Preventing Listera monocytogenes in Ready-to-Eat Prodcuts

Preventing Listera monocytogenes

in Ready-to-Eat Meat Products


Lynn Knipe, Ph. D.

The Ohio State University

USDA FSIS data show that problems with Listeria monocytogenes in fully cooked, ready-to-eat meat products are greater for smaller processors, than for larger processors.  These problems often result in public recalls of products, which is never an easy process for any company to deal with.  There are a number of techniques that the larger processors are using to eliminate Listeria from their products.

Of course there are antimicrobial agents, such as lactates and diacetates, as well as post-packaging pasteurization.  The antimicrobial agents are not difficult to add to products, however, if not added at the proper level or if added to mildly seasoned products (such as turkey breasts or roast beef), vinegar-like flavors may be noticed.

Post-packaging pasteurization involves heating the surface of packaged ready-to-eat meat products until 1 to 2 logs of Listeria monocytogenes are destroyed on the products surfaces.  The process has been shown to be more effective than the addition of antimicrobial agents in eliminating Listeria from packaged products, however, verifying the effectiveness of this treatment is not easy for smaller processors.  Also, this process may also cause undesirable product changes if the packaged products are heated too much or too long.

Some larger processors claim that proper cleaning and sanitation of their processing facilities is the most effective method from preventing problems with Listeria monocytogenes.  And if you are making products under Alternative 3, sanitation is the major means for preventing Listeria in your products.

Proper cleaning and sanitizing includes concepts as simple as using the proper cleaning compounds and sanitizers for the job that needs to be done.  Ask your chemical supplier (and his/her competitors) for advice on the best cleaners and sanitizers (as well as concentrations) for what you need to clean.  Cleaning should include the use of brushes or abrasive pads or clothes to remove all fat and protein, which will lead to development of bio-films on equipment surfaces.  It is a good idea to use different brushes for processing areas than those used on the harvest floor.  Special attention should be paid to equipment surfaces which actually come in contact with the cooked product during the packaging process.   

After cleaning and rinsing, equipment and parts should be either sprayed or submerged in a sanitizer.  Suppliers’ instructions should be checked to be sure, but most sanitizers are intended to be diluted with room temperature water, not hot or cold water.  Check sanitizer instructions for minimum times that sanitizers should be in contact with equipment.  Also, check to see if sanitizer should be rinsed from equipment before it makes contact with meat.

In addition to the pre-operational visual checks, it would be wise to monitor procedures that the cleaning crew does, such as: proper mixing of cleaning solutions and sanitizers, proper application of sanitizers, proper asceptic reassembly of equipment, etc.  These monitoring procedures could be done as part of the SSOP or a good manufacturing practice (GMP).

If you have smoke houses in your operation, some equipment could be heat or steam sanitized by placing the equipment directly into the oven and applying heat and/or steam.  Obviously, be sure that all parts of such equipment are tolerant of the temperatures or steam that you are thinking of applying to them.

Understandably, not all operations are able to totally separate their raw from their cooked products.  But, every attempt should be made to separate raw from cooked products in time or space.  It might make the process easier to color code tubs that the products are held in, knives, thermometers, workers coats or uniforms, disposable gloves, etc. 

A room or portion of a processing room should be designated for packaging of cooked products.  This is sometimes called a ‘clean room.’  Workers handling the ready-to-eat products during packaging should be properly covered (disposable gloves, aprons, sleeve covers, etc.) to protect the product from the environment.  And only the workers who are assigned to work in this area should be in this area.  Access to this room, by other workers, should be minimized, and multiple doors which lead to hallways, should be blocked off.  If possible, it is even wise to have workers wear boots or other foot wear that they only wear in this clean room, and not outside the area or the plant.  Floor sanitizers are recommended for clean rooms, and dry sanitizers are most effective, to keep the floors as dry as possible.  Listeria has more difficulty getting established in a dry room.

Drains should be cleaned on a regular basis, but only at the very end of the cleaning process, and using brushes and utensils that are only used when cleaning the drains. 

To monitor the cleaning process, most people are familiar with pre-op sanitation checks in their SSOP.  However, equipment which looks clean may still contain Listeria.  A better method for making sure that the cleaning and sanitation processes are adequate is through testing for presence of Listeria on contact surfaces.

This involves swab testing as many contact surfaces in the clean room and the chill cooler, as possible for Listeria.  This is not a popular statement, as Listeria tests are expensive.  However, swabs from multiple sampling sites could be combined in the test solution, also called a ‘composite sample,’ to lower the cost of Listeria testing.  If you combine swabs from 5 contact surfaces into one sample, and that sample comes back negative, then Listeria was not present on any of those 5 sites.  However, if a composite sample comes back positive, you then would need to sample each of those 5 sites separately to determine where the Listeria is present at.

It is best to make a list of the sites that cooked products make contact with from the time they leave the oven until they are in the package and sealed.  It is understandably difficult to sample all of these sites at one time, but each week (or each time) that you sample work down the sample site list to sample as many sites as possible as soon as you can. Negative results for Listeria, from these contact surfaces, tells you two things: you didn’t have Listeria growing at that site on the day and at the time you swabbed that site, and that you haven’t found where it is growing yet in your operation.   Negative results are not necessarily something to celebrate, and mean that you need to keep looking for the Listeria in new places, but also, occasionally return to sites that you have already swabbed to be sure that Listeria has not found its way to surfaces previously testing negative.  This should be a dynamic process, as sampling sites are likely to change with the change of seasons, volume of production, corrective actions taken with previous positives.  

Getting a positive Listeria result on a contact surface should be looked at as a positive thing, not a point to punish sanitation workers for.  This may seem like a strange approach, but Listeria is so common in our environment, that if you get a positive contact surface result, you now know where to prioritize your future cleaning efforts to get rid of the Listeria.  Think of this process as a ‘seek and destroy’ mission.  Following a positive Listeria sample, that equipment should be thoroughly disassembled again, and very carefully cleaned and sanitized, before another sample is taken.

Results of your Listeria testing should be plotted on a floor plan of your operation, to assist in detecting trends of where Listeria is showing up.  Where and how often you sample, and how you respond to negative and positive results for Listeria, is information that should be compiled to make a sanitation program, which is a part of Alternative 3 processing.  An example of a sanitation program is available on the OSU website at: .  If you would like a copy mailed to you, please contact me at (614)292-4877.

Listeria training courses have been conducted in the past at OSU, but it has been a while since the last one.  If you would be interested in the opportunity to attend a course of this type (which would go into much more detail than this article), please contact me at: or (614)292-4877.