Eradicating Listeria Monocytogenes From Mushroom Slicing and Packaging Environments

Now is the time for the mushroom industry to take all possible measures to minimize risks for Listeria contamination.
Eradicating Listeria Monocytogenes From Mushroom Slicing and Packaging Environments - Articles


For many years, it was generally accepted that concerns over Listeria monocytogenes contamination were limited to ready-to-eat meat and poultry products, dairy products, raw and smoked fish, and deli salads. Only in the last few years have we come to realize that people can become ill from eating fresh produce contaminated with this pathogen. Recent outbreaks attributed to L. monocytogenes have been traced to diced celery processed in Texas in 2010 where 10 became ill and 5 died, a 2011 outbreak in Colorado-grown cantaloupe which was responsible for 147 illnesses and 33 deaths in 28 states, a 2014 mung bean sprout outbreak in which 5 became ill and 2 died in 2 states, and a caramel apple outbreak in 2014 that caused 32 illnesses and 7 deaths in 11 states. As government agencies step up their monitoring programs, recalls of fresh produce are also on the rise. In 2014 alone, we saw several recalls of Listeria contaminated produce including fresh cut apples, and whole peaches, plums, nectarines, and mangoes.

To date, there have been no reported cases of foodborne illness attributed to fresh mushrooms. However, between 2011 and 2015, recalls of sliced mushrooms contaminated with Listeria monocytogenes have occurred each year. Although the products have primarily been linked to growers in Canada, U.S. growers should take no comfort from this fact. It is likely that Canada food safety authorities have developed more robust Listeria monitoring programs in recent years. Given the recent passage of the Food Safety Modernization Act (FSMA), it is only a matter of time before the U.S. Food and Drug Administration (FDA) ramps up its monitoring and inspection programs to a level equal to our neighbors to the north.

While we have a bit of breathing room between implementation of FDA food safety regulations for growers, packers, and processors of fresh produce, now is the time for the mushroom industry to take all possible measures to minimize risks for Listeria contamination. An understanding of the unique characteristics of L. monocytogenes is an essential first step toward taking action to keep it out, keep it from growing, and kill it before it gets onto mushrooms.

Listeria monocytogenes is one of several species of the genus Listeria and is the only one that can cause serious illness. It is unique among bacterial pathogens (e.g. E. coli, Salmonella) in that it can survive within a wide range of pH and salt concentrations, it is comparatively more heat resistant, and it can grow at refrigeration temperatures. L.monocytogenes is found in the intestinal tract of many species of mammals and birds and some studies suggest that up to 10% of humans can be non-symptomatic carriers. When not infecting animals or humans, it can exist in a spore-like state that allows it to survive for many years in agricultural environments including soil, cull piles, bins, and equipment in packing houses and fresh-cut processing areas.

Listeriosis is the name given to a range of symptoms caused by infection with L. monocytogenes. In otherwise healthy people, fever, muscle aches, nausea, and diarrhea may occur which goes away in a few days. However, “at risk” populations including fetuses of pregnant women, newborns, the elderly, and people on long term medical treatments, may suffer more serious outcomes such as blood infection, meningitis, and encephalitis. It is estimated that approximately 20 to 30 percent of severe infections result in death. We are not sure how many bacterial cells someone has to ingest to become sick, but anytime conditions are right for L. monocytogenes to grow to large numbers, the chances to illness to occur in healthy and “at risk” individuals greatly rises. For this reason and because of the potentially severe consequences of the disease, U.S. regulatory agencies have adopted a "zero tolerance" policy for L. monocytogenes in ready-to-eat foods that support bacterial growth.

Because it is ubiquitous in the environment, multiple species of Listeria and strains of L. monocytogenes are continually introduced into packing and processing plants by people’s hands, clothes, and footwear; incoming products and the containers that hold them; airborne dust through open doors and windows; and from water entering through leaky roofs. Specific strains of L. monocytogenes that are particularly well adapted to food and moisture sources within facilities tend to out-compete others and quickly colonize floors, drains, and equipment surfaces. Once established, it is very difficult to completely eradicate persistent strains. A long term commitment toward developing, implementing, and verifying sanitation programs is therefore essential for keeping this pathogen under control thereby reducing risks to consumers and to your company’s bottom line.

Now that you have a general idea of the unique characteristics of L. monocytogenes, we can begin to consider specific locations where it is likely to thrive. Think about areas in your facility that are cool and continuously wet, where there is a regular supply of food (nutrients), and where there is plenty of time for Listeria to adapt and grow. These are places that, as a consequence of incomplete cleaning procedures or poor sanitary design, allow plenty of time for Listeria to establish itself and remain persistent for years, or even decades. With all this in mind, let’s consider some ways Listeria can enter you facility, where it is likely to survive and grow, and some steps you can take to keep it under control.

Exterior grounds and buildings

Step outside and have a good look at the condition of your facility. Exterior areas should be free of tall weeds, grass, and other vegetation that can provide hiding places for insects, rodents, and birds. When it rains, water should flow away from the building. Holes or large cracks on exterior walls should be repaired so that pests cannot get in. The roof should be sloped to drain away water and regularly maintained to keep moisture from seeping inside.

Building interiors and traffic control

Take a broad look at the general organization of your operation. The flow of mushrooms through the facility should be designed so that incoming materials are physically separated from outgoing products. There should be plenty of space for workers to do their jobs efficiently and sufficient lighting to allow for continuous inspection for pests and unsanitary conditions and practices. Equipment should be 12-18 inches away from the walls and materials should be stored on pallets at least 6 inches off the ground. Movement of workers between mushroom receiving docks or other entrances should be controlled so that access to washing, slicing, and packaging areas is limited to designated individuals. Color coded uniforms for those working in product areas are a good way to monitor compliance with this policy. Make sure that everyone who works at the packing facility understands this, including office workers, maintenance workers, and visitors.

Entrances to packing and processing areas

Check to see that doors fit tightly. A good exercise is to turn off the lights and look for any signs of light coming in around doors, loading dock entrances, and the ceiling. These are potential entry points for pests and water and should be repaired or sealed promptly. The best window in a food plant is no window! But if you have them, make sure they are sealed tightly and kept closed. Plastic strip curtains are often used at entrances to areas with heavy equipment and worker traffic. But they have only limited ability to keep pests, dust, and air from moving into processing and cold storage rooms. It is best to keep doors closed during periods when traffic is light. When moving boxes of mushrooms through entrances with strip curtains, do not allow them to slide up and over the top-most box where they can contaminate exposed mushrooms. Train workers to push the curtains aside as they bring mushrooms in or cover the load to prevent contact.

Footbaths or automatic floor spray systems are good ways to prevent Listeria from entering your facility. Install them wherever people or forklifts enter the facility. Do not allow other areas, such as loading docks, to be used as worker entrance points. Check with your chemical or sanitation supply company to find the best system for your situation. Make sure the sprays are operating correctly and that footbaths are continuously supplied with sanitizer.


Research has shown that Listeria contaminated floors and drains that are close to production lines increases the chance that products and product contact surfaces will become contaminated. Because they are collection areas for water and mushroom debris, floors should be a high priority in your Listeria control program. Floors should be smooth and impervious to water so they are easy to clean and sanitize. The most common flooring material in packing houses is concrete. Unsealed concrete is not suitable for continuously moist areas such as mushroom washing and slicing areas or in walk-in coolers. These floors need to be sealed with epoxy, polyurethane, acrylic, or polyester coatings so they will not absorb water. However, even on properly sealed floors, the coating will wear down over time with continuous exposure to harsh cleaning chemicals and heavy equipment traffic. Once the underlying concrete is exposed, moisture and mushroom debris will penetrate deep within the sponge-like material and create ideal conditions for Listeria to establish itself. Incomplete or sloppy floor patching often does little more than to add to the problem. Find a credible industry floor contractor to determine the appropriate floor coating material for your operating conditions and get recommendations for how often you need to reseal it.

Be sure to include floor mats in your cleaning and sanitizing schedule. Each time the floor is cleaned, gather them up and scrub them in a wash tank. Immersing them in hot water at 170-180°F should eliminate any residual Listeria on them.


It is nearly certain that you will eventually find Listeria in your drains. Like floors, they are continuously bathed in water and mushroom debris. However, they may not be as accessible to regular cleaning and sanitizing if new equipment is carelessly installed on top of drain covers or trench drain grates, or if steam or water drain lines are hard-plumbed into them. Listeria tends to accumulate within a slime layer, known as a biofilm, that physically blocks access to sanitizers. For this reason, drains need to be thoroughly scrubbed with a dedicated brush before applying a sanitizer. Never clean drains during packing and slicing operations. This often generates aerosol droplets that can spread contaminants throughout the building. Never use hoses to direct mushroom debris on floors into the drains. This practice will eventually lead to clogging and overflowing that will spread bacteria onto the floors. Train your workers to use a floor squeegee to gather up debris and throw it in the trash.

Condensate and standing water

Anytime warm humid air meets a cooler surface, condensation occurs. Watch for signs of dripping condensate on ceilings, overhead water pipes, walls, and floors. Consider installing a dehumidifier to help keep the humidity down and keep doors closed as much as possible. Cooling units in walk-in cold storage rooms are especially prone to condensate build-up. Make sure the drip pan under the unit freely drains out of the room. This is an ideal site for Listeria biofilms to become established. Add slow dissolving QUAT sanitizer tablets to the inside of the pans to prevent clogging and overflow. Watch for signs of dripping underneath the pans. This probably means your cooling unit is undersized or that you are allowing too much humid air in. Regularly clean and sanitize underneath the pan, and avoid storing products under cooling units.

Standing water is prevented by sloping floors toward drains. If it is not possible to correct this situation, use a floor squeegee to regularly push puddled water into the drains. Wheels on forklifts, carts, and hand trucks should be shielded so that they do not spray water up off the floor and onto exposed mushrooms.

Sanitary Design of Product-Contact Equipment

Equipment that comes into contact with mushrooms includes containers such as bins, tubs, or baskets; wash tanks, conveyors and slicers; and filling or packaging equipment. Food contact surfaces should be made from materials that do not absorb moisture, resist corrosion and cracking, are smooth for easy cleaning and sanitizing. Equipment should be accessible for regular inspection and maintenance and should be easy to disassemble for regular and frequent cleaning and sanitizing. Avoid designs that include open seams, sharp internal angles, crevices, and metal on metal connections. Equipment should have enough drainage holes to permit rapid draining of water or cleaning and sanitizing solutions.

Look under conveyors for hard to clean areas and point these out to your sanitation crew for extra attention. Replace belts that are worn and frayed since they are hard to clean and sanitize and may be niche sites for Listeria. Be sure your crew cleans under the belts -- not just the top part. Many times mushroom handling equipment is modified to make product flow more efficient. Avoid changes that create overlapping metal on metal or ragged welds. These allow water, and Listeria, to diffuse into areas that are impossible to clean and sanitize. Seams should be welded shut and the welds ground and polished so they are smooth.

Mushroom baskets may be responsible for spreading Listeria and probably mushroom disease organism from farms to packing houses. Too often automatic basket washers fail to do a good job of removing caked on soils which may harbor Listeria cells. Verify cleaning effectiveness visually and with an ATP luminometer. Consider upgrading to a more effective washing system and make sure the cleaning and sanitizing chemicals are appropriate for the job.

Mushroom slicers are notoriously difficult to clean and sanitize. They violate just about every sanitary design principle and therefore require extra attention. Just about every slicing operation has its own method and schedule for cleaning and sanitizing slicer heads. The best way to sanitize them is to completely disassemble them before cleaning and sanitizing. However this may not be done enough because it is a time-consuming process that often conflicts with production schedules. General recommendations for more rapid heat sanitization treatments are to immerse equipment in circulating water that is at least 170°F so that all surfaces reach at least 160°F for 30 seconds. But the design of slicers is very complex with many interior metal-on-metal fittings that may be slower to bring to temperature than the outer equipment surface. See Method for Validating Thermal Sanitization of Mushroom Disk Slicing Equipment for more information.

Cleaning and Sanitizing

This topic has already been mentioned but there are some general considerations that are important to know.

The general sanitation sequence is:

  1. Dry clean by removing visible debris.
  2. Pre-rinse with water.
  3. Apply an appropriate cleaner.
  4. Rinse off all of the cleaner.
  5. Apply an approved sanitizer.
  6. Allow the equipment to air-dry.

Cleaning and sanitizing are therefore separate steps with two different goals. The purpose of cleaners is to remove soils from surfaces. Work with a reputable chemical supply company to select the right cleaning chemical for the soils found in your facility. The purpose of sanitizers is decrease populations of harmful microorganisms on previously cleaned surfaces to safe levels. Use only sanitizers that have been approved and registered by the U.S. Environmental Protection Agency (EPA). Sanitizers are regulated similar to pesticides. The “label is the law” for registered products and federal law requires that they be used according to label directions.

There are many chemical cleaners on the market that are suitable for soils encountered in packing houses. Chlorinated alkaline solutions are widely used to lift and break down organic soils. Quaternary ammonium compounds (Quats) and peracid products are types of sanitizer that also have some detergency properties that increases their effectiveness against biofilms. Quats also have residual antimicrobial properties that keep bacteria from growing back on treated surfaces and there are powdered forms available that can be sprinkling on floors. There are many different commercial formulations for these products. Always follow the label directions for recommended amounts of cleaners and maximum allowable levels for no-rinse sanitizer applications on food-contact surfaces.

Your Master Cleaning Schedule should indicate cleaning frequencies and procedures for all food contact and non-food-contact surfaces. Cleaning and sanitizing locations and frequencies will vary depending on the type of surface and its potential for contamination and may change after you review the results of your Listeria monitoring program. A general schedule for some Listeria hot spots might include the following:

Cleaning and Sanitizing Schedule

Waste and storage containerDaily
Condensate drip pansWeekly or monthly
WallsWeekly or monthly
HVACWeekly or monthly

Environmental sampling

A complete discussion on developing a Listeria testing program is not possible in this article. You can review the references below for more details, but here are some general observations. You should be conducting regular environmental testing for Listeria species (spp.) in your facility. This test will not differentiate between non-pathogenic species and dangerous L. monocytogenes. However, it will tell you where conditions are right for it to survive and grow and where you should make changes to improve your sanitation practices. When planning sampling locations, divide your facility into zones that move outward from food contact surfaces (Zone 1) to adjacent non-food contact areas (Zone 2) and then farther away to more distant areas (Zone 3 and 4). At first it is best to concentrate your testing on Zones 2 and 3 so that you can find and correct problems that may lead to Zone 1 contamination. Focus on high risk areas that are continuously wet, especially floors, drains, non-contact equipment framework, and areas where condensate accumulates. Correct these problems before moving to Zone 1 surfaces.

If you find Listeria spp. on a food contact surface, FDA requires you to act as if it were L. monocytogenes until proven otherwise. You must isolate and hold any product that passed over the contamination site while you conduct additional tests to confirm or refute that it was indeed L. monocytogenes. This is obviously problematic for short shelf-life products such as sliced mushrooms. If the sample contained only non-pathogenic Listeria spp., you must you re-clean, sanitize, and retest the area while you evaluate and make corrections to improve your sanitation procedures. If testing confirmed the presence of L. monocytogenes on the surface, you need to test your product to determine if it is adulterated with the pathogen and possibly subject to recall. Further details on corrective actions are given in the FDA Listeria control guidance document referenced below.

Final thoughts

Food safety isn’t cheap. It takes time and resources to establish a robust sanitation program. If you haven’t done so already, establish professional relationships with your equipment and chemical suppliers. The good ones have a wealth of knowledge on internationally established food safety and sanitation standards. When purchasing new equipment, ask questions about how it should be cleaned and sanitized. If you have issues with your current equipment or facilities, find a chemical supply company that has excellent technical support and invite a representative to your facility for a consultation. They may charge a little more, but the investment can pay off when you consider the costs involved with a product recall. There are excellent training resources on proper methods for packing house cleaning and sanitizing available on the American Mushroom Institute website. Become educated on food safety and sanitation issues by networking with colleagues and attending industry conferences, seminars, and university extension workshops. Enhancing your food safety knowledge and skills empowers you to make and defend critical and difficult decisions that you might need to take that will protect consumers and the reputation and long-term sustainability of your company.


Guidance on Environmental Monitoring and Control of Listeria for the Fresh Produce Industry. 2014. United Fresh Produce Association Food Safety & Technology Council. Free download through UFPA publications store.

Listeria monocytogenes Guidance on Environmental Monitoring and Corrective Actions in At-risk Foods. 2014. GMA. Association of Food, Beverage, and Consumer Products Companies. July 7, 2014.

Guidance for Industry: Control of Listeria monocytogenes in Refrigerated or Frozen Ready-To-Eat Foods; Draft Guidance. U.S. Food and Drug Administration. February 2008.

Additional resources

Penn State Farm Food Safety website

Training Opportunities

There are many food safety training opportunities from academic institutions and private companies. Each year the Penn State Department of Food Science presents the Food Safety and Sanitation Short Course . This two and one-half day event includes practical guidance on cleaning and sanitizing, equipment and facilities design, and pest control. For details about this and other training programs, visit the Penn State Commercial Food Processing website. Or contact Dr. Luke LaBorde, Department of Food Science, Penn State University, 814-863-2298 or at

For those who find it difficult to get away from the office, Penn State offers the Food Safety and Sanitation for Food Manufacturers Online course, a web-based introductory course on food safety and sanitary practices for commercial manufacturers of food products. This course teaches the essentials of food microbiology, sanitary design principles for facilities and equipment, worker hygiene practices, correct procedures for cleaning and sanitizing, food security, and more. This is an "independent study" course delivered online. That means you work independently and at your own pace; at work, the library, your home, anywhere! It is ideal for the busy professional on a budget and with limited time for training.

Originally printed in Mushroom News Magazine, September 2015.