Koji is made by cultivating a mold on a cooked grain or legume substrate. It is central to traditional Japanese fermented foods like Miso, Soy Sauce, Sake, Amazake, Shochu, and Mirin. This exploration focuses on using koji to transform regional ingredients from the Northeast into complex sources of flavor, sweetness, and umami, with an additional goal of developing a gluten-free option.
The principal species are [[Aspergillus Oryzae|Aspergillus oryzae]], [[Aspergillus Sojae|A. sojae]], and [[Aspergillus Luchuensis|A. luchuensis]]. Strains are selected for different emphases: proteases suit protein-rich ferments such as miso and soy sauce, amylases suit starch-to-sugar work such as sake and amazake, and A. luchuensis also produces the citric acid important to shochu.
On a suitable cooked grain, spores germinate into branching hyphae that penetrate the substrate and form a mycelial network. The mold digests externally: amylases split starch into simpler sugars, proteases split proteins into peptides and amino acids, and lipases release fatty acids. After roughly two days, these reactions make the grain sweet, fruity, and rich in umami.1 In soy-sauce koji, A. oryzae and A. sojae also secrete peptidases and cellulases as well as proteinases and amylases.2
Those enzymes make different substrates useful for more than traditional ferments. Dextrins made by amylases can slow bread staling, so amazake can bring a related effect to dough. The combination of sugars and amino acids also supplies flavor precursors that can be developed through later browning and fermentation.
While polished rice is a traditional substrate, regional grains are being evaluated as local sources of sweetness, umami, and enzymatic activity:
- Renan Wheat (pearled)
- Farro (pearled)
- Oats
- Sorghum
In one grain-substrate trial, pearled Renan wheat, pearled farro, and oats were soaked overnight and steamed for 30 minutes; all three supported strong growth. Sorghum was boiled until soft enough to press between the fingers but supported poor growth, apparently because its dense structure and intact hull limited penetration.
Cracking or lightly milling sorghum before inoculation may expose the starch-rich interior. Nixtamalization is another possible way to alter the hull and improve access, though that remains a trial rather than an established result.