Koji: An Ancient Mold and Its Modern Renaissance

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Close up of koji.

For most people, including the word “mold” in the culinary vocabulary sounds like the opposite of delicious. But for fermenters, lovers of cheese, and curators of salami, mold worship is a kindred character quirk. Aspergillus oryzae — or koji, as it’s been called for centuries in Japan — enjoys one of the most storied and multifaceted relationships with humans of any of our micro-epicurean allies. In this article, and the following articles in this series, we’ll explore koji as thoroughly as possible for the nerds around the incubator. What is koji; how does it produce a huge variety of flavors; how can we discover its potential in the kitchen; and where is our partnership with this mold headed?

Meet the Mold

Koji is a filamentous mold, much like the type you might find growing on bread or oranges. Molds are fungi, and fungi attach themselves to their homes via hyphae (singular: hypha). Think of hyphae to fungi like roots to a plant. The individual hypha together make up many hyphae, and the network of hyphae is called a mycelium. A. oryzae makes its white to yellow-green network of fungal hyphae, or mycelial mat, on the surface of foods. Its favorite substrate is grain, so it’s aided the fermentation of rice, barley, and red beans for centuries. It’s predominantly associated with Japanese cuisine; its name comes from the culture, and its uses and flavor are key throughout the cuisine. Koji is a crucial component of miso, soy sauce, and sake, just to name a few foods with which most people are familiar. This friendly, multifaceted mold unlocks the promise of possibility in the kitchen.

Jeremy Umansky experiments with koji in his development of “vegan charcuterie.” He believes that you can use the mold to put a vegetable through the same charcuterie processes as meat.

The history of koji traces back to China. Early analyses of Neolithic pottery from the second millennium B.C. reveals residue of rice-based fermented wine. The first reference to using mold in culinary production comes from 300 B.C. in the time of the Zhou dynasty, and relates to the preparations of grain-based wines and bean pastes. Koji is a saccarifying mold, meaning that its most potent use is the breakdown of starches, which are heavily present in grains.

Over time, the fusion of koji with yeasts and ash (which was added to change the pH) isolated A. oryzae from other Aspergillus strains and non-related surface molds. This achieved two things: the wealth of variants in color and enzyme production within the species A. oryzae, and likely the mutation leading to koji becoming one of the only Aspergillus molds that doesn’t produce aflatoxins, which are toxic compounds produced by certain molds.

Any time a fungus injects its hyphae into a substrate, it goes to work. This is how koji affects the flavor and texture of food. First, we must emphasize that koji in and of itself packs complex flavor, and — depending on the species variant, as well as the grain (or other) substrate — can unlock flavor notes from floral to cheesy. Koji also produces powerful enzymes that further affect its “host” — be it a mound of rice, a whole parsnip, or a piece of pork belly. Furthermore, koji’s enzymes don’t stop working when the mold itself changes conditions. This means that koji can be young, mature, in its reproductive stage, or even dead, and its enzymes will remain potent influencers of flavor, texture, and chemistry. It also means that koji-populated substrates can be manipulated in a number of ways to produce secondary and even tertiary flavor reactions; one can dry, chill, freeze, heat, or further ferment koji, and still taste its magic.

Koji produces over 50 enzymes, namely amylase, protease, and lipase enzymes, which break down sugar, protein, and fat molecules, respectively. As the enzymes split larger molecules into their smaller component parts, flavor soars, and the smaller components become available to other biological processes, such as lacto-fermentation.

The Secret in the Sauce

If you think of koji as the trunk of a tree, then the branches, twigs, and leaves are the resultant foodstuffs that koji’s primary fermentation supports. Traditional secondary fermentations produce staples such as sake, douchi (black soybeans), miso, and soy sauce.

Amazake is one of koji’s main secondary products.

Koji’s ability to release the simple sugars in starches to feed yeast has been paramount throughout its history. If there’s anything universal around the world, it’s combining yeasts with sugar to produce alcoholic ferments. Some indigenous peoples of the Americas accomplished this with saliva by chewing corn to make chicha. Malted grains did the trick for beer in the West. And in Asia, fungus is used to create fermented beverages; the most globally recognized is Japanese sake. Sake’s traditional fermentation is steeped in ritual, tradition, and lore. The production has three distinct phases. First, koji grows on polished rice. Once the koji is ready, it’s turned into sweet amazake, which is then fermented into sake. Some sake goes through yet another fermentation step: It’s left open to the air and bacteria, which transforms it into another pantry staple — rice vinegar.

Soy sauce, or shoyu in Japanese, is one of the few Asian ferments that’s also a staple in kitchens of the Western hemisphere. But soy sauce is far from an only child — there are many iterations of umami-rich dark liquids, and they all begin with koji. For example, true tamari is a by-product of miso; it’s the small bit of liquid that purges to the top during fermentation. Given there’s very little of this produced from a batch of miso, enterprising folks came up with ways to produce tamari in large quantities centuries ago. Traditionally, people made tamari with soybeans and water. The soybeans were soaked, steamed, mashed, formed into small balls, and hung outside to catch wild Aspergillus. Once the koji had grown on these soybean balls, they were dried and placed in large cedar vats with salt and water. Today’s process is similar, except that koji now grows on steamed beans that are put in a saltwater slurry. The resulting mash is then pressed under weights and allowed to ferment for at least a year, at which point the tamari is pressed from the bean paste.

The mold growing on a group of black beans.

Soy sauce differs from tamari in that it’s not a by-product, but rather its own process, and it’s one of many variations of fermenting a liquid-y mash of wheat and soy. This process has its roots in China, but has long since become a mainstay across all of Asia. For example, Indonesia makes its own version of soy sauce, known as kecap manis. This sauce — a mix of fermented soybeans, palm sugar, star anise, galangal, and other aromatics — is sweeter and thicker than traditional soy sauce.

The mass production of soy sauce uses industrialized shortcuts to get around the months, even years, it takes to achieve the rich, signature flavor of its traditionally made counterpart. Manufacturers often manipulate temperatures to hasten the process, use stronger monoculture spores, and add more salt. Lesser-quality ingredients are used, and some soy sauces are made of defatted soy grits — a by-product of soybean oil. The cheapest brands aren’t fermented at all, instead using a process known as “rapid hydrolysis” or “acid hydrolysis,” in which the defatted soy meal and cornstarch are heated with hydrochloric acid to release the amino acids. To truly appreciate what microbes bring to the table, buy soy sauce that’s unpasteurized and naturally brewed using traditional methods. Not only will the flavor be better, but you’ll have a more powerful flavor tool, because you’ll also be able to employ the soy sauce’s contribution of enzymes.

Koji is also key to the development of bean pastes, which are important staples in Asian pantries. Miso, Japan’s venerable paste, is more than just a soup at your favorite Japanese restaurant. It’s an umami-rich, salty, fermented paste, traditionally made with soybeans combined with either rice koji or barley koji. As always, the koji is tasked with slicing open the molecules to release delicious flavors.

A Culinary Revolution

As fermentation moves to the forefront of current culinary practice, traditional methods are being adopted and applied all over world, and fine-tuned with local sensibilities. The processes remain complex, both in use and in result. Koji is inspiring cooks everywhere with its ability to elevate and amplify otherwise nuanced flavors, and the ways in which koji lends itself to experimental fermentation are multiplying. For now, the koji renaissance riffs off a few koji basics:

Koji primary fermentation. This is the use of a grain or other substrate that’s inoculated with koji spores and incubated to encourage the growth and flowering of koji mold. The koji-covered food is then directly used in other foods to add and amplify flavor, or provide inoculant for further fermentation, or both.

Shio-koji. This is one of the main secondary fermentation products for which koji is well known. Shio-koji is koji-fermented grains mixed with water and salt, and then fermented a second time. This creates an umami-rich sauce similar to soy sauce, but it’s much quicker to produce, and instead of imparting its own flavor as soy sauce does, it allows the food to shine, amplifying flavor notes to their highest expression.

Ama-koji or Amazake. This is another main secondary fermentation product of koji. Koji-fermented grain is combined with water and additional cooked grain, and is then fermented again at higher temperatures. This produces either a sweeter or sour secondary ferment, as opposed to shio-koji’s salty profile.

Koji Charcuterie

Jeremy Umansky is one of the chefs leading the koji revolution in the United States. At Larder, his delicatessen in Cleveland, Ohio, Umansky uses koji to redefine Jewish deli standards. Pastrami? Cured with koji. Sour cream? Cultured with koji. Chocolate babka? Baked with koji. Pickles? Fermented in sour amazake. His vegan charcuterie board features new, innovative tastes such as carrot pastrami and beet carpaccio.

Koji is regularly employed in charcuterie.

If you think “vegan charcuterie” sounds like an oxymoron, you’re not alone. Umansky feels that if you’re applying the same procedures of traditional meat charcuterie (curing, smoking, and hanging) to vegetables to gain the texture and mouthfeel of meat, then the word to describe it should remain “charcuterie.” He argues that the definition of charcuterie has already evolved to include many approaches to preserved meats across cultures. The term describes the technique regardless of its application.

Koji unlocks flavor notes in ordinary ingredients that are hard to reach with standard cooking techniques. For example, put a vegetable through a meat-curing process — cook it until al dente, cure it with salt, herbs, and spices for a few days, and then smoke it — and you’ll have a rich, salty, smoked vegetable. But before hanging it, if you layer on koji, the hyphae will reach down into the vegetable and consume their host, secreting their powerful enzymes and drawing out flavors you recognize. The vegetables keep their identity, yet the flavors become magnified. In short, during the ensuing hang time, koji contributes an aroma boost that drives in the umami, the fifth flavor that we identify with charcuterie. In the case of these vegetables, the proteins are broken down into amino acids.

Koji-cured meat.

In addition to vegan charcuterie, koji accompanies meat in ways that push traditional culinary boundaries. Koji can be grown directly on meat to tenderize otherwise tough cuts, and to impart a dry-aged flavor to meats intended for fresh cooking. Additionally, it’s grown on salt-cured meats as a flavor and cure accelerator, and as a protective mold. Experiments with koji-cured whole muscle and salamis have shown that the active enzymes in koji allow meat to cure in about half the normal timeframe, with amplified flavor. Experimentation on drawing down water activity using koji and reduced salt content have wide applicability in the world of preserved foods, and the manipulation of koji’s probiotic metabolites could have a massive impact on the need for isolated starter cultures in fermentation.

Maitake Sausage.

Garums are a family of meat or animal protein-based sauces descended from traditional fish sauce. In fish sauce, enzymes from within the guts of fish actually digest the fish parts, and in the presence of salt, they create an amino-rich liquid. Koji replaces the work of these gut enzymes to produce the same digestive result. Now, creative manipulations of animal products are cropping up in kitchens all over, including grasshopper garums and bee pollen garum.

Beyond the Kitchen

As koji’s applications — from flavor enhancer, to enzymatic digestion aid, to electromagnetic conductivity — are explored in kitchens and labs, fungi breeders are working to further hone koji’s enzyme output to increase and broaden its possible uses. Koji has been employed to break down biodegradable plastic, specifically polybutylene succinate (PBS), found in packaging film, bags, and film for mulching. Koji is also used in laundry formulations due to its production of lipase enzymes, which break down fats.

Although Aspergillus oryzae has been co-evolving with humans for centuries, new epicurean preferences, as well as modern global problem solving, are coming together to elevate and disseminate an awareness of koji’s history and capacity. In many ways, the human journey with this ancient mold has only just begun.

While working with koji might sound intimidating, it isn’t difficult. In the next article in this series, we’ll introduce the science behind koji, and show you simple ways you can begin to harness its beguiling flavor in your own kitchen.

Meredith Leigh and Kirsten K. Shockey are fermentation fanatics. Their expertise spans fermented fruits, beans, vegetables, and meats. They’ve teamed up to spread the gospel funk and the magic of koji whenever possible. Kirsten’s books include Fermented Vegetables. Meredith is the author of The Ethical Meat Handbook.

Inspiration for edible alchemy.