In 2006, the Brewing Society of Japan designated koji mold (aspergillus oryzae) as Japan’s national fungi, or kokkin ( 国菌). Today, koji, along with traditional skills of sake-making is recognised under UNESCO’s Representative List of the Intangible Cultural Heritage of Humanity. What exactly is koji and why does it hold such significance? What does the past, present and future hold for koji and sake?

Lead Image: Koji made by Happytaro Brewery. (Credits: Kotaro Ikejima’s blog on Note)

Koji: The Heart of Fermentation in Japan

Koji, a filamentous fungus, is indispensable in the fermentation of sake, shochu, soy sauce, and miso. It is inoculated onto base ingredients like rice, soybeans, and barley by propagating koji mold spores onto them, and under the right humidity and temperature, the mold thrives, transforming these ingredients into koji. As any kojiyasan ( 麹屋さん, koji-making specialist) would tell you, the making process is physically and mentally demanding. In the kojimuro ( 麹室, koji preparation room), where temperatures are high and humid to allow koji to grow well, brewers work around the clock over a 48 to 72 hour period, with immense focus and care required in order to produce high quality koji.

The primary role of koji lies in its ability to convert base ingredients into simpler, fermentable compounds. It is often forgotten that where alcohol fermentation is concerned, starch, as found in rice, cannot be directly converted into alcohol by yeast. In its natural state, starch is unfermentable and must first be broken down into fermentable sugars. This is where koji comes in. Koji produces the amylase enzymes which convert starch into sugar, thereby enabling yeast to consume the sugar and create alcohol. Through koji too, proteins are converted into amino acids such as glutamate, responsible for that rich umami flavour we love and appreciate in sake.

From my experience, the terminology surrounding koji is often confusing for beginners. As a point to note, the koji mold itself, aspergillus oryzae, is called koji-kin ( 麹菌 ). If rice is scheduled to be inoculated with koji-kin, that rice is called koji-mai (麹⽶). After the process of inoculation is complete, the rice with koji-kin grown all over it is thereafter called koji (麹). Yes, confusing as it is, the term koji is defined not by the mold itself, but refers to a finished product or ingredient. I hope that clears things up.

White, Black and Yellow tanekoji (seed koji) provide the spores required for koji-making.

Several Koji Varietals, One Collective Purpose 

Koji comes in several varietals: black (aspergillus luchuensis), white (aspergillus kawachii) and yellow koji (aspergillus oryzae). While all essentially serve the same purpose—converting starch to sugar,— their final results vary.

Yellow koji is the most familiar, commonly used in sake, miso and soy sauce production. Its ability to facilitate fruity aromas and high amino acid production makes it the perfect engine to the fermentation of these staples. It’s also the reason why products of yellow koji, like miso and soy sauce are so rich in flavour, and the reason jukusei-shu (熟成酒, aged sake) shares similar qualities with soy sauce.

White and black koji are occasionally dabbled with in sake brewing, but are primarily used in shochu and Okinawa’s awamori production respectively. Both varieties produce bold, rich, and punchy flavors with high levels of citric acid, but yield lower amounts of amino acids. Black koji in particular, contains fewer enzymes but higher peptide levels, giving it a slightly earthier and more bitter profile compared to white koji products.

50% milling ratio sake rice (left) vs. genmai rice, unmilled (right).

Why do we actually need Koji?

Whilst other alcoholic beverages, such as beer, rely on the process of malting to release sugars from starch stored in the grain, with sake, germinating rice is generally impossible because the endosperm and germ is polished away during rice milling. As such, koji is a required component of the sake brewing process, not just for its role in developing sake’s flavour and body, but as the saccharification catalyst required for alcohol production.

Modern Sake Evolutions, Traditional Koji Methods with Matsui Brewery

[Translated] “In sake brewing, there is a saying: ‘First, koji. Second, shubo. Third, brewing.’ This highlights that koji making is the most fundamental step.”

These are the words of Matsui Jiemon, the current kuramoto (蔵元, President) of Matsui Brewery in Fushimi, Kyoto. Matsui Brewery brews the Kagura (神蔵) range of modern, structurally precise, fresh, and elegant sakes, which earned Platinum and Silver medals at the 2024 Singapore Sake Challenge.

Matsui Jiemon (second from the left), along with the team from Sake Street and myself (second from the right).

[Translated] “Koji is the foundation of sake brewing. Even with advanced technology, koji making remains the most important aspect.”

Matsui-san describes the koji-making process as one requiring the care of nurturing a child. Yet, there is an ever-present tension as koji work happens on a level invisible to the naked eye, so brewers must rely on experience and intuition to understand and imagine its state of development. Despite advancements in technology, koji making still relies on human skill and the power of nature to flourish. At Matsui Brewery, their meticulous choice over the details even extend to the precise strains of koji- kin used, and the people they trust to deliver them.

A POV view of kiri-kaeshi work at Matsui Brewery. About 10 hours into the process, koji clumps up (left side) and must be broken apart and loosened to ensure even koji growth, oxygenation and temperature. (Credits: Matsui Brewery)

"If the koji making process is done carelessly, the sake yeast cannot do its job well.” Matsui-san emphasizes. Even after dekoji (出麹, the removal of finished koji from the kojimuro), maintaining the state and hygiene of the room is of utmost importance. Only with well-made koji can the delicate balance of saccharification and fermentation be achieved during sake’s unique multiple parallel fermentation process. Naturally, the combination of high-quality ingredients with a deep understanding of koji’s interactions with yeast and rice forms the cornerstone of Kagura’s success.

[Translated] “There hasn’t been much innovation in koji making itself, so I believe innovative approaches are necessary to create new kinds of sake.” Matsui-san adds.

I can’t help but agree.

Singapore Sake Brewing & Locally Made Koji with Orchid Craftworks 

[Translated] “For me, making koji is the one thing I feel I can truly say I’m good at.” 

Hidden within Jurong Food Hub, Orchid Craftworks is Singapore’s first sake brewery, brewing sakes with a touch of terroir and tenacity to match the local palate.

Despite being a young brewery, established in 2024, their kojiyasan Yamamoto Yumika has seven years of koji making experience under her belt as a former kurabito (蔵⼈, sake brewery worker) in Japan. In addition to her work at Orchid Craftworks, she runs her own company, Kojito, which specializes in amazake and miso. Koji has always been an indispensable part of her life and career.

Yamamoto Yumika, critically assessing the quality of the rice post-steaming. (Credits: Orchid Craftworks) 

[Translated] “While sake brewing is said to be ‘First koji, second shubo, third brewing’, in the case of koji making, there’s a saying: ‘first, steaming, second, steaming, third, steaming’, emphasising the importance of properly steaming the rice.”

Yamamoto-san explains that perfectly steamed rice—firm on the outside and soft at its core—is essential for the successful propagation of koji-kin and enzymatic performance. A hard outer layer retains less moisture allowing koji-kin to attach easily, unlike a wet surface which is anaerobic. Additionally, it keeps rice grains intact during handling of the koji-mai. Conversely, the soft interior enables koji enzymes to penetrate and access the rice starch effectively.

In the midst of cooling the steamed rice to 35°C in preparation for koji-zukuri. (Credits: Orchid Craftworks)

Whilst brewing outside of Japan isn’t new, introducing koji to a new brewing space always presents unique challenges. Unlike long-established breweries across Japan, where a naturally curated ecosystem of koji-kin and beneficial microbes in the environment can offer protection and enhancement of the brewing space, Orchid Craftworks must curate their ecosystem from scratch. Adapting to Singapore’s tropical climate, with its high humidity and temperatures, demands a nuanced understanding of koji-zukuri (koji making). Yamamoto-san, however, remains undeterred.

[Translated] “In Singapore, it’s difficult to access the same facilities available in Japan. However, since the founding of the brewery, we’ve made numerous adjustments and improvements leading to better koji production.”

Currently, Orchid Craftworks produces both yellow and white koji using Koshiibuki and polished Yamadanishiki rice varietals, catering to various brewing applications. The brewery also crafts a range of doburoku as part of their expanding lineup of innovative craft sake offerings.

(Credits: Orchid Craftworks)

Doburoku: Appreciating the Flavours of Koji Deeply with Konohanano Brewery

What is doburoku? An ancient, completely unpressed version of sake that has been rediscovered in the modern era. With the introduction of special designated doburoku zones across Japan and the rise of Craft Sake brewing, doburoku is now enjoying a revival. This opaque white beverage is full of rice and koji, offering a literal mash-to-bottle fermentation experience.

Konohanano’s flagship Hanagumori The San, a white koji doburoku, showing its texture and opacity.

Just a five-minute walk from Senso-ji Temple in Asakusa, Tokyo, lies Konohanano Brewery, one of Tokyo’s forerunners in the doburoku brewing scene. This unique brewery and gastrobar in one is the perfect spot for sake lovers to explore doburoku over a glass (or three) of the porridge-like beverage. If you’re lucky, you might meet Kimura Yuzuki, the brewery’s toji ( 杜⽒, brewmaster), working the mash behind the glass. Kimura-san oversees the production of every fresh bottle you’ll find in the fridges that line the entrance.

[Translated] “Compared to traditional sake which can be drunk smoothly, doburoku allows you to directly enjoy the texture of rice and koji. The rice softened and saccharified by koji provides a drinking experience that feels almost like eating.”

“Drinking doburoku allows one to appreciate the flavours of koji more deeply and directly.”

Kimura Yuzuki covers the koji with cloth. Like a blanket, it helps keep the koji optimally warm for good growth.

Unlike sake, where rice and koji flavours are subtly present, doburoku’s unfiltered, unpressed nature retains the mash, revealing the complex interaction between koji enzymes, starch, and yeast fermentation. This results in a bold, creamy, umami-rich, edible profile, retaining all the amino acids, chestnut-ty sweetness, savories and acidity from koji fermentation — the volatile aromas and flavour compounds that are partially lost in sake post-pressing. For any enthusiast wanting to dive deeper into rice expression, koji and fermentation, doburoku offers a direct path towards a truly intimate understanding.

Giulia Maglio, Konohanano Brewery’s kurabito and koji-maker, plays an essential role in the process. Having made koji for four years and working as a contract kurabito (brewery worker), Maglio-san is the go-to person for all things koji.

Giulia Maglio (right) power posing with Kimura Yuzuki (left) at the entrance of the brewery.

“Even with extensive experience, koji remains unpredictable — it’s a living organism, after all. Each batch is unique, requiring careful attention and slight adjustments to meet its specific needs. No matter how many times you’ve made it, you can never predict the outcome with complete certainty.”

This unpredictability is what makes koji so beautifully rewarding to Maglio-san. At Konohanano, she predominantly works with Yamadanishiki and Kokuryo Miyako to make both white and yellow koji. Alongside Kimura-san, they guide the creative direction of the brewery’s doburoku, developing new craft sake concepts. Her latest project, the Yamahai Doburoku, contrasts the traditional yamahai shubo method with a modern take on doburoku brewing, which I hear might join the brewery’s series as one of the flagship labels.

“When it comes to doburoku, many people assume it’s easier to make than sake. However, now that I work at a craft sake brewery and handle doburoku daily, I’ve realised it’s actually the opposite,” Maglio-san admits.

“Since doburoku isn’t pressed, it remains unfiltered and undiluted, making it far more challenging to perfect. Any mistake during the process becomes much more evident in the final product compared to sake. This is why the ability to make excellent koji is absolutely crucial.”

Embracing Koji Fermentation Culture — Miso, Funazushi, Doburoku with Happytaro Brewery

[Translated] “I see my doburoku business as a way to carry rice koji culture into the future, which I believe sets it apart from the sake and craft sake industries.”

Nestled within Umi-no-Schole, a cultural and commercial hub just a 10-minute walk from Nagahama Station in Shiga Prefecture, lies Happytaro Brewery, founded by Ikejima Kotaro. The brewery’s name is a playful nod to Ikejima-san himself, where the kanji for “ko” (幸) in “Kotaro” means “happy.” After spending 12 years working as a kurabito across various breweries, he established Happytaro in 2017 as a handmade koji, miso and funazushi (an ancient type of sushi native to Shiga) store, with doburoku brewing commencing in 2021 after the introduction of the Craft Sake brewing license.

Ikejima Kotaro standing in his kojimuro where koji rests covered in kojibuta (koji trays).

[Translated] “For koji-making, it’s a skill that anyone can learn once they grasp the basics. These days, I don’t even use a thermometer after putting it in the koji room. It’s not because I’m a genius, but because I believe people in the past didn’t create koji with godlike skills.“

Koji is the heart of his business. Ikejima-san’s rice store and koji “culture room” (as he calls it), is almost the same floor size as the brewing space itself. His beautiful kojimuro, with wood clad walls, can culture up to 60kg of koji per session. Ikejima-san not only makes miso, but also sells his fresh koji to the public, empowering his customers to make lovely amazake, shio koji, miso and more at home.

[Translated] “People prepare the food they eat every day with all their heart as an expression of their love for those they care about. Even if you look at the quality label on rice koji, you cannot tell its true nature. I believe it is my duty as a koji-maker to provide trustworthy rice koji.”

Brewing doburoku was a natural progression of his fermentation portfolio. True to his vision, he decided to use the same strain of koji used to make miso for his doburoku—an unconventional choice for any traditional sake brewery today. Yet, historically, in the Edo era where koji making was an independent trade from sake and miso making, the miso-style soft and full koji was used universally in miso and sake fermentation. It wasn’t until sake breweries began in-house koji production, favouring a firm and harder style koji in order to create the crisp and clean taste we’re familiar with, continuing as the standard to this day. Ikejima-san‘s methods pay homage to the older, larger slice of sake brewing’s history in his world of craft doburoku brewing. This approach has resulted in his doburoku standing out as one of the most creamy, full-toned and comforting brews within the craft sake scene.

Happytaro’s doburoku in his tasting room. His labels depict the kitsune (fox spirit), guardian deity of rice fields.

In simplistic terms, miso koji is a fully ripened koji, unlike the majority of koji used for sake where koji- kin is sparsely inoculated on purpose. Fully matured, fully propagated koji has powerful enzymes stored to be able to act upon and dissolve rice in its entirety, resulting in that signature richness and generosity that is attached to the flavour and texture of Happytaro’s doburoku. Dabbling in both white and yellow koji, Ikejima-san‘s unconditional trust in his koji has allowed him to proudly produce vibrant all-koji doburoku concepts, demonstrating his prowess as both brewer and koji-maker.

[Translated] “Doburoku, brewed with trust in the rice grown by farmers who have a deep love for the soil and Lake Biwa that lies beyond, and made from fully ripened koji, tastes of love.”

Liberating Koji for the Future of Craft Sake with LINNÉ

[Translated] “If it were possible to create koji from other materials besides rice, we could potentially make craft sake using non-rice ingredients from around the world.”

Born into the Hijiri Sake Brewery family, Imai Shoya studied food biochemistry, experienced sake brewing in Japan, France and the U.S. and is no stranger to innovation. As the co-founder of WAKAZE Paris and the founder of the new phantom craft sake brewery LINNÉ, Imai-san is unleashing the full potential of koji and writing a new chapter in the history of sake.

Imai Shoya at SAKEJUMP, presenting his bottle of LINNÉ 800 Barley.

To Imai-san, koji is as essential to a brewer as a knife to a chef or a sword to a samurai; a constant companion and the extension of an artisan’s body. He recalls his days brewing at WAKAZE in Paris — though having accomplished sake brewing with rice, water, and yeast sourced from France, koji-kin, the ever so significant brewer’s tool, still had to be imported from home. This reliance underscored koji’s traditional boundaries and sparked a deeper question for Imai-san:

If we think of koji as an extension of the body, would it be possible to diversify koji as a tool for brewers to express more complex aesthetics and craftsmanship for the future of craft sake brewing?

With deep respect for traditional sake brewing, and having trained with some of the most revolutionary brewers, namely Aramasa, Masuizumi, Abe, and Hijiri Brewery, Imai-san believes that a potential future for craft sake lies behind two metaphorical doors.

An infographic outlining the differences between traditional nihonshu, craft sake and LINNÉ’s interpretation of craft sake. (Credits: LINNÉ)

The first door was unlocked by the Japan Craft Sake Breweries Association with the introduction of “botanical sake”, embracing a new brewing system that incorporates various auxiliary ingredients into the moromi (fermentation mash) whilst adhering to the multiple parallel fermentation process. From co-fermenting rice with herbs, tea, spices, fruits and more, the Association has, since 2021, challenged conventions to re-imagine a future where sake celebrates freedom of brewing whilst retaining its inherent, fundamental essence.

Behind the second door lies the heart of LINNÉ’s operation: redefining the purpose of auxiliary ingredients in craft sake. LINNÉ’s mission is to explore the possibilities where the purpose of auxiliary ingredients in craft sake transcend their supplementary role to become koji—the crucial driver of saccharification and enzyme fermentation in sake brewing.

To traditional sake devotees, the use of non-rice koji may seem sacrilegious. However, this innovation also paves the way for LINNÉ to explore sustainable brewing options, particularly as Japan faces a decline in domestically grown rice yields due to climate change. Expanding koji production to include alternative materials offers a viable path to reducing the ecological footprint of craft sake brewing, while ensuring its adaptability in an evolving agricultural landscape. Furthermore, LINNÉ’s advancements in the alternative koji space will unlock creative avenues for brewers to express umami, texture, body and structure in new, different and developed ways.

Imai Shoya making barley & soba koji at Haccoba Craft Sake Brewery in Fukushima. (Credits: LINNÉ’s crowdfunding page on camp-fire.jp)

[Translated] “Even just with rice, the diversity of sake can enrich people’s dining experiences. Applying Japanese techniques to non-rice ingredients could make the dining table more vibrant. It might even be possible to brew sake using ingredients (for koji-making) familiar to Singapore.”

To achieve this, LINNÉ’s 800 (Yao) series of craft sakes have already shown an integration of craft sake brewing with koji from imo (sweet potato), mugi (barley) and soba (buckwheat), with plans to eventually explore ingredients from around the world.

Ultimately, Imai-san believes that a re-discovery of koji from a global cross-botanical perspective is the key to shaping the future for koji culture, craft sake and fermentation on a global scale. The possibility of creating koji with the world’s limitless ingredients sparks hope for a global practice of koji fermentation, particularly in areas where rice cultivation is not a dominant agricultural activity.

At present, LINNÉ remains a phantom brewery. That is to say, a brewery without a permanent brewing space. A craft sake nomad travelling around Japan, Imai-san makes LINNÉ’s craft sake using the tanks of other breweries, eventually hoping to raise enough funds to build his own permanent brewery in Kyoto.

Experimenting with imo-koji. Normally, sweet potato is steamed, diced and dried before koji-kin is applied, however, this version appears to be kept whole. (Credits: LINNÉ on Instagram)

[Translated] “There is a wealth of literature on techniques for rice koji, but few examples involving other ingredients. I encourage you to imagine what kind of sake you’d create if you had the chance and experience the limitless possibilities with all your senses.”

OMU NOMU in Singapore will be receiving the LINNÉ 800 Barley and 800 Soba in January 2025, with plans to showcase his other works in the near future.

To A Promising Future of Koji 

In 2025 and beyond, koji continues to thrive as an emblem of Japanese fermentation culture, and as the world becomes more interested in koji’s versatility and transformative power, we’re privileged to be in an era where fresh developments are combined with the preservation of koji’s deep cultural roots. For those like Matsui-san, Yamamoto-san, Kimura-san, Maglio-san, Ikejima-san, and Imai-san, koji represents more than just a saccharifier; it embodies a living, breathing connection to tradition, creativity, and the human element that remains essential to its cultivation.

The future of koji may lie in bridging the ancient with the modern—whether through innovative techniques, new knowledge, or embracing its adaptability. By fostering a deeper understanding and respect for koji, there’s good promise for koji to flourish in exciting new contexts.

For now, I hope you’ll find an interest in koji, and I wish that your journey into sake, shochu, miso, soy sauce and more just becomes a little sweeter.

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P.S.

If any of the breweries above interest you, you’ll find them at OMU NOMU Craft Sake & Raw Bar!

OMU NOMU Craft Sake & Raw Bar

302 Beach Road, #01-08

Concourse Skyline S199600

Mondays – Saturdays, 12-2:30pm & 5:30-12am

https://omunomu.sg (Instagram: @omunomu.sg)

Words by:

Gerard Alexis

Chef-Owner and Sake Sommelier of OMU NOMU Craft Sake & Raw Bar, Representing the Japan Craft Sake Breweries Association in Singapore Craft Sake Lover; Certified Sake Sommelier; Intl’ Kikisake-shi;

Sake Scholar; Singapore Sake Challenge Judge (Instagram: @alexdrinkscraftsake)