Ike Jime Part 2 – What “closing” does to the fish?

Why Ike Jime closing is needed

Let’s look at the 3 steps of Ike-Jime closing more closely and see why we do this.

What does it actually do?

Close Fish Immediately

The instant treatment is essential. DO NOT let the fish suffer. When the fish suffers, like slowly drowning in the air, there’s a chemical reaction in the muscles. Stress is not just an emotion. Stress eats up the energy source and chemical compounds within the muscle cells which make us feel “tasty”. Stress also leaves fatigue components like lactic acid and ammonia within the meat. The recent study shows when the fish suffers to death, the chemical reactions in the meat speed up to decomposition. That also means that the tasty period for the meat comes and goes very quickly.

Some people keep the fish alive in a bucket. The intention is probably to keep the fish as fresh as possible, but this is the worst thing you can do to the fish actually. The suffering lasts longer. The meat will be tasteless, soggy, and quick to go rotten. Fishing boats have fish tanks where the fresh sea water comes in and out. This is much better than a bucket. But if you mean to eat the fish, better close it immediately.

What does it actually do?

Break the spinal cord

What this treatment does is to isolate the muscle “off-line” so that it wouldn’t know that it’s dead. This treatment slows down the breakdown process. resulting in tasty period lasting longer, giving more time for the aging process.

In several hours after fish dies, it experiences the whole body contraction, called rigor mortis. Even after death, the muscles keep looking for energy source but there’s no blood supply or no oxygen. So they start consuming the savings of energy within the cells, called ATP. ATP decrease changes pH and results in the whole body contraction. When ATP is all used up, the muscles literally die. That’s when the body is the most rigid, the state of rigor mortis. After this, the body starts loosening and the proteins start breaking down to amino acids and then eventually to decomposition.
By breaking the spinal cord, you kind of set the meat into sleep. It slows and calms down the break-down process in a way we call “aging”.

What does it actually do?

Drain the Blood

Blood is bad. Blood is where the fishy odor comes from. Blood is what incubates all the bacterias. Don’t wait. Drain completely as soon as you close the fish.
If you wait, blood will get in the meat and that’s the end. The meat will soon get smelly, look dark and go rotten.

Close first or drain first?

I have discussed this with my fellow anglers. He insisted that we should drain first while the heart is still pumping for maximum blood draining. It seemed to make sense to me. But I still have a little doubt. I did some looking-into and found a research study by Marine Net Hokkaido.

Method A: Drain blood
Method B: Ike-Jime closing
Method C: Ike-Jime first and drain

Red graph on left shows the amount of blood draining. Blue graph on right shows the amount of ATP remaining

When you just cut a gill and drain blood, you can drain the most blood but you will get the least ATP left. Apparently the stress will eat up ATP. It seems best to close first, let the suffer stop, and then drain blood.

Ike Jime makes a difference

Look at the picture of the fillets of flounders below. Left has suffered to death from being left in iced water. Right has been Ike-Jime closed. This is the picture 48 hours after closing.

It is from a research project by Hokkaido University in 2012. As you can see Ike-Jime does make a remarkable difference. Suffered fish looks dull with blood spreading in the meat. Can be smelling by now. The meat can be soggy and collapsing. Closed fish looks clean, bright, and solid.

Aging Process

Let’s look more closely at the aging process.

The proper closing, Ike-Jime, and proper storage makes the aging period longer. There’s more time before decomposition, so the meat can sit to mature and develop Umami components for rich flavor.

Umami comes from the Japanese word, a savory taste, now recognized as one of the 5 basic tastes with sweet, bitter, sour, and salty. Our tongue has receptors for L-glutamate. These amino acids in various foods are what Umami taste is based on.

Below is a chart of the aging process. It depends on what fish how fast the aging process progresses. Some are fast and some are slow. Even the same fish is different by the location, by the season, and by individuals. But proper closing maximizes each fish aging period.
Good sushi chefs senses the individual conditions and controls the aging process, and serve each fish at the right time.
The chart shows the process but the time frame is just a general idea. It really depends on various factors.

Phase 1: Right after closing
~ within 3 to 9 hours ~

The meat is definitely fresh. The meat is translucent and beautiful. But the texture is still tough, lively, sometimes too gummy and chewy. Because all the fibers and the structure of the proteins are still in a good condition. And the taste is plain, as no Umami components haven’t been produced yet.
Many people still believe that the fish is at its best right when it’s dead. But this is not very true. It’s like immature fruit.

Skipjacks and Bonitoa are exceptions.
There are exceptions like skipjacks and bonitos. They are very delicious within 2 hours after closing. Their easy-to-fade flavor is strong and the texture is soft already at this stage. Fishermen eat them sashimi on the boat all the time. After 2 hours it will go into rigor mortis and the texture is not so good during that time. You should wait another 24 hours. At the next days lunch or dinner, it will reach at its best again. Then the flavor will be gone on the following day. Bonitos are very easy to lose its flavor and go rotten. If you don’t like this fish maybe you’ve had smelly sashimi in the past. The blood is the trick. I would strongly encourage you to try again with proper closing.

The fish tank is merely for LIVE entertainment.
Many restaurants have fish tanks and serve fish almost alive. There’s a misconception that it’s the most luxury and delicious fish dining. Truth is that it is just an entertaining demonstration to cover up the fact that it’s cultivated fish. Wild fish would suffer from being alive in the tank and lose all the flavor. On the other hand, cultivated fish would stink when aged. So you would think you would better keep them alive in the tank and take advantage of “freshness”. But these days they are finding Ike-Jime works well with cultivated fish. You can’t age it for 3 or 4 days because it would still put out the odor. But Ike Jime drastically changes the quality of cultivated fish for the first couple of days. So some fish farmers have started selling Ike-Jime closed fish to the market.
Shellfish is an exception. They don’t change the quality much in the tank.

Phase 2: Toward rigor mortis
~ within 9 to 48 hours ~

When ATP is broken down, it leaves IMP, inosinic acid. This is a Umami component. At rigor mortis, ATP is all gone, meaning that the meat is full of IMP at maximum. On rigor mortis, the body gets rigid but the meat itself is not tough actually. It’s solid and elastic, but soft to chew.
This is one of the best time to eat. The taste is moderately developed with IMP and the texture is still fresh. Amberjack, halibuts, and the kind of fish that you enjoy the texture of, are at their best here.

Phase 3: Post rigor mortis
~ within 2 to 5 days ~

Once passed the rigor mortis, the meat starts loosening and IMP starts decreasing. But the decreasing pace is slow if the fish has been closed properly.
The other aging process starts taking effects here. The protein has been breaking down by its own enzyme to amino acids and peptides, all Umami components. Especially IMP and glutamic acid combined is a unmatchable Umami.
This is the tastiest period. The taste is rich, even though the texture is a bit softer. Most fish is good here. The kind of fish that is too chewy in Phase 2 would be beautiful to have at this point.

Small Mackerel family should be eaten or heated by this time. Because they are full of unsaturated fats. So good for your health but bound to go bad quickly.

Phase 4: Toward decomposition
~ within 5 days to 1 week ~

The texture has become much softer. The aging continues. Umami develops more and more. The flavor becomes full-body often with a quality of sweetness.
The surface might change to darker color due to oxidation. But the quality is the same. You just need to slice off the surface if you want to have it look good.

A big tuna over 30kg can last over 1 week. They let a 300kg tuna age up to 20 days.

Chewy meat like cods and groupers, sometimes snapper, is at its best around this time if you like rich aged flavor.


They say food is at its best just before decomposed. Do you believe that? A lot of times I find it true. But I’m talking about eating raw fish that you caught several days ago. And I’m an amateur. I’m just sharing what I and many Japanese anglers do. But you just need to be aware that you are walking on a thin line here. You need to be careful with your own responsibilities. Use your own senses. And always play safe.

You will appreciate the difference even when you cook the fish. Not as much as sashimi of course. But the closed fish smells better and tastes better. Before you might have thought that you should freeze the fish within 2 days, but now you don’t have to for a couple more days.

My sashimi preferences

Here is the list of my personal sashimi preferences, on the assumption that the fish has been properly closed and stored on my own responsibilities. Just to give you an idea what Ike Jime can do to each fish.

FishDay 1Day 2Day 3Day 4Day 5
Small TunaGoodGoodOK
Medium TunaGoodGreatGoodOK

Example With A Grouper

Look at this grouper fillet. It’s Day 6. Beautiful meat with translucent quality. Still looks good for sashimi, but it’s a cold night and we are doing fish soup.

It was caught on Wednesday. It was my first fish of that day around 8am. I think it’s Epinephelus cometae. About 40cm, 1kg.

Ike jime closing. I always do it like this. Sandwich the fish with the soles of my shoes, and put in the wire along the spine. I see the target line more clearly this way. Then it was laid in a cooler box, in the plastic bag on the ice for a whole day on the boat. When we were back in the evening, the fish body was not stiff yet, soft like live. Then I scaled, gutted, and filleted that night. Wraped it with a paper towel which sucks out the liquid but stays dry on the contact side, no plastic wrap, and put it in the fridge.

This is Day 6, Monday night. I made the base from the head and the bone, which has been frozen, along with Kombu. Cook the fillet with vegetables and tofu in the soup base. It was fabulous. The texture was still intact, a little softened but the grouper meat is pretty firm to start with anyway, it was a perfect texture. The taste was so rich. Full of Umami.

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