Once natto was made at home during the winter off-season. In the bitter cold, when there was no farm work, the people of farming households wove straw ropes and turned them into straw bundles to carry natto and household items. They also worked to prepare miso and pickled vegetables and other preserved foods, and of course natto. They prepared such things to eat themselves as well as to sell.
The basic method of preparation was simple: wrap steamed soybeans in a straw bundle and make sure that it is kept warm. To keep the bundle warm, in some cases they used only the heat of fermentation, and in other cases they added heat by means of a hot water bottle or the like. In either case, the methods that were used to keep the straw bundle warm were strongly influenced by the local culture, and they seem almost to be a battle of wits with the natto bacillus. The wealth of methods truly deserves the title "local fermentation." Some warming methods were used all the way up to the early Showa period which began in 1926.
Akita Prefecture Hiyama Natto
A straw bundle containing boiled soybeans is packed into a wooden box which is then stuffed with sawdust and covered with a straw mat, after which a weight is applied and the preparation is fermented.
Niigata Prefecture Oke Natto
Straw bundles containing boiled soybeans are lined up in a barrel which is then stuffed with straw waste and then wrapped in a straw mat.
Tohoku Region Yuki Natto
A 1m square hole is dug beneath the snow and straw waste is laid inside. A straw bundle containing boiled soybeans that has been wrapped in straw is placed in the hole. A pipe is inserted for ventilation and the hole is covered in snow to keep the interior warm.
Saitama Prefecture Hettsui Natto
"Hettsui" means keeping something warm using the residual heat from a cooking stove. After cooking, a straw bundle wrapped in a wet straw mat is placed far inside behind the ash that is still warm.
Yamagata Prefecture Tsurusu Natto
A straw bundle packed with boiled soybeans is enclosed in a straw mat. Ropes are tied around both ends of the straw mat and the mat is suspended from the ceiling. This method of warming uses the hot air rising from the irori (open hearth) and the residual heat from the cooking stove.