The launch of Mos Burger’s new limited edition Salt Kōji Burger on May 24 officially confirms that salty mold is now the flavor du jour in Japanese kitchens. Kōji, or to give it its long-winded name, Aspergillus oryzae,  is a domesticized fungus that has been used in the production of miso, sake and soy sauce for centuries. Salt kōji, made by mixing salt and kōji with water, then leaving it to ferment, is also a traditional seasoning, but one that had rather fallen out of use until it enjoyed a revival in the latter half of 2011.

According to Yomiuri Online, the craze for salt kōji started off when Komego, a miso store in Fukui Prefecture, started selling it for use as a simple seasoning back in January 2011. Word spread with restaurants around the area using the ingredient, causing it to eventually catch on with the mass media.