The science of ageing fish for sushi

One of the main misconceptions when it comes to sushi served at high end sushi establishments is that the quality of the fish is dependent on how fresh it is. In reality, only certain fishes served at these sushi establishments (sushiya/寿司屋/ 鮨屋) are prized based on their freshness whilst the majority of the fish are only served a period of time ageing in the fridge. This article will go through the scientific basis for these practices.

Before diving into this article however, an important clarification needs to be made. The ageing of fish in high end sushi restaurants is very different type from dry ageing beef, and is more akin to a kind of wet ageing. If you were to look at recipes on this site for fishes that are aged, they tend to be wrapped tightly in plastic with little exposure to air and stored in shelves. This means that moisture is trapped in the bag. In contrast, beef that is dry aged tends to be hung in the fridge with plenty of air flow to dry out the surface.

To make things more confusing, there are now people who use the same techniques of dry ageing beef to dry age fish. This modern approach is becoming more and more popular in the west. The techniques use to dry age fish are different from the wet ageing you’ll see on this blog, which is the ageing typically used in traditional sushi restaurants. More importantly, wet aged fish cannot be aged for as long as dry aged fish.

Aged tuna on display in a sushi box.

Looking at the beginnings of sushi before refrigeration, the main reason why fish was paired with vinegared rice was in order to preserve the fish, maintaining it’s safeness for consumption as it was transported. Even with the invention of refrigeration, transporting fish by plane was far too expensive and was thus mainly transported around Japan using trains. Because of this, tuna caught around Japan could be up to 2 to 3 weeks old before arriving in Tokyo. This is in stark contrast today, where tuna can be flash frozen whilst other fish arrive at the market the same day they are caught.

Back then, various techniques were used to preserve the quality of the fish such as marinating in salt or vinegar, or wrapping the fish in kombu. As the technology for keeping fish fresh advanced, the amount of salt or vinegar used formerly to preserve the fish were no longer needed and were then added in significantly smaller quantities for flavour purposes instead.

The same goes for ageing fish. Sushi chefs realised that the fresher fish that they were able to obtain due to technological and transportation advances did not have the same flavour or texture, and that it was sometimes better to age the fish themselves in the restaurant. The ageing of fish is actually only made possible due to the technique used to kill the fish known as ikejime (活け締め).The technique consists of a 2 step process, the first step being the swift insertion of a spike into the hindbrain of the fish, followed by the threading of a long thin needle through the spinal column of the fish to destroy the spinal cord. The insertion of the spike into the hindbrain instantly kills the fish and thus this method is considered to be the most humane way of dispatching fish. A special character 〆is sometimes used to denote -jime in ikejime (活け〆).

Today, the ageing of fish is still carried out in order to enhance the taste of the fish. The duration for which the fish is aged for however is dependent on the type of fish. In general, the types of fish used in sushi can be classified into these 4 categories:

  • Shiromi (しろみ/白身)- White Flesh Fish
  • Hikarimono (ひかりもの/光物)- Silver Skinned Fish
  • Nimono (にもの/煮物)- Simmered Items
  • Tuna- Which has its own category on its own.

Shiromi, or white fleshed fish, include fishes such as Flounder, Sea Bream and Snapper. These fishes are actually tasteless if eaten straight after being caught and taste their best after 3 to 5 days of ageing. This is especially true for Flounder (Hirame/ひらめ/平目), which has much more prized chewy texture due to the high amount of connective tissues which soften during maturation. The delicate taste of these fish also make them suitable by ageing using the Kombujime (昆布じめ/昆布〆) technique, where fish is kept between two pieces of kelp.

Hikarimono, or silver skinned fish, include fishes such as sardines, mackerel, sayori and gizzard shad. These fish tend to have a higher fat content with a bright silver skin. These fish are typically never aged or matured as their natural fats and oils easily go rancid. However, it is actually these fats which make them ideal candidates for ageing if done correctly (imagine dry ageing beef without any fat).

Nimono, or simmered items, include sea eel (anago), Wheel Shrimp and Mantis Shrimp. As they are cooked items, they do not benefit from ageing.

Shellfish is never aged due to the dangers of food poisoning and are always consumed fresh.

Tuna, specifically bluefin tuna used in Japan is in its own category for several reasons. Firstly, the size of a tuna is much larger compared to any other fish which lends itself well to ageing due to its smaller surface area to volume ratio. The fat content on tuna is also much higher, which also allows it to develop more complex flavours with ageing. The long distances that tuna swim also causes them to have bright blood rich flesh in order to supply the tuna with oxygen. This means the flesh of the tuna is much more similar to beef compared to fish, meaning that it’s much more robust and non-delicate texture allows it to age better. Read more about whole process of ageing tuna.

The science of ageing fish

The science behind ageing fish for up to 3 or 4 days can be found in my article on ikejime. Summarized, the key point is that as rigor mortis slowly subsides,  Adenosine Triphosphate (ATP) and Adenosine Diphosphate (ADP) start to breakdown into inosinic acid. It is this inosinic acid that improves the flavour of the fish when undergoing ageing/maturation for up to a week. This is because, inosinic acid is the building block for many kinds of salts such as dipotassium inosinate and calcium inosinate that give food it’s so called savoury umami flavour, similar to glutamic acid found in MSG.

Whilst this explains the change in flavour up when ageing fish up to a week, the body of the fish would soon run out of ATP and ADP and the production of inosinic acid would halt. Any flavour change caused by further ageing of the fish would be a result of a different process- enzymatic breakdown within the fish flesh itself. Various enzymes in the fish (proteases and amylases) start to breakdown proteins and starches into smaller amino acids and simple sugars which also increases the flavour of the tuna. Proteins are made up of long chains of amino acids. There are many kinds of different amino acids and the combinations of these amino acids joined together form the many types of proteins in our body. When broken down into singular amino acids by enzymes, the amino acid we are most interested in is glutamate. The amino acid glutamate is the same component that makes up monosodium-glutamate or MSG. Our taste buds and stomachs actually detect glutamate in the same way as MSG and thus they both have the effect of making us perceive an increase in umami taste. This means that even if the production of inosinic acid ceases after a week, the production of glutamate continues as we continue to age the fish.

As enzymes continue to breakdown the fish overtime, the texture of the fish also starts to soften, giving you the melt in your mouth texture associated with high-end sushi. This is actually one of the main differences between what is prized as sashimi in Korea and Japan. In contrast, Koreans prize the tough, chewy and springy texture of fresh fish as high quality sashimi.

Furthermore, the oxidation of fat in the fish further helps in adding complexity to the taste due to the slight increase in funky acidic taste that it contributes. As with most aged or fermented products, the point at which something can be considered to have gone bad is neither black or white but more on a spectrum. Therefore some people would prefer fish aged for a longer period of time whilst some people would prefer tuna that is not aged at all.

On the topic of moisture content and rotting

If fish is just left in the fridge without proper care, it basically starts to rot instead of age. What causes this difference?- water content. If the water content on the fish is too high, bacteria growth is promoted which then causes the fish to start to rot. The reverse is also true. A fridge is an extremely desiccating environment and if the skin dries out and desiccates too much, the flesh of the fish will be damaged and the texture will be ruined. The loss of water is bound to happen when ageing fish but a balance needs to be struck. This is because unlike beef, fishes do not have a fat cap to protect the meat during ageing.

In the restaurant, we achieved this balanced by placed fish paper in the internal cavity of the fish to soak up excess moisture whilst also wrapping the fish tightly in plastic to prevent too much moisture loss.

Moisture reduction in the internal cavity was important as too much water would facilitate bacteria growth and the fish would start to rot instead of age. This is especially so in the internal cavity where blood and guts would previously have been located.

However, we did not want to skin of the fish to completely dry out in the desiccating environment of the fridge, which is why we wrapped the fish in fish paper and then tightly in a plastic bag.

The ageing process for Isaki is a good example of how this was done.

Basic safety notes

Basic common sense goes a long way in terms of safety. Fish being aged is always wrapped in paper towel or Japanese fish paper. For simple reasons, excess water or blood equals rotting and decomposition instead of ageing. The fish should be checked and cleaned everyday, with the paper towel also being changed. The skin on fish for ageing should also not be removed as it allows for greater protection of the fish both from bacteria and desiccation.

As the fish ages, a stronger fishy smell usually starts to develop, but it should at no point start to smell rotten. Basic hygiene practices such as checking the fish and changing the paper towels will go a long way to keep the smell from going out of control.

When preparing the fish for ageing, as much blood as possible should be removed. Blood will cause a quick onset of rot because it is a liquid and is nutrient rich, the perfect environment for bacteria growth. A toothbrush and running water should be used to thoroughly clean the inside of the fish after the removal of internal organs. The goal here is to remove as much blood as possible before wrapping the fish. This is one of the crucial differences in the way fish was prepared for ageing in a sushi restaurant compared to dry aged fish in the west. When dry ageing fish, you try to use as little water as possible to clean and gut the fish as the natural slime and moisture of the fish helps prevent bad bacteria growth over long dry ageing periods. In sushi restaurants, running water is used to wash the fish thoroughly inside and outside before it is wet aged.

On the topic of removing the head of the fish, the less of the fish is touched the better it is for ageing as you will not be increasing the surface area for bacteria growth and water loss. If keeping the head on however, be sure to remove all the gills and clean the cavity well. A paper towel is usually stuffed into the cavity of the fish to absorb any gut water or blood that might leak out.

Temperature wise, the ideal temperature for ageing fish is between 1-2 ˚C as any lower might cause ice crystals to form, thus damaging the flesh of the fish, whilst any higher would cause the fish to rot. Humidity wise, you want low humidity, but not completely desiccating environment.

This article follows on into four examples:


  1. Gonna post these?
    Ageing whole Kohada lightly marinated in salt
    Ageing whole Horse Mackerel (Aji) washed with salt solution

  2. Hello, I live in Germany and most of the imported fish I observe do not use ikejime, and when the fish restaurants reach their hands, most of them are no longer fresh. Catching with ikejime technique and only stabbing its brain will chemotherapy work, and when the fish is no longer fresh at about 3 to 4 days old, will chemotherapy work?

    1. Yes almost all fish outside is never killed with the ikejime method. I don’t really understand your question, what do you mean by chemotherapy?

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