Kouji Kimura: the genius of extreme fish aging

Chef Kouji Kimura

“As fresh as it can be” is one of the most common pre-conceptions about sushi. Sushi masters in Tokyo have the world’s greatest seafood market to buy their fish, but often, depending on the fish or seafood they would mature it before serving to their guests.  Yet few of the chefs take such extreme lengths like chef-owner Kouji Kimura, who is a pioneer of fish aging. Some of his neta (sushi toppings), like blue marlin are aged for up to 2 months. I’ve known him since 2013, but only last October I had a chance to talk to him about his exceptional approach to his craft. It was a big honour for me, especially that Kimura-san rarely gives any interviews and only sleeps for 3 hours per night.

Kouji Kimura is a third generation sushi chef. His father died when he was 20 years old and it’s his grandfather, who served Edomaezushi ,where salt and vinegar was used for preserving purposes, was the biggest influence for him.When he first opened his restaurant, he did not advertise at all because he had the confidence of serving good sushi. He was confident that customers would come. When fresh sushi trend came, he switched to fresh style sushi thinking that his  grandfather’s sushi was too old fashioned. His sushi restaurant became a sushi restaurant without any characteristics located in Futagotamagawa. Because he never advertised, only locals came.  For about 3 years, his business was ok, but the number of customers started to decrease after that.  The restaurant was completely empty several days a week.  There was a famous sushi (3 Michelin star) restaurant in Kaminoge, one station away from Kimura-san place, run by Araki-san who is now in London. So it was no point competing with his sushi restaurant serving tuna.  There was also a restaurant called Irifune in Okusawa which is very nearby.  He knew, that competing with these restaurants with the quality of tuna sushi would not be a clever strategy. He did think of following his grandfather’s style, Edomaezushi style.  But he was already in his 30s without any skill that would attract customers who are already satisfied dining at Edomaezushi restaurants in Nihonbashi and other central parts of Tokyo.

Salted and fermented viscera, egg, and flesh of morotoge shrimp (shimaebi no shiokara)

Sometimes Kimura-san would end up having a block of white fish worth more than 20 thousand yen. There were days he had to throw away those expensive pieces because no customers would came to his restaurant. He thought it was such a waste and cut it into half to see whether there were any edible parts left for him to eat. The surface was all gooey. It just happened that meat around the spine had not turned black. When he tasted it, it was delicious. It did stink a lot because it was rotten, but taste-wise, it was something he had not tasted before. He thought then that if he could gather this kind of fish meat and server customers, they might come and dine at his restaurant.  Then Kimura-san realized that sushi chefs are taught how to maintain the freshness of fish but never learned about the process through which fish rots.

So, he decided to teach himself.  He started by observing how a fish rots, whether it is from gall bladder or the heart, for example. Then experimented how the rotting process goes when he removes organs that rot easily. The result was that fish lasted longer. The texture and taste of fish became something he has never tasted before. This is the kind of aged fish other sushi restaurants provide. It is actually not properly aged but result of enzymolysis just before rotting. He could detect subtle unpleasant aroma when he put the fish into my mouth. He wondered what caused this aroma. Then he came to realize that it is the blood or the moisture in the fish flesh that caused fish to go bad. So he studied to find ways to get rid of the blood and unnecessary moisture. He tried to remove innards, blood from dead fish…trying various methods as much as he could. He only had few customers coming to his restaurant, so he had time. When he was able to remove blood the stinky aroma was gone, and when he was able to control the moisture content of a fish that is one of the causes of rotting, he was able to bring out umami through aging that was different from the rotting process.

Cuttlefish ruibe. Fresh guts of cuttlefish are salted for a week, desalted for over 6 days and dried, pickled in miso (mixture of Shinshu miso and red miso) for a week, wrapped with cling film and kept frozen.

First rumors spread that Kimura-san was aging fish.  Around that, he stopped purchasing tuna.  He abandoned tuna which was one of the main ingredients of sushi omakases around Tokyo and concentrated on purchasing the most delicious fish in Japan other than tuna and age it into the most delicious fish in Japan. This rumor also spread in Tsukiji. People thought he gave up purchasing tuna because he was lacking money. Those handling other types of fish also refused to sell him their fish saying that he would only turn their fish into something “soggy, gooey and half rotten”. He endured that. Despite such situation, there was one middleman wholesaler who sold him his fish. So he asked him to come and dine at Kimura-san restaurant. The wholesaler was shocked when he ate his sushi because it was something completely different from fresh sushi that was popular then. He talked about his experience of eating aged sushi to his friends in Tsukiji then people got interested. Other middleman wholesaler came to his place and they told chefs (their customers) about his restaurant. Chefs then came to eat his sushi and told their customers about my unique sushi. Customers gradually started increasing. Michelin inspectors came and rated Kimura sushi. Mr. Nakazawa of Sushisho in Yotsuya, currently in Hawaii, introduced his restaurant as a sushi place he goes privately at one of the TV programs. Thanks to this media exposure as well as the Michelin star the restaurant received, customers came back to Kimura-san restaurant.

Sea urchin from Amakusa in Kyushu area and soba. Sea urchin has been pureed and mixed with dashi of soba.

What Kimura-san is pursuing through fish aging is an ideal form of sushi. Delicious fish on its own will not make delicious sushi. His rice is hard. Even after it’s been cooked, it is quite dry. But this is why vinegar is absorbed into the centre of the rice grains. The surface of rice grains remains dry even after absorbing vinegar. Soft and moist fish flesh would be a good combination with this hard kind of hard rice. So it is not only fish Kimura-san is thinking about. Shari (rice) is the base for every sushi. He also have to think about how to bring out umami in shari. Then he adjusts the condition of fish to the shari. If he pursues tastiness of each fish through aging, he would have to adjust the rice by, for example, changing the hardness of rice. Basically, sushi restaurants prepare only one type of shari. So he adjusts his aging to rice, also through the thickness of each slice and the way he cuts each piece. Basic rules are to avoid crunchiness or hard texture of sushi. Soft and moist is ideal so that fish mixes well with rice in a way that clings onto each rice grain in the mouth when eaten.

Sardine dumpling in clear soup with thinly sliced leek and a bit of ginger.

For Kimura-san crunchiness is kind of texture absolutely unnecessary in sushi, because sushi is not sashimi.  If it’s sashimi, what you do is put a slice of sashimi in your mouth and also rice in your mouth and eat them together.  Sushi is about harmony of sumeshi (vinegared rice) and fish. That harmony cannot be realized with crunchiness or slightly hard texture.  This pursuit of harmony was the culture of sushi in the olden times, but somehow, sashimi type of sushi where freshness was everything became popular. This is why sushi masters became lazy in treating fish.  In the olden days, though, the techniques used by sushi masters were preserving rather than maturing.  They intentionally increased the sodium content of the fish by, for example, salting it, dipping it into soy sauce or sandwiching with kelp. In the modern world, however, we have fridges.  So more focus is on how to make fish tastier.  He believes that we are at a stage where techniques to realize this are just about being established.  Matured sushi techniques would never be possible without the existence of fridges.

Shirako (milt) risotto with matured pepper.

Kimura-san has been experimenting with fish aging on trial and error basis since 10 years now and says the main difference between aged or rotten is…. if it stinks or not. Many aged sushi restaurant serve fish that have started enzymolysis and that is just before the rotting stage. This is not aging, it’s just keeping. Stinky smell is unavoidable during the aging process. It’s up to good treatment to remove that or avoid that. Because he works alone at the restaurant and has no apprentices, he used  “to test” the matured fish with his mother as he couldn’t afford to get sick. He has worked on this so much though, that he doesn’t make any mistakes now.

Sushi becomes really delicious when aged properly and matches perfectly with shari. But because it is such a risky thing, not many people want to do it. It’s difficult to acquire the skill. Each fish is different in character so you just have to learn what to do for each fish. He cannot say, this many days of aging for this type of fish. Some ask how to salt a certain type of fish, for example, but he can never give them the right answer without observing the fish. He can’t specify the amount of salt needed. Unless the person is willing to waste maybe 50 fish just to get aging right for one fish, he would advise that person to give up acquiring aging skills.

10 days aged mehikari (round greeneyes). Head, inners and the tail of mehikari are removed. Dried in the shade for 4 days. Shuto is made from the inners and mixed with sake, boiled down then smoothened. Semi dried flesh of mehikari is soaked in the shuto soup and dried for further 6 days.

Kimura-san believes one can never love a restaurant from the bottom of his/her heart unless it’s his/her own. For example, when he cleans the toilet he doesn’t mind using bare hands. He doesn’t think you can be this committed if the restaurant is not yours.  He believes if he employed someone, he would have to do everything all over again himself. Also, working hours at his restaurant would be very long, maybe about 22 hours per day.  It would be against the law, so he cannot employ anyone.

The only day off Kimura-san has is on Mondays.  He goes to Tsukiji to buy fish though only when they call him to tell him they have good ones.  In that case, he goes and buys fish in the morning, then treat fish before noon and take the rest of the day off.

I asked how is it possible to do such work with only 3 hours of sleep and Kimura-san said that it is possible, but he thinks he would die young. He said he has no stress and loves working on his own. He is a happy man when he is holding his knife, but in order to do everything himself, he has to cut his sleeping hours. This is less stressful than employing someone. If he finds some other method that makes this aging process easier, maybe he could work for less hours.

Leftover soup with a slice of baguette.(fish and vegetables)
Very thinly sliced, one week matured squid
6 days aged tilefish (kawahagi) and its liver
3 weeks matured red snapper
6 days matured bonito
3 weeks aged salmon roe (ikura)
Mantis shrimps from Kanazawa.
One month aged amberjack.
10 days aged sardine.
2 month aged blue marlin.
Conger eel
Kimura sushi exterior

Thank you Mrs Mimi Kobayashi, my “fixer” and interpreter, who made this interview happen. https://www.instagram.com/kobayashimimi2/

Kimura sushi (3-21-8 Tamagawa, Setagaya-ku, Tokyo; tel.03-3707-6355)

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