When a fish is pulled from the sea, it is put into an extraneous environment with sunlight, temperature and hot human hands that it isn’t used to, so it starts to swim violently with all its might, trying to get away, in a perpetual state of tension. If cooked in this state, blood will still be running through all the cells of the body. There is no sense of transparency in the meat at this point, especially the white meat. The body also hardens. The entire body reeks of blood. In other words, the meat of white fish is full of blood, making it unsuitable for preparing as food.

Fish are placed in a tank with enough water and oxygen, only in numbers so that they do not rub up against each other. A lid is placed on the tank to create a temporary dark space. At this time, the temperature of the water is an ideal, low temperature. This keeps the fish from struggling. After being left for half a day, it will regurgitate anything undigested that was eaten prior to being caught, become acclimated to the dark space and settle down. The bloody tone from struggling fades throughout the body and the fish relaxes.

Creating this state is called “Ikekoshi”.

Ikekoshi methods differ depending on the species of fish, but the idea is to keep this state for only one day at most. When left for two or three days, the fish loses meat, starts to swim around the dark chamber that it has now grown accustomed to, and its body, tail and fins rub up against other fish or the sides of the tank, causing damage. This hurts the meat quality and appearance so it is something the fisherman must look out for.

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Revision date: August 5, 2022

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The fish is first washed in water, then soaked in vinegar. When working with blue-backed fish, the smell of the fish’s fat is removed from the surface with vinegar that has been used once before (and thus has a low acidity). This is called Suarai. When preparing fish with Sujime, cooled Warisu is used, made either with a 10 to 3 ratio of vinegar to ice, or the same ratio of vinegar to cold water. Vinegar with a high acidity that hasn’t been cut with water only soaks the surface of the fish (denaturing it) and the vinegar does not penetrate to the inside. In addition, if the temperature of vinegar is high, such as during summer, the skin and body of the fish soften. That’s why the vinegar is diluted (acidity is between ph 3.3 and ph 3.7) and used as chilled Warisu.

How sushi chefs adjust vinegar for each type of fish

In traditional sushi preparation, vinegar is not just a preservative—it’s a seasoning tool carefully adjusted by skilled sushi chefs. The choice and dilution of vinegar used in warisu (diluted vinegar for curing) are not fixed; they vary depending on the type of fish, its condition, and even the individual preferences of the guest.

Different fish call for different vinegars. White-fleshed fish such as sea bream or flounder are delicate in flavor and low in fat. Using vinegar that’s too acidic can overpower their subtle sweetness. For these, chefs commonly use warisu made with rice vinegar, which offers gentle acidity and a clean finish that enhances the natural umami of the fish without masking it.

In contrast, blue-backed fish like mackerel or sardines are bolder in character, with strong aromas and rich oils. These require a more assertive approach. Chefs often reach for aged sake lees vinegar (akazu), which has deep complexity from long fermentation. However, because akazu can be quite sharp, it’s usually blended with rice vinegar to strike a balance—removing odor while drawing out flavor.

Fatty fish, such as toro, salmon, or yellowtail, pose a different challenge. Their richness can linger heavily on the palate, so vinegar is used to “cut” the fat and refresh the mouth. In such cases, a slightly stronger warisu—sometimes incorporating grain vinegar—provides the right amount of acidity to balance the richness and keep the dining experience light and pleasant.

But the decision doesn’t end with fish type alone. The condition of the fish also plays a vital role. Factors like the fishing method, season, and time since catch can all influence fat content and firmness. For instance, freshly caught mackerel in the morning may be mild, but by afternoon its oils begin to rise to the surface—prompting chefs to adjust the acidity accordingly. A winter yellowtail (kanburi), with its intense fattiness, might call for a bold akazu blend to bring out deeper umami.

Beyond the fish itself, experienced chefs also fine-tune vinegar based on the guest’s palate. An elderly customer might prefer a milder, smoother vinegar base, while a guest who enjoys bolder flavors or is pairing their meal with sake may appreciate a vinegar with more depth and presence. Some international guests, who may not be familiar with the distinct aroma of fermented vinegar, are served fish cured in blends that are gentler and more neutral in scent.

Ultimately, this level of customization reflects the spirit of sushi itself: it is not a standardized dish, but a conversation between chef and guest, expressed one piece at a time.

At the highest level of craftsmanship, chefs seek to create umami synergy—a phenomenon where the amino acids in vinegar (especially akazu) combine with the natural inosinate in fish to create a flavor experience greater than the sum of its parts. For example, a well-aged flounder paired with a carefully blended vinegar can produce profound depth, richness, and lingering satisfaction. In the language of food science, it’s a case of 1 + 1 = 3.

This thoughtful approach to vinegar usage is one of the quiet but essential reasons why great sushi tastes so perfectly balanced.

After the Sujime of the topping is finished, it is important to place it in the refrigerator and allow the meat to mature slowly. The unsaturated fats in oily fish are particularly prone to oxidation, which leads to unpleasant smells and off-flavors if the fish is not properly chilled and managed. For fish with strong fat, that fat oxidizes faster than the meat can mature, so it must be done very carefully.

Related contents:

TYPES OF VINEGAR

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Revision date: July 9, 2025

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Generally, saltwater fish grow in seawater, and their bodies naturally contain a certain concentration of salt. Because seawater has a higher salinity than the fish’s internal fluids, the water in their bodies is constantly being drawn out by osmotic pressure. As this water is lost, the internal salt concentration increases. To compensate, saltwater fish actively drink large amounts of seawater and excrete the excess salt through specialized cells in their gills and kidneys. This complex osmoregulation mechanism helps them maintain a balance, which is why their flesh typically contains a high percentage of water.

This high moisture content sometimes gives the impression that the fish is “somehow watery” or lacking in firmness. However, once the fish is cooked—especially through gentle heating—the texture becomes soft and fluffy, enhancing its natural qualities.

Therefore, when preparing fish for cooking, it is often sprinkled with salt approximately 30 minutes to one hour beforehand. This process, commonly practiced by sushi chefs as well, is called Shiojime (塩締め), or salting.

Shiojime serves several purposes. First, it draws out excess moisture through the process of osmosis, which in turn concentrates the fish’s natural umami (savory) flavor.

Additionally, by removing moisture, Shiojime helps eliminate the fishy odor (commonly caused by compounds like trimethylamine), allowing the clean, delicate flavor of the fish to shine through.

Another important benefit of Shiojime is that it helps control bacteria. When salt is applied to the fish, it creates a salty (hypertonic) environment that draws water out of the bacteria on the surface. Without enough water, the bacteria can’t survive or grow. In addition, salt lowers the water activity in the fish, which makes it harder for microbes to multiply. This helps keep the fish safe to eat and slows down spoilage, making it last longer.

In traditional sushi preparation, Shiojime is often followed by Sujime (vinegar curing). The amount of vinegar that penetrates the fish depends significantly on the prior conditions established during Shiojime, making it a delicate and finely tuned process that requires experience and attention to detail.

Ultimately, Shiojime is a time-honored technique that delivers numerous benefits—enhancing flavor, improving texture, reducing odors, and increasing safety—while showcasing the depth of Japanese culinary knowledge.

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Revision date: June 12, 2025

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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 is 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 deems 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 mature. 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. 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:
Technique Thursdays: Dry-Aged Fish

Everything You Need to Know About Dry-Aged Fish

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Revision date: June 2, 2020

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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 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 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

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Revision date: September 11, 2018

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Then, when the sushi chef gets the tuna, he first separates the red, lean meat and the fatty toro portion, rewraps them separately, 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.

The tuna wholesaler may also decide on the aging period independently. There is no right answer for this aging period, and it is decided based on the experience of the sushi chef.

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.

As an aside, right after the tuna cutting show, it has not yet aged enough to be delicious. The color is light, and the production of umami ingredients such as inosinic acid is not sufficient. This is a show for advertising purposes.

Related contents: TYPES OF TUNA

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Revision date: March 11, 2025

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ATP is broken down as follows, but in living fish, only the reaction up to AMP occurs. Once broken down to AMP, it is regenerated as ATP.

ATP→Adenosine diphosphate (ADP)→Adenosine monophosphate (AMP)→inosinic acid (IMP)→hypoxanthine riboside (HxR)→Hypoxanthine (Hx)

In fish after death, inosine monophosphate (IMP) is produced from AMP by an enzyme called adenosine deaminase (ADA). This is called maturing. Inosine monophosphate is not found in cells immediately after death. Maturation is required for the production of inosine monophosphate. If it goes further, it will go bad and HxR (inosine) and Hx (hypoxanthine) will be produced.

The umami created by self-digestion of ATP is “maturing (Jukusei)” and the process after that is “rotting.”

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Revision date: March 17, 2025

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Soy sauce is more than just a condiment for sushi—when used properly, it enhances the flavor of the sushi topping and brings out a perfect balance with the vinegared rice, which is already delicately seasoned. However, using it incorrectly can easily upset this harmony.

The most delicious way to eat sushi at restaurants where the sushi chef applies Nikiri shoyu (a soy sauce mixture 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 vinegared rice (shari). 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.

Related contents: SOY SAUCE FOR SUSHI

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Revision date: July 3, 2025

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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 (vinegared 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. If the sushi chef’s hands are dry, the rice grains tend to stick to them, so the rice needs to be moistened just enough.

This 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.

Also, the stickiness of rice is mainly caused by starch (especially amylopectin) gelatinizing with water and heat, but vinegar contains acetic acid, which acidifies the pH, so under these conditions, the structure of the starch changes slightly, inhibiting the formation of stickiness (the sticky component).

Additionally, vinegared rice becomes sticky if there is too much moisture, and conversely, if there is too little moisture, it becomes dry and difficult to shape. Therefore, the moisture content and temperature are carefully adjusted to achieve the perfect balance.

In short, it’s not magic or special skin — it’s the calculated use of vinegar, moisture, and temperature control that keeps sushi chefs’ hands rice-free.

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Revision date: June 16, 2025

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