On Ikejime 活け締め

What is Ikejime?

Ikejime (Ikejime/活け締め;Ikijime/活き締め) is a technique for killing fish that was developed in Japan specifically designed to preserve the quality of the fish. 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).

Find out how the Ikejime technique affects the ageing fish here.

An example of the full process can be found here, carried out on sea eel.

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What is Rigor Mortis?

In fish and in humans, energy is stored in the form of Adenosine Triphosphate (ATP) that is produced using oxygen during the process of aerobic respiration. When required, ATP is then able to bind to out muscles before breaking down into Adenosine Diphosphate (ADP) and phosphate, in the process releasing energy that is then used to power our muscle contractions. The remaining ADP molecule which is still bound to our muscles is then replaced with a new ATP molecule, which causes out muscles to relax. The process then repeats itself as muscle contraction continues.

When fish die, respiration no longer continues which means that there is no longer oxygen available to produce new ATP. This means that there is no longer a constantly supply of ATP available to replace ADP, which would result in the muscles of the fish being unable to relax. Whilst respiration would have halted completely, electrical nerve messages from the brain and spinal cord signalling for muscle contractions do not stop. As muscle contraction continues to occur, more and more ADP is not replaced, and the entire body of the fish starts to stiffen up in a process known as rigor mortis. The process of rigor mortis takes time to set in, typically 13 hours after the death of a human, and 15 to 24 hours in a fish depending on its size.

During exercise in humans, the increase breathing rate we experience during is not enough to compensate for the increase demand of oxygen in our bodies. However, our bodies are still able to generate ATP in the absence of oxygen through anaerobic respiration. This process is far less efficient and has the bi-product of lactic acid (instead of carbon dioxide during aerobic respiration). As anaerobic respiration continues, we experience a build up of lactic acid as our bodies continue to produce ATP without oxygen. This build up of lactic acid is the cause of the muscle pains we feel during exercising. In dead fish, muscle contractions continue to occur without the supply of ATP from aerobic respiration, and the dead fish is still able to produce ATP through the same process of anaerobic respiration in order to meat the ATP demand, which would result in the same build up of lactic acid, causing in a sour taste to build up in the fish.

How does the Ikejime technique affect Rigor Mortis?

During the first step when the spike is swiftly inserted into the hindbrain, the destruction of the brain would instantly halt the majority of muscle contraction signals from being sent out to the rest of the body. As the contraction signals are reduced, the following knock-on effects occur.

  • The demand for ATP reduces and the remaining supply of ATP lasts longer
  • Less muscle contractions means that the fish stiffens up slower
  • Aerobic respiration to compensate for the lack of ATP occurs later
  • The build up of lactic acid begins later

Nerve messages signalling for muscle contractions do not all originate from the brain however, but also originate from groups of neurons in the spinal cord known as Central Pattern Generators (CPGs). The signals for muscle contractions sent by CPGs are actually inhibited by the brain and thus killing the brain in the first step allows the CPGs to send signals uncontrollably. This is why inserting the spike into the hindbrain of the fish is typically followed up by the threading of a long thin needle through the spinal column of the fish, which in turn destroys the signals sent by the Central Pattern Generators for muscle contraction.

Even with these two steps, muscle contraction does not halt completely, and the fish ultimately still undergoes rigor mortis, but the time needed for the full onset of rigor mortis is significantly reduced.

How does this affect the taste of the fish?

The taste of fish in relation to the way it is slaughtered is determined by two main compounds- lactic acid and inosinic acid.

As mentioned above, if muscle contraction and anaerobic respiration continues, the buildup of lactic acid creates a sour taste in the fish. This is halted using the Ikejime technique.

The effect of inosinic acid is a little more complicated. The breakdown and degradation of ATP and ADP produces inosinic acid, which would mean that inosinic acid production would reach its peak after rigor mortis has set in. (If you’re curious about the two complicated metabolic pathways behind the production of inosinic acid, go read up on AMP-activated kinase regulation). Interestingly enough, inosinic acid in 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. Following this logic, fish tastes best not straight after being killed, but a period of time after rigor mortis.

If this was the case, wouldn’t the Ikejime technique make the fish taste worse because it prolongs the time required for rigor mortis? No, because the rate at which inosinic acid is produced also matter. When a fish is killed without the Ikejime technique, rigor mortis occurs quickly, and is soon followed by the rapid production of inosinic acid followed by rotting. If killed using the Ikejime techinque, the onset of rigor mortis takes much longer, and the production of inosinic acid occurs slowly and gradually, allowing more complex flavours to develop. A similar analogy to this would be ageing whiskey. Aged whiskey taste better due to the complex flavours resulting from the oxidation of compounds and breakdown of wood barrels over time. The same affect can be achieved by just exposing whiskey to lots of oxygen to oxidise it, but the resulting whiskey taste flat and one dimensional.

What are its implications for Dry Ageing Fish?

If you intend to dry age fish from a few days up to a few weeks, a typical fish would start rotting soon after rigor mortis has ended. If dispatched using the Ikejime technique, the prolonged onset of rigor mortis and the gradual production of inosinic acid would enable to fish the develop flavours slowly during ageing without a rapid deterioration in quality. More on fish ageing here.  

Full details of the Ikejime technique.

The hindbrain of the fish can be located just behind the eye of a fish. If the spike is inserted correctly, the shock to the brain caused by the insertion should send a electrical signal throughout the body of the fish which would cause the fish’s fins and gills to flare up momentarily.

The next step is the threading of the needle down the spinal column. This can be done two ways: through the head or through the tail. There are two way from which you can access the spinal cord through the head, one is by cutting the fish from the top of the head as you would typically behead a fish but not fully all the way through but just enough to expose the spinal cord. and then inserting the needle down the spinal cord. Alternatively , it is possible to cut though the spinal cord and insert the needle into the spinal column from beneath the gill flap if the presentation of the fish matters. This technique is rarer and harder. From the tail end, cut through the tail enough to expose the spinal column but without completely severing the tail. Insert the needle through the spinal cord. In all cases, you should pull the needle in and out of the fish multiple times to ensure the spinal cord is fully destroyed. You can actually cut off the entire tail but then you will not have the tail of the fish to grab on to when you need to de-scale it.

Next, you need to bleed the fish. Even though the brain and spinal cord have both been destroyed, the heart of the fish actually continues to beat. Using this to your advantage, you can actually drain the blood of the fish by submerging it in water and let the pumping of the heart drain the blood for you. The most effective way to do this is to bend the tail and head of the fish while keeping the body straight whilst letting it bleed (much easier said than done). After bleeding, the fish should be stored at a cold temperature (around 10ºC for a day or two).

An example of the full process can be found here, carried out on sea eel.

Whilst uncommon to see people carry out this technique outside of a fishmonger or restaurant, it has become surprisingly common in sport fishing to carry this out. Therefore if you are looking for an Ikejime spike or needle, you are most likely going to be able to find one in a fishing/hunting supply shop. Do bear in mind that the needle/wire needs to be thin enough to insert down the narrow spinal cord and thus you need to buy a small needle for a smaller fish.

Find out how the Ikejime technique affects the ageing fish here.

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