When making natto it is important that the entire process is as sterile as possible. You should take care to avoid any skin contact with the product. With our method you do not need many utensils: a few glass bottles, a pressure cooker, one teaspoon and a bit of aluminum foil. In addition you need a space where you can keep the temperature at about 40°C (104 °F).
1. For making 1 kg of natto you need 500g whole soybeans (the smaller their size the better) and 1 g natto starter.
2. Wash the soybeans and soak them in about 2 liter water for about 12 hours. It is important that the beans do not get sour during this soaking process. The Bacillus natto bacteria do not like acidic conditions. If the room temperature is high it is better to soak the beans in the fridge.
3. Drain the soaked beans and divide them over 5 glass bottles (1 liter with wide neck, for example juice bottles). They will be filled by about 1/3. Add 1 teaspoon extra water to each bottle. Cover the bottle with a first layer of aluminum foil. Make about 6 to 8 holes in the aluminum foil with a tooth pick. Cover with a second and intact layer aluminium foil. Wrap one teaspoon in aluminum foil.
4. Place the teaspoon and bottles with the soybeans in a large pressure cooker, add enough water in the pressure cooker so it will not cook dry, and cook for about 60 min at max pressure. You can adjust this time to obtain harder (50 min) or softer (80 min) natto.
5. Remove the bottles and teaspoon from the pressure cooker. Allow to cool for 8 min, the beans will be still very hot. With the sterilized spoon measure about 0.2g natto starter and add it to each bottle on top of the hot soybeans. Cover the bottle again with the foils and shake the beans very well. Use a towel or oven wants to protect your hands from the heat. Allow the beans to cool to body temperature. Shake the beans again.
6. Remove the second aluminum foil, leave the aluminum foil with holes firm on the bottle and fix it with a rubber band. Place the bottles horizontally in the incubator set at 40°C (104°F). Make sure that you shake the beans so they are spread as much as possible. Incubate for 22-24 hours. After about 8 hours of fermentation you should notice soft whitish fuzz on the soybeans and the typical natto smell should be present. If this is not the case, then something went wrong and you cannot eat the natto.
7. Your natto is now ready for consumption and can be transferred to smaller cups. Natto can be aged in the refrigerator for a few days to improve taste and stringiness. Or you can freeze it.