Tip of the Week: Control Strategies to Address Listeria monocytogenes Contamination in the Food Processing Establishment

Whether it's the practice of washing your hands thoroughly before entering a ready-to-eat (RTE) food processing area or you participate in a detailed time/temperature control data collection for refrigerated items, control strategies need to be implemented in your food production facility. After all, there always ways to make the process of Listeria prevention easier. When it comes to control strategies, AIB International has you covered with our best practices.


  • Employees must thoroughly wash their hands before entering areas where RTE foods are processed or exposed to the environment.
  • Personnel use suitable utensils (such as spatulas or tongs), or wear gloves, when touching exposed RTE foods, food-contact surfaces (FCSs), and packaging materials, and don't touch exposed RTE foods, FCSs, and packaging with bare hands.
  • Multi-use gloves are washed and sanitized before use and after the employee touches any non-food-contact surface, such as clothing, doorknobs, equipment control panels, and tools.
  • Foamers or footbaths containing liquid sanitizers or dry powdered sanitizers are used when personnel enter areas where RTE foods are processed or exposed.

Design, Construction, and Operation of Your Plant

  • Design and construct the plant so that walls, ceilings, windows, doors, floors, drains, and overhead fixtures (e.g., pipes, air vents, and lights) in areas where RTE foods are processed or exposed resist deterioration by product or cleaning chemicals, and prevent condensate buildup and harborage of microorganisms.
  • Separate areas where RTE foods are processed, exposed, or stored from areas where raw foods are processed, exposed, or stored and from equipment washing areas, microbiological laboratories, maintenance areas, waste areas, offices, lockers, and toilet facilities.
  • Maintain positive air pressure on the RTE side of the operation relative to the raw side (i.e., maintain higher air pressures in RTE areas and lower air pressures in areas where unprocessed foods are handled).
  • Consider filtering the air in rooms where RTE foods are processed or exposed. Depending on your product, it may be appropriate to use HEPA filters with efficiency of 99.97-99.99 percent at 0.3 micron size.

Design, Construction, and Maintenance of Equipment

  • RTE food-contact surfaces should be smooth, non-absorbent, sealed, and sloped, where feasible, in order to drain freely and junctures should be covered.
  • Piping used to convey RTE foods should not have dead ends or cross-connections between conveyance of raw and RTE foods.
  • The sanitary standards inventoried by 3-A Sanitary Standards, Inc. can be useful when designing and constructing equipment containing food contact surfaces.
  • Do not install stationary equipment used to process RTE foods over floor drains to prevent contamination of the equipment (and RTE foods) with L. monocytogenes that could be harbored in floor drains.


  • Design and write sanitation procedures to ensure that RTE areas, and equipment in the area, are cleaned and sanitized to reduce the risk of contamination of L. monocytogenes.
  • Do not wash equipment during times that RTE foods or packaging are exposed in the area, because of the potential formation of aerosols that could contaminate foods or packaging.
  • Sanitize equipment, floors, and (if applicable) walls and prepare the equipment for operation using ATP bioluminescence or other appropriate testing as a sanitation check.
  • Review the sanitary design of the room and equipment, and keep rooms as dry as possible as moisture fosters the growth and transfer of L. monocytogenes.

Controls on Raw Ingredients and Other Ingredients

  • Conduct a risk assessment: establish, implement, and monitor controls to reduce finished RTE foods with L. monocytogens contamination.
  • Verify that suppliers have implemented validated listericidal control measures and have programs to ensure that the process is being appropriately implemented and monitored.
  • When contamination with L. monocytogenes is reasonably foreseeable, then establish measures to prevent cross-contamination of raw materials with finished RTEs.
  • Trust, but verify. Periodically test raw materials and other ingredients received under the COC/COA to verify the efficacy of the supplier’s control programs and maintain records.

Storage Practices and Time/Temp Controls

  • Establish measures to verify time/temperature control on an ongoing basis and see that time/temperature controls are consistently applied and achieved.
  • Store RTE foods at 4°C (~40°F) or below.
  • Even though L. monocytogenes can multiply slowly at refrigeration temperatures, refrigeration is a key control measure.
  • Freezing will not eliminate L. monocytogenes from foods and cannot be relied upon as a control measure for the elimination or reduction of L. monocytogenes.


  • Establish and implement procedures to prevent contamination during transportation.
  • Inspect transportation vehicles (e.g., trailers and tankers) for structural integrity, cleanliness, and overall suitability when unloading ingredients and prior to loading finished products.
  • Use temperature-controlled vehicles for transportation and vehicles should be pre-cooled with the pre-loading temperature documented prior to loading.
  • Ensure food products are at the target temperature before loading in refrigerated trucks. In general, these vehicles are designed to maintain a temperature, but not to cool a product to the target temperature.

Environmental Monitoring Program

  • Identify the locations from which samples will be collected and the number of sites to be tested during routine environmental monitoring.
    • Identify the timing and frequency for collecting and testing samples
    • Identify the test(s) conducted, including the analytical method(s) used
  • Test both FCS and non-FCS sites at each sampling time. FDA recommends that even the smallest processors collect samples from at least 5 FCS sites and 5 non-FCS sites on each production line for RTE foods.
  • The most important time to collect environmental samples is at a time that is several hours into production (e.g., 3 to 4 hours) or preferably just prior to cleanup, because this allows time for L. monocytogenes (if present) to work its way out of harborage sites and contaminate the environment, the processing line (including FCS sites), and potentially the RTE product.
  • Periodically verify written environmental monitoring procedures by increased and intensive environmental sampling of the plant to assess whether the sampling sites are appropriate and maintain a record of all corrective actions taken.

For more information about controlling L. monocytogenes in food processing establishments, and environmental monitoring, refer to FDA’s “Control of Listeria monocytogenes in Ready-To-Eat Foods: Guidance for Industry” or schedule an environmental monitoring consultation with AIB International.