Research PR showcase! Vol.6: Meet the fish-shocking researcher!

IRCAEB Dept. of Molecular Agriculture, Takao Namihira

The first research PR in the fiscal year of 2023 will feature the works of Prof. Takao Namihira, who works on the treatment of Anisakis parasites in fish using pulsed power.

Anisakisare parasites that infect marine mammals such as whales as their final hosts. These parasites are nematodesthat are 0.5~1 mm wide and 2~3 cm long, making them visible to the naked eye. Anisakis first infect the preys of marine mammals, which include blue-backed fishes such as bonitos, mackerels, horse mackerels and sauries as intermediate hosts. Anisakis are then able infect marine mammals when the infected fishes are eaten. When humansconsume Anisakis-infected fishes raw such as sashimi, Anisakis burrow into the stomach and intestinal linings and cause excruciating pain known as anisakiasis.

Anisakiasis is well-known in the Kochi prefecture, where seared bonito (katsuo no tataki) is a famous local delicacy. Here anecdotal wisdoms such as “keep an eye on the katsuo no tataki, if worms start come out then remember to chew thoroughly” are surprisingly common. (As a Kochi native, the IRCAEB director Prof. Shinichiro Sawa can attest to this). The Ministry of Health, Labour and Welfare started to promote anisakiasis awareness during the 1990s, and with the partially amendment the Food Sanitation Act in 2012 anisakiasis cases are now better documented and publicized. Currently the only viable anisakiasis treatment is to surgically remove the invading Anisakis, however the intense pain cannot be stopped until the invading Anisakis either die or are removed. (Since humans are not Anisakis’ natural hosts, Anisakis in human digestive tracts usually die within a few days and are then excreted naturally).

Anisakis in fishes can be killed either by being heatedabove 60°C for 1 minute, or frozenbelow -20°C for 24 hours. Ultraviolet light, ultrasound and irradiation have been shown to be ineffective against Anisakis within fishes, however. While high pressure can indeed kill Anisakis within fishes, it also pulverizes the fish making it not suitable for human consumption. Therefore, currently freezing is the most recommended deworming approach for fishes, and is the mandatory treatment for fishes suspected of Anisakis infections globally. However, despite the advancements in food processing technologies, freezing inevitably reduces the commercial values of fishes. (In Japan, vendors are required by law to declare whether fishes have been previously frozen). Today fishes can be transported to essentially anywhere in Japan fresh. Since freezing is no longer obligatory for fish transportation, there are now incentives to develop new fish-deworming technologies without freezing, in order to improve fishes’ commercial values. Prof. Namihira’s group works on developing such deworming approaches, even though Prof. Namihira is neither a nematologist nor a medical doctor, but a pulsed power specialist in the Faculty of Engineering.

In simple terms, pulsed power refers to electrical energy that accumulate and become compressed over time, and then instantaneously released. One pulsed power example in nature is lightning. As such, you may picture Prof. Namihira’s research as making synthetic, controlled lightning strikes.

The pulsed power energy technology was initially developed in the USA for military uses, though research for civilian applications began in Japan during the 1990s. Pending on the magnitude, durationand frequencyof the pulses, which can be adjusted using capacitors (devices that store energy) and circuitries, devices with various applications can be made. Of course, the Anisakis elimination devices introduced here was also optimized by adjusting these parameters.

Pulsed power can have various applications. These include ozoneand medicinal nitrogen oxide gas synthesis, water purification by algae removal, concrete demolition, Nano particle synthesis, and adhesive removal. However due to the enormous amounts of energy released, pulsed power requires specialized equipment and controlled areas for its operations.

To test the deworming capability of pulsed power, slices of Anisakis-infected horse mackerel fillets were prepared, which is then treated with 350 pulses of 15,000 volts in cold brine to prevent the fillets from heating up.

All of the Anisakis in the fillet were confirmed to be dead after pulsed power treatment.

However, since the fillets did heat up during the treatment, it remains possible that the Anisakis were killed from the heat instead of electric current. To address this possibility, we compared the Anisakis morphology in pulsed power-treated fillets to those in heat-treated fillets. The cuticles of Anisakis in heat-treated fillets tend to turn opaque, whereas the cuticles of Anisakis in pulsed power-treated fillet remain transparent, similar to those in untreated fillets. From this we can at least eliminate heat as a possible cause of death for the Anisakis. No damages at the cellular level were observed in pulsed power-treated Anisakis, and Prof. Namihira is now working on deciphering the mechanisms of how high current from pulsed power kill the Anisakis.

To determine whether pulsed power is a viable deworming treatment of fishes for human consumption, we conducted various taste tests. Even with all the Anisakis removed, if pulsed power treatment reduces the flavor of fishes more so than freezing then the point of this exercise would be rendered moot. To this end, we conducted sensory tests on the visual(color), cellular(microscopic structure), mechanical(elasticity) and flavoraspects of pulsed power-treated fishes. Fortunately, we did not detect significant drops in quality in fillets after pulsed power treatment.

In addition, we were able to confirm that the fillets were sufficiently cooled in between pulses such that the fillets were not unintentionally heated during the process. With these positive results, we set up horse mackerel de-worming facilities in collaboration with Japan Seafoods, which were able to produce up to 4 tons of Anisakis-free horse mackerel fillets by September of 2021.

To date anisakiasis cases associated our horse mackerel fillets have yet to be reported, and we believe the pulsed power deworming technology has the potential to be adapted nationwide, with scaled-up trials scheduled to start by 2025. Currently our works are limited to horse mackerel fillet, though we envision in the future whole horse mackerels can be pulsed power-dewormed in harbors immediately after being caught. Meanwhile, we’re also working on developing scaled-down hardware for small vendors, as well as machineries tailored to deworm pork and beef. Sashimi is a prominent aspect of the Japanese culture, and we strive to promote sashimi and other raw foods to future generations worldwide by ensuring their safety.