Researchers have looked at the effect of different ripening temperatures and salt concentrations on the growth of Listeria monocytogenes in a traditional Norwegian fermented fish product.

Rakfisk is made from lake trout or arctic char by mild-salting and brine maturation at low temperatures for several months and is eaten without heat treatment.

Ripening temperature had the largest impact on Listeria monocytogenes growth during rakfisk production as low ripening temperatures resulted in essentially no growth. However, even freezing temperatures cannot kill Listeria.

Chosen salt concentrations and temperatures reflect the prevailing types of commercial rakfisk production, said researchers in the study published in the journal Foods.

Recent reported outbreaks
Outbreaks of listeriosis, wher batches of rakfisk were implicated, were not recorded until fairly recently. In the last six years, two outbreaks affecting three and 12 people, respectively and a recent suspected outbreak occurred.

The Norwegian Directorate of Health recommends that vulnerable groups such as pregnant women and people with weakened immune systems due to underlying medical conditions, avoid rakfisk.

Scientists behind the study are from Nofima, an institute in Norway for applied research within fisheries, aquaculture and food.

In the European unio, standard ready to eat foods that contain less than 100 colony forming units (CFU) per gram at the end of shelf life are accepted. The U.S. Food and Drug Administration (FDA) requires absence of Listeria in 25-gram samples of RTE seafood products.

Rakfisk is a seasonal product, mainly consumed from late fall through to Christmas. There are no critical control points during the production process that guarantee elimination of Listeria monocytogenes in the final product.

The most common production method is based on mild salting and spontaneous brine formation wher the gutted fish is dry-salted and layered belly up, preferably under pressure, in airtight containers.

Containers are stored at low temperatures of 3 to 7 degrees C (37.4 to 44.6 degrees F) for three to 12 months and occasionally longer. The fish is submerged in the salt brine during storage. Salt concentration in the brine varies among producers but is between 4 and 7 percent.

importance of low temperature
Rakfisk was produced with and without the addition of Listeria monocytogenes to the trout or char used. Production conditions were 4.8 percent or 6.3 percent sodium chloride with storage at 4 or 7 degrees C (39.2 to 44.6 degrees F). Brine samples were used for microbiological analysis and were collected from the containers on days 0, 3, 7, 14, 28, 42, 63, and 91.

Low temperature and relatively high salt concentrations were found to be the major hurdles for growth of Listeria monocytogenes in rakfisk production. The levels of organic acids produced during fermentation were too low to inhibit growth.

Results show temperature was the main factor influencing growth of Listeria monocytogenes in rakfisk brine. At 7 degrees C (44.6 degrees F), rapid growth occurred, especially at the lower sodium chloride concentration of 4.8 percent.

Inoculation level in the study was unrealistically high but even very low contamination levels would result in numbers of food safety concern assuming a similar growth rate, said researchers.

High sodium chloride concentration of 6.3 percent delayed growth but Listeria monocytogenes eventually reached the same levels. Changes in temperature or salt did not change the lactic acid levels but there was a significant increase in formic and acetic acid in low-salt conditions.

Low temperature of 4 degrees C (39.2 degrees F) restricted growth of Listeria monocytogenes at both salt levels. However, producers using this temperature generally employ a maturation time of minimum five months.

Researchers also investigated the effect of the anti-Listeria bacteriophage P100 on rakfisk with added Listeria monocytogenes.