Cooking Rice for Making Rice Koji

Cooking Rice for Koji Making

Katame: Firm Rice for Fermentation

If you're making your own koji—rice inoculated with Aspergillus oryzae for miso, sake, amazake, or shio koji—the rice needs to be cooked quite differently than rice for eating. Getting this right is fundamental to successful koji production.

I learned this firsthand during my twenties living in Japan, but it wasn't until I returned to Kyoto in my forties that I truly understood the nuances. I spent a week there at a Koji house intensive to verify/qualify  my knowledge was sound enough for both teaching and commercial production at The Fermentary. Watching traditional koji makers work with rice—the way they assessed texture by touch, the precision of their steaming times, the care with which they spread the rice—made me realize how much depth there is in what looks like simple technique. Where I learnt was very traditional - not a huge commercial enterprise - they cooked over wood and in wood. I don't cook like that here - but the method is the same. 

The Goal: Katame Rice

In Japanese, the rice for koji is described as katame (硬め), meaning "on the firm side" or "cooked firmly." It's similar to cooking pasta al dente, but with its own specific requirements.

Why this matters:

Koji spores need to penetrate and colonize individual rice grains. The rice must create the right environment for the aerobic mold Aspergillus oryzae to thrive. If the rice is too soft or mushy, it clumps together and creates anaerobic pockets where koji mold suffocates. If it's undercooked, the spores can't access the starches they need to grow.

You want each grain to be:

• Fully cooked through (no hard, chalky center)
• Firm enough to stay separate
• Slightly dry on the surface
• Able to hold its shape when pressed
• Providing proper air circulation between grains

Traditional Method: Steaming

Traditional koji makers always steam rice rather than boil it. Steaming gives you the firm, separated grains essential for proper koji growth.

Why steaming, not boiling:

• Prevents grains from becoming waterlogged
• Maintains structural integrity of each grain
• Creates the slightly dry surface koji spores need
• Allows for better air circulation in the koji room

How to steam rice for koji:

1. Wash & then Soak your rice for at least 6-8 hours or overnight
   • This is non-negotiable for koji rice
   • Longer soaking (up to 12 hours) is even better
   • The rice should be fully hydrated before steaming
2. Drain very thoroughly
   • Let rice drain in a colander for at least 30 minutes
   • Excess water will make your rice too soft
   • Give the colander a few shakes to remove lingering moisture
3. Steam the rice:
   • Use a bamboo steamer, steaming basket, or rice cooker with steaming tray
   • Line with muslin cloth or cheesecloth
   • Steam for 45-60 minutes for white rice
   • Check at 40 minutes and test texture
   • Brown rice needs 60-90 minutes
4. Test for doneness:
   • Take a grain and squeeze it between your fingers
   • It should compress but hold together, not turn to mush
   • Bite into it—no hard center, but distinctly firm
   • The grain should feel firm but not crunchy
5. Cool to inoculation temperature:
   • Spread steamed rice on a clean surface (traditionally a large wooden table)
   • Let cool to 30-35°C (86-95°F) before adding koji spores
   • This cooling also allows excess moisture to evaporate

The Professional Standard: Gaiko Nainan

Sake breweries aim for what they call gaiko nainan (外硬内軟)—"hard outside, soft inside." The surface of each grain should be quite firm while the center remains tender. This creates the ideal texture for koji enzymes to work their way through the grain while maintaining structural separation.

At The Fermentary, we follow this principle for all our koji production. It took years of practice to recognize the right texture by touch and sight.

About Muro: The Koji Room

In Japan, koji is made in a special temperature and humidity-controlled room called a muro (室). Traditional muro are small, cedar-lined rooms that maintain warmth (around 28-32°C) and high humidity (70-80%). The wood naturally regulates moisture while the space can be carefully monitored.

Walking into a traditional muro is an unforgettable sensory experience—the warm, humid air thick with that distinctive sweet, chestnut-like aroma of growing koji. The temperature hits you immediately, the smell is intoxicating, and you can almost feel the living koji working through the rice.

Why the muro matters:

• Aspergillus oryzae is a tropical mold that needs consistent warmth
• Humidity prevents the rice from drying out while koji grows
• The controlled environment ensures even colonization
• Traditional cedar wood helps regulate moisture naturally

At The Fermentary, we've created our own version of a muro—a controlled space where we can maintain the precise temperature and humidity koji demands. While we don't have a traditional cedar room (they're expensive and require constant care), we've adapted the principles to create consistent, high-quality koji.

Wooden Trays: Buta

Traditional koji makers use shallow wooden trays called buta (sometimes called koji-buta or koji trays) to hold the inoculated rice during fermentation. At The Fermentary, we also use wooden buta for our koji production.

Why wooden trays:

• Wood absorbs excess moisture while allowing air circulation
• The shallow depth (usually 3-5cm of rice) ensures even temperature distribution
• Multiple trays can be stacked and rotated in the muro
• Wood doesn't conduct heat the way metal does, creating a more stable environment
• Traditional Japanese cypress (hinoki) has natural antimicrobial properties

The traditional process:

1. Spread freshly steamed, cooled rice in wooden buta
2. Inoculate with koji spores (tane-koji)
3. Mix thoroughly to distribute spores
4. Smooth the surface
5. Place trays in the muro
6. After 20-24 hours, mix the koji (tane-kiri) to redistribute heat and moisture
7. Return to muro for another 24-36 hours
8. Finished koji should be snow-white with a sweet, chestnut aroma

What I learned in Kyoto:

The koji makers I observed had an intuitive understanding of their rice and mold. They could tell by touch whether the koji needed more warmth or more air. They knew by smell when fermentation was progressing well. They adjusted the stacking of their buta based on how the batch was developing. This kind of knowledge only comes from making thousands of batches—and from being taught by people who learned from their teachers before them. We try to teach all of our fermentation techniques just like this - use your senses. 

At The Fermentary

We make koji both for our own products (miso, amazake, shio koji) and to sell to home fermenters. Using the traditional buta method with properly steamed katame rice ensures consistent, high-quality results.

It's a time-intensive process—from soaking the rice overnight, to steaming it perfectly firm, to monitoring temperature and humidity for 48 hours—but it produces koji with excellent enzyme activity and that distinctive sweet aroma that tells you everything went right.

Tips for Home Koji Makers

If you're making koji at home:

• Invest in a good thermometer and humidity gauge
• Use short or medium grain white rice (easier than long grain)
• Don't skip the long soak before steaming
• Test your rice texture before inoculating—firm is always better than soft
• Create a warm, humid environment (an esky/cooler with hot water bottles works)
• Wooden trays are worth it if you're making koji regularly
• Keep everything scrupulously clean—koji mold should be your only microbe

The learning curve:

Your first few batches might not be perfect. The rice might be too soft, or too hard, or the temperature might fluctuate. That's normal. Traditional koji makers spend years developing their skill. Every batch teaches you something—how the rice should feel, how the koji should smell, when it's ready.

The Bottom Line

Cooking rice for koji isn't difficult, but it does require attention to detail. The rice must be soaked long, drained well, steamed until firm but cooked through, and cooled to the right temperature. Get this foundation right, and you're well on your way to making excellent koji.

Those weeks in Japan—both in my twenties absorbing the culture, and in my forties verifying technique—taught me that traditional methods persist because they work. The wooden buta, the cedar muro, the careful steaming of katame rice—these aren't quaint traditions. They're refined solutions developed over centuries to create optimal conditions for Aspergillus oryzae to transform rice into something extraordinary.

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Looking to make your own koji? The Fermentary stocks koji spores (tane-koji), as well as prepared Koji. We also run regular koji-making, sake and miso workshops—come learn in person!