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Food Safety Focus (106th Issue, May 2015) – Incident in Focus

Cold-smoked Salmon and Listeria monocytogenes

Reported by Ms. Janny MA, Scientific Officer,
Risk Assessment Section,
Centre for Food Safety

Recently, the Centre for Food Safety (CFS) found that a cold-smoked salmon sample from Polyfood Food Service Co Ltd contained a small amount of Listeria monocytogenes . This article discusses the risk of L. monocytogenes in cold-smoked salmon and the importance of effective control during the production.

L. monocytogenes in Smoked Salmon

Smoked salmon can be prepared by hot-smoking or cold-smoking. In hot-smoking, salmon is smoked at around 70-80°C to cook the flesh. However, in cold-smoking, salmon is smoked at no higher than 33°C to avoid cooking the flesh or coagulating the protein. Therefore unlike hot-smoking, cold-smoking is generally insufficient to eliminate but only reduce the level of L. monocytogenes, a bacterium which is ubiquitous in the environment and may also be present in raw salmon at low levels.

In addition, the extensive handling of salmon, for example slicing followed by cold-smoking, also provides ample opportunities for L. monocytogenes to cross-contaminate the products if inadequate attention is given to the Good Hygienic Practices (GHPs).

Ready-to-eat cold-smoked salmon may allow L. monocytogenes to grow slowly throughout its relatively long refrigerated shelf-life. Its contamination with L. monocytogenes , even with small quantities, is therefore of particular concern.

Various food recalls due to L. monocytogenes contamination in cold-smoked salmon have been reported all over the world. There is also indirect epidemiological evidence associating contaminated smoked fish and human listeriosis cases.

Cold-smoked salmon
Cold-smoked salmon

Importance of Effective Control to Ensure Food Safety

In general, foods containing low levels of L. monocytogenes (e.g. <100 colony-forming unit (cfu)/g) pose very little risk to consumers. However, as the number of bacteria increases, consumers will be at risk of listeriosis. Even most healthy individuals do not develop symptoms or only have mild symptoms when infected, severe complications such as septicaemia, meningitis or even death may occur in the susceptible populations, including the elderly, young children, and people with weakened immunity. Listeriosis during pregnancy can lead to miscarriage, stillbirth, premature delivery, or infection in newborns. Prevention of L. monocytogenes contamination and controlling its growth is of prime importance in cold-smoked salmon production.

Firstly, raw material control could play a role on the incidence of L. monocytogenes in incoming fish. More importantly, equipment and facilities in the production plant should be properly laid out to reduce the potential for L. monocytogenes contamination. For instance, droplets and aerosols from condensates can directly or indirectly contaminate food and food contact surfaces, it is essential to control the ventilation to minimise condensate formation. Furthermore, specified cleaning and disinfection programmes, along with an environmental monitoring programme are also critical to assuring L. monocytogenes control. Employees involved with the production and handling of cold-smoked salmon should also be trained to prevent cross-contamination.

Since temperature abuse would foster the growth of L. monocytogenes , strict temperature control for finished product i.e. never exceed 6°C (preferably 2-4°C) is also vital. Growth of L. monocytogenes in cold-smoked salmon can also be controlled by freezing or product reformulation.

Actions Taken

Subsequent to the detection of L. monocytogenes in the concerned cold-smoked salmon by the CFS, the manufacturer was instructed to recall all smoked salmon produced, regardless of the brand and batches, and suspend the relevant production line to carry out thorough cleaning and disinfection.

The Food and Environmental Hygiene Department (FEHD) will closely follow up the incident. The manufacturer should demonstrate that the remedial actions have been taken and are up to the satisfaction of the FEHD including measures addressing L. monocytogenes risk before resuming operation.

Key Points to Note:

Advice to the Public

  1. Susceptible populations should avoid consuming ready-to-eat foods with long shelf-lives under refrigeration (e.g. cold-smoked seafood including salmon, soft cheeses and deli meat).
  2. Store smoked salmon strictly in accordance with the instructions on the labels.
  3. Avoid consuming smoked salmon beyond its "use by" date.

Advice to the Trade

  1. Observe GHPs in food processing to assure that the food products do not pose risk to the public.
  2. Food businesses are recommended to implement the Hazard Analysis and Critical Control Points (HACCP) system to prevent food safety problems.
  3. Seek professional advice from food scientists if there is any doubt on whether individual food product can support the growth of L. monocytogenes . Reference can also be made to the Codex Guidelines on the Application on General Principles of Food Hygiene to the Control of Listeria monocytogenes in Foods (CAC/GL 61-2007).