Sangita Viswanathan, Former Editor-in-Chief, FoodSafetyTech

Implementing an Effective Listeria Control Plan for Dairy Products

By Sangita Viswanathan
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Sangita Viswanathan, Former Editor-in-Chief, FoodSafetyTech

Due to their high nutritional value, dairy products are highly suitable for growth of Listeria monocytogenes. Eurofins’ Dr. Doug Marshall discusses the contributing factors and conditions in which Listeria contamination of dairy products is caused, and reviews critical control points and elements of an effective Listeria Control Plan.

In the last three years, there have been four major foodborne illness outbreaks caused by Listeria monocytogenes in dairy products (Oasis fresh curd cheese; Roos raw hard cheese; Crave Brothers pasteurized farmstead cheese; and Frescolina Marte pasteurized ricotta cheese). Before 2012, there have been multiple outbreaks due to raw and pasteurized Mexican-style soft cheeses and in pasteurized milk in 2007.

Dr. Douglass Marshall, Chief Scientific Officer – Eurofins Microbiology LaboratoriesDr. Douglass Marshall, Chief Scientific Officer – Eurofins Microbiology Laboratories , recently spoke about Implementing an effective Listeria control plan for Dairy Products in a recent presentation. He described the pathogen as “a gram-positive bacteria, which is facultatively anaerobic, psychotrophic (can even multiply at refrigeration temperatures – though at slower rates), sensitive to heat processing, even found in healthy cows, raw milk and dairy processing environments, and can survive most cheese ripening processes.”

Though, listeriosis, the infection caused when a person is infected with LM, is relatively rare – only affecting about 1600 individuals a year – it has a high mortality rate, highest among foodborne illnesses, especially among high-risk individuals.

Dr. Marshall listed some of the contributing factors to a LM outbreak:

  • Inadequate thermal processing
  • Refrigeration temperature being too high
  • Inadequate product flow through processing plant
  • Inadequate personal hygiene
  • Product shelf-life too long
  • Inadequate cleaning and sanitation
  • Inadequate environmental monitoring and control
  • Inadequate end product testing

Thermal processing is a time-temperature process, and it can be inadequate if either the temperature is too low, or the process time is too short.

It is common knowledge that whether it’s during transportation, or at retail or at homes, often temperature of food storage is not adequately maintained. Dr. Marshall said that as high as 55 percent of household units and 32 of retail store units had refrigeration temperatures of greater than 9 °C. “And once you get past that temperature threshold of 10 °C, the bacteria reaches maximum population level within six days (average shelf life).”

Inadequate cleaning & sanitation is another major cause for LM contamination and this is often the battle between production & sanitation. Floor drains are a common culprit, responsible for 63 percent incidence of LM. Dr. Marshall also referred to other sources of inoculation that you are not getting effective control of such as filler heads and high pressure water sprays or air sprays, which can aerosolize bacteria and spread the contamination to other surfaces.

Inadequate product flow is usually due to the failure to segregate pasteurized product form raw product or the failure to segregate employees working in raw vs. pasteurized locations. Address this by mapping out product and employee flow (along with equipment) and look for areas where cross contamination can occur, advises Dr. Marshall, who cautions facilities to monitor and control the following direct food contact surfaces that can be cross-contaminated:

  • Fillers
  • Packaging stems
  • Conveyors
  • Chilling solutions
  • Slicers, dicers, shredders, blenders, hoops, molds
  • Collators, assemblers
  • Product movement items, such as racks, bins, tubs and buckets
  • Spiral coolers, blast freezers
  • Hand tools, gloves, aprons

Inadequate personal hygiene is another contributing factor and this can include clothing such as outerwear and gloves. Maintenance personnel should be thorough in their hand-washing and it’s recommended they use alcohol based wipes after hand-washing.

Address the issue of shelf-life being too long by determining the shelf life based on food safety, and not food quality. Also, run LM challenge test in each product, Dr. Marshall advises.


FDA, in their Preventive Controls rule proposed under the Food Safety Modernization Act, has a section on Environmental Monitoring, based on the rationale that that poor control of the environment can lead to LM cross contamination of finished product, explains Dr. Marshall, adding that inadequate environmental monitoring and control is a key component for LM contamination.

“Invest your testing dollars to find hot spots in your facility and ensure the control mechanisms are working every day,” he says, asking companies to “detect and control hot spots, measure effectiveness of general cleaning and sanitation programs, and test for Listeria species.”

Dr. Marshall asks, “If I were LM and wanted to hide, where would that be? Would it be on an easy to clean surface such as the floor, or would it be in a nook or cranny where it’s hard to reach and clean?” He lists the following as areas that commonly harbor the pathogen and advises extra caution and creativity to clean these spots:

  • Equipment framework – nuts, bolts, open tubing, spot welds
  • Floors and drains – standing water
  • Walls
  • Ceilings, overhead equipment, catwalks, pipes
  • Condensate
  • Exposed, wet insulation around pipes and walls
  • Fork lifts, trolleys
  • Cleaning tools – sponges, brushes, scrubbers
  • Maintenance tools
  • Conveyors, belts and rollers (need to be broken down and cleaned regularly)
  • Control panels and switches
  • Rubber seals (especially if they have cracks)
  • Trash cans
  • Air fillers
  • Motor/ pump housings
  • Cracked hoses
  • Ice makers

End product testing is an effective way for testing for LM, but Dr. Marshall points out that there are often arguments against this. “Companies often argue that their HACCP plan is working, their kill step is effective and that they have a history of doing end product testing, and they haven’t had any positive results so far. But this is not a convincing enough argument.” End product testing can address the failure to monitor and control high risk ingredients, and is very useful to detect gross contamination events. It should be used to assess risk of rework, and also test for LM, not just Listeria, Dr. Marshall advises.

Hear more about building an effective control plan for Listeria in your dairy processing environment, by listening to this archived webinar recording, presented by Eurofins.

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