Koya dofu is freeze dried tofu bean curd, which is a preservable food used for Japanese cuisine. It is like a dried sponge, and should be rehydrated to be used for cooking. It is often simmered with soup stock to give a savory flavor to it.
There is a famous buddhist temple in Koya mountain, Wakayama prefecture, and the buddhist monks ate vegetarian cuisine including tofu. It is told that tofu which the monks ate was found frozen in the winter with the aid of cold wind. The frozen tofu was defrosted and started to be eaten because the monks liked its unique spongy texture. It started to be called koya dofu because it was originated from the temple in Koya mountain.
In the northeast region, and in Nagano prefecture, shimi-dofu which is same as koya dofu started to be made. The word “shimi” is written 凍み which means frozen. There was a food made in winter by hanging tofu in the cold, and people in the Azuchi-momoyama period (1568-1598) found that the hanged tofu was naturally dried. People started to make shimi-dofu by using the freeze-dry technique, and stored them as preservable food.
The way to rehydrate koya dofu is to place the dried koya dofu in a bowl, and pour warm water. By soaking it in warm water for about to five to ten minutes, it will be soften all the way through. Gently squeezing the white water out, and soak again in clean water will complete the rehydrating of koya dofu. Nowadays, there are Koya dofu which does not require soaking in water or boiled, which can easily be used as an ingredient for cooking simmered dishes.