Listeria organisms are among us and form part of the environment. Control rather than eradication is the sensible approach. Therefore, managing environmental testing to validate cleaning programmes is highly recommended. Testing of finished dairy products should be conducted when required by clients or to validate a new process.

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Listeria testing should be carried out by an outside or off-sight laboratory to avoid potential propagation in the plant environment. This is the conclusion drawn in an information piece on Listeria by the Milk Quality Improvement Programme of the Department of Food Science in the College of Agriculture and Life Sciences at Cornell University, located in Ithaca, United States (US).

Threat to human health

Listeriosis is an infection caused by consuming food contaminated with the Listeria monocytogenes bacterium, resulting in the illness humans contract from this. Symptoms include fever, muscle aches and serious gastrointestinal upset. What makes listeriosis particularly dangerous is that it is an invasive infection which spreads beyond the intestinal tract to rest of the body. As the illness progresses, symptoms such as a stiff neck, confusion, loss of balance and convulsions may arise.

It especially poses a threat to pregnant women, among whom listeriosis can cause miscarriage, stillbirth, premature delivery or life-threatening infection of the newborn. The global mortality rate of listeriosis is 25% and with a hospitalisation rate of more than 95%, it ranks as the third-most serious foodborne disease in the world. This is according to a presentation by Prof Piet Jooste of the Department of Biotechnology and Food Technology at the Tshwane University of Technology at last year’s South African Society of Dairy Technology annual congress.

Since illness onset typically occurs several weeks after having consumed contaminated food, it is often difficult to trace it back to the source. While most healthy humans do not generally fall severely ill, certain groups of people are at risk for serious complications. People at higher risk include pregnant women, newborns, the elderly and people with weakened immune systems due to chronic illnesses or treatment thereof, such as human immunodeficiency virus infection and acquired immune deficiency syndrome (HIV-Aids) or cancer.

Listeriosis must be treated with antibiotics. An alarming fact is that although the infection is usually evident within three to ten days after having consumed the contaminated food, symptoms may not appear for up to two months.

Occurrence in the food environment

  1. monocytogenes is commonly present in raw foods of both plant and animal origin, such as raw milk, raw vegetables, fermented meats, raw poultry, raw meats, and raw and smoked seafood. It has been established that 2–6% of raw milk samples tested contains the organism.

It is also found in cooked foods due to post-processing contamination, such as delicatessen products, cooked sausages and poultry, pasteurised milk, cheese and ice cream. The occurrence of L. monocytogenes is frequent in the food processing environment, creating the potential for contamination of processed foods.

Listeria bacteria are widespread in the environment, according to the Cornell University report. Many animals can carry the bacteria without appearing sick and can contaminate foods, such as meats and dairy products. Listeria has been isolated from many types of foods including unpasteurised (raw) milk or foods manufactured from unpasteurised milk, red meats, poultry, seafood, vegetables and fruit. Delicatessen meats and hot dogs have also been found to contain the bacteria.

Control measures

Prof Jooste points out that the organism is ubiquitous, which makes complete elimination an unrealistic aim. Control is therefore a more practical approach. Control can be achieved by giving attention to detail in hygiene strategies and monitoring occurrence of the organism. In addition to technologies such as high-pressure processing and pulsed electric fields, novel methods for control of pathogens and spoilage organisms have, in recent years, focused on the use of natural antimicrobial agents such as bacteriocins and bacteriophage.

Dairy products

Listeria can be a common contaminant in the dairy environment, both on the farm and in the processing plant. The animals themselves can be the source on the farm, but Listeria is usually found in manure and improperly fermented silage. Various studies have found that 2–8% of raw milk samples tested contain Listeria bacteria.

In the dairy plant, Listeria is most frequently found in moist environments or areas with standing water or milk residues, including drains, floors, coolers, conveyors and washing areas. Once in food, Listeria grows readily. Human illness has been caused by Listeria growing in dairy products such as raw milk and raw milk soft cheeses.

Pasteurisation of milk effectively destroys Listeria. However, post-pasteurisation contamination can occur within the processing plant. Listeria is capable of growing at refrigeration temperatures. Therefore, even significantly low numbers of the bacteria found in processed dairy products can multiply to dangerous levels, despite proper refrigeration. An adequate sanitation programme and good hygiene practices are essential in food processing and handling areas.

Focusing on the contamination risks at the food handling, storage and processing levels, researchers have found that processing equipment such as holding tanks, storage coolers, table tops and conveyor systems can all be a potential source of contamination – as well as other sites such as drains, floors and storage areas. Listeria can be spread from processing equipment and table tops to food products through the ventilation system, from dripping and splashing when cleaning with high-power hoses and by workers themselves.

Legislation and guidelines

Food safety legislation is designed to protect the safety and quality of dairy products. Therefore, adherence to these regulations is critical. Dairy processors should also focus attention on preventing Listeria contamination in the processing environment and preventing cross-contamination to pasteurised products.

Some key prevention points are supplied by the Alaskan Office of the State Veterinarian in the US:

  • Biosecurity: Restrict access to the milking parlour and processing areas. Anyone who is on the farm (visitors and fieldworkers) should not be allowed in the processing area of a dairy plant, as they are likely to carry contaminants on their boots and clothing.
  • Use a properly constructed facility. Floors must slope to drains so no water or milk pools. Floors and walls should be smooth and easy to clean. Cracks and pits are prime areas where bacteria such as Listeria Drains should be free-flowing.
  • Separate equipment. Keep all brushes and equipment used for environmental cleaning separate from any brushes or equipment that come into contact with milk or dairy food products. Brushes, mops or cleaning equipment used for floors and drains should be used for that purpose alone.
  • Prevent splashing and spraying. This is especially important in milk storage and packaging areas where less rigorous cleaning occurs. Splashing and spraying spreads bacterial pathogens that can contaminate safe food products and ingredients. Prevent cross-contamination of raw milk to pasteurised milk and processed food products. Separate raw milk handling areas and equipment (e.g. brushes, pails/containers, utensils, piping, tanks) from those used for pasteurised products.
  • On-farm processors must enforce restrictions and procedures that prevent cross-contact of the dairy farm environment and milking parlour with the milk storage area and processing environment (limited access, good hygiene and clean clothes/boots).
  • Test your facility for Listeria and other foodborne pathogens. Have a plan in place for follow-up actions if samples test positive.
  • Cleaning and sanitation issues: Develop a cleaning plan for everything, including equipment (milking machines, pipeline, sinks, buckets etc.), utensils, tables, plant and cooler floors and walls, drains and piping exteriors.
  • Perform daily cleaning, sanitising and maintenance of all milk processing, storage and packaging equipment and rooms. Milk concentrates in filters, and separators/clarifiers are where large numbers of Listeria organisms can occur. Avoid manual contact with equipment and utensil surfaces that have been cleaned and sanitised.
  • Keep a chart or checklist available for workers to initial on, to monitor the cleaning and sanitation completed to schedule.
  • Avoid using high-pressure hoses to clean floor drains, as these are one of the common areas where Listeria is found. The water pressure sprays, splashes and spreads Listeria as well as other bacterial pathogens to other places.
  • Provide adequate training and guidance for all workers with regard to personal health and hygiene through good manufacturing practices and plant sanitation procedures. Avoid delegating tasks to any employee without proper training.
  • Develop a hazard analysis and critical control points-based programme. This should integrate prevention, validation and an action plan for possible programme deviations, and include well-defined procedures for product recalls.

As the amended South African Foodstuffs, Cosmetics and Disinfectants Act, 1972 (Act 54 of 1972) does not refer to Listeria, most of the major retailers in the country have developed their own food safety standards and audit protocols, according to Prof Jooste.

These standards are based on national legal requirements, such as Regulation R692 governing microbiological standards for foodstuffs and related matters, as well as prerequisite programmes as defined by the voluntary national standards of the South African Bureau of Standards. Companies that export also implement the relevant regulations of the country they export to.

The Dairy Standard Agency offers guidelines in its codes of practice that relate to L. monocytogenes in raw milk for final consumption, pasteurised milk, ultra-high temperature (UHT) milk, cream and salted butter. – Izak Hofmeyr, Farmbiz

For more information, contact Prof Jooste on 082 806 8589 or send an email to