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From: Food Quality & Safety magazine, August/September 2005

The Ionic Silver Lining

Incorporated into coatings, ionic silver provides a safe, long-lasting and effective way of controlling bacteria.

by Joseph Geary

Bacterial contamination of plant surfaces is a constant concern for the food processing industry. One specific bacteria, Listeria monocytogenes, accounts for 2,500 cases of illness and 500 deaths annually in the U.S., making it the bacteria of greatest concern for ready-to-eat (RTE) processors today. In 1998, one of the largest outbreaks of Listeria occurred with a large U.S. hot dog manufacturer and resulted in 15 adult deaths, six stillbirths and more than one million pounds of product recalled from the market.

In June 2003, USDA imposed a new rule to reduce Listeria in RTE meat and poultry products. The regulations encourage RTE food processing facilities to employ more effective Listeria control measures and require the facilities to verify the effectiveness of these measures through testing and sharing the results with the Food Safety and Inspection Surface (FSIS).

While USDA reports that these new regulations are having a positive impact on the reduction of Listeria in RTE food products, Listeria outbreaks continue to place the public at risk and result in costly production interruptions, plant shutdowns and significant economic losses. In the past two months, four U.S. companies were forced to recall countless quantities of sausage, sausage and chicken gumbo, chicken salad, ground beef patties, and meatballs due to possible Listeria contamination.

While plant operators and sanitation professionals employ every weapon in their arsenal of cleaning tools to control bacterial proliferation and transmission, new solutions must be developed to assist them in their efforts to keep surfaces clean and reduce the risk of food product contamination.

Floors Foster Contamination

Studies have shown that surfaces in food processing facilities – particularly floors – can harbor and transmit a variety of problem-causing bacteria, including Listeria, and increase the risk of food product contamination. Bacteria are well adapted to food processing environments since they can grow and reproduce in damp, low oxygenated, refrigerated conditions.

High foot and vehicle traffic, the movement of heavy equipment, and cleanings with hot water and harsh chemicals can cause severe damage to a floors. Damaged flooring is an ideal breeding ground for problematic bacteria since it provides a dark, damp refuge that is impervious to commonly employed cleaning techniques, such as disinfectants and sanitizers. Bacteria can live for hours¬ – or even days ¬– within the pits and crevices that can form on a floor’s surface from every day wear and tear.

Last year, researchers from the University of Wisconsin at Madison conducted an audit of 31 RTE meat and poultry processing plants to determine where Listeria contamination was a problem in post-lethality environments. As part of the study, 438 food contact and non-food contact surfaces were sampled for the presence of Listeria. The audit revealed a particularly high incidence of Listeria on floors and drains. Of the 115 floor and drain samples that were taken from these facilities, 27.8 percent tested positive for the bacteria.

When bacteria exist within the cracks and crevices of plant flooring, they are continuously exposed to small amounts of cleaning products without being killed. As bacteria grow and reproduce over time, they develop into organized and structured communities that are highly resistant to cleaners, sanitizers and disinfectants.

Researchers at the University of Georgia conducted a study to assess the ability of Listeria to survive exposure to commercial food processing equipment, cleaning solutions and subsequent treatment with sanitizers or heat. They found that Listeria can not only tolerate exposure to high concentrations of alkaline cleaning solutions, but also develop a resistance to these cleaners.

Studies have also revealed that bacteria commonly found in food processing environments are incredibly resilient. Bacteria starved of nutrients for extended periods of time have the ability to “wake up,” restore their own strength and effectively kill other cells, even if it takes them a bit longer to do it compared with healthy cells.

Researchers at the University of Arkansas starved Listeria cells for 196 days and then tested their effectiveness in killing a target population of mouse cells. While healthy Listeria cells effectively killed 90 percent of the target mouse cell population within two hours, the starved and damaged cells had the strength to kill 60 percent of the population within six hours, and 90 percent within eight hours. Based on these results, the researchers concluded that inadequate disinfecting procedures could leave some bacteria alive and virulent with the ability to regroup and contaminate.

Even if conventional disinfectants and sanitizers are used to successfully eradicate Listeria and other bacteria on floors and below the surface, they cannot control bacterial populations over an extended period of time. Conventional cleaners are based on an “instant kill” approach. When sprayed or wiped onto a surface, they kill existing microbes but do little to combat new microbes that can build up during daily use. For instance, a disinfectant could kill bacteria on a plant floor, but as soon as someone walks across that floor with shoes carrying Listeria or other problematic microbes, it is contaminated again.

Additionally, many commonly used disinfectants are only effective if they remain on a surface for a prolonged period before they are wiped away. According to the National Institutes of Health (NIH) a contact time of 20 to 30 minutes may be required to inactivate microorganisms, especially on items that are difficult to clean because of narrow channels or other areas that can potentially harbor microorganisms. In a busy food processing environment, it can be a considerable challenge to comply with these time-intensive disinfection procedures.

The longer bacteria thrive on – and below – the surface of plant flooring, the greater the risk of transmission. Shoes, vehicles, dirty equipment, hoses that lay or are dragged across floors, aeration and atomization, and even employees’ clothing have all been shown to harbor and transmit problematic bacteria. This ultimately increases the risk of food contamination.

During a six-month study conducted by Queen’s University of Belfast, United Kingdom, researchers isolated 289 Listeria strains from a poultry-processing environment and from poultry products to pinpoint sources of contamination within the plant. The analysis revealed that Listeria strains that had originated with the incoming birds had been broadly transmitted to food contact surfaces, floors and drains.

A Silver-Based Flooring Solution

The USDA has passed strict regulations that require the RTE food processing industry to develop and report procedures to prevent product adulteration by Listeria. Unfortunately, in many cases, conventional cleaning products fail to meet the needs of plant operators and sanitation professionals. Therefore, new solutions must be developed to control populations of Listeria and other bacteria on plant flooring.

One potential solution comes in the form of ionic silver. New self-cleaning coatings that contain a silver-based antimicrobial are currently being developed to help protect flooring from damage and provide a defense against problematic bacterial. These resin-rich, epoxy-based coatings are impervious to chemicals, moisture and corrosion, and shield the floor from everyday wear and tear. Since they are thermally compatible with concrete, they are not prone to the failure mechanisms of traditional epoxies, such as thermal shock and delamination. These coatings must undergo review by the Environmental Protection Agency (EPA) as part of the development process prior to introduction into the market, but surfaces treated with ionic silver have shown efficacy against a wide range of bacteria in laboratory tests, including the bacteria of greatest concern to RTE processors today.

These coatings can be used to protect floors and other food processing plant surfaces, such as walls, ceilings and overhead piping. They seal off these surfaces and protect them from damage, and the resiliency of the epoxies minimizes the formation of cracks and crevices that can harbor problematic bacteria. The added antimicrobial also prevents biofilm formation in and around surface irregularities and reduces bacterial build-up between cleanings, making it easier to remove surface microorganisms.

Ionic silver combats bacteria in three ways: Interrupting cell metabolism, inhibiting membrane transport processes and preventing cell multiplication. The ionic silver contained within the coating is released at a controlled rate, providing long-lasting protection against existing bacteria and viruses on the surface and any new microbes that come along. If the humidity in the environment increases, making conditions ideal for bacterial growth, the compound releases more ionic silver to counteract contamination. The result is a “self-cleaning” surface that provides continuous, long-term protection against problematic bacteria.

In addition to their long-lasting effectiveness against bacteria, ionic silver compounds vastly reduce the potential for the development of microbial resistance commonly encountered with traditional sanitizers and disinfectants and other products that contain organic antimicrobials, such as triclosan. Resistance occurs when a microbe finds a way to counteract the mechanism of attack. Since organic anti-microbials use a single mechanism of attack, microbes are much more likely to develop a resistance to them versus inorganic antimicrobials, such as ionic silver, which employ multiple methods of attack.


Bacterial contamination poses a significant problem for the RTE food processing industry, and conventional cleaning procedures have not done enough to address this issue. As the USDA continues to strengthen regulations designed to reduce Listeria in RTE meat and poultry products, food processors must find new and effective ways to use technology to protect plant flooring from bacterial contamination and the subsequent transmission of bacteria from surfaces to food products.

Ionic-silver is one potential solution. Incorporated into coatings, ionic silver provides a safe, long-lasting and effective way of controlling the levels of bacteria on food processing plant floors and other surfaces, breaking the chain of contamination.


  • Foodborne Diseases. National Institute of Allergy and Infectious Diseases, National Institutes of Health, U.S. Department of Health and Human Services. February 2005.
  • Food Safety Research: A focus on Listeria monocytogenes and biofilms. Food Safety Research Information Office, United States Department of Agriculture (USDA).
  • Taormina PJ; Beuchat LR. Survival of Listeria monocytogenes in commercial food-processing equipment cleaning solutions and subsequent sensitivity to sanitizers and heat. Journal of Applied Microbiology, 2002;92(1):71-80.
  • L. M. Lawrence and A. Gilmour. Characterization of Listeria monocytogenes isolated from poultry products and from the poultry-processing environment by random amplification of polymorphic DNA and multilocus enzyme electrophoresis. Applied Environmental Microbiolgy. 1995 June; 61(6): 2139–2144.
  • National Institutes of Health (NIH), Division of Safety, Office of Research Services. Available from

Joseph Geary is vice president for technology, product development and upgrade solutions for AgION Technologies, Inc. Reach him at (781) 224-7133 or .



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