Proper storage of raw seafood and good hygiene, especially early in the process from raw material to edible food, is important to avoid contaminating sushi with listeria bacteria in amounts that can cause disease.
This is the key finding in a risk assessment of Listeria monocytogenes in sushi, conducted by Norwegian Scientific Committee for Food and Environment (VKM). The Norwegian Food Safety Authority requested the assessment to update its advice about eating sushi for pregnant women and other vulnerable groups.
The listeria bacterium can cause the disease listeriosis. Most people do not get sick from listeria bacteria, but pregnant women, children, the elderly and people with impaired immune systems are more vulnerable to the bacteria. Listeriosis can be transmitted from mother to fetus during pregnancy and can cause the fetus to develop a life-threatening illness.
According to VKM, three factors in particular affect whether pregnant women and other susceptible groups exposed to Listeria monocytogenes in sushi in amounts that can lead to listeriosis, actually develop the disease.
“The 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° C 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,” explains Taran Skjerdal, the academic leader of the work on the assessment.
Handling of raw materials, storage temperatures and hygiene all have an impact on how fast the listeria bacterium multiplies in food; 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,” says Skjerdal. Sashimi differs from sushi in that it does not have vinegar-marinated rice. Vinegar inhibits the growth of listeria bacteria.
“It is therefore considerably more likely the concentration of listeria is higher in sashimi than in sushi,” says Skjerdal.” If sashimi lays on a tray with sushi and stored, the bacteria will grow faster in the sashimi than in the sushi. The bacteria can spread from the sashimi to the sushi, if the pieces are on the same tray.
There is insufficient data on the incidence of Listeria monocytogenes in raw materials and in sushi, on the realistic storage conditions at all stages of the chain, and on the consumption of sushi.
In the absence of data, VKM has assumed that all raw materials are contaminated. By using realistic scenarios for storage and processing sushi, VKM has estimated the amount of listeria taken in by consumers of processed sushi. VKM used quantitative and qualitative methods for these scenarios. Flow charts were prepared for each of the products from catch/slaughter to consumer. For all modeled products, probable values for acidity, water activity, package atmosphere and the relationship between time and temperature were stated. The assessment assumes production processes have followed requirements for hygiene, shelf life and storage, except in scenarios where the failure of production hygiene and the unknown origin of raw materials is used. Where the modeling of a scenario finds a product likely to contain listeria exceeding the threshold amount for increased likelihood of disease for pregnant women, the sushi product resulting from the given raw material and the associated storage scenario is evaluated as risky. For such products, we considered whether measures such as lower storage temperature would reduce the probability of listeriosis to preclude the development of the disease.
VKM's Scientific Panel on Biological Hazards is responsible for the assessment.