Koji (Aspergillus oryzae) is the foundational microorganism of Japanese fermentation — the same mould responsible for sake, miso, soy sauce, and mirin. Shio koji as a direct marinade represents a modern application of an ancient organism, applying the koji's enzymatic action directly to protein rather than through a fermented intermediate product. The technique became widely practised in Japanese home cooking in the 1970s–80s. [VERIFY] Whether Tsuji covers shio koji specifically or whether this belongs in the Modernist/contemporary extensions.
Rice inoculated with Aspergillus oryzae mould (koji) and mixed with salt to produce a paste that, when used as a marinade, transforms protein through enzymatic action: protease enzymes from the koji break down surface proteins into amino acids (creating natural umami and tenderness) while amylase enzymes convert surface starches to sugars (enabling Maillard browning at lower temperatures than untreated protein). The result: fish and chicken marinated in shio koji for 6–24 hours cook to an extraordinary tenderness and develop caramelisation that no other marinade produces.
- **Shio koji composition:** Steamed rice inoculated with koji spores, mixed with approximately 10% salt by weight, and allowed to ferment at room temperature for 1–3 weeks until the rice grains have softened and the paste smells sweet, complex, and faintly acidic. - **Application:** Spread directly on fish, chicken, or vegetables. No additional salt is needed — the shio koji itself is the seasoning. - **Marinating time:** 6 hours minimum; 12–24 hours optimal for fish; up to 48 hours for chicken. - **Before cooking:** Wipe off excess shio koji as you would miso in saikyoyaki — a thin film is correct; a thick layer burns. - **Cooking method:** Grilling, pan-searing, or roasting. The enzymatic activity has already pre-tenderised the surface; cooking time is shorter than for untreated protein. Decisive moment: The colour development in the pan. Because the koji's amylase has converted surface proteins to sugars, Maillard browning begins at 140°C rather than the 155°C+ required for unseasoned protein. The surface colours faster than expected — watch from the first minute rather than the second. Sensory tests: **Smell — the shio koji paste:** Sweet, complex, faintly alcoholic, slightly acidic — the characteristic smell of active koji fermentation. If it smells primarily of salt or has no complexity, the fermentation has not developed. **Sight — marinated protein:** The surface of the protein after marination should appear slightly changed — a very faint opacity or tackiness from the enzymatic breakdown of surface proteins. **Taste — cooked:** Unusually tender for the cooking time applied. More complex flavour than salt-seasoned equivalent — the amino acids produced by koji proteases provide natural umami depth that simple salt seasoning cannot match.
Tsuji