Sushi originates from Japan and is a dish that comprises boiled rice mixed with special vinegar and various toppings, usually different types of raw fish.
The sale and consumption of raw fish entails some new challenges compared to the traditional preparation of fish, something that both producers and consumers should be aware of.
Cases of salmonella and listeria infection linked to sushi have been proven in Norway and other countries. But the health risks are minimised with good routines during food preparation, good storage of the food and use of good raw materials.
Inhibits bacteria growth
Nofima scientists took sushi to the laboratory to study the quality and shelf life of the products with respect to both bacteria that may reduce the quality of the fish and bacteria that potentially pose a health risk.
An experiment was carried out to study what happens when listeria bacteria are added to pieces of nigiri sushi, which are small rice balls topped with fish. The pieces of raw salmon and halibut were then infected with the bacteria.
In order to compare, the scientists also conducted the experiment on pieces of fish without rice. The samples were then stored for a period of seven days at temperatures of 4 °C and 8 °C – a temperature range that is relevant with a view to both retail sale and storage in a refrigerator at home.
The result demonstrated that the fish that lay on the acidic rice contained far fewer listeria bacteria that the pieces of fish that did not lie on rice.
“The vinegar is God’s gift to the sushi producers. The acidic rice inhibits the growth of bacteria. It has a good effect when it is warm, for instance when the pieces of sushi are in a warm room for a while, which by the way is something the fish producers should never do,” says Nofima Scientist Hilde Herland.
“But over time the tests show that there were significantly more bacteria in the fish that was stored at eight degrees than the fish stored at four degrees, which indicates that the temperature during storage also plays an important role.”
Degree of acidity
In a separate experiment, the scientists studied whether the degree of acidity in the rice led to a change in the degree of acidity in the pieces of fish. The scientists studied both nigiri sushi, which are rice balls topped with fish, and maki sushi, rolls of rice and fish packed in dried nori (edible seaweed).
These results showed that the pH value (acidity) in the pieces of fish that were in contact with rice fell somewhat over time. The largest fall occurred in the fish that had the most contact with rice.
“It appears that the low pH value in the rice also leads to a lower pH value in the fish. This may explain why the bacteria are less able to survival in the pieces of fish,” says Herland.
Wasabi and fish
The scientists have also studied what occurs when fresh wasabi and wasabi paste are added to samples of halibut. Wasabi (also known as Japanese horseradish) is a plant used as a condiment that is known for its extremely strong flavour.
In the samples containing fresh wasabi, the scientists observed a halving of the bacteria growth after storage for a five-day period at a temperature of 12 °C. The samples containing wasabi paste also had somewhat less bacteria, but this was not as clear as when fresh wasabi was used.
“The results confirm that fresh wasabi has a bactericidal effect. But this effect is limited when wasabi is used in sushi that is eaten a short time after production,” says Herland.
“But it can still be of significance in sushi products that are produced some time before they are eaten, as is the case with ready-made sushi which is now becoming more and more common in supermarkets.”
The experiments show that there are several conditions during the production and sale of sushi that require focus in order to achieve good products.
The work is financed through Nofima’s strategic research funds.
Nofima also carries out market research on sushi and has employed a sushi scientist who engages in research on consumer habits and behaviour.