How Is Miso Made? From Soybeans to Fermented Miso Paste

Miso is a humble yet flavor-packed soybean paste that’s a cornerstone of Japanese food culture. It’s revered for its rich umami taste and incredible versatility, but how is miso made?

Miso is more than just a flavorful seasoning—it’s a tradition passed down through generations for over a thousand years. Thought to have been introduced from China by Buddhist monks in the 6th or 7^th century, it gradually took root in Japan, evolving into a wide range of regional styles. At its heart, though, traditional miso is made with just four ingredients: soybeans, salt, water, and koji. These humble elements are transformed through fermentation into the aromatic, deeply flavorful paste that’s become a staple of Japanese cooking.

How Miso Is Made

Whether it’s crafted in a rural country farmhouse or at our modern facility nestled in the mountains of Nagano, the process is grounded in the same time-honored principles: prepare quality ingredients and let nature work its magic. 

Ingredient ratios and fermentation times can vary to create different varieties of miso, but the production of miso follows these basic steps:

Selecting and Preparing Soybeans

With so few ingredients in traditional miso, the quality of each component influences the finished product. At Hikari Miso, we use only high-quality, non-GMO soybeans. Once the dried beans arrive, they’re carefully sorted and cleaned to remove impurities. Because the proteins in soybean skins can influence the final color of the miso, the skins may be milled off, depending on the type of miso being made.

Soaking and Steaming

The cleaned soybeans are then soaked in soft water to rehydrate them gradually. This step helps make their starches and proteins more accessible to the koji enzymes during fermentation. Once fully hydrated, the beans are steamed until tender. While many miso producers rely on continuous steamers for speed, we use a pressurized batch steamer. This approach allows us to fine-tune the cooking process, allowing us to produce over 40 distinct miso varieties—each with its own unique color, texture, flavor, and aroma. The steamed soybeans are cooled and mashed into a coarse paste, ready for mixing with koji.

Cultivating Koji

If soybeans form the body of miso, then koji is its soul. Koji is made by inoculating steamed grains such as white rice or barley with Aspergillus oryzae, a filamentous fungus revered in Japanese food culture for its power to break down starches and proteins into simple sugars and amino acids (such as glutamic acid). 

Over the next two days, the inoculated grains are incubated in temperature- and humidity-controlled rooms. During this time, the microbes form filaments around the grains, producing enzymes that drive miso’s fermentation and unlock its rich umami taste.

Mixing the Ingredients

With the soybeans mashed and the rice koji (or barley koji for mugi miso) fully developed, the components are finally brought together. Salt is added at this stage for flavor and to help regulate microbial activity during fermentation.

The exact ratio of soybeans, koji, and salt varies depending on the type of miso. For instance, our Saikyo Sweet Miso (a type of white miso) contains a higher ratio of rice koji and less salt for a sweeter flavor profile, while our Red Miso uses a higher soybean and salt content for a bold, deep flavor. All soybean miso, such as our Aged Black Miso, uses soybean koji instead of grains.

Once combined, the ingredients are thoroughly mixed into a uniform mash called moromi, ready to move on to fermentation.

Fermenting Miso Paste

Primary Fermentation And Monitoring

Once combined, the miso mash is transferred to large vessels where fermentation begins. This stage requires careful control—temperature, humidity, and air exposure all play a role in shaping the flavor, color, and aroma of different kinds of miso.

As fermentation gets underway, the enzymes produced by the koji break down proteins into peptides and amino acids—most notably, umami-rich glutamate—while starches are converted into simple sugars. Naturally occurring yeasts and lactic acid bacteria also thrive, contributing acidity and aromatic compounds.

Throughout the process, our master miso artisans closely monitor the mash, adjusting as needed and relying on generations of expertise to guide each batch to perfection. Some varieties of white miso with a sweeter flavor are not aged, so the miso is ready at this point, while others will be aged for months or even years.

Aging (Secondary Fermentation)

After miso completes its initial fermentation, it can be aged to develop deep flavor compounds, a darker color, and a more complex aroma.

One key process is the Maillard reaction—a non-enzymatic browning that occurs when amino acids (like glutamate) react with sugars over time. This gives longer-aged miso paste its darker hue and more complex, roasted flavors.

Meanwhile, the enzymes produced by koji continue to break down proteins into peptides and free amino acids, boosting its rich umami. At the same time, lactic acid bacteria generate mild acidity, while yeasts produce aromatic compounds like esters and alcohols. These transformations don’t just preserve the miso—they enrich it, creating a balanced depth of flavor that only nature and time can achieve.

Blending and Packaging Miso

Because fermentation is a natural process, we sample and blend each batch of miso to ensure consistent quality and flavor. Some of our miso paste is left unpasteurized to retain its live cultures, while others are heat-treated to stabilize flavor and color—so it arrives in your kitchen at its freshest. Finally, our high-quality miso products are packed into specially designed tubs, bags, or jars and shipped to markets worldwide, ready for you to enjoy in your favorite Japanese dishes.

How to Make Homemade Miso

Interested in making miso at home? It’s easier than you might think—though it does require a little patience.

Start by cleaning dried soybeans and soaking them overnight to rehydrate fully. Cook the soaked beans in a pressure cooker until they’re soft enough to mash between your fingers. Once cooled and drained, mash the cooked soybeans into a paste with store-bought koji and salt. You can do this by hand or in a food processor.

While the exact ratios depend on the style of miso you want to make, a good starting point is a 1:1 ratio of cooked soybeans to rice koji or barley koji (depending on whether you’re making rice miso or barley miso). Add salt equal to 5–13% of the total weight of the soybeans and koji. Saltier miso works better if you plan on aging it longer, while a lower salt content is best for sweeter miso.

Pack the mixed miso paste tightly into a sterilized jar or crock, pressing out as many air pockets as possible. Smooth the surface, then cover it with a generous sprinkle of salt and a sheet of plastic wrap. Weigh down the miso with a flat, heavy object and cover the container loosely with a lid—it should not be airtight.

Place your miso in a cool, dark spot and let it ferment. White miso might be ready in as little as three months, while a deeper reddish-brown miso can take over a year. Warmer temperatures(like in the summer months) will speed up fermentation, while cooler climates will slow it down.

Ready to dive deeper? Learn more about the different types of miso and how to use them in your cooking.