Put the beans into a bowl and wash them well by rubbing them together. Be careful not to break the skin. Soil and other impurities on the surface of soybeans has tons of microorganisms that have adverse effects on making Natto. Make sure to wash off all the dirt and debris.
Step ② Soak the washed soybeans in water
Soaking time differs depending on the season. In spring and autumn, you have to soak the beans for 15 hrs on 10-15 degrees celsius water temperature. In summer you have to soak the beans for 6 hrs on 20-25 degrees celsius water.When the temperature is too high, cover the bowl with plastic cling wrap and leave in the fridge for about 24 hours.The beans expand about 4 times more than the soybean weight.
Step ③ Boil the beans
Boil it until it’s easily squished, by nipping with thumb and index finger. The beans should have a slightly hard texture when fermentation starts, so make sure to boil them until tender.It takes approx 30 mins for a pressure cooker to cook. (You can boil it with regular pot as well)(The result may vary depending on a cooker. Please refer to the instruction manual for the exact cooking time)
Step ④ Dissolve Natto starter into water and make a Natto solution
Pour 10ml of water (pre-boiled and then cooled water is fine too) in a cup and dissolve 0.1g of Natto starter in it. Use this Natto solution to ferment the beans. It is easier to succeed with more Natto solution. For beginners, use a greater amount of Natto starter than this recipe is recommended.
Step ⑤ Pour the Natto solution over the beans
Discard the boiled water. Then quickly pour the Natto solution and mix thoroughly.Don’t let the temperature to drop, as it can lead to unwanted bacterial contamination.Make sure to prepare the Natto solution as soon as the beans are boiled.
Transfer the beans from the pot to a food warmer such as rice cooker, pressure cooker, or a Yogurt Maker. In this tutorial, we're using a food warmer function of a pressure cooker.Keep the soybean mixture in a warmer for 24 hours to ferment.The best temperature for Natto solution to ferment is 40 to 45 degrees celusius. Keep the temperature steady at 45 degrees and set the timer for 24 hours when using a food warmer.
After keeping the beans warm for 24 hours, let’s check the fermentation condition. It looks well fermented however it still has a strong ammonia smell at this point.
Step ⑥ Let the fermented natto cool in the fridge
Transfer the fermented beans to refrigerator to mature. By resting the beans in a fridge for a day, amino acid composition of soy protein will enrich the flavor of Natto. Let’s see how the natto is after resting it overnight.
It is perfectly fermented and there is no smell of ammonia. Tasty natto is now ready to be served. In this tutorial we used 165g of soybean The finished natto weighs 721g
If you are not planning to eat it immediately, keep it in a freezer. If you leave the natto in a place where the temperature is high, amino acids are broken down and can cause ammonia smell. When it is stored in the fridge, a white rough substance will start to grow which will give it a bad texture in a week or so.The taste and flavor of natto differs greatly depending on the fermentation time and the types of soybean. You can decide on what type of soybean and how long you want to have it in fermentation.