Grow your own koji

Before you start

You'll need

• Large bowl
• Colander
• Steamer or steamer basket, lined with a clean cloth
• Wide glass baking dish
• Clean kitchen towels
• Probe thermometer, to push into the grain
• A warm spot you can control, such as the oven with only the light on, a cooler with bottles of hot water or a dehydrator
• Disposable kitchen gloves, or very clean hands
• Containers or bags for the fridge

Step by step

1. 1

   Rinse the rice and soak it

   Rinse 500 g of rice until the water runs clear and leave it to soak. Out of the fridge, 4 hours at most; if you leave it overnight, put it in the fridge. Then drain it well in the colander.

2. 2

   Steam it for 1 hour

   Line the steamer with a clean, damp cloth, spread the rice on top and steam it for 1 hour. The grains should be cooked through and separate, without falling apart. Don't boil it: boiled rice ends up too wet.

3. 3

   Spread it out and cool it to 86–95 °F within 2 hours

   Tip the rice into the clean dish, break up the clumps with a spoon and spread it out. Check with the thermometer: it needs to drop from 140 °F to 86–95 °F within 2 hours at most.

   Tip: If it took more than 6 hours to cool, throw it out and start again (our house rule, on the guide's limit).

4. 4

   Sprinkle on the spores and mix

   With the rice between 86 and 95 °F, never above 95 °F, sprinkle on the spores at the dose the package gives for 500 g of rice. Put on the gloves or wash your hands very well, and mix thoroughly so they reach every grain.

   Tip: Always use store-bought spores and never start with koji from another batch: a wild mold, or one carried over from batch to batch, may be the wrong one.

5. 5

   Cover it with a damp cloth and keep it at about 86 °F

   Gather the rice into a mound in the middle of the dish, cover it with a clean damp cloth and push the thermometer probe into the center. Put it somewhere at about 86 °F: at first it needs a lot of humidity, close to 100%.

   The oven works switched off with only the light on and a bowl of hot water inside, with the door ajar if it gets too warm; so does a cooler with one or two bottles of hot water, swapped out as they cool; or a dehydrator set to 86 °F. Check the spot with the thermometer before you start.

   Around hour 10 the grain starts to give off heat. From then on, check the thermometer every 2 to 4 hours (our house habit) and keep the grain between 86 and 95 °F. If it reaches 104 °F, turn it right away and lower the heat of the spot.

6. 6

   Turn it at 16–18 hours and again around hour 26

   Between hours 16 and 18, uncover it, mix the rice with clean hands or gloves, break up the clumps to let the heat out and cover it again. Around hour 26, turn it again, spread it into a thinner layer and draw furrows with your finger so it can breathe. From here on, cover it with a dry cloth (our house rule): it needs less humidity now.

   Tip: Around hours 20–24 you'll see a very fine white fuzz over the grains. It smells sweet, of chestnuts or mushrooms.

7. 7

   Harvest between 42 and 48 hours

   It's ready when every grain is covered in white mold and bound into a cake that smells of chestnuts or fresh cheese. Harvest before colored spores appear. Aspergillus oryzae spores are yellow-green to green: if, from hour 40 on, a pale, even yellow-green dust appears at the edges, those are the first ones, and it's time to harvest.

   Tip: We only accept that pale green, even and from hour 40 on (our house rule). Any color before hour 36, or in patches that don't match the rest, is another mold: throw the whole batch out.

8. 8

   Break it up, cool it and refrigerate it

   Break up the cake so it loses its heat and stops growing. Keep it in the fridge for a month at most, or freeze it. Store leftover spores dry and tightly sealed: at 41–59 °F and low humidity they last a year, or frozen at −4 °F.

How to tell it's going well