidance. Not all the items listed below will apply to any particular operation, however, some will be useful in most situations.

Plant management should be committed to expending those resources that are necessary to address the problem. Employees must be trained to understand the problem, the potential sources of the organism, and the specific controls the plant is employing for L. monocytogenes. This may go far beyond normal training in GHP.

While most raw fishery products do not appear to be a significant source for L. monocytogenes, under certain conditions, these may become contaminated with the organism. In addition, the environment within processing plants will often contribute to the potential for contamination with L. monocytogenes. For cooked products, the plant should validate that the heat treatment is adequate to destroy L. monocytogenes. Once a cook step has been applied, the focus of a Listeria control programme will be on preventing recontamination of products that are subsequently handled or subjected to further processing, such as slicing or re-packaging. For those raw fishery products that have a low potential for contamination from the marine environment, most of the potential for contamination with L. monocytogenes occurs during product handling. Thus, minimization of contamination is recommended for these items as well as for cooked products.

These recommendations can also be applied to operations where there is no heat treatment to inactivate L. monocytogenes, but there is a need to minimize contamination of the product. These operations may include steps, for example washing, to remove the organism from the product. Control must focus, not only on reducing the numbers of L. monocytogenes on products by physical means, but also on preventing the establishment and growth of L. monocytogenes in the environment.

Because L. monocytogenes contamination can come from multiple sources, a comprehensive control programme may involve a combination of strategies that are compatible with HACCP and GHP.

To verify L. monocytogenes control, plants should implement an environmental monitoring programme for Listeria species. This programme, specific to the plant, should detail the areas to be sampled for Listeria species, the frequency of sampling, and the action to be taken when Listeria species is detected.

Figure 1: Proposed decision tree for the establishment of L. monocytogenes criteria in foods

a Establish a new shelf-life considering storage conditions i.e. time/temp or a new process formulation

b This numerical value should be revised based on further risk assessment

6.2. Control measures to reduce or eliminate L. monocytogenes from the processing environment

GHP and HACCP are essential in producing a safe food, and education and training can help producers and consumers apply safe handling practices. Inspection may provide evidence that the operations and practices used can consistently give a safe food while microbial testing can be used to indicate that GHP and HACCP have been effectively applied but it is recognized that sampling and testing of foods cannot guarantee their safety.

The ways in which L. monocytogenes may be introduced into fishery processing plants are numerous due to the ubiquitous nature of this bacterium. Raw marine fish does not appear to be a primary source for L. monocytogenes although slaughtered fish from colonized slaughterhouses may introduce the bacteria to a plant. Whatever the initial source of L. monocytogenes might be in each case, the main issue for the producer is to avoid colonization of the processing environment and subsequent spread to the product. This should be done by the systematic implementation of GHP and an effective HACCP programme.

Studies of smoked salmon production have identified control points for contamination with L. monocytogenes. By paying special attention to these points, including cleaning of in-process products, cleaning food-contact surfaces, separation of staff functions, personal hygiene and restriction on entry of visitors, the prevalence of L. monocytogenes in products and the processing environment was reduced in a number of plants. Including a heat treatment (hot steam, hot air, hot water at 80C) in cleaning and sanitizing procedures at various control points (skinning-, slicing- and brining equipment) has been shown to be effective in controlling L. monocytogenes in fishery processing plants.

6.2.1 Risk factors and hot spots of contamination

Research has demonstrated that sanitation and clean-up procedures appear to eliminate L. monocytogenes from the processing line and equipment, but recontamination can occur soon after resumption of processing. It has been demonstrated that reservoirs of L. monocytogenes can easily be established in the processing plant. For example, in processing of smoked salmon, the brining process and the post-brining areas have been identified as the most contaminated sites. Using pulsed-field gel electrophoresis (PFGE) for typing the isolates, it has been demonstrated that the L. monocytogenes types on the final product were similar to those associated with brining and slicing but different from the types found on the raw material.

When production facilities were in a good state of repair, research has shown that high levels of job rotation is associated with the frequency of L. monocytogenes contamination. Also, cleaning of the production line once or more during daily production may be of benefit in lowering the risk for L. monocytogenes contamination. However, there is still some debate on this latter subject, as evidence gleaned from the meat and dairy industries suggest that mid-shift clean-ups can increase the level of contamination in the plant.

While research has assisted in understanding how and where products become contaminated with L. monocytogenes, a number of issues remain unsolved. For example, more understanding of the mechanisms by which L. monocytogenes adheres to product and processing equipment and how best to kill/remove adhered cells is required. By the application of good manufacturing and cleaning practices and by the appropriate selection of disinfectants for work surfaces or sterilization by heat of critical areas, a low level of initial contamination can be maintained. At present, it may be unrealistic to expect Listeria-free products even after application of the most stringent hygienic processing.

6.2.2 The effects of various processing parameters on the survival of L. monocytogenes

6.2.2.1 Smoking

Smoking is one of the oldest methods used to preserve fish. In most cases, salting or brining precedes the smoking process. Smoking impregnates the fish with volatile smoke compounds, which impart flavour and colour, as well as bacteriostatic and antioxidant characteristics. Smoking can either be carried out at relatively low temperatures, in which case the main effect of the smoking is to deposit flavours and preservative compounds onto the fish, or at higher temperatures leading to cooking or partial cooking and drying of the product.

Under natural cold-smoking conditions (< 30oC), the frequency and levels of L. monocytogenes seem to decrease, and smoking may thus help to reduce L. monocytogenes. Hot smoking (> 65oC), seems to eliminate L. monocytogenes when smoke is applied during the whole heating process. Because smoking conditions are not standardized, the effect of smoking on bacteria, and the inhibitory effect during storage, may vary for cold-smoked and hot-smoked fish from different producers. The prevention of recontamination of both cold-smoked and hot-smoked fish is therefore of great importance.

6.2.2.2 Chlorine/Organic acids

Elimination or reduction of L. monocytogenes on products has been attempted using washing with hyper-chlorinated water (200 ppm free chlorine). However, this method could not ensure a L. monocytogenes free product. Similarly, it has been shown that depending on dose and time of exposure, dipping or spraying foods with organic acids can reduce the levels of L. monocytogenes, but its complete elimination could not be obtained. In summary, in most cases, only a 1-2 log reduction in numbers of L. monocytogenes on fish can be obtained using these techniques.

6.2.2.3 Heat

The heat resistance of L. monocytogenes varies considerably with the intrinsic properties of the heating menstrum. D60 values of 1.95 - 1.98 min for cod fillets and 4.23 - 4.48 min for salmon fillets have been observed. This difference may be due to the higher lipid content in salmon compared to that of cod. It is well known that the heat resistance may also be influenced by factors such as pH, acidulant, NaCl content, growth temperature history and heat shock.

It is generally agreed that L. monocytogenes will be inactivated by proper pasteurization. Minimal heat-processing of foods, to no less than 70C for 2 min at the coldest spot, would ensure the destruction (ca. 6 logs) of L. monocytogenes. In Australia, it is recommended that the heat applied to cook-chill products should be designed to achieve a 6-log reduction in the levels of L. monocytogenes based on a D70 value of 0.3 minutes and a z-value of 6C. However, it is unclear at the present time as to what should be the target decimal reduction value for L. monocytogenes in fishery products.

6.2.2.4 Other parameters

New preservation techniques such as the development of protective bacterial cultures targeted to inhibit the growth of L. monocytogenes in fishery products are presently being developed. Other control measures that are being used on an experimental basis include irradiation either alone or in combination with modified-atmosphere packaging and high intensity UV light.

6.3 Control strategies

6.3.1 Raw materials

In general, L. monocytogenes is not usually found on fish captured from open waters, although contamination may take place long before the fish raw material reaches processing factories. Potential sources of L. monocytogenes on fishing vessels include contamination from water and ice, soiled surfaces and boxes, as well as contamination from human and avian sources. Since L. monocytogenes is commonly found in surface waters of lakes and in coastal waters, fish captured or cultivated in these waters may possibly carry this organism. In addition, fish produced through aquaculture may come into contact with L. monocytogenes through contaminated feed, exposure to agricultural run-off or contaminated sediment in farming pens. However, there are strong indications that raw material is not currently a primary source for contamination of the final product with L. monocytogenes.

6.3.2 Processing environment

The emphasis of a control programme for L. monocytogenes should be on the more common sources of direct product contamination. The greatest risk for product contamination occurs when a product contact surface is contaminated. This risk is highest between the point where a food is cooked, pasteurized, or decontaminated and where the food is packaged. Annex III outlines several measures that can be applied to minimize the potential for contamination of fishery products with L. monocytogenes within the processing environment. Many of these should be considered for inclusion in a prerequisite programme. Some may also be useful in a HACCP programme. It is unlikely that any establishment will find utility in all of the items listed, however some of these suggestions should be useful in most operations.

6.3.3 Retail trade

Little is known of the potential for Listeria contamination of fish and fish products at the retail level. Products that are purchased in bulk and re-packaged prior to sale may be vulnerable to L. monocytogenes contamination. Establishments should consider that contamination may occur from any food contact surface as well as from secondary contamination sites such as floors, drains, ceilings, walls or equipment support structures (See Annex III). Retailers should be made aware of the risks that can arise from contamination of foods with L. monocytogenes so that appropriate measures can be instituted. Environmental sampling would not be practical or cost effective at the retail level, but diligent enforcement of sanitary conditions of food contact surfaces and handling areas, and personal hygiene practices should reduce the potential of contamination of fishery products by L. monocytogenes at the retail level.

If L. monocytogenes is present in a ready-to-eat fish product, the safety, as well as the quality is dependent upon the time and conditions of storage and display for sale. If storage temperatures fluctuate significantly, the quality will deteriorate and the risk to consumers will be increased. Thus retailers should pay strict attention to the temperature of storage. In addition, monitoring of "use-by-dates" may be one mechanism for control at retail level.

Control options for L. monocytogenes during processing is primarily a matter of having a proper cleaning and sanitation programme. Special attention needs to be paid to the presence of L. monocytogenes in plants producing ready-to-eat fishery products. A specific hygiene and sanitation programme needs to be developed in order to keep the contamination with L. monocytogenes at a low level. A total elimination of L. monocytogenes from the processing environment is impractical and may be impossible, as reintroduction of the organism is likely to occur. However, through the use of such programmes, a plant can reduce the number of L. monocytogenes contaminated products that it produces.