Fortunately, the number of people who contract an infection from consuming foods contaminated with Listeria monocytogenes is relatively low. However, this pathogen is one of the main causes of mortality due to food poisoning.
It is clear that without adequate food hygiene in the food production chain, Listeria represents a risk to health. But, even with high hygiene standards, Listeria is known for its ability to survive in environmental niches in which other microorganisms could not.
Where To Look For Listeria And How To Avoid It?
Listeria monocytogenes is able to ‘settle’ on surfaces and equipment, and remain there for long periods of time, causing recurrent cross-contamination. The five most common areas where we can find it are the soil, the drains, food processing equipment, the freezer/cooling system and the air conditioning system. Considering how difficult it can be to eradicate it, it is best to pay maximum attention to prevent it.
Listeria monocytogenes is present around us, mainly in the soil. This means that its proliferation in the processing plant is carried out most of the time through footwear. An important requirement to avoid it is to make sure that the footwear is clean in those areas of the plant that are susceptible, such as places where unpacked foods are handled. This can be achieved by changing the footwear in a hygiene hatch or change room. The use of plastic covers for shoes is less advisable, as they can accumulate dirt or even break.
Equally as important as shoe hygiene, it is to keep the soil dry at all times. Listeria, like most bacteria, needs water to grow and persist.
In the event of a spill of product on the floor, do not use excessive amounts of water to rinse it. It is much better to clean the spill with a brush and a plastic tray and only use water (and if necessary, cleaning and disinfecting agents) to remove the last remaining product from the soil.
One of the places where we can most safely find the Listeria, if it is present in the facilities, is in the drains. They provide the perfect conditions for the bacteria to enter and grow since all the dirty water passes through the drains, including the water from the cleaning processes. To this, we must add that there are many facilities whose drains do not have a suitable hygienic design and also are not properly located, at the lowest point and with all surrounding floor surfaces inclined towards the drain.
In addition to soils and drains, it is important to ensure that Listeria monocytogenes does not find places to settle and grow in food processing equipment. Here comes back into play the hygienic design of such equipment: with surfaces are smooth and designed for easy cleaning (no cracks, no sharp angles).
The places where there will most likely be a risk of contamination with the bacteria without being perceived are those that need manual cleaning, those that have cracks and, often, those that have stagnant water.
The closer the unpacked product is to those spots with frequent standing water, the greater the potential risk of introducing Listeria Monocytogenes into the production flow. It is necessary to keep standing water away from the open product always, at least two meters away. If it is not possible to maintain this rule, consider redesigning the equipment or relocating the water source.
The Freezer/Cooling system
Another area where the Listeria can hide is in the freezer or in the cooling systems. This equipment has internal evaporation plates, which are automatically defrosted on a regular basis to avoid the excessive accumulation of ice on the plates. Next, to these plates, there is a fan, which forces the displacement of the air to cool the surrounding area.
In the prevention of Listeria monocytogenes, it is essential to keep the evaporation plates clean and disinfected at all times. As we have seen, Listeria Monocytogenes can survive at very low temperatures and even grow at temperatures as low as -5˚C. For this reason, it is necessary to ensure that the evaporation plates and fans of all refrigeration and freezing systems are regularly cleaned and disinfected.
- “Listeria: Sources, Growth, and Control.” Food Safety and Inspection Service, USDA. Link
- “Listeria monocytogenes in Food Processing Facilities: Risk Factors and Control Measures.” Journal of Food Protection. Link
- “Listeria monocytogenes: A Dangerous Foodborne Pathogen.” Food Safety and Quality. Link