Seaweed is the ocean's top vegetable crop. For centuries, the plant has been prevalent in Asian cuisine, but the popularity of sushi and other Japanese, Korean, and Chinese dishes has made this ingredient—once thought of as little more than slimy shore carpet—a familiar, and sought after food.
Because seaweed is continuously bathed in mineral-rich sea water, it is one of the most nutritious vegetables around. Ounce- for-ounce, it is higher in vitamins and minerals than any other class of food. It supplies all the minerals needed for human health, including calcium, iodine, iron, phosphorus, and sodium. Seaweed also contains Vitamins A, B, C, and E.
The vegetables that "grow" in the sea's garden are plentiful and varied, and include nori (laver), kelp (konbu), wakame, and kanten (agar-agar), and an ocean of others.
Nori, known as laver in China, is notorious for its role as a sushi wrapper, but it can also be used to wrap meat or fish before roasting or baking. The thin, dark sheets are really a combination of a few related species of seaweed or marine algae. Nori production began in 17th century Japan, when the sheets were handmade. Today the process is highly mechanized, with factories in Korea, and in Maine. Nori sheets ranges from dark green to black-purple and are sold in packages of 10, 50, and 100 sheets, and by the 20-lb. case. A minced green nori is available in shakers and can be used for seasoning.
Kelp's long, olive-brown leaves average 5-6 in. and are usually coated in a salty fuzz. Don't wash this off, or the seaweed will lose its flavor. To prepare kelp, cut off as large a piece as you will need, and wipe it with a dry cloth before using. The main culinary use of kelp, also called konbu, kombu, and seatangle, is to make dashi—the all-purpose, salty Japanese stock used for dressings, soups, marinades, and sauces; it's made from kelp and dried bonito flakes (katsuo bushi).
Wakame is the green, ribbon-like seaweed commonly tossed with rice wine vinegar and sesame seeds for Japanese salads. While wakame is sold fresh in the summer months, it is most often found in dried form. Wakame can be reconstituted by soaking it in tepid water for about 20 minutes; tepid water is preferable to boiling water because it will maintain the flavor and nutritional content of the plant.
Agar-agar, or kanten in Japanese, is an essential ingredient for the gelatinous sweets found on dim sum carts. This substance is similar to gelatin, and is extracted from a number of seaweeds by a three-step process: boiling, filtering, and freeze-drying. Agar-agar is available in strands that resemble crinkled cellophane and 10-in.-long rectangular sticks. To make gelatin (generally, 2/3-oz. of agar-agar will gel 1 qt. of liquid), soak the strands or sticks in cold water for 20 minutes. Wring the pieces out and add them to cold water in a saucepan. (Sugar may be added at this point.) Bring the mixture to a boil, and reduce to a simmer until the agar-agar dissolves. Strain and stir in the remaining ingredients in your recipe. Heat again just to the boiling point and pour the liquid into a mold or dish and refrigerate. For foodservice, agar-agar is available in 1-lb. bags.
As far as storing your seaweed, if fresh, it will keep, refrigerated for 2-3 days. Dried seaweed keeps for several years when stored, tightly covered, in a cool, dry place.