In the hipster precincts of Brooklyn, New York, it's getting so you can't ride your fixie down the street without getting your mustache caught in a banjo string. The same type of cultural giggle might be had about a certain fungus, which has, for the past two years, been taking over the menus of America's top foodie establishments. It's called Aspergillus oryzae, otherwise known as koji, and chefs are slathering it on everything from salad dressings to steaks.
Koji ferments food. Japanese cooks have used it for centuries to make soy sauce, miso, and natural sweeteners. They also use it to brew sake. It's considered the national mold. Kind of like a microscopic mascot that imparts umami wherever it goes.
The science behind koji is less well-known. Its spores are fond of hot and humid environments (what spore isn't?), and grows on cooked rice. As they get bigger, they release biochemical agents-protease enzymes that break down protein and amylases that digest starch. When mixed with, say, soybeans, the ensuing culture helps transform the concoction into soy sauce.
But when applied to steak, koji does something amazing. Its powerful enzymes slowly tenderize the meat. Innovative chefs have found that in just 48 hours, koji can turn a fresh-cut piece of beef into something that resembles, in texture and taste, a 45-day-aged steak. A koji-aged New York strip, properly cooked, will offer up the same nutty and funky flavor as one that's been professionally cured, and with a touch of miso sweetness.