Never eat a wild mushroom identified only from a website, a book or an app. Have every find checked in person by an expert, such as a mycological society.
Identification
Aspergillus oryzae is a filamentous mold. On grain it first forms a white, fluffy mycelium that binds the kernels together; if allowed to sporulate it turns yellow-green and then olive or brownish. Under the microscope it shows the typical Aspergillus structure: a long conidiophore stalk ending in a swollen vesicle covered with phialides that produce chains of round, slightly rough conidia.
It is very close to Aspergillus flavus, and genomic studies indicate that A. oryzae is a domesticated lineage of A. flavus. The two cannot be reliably separated by eye; laboratories use DNA and metabolite analyses. Koji makers therefore always use selected strains from specialist suppliers.
Habitat and ecology
As a species, A. oryzae is known almost only from human use: it is maintained in koji houses, breweries and culture collections rather than as a wild population. Its wild relative A. flavus is widespread in soil, crops and stored grain. A. oryzae grows well on starchy and protein-rich substrates at warm temperatures and high humidity, which is exactly what koji making provides.
Uses and importance
Koji is grain or beans overgrown by A. oryzae. During growth the mold secretes large amounts of enzymes: amylases break starch into sugars, and proteases and peptidases break proteins into peptides and amino acids, including glutamate. These enzymes drive the production of sake, miso, soy sauce, mirin, shochu, amazake and rice vinegar. In Japan the Brewing Society of Japan adopted the proposal to call it a national fungus (kokkin) in 2006.
Chefs now use koji to make garums, shio koji and cured products. In industry, A. oryzae is a major host for producing food enzymes such as amylases and proteases. Its genome was published in 2005.
Cultivation
To make koji, rice or barley is washed, soaked and steamed so the kernels are cooked but separate. After cooling to about 35 °C (95 °F) it is inoculated with tane-koji (spores of selected strains) and incubated for about 40–48 hours at high humidity. The temperature is kept roughly in the range 28–35 °C (82–95 °F), and the grain is mixed to release heat, because the growing mold warms itself and can overheat above about 40 °C (104 °F). Koji is harvested before it turns green and sporulates.
Safety and edibility
Koji made from food-grade A. oryzae strains has a long record of safe use, and the fermented foods made with it are eaten worldwide. A. oryzae carries a version of the aflatoxin gene cluster, but in studied strains it is not functional. The main risk is contamination: wild molds on grain, including toxigenic A. flavus, look similar. Never use spontaneously moldy grain; always use commercial tane-koji and clean equipment, and discard batches with unusual colors, slime or off smells. People with mold allergy or asthma should avoid inhaling spores, for example by not letting koji sporulate indoors.
If you suspect mushroom poisoning, call Poison Help at 1-800-222-1222 or 911 in an emergency straight away, even before symptoms appear, and keep the remains of the mushrooms.
Look-alikes
The most important look-alike is Aspergillus flavus, which can produce aflatoxins and is impossible to distinguish reliably from A. oryzae by eye. Aspergillus niger is easy to tell apart by its black spore heads. Aspergillus fumigatus, a human pathogen, forms blue-green colonies and grows at high temperatures. Closely related koji molds include Aspergillus sojae (soy sauce) and the black and white koji molds used for shochu and awamori.
In the United States
Koji has moved from Japanese-American kitchens into the mainstream of the US food scene. Miso, soy sauce and sake are produced in the United States, and many chefs and home fermenters now grow koji themselves to make misos, garums and cured dishes.
Koji spores and starter cultures are sold by specialist suppliers. Anyone who wants to sell koji-based foods should check state food rules first, since home-based cottage food laws generally cover only foods that do not need time or temperature control for safety.
