Take a bite of a bowl of sweetly sour vinegared rice and delicate soft tuna slices and instantly, a delicious harmony fills the mouth.

Unbelievably, this delectable marinated tuna dish isn't found in a pricey sushi restaurant in the Ginza or the Toyosu Market. 

Topped with cuts of soy sauce-soaked, fine bluefin tuna, the bowl of cottony rice flavored with “akazu” red vinegar is a patent-registered frozen food product sold through the internet.

It is the pride and joy of Yasumitsu Watanuki, the second-generation manager of the Kouraku Sushi Yasumitsu restaurant, located in Tokyo’s Yotsuya district.

Watanuki’s prolonged struggle amid the novel coronavirus crisis ended up leading to the development of a unique cooking method to make the bowl.

Now that the brainchild of the sushi chef has proven a huge hit, 50 of the two-person packages sell out in an instant.

Kouraku Sushi started operations in 1969. Since Watanuki became its second manager in 2010, he has been obsessed with serving “quality tastiness.”

Course meals using prestigious ingredients are exclusively available at the restaurant. Customers at times have to wait for more than six months for a reservation to get a taste of Watanuki’s sushi.

However, the COVID-19 outbreak, combined with the April 2020 declaration of a state of emergency, drove away customers in droves. Due to shortened business hours and other factors, Kouraku Sushi’s sales plummeted to less than half the pre-pandemic level.

Watanuki was living hand to mouth by selling takeout “chirashizushi” dishes marked by tiny pieces of sashimi.

At this point in time, another state of emergency was issued in January 2021. Seeing that, Watanuki did not want to rely on selling chirashizushi on a continual basis.

“Continuing to do the same thing would not work,” he recalled. “I needed to consider something new to work on.”

An idea Watanuki came up with at that time was releasing a frozen marinated tuna bowl, because he had formerly been interested in making frozen sushi products.

Slashed opening hours helped Watanuki to have time for concentrating on his research following the restaurant’s daily closure at 8 p.m. He test-created tuna bowls repeatedly every day.

Watanuki put his focus on the quality of sliced tuna with the aim of reproducing the flavor served to patrons at Kouraku Sushi. He prepared dozens of slices so that various freezing times and post-defrosting treatments could be tested on each of them.

Rice was also cooked carefully with an eye on keeping it aromatic. Adjustments were made in the hopes that unfreezing the rice would not render it messy, but keeping it as savory puffy rice as much as possible.

Watanuki slept just two to three hours a night. He spent days freezing and defrosting tuna and rice again and again.

He asked the chief editor of a major gourmet magazine to sample his bowl. He re-created the dish on many occasions until it received a high evaluation.

Obtaining the full endorsement of the editor, the tuna bowl’s release was announced in mid-February 2021 on social media and Kouraku Sushi’s website.

Inquiries came in primarily from regular customers, and more bowls than initially expected were purchased. Word afterward spread about the specialty, and 50 frozen sets now sell out quickly in the span of only a minute.

As an acquaintance who tried the bowl recommended Watanuki apply for a patent for “frozen food maintaining its tastiness to such a high degree,” he sought the advice of a lawyer friend.

Watanuki was told that a patent application could be filed, so he promptly contacted a patent attorney. The patent application was submitted in late June 2022.

Though Watanuki heard that it is “really difficult to be granted a patent for a food product,” he successfully secured his patent rights in August to register his “technique of making frozen flavored tuna sashimi” as his patent certificate shows.

An all-frozen package comprised of tuna, rice, “gari” pickled ginger, soy sauce and wasabi for two people will be put on sale online for 10,800 yen ($72.50) solely from October through February.

The hefty price tag means the quality of the delicacy is as high as one provided at Kouraku Sushi. The frozen bowl cannot be mass-produced because tuna and rice are processed in a specialized way.

“The coronavirus crisis has yet to end,” said Watanuki. “I will be continually offering delicious marinated tuna bowls for those who are unable to venture out or refrain from going out.”