What is Jukusei sushi?

The fish used in sushi is generally salted or soaked in vinegar then matured for a number of days while the umami Inosinic acid component increases. This is called “Jukusei” (aging). Sushi made with toppings that have been aged in this way are called “Jukusei sushi”. The aging period depends on the type, individual size and origin of each fish, and some are even aged for over four weeks. However, the preparations are not only difficult and time-consuming, the discolored parts and inedible parts must be trimmed, so these toppings tend to be expensive. If gone too far, the Inosinic acid converts to hypoxanthine and rots. The ability to make this judgment is important. In the end, Jukusei is an evolved version of the culture of “maturing toppings” which existed in Edo-style Sushi.

How to Jukusei? (How are sushi ingredients matured?)

In order to mature seafood, after completing advance preparations (removing the head and internal organs then washing thoroughly; all blood must be removed), more than adequate considerations must be made for the fat content of the fish and management of the bodily fluids. Specifically, this includes processes like dry aging at a low temperature, removing moisture using salt, utilizing enzymes and fermentation, wet aging by putting the item in a vacuum pack, and wrapping in aging sheets, which were developed thanks to Foodism. These processes may be used alone or in combination, whichever process is most suitable for the fish.

In the initial stage of ‘jukusei’ (maturing), the increase in inosinic acid (the umami component) improves the taste. After that the inosinic acid starts to decrease, and once the long-term maturing stage (two weeks or more) starts, free amino acids such as glutamic acid and aspartic acid really start to affect the flavor. This has all been learned in research.

Let’s take a look at specific aging methods.

For example, for white flesh fish, a somewhat high amount of salt is sprinkled on the fish before it is stored in a refrigerator set at 3 to 4℃ with a humidity of at least 85%. The fish is not wrapped at this time. The fish is flipped over 3 to 4 times a day so that the moisture is extracted evenly. Several days later, the salt on the surface of the body and the body fluids that have seeped out are washed off with water (or thin saltwater). The fish is then wrapped in paper towels and then plastic to avoid contact with the air, and it is stored in a refrigerator at 1 to 2℃. Once the chef deeps the fish is ready, it is trimmed. Excess moisture is removed and then the maturing process continues.

We would like to take this time to point out that fish like Tai (Red seabream) and Buri (Japanese amberjack) are clearly more delicious when matured. However, when farmed tai and farmed hamachi are matured, the scent of the feed they were raised on comes out, so these are better eaten fresh, as sashimi, instead of maturing.

Blue-backed fish like Aji (Horse mackerel) and Iwashi (Japanese sardine) are also not suitable for maturing. Blue-backed fish lose their freshness quickly and judging the maturity is extremely difficult. Furthermore, if the fish is matured without sufficient advance preparations, bacteria breed in the remaining blood and organs. This may cause food poisoning.

These fish can be matured using the following method. The fish is put in salt-ice (water cooled with ice and salt) as soon as it is caught. It is sent to the sushi restaurant in this state and left in the refrigerator to rest for several days. Unfortunately, what happens after this is apparently a trade secret.

The easiest method is to wrap in an aging sheet and put it in the refrigerator. After that the chef trims the fish, checking the state of it. An aging sheet is a cloth made from purely breeding a ‘mold’ that is harmless to the human body, and putting cultures of its recovered spores into the cloth. Originally, it is intended to be used to age meat, but it’s just started to be used for seafood too.

Finally, in a method used for ages by sushi chefs, the akami and toro (tuna) portion are taken out and the chiai* portion is removed to be matured. This is then wrapped in paper towels, put into a plastic bag in order to prevent drying out, then put into the refrigerator to rest. The temperature setting is the most important part of this process and, obviously, this is an industry secret. The chef needs to check the state of the tuna (for example whether the white lines are soft and whether the oil has risen), and any discolored portion is trimmed. After that it is refrigerated. This process is then repeated.

*“Chiai (血合い)” is the part with the most veins, so it is a dark red color. It has a strong odor of blood and has multiple times the acidity of the lean meat, so it is not used as a sushi topping.

Related Content

https://guide.michelin.com/en/article/features/%E4%B9%BE%E5%BC%8F%E7%86%9F%E6%88%90%E9%AD%9A

https://www.foodandwine.com/cooking-techniques/dry-aged-fish-joint-sherman-oaks


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Revision date: June 2, 2020

 

What makes a “good” sushi restaurant?

It is probably cutting fish just before serving. For a big size fish, keep its skin on the meat during the process of preparing and cutting into half, and at every serving use sogigiri* as much as customers eat. Protected by the skin, the fish flesh will expose to air for the first time as it is cut. The skin blocks the oxidation process significantly because the fat in fish centers right  under the skin in general. Needless to say, even with any amazing fish, it loses flavor if the fat gets oxidized.

*Sogigiri-A method of cutting which makes a slice thinner with a greater surface, by holding the knife diagonally and cutting in line with the cutting board. Usually used for white fish with firm flesh because it is easier to be eaten when served thin.

Hirazukuri-A method of cutting which gives thickness to each slice so that the texture of sashimi can be enjoyable. This is used for akami like tuna.


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Revision date: April 1, 2020

Assessing Fish at the fish market!

One important task of sushi chefs is going to Tsukiji (Toyosu) every morning, looking at fish with their own eyes and assessing the quality. Having a good eye is important in order to get the highest quality possible, but this is cultivated by experience. They are also constantly obtaining information from the fishmongers at the market regarding what the best fish of the season and their localities. It’s almost a game as to whether they can get high-quality fish at the optimum price everyday. The skills of a sushi chef start with this assessment.


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Revision date: September 11, 2018

Tuna is allowed to rest before it is used.

Once a bluefin tuna is caught, it arrives in Tsukiji fish market within a day or two. However, that fish is not used as a sushi topping that day. No mater how good the tuna is, it starts out very stiff and is not in a state where it should be eaten. The meat is hard and the white muscle lines are left in your mouth. The odor and acidic taste of the red meat is strong and the unique sweetness of the fish is nowhere to be found. After it has rested the muscles soften, bringing out the fat.

Then, when the sushi chef gets the tuna, he first separates the red, lean meat and the fatty toro portion, rewraps them separately and, seals them in plastic and puts them on ice. Next is waiting for the “young” meat, not yet suitable for eating, to mature. The number of days the fish will be rested depends on the size of the fish and the temperature. The smaller the cut and the warmer the temperature, the shorter the rest time. Generally the time is from 3-14 days.

This “young” fish not ready for consumption is a fresh, deep color but as it matures the color darkens, the fat is brought out and becomes a fleshy color. Proper care must be taken because if it’s rested for too long, the color changes too quickly.


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Revision date: August 13, 2018

What is the difference between maturing and rotting?

When fish die, stopping the supply of ATP (Adenosine Triphosphate), the source of muscle energy, the muscle fibers gradually harden. As time passes, it gently dissolves and the ATP breaks down, changing into umami components due to self-digestion. The umami created by self-digestion of ATP is “maturing” and the process after that is “rotting.”


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Revision date: July 23, 2018

Where should soy sauce be applied to the sushi?

The most delicious way to eat sushi at restaurants where the sushi chef applies Nikiri (soy sauce perfectly evaporated with sake) for you is just the way it was prepared. However, at restaurants where sauce is not applied for you, the sushi is eaten by dipping it in soy sauce. The soy sauce used for dipping is provided for you at the counter or table. Many restaurants use the same evaporation formula for the dipping soy sauce.

Soy sauce for dipping is put into a small dish for use, but don’t put in too much. It depends on the depth of the small dish, but the diameter of the circle of soy sauce after being poured should be approximately 25mm.

When dipping sushi into the soy sauce, turning it upside down (although it will be somewhat tilted) and dipping the topping seems to be the most common method. If you keep the topping on the bottom when you put the sushi in your mouth, the flavors of the soy sauce and the fish are in complete harmony and the delicious taste spreads through your mouth. There is also an opinion that turning the sushi upside down for dipping is unacceptable practice. There are also some with the opinion that whether to eat with your hands or chopsticks depends on the situation.

Make sure not to get any soy sauce on the Shari (vinegar rice). You don’t want to add unnecessary saltiness to the Shari, which has already been seasoned. It would be a terrible waste to cancel out the exquisite balance of the topping, wasabi and Shari with the saltiness of soy sauce.


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Revision date: May 28, 2018

Why doesn’t rice stick to the sushi chef’s hands?

One of the pleasures of sitting at a sushi counter is watching the sushi master work his craft.

He holds the topping between the index finger and thumb of his left hand while simultaneously grabbing the shari (vinegar rice) with his left hand. He gently squeezes the shari and then moves the topping from his left hand to the top of that shari in a fluid motion. This entire process of shaping the shari to the finished piece of sushi takes less than six seconds. Every movement is precise and purposeful.

However, no matter how many pieces the chef makes one after another, you’ll never see a grain of rice stick to his hands. If you or I were to make even one piece of sushi, our hands would be covered in rice. So why doesn’t it happen to them? Their hands don’t look oiled. Perhaps sushi chefs have especially smooth or slick hands compared to us average Joes?

Of course not. This is actually thanks to the vinegar.

The chefs keep a bowl of vinegar close by, which they constantly use to wet their hands. This procedure is called “Tezu” or vinegared water, which both disinfects the hands and cools their palms. When the vinegar evaporates, it takes the heat from the hands with it.

Normally hands reach temperatures of 33-34 degrees Celsius (91-93 degrees Fahrenheit), but sushi chefs cool their hands to approximately 30 degrees Celsius (86 F). This transfers the heat from the hands to the shari, keeping it from getting sticky. In other words, not a single grain of rice sticks to their hands.


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Revision date: March 19, 2018

Why is freshly-caught fish allowed to age?

The most delicious time to eat fish differs depending on if it is served as sashimi, as sushi, or boiled. Fresh does not necessarily mean delicious. For example, Japanese Amberjack should be used in sashimi 3-5 days after being caught, in sushi a week after being caught and it can be used in a stew or boiled once it turns black around the edges. This is because the inosine acid, which is responsible for the umami taste, increases after rigor mortis ends and understanding the timing of the peak in flavor is up to the skill of the sushi chef.


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Revision date: March 12, 2018

Prior to WWII, there were chefs whose only job was making vinegar rice!

From the end of the Edo period through the Meiji period, rice was cooked using firewood and a pot. It is not easy to get the fire at the right temperature and the rice has to be cooked to the same texture regardless of where it came from or the size of the grains, so at the time the task required a skilled chef. Therefore, there were “Shari-ya” employed by sushi restaurants who specialized in cooking rice. “Shari-ya” focused on this single task and were not involved in the actual making of the sushi after the rice was passed on to the chefs.


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Revision date: January 29, 2018

What makes a good sushi chef?

The balance between Shari (vinegar rice) and the topping is important in sushi. No matter how good the topping, the sushi won’t be good if the Shari isn’t right for it. More restaurants have been using red vinegar lately, but even if you use a Shari with a strong taste like red vinegar, the balance will be destroyed if the topping has a weaker flavor. Seasoning that goes well with various toppings that doesn’t stand out too much is ideal.

It works the other way, too. If the Shari is too weak, the sushi won’t be delicious no matter how good the topping. Even if the topping is not premium quality, if the Shari is matched perfectly, the sushi will be perfect. In other words, a good sushi chef is someone who can make sushi with perfectly matching toppings and Shari.


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Revision date: December 18, 2017

Is Nigiri sushi made by a Master Really Transparent?!

Some of the shari drops off of the sushi placed in front of you by the chef saying, “Sorry to keep you waiting.” You may be served this kind of nigiri sushi at restaurants that have lines out the door. Of course sushi that falls apart before it even touches your lips is a failure.

Good nigiri sushi looks solid, but once you put it in your mouth the shari naturally loosens. Next the loosened rice absorbs the taste of the topping and it doesn’t stick to the roof of your mouth. On the other hand, with shari of sushi that has been pressed too strongly, the taste of the topping is left in your mouth, getting in the way of new flavors.

In other words, the sushi looks hard on the outside, but it soft on the inside. This is the perfect recipe for sushi.

When first learning, chefs are only concerned with shape and press the pieces too firmly. Next they let up on the force a bit and once they find the perfect amount of pressure, they become a real sushi chef. A master sushi chef is one rank above that and makes sushi that light can pass through. The sushi must be pressed gently enough for light to pass through, but firmly enough so that the sushi holds its shape.


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Revision date: August 28, 2017

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Once you try Kuruma ebi sushi, you’ll never want any other shrimp.

Cultured shrimp like Black tiger is imported to Japan from India, Vietnam, Indonesia, and other places and it used in a lot of nigiri sushi. The shrimp used in sushi rolls is generally the giant tiger prawn. It is used because the price is cheap and it becomes a beautiful vermilion color when boiled. Unfortunately, it doesn’t have the sweet taste normally associated with shrimp.

On the other hand, the Kuruma prawn (kuruma ebi) that is offered at Edo-style sushi restaurants has a rich aroma and sweetness than spreads over your tongue. It is also becoming more popular to boil it just before serving it to the customers. By doing this, the warmth enhances the sweetness of the shrimp.

The old Edo-style sushi restaurants will also ferment the shrimp in eggs scrambled with sweet vinegar (yolk soaked in vinegar) for several days. When the Kuruma prawn is soaked in the egg, its umami is enhanced and its pleasant acidity is delicious.

By the way, when you boil the shrimp, it normally bends towards the belly. Crooked shrimp cannot be used for sushi, so a few cuts are made in the ventral to stop it from bending and it is cut along the muscles or from head to tail and skewered before boiling. You still have to be careful it doesn’t bend when you peel off the shell. Therefore, even just the way a Kuruma prawn is boiled demonstrates the skill of a sushi chef.


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Revision date: August 7, 2017

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Why is Zuke used for lean meat?

Zuke is one of the traditional Edo-style sushi methods. It is said that it was started in the Edo period to stop tuna from rotting when there were large amounts of the fish in the market. Now that there has been advances in refrigeration technology, it’s no longer necessary, but maturing the fish gives it a completely different taste and brings out its umami. Zuke is divided into two broad methods. Here we describe the characteristics of each.

Recently, most sushi restaurants incorporate the “Single Zuke”.

Each slice of tuna is soaked separately, so it can mature quickly. The immersion time is only a few minutes. The idea is to marinate just enough so that the tuna’s aroma remains and the soy sauce doesn’t overtake it.

On the other hand, the old Edo-style method is to perform Zuke after parboiling.

Parboiling means to wrap the fish in a wet cloth, and poor boiling water on the wrapping until the color of the tuna changes color, then turn the fish over and repeat the process. The fish is then put in ice water so the heat doesn’t go too deep in the meat. It is immediately removed once it cools so that it doesn’t get too watery. The tuna is then put in Zuke soy sauce and left to marinate for about half a day. In this method, the soy sauce only soaks into the surface part where the color changed from the parboiling, so the flavor of the tuna remains.

Both methods keep the maximum tuna flavor possible. Tuna is an essential part of Edo-style sushi. There is great diversity between sushi restaurants in the parts, marinating time and flavor of Zuke, which creates a new, original flavor when the lean meat of the tuna soaks up the soy sauce. The fattiest cuts of tuna are most popular. The lean meat has only become more popular due to a rekindled interest in zuke, but in fact during the peak of the bubble economy, there was a time when high-end restaurants in Ginza didn’t know what to do with all their leftover lean tuna meat. It’s almost unbelievable to think of it now.


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Revision date: August 1, 2017

Isn’t it true that fish is all about the freshness??

It’s a well-known fact among professional chefs that some fish don’t rely solely on freshness. Of course there is importance in freshness, but that’s just one element. It’s generally understood that flavor and taste improve with time (maturity).

The umami* found in the meat of the fish is essentially inosinic acid and glutamic acid. After a certain amount of time has passed after a fish has died, the body stiffens and not long after that the rigor lets up. The inosinic acid, which the umami is composed of, comes after the fish has stiffened. It then accumulates in the process of the body relaxing. This is the same in beef and pork in which there is no umami in the meat unless it is hung and matured for a time.

Therefore, ikizukuri sashimi that is still twitching usually won’t have the taste or depth of umami. However, the firm texture of sashimi is also an undeniable enjoyable aspect. It is not all about the umami.

*Glutamic acid, Inosinic acid and Guanylic acid are representative components of umami. Guanylic acid is found in kelp and vegetables (tomato, Chinese cabbage, green tea, etc.) as well as Parmesan cheese, inosinic acid is found in fish (bonito, macheral, sea bream, etc.) and meat (pork, chicken, etc.) while Guanylic acid is abundant in mushrooms (especially dried shiitake mushrooms).


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Revision date: July 17, 2017

Why are the prices at fancier sushi restaurants and Conveyor belt sushi so different?

In some cases a two-piece sushi dish you can get for JPY 100 at Kaiten-zushi (Conver belt sushi) can cost up to 2,000 for half the volume at a fancy restaurant. Many Kaiten-zushi establishments are part of large chains so costs are kept low by buying in bulk.

Also, unlike the fancy restaurants, which procure the best seasonal catch from fishing grounds all over the country, Kaiten-zushi uses a combination of frozen and farmed fish as well as substituting some fish for certain toppings.

For example, Engawa is often thought to be from Japanese flounder, but Pacific halibut or Greenland halibut is used instead as a substitute.

While different from fancy sushi restaurants that serve various seasonal fish and edomaeshigoto, Kaiten-zushi has its own merits and offers sushi at a much lower price. It’s really up to the customer what they hope to get from their sushi experience.

 


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Revision date: July 3, 2017