Increasing Listeria Awareness

May 1, 2005

11 Min Read
Increasing Listeria Awareness

May 2005

Increasing Listeria Awareness

By Bruce FloydContributing Editor

According to FDA/CFAN's "Bad Bug Book," Listeria monocytogenes is "a Gram-positive bacterium, motile by means of flagella. Some studies suggest that 1% to 10% of humans may be intestinal carriers of L. monocytogenes. It has been found in at least 37 mammalian species, both domestic and feral, as well as at least 17 species of birds and possibly some species of fish and shellfish. It can be isolated from soil, silage and other environmental sources. L. monocytogenes is quite hardy and resists the deleterious effects of freezing, drying and heat remarkably well for a bacterium that does not form spores. Most L. monocytogenes are pathogenic to some degree."

Because Listeria survives in a wide temperature range and thrives between 40?F to 112?F, which includes refrigeration temperatures, it has become a serious threat, especially in refrigerated RTE foods like cheese, hot dogs and lunch meats.

The FDA/USDA joint task force on Listeria has set the target for serious infection rate for the year 2010 as 0.25/100,000 population. On Oct. 17, 2003, USDA FSIS released data "showing a one year, 25% drop in the percentage of positive Listeria monocytogenes samples and a 70% decline compared with years prior to the implementation of the Hazard Analysis and Critical Control Points (HACCP) system." In 2003, the infection rate was estimated at 0.7/100,000 population or about 2,000 cases.

Prevention: the best medicine Prevention involves strict adherence to good manufacturing practices (GMPs) and adequate processing. FDA and FSIS studied the risk associated with various food. Products that have the kill step in the final container have the lowest risk.

One example of high risk, according to George Flick, Ph.D., professor in the Department of Food Science and Technology, Virginia Tech, Blacksburg, would be cold-smoked fish. This product does not undergo a kill step. The manufacturer's risk is reduced by strict adherence to GMPs during processing and using fish that lack L. monocytogenes, such as fish from aquaculture. According to FDA's compliance program guidance manual, "Import Seafood Products Compliance Program," completed in 2003, high-risk products include RTE fish or fishery products using heating or pasteurization processes (e.g., cooked shrimp, crabmeat, cooked lobster, cooked crayfish, pasteurized crabmeat, surimi-based analogues, etc.) and hot or cold smoking processes. The manual notes, "The major concern is proper processing to prevent toxin formation by Clostridium botulinum or Staphylococcus aureus and infections by possible post-process pathogens, including Listeria. The concern is that these products are subject to the growth of post-process contamination by pathogens." Manufacturers can introduce lethality steps, but these can change the final quality of the product.

Considering alternatives Although Listeria is common in raw products, adequate cooking kills it. One major case of listeriosis in the 1980s involved inadequately pasteurized milk used in soft, Mexican-style cheese.

Typically, processing failures are not a general problem in the United States. The major domestic problem is with post-processing contamination of certain RTE foods. The FSIS document, "Compliance Guidelines to Control Listeria Monocytogenes in Post-Lethality Exposed Ready-to-Eat Meat and Poultry Products," was precipitated by the 1998 Bil Mar hot dog recall (see rdad/FRPubs/97013F/Lm_Rule_Compliance_Guidelines_2004.pdf). The guidelines put risks into four categories depending on the steps taken by the processor to control L. monocytogenes after the kill step. Those products not exposed to the environment after processing, products with a water activity less than 0.85, certain hot-fill products, acidified products and commercially sterile products, among others, are not considered at risk. The guidelines classify products exposed after the lethality treatment into three categories:

Alternative 1. "Alternative 1 requires the use of post-lethality treatment (which may be an antimicrobial agent or process) to reduce or eliminate L. monocytogenes and an antimicrobial agent or process to suppress or limit the growth of the pathogen. The post-lethality treatment that reduces or eliminates the pathogen must be included in the establishment's HACCP plan. The post-lethality treatment must be validated according to 9 CFR 417.4 as being effective in eliminating or reducing L. monocytogenes to undetectable level, and the validation should specify the log reduction or suppression achieved by the post-lethality treatment and antimicrobial agents."

Alternative 2. "An establishment that identifies its products in Alternative 2 must apply either a post-lethality treatment or an antimicrobial agent or process that controls the growth of L. monocytogenes. Post-lethality treatments and antimicrobial agents and processes discussed above in the section on Alternative 1 can be used for Alternative 2. If an establishment produces a product under Alternative 2 by using an antimicrobial agent or process that suppresses or limits the growth of L. monocytogenes in its product, it should maintain sanitation in the post-lethality environment in accordance with part 9 CFR 416. The sanitation program must include testing for food-contact surfaces in the post-lethality environment to ensure that the surfaces are sanitary and free of L. monocytogenes or its indicator organisms (Listeria spp. or Listeria-like organisms). Studies on antimicrobials showed growth inhibition of L. monocytogenes if present at low levels of contamination during the shelf life of the RTE product. Antimicrobials were not shown to be effective at higher levels of contamination, so an effective sanitation program, which includes verification testing for food-contact surfaces, should be implemented at the same time that antimicrobials are used."

Processes and chemicals are acceptable for use under Alternatives 1 and 2. Processes can include steam, hot water or radiant heat (Infrared Grill or the Aquaflow Food Processor, both from Unitherm Foodsystems, Bristow, OK), treatment of the product pre- and post-packaging, freezing of the product in the final package if it remains frozen until the time of use, or ultra-high-pressure processing in the final package (Avure Technology, Kent, WA).

Several chemicals are permitted either as surface sprays or in formulating the product. These treatments have advantages and disadvantages. The main problem is a change in flavor and/or appearance. These include sodium and potassium lactate (Purac America, Lincolnshire, IL), acidified sodium chloride (Bio-Cide International, Inc., Norman, OK), nisin (e.g., NOJAX AL hot dog casings, Viskase Companies, Inc., Willowbrook, IL), sodium diacetate and glucono-delta-lactone in combination with sodium lactate (see the FSIS "Updated Compliance Guidelines" for complete details). Processors have used an ozone solution (DEL Ozone, San Luis Obispo, CA) as an antimicrobial in casing soak and as a meat-surface spray prior to packaging. One advantage to acidified sodium chloride and ozone is no residual flavor.

Processors can also fight microbes with microbes. Chr. Hansen, Mahwah, NJ, has two microbial cultures, B-SF-43 SafePro(TM) and B-2 SafePro, that will suppress the growth of L. monocytogenes. The company recommends these for cooked or cured meats that are vacuum packed or modified-atmosphere packed.

Regardless of the system, manufacturers must prove procedural efficacy in actual use. There are no processing authorities as such, so each company must ensure that the person validating the process is knowledgeable about the system and the microorganism.

Alternative 3. "Under Alternative 3, the establishment does not apply a post-lethality treatment or an antimicrobial agent or process to control the growth of L. monocytogenes in the post-lethality exposed product. An establishment producing this type of product must control the pathogen in its post-lethality processing environment through the use of sanitation control measures, which may be incorporated in the establishment's HACCP plan, Sanitation SOP or prerequisite program. Because the establishment is not relying upon a post-lethality treatment or an antimicrobial agent or process to control L. monocytogenes, the product will be subject to frequent FSIS verification testing compared to the other alternatives. Examples of products in this alternative are fully cooked meat and poultry that are packaged and refrigerated, such as hot dogs, deli meats, chicken nuggets or chicken patties that did not receive any post-lethality treatment or antimicrobial agent or process."

John Stumpf, Biological Research Services, Hewitt, TX, says flavor is very important for high-value RTE products. Some treatments approved for Alternative 1 or 2 can affect flavor. In such cases, Alternative 3 is a better choice.

With alternative 3, the manufacturer must keep the environment free of Listeria -- ideally with a clean room similar to the computer or pharmaceutical industries. Stumpf mentions that in addition to the obvious issues of cleaning and sanitation, two areas can cause problems. One of these is high-air-velocity cooling systems that tend to rip bacteria from surfaces and make them airborne. One solution is cold fogging the cooling system with acidified sodium chlorite, quaternary ammonium or stable peroxides. Another potential problem area is cross-contamination between raw and cooked sides in RTE plants due to poor traffic control.

Manufacturing plants can use some of the chemicals mentioned earlier to sanitize cleaned equipment with no residual sanitizer to affect flavor. Some examples include an ozone rinse at 2 ppm and acidified sodium chloride at 25 ppm for 60 seconds.

Today's best testing Testing for L. monocytogenes verifies the effectiveness of the controls used. AOAC International, Gaithersburg, MD, should approve the method, or it should be equivalent to FSIS testing procedures (see the "Microbiology Laboratory Guidebook" on

A plant can use whatever method is available. The BA system by DuPont Qualicon, Wilmington, DE, is the only rapid method listed. According to the guidebook, labs must confirm a positive result using BAX by the traditional plating method. FDA uses the method published in its "Bacteriological Analytical Manual" (BAM; see

This does not mean that manufacturers cannot use rapid methods. According to the FSIS compliance guidelines for RTE meats: "The genus Listeria includes other species in addition to monocytogenes. Therefore, a positive test for Listeria spp. or Listeria-like organisms would indicate the potential presence of the pathogen. If these specific indicator organisms test negative, this is indicative that L. monocytogenes is not present. Aerobic plate counts, total plate counts and coliforms are not appropriate indicator organisms for L. monocytogenes. Results from these tests do not indicate the presence or absence of the pathogen, although they could provide a measure of general sanitation."

FSIS provides detailed explanation on how to take the samples. For example, make sure a similar or larger sample is taken. Interestingly, cleanliness-testing devices, such as ATP testers, are not acceptable substitutes for microbiological swabbing of food contact surfaces (see "Microbiological Testing," Food Product Design, June 2001).

Oxoid Limited, Basingstoke, England, has launched a new broth for the complete, selective enrichment of Listeria species from food samples in 24 hours. Oxoid Novel Enrichment (ONE) broth for Listeria, distributed in the United States by Remel Inc., Lenexa, KS, eliminates the need for secondary enrichment. The company's rapid identification kit can differentiate between Listeria species and similar organisms and L. monocytogenes on a general pour plate. The method is said to take 10 minutes to confirm if a colony is L. monocytogenes.

3M, St. Paul, MN, has added a Listeria plate to its Petrifilm line. This is a general plate for environmental testing.

Not a one-trick pony The controls for L. monocytogenes apply across all segments of the food industry, not just meat products. The progress made in Listeria control has come through improved sanitation, equipment design and environmental controls (including environmental testing), as well as a better understanding of processing risks through HACCP application.

In non-shelf-stable products, what happens in the product from the time it leaves the line until it is consumed is important. If post-processing contamination is possible, steps can be taken to minimize or reduce the growth of Listeria during the refrigerated shelf life, such as through improved packaging-line design, more-effective sanitation procedures, strict adherence to GMPs, reformulation of the product and/or the introduction of a lethality treatment after packaging.

Remember: One purpose of hazard analysis is to find a way to add control to a process when an uncontrollable risk is discovered.

Bruce Floyd established Process Systems Consulting, Iowa City, IA in 1997 after working more than 30 years in the food-processing industry. He has had extensive experience in sanitation, quality control, regulatory relations, and product and process development (domestic and international), and specializes in adapting ingredient and manufacturing specifications to the equipment available to create the desired finished product. A graduate of Georgia State University, Atlanta, he has successfully completed all areas of the Better Process Control School, has been qualified as an instructor by the International HACCP Alliance, and has been certified as an FPA-SAFE auditor by the Food Processors Association.

Back to top

3400 Dundee Rd. Suite #360Northbrook, IL 60062Phone: 847-559-0385Fax: 847-559-0389E-Mail: [email protected]Website:

Subscribe and receive the latest insights on the health and nutrition industry.
Join 37,000+ members. Yes, it's completely free.

You May Also Like