9.1　The handai: the wood does the work

A handai (wooden rice tub) is a shallow, wide wooden tub for mixing cooked rice with sushi vinegar and cooling it. It is also called a hangiri, or a sushi tub. It is made from sawara cypress or hinoki cypress.

A wooden handai is not used for looks. Wood is built like a sponge, made of many thin tubes bundled together. When it is damp, it absorbs extra moisture, and when it dries, it releases moisture. This is called humidity regulation.

When you move freshly cooked rice into the handai, the wood absorbs the steam rising from the rice. It also absorbs the liquid from the sushi vinegar that the rice could not take in. The surface moisture that causes stickiness is removed, so the grains of shari stand up and take on a sheen. In a plastic or metal container, the water has nowhere to go. The rice at the bottom turns sticky.

Wet it and wipe it before use

Before use, wet the handai with water and wipe it with a tightly wrung cloth so that it is damp. If it is dry, the wood absorbs the rice's moisture and vinegar all at once, and the rice sticks to the wood. If it is damp, the wood absorbs only the extra moisture, and the vinegar goes into the rice.

Before using a new handai, fill it with equal amounts of water and vinegar and leave it for 2 to 3 hours. This draws out the bitterness and smell of the wood.

Care after use

After use, wash it with hot water and a scrubbing brush before the rice grains harden. Do not use detergent. It soaks into the wood and leaves a smell. Wipe off the water, and let the handai dry completely in a well-ventilated, shaded place, out of direct sunlight. If you put it away damp, mold will grow. Hygiene for wood is a cycle: "wet it to use it, and dry it completely after use." It is the same idea as for cutting boards (Chapter 4).

Differences in wood, and caring for mold and hoops

Sawara cypress is the most common wood for a handai. Sawara is light, resists water, and has a mild aroma, so its smell does not easily pass to the rice. Hinoki cypress is hard and durable and has a strong aroma, but it is more expensive.

When you wet the handai before use, do not make it too wet either. If the wood is already full of water, it has no room left to absorb extra moisture. Wipe it until no film of water is left on the surface. On the other hand, if you use dry wood, it robs the rice at the bottom of vinegar, and the flavor becomes uneven. The wood also absorbs too much vinegar and darkens.

There is another reason not to use detergent besides the smell. Detergent stays in the wood fibers and strips away the wood's natural oils.

If you keep the handai damp in a closed space, the wood grows mold within 1 to 2 days. Black mold roots itself inside the wood, and it cannot be removed without shaving the wood. On the other hand, if the wood stays dry for a long time, the rings that hold the tub together, called hoops, become loose. If you will not use the handai for a long time, dampen it with water once a month.

9.2　The steps of shari-giri

Shari-giri (mixing vinegar into the rice) is the work of adding sushi vinegar to cooked rice, mixing it with cutting strokes, and cooling it until it settles at body temperature. The whole process takes about 5 to 10 minutes. In this short time, you do several things at once: you let the vinegar soak in, adjust the moisture, protect the grains, and bring down the temperature.

1. Prepare　Wet and wipe the handai, and measure out the sushi vinegar. Have a rice paddle, an uchiwa (flat hand fan), and a clean cloth ready at hand.
2. Move the rice to the handai while it is hot　With the rice paddle, scoop the rested rice in large portions from the bottom of the pot, and move it into the handai without crushing it (Chapter 7). Do not mix it in the pot.
3. Spread it thin　Do not heap the rice. Spread it quickly across the whole handai. If you heap it, heat and steam are trapped in the middle and make it sticky, while only the outside dries. If you spread it thin, steam escapes evenly, and the vinegar soaks in evenly too.
4. Add the vinegar while the rice is hot　Add it while the rice is hot, as a guide at 80°C or above. Pour the sushi vinegar over the rice paddle so that it spreads all over the rice. Do not pour it all into one spot. Hot rice is losing moisture rapidly from its surface, and the water that leaves makes room for vinegar to enter. On top of that, the higher the temperature, the faster the vinegar soaks in. Vinegar added to cooled rice only wets the surface, and the shari turns watery, with the acid sitting on top.
5. Cut. Do not knead　Hold the rice paddle upright, slice into the rice, and turn it over from the bottom. Never press down on the rice or knead it. The surface of cooked rice is soft. If the grains are crushed, the sticky starch inside comes out and turns into paste. The grains stick together and form a lump that does not come apart in the mouth. Stop as soon as the vinegar has spread through all the rice.
6. Fan with an uchiwa　Once the vinegar has spread, fan the rice hard with the uchiwa to drive off extra steam and heat. Turn the rice over with the paddle so that air reaches the underside too. When the surface moisture evaporates all at once, the surface is said to tighten and take on a gloss. This gloss is called teri. Also, if you cool the rice quickly, the sourness and aroma of the vinegar stay in instead of escaping. As a guide, fan for 1 to 2 minutes, until you no longer see steam. Do not cool it too much.
7. Cover with a cloth and let it settle　Once the rice is close to body temperature, gather it in the center of the handai. Cover it with a tightly wrung damp cloth and leave it for 5 to 10 minutes. The temperature and moisture of the surface and the inside even out, and the vinegar settles. Then move the shari to an ohitsu (wooden rice container) or a rice warmer.

The physics behind each movement

Each movement in shari-giri has a physical reason. At Grade 1, this certification exam asks about these reasons.

Why spread the rice thin. If you heap it, heat and steam are trapped in the center. The rice inside turns sticky with extra moisture, and only the outside dries. If you spread it thin, steam escapes evenly, and the temperature and moisture become the same throughout. This sets up the rice so that the vinegar soaks in evenly.

Why add the vinegar while the rice is hot. First, the water lost through evaporation is thought to leave room for the rice to take in liquid. Second, the higher the temperature, the thinner the liquid becomes, and the faster the vinegar soaks into the grains. These two points are the answer.

Another explanation is also widely given: "the pressure in the spaces left by escaping steam becomes lower than outside, and that difference sucks in the vinegar." This "pressure lower than outside" is called negative pressure. However, while steam is escaping outward, the pressure inside the grains does not fall below the pressure outside. If it were lower, the steam could not get out. The explanation that negative pressure sucks in the vinegar has not been confirmed, so this book does not use it.

Why you must not knead. The starch in cooked rice has absorbed water and become soft and sticky. This is called gelatinization, or alpha conversion (Chapter 7). When the surface of a grain is crushed, the gelatinized starch inside leaks out. What leaks out is mainly amylopectin, which is especially sticky. The leaked starch acts as a paste and glues the grains firmly together. That is how shari turns into a lump. The purpose of cutting is to spread the vinegar through all the rice. 20 to 30 strokes are enough. Stop once the rice paddle passes through without resistance, and do not touch the rice any more.

Why cool the rice quickly with an uchiwa. It has two effects. The first is gloss. On the surface of each grain, there is a thin layer where gelatinized starch and sushi vinegar are mixed. When you cool the rice quickly, the surface moisture evaporates all at once. This layer is then said to tighten into a smooth film that reflects light and gives a sheen. The second effect is to lock in the aroma of the vinegar. The longer the rice stays hot, the more acetic acid evaporates and escapes, and the flavor turns dull. Cooling quickly stops too much from escaping, so a pleasant sourness and aroma remain.

However, do not cool the rice too much. The target is body temperature. Below that, retrogradation begins to progress. Fan only until the steam on the surface disappears and the rice no longer feels hot when you hold your hand over it.

Finally, when you cover the rice with a cloth, a dry cloth takes moisture from the rice, and a cloth that is too wet drips water onto it. That is why you use a tightly wrung cloth.

9.3　Why nigiri is shaped at body temperature

The basic rule is to shape shari at body temperature, about 35 to 37°C. This is not a matter of taste. There are two reasons.

Because shari hardens as it cools

Starch that has become soft through cooking gradually returns to its original hard state over time. This is retrogradation (Chapter 7). Retrogradation is fastest at 0 to 4°C, which is refrigerator temperature. This is why rice kept in the refrigerator becomes dry and crumbly by the next day. Keeping rice at body temperature slows retrogradation, so the rice stays plump and soft.

Once shari has hardened, reheating it does not restore it as shari. The vinegar evaporates, and only the surface turns sticky. So the rule is not to use shari once it has cooled. A chef plans the amount and timing so that the shari never has the chance to cool.

Because it melts the fat of the neta (topping) in the mouth

The fat of toro (fatty tuna) melts at a temperature lower than the inside of the mouth. It is said to melt at about 24 to 30°C. With cold shari, the fat does not melt fully. When you eat it with shari at body temperature, the warmth of the shari melts the fat, and the umami and aroma spread (Chapter 14).

Let us go over the temperatures in this figure in words as well. Retrogradation is fastest at 0 to 4°C. Tuna fat melts at about 24 to 30°C. Shari for shaping nigiri is 35 to 37°C. The vinegar is added at 80°C or above. And 10 to 60°C is the temperature danger zone, where bacteria grow easily. Body temperature lies inside this danger zone.

Retrogradation and fat in more depth

Retrogradation is also called beta conversion. It is the change in which gelatinized starch that holds water releases that water over time, and the starch molecules line up again and set hard. It is fastest at 0 to 4°C, but it also progresses at room temperature, around 20°C. Cooled shari turns hard and crumbly within a few hours.

Retrograded starch can become soft again to some extent when reheated. It is the same as cold rice softening when you warm it up. With shari, however, reheating makes the acetic acid evaporate, and only the surface softens and turns sticky. Both flavor and texture are lost. That is why it cannot be restored "as shari."

Toro fat melts at a low temperature because it is rich in unsaturated fatty acids. Fat rich in unsaturated fatty acids does not set easily and melts easily. It is said to melt at about 24 to 30°C, but this varies with the cut and the amount of fat. Toro fat straight from the refrigerator is firm. If you put it on cold shari, it leaves a waxy texture.

For toppings with little fat, such as white-fleshed fish and shellfish, slightly cooler shari at 30 to 33°C also works. The difference in temperature between the shari and the topping is part of how flavor is designed (Chapters 14 and 24).

9.4　Firm outside, soft inside

A ball of shari that falls apart gently in the mouth has a clear structure. The grains on the outside stick together just enough to form a "wall" that holds the shape. The grains inside are not pressed together, and there is air between them. This is called firm outside, soft inside (gaiko-nainan). The moment you bite, the outer wall gives way, and the grains spread out one by one along with the topping.

This structure is not created only by the hand skills of shaping nigiri (Chapter 24). It begins at the shari-giri stage. Shari that has been kneaded into a paste forms a lump, no matter how lightly you shape it. Cold, hard shari needs force to shape, and that force crushes the air inside. Shari at body temperature is soft and takes its shape in only a few movements, so the inside is not crushed.

Shape shari whose grains stand up, at body temperature. This is where shari-giri, temperature, and shaping nigiri all come together.

9.5　Keeping shari warm, and throwing it away

Once the shari has settled, move it to a wooden ohitsu or an electric warmer. The warmer is also called a rice warmer, and it is often set to 35 to 40°C. If the temperature is too high, the shari dries out. If it is too low, retrogradation progresses. Check the actual temperature with a thermometer.

There is something you must not forget here. 35 to 40°C is right in the middle of the temperature danger zone (10 to 60°C, Chapter 2), where bacteria grow easily. Sushi is a food that gives up on controlling bacteria with temperature. Instead, it controls them with the acid of the vinegar and with time (Chapter 6). Vinegared rice only makes it harder for bacteria to grow. It does not stop them from growing. So make the following your restaurant's rules.

• Record the time when the sushi vinegar was added.
• Set a time limit for keeping shari warm. Many restaurants in Japan use 2 to 4 hours as a guide. Make it shorter in a hot kitchen.
• Throw away shari that has passed the limit, even if some is left. Do not carry it over to the next day. Do not reheat it. Do not refrigerate it to shape nigiri the next day.
• Cook small batches instead of cooking a large amount and keeping it warm for a long time. That is better for both flavor and safety.

Ohitsu and rice warmers, and how to set pH and time limits

A wooden ohitsu regulates moisture in the same way as a handai. Wood does not conduct heat well, so the temperature falls slowly, and the ohitsu keeps the shari at around 35°C for 1 to 2 hours. Many electric rice warmers hold hot water to keep the humidity up as well. The temperature inside a rice warmer varies from spot to spot, so measure in several places to check.

Vinegared rice can be handled at room temperature for a set time only because its pH has been lowered enough. With shari that has little vinegar, shari with a pH above 4.6, or in a humid place, the risk grows over time.

As a restaurant, decide "within how many hours of cooking the shari will be used up" and "what to do with leftover shari," and stick to it. Many restaurants set the limit at 2 to 4 hours as a guide. Decide this time for your own restaurant, based on temperature control (Chapter 5) and pH control (Chapter 6), and record it. When you change the blend, measure the pH again and review the time limit.

9.6　Judging good shari, and the causes of failure

Let us put the qualities of good shari into words. There are five.

1. The grains stand up. The outline of each grain is clearly visible, and no grains are crushed.
2. It has a sheen. The sheen is dry, not a wet shine.
3. It comes apart in the mouth. When you chew, it separates grain by grain.
4. The acid and salt are well balanced. The aroma of the vinegar rises into the nose, the salt gives the flavor an outline, and the sugar does not stand out.
5. The temperature is right. It is at body temperature, and the temperature difference from the topping is what you intended.

When things go wrong, work back from the symptoms to the cause. The main ones are summarized in a table.

Here are the key points of the table in words. Stickiness usually comes from kneading or too much water. Dryness comes from too little water, retrogradation, or drying out. When the sourness is dull and only the smell is strong, acetic acid has escaped. Uneven flavor comes from how the rice was spread and how the vinegar was added.

Never serve failed shari to guests. To track down the causes, keep records every day: the age of the rice, the amount of water, the soaking time, the amount of sushi vinegar, the time shari-giri took, and the finished temperature.