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Control measures for Listeria monocytogenes in ready-to-eat foods


Listeria monocytogenes (L. monocytogenes) is widely distributed in nature. It can be found in soil, sewage, vegetation, water, silage, livestock and humans. It is also well adapted to survival in the cold, moist environments commonly found in processing establishments.

Contamination of a food with L. monocytogenes is one of many biological hazards that should be considered during a hazard analysis and when developing a preventive control plan.

Certain individuals including the elderly, pregnant women and immunocompromised individuals are much more at risk if exposed to L. monocytogenes. Because of the potentially serious consequences, especially to certain groups within the population, food businesses need to pay particular attention to controlling this biological hazard.

Ready-to-Eat (RTE) foods contaminated with L. monocytogenes at levels exceeding 100 colony-forming units (CFU) of bacteria per gram of food have been implicated in outbreaks of listeriosis.

Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods aims to enhance the control and verification of Listeria in the environment where food is prepared.


The Canadian Food Inspection Agency (CFIA) created this document as guidance to help food businesses comply with the Safe Food for Canadians Regulations.

It's your choice

You may use other guidance developed by provincial governments, industry associations, international partners or academic bodies as long as they can achieve the outcomes identified in the regulations. Always ensure that the guidance you choose is relevant for your particular business, product or products, and market requirements

What is Listeria monocytogenes?

L. monocytogenes is a pathogenic bacterium that:

  • is very hardy (resistant to drying, freezing and high salt concentrations)
  • can grow readily at refrigeration temperatures
  • can be destroyed by thoroughly cooking food

What is a Ready-to-Eat food?

A food is considered Ready-to-Eat (RTE) when it:

  • does not require any further preparation before consumption, except washing or rinsing, thawing or warming
  • has received some form of processing such as cooking to render it RTE and/or to extend its shelf-life (not including food in a hermetically sealed container that is commercially sterile)

What's included

This guidance is based on Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods and is intended to help food businesses:

The examples of control measures for L. monocytogenes are not exhaustive, control measures will depend on the size and complexity of the food operation and be unique for each business.

Refer to the Tell me more! section for additional sources of information that may help you develop control measures for L. monocytogenes.

What's not included

The document does not provide information on how to:

The document does not address labelling requirements specific to RTE food. Labelling information can be found in CFIA's industry labelling tool.

Roles and responsibilities

Food businesses are responsible for complying with the law. They demonstrate compliance by ensuring that the commodities and processes for which they are responsible meet regulatory requirements. If a written preventive control plan (PCP) is required, the food business develops a PCP with supporting documents, monitors and maintains evidence of its implementation, and verifies that all control measures are effective.

The CFIA verifies the compliance of a food business by conducting activities that include inspection, and surveillance. When non-compliance is identified, the CFIA takes appropriate compliance and enforcement actions.

Listeria monocytogenes in ready-to-eat foods

When conducting a hazard analysis for an RTE food, L. monocytogenes should be identified as a hazard that can present a risk of contamination to the food.

L. monocytogenes RTE risk category

RTE foods are classified in one of three risk categories based on the health risk L. monocytogenes poses if present in the food. The parameters used to determine the risk category are established in Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods.

The risk categories are used to determine:

The following outlines the parameters for determining the L. monocytogenes risk category of a RTE food:

Control measures for L. monocytogenes

The following control measures could be applied to an RTE food to prevent or reduce the growth of L. monocytogenes.

Growth limiting parameters

L. monocytogenes has the ability to grow in an RTE food when the:

The pH and water activity (aw) of an RTE food can act as a growth limiting barrier for L. monocytogenes. The product formulation used to prepare an RTE food can be adjusted to bring these parameters to a level that can reduce or eliminate the growth of L. monocytogenes. For example, an RTE food will not support growth of L. monocytogenes when the product formulation results in a:

In order for the growth limiting parameters to be effective, and for the food to remain in the same RTE risk category, the target levels for the parameters have to be met every time the food is prepared and be maintained for the duration of the stated shelf-life. You can ensure this by:

Note: A change in the product formulation could impact the effectiveness of growth limiting parameters. The new product formulation would need to be validated and the risk category of the RTE food would have to be reviewed.

Food additives and processing aids

Some food additives can inhibit the growth of micro-organisms. The food additives authorized for use as an antimicrobial agent are those found in Health Canada's Lists of permitted food additives. For example, the Class II preservatives such as potassium lactate, sodium acetate, sodium diacetate and sodium lactate either alone or in combination can be effective in inhibiting the growth of L. monocytogenes.

The use of an antimicrobial additive can affect the categorization of a RTE food under the Health Canada policy. For instance, a Category 1 RTE food can become a Category 2B when the validation of the antimicrobial agent used demonstrates that the growth of L. monocytogenes does not increase by more than 0.5-log CFU/g for the duration of the shelf-life of the product.

If the antimicrobial additive limits the growth of L. monocytogenes to less than 2-log CFU/g for the duration of the product's shelf life, the recommended sampling frequency can be reduced based on a lower relative risk level (RRL)–see Tables 1-3.

Antimicrobial processing aids such as bacteriophages can also be used to partially or completely destroy micro-organisms such as L. monocytogenes. Health Canada maintains a List of antimicrobial processing aids for which they issued a letter of no objection. It should be noted that, while processing aids may reduce the health risk associated with a RTE food, neither the categorization of the product under the Health Canada policy or the RRL are affected, because processing aids do not remain in the product and hence do not affect growth of L. monocytogenes during the shelf life.

Durable life

The durable life (shelf life) of an RTE food is affected by factors such as the:

The durable life should take into consideration these factors and be limited to the length of time the RTE food can be stored without affecting its quality and safety. The durable life is validated to ensure that the RTE food will remain safe for consumption.

Post lethality treatment

A post-lethality treatment such as surface heat pasteurization (by steam, hot water, radiant heat or infrared technology) and high pressure processing can be applied to the finished RTE food to reduce or eliminate L. monocytogenes that may have contaminated the food after the lethality treatment. These treatments can be used alone or in combination with an approved antimicrobial agent. In the Policy on Listeria monocytogenes in Ready-to-Eat Foods, Health Canada recommends to use a post-lethality treatment that can achieve a minimum 3-log reduction in number of L. monocytogenes.

If the post lethality treatment can achieve a 3-log reduction or more of L. monocytogenes, the recommended sampling frequency can be reduced based on a lower relative risk level (RRL), see Tables 1 to 3.

Limiting exposure to temperatures favorable for L. monocytogenes

When preparing the food, the growth of L. monocytogenes in a RTE food can be reduced by limiting the length of time the ingredients and food are exposed to temperatures optimal for its growth.

The temperature of the area where the food is prepared will determine the maximum length of time that should be taken to prepare a food in order to prevent L. monocytogenes from increasing to a level that can pose a health risk by the end of its shelf life. The food may need to be refrigerated between some preparation steps.

Enhanced sanitation controls

L. monocytogenes is known to form colonies of bacterium (biofilms) that attach to a surface. They are commonly found in niches such as closed systems, areas where moisture accumulates and between close fitting materials. These biofilms are surrounded by a protective sheath of proteins and sugars that make them harder to remove. Basic cleaning and sanitization procedures may not be sufficient for the removal of these biofilms.

The following enhanced sanitation controls are more effective at removing biofilms.

Monitoring Control Measures

The effectiveness of the control measures applied for L. monocytogenes can be monitored by establishing a sampling and testing program. The document Sampling procedures provides additional guidance that you should consider.

Testing finished RTE food

Testing the finished RTE food for the presence of L. monocytogenes is used to determine whether the controls applied to prevent contamination of the food are effective.


Section 7.3 and Table 1: Sampling Methodologies and Compliance Criteria for L. monocytogenes in Ready-to-Eat Foods of Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods provide guidance on the sampling and compliance levels for each RTE risk category.

Your RTE food sampling and testing program should outline the following:

Table 1 presents recommended testing frequencies.

Table 1

Product sampling frequency
RTE risk category Antimicrobial agent used Table Note 1 No Post-lethality treatment Post-lethality treatment Table Note 2
Category 1 No Once per month Once every 6 weeks
Category 1 Yes Once every 6 weeks Once every 2 months
Category 2A No Once every 2 months Once every 3 months
Category 2A Yes Once every 3 months Once every 6 months
Category 2B N/A Once every 6 months Once per year

Note: Frequencies should be adjusted based on the trend analysis of the results.

For an establishment producing products under different RRLs, the establishment's sampling frequency will be established based on the product with the highest RRL. For a given sampling frequency, higher risk products should be sampled at a proportionally higher rate.

Environmental sampling program

Sampling and testing the areas where the food is prepared, as well as the equipment used to prepare the food, will determine the presence of L. monocytogenes or any Listeria species (spp.). This is important to confirm whether the controls you have applied are effective.

Section 7.2 and figures 1, 2 and 3 of Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods provides guidance on environmental sampling of both food contact surfaces (FCS) and non-food contact surfaces (NFCS). The MFLP-41- Environmental Sampling for the Detection of Microorganisms method ‒ available in Health Canada's Compendium of analytical methods – Volume 3 provide guidance on sampling materials, procedures, sampling sites and how samples are to be handled and transported.

Your environmental sampling program should outline:

Table 2 presents FCS testing frequencies when preparing the highest risk Category 1 RTE food.

Table 2

FCS testing frequency per production line according to the relative risk level for the highest risk Category 1 RTE foods: Deli meat products Table Note 1, Hot-dogs, smoked seafood and cooked ready to eat crustaceans Table Note 2
Production volume Table Note 3 No antimicrobial agent or post-lethality treatment Antimicrobial agent Table Note 4 Post-lethality treatmentTable Note 5 Antimicrobial agent and post-lethality treatment
Very small Once per month Once every 2 months Once every 2 months Once every 3 months
Small Twice per month Once per month Once per month Once every 2 months
Medium 3 times per month 3 times every 2 months 3 times every 2 months Once per month
Large 4 times per month Twice per month Twice per month Once per month

Table 3 presents FCS testing frequencies generally recommended.

Table 3

FCS testing frequency per production line according to relative risk level (RRL) for RTE foods not covered by Table 2
RTE risk category Antimicrobial agent used Table Note 1 Post-lethality treatment not applied Post-lethality treatment is applied Table Note 2
Category 1 No Once per month Once every 2 months
Category 1 Yes Once every 2 months Once every 3 months
Category 2A No Once every 3 months Once every 6 months
Category 2A Yes Once every 6 months Once per year
Category 2B N/A Once every 6 months Once per year

It is strongly recommended that you hold the food being produced during the environmental sampling because it may need to be tested if the test results are unsatisfactory. Having the food under your control makes follow-up activities easier.

Follow-up procedures

Follow-up procedures are set out in Figures 1, 2 and 3 of Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods. These procedures include taking corrective actions as soon as possible when an environmental or RTE food sample is found to be unsatisfactory.

You can determine in advance, and describe in your PCP, the corrective actions you would take when there are unsatisfactory results. They should include:

Unsatisfactory Food Contact Surface samples

The following are examples of appropriate corrective actions after the initial finding of Listeria:

It is recommended that:

  • you hold Category 1 RTE foods prepared on the affected production line, after corrective actions are taken, until FCS samples are negative for Listeria

Persistent contamination

If two or more FCS samples from the same production line are found positive for Listeria within a short timeframe, this is considered persistent contamination and an indication that the sanitation procedures or Listeria control measures are inadequate.

It is recommended that:

  • you test for L. monocytogenes category 1 RTE foods prepared on the affected production line and held after the initial corrective actions were taken
  • you inform the CFIA when L. monocytogenes is detected in a food sample
  • the corrective actions are considered effective only when the results for FCS and Category 1 RTE food samples are satisfactory for 3 or more consecutive days of production
    • When there is an unacceptable result, continue taking additional corrective actions and follow-up sampling of FCS and Category 1 RTE foods until the corrective actions are effective

Implement additional intensified corrective actions such as:

It is recommended that:

  • you hold category 2 RTE foods, prepared on the affected production line, after additional intensified corrective actions are taken, until FCS samples are negative for Listeria
    • take follow-up samples from the same FCS as soon as possible (within five production days) after Listeria is detected to try and identify the exact sources of contamination

If Listeria is detected again in a FCS sample, it is recommended that you:

  • test for L. monocytogenes category 2 RTE foods, prepared on the affected production line and held after the intensified corrective actions were taken
  • inform the CFIA when L. monocytogenes is detected in a Category 1 food sample or exceeds 100 cfu/g in a Category 2 food sample

Unsatisfactory non-Food Contact Surface samples

Follow-up procedures are set out in Figure 3: Sampling for NFCS especially those in proximity to food contact surfaces (FSC), linked to RTE foods in Category 1 and 2, found in Health Canada's Policy on Listeria monocytogenes in Ready-to-Eat Foods.

The presence of Listeria on NFCS is an indication that improvements are needed to the maintenance and operation of the establishment.

Follow-up procedures are implemented based on the potential for product contamination taking into consideration surfaces that routinely come in contact with an object that is in direct contact with an exposed RTE food or FCS without a sanitizing step, such as:

Unsatisfactory RTE food samples

Implement corrective actions immediately when receiving an unsatisfactory RTE food sample results, such as:

Section 7.3 of the Policy on Listeria monocytogenes in Ready-to-Eat Foods references hold and test procedures. The same food that was assessed unsatisfactory under the initial sampling plan should be sampled under hold-and-test procedures. If not available, another RTE food (preferably similar) prepared on the affected line can be selected.

Corrective actions are considered effective only when:

  • the follow-up samples of an RTE food, prepared on the affected line, test negative for L. monocytogenes.
    • For a Category 1 RTE, the samples need to test negative for a period of 5 consecutive days

Presence of L. monocytogenes at low levels in category 2 products:

  • Note that the presence of L. monocytogenes at low levels (≤ 100 cfu/g) in a category 2A or 2B product, while not unsatisfactory in most circumstancesFootnote 1, can be an indication of a possible loss of control and should prompt the operator to verify and/or re-evaluate his process and control measures

Trend analysis

Performing a trend analysis of the test results is an essential component of any sampling program designed to monitor a microbiological risk. Trend analysis allows you to identify a potential loss of control or inadequate controls that can be corrected before they result in non-compliance. For example, corrective measures could be taken when the trend analysis shows that:

The trend analysis procedures should:

Tell me more! References and further reading

The following references contain information that helps explain food safety controls, demonstrates how to develop them, and provides examples. The CFIA is not responsible for the content of documents that are created by other government agencies or international sources.

CFIA references

Other further reading

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