13.1　Washing and Tate-jio

The first fork in the road of prep work is what you wash the fish with. Fish flesh is made of small cells wrapped in membranes. When there are liquids of different strengths on the two sides of a membrane, water tries to move from the weaker side to the stronger side. This force is called osmotic pressure. It is the same effect that makes leafy greens release water and go limp when you sprinkle salt on them, and that makes a slug shrink when you pour salt on it.

What happens if you soak fish flesh in fresh water? The liquid inside the cells is strong, because it contains salts and umami. The weaker fresh water from outside flows in, and the cells swell up with water and burst. The umami flows out, and the flesh becomes watery. Fresh water is not removing dirt. It is destroying cells.

That is why you use tate-jio. It is salt water at about 3%, roughly 30 g of salt to 1 L of water. This is close to the strength of seawater, which is about 3.4% salt. People often say it is "the same strength as the fish's body fluids," but that is not correct. The body fluids of sea fish are about one-third as strong as seawater, or about 1% if expressed as table salt. 3% is a stronger liquid than that.

Because it is stronger than the body fluids, water does not push into the cells, and the flesh does not swell. If you only rinse quickly, very little water is drawn out of the flesh, and the surface just firms up slightly. Salt water also removes dirt, blood, and slime from the surface better than fresh water does. Figure 13-1 shows water flowing into the cells and swelling them in fresh water, and the cells staying intact in tate-jio.

When to use fresh water

Even so, fresh water has its place. Vibrio parahaemolyticus is a food poisoning bacterium that lives in seawater, and it is weak against fresh water (Chapter 3). This bacterium is found on the skin, scales, gills, and guts of fish. Japan's Ministry of Health, Labour and Welfare also advises washing seafood well with fresh water. Here is how to use each kind of water.

• Before the flesh is exposed　Removing scales, taking out the guts, and washing the outside. At this stage, wash well under running fresh water to remove bacteria. This is "dirty work" that brings bacteria in. So use separate cutting boards and tools for the later steps, or wash them again (Chapter 4).
• After the flesh is exposed　As a rule, do not pour fresh water on it. Wash with tate-jio or vinegar, and wipe with paper towels. If you really must use fresh water, use cold water for just an instant, and wipe right away. Never leave the flesh soaking.

An easy way to remember it: when water touches the flesh, use salt water or vinegar. Fish with soft flesh that easily gives off the smell of the dark meat, such as hikarimono (silver-skinned fish), are washed in tate-jio in particular.

13.2　Scales, Guts, and Blood

Sukibiki is a method in which you lay the knife almost flat against the skin and shave the scales off thinly, along with the very top layer of the skin. If you scrape the scales off with a tool, the force passes through the skin to the flesh and damages it. In sukibiki, the blade only slides along the surface of the skin, so no force reaches the flesh. The dirt that collects at the base of the scales comes off too, and the scales do not fly everywhere. Hold the blade almost parallel to the cutting board, and move it in small strokes from the base of the tail toward the head. If you push down, the blade cuts into the flesh. Some restaurants use a scaling tool, and practice differs by school here.

Remove the guts as soon as possible. After a fish dies and time passes, Anisakis moves from the guts into the flesh. The strong digestive enzymes in the guts also dissolve the flesh from the inside of the belly.

Blood is the main source of spoilage and odor. Along the backbone lies the blood line (chiai), which here means the kidney, and blood remains there. Cut the membrane with the tip of the knife or a bamboo brush, scrape the blood out, wash under running water, and wipe with paper towels. The flesh is not exposed yet at this stage, so fresh water is fine.

Steps of sukibiki, and why it protects the flesh

1. Get in position　Place the fish with the head to the left and the belly toward you. Set the blade against the skin at the base of the tail. Hold the blade almost parallel to the cutting board, tilted up only slightly.
2. Shave　With your left hand, stretch the skin lightly. Move the blade back and forth in small strokes as you work toward the head. The scales come up onto the blade in a band, together with a thin layer of skin.
3. Apply force sideways　Do not push down. Along the edges of the fins and the rim of the belly, stroke against the grain with your finger to check for any scales left.

A scaling tool tears the scales off. A strong force passes through the skin to the flesh, and the muscle just under the skin gets crushed. This causes the flesh to split, or to turn soft as if bruised. Also, the base of each scale is buried in the skin. Bacteria collect with the slime in the small pits around the scales. A tool removes only the scales and leaves these pits behind. Sukibiki cuts away the scales, roots and all, together with the surface of the skin. So it removes the very place where bacteria live.

Autolysis, and why blood does harm

When a fish dies, the enzymes in its guts begin to break down the flesh from the inside of the belly. This is called autolysis. If the belly meat is soft and falling apart, and the belly lining is so thin that you can see the bones through it, the guts were left in the fish for too long.

Blood is rich in nutrients, and it is neither acidic nor alkaline but in between. That makes it the best feeding ground for bacteria. The iron in blood speeds up the oxidation of fat and makes the fishy smell stronger. If you refrigerate fish with the blood line still in it, the flesh darkens from that line of blood, and the smell spreads into the flesh.

13.3　Removing Pin Bones and Skinning

In a fillet made by three-piece filleting, small bones run in a line along the dark meat. These are called pin bones. They do not grow straight down into the flesh. They slant toward the head. Grip each one with bone tweezers and pull it out at an angle toward the head, following the angle at which it grows. If you pull toward the tail or straight up, the bone turns inside the flesh as it comes out and tears the flesh around it. This is what causes the flesh to split. Press lightly on both sides of the bone with your fingertips before you pull. If a bone breaks and stays in, do not dig for it. Cut the fillet into a saku, and avoid the bone by how you slice.

When you skin with a knife, insert the blade between the skin and the flesh at the tail end, and hold the skin firmly with your left hand. Lay the knife flat on the cutting board and keep it still. Pull the skin while wiggling it from side to side. If you move the knife, the blade wobbles up and down, and it shaves the flesh or tears the skin.

Hikarimono such as horse mackerel, mackerel, kohada (gizzard shad), and Japanese halfbeak (sayori) have thin skin. So you pinch the end of the skin at the head side and peel it off by hand. Just under the skin is the silver skin layer (gin-kawa), the source of the silver shine, and you leave it on the flesh. The trick is to keep the flesh still and pull only the skin. Press the flesh onto the cutting board with your fingers. Then pull the skin slowly from head to tail, keeping it low and close along the flesh. If you pull on the flesh, or lift the skin up as you pull, the silver skin layer comes off with the skin. If the back of the peeled skin is silver, you have failed.

Outer pull and inner pull, and what the silver skin layer really is

There are two ways to skin with a knife: the outer pull and the inner pull. In the outer pull, you point the tip of the blade outward and pull the skin toward you. In the inner pull, you point the blade toward you and move it forward. The names are sometimes swapped depending on the school or the region. But the principle is the same for both: do not move the knife; pull the skin. The silver skin layer is actually a layer of crystals of a substance called guanine. These crystals reflect light and give hikarimono their silver shine.

13.4　Salt Curing

Salt curing (shio-jime) is the work of applying salt to the flesh and leaving it for a while. When you apply salt, a layer of strong salt water forms on the surface of the flesh. Then, through osmotic pressure, the water inside the cells moves out. It is the same as sprinkling salt on leafy greens. At this time, leftover blood and odor compounds also come out. The umami becomes more concentrated, and the fish keeps longer. Salt also lightly firms the proteins on the surface. That is why salt-cured flesh keeps crisp, sharp edges when cut, instead of going limp. Its effect in reducing bacteria is only a small bonus. The main point of the flavor is not saltiness. It is the umami concentrated by losing water, and the clean taste with the fishy smell removed.

There are four ways to apply salt. Sprinkled salting scatters salt evenly from high above. Heavy salting coats the flesh so thickly that you cannot see it. Tate-jio soaks the fish in salt water. Paper salting sprinkles salt over washi paper or a cloth laid on the fish. For sprinkled salting of red sea bream or olive flounder (hirame), the guideline is 15 to 30 minutes.

Fat repels salt and water. So the fattier the fish, the stronger and longer you salt it. There is one more important point. After salt curing, do not wash off the salt with fresh water. The flesh will soak back up the water you drew out. Rinse the salt off the surface quickly with the vinegar rinse described in the next section, or with tate-jio.

Drawing out free water: change the time by size and fat

What salt draws out is free water. This is water that is not bound to proteins and can move freely inside the cells. Bacteria can also use it. Along with this water, leftover blood and trimethylamine, a compound that causes the fishy smell, come out. When free water decreases, water activity (Chapter 2) goes down. Bacteria have less water to use, so the fish keeps longer. As summarized in Table 13-1, you scatter sprinkled salt from about 30 cm above, and when curing with tate-jio you may use a strength of 5 to 10%.

There is no single correct time. As a guideline, sprinkled salting of red sea bream or olive flounder takes 15 to 30 minutes, heavy salting of mackerel takes 1 to 3 hours, and kohada takes about 20 minutes to an hour and a half, depending on size (for hikarimono, see Chapter 16). The biggest factor that decides where you fall in this range is fat. Fat dissolves in neither salt nor water. So it blocks the paths through which salt water soaks in, and it also slows the loss of water. Even in the same kind of fish, fat varies by season and by individual fish. Chefs look at the thickness of the belly and how the fat shines when the skin is peeled. From this, they decide the amount of salt and the time for that day. You can tell the fish is cured when the surface has firmed up and springs back when you press it with a finger.

13.5　Vinegar Curing and Vinegar Rinsing

Soaking fish in vinegar after salt curing is called vinegar curing (su-jime). Vinegar does three jobs in the flesh.

• It turns the flesh white　The strong acid of vinegar firms the proteins on the surface, and the flesh turns white. It does with acid the same thing that heat does when it turns egg white white and firm. A raw layer remains inside. The longer you soak the fish, the thicker the white layer becomes.
• It makes it harder for bacteria to grow　It does not kill them all. It only makes it harder for them to grow.
• It softens small bones　It draws the calcium out of the bones, and softens small bones that you could not remove completely.

Before soaking, quickly wash the salt off the surface with vinegar. This is called a vinegar rinse (su-arai). If you wash with fresh water, the flesh that you drew water from soaks it right back up, and the curing is wasted. With a vinegar rinse, the salt left on the surface and the acid of the vinegar firm the surface, so the flesh does not easily soak water back up. The vinegar used for rinsing contains salt and dirt, so always keep it separate from the vinegar for the main soak. You will learn the detailed times and precautions for serving vinegar-cured hikarimono in Chapter 16.

What vinegar does inside the flesh

The acidity of table vinegar is around pH 2.5 to 3. pH is a number that shows how strong an acid is. The smaller the number, the stronger the acid. When fish muscle proteins come into contact with this strong acid, they unfold and set. This is why the flesh turns white.

Here is how vinegar makes it harder for bacteria to grow. Acetic acid, the main component of vinegar, enters bacteria while still in its molecular form. Inside the bacteria, it raises the acidity and stops their activity. This is the same principle as with vinegared rice (Chapter 6). Most food poisoning bacteria grow less easily as the pH goes down. However, the fact that growth stops at pH 4.6 or below applies to Clostridium botulinum. Some bacteria, such as Salmonella, can grow down to about pH 4. Do not rely on vinegar alone. Also control temperature and time.

Small bones soften because acetic acid draws calcium out of calcium phosphate, the main component of bone. In fish whose small bones cannot all be removed, such as kohada, Japanese halfbeak, and small horse mackerel, the bones become edible while the fish soaks. The firming of the surface in a vinegar rinse is also the work of salt and acid together. If you soak unsalted flesh in acid, the flesh actually swells.

Choosing vinegar, and timing

For vinegar curing, use mild rice vinegar as the base. When you want a sharper finish, blend in grain vinegar or spirit vinegar (Chapter 8). Chill the vinegar to around 5°C before use. The whitening then progresses gently, and the flesh is not left in the temperature danger zone. The time is mainly 10 to 30 minutes for kohada and 30 minutes to 1 hour for mackerel, adjusted up or down for size and fat. For fatty mackerel, soak longer, for the same reason as with salt. Some restaurants put a sheet of kombu in the vinegar to round off the sharp edge of the sourness. After you take the fish out of the vinegar, wrap it in paper towels and leave it in the refrigerator for several hours to overnight. The vinegar spreads evenly through the flesh, and the sourness settles.

13.6　Kobujime

Kobujime (curing between kombu sheets) is a method in which you sandwich salted white-fleshed fish between sheets of kombu and rest it in the refrigerator, so that the umami of the kombu moves into the flesh. The core of fish umami is inosinic acid (Chapter 10). The core of kombu umami is glutamic acid. When you taste these two together, the umami feels many times stronger. This is called umami synergy (Chapter 20). This effect happens not inside the flesh, but on the tongue as you eat. The milder the white fish, the clearer the effect.

Dry kombu also absorbs water from the flesh. Like salt, it firms the flesh, but more gently than salt, and it adds umami as it firms.

Before use, wipe the kombu with a cloth soaked in vinegar or sake to make it supple. If you moisten it with water, slime comes out on the surface and makes it sticky, and water gets into the flesh. As a guideline, thinly sliced flesh takes 2 to 4 hours, and a whole saku takes half a day to overnight. If you leave it too long, the flesh turns amber, becomes too firm, and the kombu aroma takes over. Once the guideline time has passed, take the fish off the kombu, wrap it in paper towels, and refrigerate it. This is work in which you rest raw fish, so manage hygiene as strictly as for aging (Chapter 10).

How strong the synergy is, and choosing kombu

Inosinic acid is a nucleotide-type umami produced when ATP breaks down. Glutamic acid is an amino-acid-type umami. The two act on the same receptor on the tongue and boost how strongly each other is tasted. The effect is said to be strongest when the two are in a ratio close to one to one, reaching 7 to 8 times. This certification exam asks about "the combination of a nucleotide type and an amino acid type." "Two amino acid types together" is wrong.

The type of kombu also changes the result. Makombu is refined and has no strong character, so it suits delicate white fish such as red sea bream and olive flounder. Rishiri kombu has a clear umami and is somewhat firm. Rausu kombu is rich in both umami and aroma, but its color comes out easily, and it tends to stain the flesh yellow. The slime that comes out when kombu is moistened with water is a substance called alginic acid. For timing, shorter suits red sea bream and olive flounder, and longer suits fish with more character, such as thread-sail filefish (kawahagi) and Japanese sea bass (suzuki).

13.7　Prep Work with Heat: Yushimo, Shimofuri, and Yakishimo

Even sushi served raw sometimes uses prep work that heats only the surface. The purpose is to firm the proteins on the surface into a thin protective film, while keeping the inside raw.

Yushimo, also called yubiki, is a method of pouring hot water over the flesh or skin, or dipping it in hot water. Muscle proteins change at around 40 to 50°C for myosin and 65 to 70°C for actin. So in water at 70 to 80°C, the surface turns white and firm in a few seconds to a few tens of seconds. The collagen in the skin shrinks with heat. With more heat, it turns into gelatin and becomes soft. Fish collagen changes at lower temperatures than that of beef or pork.

• Shirako (milt)　Dip it in water at 70 to 80°C for a few tens of seconds. The very thin membrane around the shirako firms up and becomes hard to break. The heat drives off the fishy-smelling compounds, and the inside stays creamy and raw. If the water is too hot, it firms up all the way through. If it is not hot enough, the membrane does not firm up, and the shirako falls apart.
• Red sea bream skin (matsukawa-zukuri)　Lay the fish skin side up, cover it with a cloth, pour hot water over it, and transfer it to ice water right away. The skin shrinks and ripples. The skin, which was hard and difficult to bite through, becomes soft, and you can also enjoy the rich fat just under the skin.
• Yushimo before zuke (marinating in soy sauce)　Pour hot water over the surface of a tuna saku and transfer it to ice water. Then soak it in the zuke marinade. The firmed surface limits how much soy sauce soaks in, and keeps the inside from becoming too salty (Chapter 14).

Shimofuri is a method of dipping bones, ara (fish heads and bones), or shellfish in boiling water to firm the blood and slime on the surface so you can wash them off. It is mainly done before making simmered dishes or dashi. Yakishimo is the same as aburi (searing with a flame). You apply a direct flame to the skin side or fatty parts to brown only the surface. It softens the skin, melts the fat, and adds a toasty, grilled aroma (Chapter 14).

With both yushimo and yakishimo, transfer the fish to ice water right after heating. There are three reasons. The first is to stop carryover heat. If you leave the fish, the heat on the surface keeps moving inward and cooks the inside that you want to keep raw. The second is to firm the surface and give it a clean bite. The third is hygiene. It cuts the time the surface spends in the temperature danger zone, from 10 to 60°C, down to a few seconds. When you take the fish out of the ice water, wipe off the water right away with paper towels. If you leave it in too long, water gets into the flesh, and the protective film loses its purpose.

13.8　Decorative Scoring and Hidden Scoring

Making fine cuts on the surface of a topping is called decorative scoring, and the cuts are visible. Cuts made on the underside or inside, where they cannot be seen, are called hidden scoring. Both are not just for looks. They do three things for flavor.

1. Cutting the fibers　Squid, the skin of horse mackerel and kohada, and octopus flesh are hard to bite through. If you cut across the fibers, the knife does the teeth's work in advance, and the topping gets a clean bite. This is why squid is scored in a crosshatch (Chapter 17).
2. Holding soy sauce　The cuts add surface area. On a flat surface, soy sauce runs off, but it collects in the cuts.
3. Making flavor easier to taste　Umami and sweetness dissolve out of the cuts more easily, and more surface touches the tongue. It is said that the same topping tastes stronger in umami and sweetness this way.

As a guideline, the depth is one-third to one-half of the thickness of the flesh, and the spacing is 2 to 5 mm. If the cuts are too deep, the topping falls apart when you shape the nigiri. If they are too shallow, they have no effect. Decorative scoring on the skin side of kohada also keeps the skin from shrinking, and keeps the flesh from curling up when you shape the nigiri.