Koya-dofu (freeze-dried bean curd) is a food made of preserved tofu.
In its dry form, it's light and shrunken to a sponge-like state, so it's reconstituted and seasoned by simmering in broth.
History and origin of the name
It has been said that the process was accidentally discovered when tofu was left outdoors during the winter season.
It's commonly said that the kori-dofu (frozen bean curd) manufactured at Mt. Koya spread throughout the nation as a vegetarian dish. However, in the Tohoku region there is a preserved food called shimi-dofu, for which the same process is used. It's very possible that it was introduced from China, since there is a similar food in China. In frigid regions, it was discovered accidentally regardless of the place, and therefore it seemed to be a universally manufactured food.
It was said that people started calling it Koya-dofu because it was prized as a souvenir of Mt. Koya during the Edo period. During the Edo period, the thing that was most distributed took the name of its sales territory and seller, and this is an example of the practice.
The name of Koya-dofu is known throughout the nation today, but originally it was used throughout the Kinki region. It was called shimi-dofu or kori-dufu in the Koshinetsu region, the Tohoku region and Hokkaido. The kori-dofu manufactured through the traditional process in Koshinetsu, Tohoku and Hokkaido is manufactured by hanging strings of several tofu with straw under eaves, and it's also called ren-dofu (string-bean curd) because of its shape. Additionally, it's called chihaya-dofu in Osaka. In the old days it was written as kori-dofu (ice-bean curd).
In the Tohoku region, the frozen type is sold as kori-dofu, which is at the midpoint of production, and the dried type is distinctly sold as shimi-dofu, so it needs attention.
Kori-dofu is the official name under the Japan Agricultural Standards (JAS).
Because it developed in a cold area, Nagano Prefecture is still the largest production center in Japan. Today, it's no longer manufactured around Mt. Koya.
With the most traditional process, cut tightly drained tofu into pieces of appropriate size and leave them outdoors in cold weather. By repeating the cycle of freezing overnight night and thawing during the day, it fully dehydrates and becomes a dry food. When water freezes, countless ice crystals are created inside the tofu, and when the tofu is thawed they leave small holes. This way, a porous, spongy koya-dofu is created. It's said to be the old process used in Koshinetsu, Tohoku, Hokkaido and Mt. Koya.
The above traditional process was used in the old days at Mt. Koya, but the process was altered in recent years, and a unique process has been established. In order for tofu that is frozen during the night not to thaw during the day, it's placed in a separate ice room and allowed to mature for a few days, and after thawing it is dried. Sun-drying was originally the method used, but the forced-drying method spread later by burning coal in the drying room.
Currently, the general practice is to use refrigerating machinery for freezing and drying machinery for dehydration.
Bonanzai (leavening and softening agents)
The koya-dofu processed the traditional way is hard, takes one night to reconstitute and is difficult to cook softly, so it's a time-consuming ingredient for use in cooking. Therefore, a food additive called Bonanzai is added to most koya-dofu currently on the market in order to shorten the reconstituting time and allow it to cook softly.
By utilizing the property of highly absorbent ammonia, the time needed to reconstitute the koya-dofu is shortened. The koya-dofu is placed in an ammonia processing room after drying it, and then the room is filled with ammonia gas. Once the ammonia gas spreads into the spongy koya-dofu and becomes fully absorbed, the tofu is hermetically sealed and shipped. The ammonia has no effect if it's allowed to vaporize, so it must be hermetically sealed when it's preserved.
In order to get rid of the ammonia odor, it's recommended that hot water be used to reconstitute the product. After the product is reconstituted, several changes of water are required in order to get rid of the ammonia odor completely. This method is no longer because ammonia is harmful and has no preservative effect if it's allowed to vaporize.
By utilizing the property of baking soda, which can dissolve protein and soften ingredients, it serves to soften the koya-dofu. The raw, frozen tofu is placed in thawed in a solution of water and baking soda. The tofu, having been soaked in baking soda, is then dehydrated. A portion of the protein is destroyed, so it has a short reconstituting time and will cook softly. Most of koya-dofu currently on the market is processed with baking soda.
Unlike the koya-dofu processed with ammonia, it doesn't need to be hermetically sealed. Further, it doesn't need to be reconstituted with hot water.
Koya-dofu processed with baking soda is one-fourth to one-third softer than the one produced through the traditional process, and its texture is so different as to suggest that it's a different kind of food. It's mushy due to its softness, so it loses its shape if it's cooked the traditional way. It loses its shape when it's simmered in pure water, so it should be simmered in saline broth to tighten its composition and prevent it from being mushy. Also, by cooking koya-dofu processed with baking soda in a rice cooker together with water, it can make tofu close to the ordinary type.
Its preservative quality is high since it's a dry food, but when the storage life gets longer, the fat content is oxidized and the quality deteriorates. The maximum storage life over which the flavor is retained is six months. Also, it's porous and can easily absorb odor, so attention is required in preserving it. To prevent oxidation and odor, it's preferable to put it in the hermetically sealed container and preserve it in a cold, dark place.