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Koji is a mold, not a seasoning. Propagating it means giving Aspergillus oryzae the right temperature, humidity, and oxygen to colonise steamed grain — typically rice, barley, or wheat — and produce a dense network of enzymes: amylases that break starches into fermentable sugars, proteases that cleave proteins into amino acids and glutamates, and lipases that work on fats. What you get after 40–48 hours is a grain that smells like chestnuts and warm mushrooms, tastes faintly sweet and deeply savoury, and carries enough enzymatic activity to transform whatever you bury in it next — meat, fish, vegetables, dairy. The cook's job is environmental. Soak and steam the grain until it's fully cooked but not wet on the surface — excess surface moisture drowns the mold before it takes hold. Inoculate with tane-koji (spore powder) once the grain has cooled to around 30–35°C. Spread evenly. Then manage a 40–48 hour incubation at 28–32°C with 70–85% relative humidity, aerating the mass every 12 hours or so by hand-mixing (called te-ire in traditional brewing), which disperses heat generated by the mold's own metabolism and prevents hot spots that kill the culture or push it into sporulation too early. By hour 20–24, mycelium should be visible as white filaments binding grains together. By hour 40, the mass should hold together when pressed, smell intensely of roasted chestnut and fermented grain, and feel warm and slightly dry on the surface. Sporulation — a green-grey colour — signals you've gone too long; the mold has shifted from enzyme production into reproductive mode, and enzymatic yield drops sharply. In a modern kitchen context this means owning a dedicated incubation chamber with a temperature controller, a humidity source (ultrasonic humidifier or wet towels with a probe), and a perforated tray system so airflow stays consistent around the entire mass. Koji made this way is a working ingredient — a fermentation engine — not a flavouring in the conventional sense.

Lacto-fermentation works because salt does two jobs at once: it pulls water out of vegetable cells by osmosis, creating the brine the bacteria need to work in, and it selects for the right organisms by making the environment hostile to most spoilage competitors. The microbes you want — primarily Leuconostoc mesenteroides in the early stage, then Lactobacillus plantarum as acidity builds — are salt-tolerant and anaerobic. Everything else is not. Salt concentration is your primary dial. At 2% by weight of the vegetable, you get fast, vigorous fermentation with a clean lactic bite and fragile shelf life. At 3%, fermentation slows, complexity deepens, shelf life extends. Push past 5% and you inhibit even Lactobacillus, the result coming out more preserved than fermented — salty, flat, biologically inert. The Noma Guide to Fermentation (Redzepi/Zilber) anchors their working range at 2–3% for most vegetables, and that holds across professional contexts. What you are actually tracking is pH descent. A healthy ferment drops from the vegetable's native pH of roughly 6 down through 4.5 within the first 48–72 hours, depending on temperature and salt level. That descent signals Leuconostoc handing off metabolic dominance to Lactobacillus. Below pH 4.6 — the boundary McGee identifies in On Food and Cooking as the threshold below which most dangerous pathogens cannot reproduce — the ferment is structurally safe. Below pH 3.5 the acidity starts tasting aggressive rather than bright; you have gone too far for most culinary applications. Temperature controls speed. At 18–22°C you have maximum microbial activity and the fastest pH drop but a shorter window to catch complexity. At 12–15°C fermentation slows considerably and aromatic compounds accumulate over weeks rather than days. Kitchens running hot accelerate the process unpredictably, which is why serious operations use a dedicated fermentation chamber or a cool larder with a thermometer, not ambient service kitchen temperature. Anaerobism is not optional. Oxygen above the brine invites kahm yeast and, worse, mould. Weight the vegetables, seal the crock, and if you see a white film on the surface before significant pH descent, treat it as a failure and start again.

Shio koji is a paste or brine made by combining cooked rice or barley inoculated with Aspergillus oryzae spores — koji — with salt and water, then fermenting the mixture at room temperature for seven to fourteen days. The result is a living seasoning dense with active proteases, amylases, and lipases secreted by the mould during its growth phase. When you rub shio koji onto protein, those enzymes go to work on the muscle fibres and connective tissue: proteases break peptide bonds, releasing free amino acids — glutamate in particular — and producing shorter peptide chains that read on the palate as savouriness and roundness rather than straight salt hit. Amylases convert residual starches to simple sugars that drive Maillard browning at lower temperatures than an unseasoned surface would reach. The practical result is a piece of chicken thigh or salmon collar that has been in shio koji for six to twelve hours cooks with noticeably deeper caramelisation, stays moister in the core, and carries a seasoning that reads from the inside out rather than sitting on the surface. Cure times are protein-specific: lean white fish needs two to four hours, or texture degrades visibly; chicken thighs and pork shoulder take eight to twenty-four hours comfortably; beef cuts with more intramuscular fat tolerate up to forty-eight hours in a 10–12% salt koji. Temperature during curing matters: refrigerator temperature (3–5°C) slows enzymatic activity and gives you a controlled, predictable cure; ambient curing at 20–25°C runs faster but requires close monitoring to avoid over-tenderisation. Before cooking, scrape or rinse the paste from the surface — residual sugars and koji solids scorch easily under direct heat. Shio koji also works as a straight seasoning in dressings, marinades, and vegetable pickles, where the enzymatic activity is less the point and the fermented glutamate load is. Keep a live batch refrigerated after the initial ferment; it remains active and usable for three to four months.