A CENTURIES-OLD PROCESS
Starting each year on a cold, crisp December morning, a few small tofu shops begin the ancient five-hundred-year-old process by cooking stone-ground soybeans in a caldron to make a firm tofu. This kata-dofu (hard tofu) is a traditional favorite of high mountain people who prefer its coarse texture and rich taste to "bland, watery, lowland varieties," as they say.
The fresh tofu is cooled in icy well water and skillfully cut into thin slabs. These slabs are placed on bamboo trays and allowed to freeze overnight. The water in the tofu, about 86 percent by weight, turns to ice. The protein, minerals, and other solids congeal into a firm, lacy network. The following morning, workers begin the tedious task of stringing pieces of frozen tofu with braided rice straw. Immediately, workers facilitate the freeze-drying process by hanging the tied frozen tofu from wooden frames in an open shed. There the tofu squares are left to twist and sway in the wind. During the day, temperatures are just warm enough to thaw the tofu and evaporate some of the water. At night, the tofu freezes solid again. This "aging" process is critical, and only temperature and humidity that fall within a narrow range can produce the finely textured, highly absorbent, "snow-dried" tofu that is characteristic of this region of Japan. After about twenty days, the tofu slices are feather-light and bone-dry. Only 10 percent of the original moisture remains, and nature's low-tech drying process is complete.
AN ACCIDENTAL DISCOVERY
Like many traditional Japanese food manufacturers, the freeze-dried tofu shops owe their unique way of life to their Chinese neighbors. According to William Shurtleff, co-author with Akiko Aoyagi of several books on traditional Japanese foods, frozen tofu was probably first made in the cold mountainous regions of northern China about 1,000 to 1,500 years ago. It was found that if tofu was left out in the snow overnight until frozen solid, it underwent a radical transformation. When later placed in warm water, the tofu thawed, leaving a fine-grained, highly absorbent food that had the texture of tender meat.
Although Chinese frozen tofu had vast culinary potential, it had two drawbacks. First, it had to remain frozen or, like fresh tofu, it would spoil due to bacterial action. Second, like ice, it was heavy and difficult to transport.
Leave it to the Japanese to make a good thing better. About 1225 A.D., in a temple on Mount Koya, near Kyoto, a Buddhist monk began drying frozen tofu in a heated shed. This new "dried food" came to be known as koya dofu. Because it contained little water, it kept for several months without spoiling. The relentlessly utilitarian Japanese mind, however, was still not satisfied. In the fifteenth century, aggressive war lord Takeda Shingen recognized koya dofu's potential as a military ration. To make the process more mobile, Shingen did away with the heated shed and simply let the frozen tofu dry in the sun for a few weeks. This kori (frozen) dofu, named thus to distinguish it from the monk's version, was virtually imperishable.