Dashi (出汁) is arguably the most important fundamental ingredient in Japanese cooking, the basis of misoshiru (miso soup), sauces, and simmering liquids. It is an infusion (like tea) best thought of as comparable to stock. Like stock, dashi is best made rather than purchased, and indeed most of the premade or semi-premade dashi that you can find is mediocre at best: premade dashi is to dashi as a bouillon cube is to stock. Fortunately, dashi is not difficult to make, though finding good ingredients can be tricky.
There are only three ingredients in dashi: water, kombu (a type of dried seaweed), and dried fish, usually dried bonito.
The best dashi is made from soft well water. Soft water is important because hard water will significantly impede the extraction of flavor from the kombu. Well water is not essential, but the water should be very pure and have a good flavor. Unfortunately, a great deal of bottled water is hard, despite excellent flavor. At base, any water will do, but the better the water the better the product, to a degree far greater than is true with European-style stock. A worthwhile option is to experiment with a combination of distilled water and other kinds. If at all possible, avoid chlorinated water.
Kombu is a dried seaweed that normally comes as a large folded sheet of near-black, stiff material. It usually has some whitish residue on the surface, which should not be removed. Before using kombu, it should be gently dabbed with a damp paper towel to remove dust, then broken coarsely into large pieces.
Kombu ranges widely in price, not always reflecting its quality. It is probably best, when first making high-quality dashi, to start with an expensive product so that you have a clear sense of what it should taste like. The very best kombu is stored in temperature-controlled facilities and costs a small fortune, but kombu like this is rarely available outside Japan. Once opened, a package of kombu should be stored, sealed tightly, in the freezer, retaining the little packet of moisture-absorbent crystals in the bag.
The most common dashi is made using flakes of dried, smoked bonito, known usually as katsuo-bushi. Other kinds may be made with dried sardines (niboshi), various kinds of dried shrimp, or a combination of two or more of these ingredients.
Dried bonito comes in the form of a hard block that looks and feels much like wood, which is then shaved into large or small flakes before use. Quality can vary considerably, not only by how the block is made but also by what cut of the bonito is used to make it. After shaving, the flakes begin to oxidize fairly rapidly, and lose much of their best flavor within 24 hours, such that it is best to use fresh-shaved bonito flakes for dashi. However, it can be difficult to get whole dried bonito outside Japan, and shaving it requires a special and expensive tool. Modern vacuum packing techniques have made pre-shaved bonito flakes that are of decent but not ideal quality, although they are not inexpensive by weight. For making dashi, you may use any flakes, from the very fine to the large chip-like shavings, but bear in mind that the smaller ones oxidize faster and the large ones may require somewhat longer infusion. Once opened, a package of bonito flakes should be stored, sealed tightly, in the freezer; if there is a packet of moisture-absorbent crystals, leave them in the bag with the flakes.
There are two crucial factors in making good dashi. First, the extraction of flavor from kombu occurs well below boiling temperature: the optimal temperature is 140F, and extraction ceases altogether above 176F. Furthermore, kombu will become slimy, imparting an undesirable texture (and often flavor) if it is brought too close to the boiling point. Second, bonito flakes release their flavor much closer to the boiling point. However, if bonito flakes are disturbed vigorously, as by stirring, mashing, or boiling, they will dissolve and make dashi foggy and slightly bitter.
The traditional method is to put the kombu in cold water and raise the temperature slowly. When a faint hint of a simmer is reached, i.e. bubbles begin to form around the edges of the pan, remove the kombu. Raise the temperature to a near-boil, add all the bonito flakes, and shut off the heat. Wait 30 seconds to 2 minutes, depending on the flakes used, then strain the liquid finely. Do not press on the bonito flakes in the strainer. The resulting mixture is called first dashi. You may now repeat the process exactly, using the same kombu and bonito flakes, to produce second dashi, which is weaker but also very useful. After this, the bonito flakes must be discarded, but the truly thrifty will cut and lightly pickle the kombu with soy and serve it as a side dish.
An extremely effective, perhaps ideal method for making dashi in the modern kitchen uses these recently-discovered temperature points to obtain maximal flavor extraction. Place the kombu in cold water and raise the temperature to 140F. Hold it at this temperature for an hour --- an induction cooker is excellent for this purpose; even better is an immersion circulator holding a bag containing an ideal balance of kombu and soft water at 140F. Remove the kombu, then raise the temperature to 176F. Add all the bonito flakes and shut off the heat. Wait 1-2 minutes, depending on the bonito flakes used, then strain the mixture fine. You may repeat the process, as above, to produce second dashi, though you may find that the flavor is excessively weak because the kombu has lost all its savor. If this method is used, it is generally not worth trying to pickle the kombu, which will have no remaining flavor to speak of.
This modern technique is particularly good for the cook outside Japan, who generally will not have the luxury of the very finest ingredients. By extracting every last bit of flavor from the kombu and the pre-shaved bonito, you can to a great degree make up for having somewhat lesser quality kombu and not being able to shave bonito fresh. Note that this effect will be enormously degraded by hard water.
2 L water, 35g kombu, 60g bonito flakes; the yield will be roughly 1.75L water, some having been absorbed.
Note that the quantities of kombu and bonito may seem very small, but the ingredients are dried and thus weigh very little. As a rule, a fair-sized bag of bonito flakes is only 100g, which will make only 3L of dashi.
Dashi degrades in quality fairly rapidly. If storage is necessary, it should be cooled and stored covered in the refrigerator for no more than 48 hours. It does not freeze well.
Instant dashi comes in two basic varieties: powder and teabags.
The powder is similar in most respects to dried bouillon. It is salty, usually amplified with MSG, and tastes only vaguely reminiscent of good dashi. It is easy to use and store, being a powder dissolved in hot water.
The teabags produce a markedly superior product to the powder. These are available in a wide range of qualities, so it is worthwhile experimenting with different brands. Follow the directions carefully: many of these products work best only if they are not boiled, for example. In some applications calling for dashi that is strongly reduced during cooking, as with some nimono (simmered) dishes, you can simply use water and put in one of these teabags at the start of simmering, removing it only when the dish is complete, and the results will be very good.
Your First Batch
If you have never eaten at a high-end restaurant in Japan, you almost certainly do not know what dashi is really supposed to taste like. If you generally like Japanese food --- discounting sushi, which has little to do with dashi --- you owe it to yourself to try the real thing. Set aside time and money for the project. Buy the very best ingredients you can find, which will not be cheap. Test and taste your water, and adjust. Allow 90 minutes to make the dashi, and bear in mind that you must pay attention to the temperature constantly.
Once the dashi is complete, simply drink it straight, sipping from a wide, shallow teacup or the like. The dashi may be garnished with a pinch of freshly-toasted dry-puffed rice (genmai), particularly in winter. Serve with plain white rice, excellent sake, and a little bit of quick salt-cured daikon for contrast.