Keeping the pathogen out of the plant is essentially what sanitarians preach. Sanitarians focus on minimizing the use of sanitizing chemicals. This strategy reduces costs and exposure to toxic chemicals, and by definition focuses attention on control strategies. These include identifying and studying hygienic zones, personnel hygiene, traffic patterns in the plant, and the array of sanitation steps taken over all shifts. An article by Dr. Bob Powitz describes this approach.

Eradicating Listeria when it is found is usually the result of a find it and kill it approach–often called “seek out and destroy”, an aphorism coined over a decade ago by  Dr. John Butts. The principles and techniques outlined in these mini-articles are very much aligned with this kind of approach. They are also the ones employed by most food companies in their management of Listeria risk. And that is mainly because Listeria can still find a way into a plant, in spite of best efforts to the contrary.

It is beyond the scope of this mini-article to go into all of the different chemicals that can be used to attack Listeria. Common chemicals include peroxyacetic acid, often used in plants for which an organic sanitizer must be used. Chlorine dioxide gas is used when companies want to introduce a sanitizing chemical across the entire plant, essentially as a “fumigation” technique. This can be useful since the gas can reach areas in which liquid sanitization techniques are ineffective.

The most common chemicals used against Listeria are the “quats”, mixtures of quaternary ammonium compounds. Not only are these compounds effective sanitizers, but it is generally believed that quats have a substantive property which allows them to remain on surfaces for a time. Thus they can have an ongoing effect against Listeria.

Highly experienced food plant chemical companies are well versed in sanitation chemicals and procedures. They are the best source for determining which chemicals are the most effective for your situation, at what concentrations (e.g., to achieve a no-rinse level on equipment), and with best-practice standard sanitation procedures. Highly reputable chemical companies understand sanitation, sanitization, and disinfection–and they can teach these concepts, and solve contamination issues.

FDA (“draft guidance for industry”) and USDA (“compliance guideline”) provide guidance on what corrective actions (e.g., sanitization) are required when Listeria is found. Regardless of which actions are decided upon, their effectiveness must be verified and validated. This requires well thought out statistical sampling and risk assessment, and continued testing.

Constant vigilance is very important. Use checklists that are centered on GMPs (Good Manufacturing Practices) as a start, using these to ensure that areas are sampled appropriately. But make sure that you are not solely relying on checklists (see this article on checklists that can be traps). Too often, companies find Listeria, throw some sanitizer on the area in question, sample, get a negative (the pathogen is no longer found in that location)–and then move on, convinced that they have solved the problem.

This is rarely the real outcome unless you have determined definitively that the root source has been eliminated. In addition, you have to be convinced that the transfer points around the root source are no longer in play. They have to be eliminated or highly controlled.

Said another way, you are rarely done. As frustrating as this might seem, constant microbiological vigilance is what keeps your products safe. Not doing so can end up being very costly to the company in insurance costs, recall costs, and legal costs, and in brand image to customers and consumers. All are avoidable.

It is much better to maintain and improve your environmental monitoring programs to constantly protect public health. So keep sampling and testing!

This is the 5th in a series of 6 Listeria in Food Plants articles. See the Related Articles below to read the series.

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Before you go looking for Listeria, you need to make sure that senior management across all functions is supportive of this task–especially when Listeria is found. Will someone (or some function such as QA) get blamed? Will enough resources be given? Will outward communications (e.g., to customers or FDA) be actively managed?

Or is the collective opinion “we do not need to know, because Listeria is surely not here”?

Most food companies make the correct strategic and ethical decision to test for Listeria. To that end, they typically test for the genus of Listeria species (L. spp.), and not specifically for Listeria monocytogenes (Lm). Thus, they do not end up with confirmation that Lm is present and therefore do not butt up against a regulatory zero-tolerance policy. But they get a strong indication that conditions are favorable to Listeria growth, so they can choose to assume that Lm is present and act accordingly. If corrective actions are taken to eliminate all Listeria, then one can justify that if Lm were present, it would be eradicated by these corrective actions.

Some companies test for a “marker organism” such as Listeria innocua. The thinking is the same as that above. This includes making the assumption that if Listeria innocua is found, then Lm may indeed also be present.

If your company has decided to look for Listeria, how do you go about doing so? The overall objective is to establish, validate, and execute an “environmental monitoring program” (EMP). Good starting points are the “Environmental Monitoring Handbook” published by 3M and Cornell University and a publication by the Institute of Food Technologists, “Design Elements of Listeria Environmental Monitoring Programs in Food Processing Facilities.” For seafood, the National Fisheries Institute has a publication on ready to eat seafood pathogen control.

Key to an effective EMP is sampling. Sampling is complicated to plan since the choices affect time and money. How many samples, where, and what are you sampling for (e.g., Lm, Listeria innocua, or L. spp.) ? These choices affect costs and how soon, or not, microbiological results are received. Random sampling never carries the day. Rather, sampling needs to be strategic—based on sound statistical principles, science, and  your own assessment of what might really be going on.

Building a useful EMP is hard work, and requires a lot of patience and a lot of data. Listeria does not sit still–plant conditions change all the time. Hence, a positive sample today does not guarantee that there will be a positive sample tomorrow (especially when microbiology results take days to obtain). A good EMP finds growth niches and transfer points, and helps determine the overall risk for nearby food becoming contaminated.

In addition, actual as-made equipment design is incredibly important. Equipment not made hygienically, or that has been changed over time (“to make it work better”), may end up being a root-source of Lm. A good review of proper hygienic design can be found in “Food Safety Equipment Design Principles” by the Foundation for Meat & Poultry Research & Education.

Here are some effective approaches used in the food industry:

1. Bring in an expert third-party consultant or company to do the work for you.
2. Utilize the services of a certified and vetted microbiology lab, and partner with them.
3. Purchase best-practice software which models an EMP, and directs sampling for you.
4. Review the FDA and USDA guidance documents for direction–and then speak with your local regulators. (Yes, this is not without some risk.)
5. Conduct multiple “swab-a-thons” in your plant, sampling anywhere and everywhere to gather microbiological data. As mentioned above, it can be notoriously difficult to establish patterns of harborage and movement–but having hundreds of data points can at least give you a snapshot for the microbiological cleanliness of the plant. Note that this is time-consuming and costly, but could pay dividends if this is one of your only options.
6. Study, study, study. Map and analyze traffic patterns in the plant, evaluate water use and water flows (e.g., to and from drains), sample equipment around product zones, look at data from pre-operational activities, look at past microbiological sampling data, and talk with the sanitation team about what they see.

Note that it is generally not recommended to sample actual product given the regulatory consequences if Lm is found!

Regardless of the tactics chosen, Lm is typically hard to find in a “clean” plant. And determining where it comes from (i.e., the root source) is even harder. Microbial testing is still the best way to do this. Techniques such as ATP testing, total plate count or Enterobacter testing, and PCR-assay Listeria test kits can help guide you. But those results are only as good as the level of technical thinking that is brought to the them. One must truly evaluate where Listeria growth niches might be, regardless of how easy or hard it is to access those locations.

See the Related Articles below to read the series.

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