General Considerations

To effectively manage the risk of product contamination it is necessary to assess where along the product flow the exposed food is more likely to become contaminated. This is generally wherever something has direct contact with the unpackaged product. Examples of some primary sites leading to product contamination include:

Other areas of the environment can also serve as secondary sources of L. monocytogenes. These areas may harbour the organism and under certain conditions lead to contamination of the above product contact surfaces or the food. By controlling the presence of L. monocytogenes in the environment it is possible to reduce the risk that product or a product contact surface will become contaminated. The significance of these areas will vary depending upon the facility, the process(es), the temperature and humidity of the room, and the food. Examples of such secondary sources include:

Consideration should also be given to the potential for L. monocytogenes to be brought back into the clean environment. This may be the result of traffic in the processing and packaging areas (people and equipment, such as trolleys and forklifts, that enter from more contaminated points in the operation) or unscheduled equipment maintenance.

It should be recognized that in a plant with an effective control programme L. monocytogenes contamination, when it occurs, is line or equipment specific. While it is possible to have random isolated contamination with L. monocytogenes in a controlled environment, contamination more likely will occur after the organism has become established in a niche. When this happens, routine cleaning and sanitizing become less effective in controlling the organism. As the equipment is operated, the bacteria work their way out of the niche and become deposited onto the outer surfaces of the equipment. As product moves over or through the equipment, the contamination is spread downstream to other areas along the product flow. This situation can be corrected only by identifying the source or niche of L. monocytogenes growth and eliminating it. Among the sites found to be potential harbourages are the following:

In addition to the possible establishment of L. monocytogenes in a niche, consideration must be given to certain conditions or situations that present an extra risk for contamination of finished products with Listeria contamination. When such events occur, processors may need to take extra precautions to guard against contamination. Examples of such situations include:

  1. Moving or significantly modifying a packaging line.
  2. Installing used equipment or equipment that has been in storage or used in another plant.
  3. Equipment breakdowns.
  4. Construction or major modifications in the product handling and packaging area (e.g., replacing refrigeration units or floors, replacing or building walls, modifications to sewer lines).
  5. Using employees that are not familiar with the operation, the L. monocytogenes control procedures in use, or the procedures for cleaning equipment in the production area where ready-to-eat (RTE) foods may be exposed to contamination.
  6. Personnel who handle RTE product contact surfaces or equipment that are likely to be contaminated (e.g., floor, trash cans) and not changing gloves or following other required procedures before handling product.
  7. Difficulty in meeting scheduled cleaning of the floors of holding coolers due to heavy production schedules.
  8. A drain backing up.
  9. Product being caught or hung-up on equipment. Stagnant product in a system can be a major site of microbial growth during production. The equipment should be modified to eliminate areas where product stops moving along or through a processing line.
  10. Raw or under-processed product being detected in a cooked product area. If this occurs, the process must be stopped, the unacceptable product removed, and the equipment re-cleaned and sanitized.
  11. Frequent changing of packaging film, labels, line speeds, etc. due to frequent changes of product being packaged.
  12. Personnel being used interchangeably for packaging raw and cooked products.
  13. Increased production requiring wet cleaning of down lines in the same room as lines running product.
  14. Cleaning of equipment parts, tubs, screens, etc. on the floor.
  15. Waste bins in the ready-to-eat area not being properly maintained, cleaned and sanitized. Personnel handling product may come into contact with these items and then contaminate product and/or product contact surfaces.

Specific Considerations: Processing Area

As noted before, certain types of raw fishery products as well as other ingredients used in processing may contain L. monocytogenes, although the presence of the organism and the levels of contamination vary widely. Because of this potential, steps should be taken to minimize the potential for cross-contamination from raw ingredients to products that have been treated to eliminate or reduce the contamination.

Separating raw products from semi-finished and finished products is a key factor in preventing cross-contamination.

  1. Wherever possible, there should be linear flow of product through the operation from the raw ingredients to the finished product.
    1. Plants should be rearranged, where necessary, to improve the flow of product, equipment, and people to facilitate separation of raw from cooked or treated product.
    2. It is desirable to establish positive airflow on the "clean" side of the operation relative to the "dirty" side (e.g., maintain negative air pressures in raw product areas and positive pressures on the clean or finished product side).
  2. Compartmentalize operations as needed to enhance the separation of raw ingredients and processed products.
  1. Provide separate, dedicated washing areas and clean-in-place systems for cooked or treated product equipment and raw processing equipment.
  2. Rework and trash barrels for cooked or treated product areas should be labelled or colour coded and not used elsewhere in the plant. They must be cleaned and sanitized daily or more frequently if microbiological testing data indicate this is necessary.
  3. Remove hoses from the manufacturing areas where ready-to-eat products are exposed before start of operation each day so that line workers are not tempted to use these for spot cleaning during production.
  4. Consider having separate utensils, carts, racks, totes, equipment, cleaning utensils, etc., colour coded where practical, for the ready-to-eat product area.
  5. Where possible, eliminate overhead fixtures in the ready-to-eat area.
  6. Where possible, isolate wet process areas from other production areas; at a minimum, remove standing water as soon as possible.
  1. Control traffic flow patterns between the raw ingredients and the processed products sides of the operation to prevent transfer of L. monocytogenes from the "dirty" or "raw" side of the operation to the "clean" or "cooked" side.
    1. Equipment, utensils and people in raw and cooked areas should not be interchanged during the working day.
    2. Drains from the "dirty" or "raw" side should not be connected to those in the "clean" or "cooked" side.
    3. If footbaths are installed, they must be properly maintained, or they can become a source of contamination. Maintaining clean dry floors is preferred to the use of footbaths, unless there is a specific need that cannot be addressed otherwise. Footbaths should contain stronger concentrations of sanitizer than would normally be used on equipment (e.g., 200 ppm iodophor, 400-800 ppm quaternary ammonium compound); a depth of 2 inches is recommended. Chlorine is not recommended for this use as it becomes too quickly inactivated. If chlorine is used, strict attention must be given to maintaining its strength. Footbaths will be ineffective if cleaved boots are carrying large particles of dirt/plant waste.
    4. Another option is to spray a foam disinfectant on the floor as people or rolling stock (carts, forklifts, etc.) enter the room.

Water used in processing operations that will come in contact with product should contain an antimicrobial known to be effective against L. monocytogenes and approved for the specific application at the levels used.

Specific Considerations: Packaging and Storage

Pallets entering the packaging room must be clean, dry and in good condition and preferably not made of wood.

Store and package exposed products in a clean, dry environment.

  1. Bacteria cannot multiply without water. If the environment is clean and dry, L. monocytogenes remains dormant or, perhaps, dies.
  2. There is less transfer of bacteria from surfaces if the surfaces are clean and dry.
  3. The spread of contamination by vehicular and pedestrian traffic is reduced considerably if the floors are clean and dry.
  4. The cooling units in packaging rooms and coolers for exposed product should have dehumidifying capability. To facilitate the removal of humid air and to dry floors after cleaning, it may be necessary to exhaust air outside the plant. Heating air within a room can also be effective for removing moisture at the end of the cleaning/sanitizing process.

Specific Considerations: Equipment

Equipment should be properly designed and maintained.

  1. Equipment must be designed to facilitate cleaning and to minimize sites where microbial multiplication can occur. The acceptability of the design of new or replacement equipment should be reviewed to determine its cleanability and its potential to minimize microbial contamination.
  2. Even though visually clean, previously used equipment may harbour pathogens. Plan accordingly, prior to putting such equipment into production.
  3. Properly maintain equipment to minimize breakdowns and the risk of contamination during repair.
  4. Equipment that is damaged, pitted, corroded, or cracked should be repaired or replaced.
  5. Equipment or catwalk framework should not be hollow such that water can collect and harbour L. monocytogenes.
  6. Lubricants can become contaminated with product residue and become a centre for growth of L. monocytogenes. Use lubricants that contain listericidal additives (e.g., sodium benzoate).
  7. Avoid conveyor designs and locations that are difficult to clean and sanitize. Conveyors for product prior to packaging should not contain hollow rollers. Do not locate conveyors or other processing equipment in which product is exposed near the floor, as this is a likely source of L. monocytogenes. Avoid overhead conveyors, if possible, as they are more difficult to clean, sanitize and inspect. Either provide a safety ladder or design the conveyor so it can be lowered for cleaning.
  8. Racks used for transporting exposed cooked product should have cover guards over the wheels to prevent spray from the wheels falling onto the rack and product as the racks are moved.
  9. Racks used in operations after products are cooked can be a significant source of contamination if not properly cleaned and sanitized before use. The most reliable method of sanitizing racks is with heat. Heat can be applied by (1) a hot water (82.2 C/180 F) rinse in a rack washer so the racks will reach a temperature of 71.1 C/160 F or higher, (2) steam applied in a cabinet after cleaning in a rack washer, or (3) placing the racks into an oven and applying moist heat to raise the temperature of the racks to 71.1 C/160 F or higher. When using heat to sanitize, it is essential that the equipment be thoroughly cleaned so the heat does not bake the soil on, making it more difficult to remove, resulting in more contamination problems in the future.
  10. Regular maintenance schedules should be adopted and followed to minimize the potential for harbourages and to reduce the potential for contamination of equipment due to unscheduled repair operations.
  11. For maintenance of equipment in the cooked, RTE area it may be necessary to use tools dedicated to this area or to sanitize tools prior to use in this area. Maintenance personnel should wear clean smocks that are not used in raw material areas. Equipment should be re-sanitized after maintenance work.

Specific considerations: Plant Sanitation

  1. Use sanitation procedures designed to control L. monocytogenes. The frequency of cleaning and sanitizing the equipment and environment of a plant depends upon experience and microbiological data. Routine microbiological testing allows the plant to develop a baseline for comparison purposes, observe trends, and detect a developing sanitation problem.
  2. Successful control of L. monocytogenes requires consistency and attention to detail. The steps include (1) dry clean, (2) pre-rinse the equipment, (3) visually inspect the equipment, (4) foam and scrub the equipment, (5) rinse the equipment, (6) visually inspect the equipment, (7) clean the floors, (8) sanitize the equipment and floors, (9) conduct post-sanitation verification, (10) dry the floors, (11) clean and put away supplies. Some equipment may require disassembling prior to cleaning and sanitizing, and may need to be re-sanitized after re-assembling.
  3. Quaternary ammonium compounds (quats) have been found to be effective against L. monocytogenes, and leave a residual germicidal effect on surfaces. In addition, sanitizers containing peracetic acid and peroctanoic acid have been shown to be effective against biofilm containing L. monocytogenes. Areas that should be sanitized with such compounds and a suggested frequency are as follows:
Drains Daily
Floors Daily
Waste containers & storage Daily
Walls Weekly/monthly
Condensate drip pans Weekly/monthly
Heating ventilating air conditioning (HVAC) Weekly/monthly
Coolers Weekly/monthly
Spiral freezers Semi-annually
  1. The cleanup crew should receive special training in proper procedures to control L. monocytogenes. Close monitoring and correction is essential to improve and maintain a high level of performance.
  2. Priority must be given to rooms and equipment used for holding and packaging exposed ready-to-eat product. Consideration should be given to assigning reliable personnel to areas where ready-to-eat products are handled and packaged.
  3. It is very desirable, even necessary in some cases, to have a person on the staff whose primary responsibility is to monitor the cleaning and sanitizing process at night to be certain it is being done correctly. Concerns over having the plant ready on time for start-up must be secondary to having the plant correctly cleaned and sanitized. Extensive experience indicates that, if the equipment is properly cleaned and sanitized before start-up, the risk of contamination from equipment during production is minimal.
  4. The utility of mid-shift cleanups is not entirely clear. In certain situations, they may be counter-productive by increasing the risk of L. monocytogenes contamination. There may be instances however when this is necessary to reduce contamination. Microbiological testing should be conducted to determine the utility of this practice in each circumstance.
  5. Some plants have found the following sanitizing procedure to be helpful each night. After cleaning the equipment, apply a high level of sanitizer (e.g., 800 ppm quat), allow it to stand for about 20 minutes, rinse thoroughly, and then apply the normal level of sanitizer (e.g., 200 ppm quat or chlorine). At the end of the production week the high level of sanitizer can be left on the equipment until before start-up. The sanitizer is then rinsed, the normal level is applied, and the room is prepared for start-up. Under certain circumstances, it may be beneficial to spray an aerosol of 200 ppm quat into a room as a final step in the process of cleaning and sanitizing; weekly or monthly fogging may be useful.
  6. Rotating other sanitizers (e.g., chlorine, acid-anionic, peracid and iodophors) into the sanitation programme may provide for greater effectiveness. Consider using new peracid-based sanitizers such as Matrixx, Vortexx and others where they have been demonstrated to be effective against L. monocytogenes.
  7. Modify equipment so it is simple in design, easy to clean, and has fewer maintenance problems. Breakdowns during production increase the risk of L. monocytogenes contamination.
  8. Sanitizing with high temperatures, if manufacturers’ instructions permit such application, may be particularly useful for biofilm removal.
  9. Hot water/steam sanitation is an alternative to chemical sanitation that is especially effective where equipment is difficult to clean. Wherever possible, apply steam as a final step for equipment that is difficult to clean. One method is to place a metal cover over the equipment and then inject steam. In some cases, equipment can be steamed in a cook oven. The objective is to heat the equipment so it will reach at least 71.1 C/160 F throughout. A holding period of one hour or more is desirable. For equipment that may be more sensitive to heating it may be necessary to use a lower temperature (e.g., 62.8 C/145 F) and a longer holding time. (See earlier cautions about thorough cleaning prior to application of heat.)
  10. Plastic tubs which can be stacked may be a problem unless they are cleaned and sanitized daily. They must not be placed directly on the floor, unless placed on a clean plastic mat.
  11. Infrequent cleaning of coolers used for holding finished unpackaged product is a common cause for increased L. monocytogenes problems, particularly during peak periods. These coolers should be emptied and cleaned at least once per week. Maintaining dry floors in these coolers is also important.
  12. Infrequent defrosting, cleaning, and maintenance of spiral freezers used for freezing unpackaged product is a potential source of L. monocytogenes. These freezers should be cleaned twice a year.
  13. Condensate that accumulates in drip pans of refrigeration units should be directed to a drain via a hose. Care must be taken to ensure that the hose does not become blocked. Solid forms of sanitizers (e.g., blocks or donuts of quats) can be placed in the drip pan to control microbial growth. In addition to the routine use of sanitizers, drip pans should be cleaned regularly.
  14. Using compressed air to remove debris from equipment during production can increase the risk of contamination. Compressed air can be a source of L. monocytogenes when in-line filters are not maintained or replaced with regularity. Thus, when compressed air must be used directly on product or product contact surfaces, the air should be filtered at the point of use and the filters maintained. This practice should be restricted, preferably, to cleaning certain equipment (e.g., packaging machines) at the end of production before cleaning begins.
  15. Never clean coolers or other rooms when exposed, ready-to-eat product is present. Do not rely on covering the product with plastic or paper. Remove all unpackaged product from the room before beginning to clean.
  16. Do not dismantle and wash equipment on the floor.
  17. The best method for cleaning floors is to use a powdered caustic cleaner, apply water as needed, use a dedicated, colour-coded brush to clean the floor, and then thoroughly rinse, using a low volume hose, and sanitize the floor. Newer cleaners and sanitizers may be more effective for controlling L. monocytogenes on the floor. Floor scrubbers can be helpful, particularly for cleaning large open spaces such as hallways. The equipment used for cleaning must be maintained and properly cleaned so it does not become a source of contamination. Application of powdered citric acid to certain areas of the floor may be effective for controlling L. monocytogenes, provided the floor has been properly cleaned and dried before applying the citric acid. For maximum effectiveness, the surface of the floor should be maintained at pH 5.0 or below. Litmus paper can be used to check the pH. While this may help control L. monocytogenes, the condition of the floor should be monitored, as the acid condition will cause deterioration that eventually will necessitate replacing the floor.
  18. Floor drains must be designed and maintained to prevent backups. If a backup occurs, production must cease, the drain cleared, and the area carefully cleaned with caustic, rinsed, and sanitized. Avoid splashing equipment during the process. The floor should then be dried. Never use a high-pressure hose to clear a drain. An aerosol will be created that will spread contamination throughout the room.
  19. Whenever possible, eliminate trench drains.
  20. Bactericidal drain rings are recommended.
  21. Floor drains should be cleaned and sanitized in a manner that prevents contamination of other surfaces in the room. Floor drain brushes must be at least 5 mm ( inch) smaller than the diameter of the drain opening or a splashguard must be used to prevent splashing during cleaning. Utensils for cleaning drains should be dedicated to that purpose to minimize the potential for contamination. If floor drains are cleaned first, it may be necessary to clean and sanitize them again at the end of the process.
  22. Cleaning tools should be sanitized using 600-1000 ppm quat solutions and either stored dry or in quat solutions maintained at 1000 ppm.

Specific Considerations: Personnel Hygiene

Establish personal hygiene practices with L. monocytogenes control as a major objective. The following information should become part of employee training for L. monocytogenes control.

  1. Clean gloves, smocks, and aprons are essential to protect against product contamination. Ideally there should be one colour smock for the raw side of the operation and one for the processed side. Disposable gloves and aprons should be used wherever possible in cooked product areas. Disposable paper sleeves (arm covers) can provide another barrier for those who handle exposed product. Disposable items should be discarded when leaving the work area and replaced with new when returning. Some garments (e.g., smocks) may be left in the department and re-used, provided they are still clean. Gloves should be replaced if damaged. The use of gloves does not preclude the need for employees to wash hands regularly.
  2. Everyone working in areas where ready-to-eat products are exposed must clearly understand that the purpose of wearing clean garments and disposable gloves is to protect the product from contamination, not protect themselves from getting dirty.
  3. If an unclean surface is touched, then hands should be washed and gloves changed.
  4. Equipment and soiled clothing must not be stored in lockers.
  5. If possible, assign a person in the packaging room to pick up material from the floor, remove trash, and perform other housekeeping tasks. This person must not work on a packaging line or handle product that will be packaged or replaced on the line.
  6. Rubber boots that are non-porous and easily cleaned are better for L. monocytogenes control than other footwear. Boots are necessary where footbaths are used.


An environmental monitoring programme is recommended to assess the need for additional control measures for products that may be recontaminated by L. monocytogenes. Industry experience has shown that an ongoing monitoring and control programme that uses Listeria species (Listeria spp. or "generic Listeria") as an indicator of potential L. monocytogenes contamination not only reduces the possibility of finding L. monocytogenes in finished product but other pathogens as well. Industry experience also shows that re-entry of Listeria spp. into the production environment cannot be reliably prevented. Thus, ongoing monitoring to detect the organism in the environment is necessary.

Each company should establish its own L. monocytogenes monitoring programme considering the guidelines outlined below.

General Principles for Verification of Environmental Monitoring

Environmental monitoring (microbiological testing) should focus on a non-pathogenic indicator such as Listeria spp. or Listeria-like organisms, since these indicators will be found more frequently in the environment than L. monocytogenes and because test results are available sooner. Monitoring results should alert the plant to potential problem areas, prompting further investigation and focusing of additional control efforts, as necessary. Goals for reduction of positives should be established in order to encourage continuous improvement. A detailed set of action plans should be developed to control the risk of L. monocytogenes in the event that the goals are not met.

Each plant, product, and process must be evaluated to determine the appropriate monitoring points. Each packaging line should be regarded as an independent unit for L. monocytogenes monitoring and control. It is recommended that both food contact surfaces and non-food contact surfaces that pose the potential to contaminate product be tested. One approach might be to separate testing into environmental sites, product contact sites, and product itself. Keep in mind that since L. monocytogenes will not be frequently found in products in operations following these control guidelines and because it will not be uniformly distributed, product testing will not be a reliable indicator that L. monocytogenes contamination has not occurred. Thus, the emphasis of the programme discussed here is on testing for Listeria-like organisms in the environment to verify control. There can be many variations on how this is done. Some guidelines are presented below.

Environmental testing

Food Contact Surface Testing

Product testing

Environmental Sampling Guidelines

Problem Solving

When an effective control programme for L. monocytogenes is in place, the primary source of contamination is often a niche where L. monocytogenes has become established and is multiplying. When L. monocytogenes finds a niche, the contamination will be line-specific. In general, the contamination will flow downstream along a packaging line. When seeking the source of a niche, collect and analyze sponge samples individually, not as composites. Sample additional sites along the line and sample more frequently throughout the day. Tear down suspected pieces of equipment, collecting samples of suspicious sites and materials. Clean and sanitize the equipment as it is being reassembled. If cleaning and sanitizing are unsuccessful, remove sensitive electronics, oil and grease and apply heat to 71.1 C/160 F. Small parts can be placed in an oven; larger equipment can be shrouded and steam applied under the tarp. Lower temperatures for longer times may also be effective. Also consider the possibility that employee practices may be involved in the contamination. Refresher training in the controls necessary to prevent L. monocytogenes contamination may be indicated.