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Homebrewed Sake: The Koji

Homebrewed Sake: The Koji

This is part two of a series on making sake at home, part one is here.

The first step in sake making is building a koji starter.

Um, what is koji?

Koji rice means rice grains cultured with a specific kind of mould, which breaks down rice starches as it grows.

In sake and other rice wines, two changes – starch to sugar, sugar to alcohol – take place side by side. Rice isn’t easy for yeast to eat. A lot of the fermentable sugars are locked up in the form of starches that the yeast can’t digest, so something extra is needed to do that step.

In the simple rice wines that many people make at home, the starch digestion agent is something like a yeast ball or nuruk culture, which are actually many different beasties that eat different bits of the rice and work together in sequence. Kinda like a SCOBY that you might use for kombucha, or a sourdough starter, it’s a motley crew in there.

In Japan they’ve specialised. Sake brewers have isolated one mould culture, Aspergillus oryzae, commonly known as koji kin. (There are actually a few different ones, but they are all pure strains rather than symbiotic cultures. I used white koji kin).

Quick PSA

If you are not a crazy person, you can buy dehyrated koji rice online or in a Japanese food store. A bag of the rice costs very similar amounts of money to the spores, but I have an obnoxious DIY streak and wanted to try making our own. Let’s be real, if you are seriously considering making your own sake at home, you have this streak too. OK, so:

How to make koji at home

Koji kin must be grown in specially steamed rice, under special circumstances. How special? It depends how crazy you want to get and whether you have a koji chamber in your sake brewery. Spoiler, I do not. But a bit of googling led me to decide that the big factors Aspergillus oryzae needs to be mostly happy are:

  • Conditions between 30 and 40C
  • Humidity to grow rapidly, followed by dry warmth to drive inwards to the middle of the rice grain, which, apparently, is a thing you need it to do. I don’t understand why yet, but I’m rolling with it.
  • The occasional stir (perhaps every ten hours)
  • About 30-40 hours to fully develop.

I had a google and a think, and decided that I could make something out of a picnic cooler that would do the trick.

  1. Preparing the rice

My tiny assistant and I rinsed regulation Sunrice sushi rice, soaked it for an hour and a half*, wrapped it in a very old, clean, loosely woven tea towel, and steamed it for around 45 minutes.

Every 15 minutes we turned the rice over to ensure it cooked evenly. We were looking for, and got, rice grains that were transparent rather than chalky white all the way through, and had some ‘give’ when you bit into them. They were not at all like rice you would cook to eat.

We then let the rice cool to about 35C.

Cooked rice cooling on a clean bench

*apparently this is only a minimum time, and the husk of my unpolished rice is too firm to be damaged by a long soak. Worth keeping in mind when balancing sake making with other commitments – you could soak in the fridge overnight or through the work day, and steam when ready.

Preparing the koji culture

Have you ever planted carrot seeds? The stupid things are so small that it’s impossible to plant them evenly. Experienced gardeners combine the tiny seeds with fine soil, to allow even application. So it is with koji kin. We toasted a couple of tablespoons of flour to sanitise it, let it cool, then stirred in about a teaspoon of koji kin. When the rice was ready, we stirred the flour-koji mix through.

My tiny assistant helps set up our super fancy fermentation chamber, AKA the neighbours’ beer cooler


In lieu of a special fermentation chamber, I borrowed my neighbours’ picnic cooler, sanitised it, and filled it to around 15cm with warm water. We took the water’s temperature to ensure it ended up at around 35C.

We then upended a bowl so that the rice itself wouldn’t be sitting in the water: I honestly don’t know if that was needed, but I had an idea that it would be better to have an even heat. We set the rice on top of that in a large, sanitised bowl and covered it with a previously boiled, cooled face washer. This, we reasoned, would keep in the humidity.

First day: Keep warm and humid

For the first twenty hours or so, I checked in on the rice every five to ten hours, giving it a stir with a sanitised spoon and adding boiled water to increase the temperature.

(OK, so at this point our first experiment with koji making failed. We didn’t realise that there was a step where we needed to take the wet cloth off: the top rice got a bit weird and the rice at the bottom didn’t develop. We started again. I’ll skip all that as we did it all exactly the same way and got to…)

This is how we covered the rice for step 2 (dry cloth)

Step four: Decrease the humidity

After the first day or so, we were supposed to reduce the humidity, to force the mould inside the rice spores. We took off the wet cloth and replaced it with a clean tea towel fresh out of a hot dryer.

We probably could have done even better by replacing the water bath with warm wheat bags or something, but the water was doing a great job of maintaining temperature and we decided to see if it worked anyway. It did!

Step five: Make the moto, use or store the rest of the koji

The koji grains can go straight into the next stage (moto), or you can dehydrate them or freeze them. I froze the majority, as I wanted to keep them good even if my first moto failed. A small portion went into the fridge, because I read that koji will stay good for a month in the fridge and I wanted to test the theory. Mine turned yellow, which apparently means they bloomed. Google says this isn’t fatal, but it might give the sake a rougher flavour. So the freezer seems a better bet.

Success! They are quite beautiful.

How to make koji starter at home

This makes enough for a small batch of sake, plus a little extra for projects. Koji rice has a number of different uses – it’s a starter in miso making, rice vinegar, mirin, soy sauce and even, I recently discovered, easy ‘aged’ steak. So if you’re up for another project down the track, why not make extra?

We mostly followed the instructions posted here.

To make koji rice:

Soak the rice for at least 1 1/2 hours, then drain well.

Wrap and steam for 40-50 minutes, or until grains look slightly transparent, not white. Stir every 10-15 minutes, making sure the bottom layer moves about. Bite down on a grain when you think the rice is done; if there is any crunch and has a white centre it is not ready.

Let the rice cool to 30-35C. While it is cooling, boil and cool a face cloth or similar clean cloth, and toast the flour in a hot pan. (You don’t want to burn it, just to kill off any stray bacteria or wild yeast).

When the flour and rice are both cool enough, mix the koji-kin and the flour together, then sprinkle this mix over the rice and stir. I added the spores in stages while stirring, to get even coverage.

Put all of this into a sanitised bowl.

Set up your chamber to maintain a temperature of around 35C and some humidity. I did this with water.

Set your bowl of rice inside the chamber and cover it with the wrung out cloth. Every ten hours or so, give the rice a stir and check the temperature. I maintained the temperature by removing a jug’s worth of water and adding boiling kettle water until the temperature came back up. The picnic cooler was surprisingly effective and the biggest dip I saw, overnight, was to 28C.

After about a day, remove the wet cloth and replace it with a dry one. Keep stirring and maintaining the temperature. The rice will start to smell like cheese – that’s how you know you’re on the right track. A couple of turns later and it will have grown out in fuzzy threads, and burrowed into the rice. Congratulations! Step one is done.

At this point you can stop, and store the koji. Once you start the next step, you really need to be around at least daily for about a month to grow the moto and turn it into sake.

Next: the moto.

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