16.1　What Are Hikarimono?

Hikarimono are blue-backed fish and small fish that are made into nigiri with their shiny silver skin left on. The group includes kohada, mackerel, horse mackerel, sayori, kisu, Pacific saury, and sardine. It also includes shinko (baby kohada), the young of the kohada.

The skin shines because of a layer between the skin and the flesh, called the silver skin layer (gin-kawa). The silver skin layer is made of tiny crystals of a substance called guanine, stacked on top of each other. This layer reflects light like a mirror. Keeping the silver skin layer intact is the key to how hikarimono look.

Most hikarimono spoil very quickly. There are four reasons. First, their fat oxidizes easily when it touches air, and it soon gives off a smell like old oil. Second, a fishy smell grows stronger over time. Third, the enzymes that break down the flesh are very active, so the flesh softens quickly. Fourth, in fish such as mackerel and sardine, histamine forms if temperature control fails.

Histamine is a substance that forms when bacteria change a component found in large amounts in the flesh of blue-backed fish. Eating it can cause a flushed face, headaches, and hives. Once it forms, it is not destroyed by cooking, by soaking in vinegar, or by freezing (Chapter 3).

The Edomae answer to this fast spoilage is the work of "curing." Salt draws water out of the flesh and firms it. Vinegar uses its acid to hold back bacteria and enzymes. It also turns the flesh white and firm, and softens the small bones. Curing is not killing bacteria. It is work that slows down spoilage.

The chemistry of the four kinds of deterioration

The fat of blue-backed fish is rich in polyunsaturated fatty acids, such as DHA and EPA. These fats are said to be good for your health. However, their molecules have many spots that bind easily with oxygen. So they react easily with oxygen in the air. Through lipid oxidation, they give off an "old oil" smell in a short time.

The bodies of saltwater fish contain an odorless component called trimethylamine oxide. After the fish dies, bacterial enzymes change it into trimethylamine. Trimethylamine is one of the main causes of "fishy" smell, and it increases as freshness drops. However, the amount of the starting component differs by species. In konoshiro (adult kohada), sardine, Pacific saury, and mackerel, it is said to be rather low. Lipid oxidation is thought to play a large part in the fishy smell of blue-backed fish.

Blue-backed fish have very active autolytic enzymes. These are enzymes that break down the fish's own flesh after death. As a result, the flesh softens quickly. Their muscle also contains a lot of an amino acid called histidine. If temperature control fails, for example by leaving the fish at room temperature, bacteria change histidine into histamine. Histamine can form even at refrigerator temperatures if the fish is kept for a long time.

In short, four kinds of deterioration happen at once in hikarimono, and they happen fast. They are fishy smell, oxidation, softening, and histamine. Salt draws out water and reduces the water that bacteria can use. Vinegar lowers the pH, which holds back bacteria and enzymes. It also firms the proteins and neutralizes the components that cause fishy smell. Both are tools for slowing down this deterioration.

16.2　Work Common to All Hikarimono

Wash the flesh in tate-jio

After filleting hikarimono, never wash the flesh in fresh water. Wash it in about 3% salt water, called tate-jio. Fresh water is much more dilute than the fluid inside the fish's cells. Because of osmotic pressure, water flows into the cells. The cells swell with water, the flesh becomes watery and falls apart, and the umami washes out.

Salt water at about 3% is close to the strength of seawater. Water does not flow into the cells the way it does with fresh water. So you can wash away surface dirt, blood, and slime without making the flesh watery. However, it is not the same strength as the fish's body fluids. The strength of body fluids is about one-third that of seawater.

On the other hand, before the flesh is exposed, wash the outer parts of the fish, such as the scales and the guts, well under running fresh water. The reason is that Vibrio parahaemolyticus, a food poisoning bacterium from the sea, is weak against fresh water. Also wash cutting boards and tools in fresh water (Chapters 3, 4, and 13).

Curing with salt: the fattier the fish, the longer

When you sprinkle salt on the flesh and let it sit, osmotic pressure draws water out, and the flesh firms up. At this stage, the fattier the fish, the harder it is for salt to get in. Fat repels water, so it acts as a wall that blocks salt and water from moving in and out. It is just like a raincoat repelling water.

So chefs change the time even for the same kind of fish, after checking how much fat it has. Fatty fish get longer times in both salt and vinegar. If you give a small, lean fish the same time, it becomes salty and tough.

Rinse off the salt with vinegar: su-arai

After salt curing, do not use fresh water to remove the surface salt either. Instead, rinse it quickly with vinegar. This is called the vinegar rinse (su-arai). If fresh water touches the flesh, the water you worked to draw out flows back into it, and it turns watery. Vinegar does not push water back in. It works to firm the flesh. Throw away the vinegar used for rinsing, and use fresh vinegar for the main soak.

Curing with vinegar

When you soak the fish in vinegar, the acid makes the proteins in the flesh set. The flesh turns cloudy white, starting from the surface. At the same time, the many small bones soften. This is why you can eat kohada without pulling out the small bones. Today's preference is for the surface to be white and firm, with raw translucency left in the center. If you turn it white all the way through, it becomes sour and dry.

Pull the skin while holding down the flesh

When you pull the thin skin off horse mackerel or mackerel, hold the flesh down and pull the skin. Pinch the edge of the skin at the head end. With the fingers of your other hand, or the back of the knife, press the flesh against the cutting board. Then pull only the skin toward the tail. The silver skin layer lies between the skin and the flesh. If the flesh moves, the silver skin layer sticks to the skin and comes off with it. If you hold the flesh still and move only the skin, the silver skin layer stays on the flesh and leaves a beautiful silver pattern.

Cured fish is still raw

Even after vinegar curing, the fish is still raw food. Wipe the cured fish dry, wrap it in paper, and store it at 0 to 4°C. Set a time limit in your restaurant and keep to it. The idea that "it's cured in vinegar, so room temperature is fine" is wrong.

A closer look at the logic of salt and vinegar

Here is how to tell that the salt has gone in. The salt has dissolved and starts to drip. The surface of the flesh has firmed up. When you press it with a finger, it springs back. In a fatty fish, the fat repels the water, so the salt stays undissolved on the surface. This is also a sign that it needs a longer time.

The cloudy white color caused by vinegar is called turning opaque white (hakudaku). It is the same kind of change as flesh turning white when heated, and it moves from the surface inward. You can check how far it has gone by cutting the flesh and looking at the cut surface.

The small bones soften because the acid dissolves calcium out of the bones. Also, trimethylamine, which causes fishy smell, is an alkaline component. The acid in vinegar neutralizes it and turns it into a salt that does not evaporate easily. This is the chemistry behind the saying that "vinegar removes the fishy smell."

The time limit for storage depends on each restaurant's style, but there are general guidelines. Kohada is used from the next day through day 3. Shime-saba is used from the same day to the next day, or within 3 days at most. Leaving vinegar-cured fish at room temperature is the most common cause of accidents at restaurants in hot countries outside Japan.

16.3　Kohada

Kohada is a young konoshiro, a fish also known as gizzard shad. It is often called the symbol of Edomae sushi, and its name changes with its size. The smallest is called shinko. Next comes kohada, then nakazumi (medium kohada), and the large ones are called konoshiro. Shinko appear in summer, and the hashiri (first of the season) fetch astonishingly high prices. Kohada with firm flesh and good fat come in autumn and winter.

The preparation goes like this. Remove the scales, cut off the head, and take out the guts. While washing in tate-jio, cut in from the belly side to open the fish (opening from the belly). Then shave off the backbone and the belly bones. Sprinkle with salt, rinse with vinegar, and soak in fresh vinegar. Wipe off the moisture and let it rest in the refrigerator. Right after it comes out of the vinegar, the sourness is sharp, so use it from the next day on. Decorative scoring on the skin side hides the feel of the small bones and helps the fish sit well on the shari (sushi rice).

The bigger the fish, the longer both the salt and the vinegar. As a guideline, shinko takes a few minutes to a dozen or so minutes in salt, and kohada takes 30 minutes to 1 hour. But these times are only a starting point. Judge the salt by how firm and springy the surface is. Judge the vinegar by how deep the white layer is on the cut surface.

A detailed look at kohada prep

Shinko appear from early summer through summer, around June to August. Kohada come in all year, but they are best from autumn to winter. The main sources include the Ariake Sea (Saga and Kumamoto), Lake Hamana, Mikawa Bay, and Tokyo Bay.

1. Prep work　Scrape off the scales with the tip of the knife. Cut off the head, slice the belly at an angle, and scrape out the guts. Wash off blood and dirt in tate-jio, and wipe the fish dry.
2. Open the fish　Cut in from the belly side along the backbone, and open the fish, leaving it joined by the skin along the back. Shave off the backbone, and thinly shave off the belly bones.
3. Salt　Lay the fish skin side down in a colander, and sprinkle salt on the flesh side. Use more salt for fatty fish and less for small ones. Some restaurants cure with heavy salting, coating the fish in plenty of salt. Styles differ (Chapter 13). Leave large, fatty fish for 1 hour or more.
4. Vinegar rinse　Quickly rinse the surface salt off with vinegar. Do not use fresh water.
5. Soak in vinegar　Soak in fresh vinegar. Take the fish out when the cloudy white layer on the cut surface reaches 2 to 3 mm in from the surface, with translucency left in the center.
6. Rest　Wipe off the moisture, lay the fish skin side up, wrap in paper, and refrigerate.
7. Slicing and decorative scoring　Depending on size, use 1 whole fish per piece, or half a fish per piece. For shinko, use 2 to 4 fish per piece. Make diagonal or crosshatch decorative scoring on the skin side.

How you make the nigiri depends on the size. For shinko, leave the small bones in, and use 2 to 4 fish per piece. For kohada, use half a fish or 1 whole fish per piece. For nakazumi, use half a fish, and either pull out the small bones or score through them. For konoshiro, cut angled slices and pull out the small bones. Konoshiro is also used for grilled dishes.

There are three key points for judging. First, judge how the salt has gone in by the firmness of the surface, how springy it is when pressed, and how the bones feel after you pull out the backbone. Next, judge how the vinegar has gone in by how the flesh curves when you bend it slightly, and by the depth of the cloudy white on the cut surface. Finally, eat it the next day to confirm. The numbers are a starting point, and the only real answer is in the next day's taste.

The flavor changes day by day. On the next day, the vinegar settles into the flesh. The salt, vinegar, and fat come together into one flavor, and the fish is at its best. On day 3, the vinegar aroma fades and the umami comes forward. Chefs build their restaurant's flavor around this change. Shinko season is when prep takes the most time.

16.4　Mackerel

Mackerel spoils so fast that there is a Japanese saying, "saba no ikigusare," meaning "mackerel rots even while it's alive." It also causes many histamine incidents. Chub mackerel (masaba) gets fatty in autumn and winter. Shime-saba developed as a technique for eating this fish safely and deliciously.

1. Three-piece filleting　Cut off the head and take out the guts. Wash off the blood in tate-jio, and do three-piece filleting (sanmai oroshi). Remove the guts as quickly as possible.
2. Heavy salting　Spread salt in a tray, lay the fillets on it, and cover them with salt until the flesh is hidden. This is called heavy salting. The time is 1 to 3 hours. Use a longer time for fatty fish, and a shorter time for small fish or summer mackerel.
3. Vinegar rinse　Rinse off the salt with vinegar. Do not use fresh water.
4. Soak in vinegar　Soak in fresh vinegar for about 30 minutes to 1 hour. Some restaurants soak it longer.
5. Remove the bones　Thinly shave off the belly bones, and pull out the small bones with bone tweezers.
6. Pull off the thin skin　Pinch the edge of the skin at the head end. Hold down the flesh and pull only the skin toward the tail. The clear thin skin comes off, and the silver skin pattern stays on the flesh.
7. Rest　Wrap in paper and leave in the refrigerator overnight, so the vinegar settles in.

Kinds of mackerel, and the range of shime-saba styles

Chub mackerel is in season in autumn and winter, while blue mackerel (gomasaba) is in season in summer. Atlantic mackerel from Norway is fatty, and it comes in frozen with a steady supply.

The vinegar soaking time ranges from 30 minutes to several hours, depending on the restaurant's style. Styles run from a "rare" finish, where only the surface is white and the center stays reddish, to the old style, where the fish is cured all the way through. Some restaurants add kombu to the vinegar. Some remove the belly bones and small bones before salting.

Matsumae and battera taste richer because of umami synergy. When you taste the umami of kombu, glutamic acid, together with the umami of mackerel, inosinic acid, the umami feels much stronger (Chapter 15).

16.5　Horse Mackerel, Sayori, Kisu, Pacific Saury, and Sardine

Horse mackerel

Japanese horse mackerel is in season in summer. Do three-piece filleting, wash in tate-jio, and shave off the belly bones. The hard scales near the base of the tail are called scutes (zeigo). Lay the knife almost flat and shave them off from the tail toward the head. Pull the skin from the head end, and pull out the small bones. Today, very fresh horse mackerel is often made into nigiri raw, topped with ginger and green onion. For a light cure, keep the times short: 10 to 20 minutes in salt and a few minutes up to 10 minutes in vinegar. This keeps the raw bite.

Sayori

Sayori is a slender, see-through fish that is in season in spring. In the Kanto region, the season is around February to April, and it shifts earlier or later by sea area. The black lining inside the belly causes bitterness and a fishy smell, and it also looks dark. Always scrape it off. The flesh is thin, so salt for only a few minutes. Either just dip it through the vinegar, or make it into nigiri raw.

Kisu, Pacific saury, and sardine

Kisu is in season in summer, and Pacific saury in autumn. Sardine has two seasons, the rainy season and autumn. Along with mackerel, sardine spoils especially fast, even among hikarimono. If you serve it raw as nigiri, use it all up on the same day.

Detailed work for each fish

Horse mackerel　The best are said to be "ki-aji" (yellow horse mackerel). These are strongly yellowish fish that settle and live inside a bay, and they are fatty. Tataki is also one way to prepare hikarimono. The fish is finely chopped with a knife and mixed with ginger and green onion. Cutting through the fibers of the flesh lets the components of the fishy smell mix more easily with the condiments.

Sayori　Also peel off the thin black film on the surface of the skin. Kobujime (curing between kombu sheets) is another good method, because it adds umami.

Kisu　Japanese whiting (shirogisu) has light, delicate white flesh, and it is made into nigiri with the silver skin layer on. Its umami is mild, so kobujime suits it well. It is sometimes lightly cured in vinegar too.

Pacific saury　Its peak season is September to October, and its fat can be over 20%. Very fresh fish from Hokkaido or Sanriku is skinned and made into nigiri raw, topped with ginger and green onion. Curing a very fatty fish with salt and vinegar cuts the richness of the fat. The yellow color at the tip of the mouth and the firmness of the body are signs of freshness.

Sardine　For the Japanese sardine, the "nyubai iwashi" (rainy-season sardine), caught in the rainy season (June to July), is fatty. It also has a season in autumn (September to October). Both its enzyme activity and its lipid oxidation are strong. Open it by hand with your fingers along the backbone (opening by hand), and also peel the skin by hand. If you serve it raw, top it with extra ginger. It is sometimes cured in vinegar too.

This table sums up the peak season of each fish, its salt and vinegar times, and the condiments that go with it. The basic rule is ginger and green onion for fish served raw, and wasabi for cured fish.

16.6　Condiments: Why We Top Hikarimono with Ginger and Green Onion

White-fleshed fish get citrus and salt. Hikarimono get ginger and green onion. There is a reason for each pairing, and their purposes are opposite. The condiments for white-fleshed fish are there to bring out their delicate umami (Chapter 15). The condiments for hikarimono are there to hide smells.

The heat of ginger and the aroma of green onion are strong. They make the fishy smell of blue-backed fish and the smell of oxidized fat harder to notice. This is called masking (covering up). The acid in citrus and vinegar neutralizes the components that cause fishy smell, and reduces the smell itself. That is why wasabi alone is often enough for vinegar-cured hikarimono. Horse mackerel, Pacific saury, and sardine served raw as nigiri need ginger and green onion.

Condiments are tools for hiding fishy smell, not for hiding poor freshness. Wasabi and ginger do have some power to hold back bacteria, but it is only a support. Safety is protected by managing temperature and time (Chapter 5).

The components that make condiments work

The hot component of ginger is gingerol. Heating or drying also produces a component called shogaol. In green onion, compounds in the allyl sulfide family work in the same way. Their strong aroma and heat make the smells of trimethylamine and oxidized fat harder to notice. This is an effect on how we perceive smell. On the other hand, the acid in citrus and vinegar neutralizes trimethylamine, which is alkaline, and makes it less likely to evaporate. This is a chemical effect that reduces the smell components themselves.

16.7　Hikarimono Outside Japan

Outside Japan, too, you can buy saba under the name mackerel, and iwashi under the name sardine. The problem is the temperature during distribution. If the cold chain breaks even once, mackerel and sardine become the most dangerous fish of all.

Blue-backed fish around the world, and the wisdom of curing

Salted and pickled herring in Northern Europe and anchovies in the Mediterranean use salt and acid to preserve blue-backed fish. These foods are based on the same principle as Edomae curing.

With blue-backed fish, the histamine risk can reach a stage that cannot be undone. This happens the moment the fish is not put on ice on the boat. It also happens when the fish sits for a few hours on a room-temperature stand at the market. Do not buy blue-backed fish that are not buried in ice at the market, or that have sunken eyes and a soft belly.

Mackerel frozen on board the ship has the Anisakis problem solved by freezing. Because the fish is cured in vinegar, the loss of texture from freezing is hardly noticeable. Thaw it in the refrigerator. After thawing, assume it spoils as fast as raw mackerel, and salt it that same day.