Koji [ko- jee] is an abbreviation of kabi-tacki meaning Bloom of Mold. Koji is an important culture-food product of Japan and like much of Japanese culture, koji was introduced from China some 200 A.D. A variety of products rely on koji for their preparation, including the production of Miso, Soy sauce, Amazake and Sake. Koji is mainly used as a starter-process for producing the latter culture-food products, through the action of enzymes, generously provided in abundance by koji. Variations of koji prepared with mixed strains of specific molds, yeasts or Lactic acid bacteria [LAB] are specially prepared and incorporated in Japan. For optimisation, a tailor-prepared koji with specific enzyme activity is used in accordance with the product intended to be prepared. Enzymes mainly convert starch into simple fermentable sugars, so that during a secondary fermentation [if required], the sugars become available for yeasts e.g., for preparing specific varieties of miso or sake. In addition, for miso production, proteins are converted to peptides [and amino acids] by the action of proteolytic enzymes [see below for a list of enzymes of koji].

Koji is prepared from steamed short grain white rice, barley or soybeans. The steamed ingredients are cooled to 35°C [95°F] and inoculated with tanae koji [seed of koji]. Tanae koji is an inoculant prepared from mold spores of Aspergillus oryzae or A. sojae. The inoculant may also contain yeast and Lactic acid bacteria. The warm substrate is incubated for 3 to 4 days at approx. 42° C [107° F] at 90 - 95% humidity. The koji is stirred regularly during incubation, to reduce or eliminate excess heat build up. This encourages the proliferation of mold growth to grow evenly throughout the entire rice substrate, while inhibiting clumps of rice from forming. A white mycelium of new mold growth should fully cover each grain of rice. Koji is harvested prior sporulation, to ensure peak enzyme activity and good aroma.

Enzymes of koji - Koji contains high amounts of catalytic enzymes, including alpha-amylase [a starch to simple sugar converter], proteolytic enzymes, including protease's [3 types are known, one is active at acid, one at alkaline and one at neutral pH]. Other enzymes include; sulfatases, nucleases, phosphatases, transglycosidases, peptidases, acylase, ribonucleo-depolymerase, mononucleotide phosphatase, adenyl-deaminases and purine nucleosidases.