18.1　Shellfish hygiene: where "fresh means safe" does not apply

Bivalves such as Manila clams (asari), hamaguri (Japanese hard clams), oysters, and scallops live by filter feeding. They draw in seawater and strain out tiny bits of food with their gills. A single oyster can filter more than 100 L of seawater in 1 day.

If the seawater holds viruses or toxic plankton, the shellfish build them up inside their bodies. They collect most of all in the digestive gland (midgut gland). This is the organ that handles digestion. In Japanese kitchens it is called the "uro" or "wata." For this reason, shellfish are different from fish. You cannot say a shellfish is safe just because it is fresh. Safety depends on which waters it came from.

Norovirus

Norovirus is a virus that multiplies only inside the human intestine. It leaves infected people in their stool, travels through sewage to the sea, and is then taken up by bivalves. A very small dose can make people sick, said to be only about 10 to 100 virus particles. Outbreaks happen in winter, mainly from November to February. Freezing does not kill the virus, and alcohol sanitizer does not work well against it.

• If you serve shellfish cooked, heat them until the core reaches 85 to 90°C for at least 90 seconds.
• To disinfect tools and cutting boards, use sodium hypochlorite (200 ppm) (see Chapter 4).
• After handling shellfish, do double handwashing with soap (see Chapters 4 and 6).

There is one more thing you must not forget. Most norovirus food poisoning does not spread from shellfish. It spreads from the hands of the people preparing the food. A chef who has had norovirus does not touch food directly, even after the symptoms are gone, until a stool test confirms the virus is no longer present (see Chapter 3).

What "for raw consumption" and "for cooking" mean

In Japan, oysters are labeled either "for raw consumption" or "for cooking." People often think this is a difference in freshness. In fact, the difference lies in where the oysters were harvested and how they were processed.

Oysters for raw consumption are harvested in designated sea areas that meet water-quality standards. Or they are oysters that have gone through depuration in clean seawater. Depuration means letting the oysters take in clean seawater so they expel the bacteria inside them. Oysters for cooking are those harvested in all other waters. Oysters for cooking often grow in waters richer in nutrients, and they are often larger.

Put simply, the table says this. Do not let your guard down even with oysters for raw consumption, and never serve oysters for cooking raw. Shucked meat of other shellfish, such as scallop adductor muscles, also carries a "for raw consumption" or "for cooking" label. With any shellfish, a bivalve without a "for raw consumption" label must always be cooked before serving.

Vibrio parahaemolyticus and storing shellfish

In summer, seawater contains Vibrio parahaemolyticus. It is a food-poisoning bacterium that lives in the sea. It multiplies very fast, but it cannot stand fresh water. Wash the shells of whole shellfish well under running water. Rinse shucked meat quickly in fresh water, then chill it.

Store live shellfish in the shell at about 5 to 10°C, close to the temperature of seawater. If you chill them to near 0°C, as you would fish, they weaken and die. Shellfish with any of these signs are dead. The shell stays open and does not close when touched. It feels light in the hand. It smells of sulfur or ammonia. Dead shellfish spoil very quickly, so throw them away. Do not use them even for cooking.

Standards for oysters for raw consumption

In Japan, oysters for raw consumption have compositional standards set by the national government. The bacterial count must be 50,000 or less per gram. E. coli must be 230 or less per 100 g, as a most probable number. The most probable number is a bacterial count estimated by a statistical method. Vibrio parahaemolyticus must be 100 or less per gram, as a most probable number. On top of this, there is a storage standard: the oysters must be kept at 10°C or below.

The key point is that these compositional standards cover bacteria only. Norovirus is not included. So even oysters for raw consumption still carry a norovirus risk. A restaurant that serves raw oysters in winter should be ready to explain this to its guests.

How shellfish toxins work and how they are regulated

Shellfish toxins build up in bivalves that feed on toxic plankton such as dinoflagellates. There are two main types.

• Paralytic shellfish toxin (PSP)　This includes toxins such as saxitoxin. It starts with numbness of the lips, hands, and feet. In severe cases, breathing stops and the person dies.
• Diarrhetic shellfish toxin (DSP)　This includes toxins such as okadaic acid. It causes severe diarrhea and vomiting.

Heat does not destroy either type. In Japan, each prefecture measures the amount of toxin in shellfish area by area. The regulatory limit for paralytic toxin is 4 MU (mouse units) per gram. For diarrhetic toxin, it is 0.16 mg per kilogram, measured as okadaic acid. When levels go over these limits, shipments stop. Shellfish sold at market are safe because of this monitoring.

In scallops, the toxin gathers in the digestive gland and does not easily build up in the adductor muscle. So even during a shipping ban, some products may still be shipped. These are limited to products such as adductor muscles with the digestive gland removed, prepared at approved processing plants. This is why people do not eat the scallop "uro." It is to avoid these toxins and also the cadmium that collects in the same place.

The nature of Vibrio parahaemolyticus, and storage by type of shellfish

Vibrio parahaemolyticus multiplies in the sea in summer, when the water goes above 20°C. Under good conditions, its numbers double in about 10 minutes. At 10°C or below, its growth slows sharply. Fish fillets are washed in tate-jio, a 3% brine (see Chapters 13 and 16). Shucked shellfish, however, are rinsed in fresh water. This is because the weakness of Vibrio parahaemolyticus against fresh water takes priority. Still, keep the rinse short so the meat does not become watery.

To sum up the table, most shellfish keep at 5 to 10°C, and scallops a little colder, at about 5°C. All of them dislike drying out and fresh water. You can also spot dead shellfish by the dull sound they make when you tap two shells together. Dead bivalves spoil quickly from their own enzymes and from bacteria.

18.2　Akagai: slap it just before shaping nigiri

To prepare akagai, remove the meat and separate it from the himo (mantle edge). The himo is the frilly part around the meat. Open the meat, remove the black liver and guts, and rub it with salt to remove the slime. Make shallow crosshatch cuts on the surface. Then, just before you shape the nigiri, slap it hard against the cutting board.

When you slap it, the stimulated muscle contracts in an instant. The meat curls up and takes on a crunchy bite and a three-dimensional shape. This is the reaction of muscle that is still alive. Meat that has been sitting for a while does not curl, even when slapped. Calling this "rigor mortis" is a mistake. Rigor mortis is a different event, in which the muscle slowly stiffens over time after death (see Chapter 10).

Origin and season, shucking and preparation

Akagai from Yuriage in Miyagi Prefecture is considered the finest. There are also producing areas in the Seto Inland Sea and Kyushu, but most akagai on the market comes from China and South Korea. Its peak season (shun) is winter to spring. Choose akagai with thick, deep orange meat and a thick himo. In most shellfish, the blood pigment is hemocyanin, which contains copper. Akagai has hemoglobin instead, so its blood is red and it has a distinctive flavor.

1. Open the shell　Insert a shell knife into the seam of the shell, on the side opposite the hinge. Run it along the inside of the top shell to cut the adductor muscles. Then cut the muscles on the bottom shell too, and remove the meat.
2. Separate the meat and himo　Cut the himo away from the meat (the foot). Cut the adductor muscles and liver off the himo.
3. Remove the liver　Open the meat halfway through its thickness. Scrape out the black liver and guts with the tip of the blade. The liver holds sand and bitterness, and if you leave it in, the meat spoils faster.
4. Salt-rub and wash　Sprinkle the meat and himo with salt and rub them to remove the slime and blood. Rinse quickly under running water and pat dry.
5. Decorative scoring　Make shallow crosshatch cuts on the surface of the opened meat. This makes it easier to bite through and helps the soy sauce cling.
6. Slap　Just before you shape the nigiri, slap the meat hard against the cutting board 2 or 3 times.

18.3　Scallops: slice horizontally

The part of the scallop used as a sushi topping is the adductor muscle. This is the muscle that closes the top and bottom shells. Because the adductor muscle connects shell to shell, its fibers run the same way, from shell to shell. In other words, they run vertically. So you cut through it horizontally, across the fibers.

More cut surface lets the amino acids that give sweetness, such as glycine and alanine, touch the tongue more easily. That is why the scallop tastes rich and sweet. If you slice it vertically, you cut along the fibers. The meat is then hard to bite through, and the sweetness is weaker. Figure 18-1 shows the horizontal cut and the vertical cut side by side.

The source of sweetness, and preparing scallops

The flavor of scallops comes from the sweet amino acids glycine and alanine, and from umami substances such as glutamic acid and AMP. Scallops also contain a lot of glycogen, but glycogen itself has no taste. It is thought to add body and texture to the meat. Saying that "glycogen is sweet" is a mistake.

Open adductor muscles thicker than 2 cm into 2 or 3 pieces, and make shallow cuts on the surface. For live scallops in the shell, place the flat shell on top and insert a spatula. Run it along the inside of the top shell to free the adductor muscle. Serve shucked meat raw only if it carries a "for raw consumption" label. Thaw frozen adductor muscles (tamarei) overnight in the refrigerator, and wipe off the liquid that seeps out (the drip).

Rub the himo with salt to remove the slime, then blanch it briefly or sear it. Searing triggers the Maillard reaction between the sugars and amino acids on the surface, which makes it fragrant. In this reaction, heat makes sugars and amino acids combine, creating brown color and aroma. If you cook it all the way through, the sweetness disappears, so sear only the surface.

18.4　Abalone: it all depends on how you apply heat

Raw abalone has a firm, crunchy texture. Cut it into thin angled slices and make fine cuts in them.

For simmered abalone, keep the liquid at 70 to 80°C without letting it boil, and simmer for 3 to 5 hours. The firmness comes from a tough protein called collagen. At a low temperature, over a long time, it turns into gelatin, and the abalone becomes tender. If you let it boil, the meat shrinks and turns hard.

Many restaurants add daikon radish to the pot. People have long said that "the enzymes in daikon make it tender." But the main enzyme in daikon breaks down starch. It does not act on protein. It is also easily destroyed by heat. There is no evidence for this explanation. The main reasons abalone becomes tender are temperature and time.

Types of abalone, their season, and handling the liver

Kuro-awabi (black abalone) has firm meat and a strong aroma, and it is considered the finest for eating raw. Megai-awabi has soft meat and suits steaming and simmering. Madaka-awabi is the largest species and is rare. Ezo-awabi is the northern form of kuro-awabi and is found mostly in the Tohoku region and Hokkaido. Tokobushi looks very similar, but it is a different, smaller species. The breathing holes in the shell number 4 to 5 on abalone, and more, 6 to 8, on tokobushi. Abalone is in peak season in summer before spawning, from about June to September. In many areas, fishing is banned from autumn through winter.

To remove abalone from the shell, insert a spatula between the meat and the shell, starting from the thin side of the shell. Run it along the inside of the shell to free the adductor muscle. Lift the meat out, taking care not to break the liver. Cut off the liver, and cut away the mouth at the tip of the meat. Coat the meat with salt, rub it, and scrub off the black slime with a scrub brush. As with salt-rubbing octopus, this removes the slime through the osmotic pressure of the salt and the force of scrubbing (see Chapter 17).

The liver is made up of the digestive gland and the gonads that wrap around it. Depending on the color of the gonads, it looks green in females and cream-colored in males. The liver is cooked and made into liver soy sauce or a sauce. However, from February to May, pyropheophorbide a can build up in the digestive gland. This substance comes from the seaweed the abalone eats. If a person eats it and is then exposed to light, it causes skin inflammation. Do not serve large amounts of liver in spring.

Preparing simmered abalone, and how it becomes tender

The simmering liquid is sake and water, lightly seasoned with soy sauce. Add rounds of daikon radish, keep the liquid at 70 to 80°C, and heat until a skewer slides into the center with no resistance. Collagen starts to take up water and turn into gelatin from around 65°C, but the change is slow. With a low temperature over a long time, the muscle proteins do not shrink too much. This avoids the same shrinking that happens when shrimp is overcooked (see Chapter 17), so the abalone becomes tender while keeping its moisture. If you boil it for a short time, the outside shrinks and turns hard while the inside stays raw.

When the abalone is done, let it cool in the simmering liquid. Refrigerate it in the liquid at 4°C or below, and use it within 2 to 3 days. When serving, add tsume (sweet reduced sauce), a liver sauce, or salt.

18.5　Other shellfish, and the toxin in tsubu

Hamaguri meat shrinks suddenly and becomes hard above 60°C. So you do not simmer it for long. Take the clams out as soon as they open, then soak the meat in the cooled liquid to let the flavor in. You will learn the steps of this tsukekomi (marinating in cooled liquid) in Chapter 19.

Among shellfish, the one that needs special care is the sea snail called tsubu. Tsubu are whelks related to the Neptunea group. Inside the meat, they have yellowish-white salivary glands.

The toxins in tsubu and baigai, and key points for the main shellfish

Tetramine is a toxin that acts on the nerves. Besides dizziness, it causes headaches and symptoms like seasickness. The symptoms pass within a few hours, but there is a risk of falling, or of having an accident while driving. The salivary glands come in one pair. Open the meat lengthwise, pinch the glands with your fingers, and remove them completely. In baigai (Japanese ivory shell), pufferfish toxin has been found in the digestive gland in some cases, so remove the internal organs.

From this table, there are three things to remember in particular. Asari are almost all for cooking and are not served raw. With tsubu, always remove the salivary glands. The black skin of torigai rubs off easily, so handle it gently.

18.6　Handling shellfish: choosing, shucking, and slicing

The freshness of shellfish drops sharply once they are shucked. So the basic rule of handling is "let the shell protect them until you shuck them."

Four checks for choosing shellfish

When judging the quality of live shellfish (mekiki), look at four things: how strongly they close, their weight, their smell, and the gloss of the shell. A good bivalve closes quickly and tightly when you touch its shell. For sea snails, choose ones whose lid pulls back deep inside, or that move when touched. Shellfish that feel heavy are holding water in their bodies. Light ones are weak. The only good smell is the smell of the sea. If there is any smell of ammonia or sulfur, they are dead. A moist, glossy shell means the shellfish is fresh. A dry, whitish shell means time has passed.

The basics of shucking and slicing

The key to shucking is knowing where the adductor muscles are. Scallops and oysters have 1 adductor muscle. Akagai, asari, hamaguri, hokkigai, torigai, and mirugai have 2. If you run the blade along the inside of the shell and cut only the adductor muscles, the meat is not damaged. The tool differs for each shellfish. Use a slim shell knife for akagai, a short, thick oyster knife for oysters, and a wide spatula for scallops and abalone.

For kiritsuke (slicing the topping), cut firm shellfish into thin angled slices. These include abalone, mirugai, hokkigai, and tsubu. Make shallow crosshatch or fine cuts so they are easier to bite through. Akagai is slapped, but torigai is not, because its black skin would come off. Hamaguri and torigai are opened with the knife to give a larger surface, and shaped into nigiri so that they wrap around the shari (sushi rice).

Blanch torigai and mirugai all at once before service. Chill them to the core in ice water, drain well, and refrigerate them between dry cloths. Store live shellfish in the shell at 5 to 10°C, and here too, shuck them for each order.