Month: July 2023

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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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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We hope this information will be helpful.

Revision date: July 24, 2023

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