Fighting Listeria

Rising consumer demand for convenience and ease of preparation has increased the number and variety of refrigerated ready-to-eat (RTE) foods. Improved nutrition awareness has also increased consumption of fresh and minimally processed fruits and vegetables. Those responsible for developing and producing these products face a daunting task in attempting to review and assimilate the large volume of literature about Listeria monocytogenes and its control. However, a few basic bits of knowledge can be helpful in planning a defensive strategy.

A formidable foe 

L. monocytogenes, a bad bug and a formidable foe, was recognized as a significant foodborne pathogen in the early 1980s when major listeriosis outbreaks caused by coleslaw and Mexican-style cheese occurred. More recently, major recalls of deli meats and hot dogs in Dec. 1998, meat and poultry products in Jan. 1999, and a 10- state outbreak linked to deli turkey meat in 2000 have occurred. These incidents spurred renewed research efforts to better understand the organism and ways to control its growth in RTE foods. They also led government food-safety agencies to review and strengthen their procedures and regulations pertaining to controlling potential L. monocytogenes contamination in the nations food supply.

L. monocytogenes is one of the leading causes of death due to foodborne illness. While listeriosis accounts for only about 0.06% of all foodborne illness in the United States, 28% of food-related deaths are attributed to the organism. Death rates are significantly higher (from about 15% to over 60%) within susceptible segments of the population: pregnant women and their unborn or newborn children, the elderly, and immunocompromised individuals.

Listeriais one tough bug: The bacteria can grow and reproduce under a wide variety of conditions and are found almost everywhere in the environment, including soil, decaying vegetation, water, most mammals and some birds. While it prefers to grow at temperatures of 33°C, L. monocytogenes can grow in cool, damp locations like coolers, processing areas, floor drains and home refrigerators. It is critical to keep refrigerated RTE foods at 4°C or below at all times to prevent its rapid growth. The organism survives freezing, but pasteurization and other standard thermal processes kill it. It can grow within a pH range of 4.0 to 9.5 and is salt tolerant in concentrations up to 10%, and its growth may increase at low levels of oxygen in the presence of carbon dioxide.

Zero tolerance 

Because of the severity of L. monocytogenes infections and high mortality rates, FDA and USDAs Food Safety and Inspection Service (FSIS) have established zero-tolerance policies for it. Any RTE food that contains any amount of L. monocytogenes or any product that may have come in contact with a surface contaminated with the organism is deemed adulterated.

In June 2003, FSIS published Title 9 Code of Federal Regulations (CFR), Part 430, Section 4(a), requiring establishments producing certain RTE products to prevent adulteration by L. monocytogenes from environmental sources. A plant producing RTE product exposed to the environment after lethality treatment must recognize and control L. monocytogenes through its HACCP plan or prevent it in the processing environment through a sanitation SOP (SSOP) or other prerequisite program.

The rule establishes three alternative ways to address the potential for post-lethality contamination of regulated products. Each method has necessary testing requirements and verification procedures. FSIS testing and verification policies are also specified. In May 2006, USDA published updated compliance guidelines to assist covered establishments in complying with the rule (see www.fsis.usda.gov).

In Sept. 2003, the results of a risk assessment of the publichealth impact of foodborne L. monocytogenes conducted jointly by the Centers for Disease Control, FDA and USDA were published. The common factors generally associated with higher risk foods are: the potential for contamination with the organism; the ability to support growth to high numbers; and an RTE product that requires refrigeration for an extended period of time before consumption. If developing or manufacturing products with these characteristics, L. monocytogenes control must be a component of the overall program.

Methods of control 

While current regulations allow alternative methods in controlling L. monocytogenes in post-lethal processing, caution must be exercised in selecting which method or methods to implement. Extensive research has been, and continues to be, conducted on use of organic acids and their salts, plant extracts, and other preservatives to control L. monocytogenes in formulated high-risk products. While some of the materials usede.g., mixtures of sodium lactate and sodium diacetate in cured meat productshave shown some promise, no single compound or mixture is effective across a broad spectrum of products. Each material must be evaluated in each formulation and associated set of processing, packaging and distribution parameters to verify its effectiveness.

Ionizing radiation, high-pressure processing, pulsed electric fields, ultraviolet light and several other novel technologies have been investigated for effectiveness in eliminating or reducing post- processing populations. These methods have shown some success, but L. monocytogenes control generally seems to require moresevere treatment parameters than for other bacteria. Significant variation in the effectiveness of these processes may exist with changes in temperature, product formulation and the strain of L. monocytogenes present.

Appropriate good manufacturing practices and proper sanitation procedures are the best defense. A partial list of some important practices follows:

Some key sanitation considerations are:

Chlorine, quaternary ammonium and peracid-based disinfectants are all effective against L. monocytogenes. Each has an approved regulatory limit for norinse applicationse.g., 200 ppm for hypochlorite compounds. When niches of contamination are found or if biofilms are present, higher levels of the sanitizer in conjunction with mechanical action may be necessary to eliminate the hazard. After using the higher concentration, the equipment should be rinsed and resanitized with a solution at the no-rinse level.

Verifying effectiveness 

Establishing and maintaining a microbiological-monitoring program is critical to verify the effectiveness of sanitation procedures. The testing program should focus on areas where Listeria is most likely found and must also include action plans for when the bacteria is found.

While L. monocytogenes contamination of RTE refrigerated foods is most likely to occur after processing and before packaging, control in distribution and warehousing is necessary to prevent the likelihood of bacterial growth and to minimize contamination potential. Maintain the temperature of high-risk foods at 4°C or below, avoid handling and monitoring procedures that may compromise package integrity, and follow proper inventory practices first in, first out. Establish these practices at each step in the cold chain, from processor to consumer. The key factor in preventing listeriosis is eliminating or reducing to the absolute minimum the number of L. monocytogenes organisms present in foods at the point of consumption.

Weve only scratched the surface of the mass of research and information available on the characteristics of L. monocytogenes and on methods for its control. Each product type, each process, each processing facilityin fact, each individual formulation of a highrisk productmust be thoroughly tested and challenged to ensure that all of the necessary components are in place to prevent contamination and eliminate conditions which may allow the organism to grow before it reaches the consumer. Simply put: Keep it clean, keep it cool, and keep it fresh. 

Stan Iwanicki has a B.S. and M.S. in Food Science from the University of Illinois, Champaign-Urbana and established Hill Country Food Consultants, Fredericksburg, TX, in 2005 after more than 35 years in the food industry. Hill Country Food Consultants (www.hillcountryfoodconsultants.com) reviews current company practices to find holes in the safety net, including assurance that the company complies with all the necessary regulatory program, while discovering the overlap among them, resulting in streamlined data-collection efforts, elimination of duplicate work, enhanced efficiencyand most importantlya safer product. He can be reached at 830/990- 1778 or by e-mail at [email protected].