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How does Listeria monocytogenes contamination occur in Vacuum-Packed Smoked Fish Products?


(CMA Testing and Certification Laboratories)

Last week the Centre for Food Safety (CFS) announced that a ready-to-eat smoked salmon sample made by a local food processing plant, was detected with Listeria monocytogenes. The CFS advises the public not to consume all smoked salmon produced by the manufacturer concerned, regardless of the brand, batch or packaging.

What is Listeria monocytogenes?

L. monocytogenes a bacterium commonly found in soil and water. Chilled ready-to-eat foods including raw / smoked fish and seafood, inadequately pasteurized milk, soft cheeses, ice cream, raw vegetables, etc. are often contaminated with the bacterium.

Unlike many other germs, L. monocytogenes can grow in refrigerated temperatures, which makes this organism a particular problem for the food industry.

Although healthy people develop few or no symptoms when infected, L. monocytogenes could be dangerous to pregnant women, newborns, the elderly and people with low immunity. In United States, foodborne L. monocytogenes causes 255 deaths annually.

Bacterial Contamination in a Food Processing Plant

Tocmo et al., 2014 has identified routes of contamination of L. monocytogenes in vacuum-packed smoked fish products.


Raw Materials

Raw fish itself can be a source of contamination since L. monocytogenes is a naturally occurring bacterium in the environment. However, the occurrence of L. monocytogenes in raw fish is rather low but cannot be ruled out.

Head Cutting, Evisceration and Filleting

Processing surfaces (e.g. contact surfaces of machines) were found positive for L. monocytogenes contamination, even after cleaning and disinfecting with sodium hypochlorite and peracetic acid. This suggests that specific resistant strains have adapted better to specific areas in the fish processing plant.

It is therefore essential to employ more stringent cleaning and disinfection regimes and to constantly monitor levels of contamination in food contact surfaces.


Brine containers and the brine used in making smoked fish may also serve as reservoirs for halotolerant L. monocytogenes.

Therefore, a thorough cleaning and disinfection protocol of all contact surfaces in the brining area should be put in place.

Moreover, the personnel running the salting area may serve as reservoirs for this bacterium, thus enhancing its chance of transmission into food product.

Cold Smoking

The temperature (≤30 ◦C) during cold smoking is too low to inactivate this pathogen. Hence, the chance that L. monocytogenes survives and proliferates after the smoking process is rather high.

Other potential sources of contamination during the smoking process are the food contact surfaces in the smoking room as well as personnel safety items. Samples obtained from smoking trays, personnel gloves, and protective clothing were found positive for L. monocytogenes and are, therefore, potential sources of cross-contamination.

As such, personnel hygiene and stringent cleaning & decontamination of contact surfaces in the smoking area are crucial in controlling L. monocytogenes contamination.

Skinning and Slicing

L. monocytogenes were repeatedly detected in slicing machines (e.g. slicer belts, slicing covers) and working tables in a smoked-salmon production facility.

To minimize contamination, it is suggested to dismantle blades and clean and sanitize them regularly to prevent attachment of L. monocytogenes, and discard slices which may contain higher counts.


Vacuum-packaging i.e. lack of oxygen, cannot inhibit the growth of L. monocytogenes. In addition, the prolonged shelf life of vacuum-packed ready-to-eat fish products shall give sufficient time for L. monocytogenes to proliferate to infective levels.

Prevention of L. monocytogenes from reaching hazardous levels at retail establishments can be attained by reducing shelf-life and stringently adhering to the optimal temperature requirement.

Lack of Food Analysis

According to another study revealing the practices used by the Scottish smoked salmon industry, the risk of ceiling condensation dripping onto product was a common problem, especially in the smaller premises.

However, it was found that the smaller producers did not undertake product testing for Listeria because of high test costs and lack of technical expertise.