Koji: The Mold You Want In Your Kitchen

16:49 minutes

three pastes on spoons, one is a orange-red, the middle is brown and grainy, the third is a yellow-mustard color
Amino pastes, made with koji. Credit: Andrew Wang

When chef Jeremy Umansky grows a batch of Aspergillus oryzae, a cultured mold also known as koji, in a tray of rice, he says he’s “bewitched” by its fluffy white texture and tantalizing floral smells. When professional mechanical engineer and koji explorer Rich Shih thinks about the versatility of koji, from traditional Japanese sake to cured meats, he says, “It blows my mind.”

Koji-inoculated starches are crucial in centuries-old Asian foods like soy sauce and miso—and, now, inspiring new and creative twists from modern culinary minds.

And Shih and Umansky, the two food fanatics, have written a new book describing the near-magical workings of the fungus, which, like other molds, uses enzymes to break starches, fats, and proteins down into food for itself. It just so happens that, in the process, it’s making our food tastier. (Check out a recipe for amazake, the foundation of sake and rice-based drinks, in an excerpt of Shih and Umansky’s book Koji Alchemy.)

You can grow koji on grains, vegetables, and other starchy foods, and make sauces, pastes, alcohols, and vinegars. Even cure meats. Umansky and Shih say the possibilities are endless—and they have the koji pastrami and umami popcorn to prove it.

popcorn covered in white fuzzy mold
Popcorn Koji. Credit: Andrew Wang
a close up of tiny beads attached to long white strands. they are the fruiting bodies of the mold
Koji spores on the popcorn. Credit: Andrew Wang
ginger, a root vegetable, and collard greens
Kojizuke. Credit: Andrew Wang
white cloth around some light brown cheese
Ricotta Miso Cheese. Credit: Claudia Mak
three slices of meat. one is raw and plain, the second is raw and dusting with white powder and brown mold, the third is raw but the seasoning is rubbed in
Meat coating, with koji. Credit: Peter Larson

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Segment Guests

Rich Shih

Rich Shih is an exhibit engineer at the Museum of Food and Drink and co-author of Koji Alchemy: Rediscovering the Magic of Mold-Based Fermentation (Chelsea Green, 2020). He’s based in New York, New York.

Jeremy Umansky

Jeremy Umansky is co-owner and co-chef of the Larder Delicatessen and Bakery, and co-author of Koji Alchemy: Rediscovering the Magic of Mold-Based Fermentation (Chelsea Green, 2020). He’s based in Cleveland, Ohio.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. If anything is certain in this stay-at-home pandemic, it’s that people are cooking, like these listeners who called us to share their proud creations.

MORGAN: Since the onset of the pandemic, I’ve brewed three big batches of kombucha using a variety of teas– chai, [INAUDIBLE], and white. I’ve grown oyster mushrooms from a mail ordered bag of mycelial spawn and fermented two jars of sauerkraut.

CAMILLE: Through this quarantine, I’ve done a lot of bread baking and a lot of fermenting. I recently made a sauerkraut with red cabbage, beets, and carrots. And it was really, really good. And I have a whole bunch of ginger that I accidentally bought too much of. So now, I’m going to make a fermented ginger paste and see how that works.

TANYA: So I made dandelion jam, pesto from the chickweed and dead nettle in my yard. I have been brewing kombucha. I’m trying to make [INAUDIBLE]. I have been using a mesophilic yogurt starter to make my own yogurt. It’s really been great fun and a good way to pass time.

IRA FLATOW: Those were the voices of Morgan from Portland, Camille and Tanya from Arkansas. Thank you for your submissions to this Science Friday VoxPop App. And as we face still uncertain opening dates for workplaces and recreation around the country, what better to do than play with our food?

Maybe you’ve already done the sourdough, or the pickles, or the yogurt, right? My next guests have the perfect next step for you. It’s called koji. It’s a white fuzzy mold, and it smells like fruit. And we can thank it for a splendid array of foods from East Asian cultures, including soy sauce, miso, and sake.

And my next guests want you to join in to make culinary delights with the help of this magical mold, even beyond the traditional uses. Think koji charcuterie or miso peanut butter. Here to talk more about the transformation power of Aspergillus oryzae are my guests, the co-authors of the book, Koji Alchemy.

Rich Shih is a mechanical engineer by day and a exhibit engineer for NYC’s Museum of Food and Drink. And Jeremy Umansky is a co-chef and co-owner of Larder Delicatessen and Bakery in Cleveland, Ohio. Welcome to Science Friday.

RICH SHIH: Thanks for having us on.


IRA FLATOW: I know I’ve already giving the overview that koji is a mold and that it’s been used for thousands of years. But please, you got to sell me on the idea of using a mold to make food. Jeremy, why is this so delicious?

JEREMY UMANSKY: Well, one thing to keep in mind is you’re already eating this mold in one way, shape, or form. I’m willing to bet that virtually every listener today has some soy sauce in their house, whether it’s a bottle of it or a little packet from Chinese takeout. And that soy sauce cannot be made without this mold.

So we already have it in our lives. We already eat so many different fermented foods that rely on molds, things like cheeses and charcuterie. We can lump yeast into that. They’re both types of fungi. So bread– it already exists there. So using it to make foods more delicious is pretty simple and straightforward.

IRA FLATOW: And is it the rice that we’re really cooking with it? Right?

JEREMY UMANSKY: Exactly. You can’t just take the spores of the mold and make something delicious. You have to get it to grow first. So you grow it on rice, or barley, or actually any starchy substrate. So it could be wheat berries. It could be rye. It could be hominy.

IRA FLATOW: Give me an idea what it looks like. I’ve seen photos. It looks very pretty growing there on the rice.

JEREMY UMANSKY: I think “pretty” is a lackluster word, Ira. It is sultry and stunning. I mean, it’s sultry and stunning. It really is. You look at it and you get lost in it. It is just so white, and fuzzy, and fluffy. It’s inviting, almost like you look at a picture of a sky with beautiful cumulus clouds in it. And you’re like– it just makes you relax and feel at home. And it just– oh, I could hug one of those clouds. Or I could lay down on it and take a nap.

Koji has that same effect on you when you [INAUDIBLE] and it’s growing fresh. And then you throw its aroma on top of that, which its aroma is green apple, and champagne, and honeysuckle, tropical fruit like mango and pineapple with a little bit of mushroom-iness there. Some people even say they pick up roasted chestnut. I mean, it’s just absolutely bewitching and intoxicating from how it looks, to how it smells, to how it tastes. It’s just absolutely incredible.

IRA FLATOW: You sound like you’re describing a fine wine. Rich, does it do that for you, too, make you feel that way?

RICH SHIH: Yeah. I mean, I am not as externally excited about koji as Jeremy is. But internally, it just blows my mind. Koji is so simple. All you need is a warm space with a little bit of humidity that can be achieved in an array of ways, very similar to setting up for bread proofing.

And you just basically boil some water or you set up a steamer. And you mix these ingredients in, dust on some spores, and you mix every 12 hours. And who can’t do that over the course of two days? I mean, you can go to work, come back home, and do your mix. And at the end of the day, it’s done and ready to go.

And then all you have to do is add some water to it and some grain like cooked grains. And you can make this amazing sweet porridge that you can also use as a marinade. That’s the perfect marinade for any piece of meat or protein you put it on. Because we often go through the exercises of making a marinade such that you enhance the flavor of the core component itself.

Koji does that by default, by taking the enzymes to break down the constituent parts of this food and creates basically what I like to refer to as an automatic barbecue sauce that has nothing– that is made with everything that’s part of the food that you create it with.

IRA FLATOW: And, Jeremy, does it have a taste of its own? If I put it on rice, for example, and I’m growing it on rice, I know what rice tastes like. Will then the other flavor there be that of the mold?

JEREMY UMANSKY: Yeah, so it’s actually going to transform the rice itself. So oftentimes, we talk about one of the molds, Aspergillus luchuensis. That produces citric acid as it grows. Not as a byproduct, but actually produces it as it grows. And if you were to eat some rice that had this mold growing on it, it would taste like Sour Patch Kids.


And we’re talking just plain old rice here.

IRA FLATOW: And, Rich, are there different strains of koji that produce different flavors?

RICH SHIH: There are specific kojis that create different flavors based on their enzyme engines. Sojae has an enzymatic engine that is more on the protease side to break down proteins into amino acids. As these enzymes become active in terms of breaking down the basic food substances, you get all sorts of interesting and funky flavors. I recall every time that I grow it on rye or even teff, I get these mushroomy aromas as well as flavors.

IRA FLATOW: We’re all familiar with soy sauce, for example, which is one of the many things koji is used to make as you said before. How does that process work? I mean, how do you start from a mold and get to a tasty soy sauce, Rich?

RICH SHIH: Yeah, so to make soy sauce, you basically cook some soybeans, whether you soak them and steam them or you boil them, such that in this state where they’re cooked all the way through. So that’s one part of it. The other part is you have toasted cracked wheat.

So what you do is you basically have a one to one ratio of these two ingredients. You mix the ingredients together. You allow it to cool to a temperature that’s pretty much to your body temp. And that’s how a lot of Japanese makers gauge when you can actually inoculate it with the spores.

Then you just basically sprinkle it lightly with spores. Once you grow the koji, you put it in basically a salt water brine. And you allow it to ferment. And then you have a specific mixing schedule based on the temperature conditions and the stages of making it.

IRA FLATOW: So that’s how it happens.

JEREMY UMANSKY: Yeah. And the only difference between miso or an amino paste, as we call them, because they’re pastes and they’re rich in amino acids and an amino source like soy sauce, is water content. So whether you decide you want to make a miso, or gochujang, or any of these pastes, or you want to make an amino sauce, it’s just varying degrees of water that they contain. So you can go in either direction just as easily.

IRA FLATOW: And sake? Can you get alcohol?

JEREMY UMANSKY: Oh, my God. Can you? It’s really interesting, because of the breadth, the types of sugars like the [INAUDIBLE] sugars, and the glucose that’s formed in amazake, which is a mixture of a cooked starch, the koji inoculated starch, and water. You can get a lot of alcohol.

I mean, you can get upwards towards a 12% ABV without doing anything extraordinary, literally just putting some yeast in and letting it sit and be happy. So you can get some fantastic alcohols and some of the kojis that produce these different flavors, and aromas. And some of that produce citric acid– can just add infinite layers of complexity to any of the alcohols you make.

IRA FLATOW: I understand you can make popcorn. Rich, how did you come up with that?

RICH SHIH: So I was just looking around in my pantry to figure out, hey, what could I really play around with to create this accessible starch that could have these gaps, such that the koji– the mold would grow in between. Because you need a level of air in between the grains.

And I just saw this popcorn. And I said, well, when you pop popcorn, you’re basically– you create– it’s basically a pressure cooker for each kernel. And when it blows up, it uses the internal steam to blow it up to create a puff condition, such that the starch is very accessible.

As we all know, when we eat it, it dissolves in our mouths. So I just decided that, hey, I don’t have to cook it. I can make it explode and create this accessible starch. And all I needed to do was not to make it too wet. It’s just to mist it with a little bit of water. And there I had my accessible starch.

I had plenty of air for the mold to grow. And I just– to assure that it would grow well, I just dusted it with a little bit of flour and the spores. And it took off like gangbusters. And for somebody who doesn’t necessarily want to sit and wait for your grains to soak or your beans to soak and you just want to try something, it’s a pretty cool starting point.

IRA FLATOW: Can you use koji as a quarantine experiment for all of us now?

JEREMY UMANSKY: You most definitely can. Whatever level you want to plug in with, you can. So for example, traditionally throughout Japan, they use a product called shio koji, which is this porridge of the molded rice or barley with salt added to it.

And that is used as a general all-purpose seasoning. So you can easily find that online. And you can order a little package of it. It’ll show up at your doorstep. You can rub it on some chicken or some steak or saute some vegetables with it and see instantly the short-term drasticness that koji brings to amazing flavor as you’re working with it.

So while it is straightforward, should you not want to grow the mold, there are several great companies out there. Most of them are really small family-owned businesses like Cold Mountain Koji. And you can buy pre-inoculated rice or barley from them.

So if you wanted to go ahead and make an amino paste like a miso or gochujang, you could simply buy the inoculated grains, mix them with a little water to hydrate them, open up a can of beans, mix those together with the inoculated grain and a little bit of salt.

We use 3% of its weight as a very minimum on the base, but you can go 5%, 7%, 10% and let it sit. And you will have your own amino paste, something like a miso. Or you could simply order spores and just go all in and start growing the mold on everything like we do. There really are no barriers to entry for working with koji.

IRA FLATOW: I’m Ira Flatow. And this is Science Friday from WNYC Studios. Rich, does playing with molds give you a new dimension about creativity with food? We’re talking about jams, and pickles, and pestos, and vinegars. Now, you have something new to play with.

RICH SHIH: I think it’s just the fact that koji allows you to do so many multi-faceted things. A lot of us focus on very specific fermented products like kraut, kimchi, or yogurt, or a very specific beer from a very specific region with all these incredible malts and hops, and specific water, and a specific alcohol content. What we have to think about is they got to that point.

Because somebody just left something out for a period of time, such that it can be consumed at a specific time for survival. And people got tied into this idea of, hey, I really love that. I want to keep making it the same exact way. Because not only is it safe to consume, but it’s delicious. But our idea is to think about this in a much larger scheme in terms of that specific discovery– can be with any food that you apply it to. I mean, granted, things can go wrong.

But it’s the adventure of all these possibilities that we have access to, whether it be the ingredients, technology, ideas, or even past products that we know and love that we can change up and play around with. And that’s the nature of what we love to do.

IRA FLATOW: How does it go with pastrami?

JEREMY UMANSKY: Oh, man. If I describe it, I’ll be bragging. So I’ll let Rich who’s eating my pastrami describe it.

RICH SHIH: So I think one of the things to understand about it is that through these– through usage of creating these amino acids through the enzymes and these sugars, you get this amazing flavor that bounds– that’s above and beyond what you could do in a traditional method.

With a traditional method, you have the slow process of heating, such that you can create a food that is quite unctuous by breaking down the connective tissue. And it makes it moist. And it makes it very pleasing. And then you also have a brine to create this salinity. But with the koji, you can actually create this level of tenderness and depth of flavor without doing any sort of manipulation. And it’s just bringing it up a level of what it already is.

IRA FLATOW: I’m coming over.

JEREMY UMANSKY: We’ll save a seat on the patio for you. We’re not open in the dining room for a little bit.

IRA FLATOW: Yeah. Thank you both. This was quite fascinating. I hope we have inspired a lot of people to try some new cooking ideas. Rich Shih is a mechanical engineer by day and an exhibit engineer for NYC’s Museum of Food and Drink.

And Jeremy Umansky is a chef and owner of Larder Delicatessen and Bakery in Cleveland, Ohio. And they’re co-authors of the book, Koji Alchemy– Rediscovering the Magic of Mold-Based Fermentation. Thank you guys for enlightening us and giving us something more to do as we stay home.

JEREMY UMANSKY: You’re welcome, Ira. Thank you so much. This has been a dream come true.

RICH SHIH: Yeah, this was [INAUDIBLE]. Yeah, thank you so much.

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