Shoyu Koji 醤油糀

In this post, I’m going to introduce to you yet another umami seasoning made with koji. Previously, I’ve introduced shio koji 塩糀/塩麹 to my blog readers. Have you read it yet? Do run through it if you have not as it contains information on koji which will help you understand this post better.

Brewing my own traditional fermented soy sauce is beyond my reach right now. So I was excited to learn about shoyu koji 醤油糀/醤油麹/しょうゆ糀/しょうゆ麹. Shoyu koji is made by fermenting koji in shoyu (Japanese name for soy sauce). With the help of the famous Japanese fungus called Aspergillus oryzae aka koji, regular shoyu is transformed into super umami shoyu.

Shoyu Koji 醤油糀
Delicious and umamilicious shoyu koji 醤油糀! 😋😋😋

Because shoyu already contains quite a good amount of glutamate (shoyu is made by fermenting soybeans &/or wheat and salt with koji), fermenting shoyu with koji further increases its umami. Compared to shio koji, shoyu koji has 10x more umami.

Let’s learn how to make shoyu koji. It’s even simpler than shio koji.

I’d made shoyu koji with dried koji rice/kome koji before. Since I’ve never tried it with dried koji brown rice/genmai koji, I’ll be using it for this recipe. Feel free to use either one for this recipe. Measurements are the same. If you’re using fresh (raw) koji, then the measurement is different. I will highlight the weight difference in the recipe below.

Dried Koji Brown Rice
Place dried koji brown rice in a cleaned glass jar. Mine is a clamp lid glass jar. Use a spoon or a chopstick to break apart the dried koji brown rice if they stick to each other.

By the way, do you notice the white specks on the brown rice? They’re koji! If you go back to the shio koji post and scroll to the 3rd and 4th photos, you’ll see the fungus on the white rice too, though it may be a little difficult to see via photos as they are white/light yellowish in color.

Mix Dried Koji Brown Rice With Soy Sauce
Pour good quality shoyu into the glass jar and mix well. Make sure the grains have all been separated. If not, it will not be fermented evenly. Btw, I’m using Kikkoman koikuchi shoyu to make this shoyu koji.

Good quality shoyu are naturally brewed/fermented and is free from artificial preservatives, artificial flavorings, artificial colorings and additives. If you are able to obtain raw unfiltered unpasteurized naturally brewed/fermented shoyu, use it to make shoyu koji as it’s the best selection.

Next, screw the lid loosely if you are using a regular glass jar. For clamp lid glass jar type, remove the rubber gasket before clamping the jar. This is because gas may be released during fermentation so you need to have a pathway for the gas to escape. For the clamp lid glass jar, I use a thin cloth to cover the jar’s mouth before clamping it. It’s to prevent bugs from entering the jar (encountered a bug problem in my shio koji solution once and I had to discard it).

Ferment in a cool place at room temperature. Similar to shio koji, you must stir the solution daily. You can use the shoyu koji when the koji rice in the shoyu becomes soft and the solution thickens. The best is to ferment for a month and if you would like a deeper and richer shoyu koji, ferment for two months to a year or more! Shoyu koji will not go bad as the solution is salty enough. If you’re fermenting for over a month, then it’s okay to stir every 2-3 days after the 1-month timeline has gone by. I’ve seen recipes suggesting 1-2 weeks of fermenting timeline (depending on warm/cold weather). From my experience, the shoyu koji is still too young after a week of fermenting in warm weather.

If you taste the solution during daily stirring, you’ll experience the metamorphosis of the shoyu from a regular one to an extraordinary shoyu full of complexity and rich umami, and when you think it can’t give any more, it delivers more miracles. Do note that even when you store the shoyu koji in the fridge, it will still ferment albeit a little slow and you’ll surely notice its changes.

Shoyu Koji 醤油糀
This is the shoyu koji made with koji brown rice/genmai koji above that is now nearly 4 months old.

Once the shoyu koji is ready to be kept in the refrigerator, pour it into a clean air-tight glass jar. Store for ~10 months in the refrigerator or a year in the freezer. You can also blend the shoyu koji into a puree before storing but I don’t see any point in doing that.

Shoyu Koji 醤油糀
Because I’ve fermented shoyu koji for a month before, this time I tried fermenting it for a longer period. This almost 4 months old shoyu koji made with koji brown rice/genmai koji is ready for storing. The shoyu koji is thick and dark brown in color, extremely rich and savory, has a very deep taste and some sweetness to it. It no longer smell like regular shoyu but koji-fied shoyu. The koji brown rice grains are mostly still in their whole form but soft. It truly feels like I’ve created a top-notch shoyu which is indeed the case!

Shoyu Koji 醤油糀
This is the first-ever shoyu koji I made last year.

Shoyu Koji 醤油糀
It was made with koji rice/kome koji and fermented for a month. It has a lighter brown in color, rich in umami, some sweetness and also has a depth to it yet mellower than the one made with long-fermented koji brown rice above. It also has a distinct aroma that is rightly shoyu koji. The koji rice for this one has broken down just like the case with shio koji made with koji rice.

What can you use shoyu koji for? Use it just like regular shoyu, as a seasoning, and as well as in condiment, dipping sauce, sauce, topping, dressing and as a pickling agent. Shoyu koji contains living enzymes that will help tenderize ingredients just like shio koji. I’ve used shoyu koji as a substitute for shoyu in cooking (stir-fry, braise, stew, soup, fried rice, pasta, yakisoba, etc), in meat marination and as a dipping sauce for sushi (always inform the sushi restaurant not to give me their green packets of shoyu. Only wasabi). Other ideas are to put it on rice with raw egg, onigiri, tofu (chilled, steamed, fried), sashimi and natto.

Shoyu Koji 醤油糀

One of the biggest advantages of homemade shoyu koji is that the enzymes and probiotics are alive and active and the nutrients are still intact. Unlike the ones that have been heated and/or pasteurized in the market which greatly decrease or destroy the beneficial communities in the shoyu koji.

You don’t have to use a lot of shoyu koji compared to shoyu as shoyu koji has a deeper taste, more umami and is sweeter. Compared to shio koji, shoyu koji is more savory and has a stronger taste. Please gauge/estimate how much shoyu koji you need for cooking initially. Once you’re used to it, you’ll know how much to use by feel. Since I jumped onto the koji bandwagon, I’ve been using shio koji and shoyu koji non-stop while sea salt and shoyu were no longer a part of my cooking. Thus far, only sea salt has made an appearance when I was making bacon. I use way more shio koji than shoyu koji in my cooking, just like how I use more salt than shoyu (a large part of my cooking uses sea salt).

Together with shio koji, they make an indispensable umami seasoning team that is loved by those that have personally experienced its miracle. They are wonderful all-around magic helpers that are a definite PLUS in anyone’s kitchen arsenal.

Give it a try because you’ll love it!

Shoyu Koji 醤油糀
Inspired by & adapted from Cooking With Koji
Yields about 570g

200g dried koji rice (kome or genmai)
400g good-quality shoyu
*If using fresh (raw) koji rice (kome or genmai), use 300g shoyu for 200g of fresh koji rice

800ml or higher capacity regular glass jar or clamp lid glass jar

*Do note that when using different types of koji rice and shoyu brands/manufacturers, the final outcome (flavor, texture and scent) of the shoyu koji will be different.

1. In a cleaned glass jar, add koji rice. Break up the clumped koji rice (if any) with a spoon or chopstick.

2. Pour shoyu and mix thoroughly.

3. To release gas during fermentation, loosely screw the jar lid of a regular glass jar. For the clamp lid glass jar type, remove the rubber gasket and cover the jar mouth with a piece of thin cloth before clamping the lid.

4. Put the glass jar in a cool area at room temperature away from heat. Ferment for a month at least to age the shoyu koji. Ferment for more than a month to a year or more for a deeper, richer and more complex shoyu koji.

Important: During fermentation, stir the solution daily for at least a month with a clean wooden/ceramic spoon. If you are to extend the fermentation to more than a month and more, you can opt to stir the solution every 2-3 days. As you taste the solution during daily stirring, you will learn how the mixture come together and became the delicious shoyu koji.

5. Once shoyu koji is ready to be used, transfer the living enzymes shoyu koji to a clean air-tight glass jar. Store the shoyu koji in the refrigerator for ~10 months or in the freezer for a year. Whenever you need to use the shoyu koji, stir the solution with a clean wooden/ceramic spoon before scooping it for use. This is because the koji rice will sink to the bottom of the jar. Fermentation is still ongoing but slow when shoyu koji is kept in the fridge. The koji rice will break down further as time went by.

6. Kindly gauge/estimate (agak-agak) when you first start using shoyu koji. You’ll know how much to use by feel once you’re used to it. I’ll be sharing some recipes using shoyu koji in the future, so start making it ya.

I hope you’re koji-fied by now. Welcome to the koji club!

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  • Reply Lisa April 10, 2022 at 12:15 pm

    If we can’t get any of the special rice you have, the koji one, can regular brown rice work too? How would you use this after, it’d be great to know.

    • Reply Che-Cheh April 11, 2022 at 11:46 am

      Hi Lisa, regular brown rice will not work. You need to get koji rice (whether it’s koji white rice aka koji rice or koji brown rice aka genmai koji). This rice has been inoculated with the koji fungus and it’s this fungus that will transform your ingredients into umami & sweetness. Read more about koji in the shio koji post: https://messywitchen.com/recipe/japanese-recipe/shio-koji-塩糀 or you can find more details in the WWW.

      I’m not sure where you are, but you can get koji rice from some Japanese specialty shops and online. I mentioned the usage in the post above. Simply use it in place of regular shoyu.

      • Reply Alberto Darce November 27, 2023 at 12:41 am

        Is Aspergillus Oryzae from a bottle of unpausterized shoyu still alive and reproducing and breaking down proteins and starchs? Or is just the lactic bacterias that are working on this task during months of fermentation?
        If Arpergillus is alive, shouldn’t it ferment regular rice when added to it?
        Of course koji rice when added will boust everything, as there are extra enzymes ready. But I wonder if regular rice if put on unpausterized shoyu would also create something similar, but less strong.

        Also, people say shoyu koji is 10x stronger than shio koji. But how much stronger is shoyu koji compared to regular shoyu?

        What happens if we add soy protein isolate powder to shio koji ingredients? Will it break the proteins into aminoacids and create something closer to shoyu koji?

        May I ask your help to understand?

        • Reply Che-Cheh December 10, 2023 at 7:53 pm

          Hi Alberto, if the shoyu is not heat treated and/or pasteurized, then yes, the fungus is still alive & will continue break down the proteins & starches.

          As for adding regular rice to unpasteurized shoyu, sorry this one I have no idea. My guess is it will not work. If it did, the Japanese would have done it long ago. Every Japanese fermented food (real ones, not factory made ones) had and always will be made with inoculation of koji with a substrate.

          If you want to know how much stronger shoyu koji is than shio koji, do try making both yourself. I can attest that shoyu koji is way way more umami.

          I’m not familiar with soy protein isolate powder, so can’t give any comments. I know it’s not real protein.

  • Reply Alberto Darce November 29, 2023 at 11:49 pm

    How much more umami shoyukoji have compared to a quality shoyu souce?
    I heard that compared to shiokoji the shoyukoji have 10 times more umami, but I’m curious when comparing shoyu with shoyukoji.

    • Reply Che-Cheh December 10, 2023 at 7:58 pm

      Hi Alberto, I have no way of measuring it. Neither I can say exactly how many times more umami. Since you’re curious, try making both and see.

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