This is a fool-proof guide for growing koji at home — the Koji Recipe for Dummies. In addition to providing an easy-to-follow koji growing recipe, I also share the reasons why this particular process is so effective. Let’s go.

Koji 101

WHAT’S KOJI?

Koji is a mold also known as aspergillus oryzae. It’s a magical ingredient that can transform your cooking. Koji grows when koji spores (which are a powder that can be purchased online) are applied to cooked starchy substrates such as barley or rice. Once grown, koji is an essential ingredient in soy sauce, miso, sake, and many other umami-rich foods.

HOW IS KOJI GROWN?

Koji takes about two days to grow. But it only grows under certain conditions. To grow koji, you need:

The Noma Guide to Fermentation is the best koji guide on the market (and it’s how I learned to grow koji), so if you want to dive deep into the koji-growing hobby, the Noma Guide is essential.

• The right substrate: The barley or rice must be polished (or “pearled”), which means the substrate’s protective layers (the husk and bran) have been removed. This is because koji cannot penetrate these layers. Thankfully, just about all rice you buy is polished, and it’s easy to buy pearled barley.

• The right cooking method: The substrate must be cooked so that it is soft but not soggy. And it can’t be too wet. To achieve this, the barley or rice is typically steamed.

• The right temperature: Koji grows best at around 86ºF, and it dies at around 108ºF. To ensure the koji is kept at the right temperature, the koji is typically placed in a temperature-controlled environment with plenty of airflow.

• The right humidity: Koji grows best at around 70% humidity. To achieve this, the koji is typically placed in a humidity-controlled environment.

To control temperature and humidity, most people make a fermentation chamber. Building a fermentation chamber for growing koji is easier than you’d think. With the right equipment, it only takes 10 minutes.

The koji growing process

At a high level, koji recipes are only a few steps long. I’ll describe the high-level steps then get into the details. Moving forward, I’ll refer to the substrate used as only barley, because that is my favorite substrate to use.

1. Cooking the barley: The barley is soaked for a couple hours then steamed until it is soft but not soggy.

2. Inoculating the barley: The barley is placed in a kitchen towel-lined hotel pan and then dusted with koji spores.

3. Growing the koji: The barley-filled hotel pan is placed in the fermentation chamber for two days for the koji to grow. The koji is periodically monitored and adjusted to ensure it doesn’t overheat.

To do it well, you’ll need to know a bit more about the process. We’ll get there.

KOJI RECIPE

What you’ll need

1. A fermentation chamber. See my full guide here.

2. Koji spores. I highly recommend the white koji spores from fermentationculture.eu. They’re a small company of koji-obsessed entrepreneurs selling the best quality spores I’ve been able to find. They are based in Europe but ship almost everywhere.

3. Thermometer. You need to use this before adding the koji spores to the barley. Otherwise, you risk adding the spores when the barley is too hot (which kills the spores). I have this one.

4. Pearl barley. You’ll use about 500 grams per batch of koji, so I buy this pack of 4.

5. Kitchen towels: You need 2 kitchen towels, preferably 2 you don’t use for anything else. I use these. Be sure to wash them before using them for the first time.

6. Kitchen scale: If you use too much barley in one batch, you risk the koji overheating. I weigh 500g of barley for each batch of koji with a kitchen scale. I also use the scale to create my koji/flour mixture.

Step 1: Cooking the barley

In this step, we aim to cook the barley until it is soft but not soggy.

You’re probably used to cooking barley (or rice) by simmering it in water. This will not work. It will make the barley too soggy (even if it doesn’t look soggy) and the koji won’t grow.

Instead, you’re going to have to soak and then steam the barley. Don’t worry, you don’t need a steamer to steam. Here’s what you do:

1. Put 500g of barley in a bowl and fill the bowl with water. Use more water than is necessary to merely cover the barley, because the barley will soak up some of the water.

2. Leave the barley in the water for 2 hours at room temperature. For food safety reasons, if you plan on soaking for more than 2 hours, do it in the fridge instead. I’ve soaked in the fridge for up to 8 hours before.

3. Rinse the barley in the sink with clean water multiple times until the water is no longer cloudy.

4. Put the barley in your steamer and begin steamer. If you don’t have a steamer:

• Boil water in a dutch oven or a pot with a lid.

• Put the barley in a strainer on top of the pot, ensuring the water does not reach the strainer.

• Put the dutch oven/pot lid on top of the pot.

• Continue to boil (ensuring the water never fully evaporates).

5. After 20 minutes of steaming, use a spoon to mix the barley. At this point, you can also take a single piece of barley to see how done it is (see next step for how).

6. Continue cooking the barley until it is done, checking and mixing every 5-10 minutes. I’m not going to give you a time estimate for this, because it is very variable! Instead, you can check if the barley is done by biting it. If there is a white inner center, the barley isn’t done. If the center is a uniform brown, you are good to go.

7. As soon as you’ve confirmed the barley is cooked, remove it from the heat (as further cooking will make it soggy).

8. Immediately pour the barley into a kitchen towel-lined hotel pan. Mix it around with a spoon a few times to keep the excess steam from overcooking the barley.

Step 2: Inoculating the barley

In this step, we aim to dust the koji spores on the barley so that they will produce koji.

Since the koji spores are very concentrated, this step is easiest (and least wasteful) when you combine the koji spores with flour. I recommend combining all your koji spores with flour as soon as you receive them so that you only have to do this once.

1. Weigh the spores.

2. Put 5x the weight of the spores in flour into a microwave-safe bowl. Microwave the flour for one minute. This pasteurizes the flour (by killing salmonella), which will allow you to use the koji in raw preparations in the future.

3. Once the flour cools down to 86ºF or lower, combine it with the spores. Store in a cool place in an airtight container.

Once you have your koji/flour mix, here are the steps to inoculate the barley:

1. Wait until your barley has lowered in temperature to 86ºF. Koji dies at 108ºF, so you risk ruining your batch if you add your koji spores too early.

2. Dust the barley with your koji/flour mix. I do so as follows:

• With your hand one foot above the barley, dust 1/2 teaspoon of koji/flour.

• Mix the barley around with a spoon. Ensure that you mix from the corners too.

• Dust a second 1/2 teaspoon of koji/flour.

• Mix the barley around with a spoon a second time, and then spread the koji out into an even thin layer.

Step 3: GROWING the Koji

In this step, we aim to keep the koji at the perfect temperature and humidity for growth. The fermentation chamber helps, but you’ll need to do a bit of work, too.

Here are the steps for hours 0-24:

1. Place the hotel pan of koji/flour-dusted barley into the fermentation chamber.

2. Put the thermostat’s probe into the barley (i.e. buried halfway into the barley).

3. Cover the barley with a barely damp kitchen towel. This serves to prevent over-drying at the top level. The towel should be just the tiniest bit damp and certainly not dripping wet.

4. Place the humidistat’s probe on top of the kitchen towel.

5. Turn on the thermostat (set to 86ºF with 2ºF plus/minus) and humidistat (set to 72.5% with 5% plus/minus).

6. Close the lid of the cooler. I don’t close it all the way so that there is enough airflow. The way i do this is by stacking the probe wires from the thermostat and humidistat on top of one another, which keeps the lid raised by just a centimeter.

7. Leave the koji just like this for the next 24 hours.

I often find that the humidity rises well above 80% during this first day. To regulate the humidity, I occasionally open the lid of the cooler further or even wipe the insides of the cooler with a paper towel.

After 24 hours have passed, you’ll start to see some koji growth. Just tiny specs. You’re making progress.

1. At hour 24, mix the barley around so that it is evenly distributed. Mixing it (and spreading it out in an even thin layer) also helps to prevent overheating. I mix it while wearing rubber gloves.

2. If the kitchen towel has fully dried, lightly rewet the kitchen towel and place it on top of the koji again.

3. Periodically from hour 24 to around 48, monitor the koji to ensure that the temperature does not creep too high beyond 86ºF. Maintaining temperature becomes increasingly difficult as the hours go on, because the koji begins producing its own heat! It produces so much heat that it can actually reach 108ºF, thus killing itself. Some temperature increases are inevitable (and my koji batches usually end with a temperature around 103ºF), but the better you are able to control the temperature the more evenly the koji will grow.

4. When your koji temperature rises too high, there are steps you can take to address it:

• Break up and mix the koji again (like you did at hour 24). If you do this too often, the final result won’t be a single block of koji. But that’s fine! Optimize for good koji over pretty block of koji.

• Temporarily remove the hotel pan and place it on a cool countertop. Even 5 minutes like this can lower the temperature significantly.

• Add a fan! I don’t have a small fan, but if you do, you can add it to your cooler (below the hotel pan) to provide additional air from below to cool the koji.

5. Your koji will probably be done at around hour 48, but this is just an estimate. The best way to know when it is done is to simply look at it and smell it:

• If a blanket of white fully covers the koji, it is done. You did a great job!

• If the koji starts to show green or yellow spores, it is done. That means it is sporulating and you probably struggled with temperature control. Heavily sporulated koji doesn’t taste as good, so you should end the process as soon as you see sporulation.

• If the koji smells like ammonia, the wrong bacteria took hold. You should throw the batch away.

Here is my finished koji batch. When all goes well, it is a single block of white goodness.

FINISHING AND STORING YOUR KOJI

Once you take your finished koji out of the fermentation chamber, put it in the fridge in an airtight container. The flavors improve with a few hours in the fridge, so I recommend doing this regardless of what you plan to do with it next.

You can keep the koji in the fridge for up to a week before you use it in something else. Otherwise, you can:

• Freeze it: It will last multiple months frozen.

• Dehydrate it: I dehydrate it at 140ºF for a few hours.

Troubleshooting

Having issues? Comment below and I will address them in this section.