I’ve made miso, sake (doburoku), amazake, or other fermented products using rice koji. But I’ve never made koji itself. Koji is cereal covered with cultured mold. Most common koji is rice koji, which can be bought at a Japanese grocery store or through several online shopping sites.
There are other types of koji exist, such as barley koji and soy bean koji, for instance, but probably due to the low demand and short shelf lives, they are not sold in the United States. Fortunately, several koji spores, which are starters to grow particular mold on particular cereal are sold from several online sellers.
The process is simple. Sprinkle the powder over cooked cereal (rice in this case), and wait for the particular mold to develop under regulated temperature. It’s somewhat similar to yogurt making, because yogurt is also fermented food.
For those who live in the 21st century, the hardest part of koji making is waiting, because the entire process takes about 5 days. But actual working time is probably about 3 hours or so. We just have to wait while the micro organisms are working hard to turn the cereal into koji.
When it’s successful, you can smell sweet alcohol, and recognize mold growing around the grains. I believe the color of the mold is different depending on the kind of koji.
After koji is done, it can be used to make sake, miso, amazake, or other fermented products. I am going to challenge koji making and post the process later.