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What is natto?

In case you’ve never heard of it, natto is a traditional Japanese food made from fermented soybeans. It’s characterized by its strong, pungent smell, and sticky, stringy texture.

Natto is not typically eaten as a standalone food. Rather, it’s usually seasoned with sauces like ponzu, soy sauce, or mustard, then served atop rice with other seasoned or fermented veggies. Some people love natto, some people find it less than agreeable. People who grow up eating it (like the Japanese) tend to love it.

Natto is traditionally made from soybeans, but keep reading to find out how to make your own natto from chestnuts! This batch of natto chestnuts is flavored with ponzu sauce, then mixed with thin-sliced kumquats and garnished with green onions and wood sorrel leaves & flowers.

How do you make natto?

The natto fermentation process is carried out by a specific strain of the bacteria Bacillus subtilis (formerly named Bacillus natto). Dried soybeans are first soaked, then boiled to create an ideal, sterile medium before being inoculated with a natto culture. From there, the soybeans are maintained at a temperature around 105°F (40.5°C) for 24 hours. Finally, the natto is refrigerated for 24-48 hours before being eaten.

Health benefits of natto

Natto fermentation significantly improves the health benefits of soybeans. First, it enhances their digestibility by breaking down their anti-nutrients and making the proteins and minerals in soybeans easier for the body to absorb. Additionally, natto fermentation actually increases some of the key macro and micronutrients. A 2023 study found that natto “contains more proteins, cellulose, calcium, iron, potassium and vitamin B2” than the base soybeans they’re made from.

The natto fermentation process also creates new beneficial compounds such as vitamin K2 and other bioactive compounds like nattokinase. The specific type of K2 in natto is menaquinone-7, which is also present in other foods such as hard cheeses.

Menaquinone-7 is a fat-soluble nutrient essential for immune function, bone and cardiovascular health, brain function, and cancer prevention. Interestingly, natto contains 100 times more menaquinone-7 than most cheeses!

Partly for the reasons above, natto is considered one of the secrets to the incredible health and longevity of Japanese people, who regularly consume it. In other words, you need natto in your life!

Making natto from chestnuts

In trying to find interesting fermentation methods for our home-grown chestnuts, I stumbled on a 2020 study from China published in the journal Food Research International titled, “Development of fermented chestnut with Bacillus natto: functional and sensory properties.” The study authors provide a biochemical analysis of the natto chestnuts, which includes the following information:

…the fibrinolytic activity [enhanced blood circulation] of the natto-chestnut reached 6479 IU/g. Meanwhile, higher antioxidant activity of the natto-chestnut was obtained due to the increased contents of total phenolic, total flavonoid and VC [vitamin C]. In addition, α-glucosidase inhibition activity was also improved in the natto-chestnut. These results indicated that fermented chestnut could be a new dietary supplement with higher quality and better activities for people′s health.

Thus, we learned that making natto chestnuts was not only possible, but produced great culinary and health-promoting results.

We immediately ordered Nattomoto brand natto spores and began experimenting on our chestnuts. Below, we share our final recipe and process with you!

How to make natto chestnuts from fresh chestnuts: steps and process photos

Step 1: Make sure you have what you need

Here’s what you’ll need to make natto chestnuts:

dehydrator, yogurt maker, or some other device that can maintain a temperature of ~105°F / 40.5°C (we use an Excalibur dehydrator)
100 grams of chestnuts if using a food scale or about 1.5 cups when measured with de-shelled chestnuts cut in half (this isn’t baking – you don’t have to be super-precise on these measurements)
Nattomoto brand natto spores
*glass container with airtight lid – we use a 5 1/2″ x 8″ storage container (*Why glass? We don’t recommend cooking or fermenting in plastic containers due to the risk of harmful compounds leaching into your food.)

Step 2: Prep your chestnuts

Carefully cut your chestnuts in half using a chef’s knife. Discard any discolored or weevil-damaged chestnuts.

Next, place your cut chestnuts a single-row deep on a sheet pan. Bake at 350°F (177°C) for 40 minutes.

Remove pan from the oven and let the chestnuts cool to room temp. Once cooled, you’ll be able to easily remove the nuts from the shells and testa (the papery inner coating).

Now, cut the halved chestnuts again so you end up with pieces about the size of large soybeans (roughly equal to 1/8th of a chestnut). The point of this step is to make the nuts smaller and increase the surface area for microbial inoculation.

Here’s our chestnuts cut to the desired size.

Step 3: Boil and sterilize the chestnuts

Bring a pot of water to boil. Place chestnut pieces into a strainer. Dip chestnuts into the boiling water for about 75 seconds, then remove.

Boiling the cut chestnuts to sterilize them prior to natto inoculation.

The point of this step is to:

sterilize the chestnuts so the natto inoculant has minimal microbial competition, and
add extra moisture to the chestnuts prior to inoculation.

Step 4: Transfer chestnut pieces to clean, sterilized glass container.

Using clean utensils washed in hot soapy water (not your hands), transfer the chestnuts into a sterilized glass container. The chestnuts should be touching, but not smashed together. They should also be no more than 1/2″ deep in the container in order to allow for optimal inoculation.

Don’t pile your chestnuts too deep or smash them into a single blob.

Step 5: inoculate your chestnuts with natto spores

Nattomoto brand natto spores do NOT come with English instructions, so we had to gather instructional information from as many other reliable sources as possible, plus do our own experimentation.

With soybean natto, the soybean-to-inoculant ratios are: 100gm soybeans to 1cc natto starter culture. (For reference, one of the small spoons provided in the Nattomoto inoculant kit is 1cc.) With chestnuts, we use more natto spores than recommended. Per 100gm of our natto chestnuts, we use 2 cc (two scoops from the provided spoon). As mentioned earlier, 100 grams of chestnuts is about 1.5 cups measured cut in half with no shells.

Many people say that natto spores need a thermal shock to reactivate properly. Thus, add the natto culture to a small sterilized bowl with about 1 tablespoon of very warm (not hot) water.

Mixing the natto spores into warm water to reactivate them. — Mixing the natto spores into warm water to wake them up.

Stir the natto powder into the warm water with a sterilized spoon, then pour the liquid over warm (not hot) chestnuts. Stir the water-natto mixture into the chestnuts to evenly coat the nut surfaces.

Step 6: warm and wait, then refrigerate.

Immediately cover the inoculated chestnuts with an airtight lid. Place them in a dehydrator or yogurt maker set to 105°F / 40.5°C for 24 hours.

Natto chestnuts won’t taste or smell great when you first open the container after 24 hours, nor will the characteristic natto “strings” form very robustly when you lift the nuts with a spoon or fork. Ergo, they’re not ready to use yet!

Instead, put the container in your fridge for at least 24-48 hours before using. After about a week in the fridge, they’ll be quite slimy and stringy (yes, this is a good thing when it comes to natto!).

Use them within ~14 days.

Natto chestnut strings after 12 days in the fridge. Stir them prior to eating to get them extra stringy!

Serving notes:

Chestnut natto is not ideal eaten by itself. Instead, think of it as a primary ingredient. As the pictures at the top of this article show, our recommendation is to toss them with ponzu and pieces of bright-flavored citrus (satsumas, sliced kumquats, Australian blood limes, etc.) then top them with green onions and other seasonal garnishes.

Natto chestnut recipe:

Natto chestnuts (fermented chestnuts)

Course: Dinner, lunch, Side Dish

Cuisine: Chinese, Japanese

Keyword: fermented chestnuts, natto, natto chestnuts

Prep Time: 1

Fermentation time: 1

Servings:

Author: Susan von Frank

An Asian-inspired natto fermentation utilizing chestnuts instead of soybeans.

Print Recipe

Equipment

dehydrator or yogurt maker (or other device that can maintain temperature of 105F for 24 hours)
small glass container with lid (For reference, we use a 5 1/2" x 8" glass storage container)

Ingredients

100 grams chestnuts, weighed with shells removed (approximately 1.5 cups measured when nuts cut in half)
2 cc Nattomato brand natto spores (2 scoops using spoon provided by Nattomoto company)
1 tbsp warm water to reactivate spores

Instructions

Carefully cut chestnuts in half using a chef's knife. Bake at 350°F (177°C) for 40 minutes. Remove from oven and let cool.
Once cooled, remove nuts from shells and testa (papery inner coating). Cut the halved nuts until you end up with chestnut pieces about the size of large soybeans.
Bring pot of water to boil. Place quartered chestnuts into strainer and dip into boiling water for about 75 seconds, then remove.
Using clean utensils (not your hands) transfer chestnuts into sterilized glass or metal container(s). Chestnuts should be touching, but no more than 1/2" deep in container.
Add the natto culture to a small bowl with tablespoon of very warm (not hot) water. Stir natto powder into water with sterilized spoon then pour over warm (not hot) chestnuts. Stir water-natto mixture into chestnuts to evenly coat the nut surfaces. Immediately cover chestnuts with an airtight lid and place in dehydrator or yogurt maker set to 105°F (40.5°C) for 24 hours.
Do not eat natto chestnuts immediately after fermentation. They need to develop. Instead, put the container(s) in your fridge for at least 24-48 hours before consuming. Keep refrigerated and use within ~10-14 days.
Serving tips: toss in soy or ponzu sauce, then add cut citrus + garnish of green onions or chives.

Now you have yet another way to use your chestnuts — enjoy!

KIGI,