Shio Koji: A Fermented Moldy-Rice Marinade That Makes Food Taste Great

The same ingredient that gives us miso, soy sauce, and sake is also the key to this versatile marinade.

Pouring shio koji over a prime rib roast.
Photographs: Vicky Wasik

It's rare for mold to achieve foodstuff celebrity status in the United States, but koji—the ancient mold responsible for miso, soy sauce, sake, and makgeolli—has done just that in recent years. And yet, while famous chefs and restaurants with dedicated test kitchens and fermentation labs have been espousing the virtues of koji and the delicious products that can be made from it, it hasn't made the leap to most American home kitchens, where "mold" is usually thought of as the enemy and six-month fermentation projects are only taken on by a small number of enthusiasts. But there are plenty of delicious ways to begin cooking and experimenting with koji that don't require earthenware crocks and months of waiting. Shio koji is the way to start down the koji rabbit hole.

What Is Koji?

Closeup of rice inoculated with koji-kin.

To quote from Sho's Obsessed profile of koji apostle Rich Shih:

"Even if you've never heard of koji, you've put it in your mouth in one form or another. 'Koji' refers to any grain that has been inoculated with the mold Aspergillus oryzae, although it can, confusingly, also refer to the mold itself. In Japanese, the mold spores are referred to more specifically as koji-kin. Grains inoculated with koji-kin can produce a range of flavors and products with which you're likely very familiar, from slightly sweet sake to nutty miso to intensely savory soy sauce."

One of the simplest ways to tap into the flavor potential of koji is to combine it with water and salt to make shio koji.

What Is Shio Koji?

Overhead of two containers of shio koji.

Shio koji is primarily used as a marinade for poultry, meat, and seafood. It's made by fermenting a mixture of grain koji (cooked grain, most commonly rice, that has been inoculated with Aspergillus Oryzae and then dried), salt, and water to create a porridge-textured product with a sweet, funky aroma.

As with other koji kin-derived ingredients like soy sauce and miso, shio koji lends savory depth to foods it comes into contact with, thanks to protease enzymes that break down proteins into amino acids, which we perceive as umami. It's also rich in amylase enzymes, which break down starches, making it equally effective for marinating vegetables. This enzymatic activity not only boosts flavor but also has a textural effect on ingredients, making meat and poultry incredibly juicy and tender and firming up fish in a short amount of time. But shio koji isn't just a marinade; it can be used to boost the flavor of sauces, dressings, pickles, baked goods, and even ice cream bases.

How to Make Shio Koji

Stirring an in-progress batch of shio koji.

To make shio koji, you will need some form of grain koji. As mentioned earlier, grain koji is made by inoculating some form of cooked grain (most commonly rice) with koji kin and then drying it. If moldy long-term science projects are your bag, you can pick up a copy of The Noma Guide to Fermentation to learn how to make grain koji for yourself. Or you can purchase ready-to-use, granular rice koji—dried, koji kin-inoculated rice—online or in Japanese markets and even in some high-end supermarkets (look for it in the same refrigerated section where you'd find miso).

Once you have some rice koji on your hands, the process for turning it into shio koji couldn't be simpler, but it does take at least a week to ferment. Combine rice koji with kosher salt in a lidded container and then stir in water until the salt has dissolved. The general ratio for shio koji is 5:4:1 by weight of water to grain koji to salt (the ratio is adjusted slightly in the attached recipe to accommodate volumetric measurements).

Stirring in-progress shio koji.

Once the mixture is well-combined and the salt is dissolved, pop the lid on your container of shio koji in the making and find an out-of-the-way spot in your home to ferment it at room temperature. The only hands-on work you have to do during the fermentation process is to stir the mixture once per day. This ensures that the rice grains are being evenly coated with the liquid, which will take on an increasingly milky color as well as a sweet and funky aroma.

After a week, the shio koji will thicken to a porridge-like consistency and will smell fruity and pleasantly fermented. Depending on the time of year and the temperature in your home, this fermentation process might take a day or two longer, but it will be good to go between seven and ten days. At this point, pop the shio koji in the fridge and store it there until you'r ready to use it.

How Long Will Shio Koji Keep?

Side view closeup of two containers of shio koji.

Like miso and soy sauce, shio koji will keep for a long time in the fridge in an airtight container; in the above photo, you can see that the batch on the left was 10 months old when I started the one on the right. For this reason, I like making big batches of shio koji that I can dip into whenever I want to give a protein the shio-koji marinade treatment or add some funky sweetness to a sauce or dressing.

Most recipes for shio koji claim that it can be refrigerated for up to six months, but the 10-month-old batch I had was still perfectly good and showed no signs of degradation in quality. It's hard to imagine a batch of shio koji going bad before you use it up (provided you store it properly).

Should You Buy Prepared Shio Koji?

Prepared shio koji is available for purchase, but I don't recommend using it. When we were experimenting with shio koji at Cook's Science, we tested a lot of store-bought shio koji and found that most commercial versions were overly sweet and often had alcohol added to prolong their shelf-life. The added sugars in these versions of prepared shio koji change their flavor and make them harder to cook with, causing foods to brown and burn much too quickly. While making it yourself requires some advance planning, it's worth it and doesn't require any significant effort.

How to Use Shio Koji

Roast koji prime rib on a wire rack-lined baking sheet.

So you've made a batch of shio koji, and it's now sitting in the back of your fridge waiting for you to unleash its potential. So how should you go about using it? First and foremost, it's one of the easiest and most effective marinades around. But shio koji has other uses as well! Add it to sauces, dressings, baked goods, lacto-fermented vegetables, and more! Here are some of our favorite uses for shio koji, which I will continue to add to as we experiment more and more with it in the test kitchen.

Marinade for Poultry, Meat, and Fish

Rubbing shio koji over a prime rib roast.

The most common use for shio koji is as a marinade or cure for poultry, meat, seafood, and even vegetables. For proteins, slather them up with shio koji and let them hang out for as little as 30 minutes and up to 24 hours, depending on the size and type of ingredient you are working with. Generally speaking, the larger the piece of food, the longer you should marinate it. Over time, shio koji will begin to cure the ingredient it's in contact with, so delicate foods, like fish, should be marinated for a shorter amount of time.

You can decide whether to leave the texture of the shio koji as-is, in its porridge-like state, or buzz it up with a blender to make a smooth, creamy marinade. It will be effective either way. I usually blend it when I am looking to get a smooth, burnished surface on the end product (I have a recipe for koji roast duck dropping soon), and leave it coarse when marinating something that will have a crust-like exterior (such as a beef roast).

Wiping excess shio koji off of a prime rib roast with a paper towel.

When you're ready to cook, wipe off excess shio koji from the surface of your food to prevent it from scorching and then cook as you normally would, although you do have to keep an eye on things since koji-treated ingredients do take on color much faster (as is the case with most marinades that usually contain some form of sugar).

Individual fish fillets, scallops, and shrimp can be marinated for as little as 30 minutes with shio koji, which will firm them up, keep them succulent, and season them with a perfect balance of savory and sweet flavor.

Steaks, chops, chicken breasts, and the like should be marinated for at least an hour and up to a few hours.

Shio koji-marinated prime rib on a wire rack-lined baking sheet.

Larger cuts, especially bone-in pieces of meat or poultry, are best marinated overnight (at least 12 hours), giving the protease enzymes time to do their magic, breaking down proteins and seasoning the meat. Treat large roasts as if you are dry-brining them: set them on a wire rack-lined baking sheet after slathering them with shio koji. As with regular kosher salt, the salinity in shio koji will work its way into the meat and dry out the surface of the roast at the same time.

For smaller items that you aren't searing or roasting (like the koji duck confit recipe that I will be publishing soon), you can marinate them in sealed zipper-lock bags.

Flavor Booster for Sauces

Adding blended shio koji to reduced beef jus.

Shio koji can also be used to bump up the flavor of sauces and dressings. Add savory depth to a gravy or jus by stirring in a little blended shio koji. Try it in salad dressings—koji Caesar salad is killer, I promise you—or whisked into your favorite pan sauce. Shio koji has a can't-quite-put-your-finger-on-it magic flavor that toes the line between savory and sweet in the best way, and there is so much room for exploration with it. So go make yourself a batch, and start koji-cooking!