What is K value, a formula to evaluate the freshness of fish?

a photo of Fish market
A fish market is a marketplace used for marketing fish products. It can be dedicated to wholesale trade between fishermen and fish merchants, selling seafood to individual consumers, or both.

As fish and meat age, their components change, losing their deliciousness while at the same time producing dangerous components that can be harmful to the body. Freshness is determined by the amount of time that has passed since the fish was caught. By knowing the freshness, we can tell how fresh the food is, and at the same time, we can tell if it tastes good and is safe.

To swim, fish consume adenosine triphosphate (hereafter referred to as ATP) in their muscles to generate kinetic energy. While the fish is alive, ATP is resynthesized through respiration and the breakdown of glycogen in the body, even if the muscles are fatigued.

However, if they die and are no longer supplied with new ATP, the binding of actin and myosin, the proteins that make muscles stretch and contract, proceeds unilaterally. In combination with the acidification caused by the breakdown of ATP and the formation of lactic acid, the muscles remain contracted. This is the onset of rigor mortis. Finally, ATP is lost and rigor mortis is completed. If a fish dies while its body is exhausted from strenuous exercise, rigor begins earlier than normal. This means that ATP is consumed when fish resist violently to avoid being caught.

K value is a scientific and objective standard for judging the freshness of fish and other products.

To begin with, the postmortem flesh of fish is said to be alkaline. It becomes more acidic over time. In the process of this change, ATP is broken down into several acids as shown below, and the K value is a mathematical formula that captures some of these acids and calculates them.

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

The formula is as follows:

K value (%) = (HxR+Hx)/(ATP+ADP+AMP+IMP+HxR+Hx)×100

The rate of degradation varies among fish species, but the pathway to HxR is constant. Fish that die quickly after catching have more ATP, ADP, and AMP. Then, as time passes, IMP increases and finally HxR and Hx increase. These HxR and Hx present a tasteless or bitter taste, which causes a decrease in deliciousness. In other words, a high amount of HxR and Hx is evidence of reduced freshness.

In general, fish with a K value of up to 20% can be eaten raw, i.e., as sashimi. In the case of fish, the K value is about 5% immediately after ikejime. 20-40% is considered to be good freshness. Sushi toppings are around 40%, which is not low by any means, but it is at this value that the flavor of the ingredients is most likely to be extracted. In other words, it is a state of maturing and increased IMP. And if it is 40-60%, it is edible if cooked. In other words, it is better to eat simmered or grilled fish. If it is over 60%, it is in a state of decomposition. Naturally, it is not suitable for eating. It is also known that the rate of increase in K value is large for cod and red-fleshed fish, while the rate of increase in K value is small for white-fleshed fish such as red sea bream and flatfish.

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Revision date: July 25, 2023


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The rating of fish in the Edo period was totally different from that of today!

a photo of Rating
In the Edo period, people rated various things. It seems that there was such a rating chart.

Tuna, which is now a representative of high-class fish, was one of the lower ranks of Gezakana (ge means ‘cheap’, and zakana means ‘fish’) in the Edo period. According to an encyclopedia of the Edo period, “maguro is a very vulgar fish, and even common people who live in houses facing the main street would be ashamed to eat it. Maguro was so low-ranked fish that was said to be the food of poor people living in houses on back streets. However, due to the effects of global warming and overfishing, the number of fish called Gezakana has been decreasing. Therefore, meat is definitely cheaper than fish in supermarkets.

Next, please take a look at the ratings of fish in the Edo period as listed in Chiba University‘s study of classical cuisine.

The following fish are listed as Jyozakana (Top-grade fish)

Tai、Ankou、Amadai、Sayori、Shirauo、Suzuki、Sawara、Ishigarei、Ayu、Tara、Akagai、Kaki、Karei、Kisu、Kurumaebi

 

The following fish are listed as Chuzakana (Middle Fish)

Tako、Kochi、Hirame、Aji、Ara、Ika、Katsuo、Houbuo、Ishimochi、Ungai、Ainame、Asari、Hamaguri、Bora

 

The following fish are listed as Gezakana (Lower fish)

Buri、Kurodai、Saba、Iwashi、Fugu、Kohada、Mutsu、Okoze、Shimaaji、Maguro、Nishin、Same

 

Again, maguro is classified as Gezakana, along with today’s high-end fish such as Kurodai and Fugu, and it seems that most Jyozakana are white fish, and the oily fish, so much appreciated today, was not favored by the people of Edo. In addition, maguro was also called shibi (pronounced “shi,” which is a reference to death), and some believe that it was not popular because it was associated with death.

When large schools of maguro appeared in the seas around Edo (Tokyo) between 1340 and 1844 and were caught in large numbers, they were sold for free because they were not popular.

As a side note, a sushi restaurant made a fortune by purchasing maguro red meat at a low price, marinating it in soy sauce, and selling. This was the beginning of what is now called Zuke.

n this way, red meat was still sometimes preferred, but Toro, which tends to spoil easily, was not looked upon and was often thrown away. Even if it was eaten, it was only in Negima nabe (hot pot) and was rarely eaten raw.

It was not only in the Edo period that toro was not favored.

It was not until the development of refrigeration and freezing technology in the 1945s that otoro and chutoro became as much appreciated as they are today. It is only recently that Otoro and Chutoro have become the kings of fish. Fish grading, in fact, has a great deal to do with safety. Saba, which suddenly loses its freshness, and Iwashi, which is tender and easily loses its freshness, are both Gezakana. In other words, fish that lose their freshness easily were likely to be classified as gezakana.

Another reason was the taste of the Edo people, who preferred light fish such as Amadai, Sayori, and Suzuki to oily fish such as Saba and Buri. It is understandable that Maguro Toro, which is both fatty and perishable, is frowned upon.

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Revision date: July 24, 2023


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Here’s a Michelin sushi restaurant where photography is not allowed!

a photo of Kuromutsu nigiri sushi
Kuromutsu were disturbers a long time ago when we were fishing Splendid alfonsino (Kinmedai). And they were not really liked by Edo people who don’t really like oily taste.

In this age of social networking, there is a certain discomfort in not being able to take pictures of your food. But sushi restaurants don’t want people to write bad things about them, and most of all, other customers may feel uncomfortable about the sound of the shutter. Especially for regular customers, this behavior is not funny. On the other hand, it is human nature to want to show off on social media or record your visit to a restaurant where reservations are hard to come by. It is the policy of the restaurant, so if you cannot follow it, just don’t go there.

Therefore, we investigated Michelin sushi restaurants where photography is not allowed.

a photo of michelin-guide-tokyo-2023

Of the 30 sushi restaurants that received stars in the Michelin Guide Tokyo 2023, the following five are among those where photography is prohibited. Of these, Sawada in Ginza is the only one that has not been photographed, probably because it has been prohibited ever since it moved from Nakano to Ginza. The other four have been banned at one point or another. This means that at the remaining 25 establishments, Oyakata (Sushi master) is still allowed to take pictures, albeit reluctantly. However, the sound of the shutter is not very pleasant, so care must be taken when taking pictures.

Sushi Yoshitake

Sawada

Sushi Kanesaka

Takagaki-no-sushi

Sushi kuwano

Next up is a sushi restaurant previously awarded a star in the Schlumberger Guide Tokyo where photography is not allowed. Sushi Arai will open a new restaurant on the second floor of the same building in spring 2021, in addition to its previous location on the basement floor. Photography is not permitted at the new restaurant. Sukiyabashi Jiro will be replaced by the eldest son, and in my opinion, it will become OK (it is still NG). The reason is that the customers will be replaced by new ones.

Sukiyabashi jiro

Sushi Arai

In summary, if the restaurant offers the Omakase course, which is strongly recommended, we would like to be allowed to take pictures. The reason is that we are eating at the convenience of the restaurant, so “we are cooperating, right? Let me at least take a picture”. As one example, Sushi Saito, which has been awarded three Michelin stars for 10 consecutive years, has a section that considers photography as part of its service. On the other hand, if you eat at Okonomi, the sushi chef and the restaurant are both forced to work hard, and you are told to “Eat quickly” before taking any pictures. For your information.

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Revision date: July 22, 2023


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What are Komono-ya and Oomono-ya?

a photo of seafood wholesalers in Tsukiji
As of April 2020, the Toyosu market will have 481 seafood wholesalers.

As of April 2020, the Toyosu market will have 481 seafood wholesalers, a decrease of 55 from two years ago, when the old Tsukiji market was still in operation. More specifically, it is down about 30% from 664 in 2014. As of June 2021, the demand for seafood from restaurants and other businesses has decreased significantly due to the new coronavirus, and the business conditions are worrisome. In case you are wondering, there were about 1,600 middle wholesalers in the Tsukiji market at its peak.

Nevertheless, the reason there are so many of them and they are all doing business in their own way is because they are all specialty stores. There are many types of seafood, such as tuna, fresh fish, shrimp, shellfish, and dried fish, which require specialization. Recently, seafood wholesalers have begun to offer other specialties in order to expand the scale of their business. In the old days, however, there were “Akamono-ya” (aka means “red” and mono means “fish”) that dealt only with tai, “Jomonoshi” (jo means “good quality”) for ryotei, “Kajiki-ya” for Kajiki (Marlin), and so on, all of which were divided into many different categories. Incidentally, “ya” is added after a product to indicate the store or occupation that handles that product.

“Komono-ya” refers to restaurants that specialize in sushi items and tempura ingredients. Sushi items are often small, such as Kohada, Kisu, Anago, Akagai, and Tako. That is why it is called Komono-ya. If there are small items, there are also big ones. Oomono-ya (Oo means Big) deals in tunas. Tuna is a big fish, and Oomono-ya is the name of the seafood wholesalers because they deal in tunas. There are about 200 tuna wholesalers in Toyosu, but only a few wholesalers sell only fresh bluefin tuna caught in the waters around Japan throughout the year. Fujita-suisan (藤田水産), which deals with Sukiyabashi Jiro, Hicho (樋長), which deals with Nishiazabu-Taku, Yamayuki (やま幸), which deals with Harutake, and Ishiji (石司), which deals with Takaichi-no-sushi are the most famous. Almost all Michelin and Gault & Millau-rated sushi restaurants purchase their raw tuna from these four wholesalers.

In summary, these words should be understood only by those who are related to the Toyosu market, and there is no need for pretentious names. In fact, these words are very clear.

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Related contents:

Toyosu market Information


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Revision date: July 21, 2023


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