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From: Food Quality & Safety magazine, December/January 2009

Prevent and Control Listeria

Tools are available to help keep bacteria away

by Peter G. Demakos

Recognized as an important public health problem in the United States, Listeria monocytogenes is a ubiquitous, intracellular pathogen known to cause food-contaminating outbreaks. These microorganisms have the ability to multiply within host cells and spread from cell to cell.

Listeria is commonly found in dairy products, meat, poultry, and seafood. Foods that are ready to eat, require refrigeration, and are stored for an extended period of time have a higher likelihood of contamination than other products. L. monocytogenes, the bacterium that causes listeriosis, has the ability to survive while refrigerated and will grow at temperatures as low as 34ºF.

Outbreaks can result in plant inspections, a compromised company or brand reputation, product recalls, and plant shutdowns. Though it is impossible to completely eradicate L. monocytogenes from a plant, steps can be taken to prevent and control the bacterium in order to assure consumer safety while protecting business.

Avoiding Outbreaks

A clean, dry environment is of utmost importance in controlling Listeria. Common processing facility contamination sites include floors, walls, ceilings, food contact surfaces, cleaning aids, drains/wash areas, and heating, ventilation, and air conditioning (HVAC) systems. Potential problem areas should be identified at each processing facility.

Without meticulous and proper cleaning, plant equipment, including any equipment used for refrigeration and HVAC, can become breeding grounds for microorganisms. Intermittent defrost systems, which are subject to frequent washdowns, encourage microbial growth. Because they often contain moisture from standing water or condensation, special attention should be paid to these systems to prevent their contamination.

Among the many general guidelines for keeping a food processing facility clean, visual inspection and routine testing are important. While regular cleaning can help prevent outbreaks, continuous monitoring is needed to assess further control measures. Determine sample points and frequency for routine environmental testing, food contact surface testing, and product testing. Also, provide personal hygiene training and make at least one staff member responsible for overall facility sanitation.

Cleaning Products

A variety of antimicrobial sprays are available to help prevent outbreaks. While some of these products are applied to air conditioning and refrigeration systems to prevent the spread of microorganisms through circulating air, food additive sprays can also be effective against Listeria on ready-to-eat foods. A bacteriophage-based decontaminant was recently approved by the Food and Drug Administration (FDA) for use in food processing facilities. A similar product, also approved by the FDA, is being marketed for use in grocery store meat.

Air-cleaning devices installed in HVAC ductwork offer another option. These products are intended to remove microorganisms and other particles from air. Some of these systems utilize ultraviolet rays to sterilize air, while others employ high efficiency particulate air (HEPA) filtration to remove airborne particles of a certain size as air passes through.

Liquid Dehumidification

The chances of contamination increase with wetness in plants and products. Because bacteria are unable to multiply without sufficient moisture, a hygienic and arid environment can support prevention efforts. But first, a bit of background is necessary. Dry bulb, wet bulb, and dew point temperatures are often used to determine the state of humid moist air. Dry bulb is the temperature of air measured by a thermometer exposed to the air but shielded from radiation and moisture. Wet bulb is the temperature measured by a moistened thermometer bulb exposed to the air flow. Dew point is the temperature at which water vapor starts to condense out of the air; above this temperature, the moisture will stay in the air.

Liquid desiccant dehumidification systems provide consistent dry-bulb and dew-point temperatures for contamination control. With the ability to handle large moisture loads, such systems provide the low air dew-point temperatures required to maintain dry surfaces and facilitate quick recovery from washdown.

Examples of liquid desiccant dehumidification systems include Niagara Blower Company’s (Buffalo, N.Y.) No-Frost system, comprised of a conditioner and a liquid concentrator. In the conditioner, a continuous spray of No-Frost liquid, approved by the United States Department of Agriculture (USDA), is pumped over an evaporator coil, while a fan pulls air from the space through the spray coil chamber and back into the areas. The liquid spray cools air, controls dew point, captures airborne particles (including Listeria, Salmonella, and Yersinia), deodorizes air passing through the spray chamber, and picks up moisture that would otherwise become frost on the coil surface.

A spray of desiccant liquid continuously sanitizes cooling coils, so no further cleaning is required. As the No-Frost liquid dilutes, it is circulated to a No-Frost concentrator, which uses a heat source to remove excessive moisture from the liquid. The concentrator maintains the liquid solution at its proper concentration and effectively eliminates any liquid losses.

Buffalo Testing Laboratories, Inc., determined the effectiveness of a No-Frost conditioner in removing dust and microorganisms to be 98% of all airborne contaminants. Of that 98%, the No-Frost liquid itself removed 99.9 to 100%, depending on concentration. In addition, the State University of New York at Buffalo department of microbiology also tested the antibacterial effects of No-Frost liquids and found that Listeria was inhibited by 99.4 to 99.9% within 24 hours or less, depending on the concentration.

Another benefit of the No-Frost system is that all interior parts are readily accessible for inspection and cleaning. The system’s interior surfaces are designed to eliminate any pockets that might collect dirt, lint, or bacteria.

While cleanliness and sanitation are of great importance, if ventilation systems circulate air containing microorganisms, the time and resources spent on cleaning are wasted. Combining several available options may be the best way to protect a company, its brand reputation, and its consumers.

Demakos is the president of the Niagara Blower Company. For more information, reach him at or (716) 875-2000.



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