How long does natto take to ferment?

By Paulo de VriesLast verified 2026-05-204 sources~3 min readhigh consensus

Natto ferments 22–28 hours at 100–104°F (38–40°C), followed by 24+ hours aging in the fridge. Total: ~2 days from cooked soybeans to ready-to-eat. The bacteria need warmth + humidity.

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The full answer

Natto is Japanese fermented soybeans, distinguished by its sticky stringy "neba-neba" texture and pungent smell. The fermentation uses Bacillus subtilis var. natto, an aerobic bacterium that requires warmer, more humid conditions than most other ferments.

**Standard timing (home batch, ~250g cooked soybeans):** - Step 1: Cook soybeans (pressure cooker 30 min OR boil 4–6 hours): until very soft, squashable between fingers - Step 2: Drain hot, inoculate with natto starter (commercial spores or 1 tsp existing natto) - Step 3: Spread thin (≤1.5cm deep) in glass dish, cover loosely with cloth - Step 4: **Incubate at 100–104°F (38–40°C) for 22–28 hours** - Step 5: Refrigerate 24+ hours (the "aging" — develops full stringiness + flavor) - **Total: ~50 hours from start to ready**

**Why the specific temperature range (100–104°F):** - Below 95°F (35°C): bacteria barely grow; no fermentation - 95–110°F: optimal Bacillus subtilis natto growth - 100–104°F: sweet spot (most home methods) - Above 115°F (46°C): kills the bacteria

**Why 22–28 hours:** - 18 hours: barely fermented, no stringiness - 22 hours: visible white biofilm forms on surface - 24 hours: classic stringiness develops (standard target) - 28 hours: deeper flavor, more ammonia notes - 36+ hours: over-fermented, harsh ammonia, slimy texture

**The "done" test:** - White biofilm covers all beans - When stirred, beans connect with stringy threads (neba) - Smell is pungent but not ammonia-sharp - After fridge-aging, threads become elastic and pull 6+ inches

**Home incubation methods:** - Oven with light on (~100°F): standard method, works for most ovens - Dehydrator at 100°F: precise + reliable - Insulated cooler with warm water bath: low-tech but effective - Yogurt maker (Instant Pot yogurt setting): works at 100–104°F - Sous-vide setup at 100°F in plastic bag: very precise

**Don't:** - Use airtight container during fermentation (Bacillus subtilis is aerobic; needs oxygen) - Skip the fridge-aging (texture and flavor underdevelop) - Pile beans more than 1.5cm deep (uneven temperature + texture) - Use unboiled-enough beans (texture stays hard)

**Storage:** - Refrigerated: 1 week peak flavor, 3+ weeks edible (just becomes more pungent) - Frozen: 3 months without quality loss - Texture stabilizes after Day 2 in fridge

Most published references (Sandor Katz "The Art of Fermentation", Karen Solomon "Cultured Foods for Your Kitchen", Japan Society publications) converge on 24-hour active fermentation + overnight refrigeration as the home-cook standard.

Time ranges by condition

Condition	Duration	Note
Active fermentation at 100–104°F	22–28 hours	—
Refrigerated aging (mandatory)	24+ hours, 48h ideal	—
Total time from start to ready	~50 hours	—
Cool incubator (95°F)	30–36 hours	Less reliable; risk of unwanted bacteria

What changes the time

Temperature. 100–104°F = sweet spot; cooler = slower + more contamination risk; warmer = bacteria die
Soybean variety. Small-bean natto (mini-natto) ferments slightly faster; large-bean (regular) needs full 24h
Starter type. Commercial Bacillus subtilis natto spores (Mitoku, NattoMoto) = reliable; reused natto = sometimes underperforms after 3 generations
Humidity. Cover with cloth lets bacteria breathe but keeps moist; airtight = anaerobic + failed batch

Common questions

Can I make natto without a starter?

You can try with rice straw (traditional method — wild natto bacteria live on it) but reliability is low. Commercial Bacillus subtilis natto spores from Japan-import shops or Amazon are cheap, reliable, and last years dried.

Why does natto smell like ammonia?

Mild ammonia is normal — it's a byproduct of protein breakdown. Strong sharp ammonia means over-fermented (past 30 hours warm). Reduce next batch to 22–24 hours.

Is natto safe if it didn't get stringy?

No — if no stringy biofilm formed, the Bacillus subtilis didn't take. Whatever you have is not properly fermented natto and could contain unwanted bacteria. Discard.

Sources

We cite primary research, expert practice, and authoritative reference. Higher-tier sources weighted heavier. See methodology.

Tier 1 · peer-reviewed / governmentalTier 2 · editorial referenceTier 3 · named practitioner

T3Sandor Katz, "The Art of Fermentation" — Detailed home-natto chapter with troubleshooting + temperature ranges
T2Karen Solomon, "Cultured Foods for Your Kitchen" — Accessible home reference: 24h incubation + overnight age
T2Japan Natto Cooperative Society research papers — Industrial standards: 18-24h at 40°C, then 24h cool-aging
T2William Shurtleff + Akiko Aoyagi, "The Book of Tempeh + The Soyfoods Center" — English-language reference covering related Bacillus subtilis fermentations

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What could change this answer

honest uncertainty

We don’t know your specific equipment, ingredient batch, or environment. This answer is the best directional estimate from cited sources. Here’s where it’s less certain:

4 factors shift the answer. Your specific temperature, soybean variety, starter type (and more) can move this estimate in either direction. The table below shows the documented ranges.
Range spans 4 documented conditions. Pick the row that matches your scenario; halfway between two rows is your best estimate. The widest condition you can imagine is the safer upper bound.
Treat as a directional starting point. Real-world equipment, ingredient batch, and ambient conditions cause natural variance the table cannot capture. Use sensory checks (sight, smell, touch, internal temperature when relevant) alongside this answer — not instead of it.
Backed by 4 cited sources — verify if precision matters. See the Sources section for primary references. Cross-check the cited materials when stakes are high: commercial production, food safety, allergens, dietary or medical considerations.
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de Vries, P. (2026). How long does natto take to ferment?. AskedWell. Retrieved 2026-05-21, from https://askedwell.com/pages/how-long-does/natto-ferment

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Last verified: 2026-05-20 · Published 2026-05-20

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