How to Make Natto
Natto is a traditional Japanese food made from soybeans fermented with Bacillus subtilis bacteria. It is famous for its strong smell, sticky stringy texture, and powerful probiotic and nutritional properties, including being the richest food source of vitamin K2.
Category: Legumes & Soy · Method: Alkaline · Time: 24h · Difficulty: 3/5 · Yield: ~500g · Temp: 38-42°C
History
Natto originated in Japan over 1,000 years ago, possibly discovered when cooked soybeans were stored in rice straw, which naturally contains Bacillus subtilis. It remains a beloved breakfast food in Japan despite being challenging for many non-Japanese palates.
Ingredients
- 500g Dried soybeans — Small to medium sized
- 1/8 tsp Natto starter spores — Bacillus subtilis var. natto
- For soaking and cooking Water — Enough to cover beans
Equipment
- Large pot
- Colander
- Shallow containers (traditional uses rice straw)
- Oven with light, dehydrator, or other warm spot
- Thermometer
- Plastic wrap or breathable covering
- Chopsticks for stirring
Instructions
Cook soybeans
Drain and rinse. Cover with fresh water and bring to boil. Reduce heat and simmer for 3-4 hours until very soft. For best results, pressure cook for 35-45 minutes at high pressure. The beans must be soft enough to be easily crushed between your thumb and pinky finger without resistance.
Time: 45 min - 4 hours
Tip: Pressure cooking is the preferred method as it ensures the core of the bean is softened without the skins becoming waterlogged or falling off.
Sterilize equipment
While beans are cooking, sterilize all containers, spoons, and mixing tools. Boil them in water for 5-10 minutes or use a high-heat steam setting. This is critical because Bacillus subtilis needs a clean environment to outpace wild bacteria.
Time: 10 min
Tip: Even a small amount of contamination can ruin the batch. Do not skip this step.
Inoculate with spores
Drain beans while still hot. Immediately sprinkle natto spores over beans and mix thoroughly with sterilized chopsticks or a spoon. Work quickly to maintain the heat of the beans, which helps activate the spores and prevents contamination.
Time: 5 min
Tip: Spores are heat-resistant and are actually activated by the thermal shock of the hot beans. Ensure all tools touching the beans at this stage have been boiled.
Fermentation Timeline
- Hour 0-8: Initial growth — Little visible change. Spores are germinating.
- Hour 8-16: White film appears — Thin white coating develops on beans.
- Hour 16-22: Sticky threads form — When stirred, beans produce characteristic stringy threads.
- Hour 22-24: Ready — Strong ammonia smell. Sticky, slimy coating. Refrigerate.
Signs of Success & Spoilage
Good signs
- White film coating beans
- Sticky strings when stirred
- Ammonia smell (strong but not putrid)
- Beans are soft but not mushy
- Uniform fermentation across batch
Warning signs
- Colored mold (black, green, pink)
- Rotten or putrid smell
- Slimy dark spots
- No stickiness after 24 hours
- Dry, crusty beans
Troubleshooting
- Not sticky
- Cause: Temperature too low, old spores, or beans cooled before inoculation Solution: Maintain 40°C. Use fresh spores. Inoculate while beans are hot.
- Mold growth
- Cause: Contamination or temperature too low Solution: Discard batch. Sanitize equipment. Maintain proper temperature.
- Too strong ammonia smell
- Cause: Over-fermented Solution: Reduce fermentation time. Refrigerate promptly when ready.
Storage
Refrigerate in airtight containers. Can be frozen for long-term storage. Keeps 1 week refrigerated, months frozen. Freezing preserves quality well. Thaw in refrigerator before eating.
Health Benefits
- Highest food source of vitamin K2 (for bone and heart health)
- Contains nattokinase enzyme (blood clot prevention)
- Complete protein source
- Rich in probiotics
- High in fiber and minerals
- May support cardiovascular health
Serving Suggestions
- Traditional: over hot rice with soy sauce and mustard
- Mixed with raw egg and green onions
- On toast with avocado
- In miso soup
- Sushi rolls (natto maki)
- Fried as crispy topping
Related Legumes & Soy Ferments
Other Alkaline Ferments
- Soybean oil cake — Legumes & Soy
- Hákarl — Fish & Seafood
- Century egg — Eggs
- Dawa-dawa (African locust) — Nuts & Seeds
- Ogiri (melon seed) — Nuts & Seeds