In academic terms, “fermentation” is a reaction that uses no oxygen to break down carbohydrates to obtain energy. On the other hand, the reaction that uses oxygen to break down organic matter to obtain energy is called “respiration”.
In more familiar terms, microorganisms such as lactic acid bacteria, koji-mold, and yeast break down organic compounds such as carbohydrates and proteins to produce a variety of by-products to obtain energy in their own life activities. Among these, “fermentation” is a phenomenon in which beneficial substances are produced for humans, while “putrefaction” is a phenomenon in which harmful substances are produced for humans. Food spoilage can be detected by the five senses, such as smell, appearance, and taste.
What is the difference between fermentation and maturing?
While “fermentation” and “putrefaction” are caused by microorganisms, “maturing” is a process in which the food itself is transformed by enzymes and other substances to produce something beneficial to humans. Or, “maturing” is the process of improving the flavor and quality of food by allowing it to rest under controlled temperature and humidity after fermentation is complete. Maturing is said to be beneficial to humans because it changes the texture and taste of the fish, making it tastier.
In case you are wondering, “enzyme” is mainly composed of protein, which promotes chemical reactions such as digestion, absorption, and metabolism that are necessary for all living things, including humans, animals, and plants, to survive. It is said that there are approximately 5,000 enzymes in our body, but each enzyme is a specialist that performs only one function and is largely divided into “Digestive enzymes” and “Metabolic enzymes.
Katsuobushi (鰹節) appears frequently in documents from the Muromachi period (1333-1573) and later, and was used then, as it is now, to take dashi. The name Tosa-bushi is also found in documents from the early Edo period (1603-1868), but the method of making it seems to have been to boil it down and then dry it in the sun, and it is said that the current molding method was invented around 1673~81. In theTosa Domain, which has been famous for bonito fishing since ancient times, Harimaya Sanosuke of Usa and Yamazaki Giemon of Nakahama worked to improve and popularize Tosa-bushi, and the name Tosa-bushi spread as a specialty of the domain in Edo and Osaka.
The process of making katsuobushi begins by boiling the formed bonito meat in boiling water, removing the bones and some of the skin, placing it in baskets, and then placing it in a chamber to be heated and dried over a fire made of oak, sawtooth oak, or kashiwa (oak tree). This is called Baikan (焙乾).
At this stage the product to be shipped is called Namabushi. The first Baikan is called Ichiban-bi. After that, the cracks and missing parts of the Fushi are repaired with bonito surimi. The baikan is repeated once a day for 2 to 12 times. In case you are wondering, Katsuobushi before shaving is called Fushi.
After Baikan, it is called Arabushi (荒節) or Onibushi (鬼節). After drying in the sun for a few days, the surface is scraped with a small knife and called Hadakabushi (裸節), Akamuki (赤むき), Wakabushi (若節), Shinbushi (新節), etc. After drying in the sun again, it is placed in a wooden box called Kabi-tsuke-bako for 15 to 17 days in a cool and dark place, and the surface of the Fushi is covered with blue-green mold. This initial molding is called Ichiban-kabi (一番黴). This process is repeated, and those that have been processed for Niban-kabi are called Aokarebushi (青枯れ節).
This process is usually repeated four times, and when the Yoban-kabi (四番黴) process is completed, the product is called Hongarebushi (本枯れ節). This process is very effective in reducing the fishy smell and fat content of the Fushi and improving its flavor and color.
The best Katsuobushi is the one that is well-dried, has a tortoiseshell-like color, and a clear metallic sound when tapped. When buying katsuobushi with high-fat content or oxidized fat, the surface color may be white or yellowish-brown, so care should be taken when purchasing katsuobushi.
Kezuribushi made by shaving Hongarebushi is called Katsuobushi-kezuri (鰹節削り). On the other hand, shaved Arabushi is called Katsuo-kezuri (鰹削り). Katsuobushi-kezuri has a milder fragrance than Katsuo-kezuri and is relatively light. This is because the mold softens the smoky smell of Baikan and the fishy smell of fish.
Generally, Arabushi is used mainly in Kansai, while Hongarebushi is preferred in Kanto. The reason for this goes back to the Edo period (1603-1867). At that time, Katsuobushi was transported to Edo by sea from western Japan, including Tosa, Satsuma, and Kishu. However, because mold grew during the voyage, it was dried in the sun and eaten, which added a mild aroma. Since then, mold-dried Katsuobushi has been favored in Edo.
Kezuribushi varies in thickness. Usukezuri (薄削り) is 0.1 mm or less and is used as Hana-katsuo (花かつお) for decoration, and is not suitable for making dashi. Nakakezuri (中削り) is around 0.2 mm thick and is generally used at home because it can be used to make dashi in a short period of time. Atsukezuri (厚削り) is about 0.7 mm thick and should be boiled for about 20 minutes to make dashi. Atsukezuri is rarely used by itself but is often blended with several types of Fushi for commercial use.
Kezuribushi is all about the aroma. To prevent volatilization and oxidation of the aroma, it should be sealed in a plastic bag, with the air inside pushed out, and stored in a refrigerator or freezer. Even though it is dry food, it needs to be handled in the same way as fresh food.
Shiokara (salted fish guts) and Gyosho (fish sauce) are widely produced throughout East Asia and are very similar foods in terms of their ingredients and production methods.
Gyosho is made by preserving raw seafood in salt. It is a fermented food in which the raw materials are broken down into amino acids, mainly by the action of enzymes contained in the raw materials, and the umami is intensified. There are solid and liquid products leached from it.
The production process is not much different from that of salted fish, but salted fish is intended for long-term preservation of protein sources, whereas gyosho is produced for use as a seasoning. It is originally obtained as a byproduct of salted fish, but nowadays, it is produced only for the purpose of obtaining Gyosho.
However, most of the Shoyu produced in Japan is made from cereal grains, and the amount of gyosho produced for local consumption is minimal.
Types of Gyosho
Shottsuru has long been produced in the Akita area. Sailfin sandfish (Hatahata) is the most well-known fish used, but Sardine and Pacific sand lance are also used. First, the fish’s head, entrails, and tail fins are removed, and the fish is washed in water. Then the fish is drained, and about 10 kg of fish is mixed with about 1,800 ml of rice malt and 1,800 ml of salt, packed in a wooden barrel, covered with a lid, and weighted down. After maturing in a cool, dark place for about three years, the fish is filtered and boiled to make the product.
Ishiru is made in the Okunoto region. There are other names such as Ishiri, Yoshiru, and Yoshiri. The fish used are sardine, round herring, horse-mackerel, etc. The meat is often processed into dried fish, and the surplus heads and entrails are used. About 30% salt is added to this, packed in miso barrels, covered with a sheet, and aged for six months to a year. After that, it is boiled and filtered to make the product.
Ishikawa Prefecture also produces “Ishiri,” which is easily mistaken for “Ishiru,” but it is made from the entrails of the Japanese common squid.
Nam pla (น้ำปลา)
Nam pla is an essential seasoning for Thai cuisine. In Thai, nam means liquid and pla means fish. In the traditional method, small marine fish, starting with sardine, are mixed with 30~40% salt by weight and placed in a large jar for maturing.
After about a year, a long, thin bamboo basket is inserted into the jar and the liquid that has leached out is drawn out and bottled to make the product. Inland, freshwater fish such as Carp and Loach are used. There is also a theory that the origin of Nam pla is to use freshwater fish.
Gyosho, called garum, was widely used in ancient Rome around the 1st century BC. Bluefish such as mackerel would be pickled in salt, stuffed into unglazed jars, and maturing under the sun. The garum that flowed out through a hole drilled in the bottom of the jar was then used as a seasoning. In Italy, garum production stopped around the 16th century.
Anchovy sauce, a similar product made by maturing salted round herring for six months or more, grinding it, and adding spices, is now used as a secret ingredient in spaghetti and other Italian dishes.
Nước mắm is mainly made from Round herring and Amberstripe scad in Vietnamese Gyosho, and Carp, Loach and Catfish fry are also used around the Mekong Delta. The cleaned fish is placed in a container with 10~15% salt by weight, stirred every morning, and salt is gradually added until it reaches a concentration of about 30%, depending on the progress of maturing. Those aged for one year or more are more delicious. Gyosho from Đảo Phú Quốc is considered the best.
Colatura is an Italian Gyosho. It is made from nothing but round herring and salt. The round herring is first removed from the head and entrails and placed in a barrel, alternately layered with salt. Then, a wooden lid is placed on the herring, and weights are placed on top of it for maturing.
After maturing for three to four years, a hole is drilled in the bottom of the barrel and the Colatura is slowly extracted, drop by drop, over time. The slow aging process in the barrels concentrates the flavor of the fish and produces the amber-colored Colatura.
Ending a fish’s life without stress is the key to the delicious flavor. When fish are left out in air, and die in agony it promotes rapid Adenosine triphosphate (ATP) degradation and increases in Inosinic acid. This is the worst way to kill a fish.
So what method can control the fish’s flavor or “umami”?
The delicious flavor of fish comes from the postmortem breakdown of ATP, which is the source of biological energy, and the generation of Inosinic acid, which is the component of the umami. Rigor mortis after the fish has died, progresses with the decomposition of ATP. When the oxygen supply is cut off after death, the energy required for muscles to move is no longer supplied, so the flexibility of the muscles is lost and the body starts to contract. This is postmortem rigidity. This state depends on the fish, but it is the lowest point for flavor and after that, the fish is managed at an appropriate temperature to increase the Inosinic acid.
This method has been the norm among sushi chefs who didn’t know about the mechanism that produces umami. Since long ago, the daily routine was to purchase fish that were killed first thing in the morning in the market, then increase the umami by keeping the fish in a refrigerator with a controlled temperature for 12 to 48 hours. Nowadays, the method has become a common practice among fisherman, distributors and fish handlers at the market.
Now I will explain a number of methods that maintain peak flavor.
Ikezukuri means to take a fish that’s swimming in a tank at the restaurant, and immediately making sashimi after killing it, while the meat is still super fresh. In this state, there is no Inosinic acid, but there is quite a show with movement still in the fish and this preparation method gives the best-tasting texture.
Nojime* means using a massive amount of ice to suddenly reduce the temperature, resulting in the death of the fish, either at the fishing site or at the local market. This method is used for small, cheaper mass-market fish that are caught in large amounts. Nojime starts with a reverse calculation for flavor from the day after fishing, but if the process isn’t thorough or there are any deficiencies, the fish won’t stay as fresh, so detailed care must be taken in temperature management.
Hamajime* means to cut the spinal cord at the production site, drain blood and spinal fluid, then pack in ice from the afternoon the next day and wait for the peak flavor, which will be about two days later. That’s when the seafood is shipped to make it to the consumer auction the next day. As time passes, inosinic acid is generated and the aim is to use the fish in sushi at peak flavor.
Ikejime is used so that the peak flavor will be reached during afternoon and evening business hours. The spinal cord of the live fish is cut and spinal fluid drained at the early morning market. This result is a firmness from the remaining ATP, and delicious flavor from the Inosinic acid that is generated as time passes. After some time has passed, even fish for which Ikejime is applied, can reach the same state as Hamajime if used after being refrigerated for one to two days, in order to maximize the Inosinic acid generated. Of course, sushi chefs find their own balance of firmness in meat or added flavor, and incorporate this balance for the optimum combination with their shari in each piece of sushi.
Finally, maintaining freshness by using Ikejime, has become common practice overseas, and the term “Ikejime” has also become standard among the fishing industry. There are many websites that go into further detail on Ikejime, which you can reference from the links below.
*At markets in Japan, the term “kill” is not used for living fish, instead the word “shimeru” meaning “close” or “tighten” is used. This expression is thought to have come from the sentiment of showing respect and appreciation for all living beings, not only humans.
Related Contents: Première poissonnerie ikejime à Paris
The fish used in sushi is generally salted or soaked in vinegar then matured for several days while the umami Inosinic acid component increases. This is called “Jukusei” (aging). Sushi made with toppings that have been aged in this way is 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, but the discolored parts and inedible parts must also 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 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.
Umami describes the delicious taste of savory flavor essences. For many years, people held to the belief that humans can taste only four basic flavors (sweet, sour, salty, bitter) unitil a japanese chemist named Kikunae Ikeda discovered a fifth flavor (glutamic acid) in the early 20th century.
The main umami ingredients are glutamic acid in seaweed, inosinic acid in dried bonito and meats, succinic acid in shellfish, guanylic acid in shiitake mushroom.
Many ingredients contain a wide variety of umami essences. In combination, they create a synergistic effect which produces an even more potent savory flavor.
Tuna caught in the coastal regions of the Japan Sea is notable as the best bluefin tuna in January. Iki, a small island in Kyushu area is one of the famous ports for tuna.
In February and March, the tuna auction market becomes slack due to rough weather. Just a few tunas from Nachikatsuura where is also the famous port for tuna are on the market.
In March and April, tunas become thin because their eggs need many nutrients.
In May, large tuna is seldom seen in the Tsukiji Fish Market. Even if there is, its body is really thin. “Kinkaimono” which means a shore-fish is generally considered as high-class tuna, but in this season, imported tuna is useful instead.
It is said that Pacific Bluefin tunas spawn around Japanese waters between Taiwan and Okinawa in April and May. And then, they go up to fertile, north sea along the eastern coast of Japan.
In June, “Chubo” which is young and small tuna is taken hugely off the coast of the Sea of Japan. The school of Chubo begin moving northward in this season.
In July and August, tunas can be seen occasionally but their bodies are still thin. Instead, Boston Tuna which is caught in the Atlantic Ocean and nicknamed “Jumbo” is on the market. Its fresh is softer than “Kinkaimono” and it doesn’t have medium-fatty part which “Kinkaimono” has.
In September, Boston Tunas are at their best with plenty of fat on them. The best season of Boston Tuna is limited and ends in October. But fortunately, Japanese tunas come into season.
The school of tunas split up into two groups, the one takes Pacific Ocean route and another takes the Japan Sea route and both of them move northward along the Japanese Islands. Some of them reach the Tsugaru Strait where and the season of Tuna begins from September to next January. Oma town and Toi town is famous nationwide for its catch of tuna from the Tsugaru Strait. The flavor of tuna in September is still weak but it becomes stronger in October. In November, feed of tunas such as Pacific saury or Japanese common squid with plenty of fat increase and flavor of tuna also gets stronger. In December, the peak season comes around.
A catch of tuna falls off in January and it enters the final season. The temperature of sea water gets cold and feed of tuna, squids decrease and the fishing season in this area ends.
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.”
Large fish that are caught are always kept and transported on their side with their heads facing left from the port to the market and to the restaurant where they are served. The part of the fish facing down when in this position is called “Shitami” or the “bottom body” and the part facing up is called “Uwami” or the top body. The Uwami costs more than Shitami. This is because the Shitami takes on the weight of the Uwami, reducing the freshness and possibly causing cracks in the body (cracking occurs on the edges of the muscles).
This mostly applies to Pacific bluefin tuna (tuna that is consumed without any freezing after being caught). At any rate, since this fish costs hundreds of dollars per kilogram, a full-grown fish may be worth more than a luxury sports car. Therefore, from the time they make their catch, the fishermen work quickly, which affects all aspects of the quality. Most of all, this work affects the price. A Pacific bluefin is never placed directly on the deck of the ship. If a fish weighing 100 kg or more is set directly on the hard deck, its own weight would cause injury to its surface. Naturally, any damage or injury to the fish reduces the price. Instead, each fish is laid on a soft, spongy mat to protect its skin surface. Next, the blood is drained, the spinal cord nerves are destroyed and the fish is submerged in ice water. It might be easier to understand if you imagine handling a luxury vehicle, like a Ferrari, rather than a tuna fish.
Incidentally, the idea behind keeping the “Left side up (or prioritized), right side down,” is a fixed Japanese etiquette, passed down through China from the Asuka period. Perhaps the reason fish are also served for eating and photographed with the head on the left side is due to this influence as well. However, this is only a fixed practice within Japan.
At sushi restaurants, one of the many things the chef teaches their apprentice is to, “Start using purchase fish from ‘Shitami’.” Once a sushi topping is prepared, it may be served for several. It’s hard to imagine that the Shitami would start to go bad during these several days, but it is attention to these small details that make a master sushi chef.
There is a part on the tuna belly called “Sunazuri (gizzards)” or “Zuri” . Normally “Jabara,” with the diagonal white lines is the king of tuna, but the fatty tuna is spoiled if the white lines are left in your mouth. Also, on the dorsal side there is a part that produces chutoro called wakaremi. This part is also complex with hard, white lines throughout that we want to avoid eating. Instead, the knife cuts along those lines, gently removing the fish meat from them, making “Hagashi.” If the chef is not skilled, this cut will take time and extra meat is left behind. This is delicate work, making for a delicious and satisfying experience.
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.
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.