Soak the beans in twice their volume of cold water. Let them stand for at least 12 hours, or 24 hours if the ambient temperature is cold.
Drain the beans and place them in a large saucepan.
Cover the beans with water and cook until tender, this will take about 4 hours. Alternatively, use a pressure cooker and steam them for 45 minutes.
The beans are ready when you can easily crush them between your fingers.
Sanitizing
Wash and rinse your kitchen sink. Put the sink stopper and fill it with a gallon of water.
Add 1 teaspoon of Star San to the water. If you don't have Star San, use an alternative method to sanitize.
Soak all utensils (bowl, fermentation dish, spoon) for at least 2 minutes. Then remove the stopper and let them drain without rinsing.
Adding the Culture
Bring water to boil in a kettle. Pour ½ cup of water into a small bowl and allow to cool for 5 minutes.
Add 1/5 teaspoon of natto culture powder (0.1 g or 1 ml) to the water and mix well.
While the beans are still very hot (around 80°C), add the water inoculated with the culture. It is important to add the natto culture while the beans are still very hot, as the thermal shock will allow the Bacillus spores to germinate while eliminating the undesirable bacteria.
Mix well with a large spoon.
Fermentation
Preheat your incubator at 40°C (oven, dehydrator, yogurt maker, etc.).
Add a thin layer of beans to each container (about 1 to 2 cm).
Place a cloth over the top of each container and fix a plastic film over it.
Place the containers in the incubator and allow them to ferment for 12 hours.
For a stronger flavour, let them ferment for up to 24 hours. Make sure that the temperature of 40°C remains constant throughout the fermentation period.
Notes
You can eat natto straight away, but its quality will improve if you keep it in the fridge for a few days. Eat it the next morning as a Japanese-style breakfast!
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