Homemade Natto Starter: Your Complete Guide to Fermenting Natto from Scratch

Making natto at home is one of the most rewarding fermentation projects you can take on, and it all begins with the right starter culture. A natto starter — containing the beneficial bacteria Bacillus subtilis var. natto — is the secret ingredient that transforms plain cooked soybeans into sticky, pungent, nutrient-dense natto. Whether you want to save money, experiment with fermentation, or simply ensure a steady supply of this Japanese superfood, learning to prepare and use your own homemade natto starter is an essential skill.

What Is Natto Starter and Why Does It Matter?

Natto starter is a culture of Bacillus subtilis var. natto, a spore-forming bacterium that ferments cooked soybeans at high temperatures (around 100–110°F / 38–43°C). Unlike many fermentation cultures, B. subtilis is a thermophilic organism, meaning it thrives in warm conditions where most other bacteria cannot compete.

The starter matters for several important reasons:

• Consistency: A reliable starter ensures every batch develops the characteristic sticky strings (neba-neba), savory flavor, and creamy texture.
• Safety: A robust culture outcompetes unwanted microbes, keeping your ferment safe.
• Nutritional value: B. subtilis produces vitamin K2 (MK-7), nattokinase enzymes, and a wealth of probiotics that make natto a functional food powerhouse.
• Cost savings: Commercial natto starter packets can be expensive. Once you have a culture, you can propagate it indefinitely.

Types of Natto Starter You Can Use

There are three main approaches to sourcing your natto starter, each with its own advantages:

1. Store-Bought Fresh Natto (Best for Beginners)

Fresh, refrigerated natto from a Japanese grocery store or Asian market contains live, active B. subtilis spores. This is the most accessible and reliable starting point for most home fermenters.

• Pros: Readily available, proven live culture, free from guesswork
• Cons: Must be refrigerated or fresh (not pasteurized), culture strength can vary by brand

2. Commercial Natto Starter Powder

Freeze-dried starter packets are sold online and in specialty stores. Brands like Mitoku and Bio Natto are well-regarded. These contain a concentrated, standardized dose of spores.

• Pros: Long shelf life, precise dosing, consistent results
• Cons: Ongoing cost, requires purchasing repeatedly

3. Homemade Propagated Starter

Once you have made a successful batch, you can save a portion as your starter for the next batch — essentially creating a perpetual culture.

• Pros: Zero ongoing cost, becomes increasingly adapted to your environment
• Cons: Requires careful storage; spore viability decreases over time if not stored properly

How to Make Homemade Natto Starter from Store-Bought Natto

This method uses fresh store-bought natto to create a concentrated liquid starter you can use for multiple batches.

What You'll Need

• 1 package (40–50g) of fresh, refrigerated natto (check that it contains live cultures — avoid pasteurized)
• 100ml of filtered or boiled-then-cooled water (room temperature)
• A clean glass jar with a lid
• A small fine-mesh strainer or cheesecloth (optional)

Step-by-Step Instructions

Step 1: Check Your Natto Open the natto package and inspect it. It should have visible sticky strings when stirred. Avoid any natto that smells strongly of ammonia (over-fermented) or shows signs of mold.

Step 2: Dilute the Natto Place the entire contents of one natto package into a clean jar. Add 100ml of room-temperature filtered water. Stir vigorously for 1–2 minutes. The water will become slightly cloudy and viscous — this is your liquid starter.

Step 3: Strain (Optional) If you want a cleaner liquid starter without bean solids, strain the mixture through cheesecloth or a fine-mesh strainer into a second clean jar.

Step 4: Use Immediately or Store

• Immediate use: Use 1–2 tablespoons per 500g of cooked soybeans right away.
• Short-term storage: Refrigerate in a sealed jar and use within 5–7 days.
• Long-term storage: Freeze in ice cube trays. Each cube (approximately 1 tablespoon) can be used directly from frozen, as B. subtilis spores survive freezing well.

How to Use Your Natto Starter for Fermentation

Preparing Your Soybeans

1. Soak 500g of dried soybeans in plenty of cold water for 18–24 hours.
2. Drain and rinse, then cook until fully tender — pressure cook for 45 minutes or simmer for 4–5 hours.
3. The beans should crush easily between your fingers with light pressure.

Inoculating the Beans

1. Drain the cooked beans thoroughly and allow them to cool slightly to around 110°F (43°C) — hot to the touch but not scalding.
2. Add 1–2 tablespoons of your liquid starter (or the equivalent of 1 frozen cube) per 500g of beans.
3. Mix gently but thoroughly, making sure the starter coats all the beans evenly.

Fermenting

1. Spread the inoculated beans in a thin layer (no more than 1.5 inches / 4cm deep) in a shallow container. Poke small holes in a layer of plastic wrap placed over the beans to allow airflow.
2. Ferment at 100–110°F (38–43°C) for 22–24 hours. A yogurt maker, Instant Pot on the "yogurt" setting, or a dedicated fermentation chamber works perfectly.
3. After fermentation, you should see a white, powdery coating on the beans and sticky strings when stirred — signs of success.
4. Refrigerate for at least 24 hours before eating to allow flavors to develop and mellow.

Saving and Propagating Your Starter

Once your batch is successful, set aside 2–3 tablespoons of freshly made natto before refrigerating the rest. This becomes your starter for the next batch. Follow the same dilution method above, or freeze it in portions.

Tips for maintaining a strong culture:

• Always use your starter within 3 months if frozen
• Refresh the culture by making a new batch every 4–6 weeks
• Label your jars with the date so you always use the freshest starter
• Store frozen starters in airtight containers to prevent freezer burn

Frequently Asked Questions

Can I use natto that has been in my fridge for a few weeks? It depends. B. subtilis spores are hardy, but if the natto has developed a strong ammonia smell or unusual texture, the culture may be weakened or contaminated. For best results, use the freshest natto available. If in doubt, pick up a fresh package to start your culture.

Why didn't my natto develop sticky strings? This is the most common sign that fermentation was incomplete. The most likely culprits are: fermentation temperature too low, fermentation time too short, starter culture that was too old or weak, or beans that were too wet or too dry. Check your temperature setup with a reliable thermometer and try again with fresh starter.

How much starter do I need per batch? A general guideline is 1–2 tablespoons of liquid starter (or the equivalent of one ice cube portion) per 500g of dry soybeans. Using more starter does not significantly improve results and can occasionally create off-flavors.

Is homemade natto starter safe to eat? Yes. Bacillus subtilis var. natto has a long history of safe consumption in Japan and is classified as GRAS (Generally Recognized as Safe) by food safety authorities. As with all fermentation, cleanliness of equipment and proper temperature control are your best tools for a safe, successful ferment.