Natto Recipe: How to Make Japanese Fermented Soybeans at Home

Let’s be direct about natto: it’s one of the most polarizing foods in the world. The sticky strings that pull between chopsticks when you stir it, the pungent ammonia-tinged smell, the flavor that hits you in places you didn’t expect — natto is not for the uncommitted. It has been said, with some accuracy, that it takes Japanese children years to acquire the taste, and that most people outside Japan never do. And yet.

Natto is also one of the most research-backed health foods in existence. Nattokinase — an enzyme unique to natto — has been studied extensively for cardiovascular benefits, with multiple clinical trials showing it dissolves blood clots, reduces arterial stiffness, and lowers blood pressure. Natto contains more vitamin K2 (specifically MK-7) than virtually any other food. The Bacillus subtilis var. natto cultures in it are among the most robust probiotic organisms studied. If you can develop the taste — and you can, given time and the right preparation — you have access to one of Japan’s great functional foods.

This guide covers everything: the fermentation science, how to make natto at home from scratch (it’s more achievable than you think), how to develop a palate for it, and the multiple ways to serve it so it’s actually delicious rather than just challenging. The homemade version is significantly better than most commercially imported natto — fresher, more active, more alive.

Natto in Japanese Food History

The origin stories for natto are multiple and somewhat contradictory, as origin stories tend to be. The most widely repeated: Minamoto no Yoshiie, an 11th-century samurai general, boiled soybeans to feed his horses during a campaign, bundled the leftovers in rice straw, and discovered several days later that the straw’s natural Bacillus subtilis bacteria had transformed the beans into something entirely new — sticky, fermented, nutritious. His soldiers ate it anyway and noted its benefits.

The rice straw element is critical. Bacillus subtilis — the bacteria responsible for natto fermentation — lives naturally on rice straw. Traditional natto was made by packing cooked soybeans into rice straw bundles, which inoculated the beans with naturally occurring B. subtilis. In Japan, you can still find “wara natto” (straw natto) from artisan producers in Ibaraki Prefecture. The straw-wrapped version is considered the gold standard by natto connoisseurs and has a more complex, slightly earthier flavor than modern natto made with pure starter cultures.

Industrial natto production began in earnest in the early 20th century after researchers at Tokyo Agricultural University isolated pure Bacillus subtilis var. natto starter cultures, making consistent production possible without straw. Today, Japan produces approximately 200,000 tons of natto annually, consumed primarily at breakfast — the iconic small polystyrene container with its mustard and tare sauce packet is a fixture of Japanese convenience stores and home refrigerators alike.

Natto consumption correlates strongly with geography within Japan. The Kanto region (Tokyo and surrounding areas) consumes dramatically more natto than the Kansai region (Osaka, Kyoto). Osaka food culture famously dislikes natto, and the regional preference divide has become a cultural shorthand — similar to how Americans might invoke regional food divisions as a form of identity.

The health research community became seriously interested in natto in the 1980s when Dr. Hiroyuki Sumi at the University of Chicago Medical School discovered nattokinase — a fibrinolytic (clot-dissolving) enzyme unique to natto — by literally spilling natto onto a fibrin test plate and watching it dissolve faster than any known enzyme. That accidental discovery launched decades of cardiovascular research and drove significant global interest in natto as a functional food.

The Science Behind Natto’s Extraordinary Health Properties

Natto’s health credentials are unusually well-supported for a traditional fermented food. Let me break down the key evidence.

Nattokinase and cardiovascular health: Nattokinase (NK) is a serine protease enzyme produced by B. subtilis var. natto during fermentation — it doesn’t exist in the raw soybeans and is not found in any other food. Multiple randomized controlled trials have examined NK’s effects: a 2018 study in Scientific Reports (Chen et al.) found that nattokinase supplementation significantly reduced carotid artery plaque volume over 26 weeks. A 2022 meta-analysis in Hypertension Research (Ren et al.) found NK supplementation reduced both systolic and diastolic blood pressure. NK works by directly dissolving fibrin (the protein that forms blood clots) and by inhibiting plasminogen activator inhibitor-1 (PAI-1), a key clot-formation factor.

Vitamin K2 (MK-7): Natto is the richest dietary source of menaquinone-7 (MK-7), a long-chain form of vitamin K2. A single 100g serving contains approximately 1,000 µg of MK-7 — compared to roughly 10–50 µg in aged cheeses and 0–5 µg in most other foods. MK-7 has the longest half-life of any vitamin K form (72 hours versus 1–2 hours for K1), making it particularly effective for calcium regulation in the body. Research published in Osteoporosis International (Knapen et al., 2013) found that MK-7 supplementation equivalent to that in a 100g natto serving significantly improved bone strength in postmenopausal women over 3 years.

Probiotic properties: Bacillus subtilis var. natto is a spore-forming bacteria, which makes it unusually resilient — unlike lactobacilli, it can survive stomach acid in spore form, germinating in the intestine where it exerts its effects. Research published in the World Journal of Gastroenterology (Cutting et al., 2011) reviewed evidence for B. subtilis as a probiotic, finding strong data for immune modulation and competitive exclusion of pathogenic bacteria.

Protein and isoflavones: The fermentation process partially breaks down soybean proteins into more digestible peptides, improving protein bioavailability compared to raw or simply cooked soybeans. Soy isoflavones in natto — particularly genistein and daidzein — are also transformed by the fermentation into more bioavailable forms, with specific forms showing stronger estrogenic activity that may benefit bone density and cardiovascular health in postmenopausal women.

One honest caveat: natto’s high vitamin K2 content means people on warfarin (Coumadin) or other vitamin K-sensitive anticoagulants should avoid it or discuss it with their physician, as it can significantly affect anticoagulant dosing requirements.

What You Need

Ingredients:

• 2 cups (400g) dried whole soybeans — use whole, not split. Japanese soybeans (daizu) are ideal if available; regular yellow soybeans from any grocery work fine. Organic preferred, since pesticide residues may inhibit B. subtilis.
• 1/4 teaspoon natto starter spores (Bacillus subtilis var. natto) — sold as powder in small packets from Japanese grocery stores and online fermentation suppliers (South River Miso Company, Cultures for Health, Amazon). A single packet typically makes multiple batches.
• 2 tablespoons filtered water — for dissolving the starter

Equipment:

• Pressure cooker or large pot — for cooking soybeans
• Fermentation container — shallow trays or small glass/ceramic dishes that allow thin layers of beans. Thin layers (1–2 inches) are critical for even fermentation.
• Controlled-temperature environment: 104–109°F (40–43°C) for 22–24 hours. Options: yogurt maker, Instant Pot on yogurt setting, food dehydrator, oven with just the light on (check temperature first), or an insulated box with a heat mat. A stable temperature is critical.
• Cheesecloth or damp cloth — to cover trays during fermentation, preventing surface drying while allowing gas exchange
• Thermometer

How to Make Natto: Step-by-Step

Day Before: Soak the Soybeans (12–18 hours)

Pick through the soybeans and discard any damaged or shriveled ones. Rinse thoroughly, then cover with 3–4 times their volume in cold water. The beans will expand dramatically — expect them to roughly double in size. Soak 12–18 hours in the refrigerator to slow fermentation of any naturally occurring bacteria on the bean surface.

Cook the Soybeans (30 minutes pressure / 3–4 hours simmering)

Drain and rinse the soaked beans. Cook until very soft — you should be able to crush them easily between thumb and forefinger with almost no resistance. Undercooked beans produce inferior natto with less stringiness. Options:

• Pressure cooker: 30–40 minutes at high pressure. Easiest and most consistent.
• Stovetop: 3–4 hours, keeping at a gentle simmer and adding water as needed. Time-consuming but no special equipment needed.
• Instant Pot (bean/chili setting): 40–45 minutes. Works reliably.

Once cooked, drain immediately and work quickly — you want to inoculate the beans while they’re still hot (around 140–160°F / 60–70°C). The heat helps kill competing bacteria on the bean surface.

Inoculate the Beans

Dissolve 1/4 teaspoon natto starter spores in 2 tablespoons of filtered water. Pour over the hot, drained beans and toss thoroughly to distribute the starter evenly. Every bean should be coated.

Work quickly and cleanly. This is the step where contamination is most likely — everything that touches the beans from here on should be very clean or sterile. B. subtilis is competitive and robust, but giving it a clean environment at the start produces more consistent results.

Pack into Fermentation Containers

Transfer the inoculated beans into shallow trays or dishes. The depth matters: keep the bean layer to 1–2 inches maximum. Thicker layers result in uneven fermentation — the bottom beans can over-ferment or turn anaerobic while the top layer is still fermenting.

Cover loosely with damp cheesecloth or a clean damp towel. This maintains humidity (preventing the surface from drying out) while allowing the fermentation gases to escape. Natto fermentation is aerobic — B. subtilis needs some oxygen to produce its enzymes.

Ferment at 104–109°F for 22–24 Hours

Place in your temperature-controlled environment and leave undisturbed. The ideal temperature range is 104–109°F (40–43°C). Too cold (below 95°F/35°C) and fermentation stalls. Too hot (above 115°F/46°C) and the bacteria die and you get cooked, not fermented, soybeans.

Between hours 12–16, you should see the first signs: a white, powdery or slightly fuzzy coating beginning to form on the beans — this is the B. subtilis colony establishing itself. By hour 20, the beans should have a visible white coating and you should be able to detect the distinctive natto smell (earthy, ammonia-tinged, and surprisingly complex). By hour 22–24, the beans should pull into characteristic sticky strings when you stir them with chopsticks or a fork. This stringing is caused by the poly-glutamic acid produced by the bacteria — it’s the visual hallmark of successful natto.

Refrigerate and Age (24 Hours)

This step surprises many new natto makers: the fermentation is not done when you remove the beans from the warm environment. Transfer to the refrigerator and let them cold-age for at least 24 hours, and ideally 48–72 hours. During this stage, the flavor mellows and deepens considerably, the enzymes continue working at a slower rate, and the overall character of the natto develops from harsh and sharp to rounded and complex.

Fresh (not yet aged) natto is noticeably harsher and more one-dimensional than properly aged natto. This cold-aging step is not optional if you want good results.

How to Eat Natto: Getting Past the First Impression

The most important advice for anyone new to natto: don’t eat it plain for the first time. The classic Japanese preparation softens the impact considerably and is genuinely delicious.

Natto over rice (Natto gohan): The standard breakfast preparation. Place one container or 100g of natto in a small bowl. Add the included mustard (or wasabi) and tare sauce. Stir vigorously — 30–50 times, creating maximum stringiness, which aerates the natto and mellows the flavor. Add a raw egg yolk if desired. Pour over a bowl of hot rice. Top with chopped green onions, drizzle with soy sauce. The rice absorbs the natto, the egg yolk rounds out the flavor, and the green onions provide freshness. This is the definitive preparation and a good first experience.

Natto maki (natto sushi rolls): Natto rolled with rice and nori, often with green onion and cucumber. The nori and rice temper the intensity significantly. This is how many Japanese children first encounter natto — in a form that feels less confrontational.

Natto on avocado toast: A genuinely excellent modern preparation. Spread ripe avocado on toast, top with stirred natto (mixed with a little soy sauce and grated ginger), finish with sesame seeds and chili flakes. The fat in the avocado softens the natto flavor dramatically. I’ve converted several natto skeptics with this preparation.

Natto pasta: Toss cooked pasta with natto, butter, soy sauce, and a raw egg. The heat from the pasta warms but doesn’t cook the natto, and the result resembles a savory, umami-rich carbonara with fermented depth.

Natto in miso soup: Stir a small amount of natto into miso soup just before serving. The natto distributes through the soup, softening its impact while adding protein and fermented complexity.

Developing a Palate for Natto

Unpopular opinion: natto is an acquired taste, and acquisition takes intentional effort. Here’s how to develop it strategically rather than forcing yourself through unpleasant experiences.

Week 1: Start with natto maki from a Japanese restaurant, or natto stirred thoroughly into miso soup. The goal is exposure with buffers.

Week 2: Natto gohan with all the traditional condiments — mustard, tare, raw egg yolk, rice, green onion. This is the definitive preparation for a reason.

Week 3: Natto on avocado toast. Notice whether the flavor seems more interesting now than it did initially.

The key insight: The brain’s response to natto changes with repeated exposure. Initial reactions are often dominated by the smell and the unusual texture. By weeks 3–4 of regular consumption, the flavor complexity — the deep earthiness, the savory depth, the underlying nuttiness — becomes more prominent. The same process happens with blue cheese, Vegemite, and virtually every other strongly flavored traditional food. You’re not overcoming disgust; you’re training pattern recognition.

Common Mistakes to Avoid

Beans not soft enough. Undercooked soybeans don’t ferment properly. The B. subtilis enzymes need soft cell walls to penetrate. If your beans aren’t crush-soft when you bite them, cook longer.

Temperature too variable. B. subtilis var. natto is less temperature-tolerant than many fermentation organisms. Swings below 95°F or above 115°F during the 24-hour window produce poor results. Invest in a thermometer and check your setup before you start.

Layers too thick. A 4-inch layer of beans will ferment unevenly — outer beans over-ferment while inner beans stall. Keep layers at 1–2 inches maximum.

Skipping the refrigerator aging. Natto taken straight from the fermentation environment is harsh and unpleasant. The 24–48 hour cold aging is not optional for good flavor.

Contaminated equipment. B. subtilis is robust but can be overwhelmed by competing mold if equipment has residual organic material. Clean everything thoroughly and use very hot water or brief oven sterilization for equipment that will contact the beans directly.

Troubleshooting Natto

No white coating after 24 hours

Check temperature — most likely it dropped below 95°F (35°C). Also verify that your starter culture was stored properly (usually frozen) and is still viable. Return to a warmer environment for another 6–8 hours and check again. If still nothing, the starter may be dead; source fresh culture and start over.

Black or green mold spots

Discard the batch. Mold contamination in natto happens when competing bacteria overwhelm the B. subtilis colony, usually from contaminated equipment or undercooked beans (mold can survive undercooking). A white powdery coating is normal B. subtilis growth; colored mold is not.

Very strong ammonia smell

A slight ammonia note is normal and characteristic of natto. A very strong, overwhelming ammonia smell indicates over-fermentation — the beans fermented too long at too high a temperature. The natto may still be edible (taste it — if it tastes aggressively harsh rather than earthy-complex, it has over-fermented), but reduce fermentation time by 2–3 hours next batch.

No stringiness when stirred

Insufficient fermentation. Poly-glutamic acid (the compound creating the strings) is produced in the latter stages of fermentation — if strings haven’t formed by hour 22, return to the fermentation environment for 4–6 more hours. Also check that the starter culture was correctly distributed over all beans.

Storing and Freezing Natto

Finished natto keeps in the refrigerator for 1 week; flavor continues to develop during this time and is often at its best at day 3–5. Beyond a week, the flavor becomes stronger and increasingly ammonia-forward — still safe to eat but less pleasant.

For longer storage, natto freezes excellently. Pack in small portions (100g per container works well — that’s a typical serving), freeze immediately after the cold-aging period, and use within 3 months. Thaw in the refrigerator overnight. Frozen natto loses a bit of its fresh springiness but retains its nattokinase activity (the enzyme is freeze-stable) and all nutritional properties. Many natto enthusiasts make large batches and freeze most of them.

Frequently Asked Questions About Natto

What is natto?

Natto is a traditional Japanese fermented soybean food made by inoculating cooked soybeans with Bacillus subtilis var. natto and fermenting at 104–109°F (40–43°C) for 22–24 hours. The bacteria produce a sticky, string-forming coating and enzymes including nattokinase, making natto uniquely rich in vitamin K2 and cardiovascular-beneficial compounds.

Is natto healthy?

Exceptionally so, with one caveat. Natto is the world’s richest source of vitamin K2 (MK-7), contains nattokinase (a documented fibrinolytic enzyme), and delivers robust spore-forming probiotics. The caveat: people taking warfarin or other vitamin K-sensitive anticoagulants must avoid natto or consult their physician, as even a single serving significantly affects anticoagulant dosing.

What does natto taste like?

Intensely savory and earthy, with a strong umami depth, mild bitterness, and a distinctive ammonia-tinged fermented note that some find unpleasant and others find complex. The texture — sticky, slimy, string-forming — is as distinctive as the flavor. Natto tastes unlike anything else; the closest analogy might be a very pungent aged cheese, but even that understates how unique it is.

How long does natto last?

Refrigerated, 1 week. Frozen (in airtight containers), up to 3 months. Nattokinase enzyme activity is preserved through freezing, making frozen natto nutritionally equivalent to fresh. Thaw overnight in the refrigerator before use.

Can I make natto without special starter?

Traditional wara natto was made using rice straw, which harbors wild B. subtilis. If you can source clean, pesticide-free rice straw, you can attempt this — soak straw in boiling water briefly to reduce competing organisms, then bundle cooked soybeans in the straw and ferment at 104°F (40°C) for 24 hours. Results are less consistent than pure culture but produce more complex flavor. For reliable results, use commercial starter.

Why does natto smell like that?

The distinctive ammonia-like note in natto comes from pyrazines and other nitrogen-containing compounds produced by B. subtilis during protein breakdown. This is the same class of compounds responsible for the sharp notes in aged hard cheeses and certain fermented meats. The smell is strongest when natto is first opened; it mellows on stirring and is much less prominent when natto is incorporated into food.

Is natto vegan?

Yes. Traditional natto contains only soybeans and the bacterial starter culture. The tare sauce and mustard included in commercial packages are typically vegan as well, though check labels if this is important to you. Natto is an excellent high-protein, high-K2 option for plant-based diets.

Ready to Start

Natto is the one fermented food where the health case is strong enough that it’s worth developing the taste for, even if your initial reaction is skepticism. The nattokinase research is real and well-replicated. The K2 content is genuinely remarkable. And the fermentation process, while requiring attention to temperature, is fundamentally simple.

Order a packet of starter culture, soak your soybeans tonight, and give it 48 hours. Your first batch may not convert you immediately — that’s fine. Make it again. Try it with avocado toast. Keep going. By batch three or four, something usually clicks, and you’ll understand why 10 million Japanese people eat this for breakfast every morning.