What Is Natto? The Japanese Fermented Soybean That Divides the World

Natto (納豆) is the food that more than any other divides Japan itself. Japanese people from western Japan often find it repulsive. People from eastern Japan and Tokyo eat it for breakfast every day. Outside Japan, it's either a revelation or the most challenging thing you'll encounter in Japanese cuisine.

It is fermented soybeans. It is sticky — aggressively, dramatically, with strings that stretch several inches when you lift the beans. It has a strong smell, variously described as ammonia, old gym socks, or aged cheese. The flavor is savory, nutty, slightly bitter, and deeply umami.

It is also one of the most nutritionally concentrated foods in the world.

How Natto Is Made

Natto is made from whole soybeans inoculated with Bacillus subtilis var. natto — a bacterium that ferments the beans, breaks down their proteins, and produces the characteristic stickiness (poly-glutamic acid — a natural polymer that creates the stringy texture) and the enzyme nattokinase (which has attracted significant research interest for cardiovascular health).

The process: Soybeans are soaked overnight, steamed, inoculated with the Bacillus subtilis culture, and packed into small containers — traditionally rice straw, which naturally harbors the bacteria; modern production uses polystyrene cups. The beans ferment at 40°C (104°F) for 17-24 hours, then rest in cold storage for another day to develop flavor.

The result: sticky, slightly darkened soybeans in a web of white stringy material (the poly-glutamic acid), with a pungent aroma.

What Natto Tastes Like

Natto tastes like concentrated soybean with fermented depth. The flavor is:

• Savory — high in glutamates and free amino acids from protein breakdown during fermentation
• Nutty — from the cooked soybean base
• Slightly bitter — from compounds that develop during fermentation
• Funky — from the ammonia compounds that account for the distinctive smell
• Umami-forward — perhaps the highest naturally occurring umami of any whole food

The texture is the most challenging element for newcomers. The strings form between any two surfaces the natto touches. Eating natto means accepting the strings — either learning to control them with chopsticks or leaning into the chaos.

How Japanese People Eat Natto

Standard breakfast preparation:

1. Open the natto package
2. Add the packets of soy sauce and karashi (Japanese hot mustard) that come inside
3. Stir vigorously — this is important. More stirring (50-100 times) develops the flavor and distributes the seasoning
4. Pour over hot white rice
5. Add optional toppings: sliced green onion, a raw egg yolk, chopped kimchi, or a drizzle of sesame oil

The stirring is not optional. Natto stirred 50 times has a different flavor than natto stirred 10 times — more concentrated, more savory, the strings more fully integrated.

Other serving methods:

• In miso soup (dissolved into the broth, invisible but adding depth)
• In okonomiyaki (savory pancake) batter
• On toast (a modern preparation — natto on buttered toast is more common than it sounds)
• In onigiri filling (tightly packed into rice balls)
• In sushi (natto rolls are common at kaitenzushi conveyor belt restaurants)

The Nutritional Profile

Natto is nutritionally exceptional in several ways:

Protein: 100g of natto provides 18g of complete protein — all essential amino acids, from a plant source.

Vitamin K2 (MK-7): Natto is the single highest dietary source of vitamin K2 menaquinone-7 in the world. K2-MK7 supports bone density (directs calcium into bones rather than arteries) and cardiovascular health. A single serving of natto provides 10-20× the daily recommended K2 intake.

Nattokinase: The enzyme produced during natto fermentation has been studied for its ability to break down fibrin (a protein involved in blood clot formation). Research is ongoing, but nattokinase supplements have become popular in wellness communities.

Probiotics: The Bacillus subtilis bacteria in natto survive digestion and colonize the gut, making natto one of the more effective probiotic foods.

Iron and calcium: Meaningful amounts of both, which are often deficient in plant-heavy diets.

Why Eastern Japan Eats It and Western Japan Often Doesn't

The regional divide is genuine. In Tokyo, Tohoku, and the Kanto region, natto is eaten daily — it's the cheap, convenient, nutritious breakfast that sustained Japan through periods of food scarcity. Production and consumption historically clustered in eastern Japan where the cooler winters are better for the fermentation process.

In Osaka and western Japan (Kansai), the culture developed differently — better access to fresh fish, different preservation traditions, different fermented foods (Kyoto's pickles, various miso styles). Natto arrived later and never embedded in the same way.

Acquiring the Taste

For people who find natto challenging, the path usually goes like this:

1. Mix in strong flavors first. Kimchi, strong mustard, raw garlic, and enough soy sauce mask the funkier notes while you acclimate.

2. Start with hikiwari natto. Finely ground natto with a more subdued flavor and less dramatic strings than whole bean natto.

3. Use it as an ingredient rather than eating it straight. Natto dissolved into miso soup adds umami depth without the full sensory impact.

4. Give yourself three sessions. The first time is about the shock. The second is about noticing the flavor under the funk. The third is usually when it starts to make sense.

Most people who persist with natto eventually understand what Japan sees in it. It is genuinely one of the most flavorful and nutritious whole foods in existence. The smell is real. But so is the reward.

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Related reading: What Is Doenjang? | How to Make Miso | How to Make Kimchi