Methods for Making Natto in North America

There are various methods for making natto, but the method here will focus on the one that uses natto bacteria (bacillus natto) for the people in Canada and the United States. It is essential to sterilize the utensils being used to make natto to reduce the chance of contamination by harmful germs as well as to maintain a temperature of approximately 40°C (104°F) for successful fermentation of natto. So the large ovens built into most of the North American homes are ideal to satisfy these two requirements.

On the other hand, an oven is just used as a "box" during the fermentation period. As long as you can maintain the temperature between 37°C (99°F)  and 42°C (108°F) you can use any method such as a "cooler box" kept warm with a few PET bottles filled with hot water. One lady told me she has a good result using a Jacuzzi (hot tub) to keep natto warm. If we can disregard the wastefulness of energy, this seems to be a fine idea.

I strongly recommend you to use the spores of natto bacillus instead of a commercial package of natto as the source of natto bacteria since using natto bacillus spores is more economical and runs lower risk of contamination by harmful microbes. Some of the commercially produced natto may have been partially sterilized to reduce the aroma and stickiness of natto, thus making it questionable as a source of natto bacillus. You can obtain natto bacillus spores from several vendors in North America and Japan. The spores can be kept in a refrigerator for a long time. If you are going to use commercially-produced natto instead of natto bacteria, substitute 0.1g of natto bacteria with one pack of natto in the recipe bellow. Some people have had good results with this method.

Ingredients (makes 1.2kg of natto):

• 500g soybeans (the smaller their size the better, so the natto fermentation will permeate to the center of beans.)
• 0.1g Bacillus Natto bacteria
• 1/4 teaspoon salt (natural salt, if possible) A little salt increases stickiness and improves the flavor.
• 1/2 teaspoon sugar (brown sugar or molasses, if possible) Sugar aids the activation of natto bacteria spores and will help fermentation.

Utensils:

• One bowl to immerse soybeans
• One sieve to drain soybeans
• One steamer.  (You can boil soy beans instead of steaming, but the nutrition and flavor will escape, and the natto may become soggy.)
• Two casserole pans, about 25cm in diameter and 5cm deep
• Two sheets of aluminum foil (should be large enough to cover the casserole pans)
• Two deep plates to hold water, 20 - 30cm in diameter
• One thermometer
• One family-sized oven
• One work lamp or lamp stand without its shade   (This is not absolutely necessary, but the heat may not be evenly distributed throughout the oven unless it is placed at the bottom of the oven.)
• On each of 40W and 60W light bulb
• One each of heat-resistant cup, teaspoon and spatula
• One pair of rubber gloves (should be used as much as possible to prevent food poisoning.)

Preparation Method:

1. Wash 500g of soybeans thoroughly, and soak them in more than three times as much water. The amount of time to soak soybeans is twelve hours in the summer and 20 hours in the winter, but in the case of North American central heating, 12 hours may be sufficient even in the winter. One of my friends suggested when the surface of water becomes partly bubbly due to germination, enough soaking is done. This seems to be a good indicator. Make sure to soak soy beans well.

 

2. When the soybeans have absorbed enough water and swollen to twice their dry size, put them into the large steamer pot and steam them for 6 hours. The soybeans are finished steaming when a bean can be easily mashed between your tongue and palate of your mouth. They say it would take between 10 and 15 minutes to boil soybeans with a pressure cooker, but I have not tried this method yet. Make sure that cooked soy beans are soft enough; otherwise natto bacteria cannot penetrate to the center of the beans.

 

3. Two hours before the soybeans are finished steaming, cover the casserole pans with the sheets of aluminum foil, make air holes with a pointed object such as a chopstick, and place them on the top oven rack. Half fill the deep plates with water, and place them on the middle oven rack. The water will keep the natto from drying out. Also add the heat-resistant cup and teaspoon. Heat the oven to around 120°C (250°F) to sterilize the utensils. When the oven has been adequately heated, turn off the switch and allow it to cool naturally. Before using the rubber gloves to mix the natto, disinfect them in hot water. Make sure to sterilize everything you use to prevent food poisoning.

Work quickly from this point on to prevent introduction of harmful bacteria into your natto.

  4. When you have finished steaming soybeans, strain out the water by keeping the lid in place while tipping the steamer.

5. Remove the lid and steamer, leaving the soybeans in the pot. Put the lid back quickly to avoid contaminating soybeans and losing heat.

 

6. Pour 10ml of pre-boiled water into a cup, and mix in salt, sugar and 0.1g of natto bacteria (See the note below in this section). Natto bacteria spores are very resistant to heat. It takes one hour at 140°C (284°F) to kill all the natto bacteria. But make sure the water temperature is bellow 80°C (176°F). Also, if it is hard to handle the small 10ml volume of  pre-boiled water, increase the amount 2 - 3 times. The amount of water will also affect the stickiness and sogginess of natto. So please experiment with it. The photo to the right shows a 3g package. With this small package, 30kg of natto can be made. A tiny spoon to measure the natto bacillus is included. One spoonful of natto bacteria equals 0.1g. When you have become accustomed to this preparation method, you will be able to make good natto with even a smaller amount of natto bacteria.

7. Remove the lid from the pot, sprinkle the solution of salt, sugar and natto bacteria over the soybeans, and stir so that the natto bacteria will be evenly distributed.

Make sure to mix steamed soybeans with natto bacteria solution very well, but do it quickly to avoid losing heat and introducing harmful microbes into your natto.

8. Take the aluminum foil off the casserole pans and spread the soybeans to a 2-3 cm thick layer. (Ideally there should be no more than three layers of soybeans.) Put the aluminum foil covers back on again. The beans are covered with aluminum foil, but if there is not enough air circulation, natto may become bitter. With too much air circulation, its surface may become dry.

 

9. Make sure the oven is turned off first. Then put the aluminum foil-covered pans on the top oven rack. The plates filled with water should be left on the middle rack. The water should have cooled down to around 40°C (104°F). Place the work lamp, fitted with a 40W bulb, in the lower part of the oven. You may use the built-in light in the inner left, upper part of the oven. But it does not warm the oven evenly. You can substitute the work lamp with a small desk lamp stand as long as it fits in the oven. Place the thermometer on the upper rack and close the door.

10. Check the temperature after a few hours. In order to keep the temperature between 37°C (99°F)  and 42°C (108°F) , you may have to swap the light bulbs with different wattage, or you may need to keep the oven door propped open a crack. After a few adjustments it will become easier to maintain the proper temperature between 37°C (99°F) and 42°C (108°F). Maintain this temperature for 20 to 24 hours.

Maintain temperature between 37°C (99°F) and 42°C (108°F) for 20 to 24 hours.

11. After 20 to 24 hours at between 37°C (99°F)  and 42°C (108°F), turn off the lamp, and wait a few hours until the natto becomes cool, stopping the fermentation. When the aluminum foil is removed, you may see some white film on the surface of soybeans. And the kitchen will be full of the aroma of natto. Some smell of ammonia is normal, but if it is too strong, undesirable germs may have flourished. Note: if you would prefer well-fermented natto, you might want to leave the natto in the oven for up to 24 hours after turning off the lamp. If you do, do not remove the aluminum foil. Since leaving food at room temperature for prolonged time increases a chance of food poisoning, avoid disturbing the oven and its contents.

 

The two photos to the right were taken on separate occasions. Natto is very sensitive to air circulation, temperature and moisture. This can cause differences in the outcome of fermentation. Observe the outcome of your natto making, and make adjustments when you make natto the next time.

 

12. Keep the natto in the refrigerator for a few days to one week for aging.  It will develop a nice stringiness and improved taste. If it is left for a long period even in the refrigerator, too much amino acid will crystallize, creating a sandy texture. So after aging natto in the fridge, store it in several smaller packages, and freeze them. Making natto is easy and fun. I hope you enjoy it.

About aging: An enzyme called protease is created in the process of natto bacteria fermentation. At around 0°C (32°F), natto bacteria form spores and become dormant, but the protease keeps breaking down soybean protein to amino acids. So keeping newly fermented natto at around 0°C (32°F) will age natto and give it a richer taste.

Note: If the natto fermentation is not successful, the finished natto may not be sticky enough, may not be stringy, or may be bitter or have strong smell of ammonia. On the other hand, a white film on the surface of natto as well as its stickiness does not guarantee the successful fermentation. Let's be careful about being sanitary when making natto so as not to introduce undesirable germs that contaminate natto.

Common Problems Q&A

Q: My natto is not sticky and does not taste very much like natto.
Experiment with amount & type of sweetener used in the natto microbe solution. Sugar is “baby food” for dormant bacillus natto. The more sugar you add, the better the natto fermentation, but it can become too sweet if too much is used. You might want to experiment with other types of sweetener such as brown sugar and molasses. Salt can affect fermentation as well. Also experiment with the amount of water used. Too little water hinders fermentation, but too much makes your natto soggy. You might want to try a longer fermentation time or leaving freshly fermented natto in the oven for up to a day. (Refer to the note at step 11.) Aging can also help.

Q: I cannot buy natto bacillus. Is there any other way?
Some people use commercially available frozen or refrigerated natto as the source of natto bacillus. If one brand does not work, try another as they might use a different variety of natto bacillus. I would suggest you use a fresh pack of natto every time instead of reusing your fermented natto as the source of the bacillus natto to avoid contamination. In the long run, however using bacillus natto spores is cheaper and more reliable.

Q: Where can I buy commercially produced natto?
Most of the Japanese and many of Asian grocery stores sell frozen or refrigerated natto in convenient small packages for a couple of dollars. You might want to try the commercially produced natto first so that you can check the quality of your natto against it. The people who want to try natto are usually motivated by health reasons. But natto, like some kinds of cheese, is an acquired taste. Quite a few Japanese people cannot stand its smell and slimy texture. So you might want to try a few commercial packages first.

Q: Can I just take natto bacillus instead of natto for health benefit?
Yes and no. Some research indicates natto bacillus becomes active in our digestive system and produces vitamin K and B12, and possibly some other nutrients and enzymes, too. However it might depend upon what kind of food you eat and what kind of intestinal flora you have. Natto on the other hand is full of nutrients and propagated natto bacillus. Besides, taking natto bacillus itself could be quite expensive.

Disclaimer: As making natto involves food processing, there is a chance of food poisoning. Even with the best care it could occur. The information contained on this web site is accurate to the best of my knowledge. However there is no guarantee to its accuracy. The reported health benefits might not apply to you as well. Please make good use of the information on this web site at your own risk. 

Happy natto making and enjoy your natto!

Kazuo Shiroki

by Kazuo Shiroki Gaia Enterprises Inc. Suite 708, 1155 W. Pender St. Vancouver, B.C., Canada   V6E 2P4 Phone 604-605-7017 E-mail kshiroki@gaia21.net Created on 1998/10/31 Last revised on 2003/07/18

This natto homepage is mainly composed of information available on the internet. I appreciate the efforts of these predecessors who published it. I also received some help from Takahashi Yuzo Research Facility. I would like to thank the staff for their help. Please feel free to copy, use, and distribute this information. If you do so, however, please drop me a line as courtesy. Let's share helpful information through the use of the Internet.