Proper storage of raw seafood and good hygiene is important to avoid contaminating sushi with Listeria at amounts that can cause illness, according to a risk assessment in Norway.

The Norwegian Scientific Committee for Food and Environment (VKM) looked at Listeria monocytogenes in sushi after a Norwegian Food Safety Authority (Mattilsynet) request, so the agency can updat its advice on eating sushi for pregnant women and other vulnerable groups.

The likelihood of vulnerable groups being exposed to Listeria in concentrations that can cause illness varies according to whether the contaminated item is eaten as sushi or sashimi, storage conditions of raw materials, and whether the product is homemade or purchased.

Three factors affect whether pregnant women and other susceptible groups exposed to Listeria in sushi in amounts that can lead to listeriosis, actually develop the disease, according to the VKM assessment. These are food safety hygiene; storage conditions for the raw seafood from producer until preparation and before being eaten; and whether the raw seafood is prepared as sushi or sashimi.

Sashimi differs from sushi as it does not have vinegar-marinated rice. Vinegar inhibits growth of Listeria, but it fails to work unless it is used properly. Specific dilution, percent of acidity, and duration of immersion are mandatory for the effective use of vinegar.

How to minimize risk
Taran Skjerdal, who led work on the assessment, said handling of raw materials is crucial for the development of Listeria monocytogenes.

“Raw foods should be processed under good hygienic conditions, cooled down quickly and stored at 4 degrees Celsius or colder for a maximum of seven days before the sushi is prepared and eaten. Then the sushi will not contain the bacteria in quantities that exceed the threshold value for increased likelihood of developing listeriosis for vulnerable consumers.”

Whether sushi is homemade or purchased affects the likelihood of exposure to the bacterium.

“This is partly because the temperature in the consumer’s refrigerator is often higher than that of professional providers, and partly because professionals generally have a better opportunity to choose suitable raw materials. If professional providers and consumers use raw material stored for the same length of time and at the same temperature, the difference with regard to the amount of Listeria in the sushi is very small,” said Skjerdal.

Skjerdal added it was likely the concentration of Listeria is higher in sashimi than sushi. If sashimi is on a tray with sushi and stored, the bacteria will grow faster in sashimi than sushi. Sashimi will also release more fluid, which may result in contamination of sushi if pieces are on the same tray.

From 2005 to 2018, 661 imported seafood samples were examined for Listeria monocytogenes. It was detected in 20 samples at a concentration always below 2 log colony forming units (cfu) per gram.

Lack of data in literature decides VKM methods
Data on the occurrence of Listeria in the raw constituents and sushi, realistic storage conditions at all stages of the production chain, and consumption of sushi was unavailable. So researchers set up scenarios to provide the least suitable, moderate suitability, and optimal conditions for growth of Listeria monocytogenes.

By using scenarios for storage and processing sushi, VKM estimated the Listeria exposure in consumers of processed sushi. They assumed the fish or seafood was already contaminated when killed, as this would result in the greatest possible growth of Listeria in sushi until consumption.

The assessment assumes production processes have followed requirements for hygiene, shelf life and storage, except in scenarios wher the failure of production hygiene and the unknown origin of raw materials is used.

Sushi made from raw materials processed under good hygienic conditions, then rapidly cooled and stored at 4 degrees Celsius or colder for up to a week before preparation and consumption will not result in concentrations of Listeria that exceed 3 log cfu per gram (1,000 cfu per gram), based on assumptions in the assessment.

Fish species that can be stored for prolonged periods at cool temperatures, such as halibut and tuna, may have higher concentrations of Listeria than those species stored for a shorter period.

wher the modelling scenario indicated the product was likely to have concentrations of Listeria exceeding above 1,000 cfu per gram at consumption, the product was associated with an increased likelihood of listeriosis in pregnant women and other vulnerable consumers. The sushi product resulting from this raw material and the associated storage scenario was evaluated as risky.

For such products, scientists evaluated whether lower storage temperature would reduce the increased likelihood of developing listeriosis to such an extent that disease would be avoided.