Processing is a stage in coffee production which is necessary to prepare the beans for roasting, as well as having a significant impact on coffee freshness and flavour. 

In recent years, experimental processing methods which leverage the power of fermentation (such as anaerobic fermentation and carbonic maceration) have become increasingly common. However, there is still room for further innovation in coffee processing.

One of the newest emerging trends in processing is using something called “koji mould” as a unique priming agent during the fermentation stage. This new method is commonly referred to as “koji coffee,” “oryzae coffee”, or “koji supernatural processing”.

To understand this process, I spoke with several coffee professionals who helped design it. Read on to learn more about koji mould and its effects on coffee flavour.

You may also like our article on how microorganisms affect fermentation & the sensory profile of coffee.

Koji: Japan’s national mould

Koji (Aspergillus oryzae) has been widely used across southeast Asia for centuries. It’s perhaps most notably used in Japan to make sake, amazake, miso, and other umami products – granting it the title of “Japan’s national mould”.

Kaapo Paavolainen is the founder of One Day Coffee Co. and the 2021 Finnish Barista Champion. Kaapo used a koji processed coffee in his performance at the 2021 World Barista Championships in Milan.

“Koji is an ancient, beneficial fungus which is used to saccharify starches,” he tells me. 

Saccharification is the process by which enzymes turn starches into sugars and dextrins to be used during fermentation. 

When added to certain food and beverage ingredients (including rice, sweet potato, barley, and soybeans) the enzymes in koji mould break down starches, which helps to develop new flavours.

Over the past few decades, the use of koji mould in cooking has become increasingly prominent in western cultures, too. Author of Koji Alchemy and chef Jeremy Umanski opened the delicatessen Larder in Ohio to showcase the new flavours that can be created by using koji mould.

So, if it works so well in the fermentation process for other food and beverage products, what about coffee? 

Christopher Feran is the Director of Coffee for Phoenix Coffee Co. and an independent coffee consultant. He tells me about some of the initial challenges with using koji mould during fermentation.

“It’s difficult to do successfully because there is very little starch available in green coffee for the koji to grow,” he says. “You have to find some way to make the mould stick to the fermentation medium.”

Christopher tells me that to support with this, he reached out to Koichi Higuchi of Higuchi Matsunosuke Shoten. The Osaka company provides koji fermentation starters to manufacturers of traditionally fermented foods.

Christopher requested Koichi’s assistance and expertise for the preliminary koji coffee processing trials. Koji mould was directly applied to the green coffee as part of this. 

The initial results created prominent umami flavours, which aren’t considered desirable to most coffee drinkers. However, when koji spores were applied to the coffee cherries, the results were much more promising. 

After achieving these results, Christopher and Kaapo collaborated with Elias Bayter Montenegro – Head of Processing at El Vergel Estate in Colombia – to develop the first formal koji coffee process.

Fermenting coffee cherries with koji

“Coffee is almost always fermented at origin because you need to extract the seed from the fruit,” Christopher tells me. “Traditionally, this is done using yeast or bacteria – similar to how you would when producing wine or beer.”

When making beer or wine, the grains or grapes produce sugars, which live microorganisms consume. This produces alcohol, as well as carbonating the beverage and creating other compounds which create the flavours we associate with beer and wine.

“Koji mould breaks down larger starches into fermentable sugars, as well as producing amino acids and glutamates,” he continues. “This makes the coffee feel heavier on your tongue and enhances the mouthfeel.

“Koji also produces esters and aldehydes, which increase the fruity aromas in coffee.”

Kaapo explains that koji helps to break down complex sugars into simpler ones, which has great potential for new flavour profiles.

“Around 70% of the sugars in coffee are broken down in other processing methods – from washed to carbonic maceration and anaerobics,” he tells me. “However, koji allows us to use more sugars in the following fermentation stages, so we are able to intensify the flavours in coffee.”

So, how did Kaapo, Christopher, and Elias achieve these results?

Kaapo says: “We first tried it with a Red Bourbon. We created a natural koji fermentation process as outlined in our processing manual and compared it against a classic natural processed coffee of the same variety. 

“We then sent an A-B test to Japan, the US, Netherlands, and Finland and we all cupped the samples blind,” he adds. “The koji processed coffee scored higher, and had more body, more aftertaste, and a better and more refined acidity.”

Elias agrees, saying that “the results were outstanding”. 

He adds that the koji doesn’t necessarily create new flavours, but rather that it enhances the existing complex components of the coffee.

“The main change was in the body,” he says. “The structure of the coffee changed a lot. For example, a soft body became more creamy and well-rounded.”

Following the success of this experiment, El Vergel has added koji to many other different varieties, including Geisha, Bourbon, Caturra, and Java. Elias emphasises that each varietal had promising results, but says that Java in particular responded especially well to koji processing.

What are the implications for producers?

When asking Christopher if koji could provide added value for fermented coffee, he remains cautious.

“Taking an 85-point coffee to 87 points might be compelling for producers who want to receive a premium for their coffee,” he says. “However, coffee farmers producing 82 or 83-point coffees are likely to see less benefit with koji in comparison to other, more intensive improvements (such as selective picking, sorting, and tank cleaning, for instance).”

Elias points out that scaling the koji fermentation process is also a challenge. In Colombia, he says most coffee farmers are more used to traditional processing methods, which result in more traditional flavour profiles. Moreover, consumer demand for koji coffee remains minimal, as it is largely unknown.

Furthermore, he notes that controlling the koji culturing stage can be difficult, as it requires paying close attention to time and temperature.

“However, once you learn how to manage these steps, it’s a simple process that can help to increase scores by one to two points,” he says. “Farmers don’t need a lot of equipment, mainly just a shaded area to store the coffee where they can spread the koji spores.”

Elias adds: “The most difficult part will be changing people’s perceptions about growing koji and using it in fermentation.”

However, Kaapo believes that koji mould has great potential for coffee producers around the world. 

“The reason we created this processing method is because we knew it was possible and we wanted to test it,” he says. “We noticed it is beneficial for coffee producers, which is why we made the information available on an open-source basis in both English and Spanish.”

Making koji more accessible

Whether or not farmers use koji to increase the scores of their coffee or to attain more premium prices, there’s no denying its potential. But that doesn’t mean that there aren’t challenges in adopting these more experimental techniques.

“Cost is going to be the biggest issue,” Christopher says. “It’s expensive to buy the koji spores. The spores we used for Kaapo’s competition coffee cost around US $17 for 30g.

“We were dosing 2g per 1kg of cherry, so 1kg of cherry might yield between 18g to 20g of green coffee.”

However, Christopher explains that El Vergel attempted to create a more affordable method of growing koji spores.

“We were growing what is called rice koji,” he tells me. “You partially cook rice by steaming it.

“You can then disperse the koji spores on the rice and culture more koji mould, which makes these ‘cakes’. You can then pulverise these cakes and disperse them accordingly.”

Christopher says this can significantly increase the amount of koji mould available to producers.

“From 30g of koji starter you can produce about 240kg of rice koji. Ultimately, this could address some barriers in terms of scalability and affordability.

“But we are still making sure that koji fermentation is a fully safe process and that we are selecting the right strain of koji for broader commercial use,” Christopher adds.

“Rice grows in many places along the Bean Belt, so hopefully koji will become a more accessible ingredient for coffee fermentation.”

What might the future hold?

“Koji processing is still so new,” Kaapo says. “No one has really established a new processing method since Saša Šestić pioneered the carbonic maceration method in 2015.

“We’re still experimenting with the method at El Vergel for my 2022 Finnish Barista Championships coffee,” he tells me. “In addition to koji mould, we’re also using a pre-process technique to increase the sugar content for subsequent processing.”

Elias, meanwhile, says that more research is needed to identify the sugars present in the process, as well as experiments with other varieties and processing methods to explore the full potential of koji fermentation.

“There are a lot more things we can do with koji; we are just scratching the surface of this new processing method,” he says.

Christopher, Elias, and Kaapo are all also focusing on how koji cultivation and fermentation can be scaled for commercial use. Christopher, however, underlines the importance of “starting small”.

“It’s a matter of convincing two or three farmers to produce enough koji coffee that it can be exported and served to consumers to start increasing demand,” he says.

For now, he believes koji coffee will be more prevalent in competitions than coffee shops, but Elias hopes that this will help to boost demand in time. 

“When koji coffees start to win competitions, that’s when the word will spread,” Elias concludes.

As of now, the practice of koji fermentation remains prevalent only on a handful of farms in coffee-producing countries. El Vergel currently only sells pre-orders of its koji coffee through Forest Coffee, a Colombian wholesale platform.

There is potential for the demand for koji fermentation to grow over the next few years, but the only way this will happen for sure is if consumers try it. If you happen to come across koji coffee, take the opportunity to try this rare processing method – it might just become your new favourite.

Enjoyed this? Then read our article on how to ensure consistency in coffee fermentation & processing. 

Photo credits: One Day Coffee Co., El Vergel Estate

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