Koji and natto making in Wales

We had been lured to a remote valley in mid-Wales in search of the secret to Japanese cuisine:  koji. Arriving on a sunny Saturday afternoon in April, it felt as though we had stepped out of the Welsh countryside and into Edo period Japan. Reiko, our host, is a really remarkable woman as well as being a highly talented chef.

Cwm Mawr

Our reasons for seeking out Reiko stemmed from our growing understanding of and appreciation for koji. We wanted to learn more about this miracle foodstuff from someone steeped in Japanese culture and culinary tradition. Reiko was a very welcoming host, and straight away we felt very comfortable sharing food and stories in her spacious and gracefully decorated barn conversion. Despite the fact that she is approaching the age of 70, Reiko could easily pass for a woman at least 20 years younger. This seems to be at least partly due to her upbringing in the countryside of post-war Japan, where junk food was non-existent and sugar, meat and even fish were very rare indeed.

Reiko and Georgina

Reiko explained over some of her extraordinary dishes how she had begun to learn to cook at the age of ten. This was macrobiotics without the classes, books or teachers which came along in later years. It’s an authentic, artisanal form of macrobiotics and we were learning at the feet of a master.

That evening we helped her start a batch of koji made from organic short grain rice. The process takes about three days, (including pre-soaking the rice) so the plan was that the koji would be ready just before our return to the Peak District on Monday afternoon. We also began work on a large batch of organic natto for one of her customers.

Koya tofu

Reiko has spent many years perfecting her recipes and methods for the koji, miso, natto, tamari and amazake she makes herself, and it was difficult to keep up with her as she buzzed around her comfortable kitchen.  Like an artist in the kitchen,  she wasn’t one to carefully measure out ingredients or amounts, but she took the time with us to try and come up with approximate measurements. Fermentation is a type of cooking better suited to the artist than the scientist, as outcomes depend so much on a symphony of time, temperature, environment and ingredients.

The wonderful Reiko with her wonderful koji
Fresh koji
Natto preparation
Fresh natto!

But fermentation is more about waiting than it is about rushing around the kitchen, so once the koji and natto were bedded down for the night, we were able to learn more about the central part that fermentation (especially in the form of products based on koji) plays in traditional Japanese food. Koji (which can be either made with rice or barley) is a key ingredient in many important foods, most notably miso, amazake and tamari. It seems that there is hardly a single Japanese foodstuff that does not involve at least some basic type of fermentation, whether it be natto, miso, tofu, pickles or soy sauce. It feels as though the health benefits (not to mention the taste benefits!) of fermented foods are now only beginning to be better understood here in Western countries, even though many, if not most, traditional food cultures have long treasured and understood their life-giving benefits.

Our first batch of home made miso, day 1

Perhaps our modern aversion to ‘germs’ has a lot to do with this shyness for fermented foods in the Western diet. Or perhaps it’s simply down to the fact that fermented foods are rebellious and don’t take kindly to mass production and homogenization. In any case, there is something of a rebellion taking place, and Cwm Mawr is at the heart of it.

Serving lunch

On Sunday the more traditional Welsh rain had returned, so we cancelled our plans for a short hike in the surrounding hills to focus more closely on the making of other macrobiotic mainstays, namely miso and barley and rice amazake. Reiko guided us in our endeavours as we made a start on a large batch of organic miso and began fermenting the amazake with some koji we had prepared earlier. By now the natto was ready, so we were also able to watch Reiko whiz together a very tasty natto condiment which was a great accompaniment to the afternoon’s late lunch.

Natto condiment

We rose fairly early on Monday for the final day of our trip. By now the koji would be ready, allowing us to finalise our first batch of miso. The koji smelled quite extraordinary, a smell that is too elusive for words. An unusual but also pleasant smell, quite unlike the dried koji we were more familiar with.

Miso soup with mochi and noodles

And so later that day we set off north, the car packed with an array of exciting foods awaiting further fermentation and experimentation. We were both buzzing from this brief but deeply interesting culinary journey. Check back in four months when the miso will be approaching maturity! And we will soon be returning to Cwm Mawr to learn even more in our next lesson.

How to make amazake with our koji

Amazake is cooked rice mixed with koji yielding, after a period of fermentation, a naturally sweet aromatic thick beverage. Amazake is great as a wholesome sweetener for cakes, muffins or other desserts. Add a touch of unrefined sea salt and a small piece of any sea vegetable, to boost nutrients, minerals, vitamins, and to enhance Amazake’s flavour. Spirulina, blue green algae, wheat grass, nut- and seed butters, flax, vanilla, cinnamon, coriander, coconut, are just some of the foods you can add to enrich or flavour amazake.

Koji is steamed rice that has been inoculated with a variety of the Aspergillus oryzae mold that is then used to make amazake, miso, sake, vinegar, mirin, and more.

Tools needed:

  • Medium pot with lid
  • Double boiler or large pot with lid
  • Probe thermometer
  • Flame diffusers
  • Blender (optional)
  • 5 Mason jars with lids

(Note: do not use aluminium pots or tools)


  • 5 cups organic brown rice or any other whole grain
  • 7 cups water
  • Pinch of sea salt
  • 1 cup Rhapsody amazake koji (double or triple for increased sweetness)

Soak grain overnight to reduce phytic acid content (skip when using white rice). Discard soaking water (skip when using rolled oats). Bring water, grain, and salt to a boil. Simmer covered for 50 minutes.

Let grain cool down to 135⁰F. Add koji and stir well. Cover. Insert this pot into the large pot with water (double boiler style) that’s between 135⁰F and 140⁰F. Use a thermometer to be able to control the fermentation process closely. Above 140⁰F the koji will get killed, so that’s a critical temperature to stay below. Slip several flame diffusers under the double boiler, to maintain the water bath at the right temperature on a low flame. This is the trickiest part of it all, but once you have figured out what the right number of diffusers is and the setting of your stove, not much can go wrong. We use a cast iron skillet with two diffusers on the smallest burner turned down to its lowest setting.
When doing this for the first time, start in the morning so you can keep an eye on the temperature. Once you know how to keep the temperature constant (within the range of 125⁰F to 139⁰F), make it in the evening and have a tasty pot of amazake ready for you the next morning.

Check temperature of water bath regularly in the beginning to get the temperature right. You can adjust the temperature by the number of flame diffusers.

Let the mixture sit overnight (8-12 hours). Stir occasionally. Speed up the fermentation to only 2 or 3 hours by running the mixture through a blender first.

Fermentation is done when the slurry smells nice and tastes sweet. Cool it down quickly and consume within a few days or bring to a boil to stop the fermentation process to prevent souring; stir continuously to prevent burning or bring the water in your double boiler up to boiling and give the slurry a few hours to rise to above 200⁰F.

Blend until smooth if desired and pour in a mason jar while boiling hot. (Rinse jar with hot water to prevent the glass from cracking.) Or pour into sturdy plastic containers and freeze.

To make rice milk, blend 2 parts amazake with 1 to 2 parts water depending on desired sweetness and consistency. Strain through cheese cloth for smoothness. Bring to a boil and also bottle hot. Refrigerate when cool.

Unopened amazake/rice milk lasts for at least 4 months when refrigerated.