Natto is essentially made of boiled soybeans with natto yeasts growing on the surface. This is what produces the characteristic "natto strings" and natto odor. This very nutritious food undergoes a relatively short fermentation process of 20-22 hours from the start compared to miso paste or soy sauce. Essentially, the soybeans are washed, soaked in water, cleaned, steamed, sprayed with natto yeast and then placed in a warm environment to allow fermentation.
Natto can be purchased or made at home. When mass produced, soybeans are first put through a sorting machine to eliminate worm-eaten beans, cracked beans, colored beans and foreign substances. Afterwards, they are washed thoroughly in a bean-washing machine. These clean soybeans are then soaked in water, which allows them to be steamed and boiled with even flavor and softness. The steaming process kills bacteria on the surface of the soybeans and creates an ideal environment for natto yeasts to grow.
Next, the boiled soybeans are sprayed with natto yeasts, and the batches are stored in containers in a fermentation room at a temperature between 38-42 degrees Celsius for 16-24 hours. After this time period, the batches are taken out of the room to be cooled in a refrigeration room. This cooling process is called maturation, and subsequently, the product is packaged in a container to be shipped out. This entire process can be replicated at home with special store-bought natto yeast, an oven and a refrigerator.