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What Natto Is and Why People Ferment It

Natto is a traditional Japanese food made from soybeans that have been fermented using a specific bacterium called Bacillus subtilis. The fermentation process transforms whole soybeans into a sticky, stringy food with a distinctive strong smell and umami flavor. Natto has been part of Japanese cuisine for over 1,000 years, with historical records showing it was consumed during the Edo period as an affordable protein source for common people.

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The fermentation process breaks down the soybean's complex proteins and carbohydrates, making nutrients more bioavailable for human digestion. During fermentation, the Bacillus subtilis bacteria produce enzymes called proteases and amylases that work on the soybeans' structure. This creates the characteristic sticky texture and the strong ammonia-like smell that many people notice. The process typically takes 18 to 24 hours at the right temperature.

People choose to ferment natto at home for several reasons. Store-bought natto can be expensive, especially for those who consume it regularly. A package of commercial natto typically costs between three to eight dollars and contains only a few ounces. Home fermentation allows people to make larger batches at a fraction of the cost. Additionally, some people prefer making natto themselves because they can control ingredients, avoid additives, and understand exactly how their food was prepared.

Natto contains vitamin K2, which plays a role in bone metabolism and cardiovascular function. Research published in the American Journal of Clinical Nutrition has examined fermented soy products and their nutritional profiles. One study found that natto can contain between 100 to 300 micrograms of vitamin K2 per 100-gram serving, significantly higher than most other food sources. Home fermenters find that making natto allows them to consume this nutrient-dense food regularly without high costs.

Practical Takeaway: Understanding what natto is and why fermentation changes its properties helps you appreciate the purpose of each step in the fermentation process. Knowing that fermentation takes 18 to 24 hours and that the strong smell is a normal part of the process sets realistic expectations for home fermentation.

Understanding the Science Behind Natto Fermentation

The science of natto fermentation centers on creating the right environment for Bacillus subtilis to thrive and transform soybeans. This bacterium is not pathogenic to humans and has been used safely in food production for centuries. The bacterium produces sticky polysaccharides that give natto its characteristic stringy texture. These polysaccharides, primarily composed of polyglutamic acid, are responsible for the viscous substance that strings when you lift natto with chopsticks.

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Temperature control is the most critical factor in successful natto fermentation. Bacillus subtilis grows optimally between 40 to 50 degrees Celsius, which is approximately 104 to 122 degrees Fahrenheit. Most home fermenters maintain temperatures around 40 to 45 degrees Celsius (104 to 113 degrees Fahrenheit) for steady fermentation. If the temperature drops below 35 degrees Celsius or exceeds 55 degrees Celsius, fermentation slows dramatically or stops entirely. Too much heat can kill the bacteria, while too little heat results in very slow or incomplete fermentation.

Moisture content also affects fermentation success. Soybeans should be cooked until soft but not mushy before introducing the Bacillus subtilis culture. The cooked soybeans should maintain enough moisture for bacterial activity but not so much that they become waterlogged. When soybeans are too dry, fermentation proceeds slowly. When they are too wet, mold or unwanted bacteria may grow instead of natto bacteria.

The fermentation environment requires anaerobic or near-anaerobic conditions, meaning low-oxygen environments. Many home fermenters use specialized fermentation boxes that maintain stable temperatures and moderate oxygen levels. Others use coolers with heating pads, thermostats, and breathable coverings. The guide explores various methods of maintaining these conditions using common household items. Some fermenters report success using incubators designed for baking bread or yogurt, which naturally maintain consistent warm temperatures.

Practical Takeaway: Natto fermentation success depends primarily on maintaining temperature between 40 to 45 degrees Celsius for 18 to 24 hours. A reliable thermometer and a heating source that you can control are the most important tools you need before starting fermentation.

Preparing Your Equipment and Creating a Fermentation Setup

Before beginning natto fermentation, you need to gather specific equipment and create a space where temperature can be controlled reliably. The most basic setup requires a heat source, a way to measure temperature accurately, a container for fermentation, and sterilized materials. Most home fermenters spend between thirty to one hundred dollars on initial equipment, depending on whether they already own some items.

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A reliable thermometer is essential. Digital thermometers with probe sensors allow you to monitor temperature continuously without opening your fermentation container repeatedly. These thermostats typically cost between ten to thirty dollars and provide accurate readings within 0.5 degrees. Some fermenters use thermometers designed for aquariums or indoor gardening, which work equally well for natto fermentation. Analog thermometers can work but require you to open your fermentation setup to read them, which disrupts temperature stability.

For heating, many home fermenters use heating pads designed for seedling propagation or reptile enclosures, which cost between twenty to fifty dollars. These pads are placed under or beside fermentation containers and controlled with a separate thermostat. Other options include using an oven with the light on and door slightly open, a dehydrator set to low temperature, or a yogurt maker converted for natto use. The key requirement is that your heat source maintains consistent temperature without significant fluctuations.

Fermentation containers should be breathable but clean. Many fermenters use small glass jars with breathable cloth covers held in place with rubber bands. Others use ceramic dishes covered with sterile cotton gauze. The container should allow minimal air exchange but prevent contamination from dust and airborne microorganisms. Sterilization involves either pressure cooking the containers at 15 PSI for ten minutes or boiling them for twenty minutes. After sterilization, containers must cool completely before use to prevent premature bacterial death when the culture is introduced.

Practical Takeaway: Before purchasing expensive equipment, you may already own suitable items at home. A heating pad, digital thermometer, small glass jars, and a cooler or enclosed space can create a functional fermentation setup for minimal cost. Test your setup by maintaining stable temperature for 24 hours before attempting actual fermentation.

Step-by-Step Natto Fermentation Process

The natto fermentation process begins with selecting high-quality soybeans. Dried soybeans should be stored in cool, dry conditions and used within one year of purchase for best results. Soybeans purchased from bulk bins at natural food stores or Asian markets typically cost between two to four dollars per pound. Before cooking, rinse the dried soybeans thoroughly under running water several times to remove dust and debris.

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Soaking soybeans overnight or for at least six hours softens them and reduces cooking time. Use approximately three parts water to one part soybeans by volume. After soaking, drain the water and cook the soybeans in fresh water until they are completely soft but not falling apart. Cooking typically takes between four to six hours in a regular pot, or about one hour in a pressure cooker. The soybeans should be easily mashable between your fingers but should maintain their individual shape. This stage is critical because undercooked soybeans will not ferment properly, while overcooked ones become mushy and may not develop the proper texture.

Once cooked soybeans cool to room temperature, they are ready for inoculation with Bacillus subtilis culture. Natto spores can be obtained from several sources: online fermentation suppliers, Japanese specialty grocery stores, or by using a small amount of store-bought natto as a starter culture. If using store-bought natto, mix it thoroughly with a small amount of cooled cooked soybeans, then add this mixture to your larger batch. If using dried spores, follow the supplier's instructions for dilution and application. Generally, one teaspoon of starter culture is sufficient for one to two pounds of cooked soybeans.

After inoculation,