Koji

Koji is a molded substrate, not a sour vegetable. Allwood et al.: mold spores are inoculated onto rice, barley, wheat, or soybeans and grown as a low-moisture solid-state culture; the product is named for the substrate (rice koji, and so on) ¹. The worker is usually Aspergillus oryzae. FDA, reviewing that organism as an enzyme host, records more than two thousand years of use in koji and in enzymes for soy sauce, Miso, sake, baked goods, and brewing, and cites JECFA’s conclusion that A. oryzae amylases and proteases in that review should be regarded as foods ².

This is Fermentation by a filamentous fungus. It is not Lacto-fermentation. The acid you taste later in miso is a different stage.

The mold’s job is enzymes #

Allwood: the important feature of koji is the range of catabolic enzymes the mold makes. Strains are chosen for more proteolytic activity (a more savory direction) or more amylolytic activity (a sweeter direction) ¹. Those enzymes later attack starch and protein in the salted mash. Enzymes is the chemistry; koji is the culture that secretes them.

What to notice on a finished koji (industrial and craft descriptions agree on the sensory claim, not on a kitchen clock): the grain should be bound by white mycelium, smell cleanly sweet-chestnut rather than ammonia or rot, and feel neither slimy nor dried to dust. Off colors that are not the culture you inoculated are a reason to discard, not to “cut away the bad bit” as if this were a hard cheese.

Starter and pH are not interchangeable with kraut rules #

Modern koji uses a pure, food-safe, nontoxigenic A. oryzae culture. Allwood: wild spores are no longer the method; uncontaminated starter matters. They note a 2012 outbreak in which a fungal tempeh starter was linked to Salmonella Paratyphi — a different mold food, same lesson about buying culture ¹.

pH 4.6 is the canning split for acid foods. It is not the hurdle that makes koji safe. Allwood: koji and miso are non-acid foods; western fermented-food guidelines built around pH do not map onto them ¹. Do not “fix” koji with vinegar and call it a tested preserve. Acidity still explains why that temptation exists.

Process controls #

Allwood discusses temperature and water activity as process-validation controls for commercial production, citing craft and lab sources for those numbers. They are not a USDA/NCHFP home formula. Safe koji production depends on an uncontaminated starter and controlled temperature, humidity, and time. See Food safety.

Tempeh is a Rhizopus cake on beans, not Aspergillus on rice. Rice is a common koji substrate; soy is the other classic.