How to make koji

Koji is a type of Japanese fermentation starter – a grain (usually rice) that has been inoculated with a type of mould called Aspergillus Oryzae. While yeast is the magical key to many western food favourites – bread and beer being just two examples – koji serves a similarly vital role in Japanese cuisine, kicking off fermentation in sake, soy sauce, miso and mirin, to name a few.

The wording around koji can be a little confusing at first, as it refers both to the specific mould and to the grain that has been inoculated with the mould. Although rice is the most common grain used to make koji, other grains like barley can be used, or even legumes like soy beans. Nuts and seeds can also be used to make koji, if you’re feeling particularly experimental!

To make koji, partially steamed rice is inoculated with koji grains or spores and left to ferment in a warm, humid atmosphere for 5–7 days, where the mould feeds on the rice and breaks it down until the carbohydrates transform into sugars and produce glutamate (also known as that magical ‘fifth flavour’, umami). This can then be used as a ‘starter’ to make many fermented products such as sake, miso, soy sauce or a seasoning paste known as shio koji.

The recipe below comes from Dean Parker, head chef at The Manor restaurant in Clapham. Use this to try making your own miso paste, including Dean's cacao nib miso or toasted sesame miso. You can also use koji in porridge, blend it into smoothies or add it to rice pudding – the possibilities are endless.

How to make miso

Cacao nib miso

by Dean Parker

How to make koji

Ingredients

900g of white sushi rice, or pearl barley
90g of dried rice koji
80g of rice powder

shopping List

Equipment

For this recipe you'll need a fair amount of kit, including:

1 x spice grinder, powerful blender or food processor to grind the koji rice

1 x shallow perforated gastro tray with well-fitting lid

1 x deeper gastro tray for the perforated gastro tray to sit in

Large pieces of muslin cloth

A steam oven or rice cooker

Lengths of elastic or string

Cling film

A temperature-controlled environment, such as a dehydrator

Sterilised utensils

To begin, soak the rice for 3–6 hours then rinse until the water runs clear

Now steam the rice, either in a rice cooker or in a steam oven using the following method. Whichever method you choose, it’s critical that you don’t overcook the rice, you want it to only be half cooked. If you overcook the rice, it will stick together and be difficult to mix and inoculate evenly with the koji spores

Place a wire rack in a steamer gastro tray and line with a sheet of muslin cloth. Evenly spread the washed rice in an even layer, then place the gastro tray in a larger gastro tray filled with boiling water a third of the way up

Cover the top of the tray in cling film, then place a lid on top. Place in a steam oven set to 100% steam on full fan for 1 ½ hours – the rice should be only half-cooked

Remove the tray from the oven and leave to cool with the cling film intact to ensure that the rice doesn’t dry out while cooling

Once completely cooled, place the rice in an empty gastro tray in an even layer, sift over two thirds of the rice and koji powder and mix well with a spatula

Return the rice to the muslin-lined steamer gastro tray and spread out in an even layer. Place a damp cloth in the base of the other gastro, ensuring it doesn’t touch the base of the rice tray

Sift the remaining koji mixture evenly over the surface of the rice and drape a large, clean napkin over the surface of the tray and secure tightly with elastic or string, ensuring the surface is fairly taut

Place a lid over the top to seal and place the rig in the dehydrator at 37°C for 5–7 days, mixing the rice every 2 days with sterilised utensils

The rice will have a 1 inch layer of clean white mould layered on top (or green mould, if using barley instead of rice). It should smell sweet and floral, and will be ready to use to make miso, rice pudding and porridge

Alternatively, you can keep the koji in the freezer for up to 3 months, or allow to dry out for future use

How to make kombucha

Toasted sesame miso

by Dean Parker

How to use koji

You can use your koji to make homemade miso. While miso is traditionally made by mixing koji with soy beans, you can have fun experimenting with any bean, nut, seed or legume. Dean Parker’s recipes for cacao or toasted sesame miso are delicious used in sweet or savoury dishes.

Another popular and easy use for koji is koji salt or shio koji, a sweet, mild miso-flavoured paste often used as a marinade, pickling solution or low-salt, umami rich flavouring for stews and soups, such as Rosana McPhee's Japanese cream stew recipe.