Pour ¼ cup of water (pre-boiled and then cooled water is fine too) in a cup and dissolve 1.5g (1 teaspoon) of natto starter in it. We use a little more on the first batch to get things going.
Strain the water from the beans and place the beans in the yogurt maker.
Pour in the natto solution while the beans are hot and stir to mix thoroughly. The bacteria must be added while the beans are hot, as B. subtilis is heat-activated.
Natto bacteria require oxygen to survive and reproduce, so do not put the inner lid on the jar. Place the open jar into the water bath then put the clear cover lid on the maker. The cover lid has ventilation holes allowing oxygen in. Set the temperature to 110°F and the timer to 24 hours and then press confirm.
Condensation will have collected under the cover lid. Please take care removing it and be careful not to get water in the natto. At first, the natto might have a strong ammonia smell, (mine did not) but this will subside with a second fermentation by placing it in the fridge. Stir the beans and check the stringiness which is a sign of good fermentation.
Place the lid on the jar and transfer the fermented beans to the fridge. This is the second fermentation and will increase the signature stringy texture and reduce the ammonia smell. By resting the beans in the fridge for a day, the amino acid composition of soy protein will enrich the flavor of natto.