How to tenderize Wagyu with shio koji

What is shio koji?

Shio koji is a Japanese seasoning made from three ingredients: rice koji, salt and water. Rice koji is cooked rice that has been grown with Aspergillus oryzae, the same mold behind miso, soy sauce and sake. After about a week of fermenting, the mix turns into a loose, porridge-like paste that smells sweet and a little fruity ^[2].

Japanese cooks use it to cure fish, to pickle vegetables overnight, and as a marinade for meat ^[2]. You can buy it ready-made from Japanese grocers and online, or make it yourself.

Why use shio koji on Wagyu?

Tenderness is mostly about the muscle, not the marbling. A cut from a muscle the cow used all day, like the shoulder, the brisket, the shank or the round, has more connective tissue and a firmer bite than the ribeye or strip, however well it's marbled. Those cuts carry plenty of flavor, and shio koji can help close the gap in texture.

The koji's protease enzymes do the work. America's Test Kitchen describes them as scissors, snipping long protein chains into shorter ones ^[2]. Marukome, one of Japan's largest miso makers, measured the force needed to cut beef after 30 minutes in shio koji and found it took 38% less than unmarinated beef ^[1].

There's a flavor bonus too. The same breakdown releases amino acids, glutamic acid among them, which add savory depth and help the surface brown ^[2]. And the salt in shio koji moves into the meat and seasons it, while helping it hold on to its juices as it cooks ^[2].

Which Wagyu cuts benefit most from shio koji?

The harder-working ones. A 2026 study marinated Angus chuck and round in koji for 72 hours. The shio koji chuck came out significantly more tender than the control, while the round came out juicier but not measurably more tender ^[3]. The authors credit the chuck's extra collagen and fat for the difference ^[3]. It was a small study, so it's a guide, not a rule.

From our current cow, these are the cuts we'd reach for first:

The ribeye, strip, filet and zabuton don't need the help. Shio koji still adds savory depth to them, but they're tender to begin with.

How do you make shio koji at home?

It takes about ten minutes of work and a week of waiting. You'll need dried rice koji, which Japanese grocers and online shops sell, along with non-iodized salt and clean water.

This version from Great British Chefs is easy to scale [4]:

1. Rub the koji between your hands to break up any clumps, then stir it together with the salt and water in a clean jar. America's Test Kitchen warms the water to 140ºF first, which helps the salt dissolve ^[5].
2. Cover the jar loosely, with a cloth and a rubber band, so gas can escape ^[4].
3. Leave it at room temperature, out of direct sun, and stir it once a day with a clean spoon. Add a little water if the koji is no longer covered ^[4].
4. It's ready when the grains are soft, the mix has thickened, and it smells sweet, fruity and slightly funky. That's about 5 to 7 days in a warm kitchen ^[4] and up to 14 in a cool one ^[5].
5. Move it to the fridge, where it keeps for up to six months ^[5].

How do you marinate Wagyu in shio koji?

Use 10% shio koji by weight. Weigh the meat, multiply by 0.1, and that's how much shio koji to use. A 1 lb (454 g) steak takes about 45 g. Marukome's own guidance works out to the same ratio, about 10 g per 3½ oz of meat ^[1].

1. Thaw the meat in the fridge first, and pat it dry.
2. Weigh out the shio koji and rub it evenly over every surface.
3. Seal it in a zip-top bag with the air pressed out, or a covered container, and refrigerate.
4. Before cooking, wipe the shio koji off with a paper towel or the back of a knife, then pat the surface dry.

The wiping matters for two reasons. The rice grains burn quickly in a hot pan, well before the meat has a crust ^[1], and a dry surface browns much better than a wet one.

Shio koji is salty, so the meat comes out seasoned. You can skip the salt before cooking, then taste and add more at the table if it needs it.

How long should you marinate in shio koji?

It depends on how thick the piece is. The salt works its way into the meat over time, but the enzymes do most of their work near the surface, so a large piece needs longer than a small one, and a thin slice needs very little.

For a whole subprimal, such as a 10 to 15 lb packer brisket or a whole bottom round, a longer marinade of two to three days is worth considering, turned once a day so the shio koji stays in contact. The study above marinated its beef for 72 hours ^[3]. Another option is to cut the subprimal into steaks or roasts first, which gives the shio koji more surface to work on.

For retail cuts, a simple overnight marinade of about 24 hours is a good place to start. At the Wagyu Sommelier Level 1 course in New York in October 2026, San-J, the tamari maker, put the gain from a 24-hour shio koji marinade at roughly 50% more tenderness ^[6]. Here's what we'd suggest for ours, at listed pack weights. Packs vary, so it's worth weighing yours.

Thin yakiniku slices are mostly surface, so a short marinade does the job, close to the 20 to 30 minutes Marukome suggests for meat ^[1]. Left much longer, they can turn soft and pasty.

How do you cook Wagyu after a shio koji marinade?

Much the way you'd cook it otherwise, with an eye on the pan. The amino acids that shio koji releases brown faster than plain beef ^[2], so a crust forms sooner.

For a steak, wipe and dry it, then sear it in a dry pan at 450–500ºF. If the surface darkens before the inside is where you want it, turn the heat down a notch. For stew cubes and shank, wipe them and brown them in a dry pan, then stew as usual. The brisket flat goes on the smoker once it's wiped and dry.

Slice the round and brisket steaks thin and across the grain. Shio koji helps, and so does a short fiber.