What is ikejime and is it a humane way of preparing sushi? The traditional Japanese method of clo

One of the techniques that creates viable conditions for ageing is known as ikejime. The technique, which means “closing of the fish” in Japanese, was first developed some two centuries ago in Edo, now Tokyo, and is designed to limit the waves of stress a fish experiences after being removed from the water.

“The moment you take fish out of the water,” Tsui says, “a number of things happen for that fish. First, they asphyxiate. They’ve also never experienced gravity out of water before, so they’re feeling their bodyweight for the first time. They have also never been exposed to unfiltered light, so they’re effectively blinded. There is absolutely going to be some stress and hyperactivity in response, and that stress is measurable in a lab.”

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These death conditions produce a chain reaction of responses in the fish that the ikejime method is designed to mitigate. Cortisol and adrenaline levels rise dramatically, as do breathing and heart rates as the fish tries to desperately move oxygen that it cannot acquire outside of water. Core temperature increases as a result. This hyperactivity leads to tremendous lactic acid build up in muscle tissue, which is already delicate to start with.

“When a fish is caught and left on the deck to passively suffocate to death, what you get is a fish that is hot, acidic, and full of blood – which is the perfect breeding ground for rapid bacterial spoilage,” adds Tsui.

The three steps of ikejime seem gruesome at first, but they go a long way to helping a fish experience as little stress as possible during handling, ensuring it does not effectively become a biohazard, and can be prepared in a way that creates the flavours we know and love.

“Treatment devoid of unnecessary stress and pain is a prerequisite to quality,” Tsui said.

The ikejeme method explained

(as demonstrated by Andrew Tsui – and not for the faint-hearted)

The first step is a spike to the head that renders the fish immediately brain-dead so it does not even experience the stress associated with being out of water.

The second step, called exsanguination, makes use of the still-beating heart to push out the nutrient-rich blood that could become a breeding ground for bacteria were it to seep through the capillaries and into the muscle tissue.

The third step, perhaps the most gruesome of all, uses a shinkei wire to rupture the spinal cord of the fish to prevent any remaining signalling of death to the muscle tissue, as well as delay the onset of rigor mortis, which happens when muscles exhaust all their cellular energy and become inedible.

“Following rigor mortis, the muscles will only relax again during autolysis,” Tsui explains, “where enzymes start to break down the muscle tissue. Autolysis creates new amino acids such as glutamic acid in the process.” Glutamic acid is one of the key components of umami and is present in the fatty tuna we all know and enjoy.

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Is ikejime ethical?

Despite the science backing the traditional method that has put Japan at the forefront of pescatarian cuisine, adoption of ikejime elsewhere in the world faces obstacles.

“There are detractors,” Tsui says, “who feel that ike jime amounts to ‘doing surgery’ on a boat, but at the same time, animal rights activists who view the practice as cruel. It’s a tough vantage point to advocate from, for sure.”

But in both Hong Kong and Japan, belief in ikejime is high and at the restaurant level, masters of the craft, such as chef Kin from Sushi Kumogaku, are able to select the ideal handling method for each kind of fish.

“Most fish are handled using ikejime, but some ‘shiny’ fish [such as mackerel or saury] benefit from the koorijime [flash freezing] method. I also prefer asajime, which keeps the fish alive in a tank after fishing, processing it only when the fish needs to be used,” Kin says.

Other method of ageing fish

Asajime is similar to Hong Kong’s method of handling local fish, which is to keep them in a tank until they are bought or required for steaming in a restaurant.

Tsui explains that local varieties with marbled flesh, like grouper, are better steamed when fresh rather than aged via the ikejime method. This is because the moisture content in a grouper is essential for a light texture after steaming, whereas ageing for glutamic acid dries the fish out.

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“Aging fish intensifies flavour. Cooking fish also intensifies flavour because it drives water out,” Tsui explains. “Using the iconic steamed grouper as an example then, aging that fish before steaming would certainly result in an enhanced flavour, but the mouthfeel might suck because it’s lost too much water by that point. Aging therefore creates a culinary tradeoff, which sometimes ends up in no cooking at all.”

According to Tsui, ikejime’s value ultimately lies in bypassing the natural consequences of stress, suffocation and death. “In the US we have absolutely incredible freshwater species, but [lacking knowledge of ikejime] we can’t get them into a position where they can travel the distances that Japanese fish tend to travel,” Tsui says.

“Fine dining also tends to prefer imports because local fish are not at the quality standards that are achievable by buying overseas. So these restaurants lose the chance to tell the story of American seafood.”

For end users like Kin, who mainly works with fish imported from Japan, quality and time are of the utmost importance. “Ikejime is used in Hong Kong markets, but the difference is whether the finishing quality is high or low,” Kin says.

“The factors that determine the level of the sashimi are weather, origin, processing method and maturation time, which all have unique effects on fish texture and flavour.”

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