Koji Prime Rib

Prime rib is an iconic holiday centerpiece that doesn't need much gussying up; the beauty of a beef roast lies in its simplicity. Here at Serious Eats, Kenji has covered this cut exhaustively, making it so that nobody should feel intimidated by the prospect of cooking this very expensive piece of beef. But that doesn't mean there isn't room to give a rib roast a helping hand, so that it achieves its full flavor potential.

This year, we wanted to try something new with prime rib, to up the ante on its rich, beefy, savory flavor. So I turned to one of the best flavor amplifiers I know, shio koji, to make the ultimate holiday roast: koji prime rib.

Shio koji is a Japanese marinade, commonly used with poultry, meat, and seafood, that taps into the flavor potential of Aspergillus oryzae (aka koji-kin or just "koji" if you're already on a first-name basis), the mold that also gives us miso, soy sauce, and sake. Shio koji is made by fermenting a mixture of grain koji (cooked grain, most commonly rice, that has been inoculated with koji spores and then dried), salt, and water to create a porridge-textured product with a sweet, funky aroma.

This koji prime rib is the most savory, meaty, juicy, and delicious beef roast that I've ever cooked. And it's so easy to make. So if you're looking to get wild this holiday season, do yourself a favor—stop reading this, and go start a batch of funky, salty-sweet shio koji right now (if you don't have some in the works already).

The Ins and Outs of Koji Prime Rib

The process for making koji prime rib is almost identical to the oven reverse-sear method that Kenji uses in his primer for perfect prime rib. While we don't need to rehash each step in minute detail, it's worth going over the basics, as my iteration has a few little tweaks and innovations of its own.

Take Roast Apart, Slather It With Shio Koji, Tie It Back Together

First and foremost, a roast needs to be seasoned before cooking. I recently wrote a guide to our favorite method for seasoning both large and small cuts of beef, the dry brine. The TLDR version of that article is "season the meat with salt, let it rest, cook it." For large roasts like pork shoulder and prime rib, you'll achieve the best results by seasoning a day ahead of cooking, then allowing the roast to rest, uncovered in the fridge, overnight. Dry-brining produces well-seasoned meat that browns beautifully while holding onto its juices.

For this recipe, I took the same principles of dry-brining and replaced the salt with shio koji. Shio koji is plenty salty ("shio" is Japanese for "salt" after all), but as with other koji-derived ingredients like soy sauce and miso, it doesn't just provide salinity; it lends savory depth to foods that it comes into contact with, thanks to protease enzymes that break down proteins into amino acids, which we perceive as umami. This enzymatic activity not only boosts flavor but also has a textural effect on ingredients, making meat and poultry incredibly juicy and tender.

In order to maximize the effect of shio koji on the prime rib, I start by separating the roast from the rib bones, which increases the surface area on which to rub the shio koji by exposing the interface between the meat and bones. Along with providing more even seasoning, cutting the roast off the bone and tying the two pieces back together before cooking makes carving much easier later.

As Kenji notes in his guide to prime rib, "The best way to cook your beef is to detach the bone and tie it back on. You get the same cooking quality of a completely intact roast, with the advantage that once it's cooked, carving is as simple as cutting the string, removing the bones, and slicing." Why keep the bones at all? Because they help insulate the meat, promote even cooking, and are also delicious to gnaw on at the table!

So slather up a prime rib with shio koji (you can blend shio koji until smooth if you'd like, but it's not necessary for this application, as it will still adhere to the meat just fine in its porridge-like state), before tying it back to its rib bones with butcher's twine.

Overnight Rest

With the seasoning and trussing taken care of, it's time to let the roast rest, uncovered, on a wire rack–lined sheet tray in the fridge overnight. This gives enough time for the shio koji to do its thing. When you pull it out the next day, you'll notice how dry the surface of the roast has become—this, along with the sugars in the shio koji, will make for a deeply browned crust at the end of the roasting process.

Before popping the prime rib in the oven, I use a paper towel to lightly brush any excess bits of rice from the surface of the meat. There's no need to obsess over it by removing every last grain; just get any that stick out. Those pieces can easily burn when the prime rib gets its high heat blast at the end, and we don't want that. (This isn't an issue if you blend the mixture first, which is one small argument in favor of doing it.)

Slow-Roast for Even Cooking

The roasting process follows our standard low-and-slow reverse-sear method. Cooking the roast at a low temperature allows us to achieve beautiful rosy meat from edge to edge.

Depending on how low a temperature your oven can go, along with how well-done you like your prime rib cooked (I'm with Daniel, who favors prime rib cooked closer to medium than mid-rare; with so much interior real estate in a prime rib, I don't love chewing on thick bites of medium-rare beef with no crust to break up the textural monotony), the roast will take four to five hours to cook.

Blast It for a Browned, Crispy Crust

Get the roast out of the oven and make a little foil fort for it while it rests. While it hangs out, turn the heat all the way up on your oven to give it a final blast for crispy, crusty, koji perfection. As I mentioned earlier, the sugars in the shio koji will accelerate browning, so keep an eye on the roast once it goes in the oven for this final step.

Also, once the roast comes out of the oven and moves to your carving board, don't forget to save the jelly-like beef juices that will have dripped down onto the foil-lined baking sheet. That's the good stuff! Spread it on bread, eat it with a spoon, fold it into a sauce. Just don't let it go to waste.

Untie and Slice

Having already separated the meat off the rib bones before roasting, carving is a breeze. Just snip away the butcher's twine to release the roast from the bones once more.

Cut between the bones to separate them into gnawable individual pieces, and start slicing up that juicy beef. Whatever you do, promise me you won't throw out any delicious (and expensive) prime rib fat, even if you're not planning on serving or eating all of it. I'll show you what to do with it in just a moment.

But first, let's take a moment to appreciate the finished koji prime rib. It's beautiful to look at, sure, but just wait until you take that first bite. It's hard to describe what the flavor is, if not simply "extra." The koji amplifies the already pronounced beefy flavor of prime rib, while also lending a very subtle sweetness to the meat.

It's intensely savory, and impossible to stop snacking on. When I cooked the roast for these photos in the Serious Eats test kitchen, there were at least five people milling around, waiting for their time to pounce and snack, who then found they couldn't tear themselves away. Between mouthfuls, Daniel kept reminding himself out loud how he usually isn't a big fan of prime rib. And then he would grab another bite. This prime rib is different, in the best way.

And if you’re already going over the top with this prime rib, why not accompany it with an equally luxe shio koji-spiked jus?

Render and Save Your Beef Fat

If you've bought a prime rib roast before, you're well aware that they don't come cheap. So if you are tempted to throw out any scraps of meat and fat that crop up when cutting the loin off the ribs, or any of the fat from the finished roast, then understand that you are throwing money—and flavor—away. (This applies to all food waste of course.)

That fat is delicious, especially if you sprung for a dry-aged piece of beef. It's your duty to put it to good use! The simplest way to process any scraps of fat is to render them in a saucepan with woodsy aromatics like rosemary and thyme. That rendered fat is gold. Strain it and keep it in the fridge, deploying it whenever you want to give a meaty boost to cooking projects.

In recent weeks, I've been using my rendered dry-aged beef fat for simple pleasures like dipping crusty bread but also for cooking. The past two dinner parties I've hosted at home have featured a dish of young carrots roasted in dry-aged beef fat with an herby tahini sauce. They're pretty phenomenal, and I've gotten a couple of messages from friends asking for the recipe. I plan to write it up soon, but for now this koji rib roast will have to tide them over.