Close to calling it quits six years ago after more than 320 years in business, a small producer and retailer of "koji" in Saiki, Oita Prefecture, is now thriving thanks to a flood of orders for a "magical ingredient" — a salted ("shio") version of the traditional fermented seasoning made from rice malt.

Used for centuries in Japan to produce healthy fermented food products such as soy sauce and miso, the use of koji has been on the decline as a result of changes in the Japanese diet.

According to government data, the per capita consumption of miso plunged to 2,016 grams in 2011 from 3,960 grams in 1970 and that of soy sauce fell to 6.5 liters from more than 10 liters.