I have made many bad batches of natto. Hopefully this troubleshooting guide will help homemade natto makers out there make better natto. If you have any questions that are not listed here, please email me at "Contact Me" or directly at my email if you want to attach pics.
Hi all again, I have updated this as of May-2020, so please take a look if you had visited the guide before.
Before going through the Q&A, here are the most basic requirements to make good natto.
1. Fully hydrate soybeans. 16-20 hours for Summer time and 20-24 hours for Winter time.
-Update: Better to soak for longer than for shorter if time allows. So even in the warmer months, soak for 20 hours. For me, I will wash and soak the beans before I go to sleep and then make natto after coming home the next day.
2. Cook beans by steaming under pressure. This means putting beans in a basket and steaming them in a pressure cooker making sure that most of the beans do not touch the water. The cooked beans should be soft enough to squish with your thumb and pinky and should have the consistency of a thick paste.
-Update: For my pressure cooker, it takes about 40 minutes, but this may vary with your setup.
-Update: The beans should be light beige in color and not yellow after proper cooking time.
-Update: If the soybeans are hard after fermentation, the beans did not cook for long enough.
-Update: Boiling under pressure (beans immersed in water while cooking) results in wet beans and can cause a waxy low thread natto and/or high ammonia.
3. Use frozen packaged natto as starter.
-Update: you can also use spore starter and
but make sure to dissolve the spore in 80 celcius water. Using spore starter for me, tends to result in higher failure rates when compared to frozen packaged natto. Refer to the 'Spore Starter Natto' on Youtube for optimized instructions.
4. Update: Ferment beans and hit at least 42C (108F) earlier in the fermentation, with a temp range of 40-42C (104F-108F). Total fermenation time is 18-22 hours. Fermenting longer will not fix any issues or produce more threads.
Q: My natto is not stringy, what's up?
A: This one is the hardest question and probably deserves its own post.
A: Low temperature will result in the natto not growing properly. It is important to reach 42C (108F) during the fermenation. Shoot for 40C - 42C (104F-108F) for 18-22 hours. Natto is okay with temperature fluctuation and will survive heat spikes up to 45C (113F) and maybe even up to 50C (122F) for short amounts of time.
A: The soybeans were not cooked long enough. The cooked beans should be easily squished with the thumb and pinky and have a creamy texture. Steamed under pressure takes 40 minutes to reach this with soybeans.
Update: If the soybeans are hard after fermentation, the beans were not pressure steamed for long enough.
A: Another possibility noted on some natto blogs is that the fermentation temperature is too high.
-Update: Usually 9 out of 10, the fermentation temp was too low which made the fermentation fail, not because the fermantation was too high. Natto is okay with short heat spikes up to 113F and maybe higher.
A: I used to think that overcooking the soybeans somehow destroyed a nutrient to make the silky threads, but this is not the case. Overcooking should not be a concern from what I have seen. As a note, I have noticed that boiling soybeans will make the natto have a non-stringy rubbery growth on the beans.
Q: My natto looks white and leathery.
A: This is related to the question above. When the natto is not stringy, it usually turns out leathery and white. This might be related to lower fermenting temps.
-Update: Make sure you are reaching at least 42C (108F) in the fermenation.
A: The soybeans were too wet which can happen if you boil or pressure boil the beans. Pressure steaming is the best way to cook the beans.
Q: My natto stinks of ammonia.
A: I associate this with temperature problems. One of my earlier batches smelled strongly of ammonia and the natto was not stringy. If I remember correctly this was because the temperature was too low and I left the natto batch fermenting for more than 24 hours waiting for the stringyness to appear.
Q: My natto does not have flavor.
A: Boiling the soybeans is the reason why natto turns out flavorless. Boiling them in a regular pot or boiling them under pressure will do this. Curiously the skin of the soybean will come off when boiled. To resolve this, you must cook the soybeans in a steamer basket or strainer above the boiling water so most of the beans do not touch the water. I use a small glass bowl to suspend the steamer basket over the boiling water.
Q: My boiled soybeans are soft, but the natto is hard.
A: The reason is that the soybeans were not cooked long enough. This is hard to accept when you spend 4-6 hours of your valuable time to steam (without pressure) the soybeans. I have done this and there is that transitional stage when they are almost soft enough, but not quite. The soybeans should be a dark beige color and not yellow. Also, when eaten, it should have an almost creamy texture. I recommend steaming them under pressure which only takes 40 minutes.
Q: My natto is dry.
A: I initially thought that natto needed a lot of ventilation for the natto to breathe and to blow off the ammonia produced in the fermentation. So I covered the natto with paper towels and newspaper and then put it in a paper bag. This was a very long path to a dead end which led to not so tasty dry natto. Turns out that natto does not need that much air. So now I use the Double Cling Wrap Method (scroll down to the middle of the post). I poke holes on the cling wrap with a tooth pick so there is a hole every 1-2 cm (1/2" to 1") apart. This is tedious, but I enjoy this mindless repetitive work.
Q: My fermented natto tastes like boiled soybeans and not natto.
A: The temperature of the fermentation was too low. Make sure you are reaching 42C (108F).
A: I do not have a vendetta against the Mitoku Natto Spore Starter, but the spore starter has given me this result. Mitoku Natto Spore Starter has resulted in leathery white natto or unfermented natto that tastes like cooked beans. Make sure to dissolve the spore starter in 80C (176F) water. Or use packaged frozen natto as the starter.
A: Packaged frozen natto starter was weak. Try a different brand of packaged frozen natto if available.
Q: How much packaged natto starter should I add?
A: When I use the frozen packaged natto, I chop up the package into 9 cubes. For Okame Natto, one package is 50 grams, so one cube is 5-6 grams. One cube will easily ferment 400 grams of dry soybeans (a little less than 1 lb). So a 3-pack of 150 grams will yield 27 cubes, or 27 batches of natto!
Q: What is the conversion rate of Dry soybeans to fully hydrated soybeans (after 18 hours).
A: For soybeans, I use x2 as the conversion factor. So 400 grams of dry soybeans will become 800 grams of fully hydrated soybeans.
Note: Please use the "Contact Me" or email me directly at nattodad[at]gmail.com if you want direct replies.
