How to make koji at home

If you haven’t heard of koji, then you’ve almost certainly eaten it. A fungus that goes by the full name of Aspergillus oryzae, it brings umami depth to lip-smackingly tasty things such as soy sauce and miso by kickstarting the fermentation process, and is a fundamental ingredient in Japanese cuisine. 

Did you know you can make your own koji at home? It’s complicated but achievable, and who better to guide you through the process than Masaki Sugisaki, Executive Chef at London’s Dinings SW3, a Japanese restaurant in Knightsbridge, who knows a thing or two about DIY koji-making.

At the restaurant, Sugisaki cultivates his own koji in-house, using it to produce his own shio-koji – “a natural seasoning used to tenderise and enhance the richness of ingredients” – and amazake, made by mixing koji and cooked glutinous rice, which is used as a natural sweetener in the restaurant.

So if you’re a home-fermentation nut, you’ll love these tips for how to make koji at home as well as a few ideas for how to use it. 

How to make koji at a restaurant

_{Masaki Sugisaki at Dinings SW3}

“Timing is paramount when producing koji,” says Sugisaki. “[At the restaurant] we start with koshihikari sushi rice, which is washed eight or nine times in the late afternoon, then left to soak in filtered water overnight. In the morning at 9-10am, water is drained and left to dry until 4pm. The rice is then wrapped in a cloth from Japan that allows steam to evaporate and prevents moisture from entering. After one hour of steaming, we then leave the rice to cool. Its core temperature needs to be less than 45°C/113°F otherwise it will kill the fungus that's added.. (ideal temp is 39-41°C/102-106°F). 

“We then sprinkle fungus evenly to coat the rice, mixing this well, and quickly so the temp doesn’t drop below 35°C/95°F It's then wrapped in a koji cloth to help the fungus stay active. It's then placed in our house-made hot holder cabinet which is set to 35°C/95°F exactly and is regularly monitored until the next afternoon. We then remove it from the cabinet and spread the koji onto wooden trays. At this point, the fungus is fully activated and will have grown a white 'fluff' layer. If the fluff is any other colour then it's not usable. We leave the rice for 24 more hours to achieve nama koji (We use it as a marinade, bringing sweetness and deep umami flavours), and from here we can then produce amazake.” 

How to make koji at home

“Different styles of fermentation are used across the world and we are all aware of the health benefits. Koji is the most widely used fermented product in Japan, we use it in everything that we do and it’s the base of so many ingredients native to Japan,” says sugisaki.

_{A dish using koji at Dinings SW3}

“As seen above developing koji requires lots of patience and attention. Whilst it’s rewarding, the process is very complicated. If you’re making koji at home and want to cut down on a few steps you can buy dry koji rice (which is already dehydrated). You can activate the dry koji at home by creating the right temperature/atmosphere for the dry koji rice. This can be done by either adding the dry rice to water at 60°C/140°F and keeping it in a rice cooker on the ‘warm setting’ with the lid open. 

“Once the koji is fully activated you can use it in a variety of dishes – store any unused koji in an airtight container in the fridge and it will be good to use for another few weeks. At home, I add koji to passata to make tomato koji, it’s not conventional but there are lots of ways you can experiment with it at home.”

How to make tomato koji at home

“To make the tomato koji, add a small amount of your activated koji rice to your passata and leave this to slowly cook in a pan at a low temperature for eight hours. Alternatively, store the mixture in a warm location in an airtight jar and give it a mix every day for two weeks. The koji fermentation can have a significant impact on the flavour and aroma of the passata, it acts as an additional seasoning that can work beautifully in a tomato-based soup or pomodoro-style sauce.”

How to make amazake at home

“Koji can also can work extremely well as a natural sweetener and it’s very easy to make at home,” says Sugisaki. “This style of koji is called amazake. It’s a healthy sweetener that dilutes well into oat milk or any milk of choice for a hot beverage. It’s very commonly used in Japan as a store cupboard ingredient that you can buy off the shelves, but here’s how to make it at home.”

Ingredients

300g/10.5oz rice koji

600g/21 fl. oz water (55-60°C/131-140°F)

Method

1. In a clean container, mix warm water and rice koji. 
2. Keep this stored between 55-60°C/131-140°F for 6 to 8 hours by using the ‘keep warm’ setting of your rice cooker with the lid open.

Amazake marinade recipe 

Use for oily fish like salmon belly or black cod.

Ingredients

300g/10.5oz amazake

230g/8 fl. oz soy sauce

150g/5.3 fl. oz mirin

150g/5.3oz sugar

50g/1.8oz ginger juice

Method

1. Place all ingredients in a blender and blend well until achieving a smooth texture. 
2. Weigh the fillet of fish and add half of the weight of the marinade in a clean plastic bag. Remove air inside of bag and keep in the fridge overnight.
3. Take out the fish and cook it in a hot oven to your liking.