Koji-kin (麹菌), in scientific term Aspergillus Oryzae, is a type of mold spores that has been a foundation for the fermenting food culture of Japan for many years. It is a beneficial and safe variety of bacteria used for Miso, Shouyu (醤油, soy sauce), Sake, Mirin (味醂, sweet sake for cooking), rice vinegar, Shochu and various other ingredients in Japan. 

Grape juice contains sugars, which ferment in the presence of yeast, but with beverages made from grains, such as sake and beer, it is first necessary to use enzymes to break down the starch in the grain to convert it to sugar before yeast fermentation. In beer brewing, malt is used as the source of these enzymes, but for making sake, Kome-Koji (米麹) is the key player. Kome-Koji is steamed rice inoculated with koji-kin, it creates enzymes that convert rice starch into sugar, which the kobo (酵母, yeast) feeds on.
Koji also produces the other type of enzyme that breaks down protein and produces amino acid and peptide, which create unique characteristic of each sake.

Koji production is the heart of the sake-brewing process, and this process is most exercised, in the mind of master brewer. It requires constant control and adjustment of temperature throughout its 40 to 48 hours process in Koji-muro (麹室), a special temperature-controlled room and traditionally covered with cedar wood with electric heating wire or convection heater. (In modern settings more and more stainless-steel covered koji-muro can be seen.) The koji itself releases heat and koji temperature has to be checked every 2 hours in day and night.