Cultural Context:

Koji is the foundation of Japanese fermentation - the ingredient that makes miso, sake, soy sauce, mirin, amazake, and shio koji possible. It's rice (or sometimes barley or soybeans) that's been inoculated with Aspergillus oryzae, a mould that produces powerful enzymes to break down proteins and starches into amino acids and sugars. The result is intensely umami, subtly sweet, and utterly transformative.

I have to admit - in all those years I spent living in Japan in my twenties, I never encountered anyone making koji, nor did I think about how to make it myself. That's just how it is these days. Modern Japanese people buy koji at the supermarket (usually for amazake, shiokoji and the like as they also often just buy their miso as well). It's the same way people everywhere buy bread, pickles, cheese or cured meats. They don't need to know how to make it any more than most of us need to know how to make our own charcuterie. We've all changed our relationship with traditional food knowledge - it's been handed over to commercial producers, and for the most part, that's worked fine.

But things are shifting. We're becoming aware that we need more involvement in our food systems - to understand them, to support small makers, and to ensure we don't lose these traditions entirely to highly processed food manufacturers. Learning to make koji yourself is part of that reconnection. Plus - it's fun :)

Here's the truth: you don't need to make koji. It's easier to buy it, especially when you're starting out, and there's no shame in that. But making it yourself is as satisfying as baking a perfect sourdough loaf. The aroma alone - that sweet, chestnut-like smell as the white mycelium blooms across the rice - is something else entirely.

And it's not as hard as you might think. Mould wants to grow. That's what it does. You don't need to buy all kinds of fancy equipment or overcomplicate the process unless you enjoy that sort of thing. A simple, clean environment and attention to temperature is really all you need. Don't let the mystique intimidate you.

When I finally learned to make koji properly - spending time in Kyoto in my forties to verify technique for teaching and commercial production at The Fermentary - I realised the traditional methods (wooden trays, controlled rooms, careful monitoring) aren't about making it difficult. They're just refined solutions for creating consistent results. But you can adapt these principles to your home kitchen and still make beautiful koji.

Ingredients

• 1kg short or medium grain white rice (sushi- koshihikari rice - works beautifully and we have lovely biodynamic, rain fed rice in Australia)
• 2-3g koji kin (tane-koji/koji spores) - available at The Fermentary
• Clean water for soaking

Equipment

• Large bowl for soaking
• Steamer setup (bamboo steamer, steaming basket (it's best not to use a rice cooker with steam function as it makes it a little too wet)
• Muslin or cheesecloth
• Clean tea towels
• Wooden trays, baking trays, or shallow containers (ideally wooden buta if you have them)
• Incubation setup: insulated box/esky with heating pad, dehydrator, or wine fridge
• Thermometer and humidity gauge
• Clean hands!

Method

Stage 1: Prepare the Rice (Day 1, morning)

Soak the rice: Rinse your rice thoroughly until the water runs clear, then cover with plenty of cold water and soak for 8-12 hours or overnight. This long soak is essential - the rice needs to be fully hydrated before steaming.

Drain thoroughly: After soaking, drain the rice in a colander for at least 30 minutes. Give it a few good shakes to remove excess water. This step matters - too much water creates mushy rice.

Steam the rice: This is crucial, and we've written a detailed guide on cooking rice for koji that covers the technique in depth. In brief: Line your steamer with muslin, spread the drained rice in a layer no more than 5cm deep, and steam for 45-60 minutes. You're aiming for katame - firm rice. Each grain should be fully cooked through but hold its shape when pressed. Test by squeezing a grain between your fingers - it should compress but not turn to mush.

For the full technique including the gaiko nainan principle and professional tips, see my "Cooking Rice for Koji" guide here on the website

Stage 2: Inoculate (Day 1, afternoon)

Cool the rice: Spread your steamed rice on a clean surface (we use stainless steel trays, but traditionally it's done on large wooden tables). You need to cool it to 30-35°C - warm enough for the spores to activate, cool enough not to kill them. This takes 20-30 minutes. The cooling also lets excess steam evaporate, which is important.

Inoculate: Once cooled to the right temperature, sprinkle the koji kin evenly over the rice. Use your hands to mix thoroughly for at least 2-3 minutes - every grain should have contact with the spores. It seems like very little spore powder for so much rice, but trust the process. I think it helps if you pile it all into a large parcel - wrapping this up to make a warm almost compost like environment for the first 12-24 hours - making sure to spread it out and cool it down on time -  as it can get pretty hot in there!

Transfer to trays: Spread the inoculated rice in your wooden buta or shallow trays, about 3-5cm deep. Don't pack it down - you want air circulation. Smooth the surface gently. Cover with a clean, damp tea towel (not dripping wet, just damp enough to maintain humidity).

Stage 3: Incubation - First Phase (Day 1-2, 20-24 hours)

Create your muro: Place your trays in your incubation setup. You need:

• Temperature: 28-32°C (ideally holding steady at 30°C)
• Humidity: 70-80%

Temperature matters for what you're making:

• For sake koji: Hold at 28-30°C - cooler temperatures favour amylase enzymes that break down starches into sugars
• For miso koji: Hold at 30-32°C - slightly warmer temperatures favour protease enzymes that break down proteins
• For general use (shio koji, amazake): 30°C is perfect

Home muro options:

• Esky/cooler with hot water bottles, changed every 6-8 hours
• Dehydrator set to 30°C with trays of water for humidity
• Wine fridge or fermenting chamber if you have one
• Heating pad wrapped in towels in an insulated box

The first 12 hours are quiet - not much seems to be happening. Then you'll start to see and smell the koji activating. By 20 hours, you should see fine white fuzz beginning to appear, and the rice will smell sweet and slightly mushroomy.

Check periodically: Peek in (don't let all the warmth escape), sniff, observe. The rice should stay slightly damp but not wet. If it's drying out too much, mist the tea towel. If it's getting too wet, remove the towel for an hour.

Stage 4: Tane-kiri - The Mix (Around 24 hours)

Mix the koji: After 20-24 hours, it's time for tane-kiri - literally "cutting the seed." The koji generates its own heat as it grows, creating hot spots. Mixing redistributes this heat and ensures even colonisation.

Wash and dry your hands thoroughly. Gently but thoroughly mix the rice, breaking up any clumps, bringing rice from the edges to the centre and vice versa. You should see white mycelium forming - it's fragile, so be gentle but don't be scared of it. The rice should smell beautifully sweet, almost like chestnuts or popcorn.

Reform and return: Spread the mixed rice back into your trays, smooth the surface, re-dampen your tea towel if needed, and return to your incubation setup.

Stage 5: Final Fermentation (Day 2-3, another 20-30 hours)

Monitor closely: The koji enters its growth phase. Over the next 20-30 hours, the white mycelium will spread across and through the rice. The temperature will rise as the koji works - this is normal, but don't let it exceed 35-36°C or the enzymes can denature. If it's getting too hot, remove the cover for a while or reduce your heat source.

What you're looking for:

• Snow-white mycelium covering most of the rice
• Sweet, chestnut-like aroma (not sour, not musty)
• Rice grains still separate but bound together with fine white threads
• When you pick up a clump, it should hold together slightly then break apart
• The rice should feel warm to touch from its own heat generation

Timing: Most batches are ready at 40-48 hours total. Trust your nose - if it smells sweet and nutty, it's ready. If it starts to smell musty or develops yellow/green patches, something's gone wrong (but don't throw it out - see troubleshooting below).

Stage 6: Finishing

Cool the koji: Once ready, spread your finished koji out to cool to room temperature. This stops the fermentation process.

Dry slightly (optional but recommended): Let it air-dry for a few hours or overnight. This extends shelf life and makes it easier to store. The rice should feel dry to touch but not bone-dry.

Store: Transfer to clean jars or bags. Fresh koji keeps in the fridge for 1-2 weeks, or freeze for up to 6 months. Dried koji (if you've dehydrated it completely) keeps for months at room temperature.

Troubleshooting

No mycelium growth after 24 hours:

• Temperature too low - koji needs steady warmth
• Rice was too hot when inoculated (killed the spores)
• Old or dead koji kin
• Not enough moisture

Yellow or green spots:

• This is likely Aspergillus flavus or other contaminating moulds
• Usually means temperature was too high or inconsistent
• Still safe to use if just a few spots - pick them out
• If extensively contaminated, compost and start again

Black spots:

• These are spores from your koji reaching maturity
• A few are fine; lots means you left it too long
• Still usable but enzyme activity may be reduced

Sour or ammonia smell:

• Bacterial contamination
• Rice was too wet or temperature control failed
• Discard and start again - fermentation has gone wrong

Koji feels slimy:

• Too much moisture
• Next batch: drain rice more thoroughly, reduce humidity slightly

What to Make with Your Koji

Once you've made koji, a world of fermentation opens up:

• Shio koji: Mix 100g koji + 30g salt + 120ml water, ferment 1-2 weeks
• Amazake: Blend koji with warm water, hold at 55-60°C for 8 hours
• Miso: Combine koji with cooked soybeans and salt, age for months
• Sake: If you're feeling ambitious (and have the right equipment!) we also do classes in this.

You can find the above recipes in my books - and here on the website. 

We stock everything you need to start your koji journey at The Fermentary - fresh koji kin, sometimes we have the wooden buta trays. We also run koji-making workshops once or twice a year if you'd like to learn in person.