What is miso made of?: A Deep Dive into the Fermented Japanese Staple

October 3, 2025 •

2 min read

With a history spanning over 1,300 years, miso is a staple of Japanese cuisine, with an average person in Japan reportedly consuming around 4kg of miso per year. This traditional seasoning has a complex, savory flavor known as umami, which stems from a meticulous fermentation process.

Quick Summary

Miso paste is created by fermenting soybeans with salt and koji, a mold cultured on grains like rice or barley. The specific ingredients and fermentation time dictate its distinct flavor, color, and texture, resulting in a wide array of varieties.

Key Points

Core Ingredients: The base of miso includes soybeans, salt, water, and the essential koji culture, which acts as the fermentation agent.
The Role of Koji: Koji, a mold cultivated on grains like rice or barley, produces enzymes that break down soybeans and other components, creating miso's unique flavor profile.
Fermentation Time: The aging period is a key differentiator, with shorter fermentation resulting in milder, lighter-colored miso, and longer periods leading to deeper, saltier, and darker varieties.
Ingredient Variations: While soybeans are standard, different grains for koji (e.g., barley vs. rice) and even other legumes like chickpeas can be used, creating a wide range of flavor profiles.
Health Benefits: Miso is a fermented food known for providing probiotics that support gut health, though it is also high in sodium.
Vegan and Gluten-Free: Many types of miso are naturally vegan and gluten-free, though varieties using barley or other gluten-containing grains should be avoided by those with sensitivities.

The Core Ingredients of Miso

Miso is made from a few basic ingredients: soybeans, koji, salt, and water.

Soybeans

The primary ingredient is typically soybeans, which are soaked, cooked, and mashed. They provide protein and fat that are broken down during fermentation. While soybeans are traditional, some miso uses other legumes like chickpeas.

Koji (Aspergillus oryzae)

Koji is a mold (Aspergillus oryzae) cultivated on grains such as rice or barley. It contains enzymes that are essential for breaking down the starches and proteins into simpler compounds, creating miso's characteristic flavor. The grain used for koji significantly influences the final miso.

Salt

Salt is vital for both flavor and preservation. It regulates fermentation by preventing the growth of harmful bacteria while allowing beneficial microbes to flourish. The salt content affects the flavor and aging time.

Water

Water is added to achieve the desired consistency. Sometimes, a small amount of 'seed' miso is included to introduce beneficial microorganisms from a previous batch.

The Transformative Power of Fermentation

Miso is produced through a two-stage fermentation process that can last from weeks to years.

Stage 1: Making the Koji Grains like rice or barley are steamed and inoculated with Aspergillus oryzae spores, then incubated to create koji.
Stage 2: Miso Fermentation Koji is mixed with cooked, mashed soybeans, salt, and water. This mixture is packed into containers and weighted to remove air. Enzymes from the koji and other microbes break down the ingredients during aging, developing the umami flavor. Longer fermentation leads to darker color and stronger flavor.

A Comparison of Popular Miso Varieties

Miso varieties differ based on the koji grain, ingredient ratios, and fermentation duration.


Feature	White Miso (Shiro Miso)	Yellow Miso (Shinshu Miso)	Red Miso (Aka Miso)
Koji Grain	Primarily rice	Barley and/or rice	Barley and/or other grains
Fermentation Time	Short (weeks to 6 months)	Medium (weeks to 1 year)	Long (1 to 3 years)
Color	White to light beige	Yellow to light brown	Deep red to dark brown
Flavor Profile	Mild, sweet, less salty	Earthy, well-balanced, mild	Deep, rich, pungent, salty
Best Uses	Dressings, light sauces, light soups	Marinades, glazes, general use	Hearty soups, stews, braises

Health Benefits and Dietary Considerations

Unpasteurized miso can offer health benefits as a fermented food.

Probiotics: Contains beneficial microbes that may support gut health.
Digestibility: Fermentation breaks down components, potentially aiding digestion.
Nutrients: Provides protein, vitamins, and minerals.
Gluten-Free: Many rice-based misos are gluten-free, but check labels as some use barley.
Sodium: Miso is high in sodium and should be consumed in moderation.

Conclusion

Miso is a foundational Japanese ingredient born from fermentation. Knowing what is miso made of – soybeans, koji, and salt – clarifies how its umami flavor and potential health benefits are developed. The variation in these simple ingredients and aging times results in a spectrum of miso types, from mild white to robust red, each capable of adding depth to diverse dishes.

Frequently Asked Questions

While soybeans are the most common base, some miso varieties are made with other legumes such as chickpeas or adzuki beans, offering a soy-free alternative.

Koji is a mold, Aspergillus oryzae, that is grown on a grain like rice or barley. It produces enzymes necessary for the fermentation process that transforms the raw ingredients into miso.

Not all miso is gluten-free. Miso made with koji cultured on rice is typically gluten-free, but varieties using barley or wheat, such as mugi miso, contain gluten. Always check the product label.

The main differences are the ratio of ingredients (especially the type and amount of koji) and the fermentation time. White miso is mild and sweet with a short aging period, while red miso is saltier and pungent, aged for a longer time.

Most miso paste is naturally vegan, made only from soybeans, grains, salt, and koji. However, traditional miso soup might be made with a dashi stock containing bonito flakes (dried fish), so vegans should ensure the dashi is plant-based.

Miso should be stored in an airtight container in the refrigerator to maintain its quality and prolong its shelf life, which can last for a year or longer.

Yes, unpasteurized miso contains beneficial microorganisms and is a source of probiotics that can contribute to gut health. However, heating miso to a boil can kill these beneficial bacteria.