Why Does a Fish Taste Better If You Kill It Right Away?
― The More It Thrashes, the More Its Muscle "Fuel" Runs Out
When anglers and fishers land a fish, they quickly spike its head and drain its blood. It looks cruel, but the effort is science in the service of taste. A fish's muscle keeps carrying out chemical reactions for hours after death. Whether the fish thrashes or not changes where those reactions start.
On an autumn breakwater, horse mackerel and mackerel are coming in one after another. The veteran next to you drops each catch straight into a bucket of ice and seawater. For the big ones, he pushes a thin tool into the spot between the eyes and slips a blade in at the base of the gills.
Meanwhile, someone else leaves the fish in a bucket and watches them leap around. At home, the sashimi from those fish turns out mushy and fishy. The veteran's fish has translucent flesh with a firm bite.
Same day, same sea. What made the difference was the few minutes after the fish left the water.
There are two reasons
Muscle stays soft only while a certain fuel lasts. Thrashing burns through it all at once, so the flesh stiffens early and starts to spoil early.
After death, the leftover fuel slowly changes into umami compounds. The more fuel you leave at the start, the longer the umami lasts.
So "ikejime" (killing the fish quickly) means stopping its life fast so its muscle fuel isn't wasted. Let's go through it step by step.
When the muscle's fuel runs out, the flesh stiffens
Muscle shortens when two kinds of thin filaments slide past each other. Pulling the filaments apart again also takes fuel. This fuel is a substance inside cells that works like an energy currency.
While a fish is alive, this fuel is constantly made anew. When life stops, the supply eventually stops too. Even so, a stock of fuel stays in the muscle for a while. As long as the stock lasts, the filaments come apart and the flesh stays soft.
Once the stock runs out, the filaments stay stuck together. This is rigor mortis. A fish that thrashes after being landed can use up its stock in minutes. Rigor then starts early, ends early, and the flesh begins to loosen.
A thrashing muscle also piles up the acid that builds when you tire. When the flesh tips toward the acidic side, the proteins inside are thought to hold water less well. This is believed to be why sashimi can crack or turn watery.
Look at Figure 1. Even for the same kind of fish, the fuel left over is very different in a fish killed quickly and one that thrashed.
Over time, the fuel turns into "umami"
The leftover fuel doesn't simply vanish. Inside the muscle after death, it changes form bit by bit. One product along the way is inosinic acid, the same savory compound found in dried bonito flakes.
But the change doesn't stop there. Inosinic acid is also slowly broken down, and in the end it becomes bitter-tasting substances. Umami is, so to speak, a "waypoint" on the road from fuel to bitterness.
Try moving the slider in Figure 2. As you advance the time since the fish was killed, the fuel band shrinks, the umami band grows, and eventually the bitter band at the right end builds up.
Two things follow from this. First, a freshly killed fish has not yet built up much umami. This is said to be why some restaurants let prized white fish rest for half a day to a day after killing before serving it.
Second, if the starting fuel is low, the umami peak tends to be small too. A fish that thrashed has already diverted part of its fuel into other substances while alive, and its rigor and spoilage move faster. Ikejime is meant to make this "clock" tick as slowly as possible.
First, spike the spot between the eyes to stop the brain (brain spiking). Next, cut the base of the gills and bleed the fish in seawater (bleeding). Blood is thought to cause fishy smells and discolored flesh. There is also "nerve spiking," in which a thin wire is threaded through the nerve inside the backbone. Even after death, signals from the nerves can make muscle twitch and burn fuel, and the wire stops that.
Spiking small fish like horse mackerel or sardines one by one is a lot of work. So they are dropped into cold water of ice and seawater, which stops their movement all at once. Cooling also slows the chemical reactions themselves, so the clock in Figure 2 ticks more slowly.
Summary
A fish's muscle keeps using fuel after death, makes umami, and eventually moves on to bitterness. If the fish thrashes, it uses up its fuel at the very start of this process, and rigor and spoilage come sooner. Killing quickly, bleeding and chilling are all practical wisdom for slowing this chemical clock.
Ikejime is the work of stopping a life,
and also of slowing the clock inside the flesh.
For more on the umami compounds themselves, see the article on MSG seasoning. For why autumn fish taste so good, see the article on the fat of Pacific saury.
- Compare whole fish side by side. Look for clear eyes and bright red gills. With time, eyes turn white and cloudy and gills turn dark.
- If you find a fish labeled "ikejime" or "nerve-spiked," compare the translucency of its flesh with an ordinary fillet.
- Eat a little of the same fish as sashimi on the day you buy it and again the next day (kept in the fridge), and compare the bite and the umami.
When eating fish raw, use fish sold for sashimi and keep to the date shown on the label.
For those who want to know more ― terms, formulas, and links to textbooksShows which level each topic belongs to, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school biology and chemistry
- High school+Advanced high school material, or textbook sidebar content
- UniversityUniversity specialist courses (fisheries chemistry, food chemistry, biochemistry), not taught in high school
- ResearchNot taught even at university as settled fact; researchers are still working on it
Junior highTerms: this phenomenon has names
- ATP (adenosine triphosphate): The substance called "fuel" in the main text. It is the energy currency used when muscle contracts and relaxes.
- Rigor mortis: The state in which ATP has run out and the muscle filaments (actin and myosin) stay bound together.
- Inosinic acid: A savory compound made partway through the breakdown of ATP. It is also the main source of umami in dried bonito.
- K value: An index that expresses fish freshness as a number. It shows, as a percentage, how far the breakdown of ATP has gone.
Junior highHigh schoolCheck with a formula: measuring freshness with the K value
The K value is calculated by measuring the amounts of substances produced as ATP breaks down. It looks at what percentage of the total is made up of the substances that have gone past umami (inosine and hypoxanthine). The unit of amount is the quantity of substance per gram of muscle (μmol/g).
| In symbols | K ＝ ( HxR ＋ Hx ) ÷ ( ATP ＋ ADP ＋ AMP ＋ IMP ＋ HxR ＋ Hx ) × 100 |
| In words | K value (%) ＝ amount of staleness components ÷ amount of all components made from ATP × 100 |
| Where the formula comes from | ATP breaks down one way, in the order ADP → AMP → IMP → HxR → Hx, and the total number of molecules stays almost the same. So the idea is to read the share of that total that has reached the last two steps as the "hand of a freshness clock" (said to have been proposed in Japan in 1959). |
| Symbol | Meaning and unit |
| ATP, ADP, AMP | The fuel and the first stages of its breakdown (μmol/g) |
| IMP | Inosinic acid. The umami compound (μmol/g) |
| HxR, Hx | Inosine and hypoxanthine. The staleness components (μmol/g) |
| K | K value. A freshness gauge (%). The smaller, the fresher |
| Total of ATP, ADP, AMP | 1.0 μmol/g |
| IMP (umami) | 6.0 μmol/g |
| HxR (inosine) | 1.0 μmol/g |
| Hx (hypoxanthine) | 0.5 μmol/g |
| Staleness components (top of the formula) | 1.0 ＋ 0.5 ＝ 1.5 |
| Fuel and umami together | 1.0 ＋ 6.0 ＝ 7.0 |
| Total (bottom of the formula) | 7.0 ＋ 1.5 ＝ 8.5 |
| Share of staleness | 1.5 ÷ 8.5 ≒ 0.176 |
| K value (%) | 0.176 × 100 ＝ 17.6 |
| Share of umami | 6.0 ÷ 8.5 ≒ 0.71 |
The K value in this example is about 18%. In practice, about 20% or less is one guideline for sashimi, and a fish just after ikejime is often said to be below 10%. Here about 70% of the components made from ATP are at the umami stage, which is close to "ready to eat." In the sketch of Figure 2, this is around the start of day two.
High schoolHigh school+How muscle stiffens, and how acid builds up
High schoolMuscle contracts when the heads of myosin pull on actin filaments. For myosin to let go of actin, one ATP must bind to it. When ATP runs out, myosin stays gripping actin and cannot move. This is what rigor mortis is.
High school+When oxygen no longer reaches the muscle, it tries to make ATP through glycolysis, which breaks down glycogen. Lactic acid builds up, and the muscle's pH (hydrogen ion exponent) falls. In a fish that has thrashed, this reaction was already well under way before death, so the pH drops easily and the flesh is said to hold water less well. Muscle also has a reserve fuel called creatine phosphate, which quickly rebuilds ATP, and this too is used up first when the fish thrashes.
UniversityFreshness loss as a sequential reaction
The flow ATP → IMP → HxR → Hx can be approximated as a sequential (consecutive) reaction in chemical kinetics. If each step is treated as first order, the amount of the intermediate, IMP, rises to a peak and then falls. Figure 2 shows this shape as a sketch. In fish, the step that breaks down IMP (IMP → HxR, by a phosphatase) is slow, so the umami is said to last relatively long. The speed of this step differs a lot between species, which is why the "ready to eat" time differs by species even at the same K value. The progress of rigor mortis is often assessed with the rigor index, measured by placing the fish body horizontally and recording how far the tail droops.
📖 Derivations and further reading: Rigor mortis (Wikipedia, in Japanese) ／ Inosinic acid (Wikipedia, in Japanese)
ResearchWhat is still not well understood
- How far does nerve spiking work? Its effect of stopping muscle twitching and slowing the loss of ATP has been reported, but how much difference it makes across species, sizes and methods has not yet been sorted out.
- Is "tastiness" decided by the K value alone? The K value only shows how far the breakdown has gone. Changes in texture, fat and smell happen separately. Evaluating eating quality as a whole is still being researched.
- How can fish stress be reduced? How stress between landing and killing affects flesh quality, and how to reduce the fish's suffering, are being studied from both fisheries and animal welfare angles.
In other words, this article too is "an explanation of what is known so far." Numbers vary a lot with species and storage temperature, so read them as trends.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: structure of animal bodies (muscles and movement) | Muscle uses fuel to contract and relax |
| High school | Biology: muscle contraction and ATP | When ATP runs out, rigor mortis sets in |
| High school+ | Biology: respiration and glycolysis | Lactic acid builds up in a thrashing fish and pH falls |
| University | Fisheries chemistry, chemical kinetics | K value, sequential reactions, rigor index |
| Research | Seafood utilization science, food science | Effects of nerve spiking, overall evaluation of taste |
| ― | Links to daily life | Handling a caught fish, choosing fish at the counter |
- Wikipedia, "Ikejime" (活け締め)
- Wikipedia, "Rigor mortis" (死後硬直)
- Wikipedia, "Inosinic acid" (イノシン酸)
- Saito, T., Arai, K., Matsuyoshi, M. (1959) A new method for estimating the freshness of fish. Bulletin of the Japanese Society of Scientific Fisheries, 24, 749–750.
- Shugo Watabe (ed.), Fundamentals of Fisheries Utilization Chemistry (渡部終五 編『水産利用化学の基礎』), Koseisha Koseikaku (恒星社厚生閣)
※This article is a general-audience science explainer. The figures given are rough guides meant to help you understand the mechanisms. K values and the time rigor mortis takes vary greatly with the species, size and storage temperature of the fish. When eating fish raw, choose fish sold for sashimi and follow the labels and the seller's instructions.