If you’re curious about koji—which seems to be everywhere, all of a sudden, in American food culture—this recipe is an excellent way to introduce yourself to this ancient and essential ingredient. A large-format roast like prime rib benefits from koji’s transformative powers, and mixing koji-inoculated rice with salt is the easiest way to unlock them.
If you’ve never heard of koji before, then congratulations! You’re about to discover a new ingredient that can transform your home cooking. Until then, know this: koji is microbe (Aspergiullus oryzae) that grows on rice or soybeans, and has been used for thousands of years to make many umami-rich foods, including soy sauce, miso, and fish sauce, as well as alcoholic beverages, especially sake.
Consider this prime rib your first baby step into the world of koji. The koji-based dry brine will both tenderize and subtly flavor the roast—making it taste extra-beefy—and promote browning, which is especially useful when using the reverse-sear method. If this recipe hooks you, we strongly recommend reading Koji Alchemy to discover all of its alchemic applications.
Prime rib is expensive, so it’s best to scale this recipe up or down to your needs. Luckily it’s easy: factor one rib for every two to three people (depending on their appetites). The slow-roasting time will increase as you go up in size, so time your initial roast accordingly.
Koji rice is surprisingly easy to source and not very expensive (you shouldn’t have to pay more than $10 per pound). You’ll find it in many Asian or Japanese markets or online.
The longer you dry brine your prime rib, the deeper the salt will penetrate the meat. That said, if you don’t have the time or space to dry-brine your roast for at least a day, skip the koji brine altogether and simply give your roast a generous seasoning of salt just before roasting.
Want to play around more with koji? Make some shio koji (a koji rice-based marinade) and rub it on your next roasted protein—it’ll enhance everything from steaks and chops to whole chickens, turkeys, and large-format roasts.
Recipe: Koji-Brined Standing Beef Rib Roast (Prime Rib)
¼ cup koji rice
2 tablespoons kosher salt
One standing rib beef roast (prime rib), 4 ribs, about 8 pounds
The Field Method for Cast Iron Care
If time and refrigerator space allows, it’s best to season your roast at least 1 day ahead of time. In a spice grinder or blender, grind the koji rice into a fine powder. Transfer to a bowl and stir in the salt. Use this mixture to generously season the roast all over, then place in a wire rack set inside a rimmed baking sheet. Refrigerate, uncovered, at least 1 day and up to 5 days.
Preheat the oven to 225°F and position a rack in the center. If you weren’t able to season your meat ahead of time, do so now by generously seasoning it all over with kosher salt. Place the rib roast, bone side down, in a No.10 Field Skillet.
Transfer to the oven and roast, rotating the skillet 180° every hour or so, until an instant-read thermometer inserted into the center of the meat registers 125°F (for rare, or 130°F for medium-rare), 4 to 5 hours. Remove the skillet from the oven and tent the roast with foil; let rest at room temperature for at least 30 minutes and up to 2 hours. Pour off any accumulated juices and use for making gravy or au jus.
About 30 minutes before you’re ready to serve the roast, preheat the oven to 500°F. Uncover the roast and return the skillet to the oven; roast until well browned all over, 10 to 15 minutes.
Transfer the roast to a cutting board and cut away the ribs, following the contours of the bones. Slice the roast and serve right away.
Seasoning Rating: Better
Better. All that fat that renders out of the beef as it slowly roasts will add a good amount of seasoning to your skillet.
Best—These dishes are the best options for building resilient seasoning, and surefire choices for getting tricky pans back on track.
Better—The best way to keep your skillet in great shape is to cook frequently, and cast iron-friendly dishes like these are your bread and butter.
Safe—These recipes won't strip seasoning away from your pan, but won't really add any, either.
OK—Be sure to clean up promptly. Recipes with this rating might feature acidic ingredients which can affect seasoning if not washed soon after cooking.