Japanese Izushi: Lactic-Fermented Fish Rice of the North and the Narezushi Continuum

Ingredients

• 600 g salmon, saury, or herring fillets, salted and pressed overnight to draw moisture; bones removed
• 400 g cooked short-grain white rice, cooled to room temperature
• 200 g rice koji (kome koji), fresh or dried; the enzymatic driver of the ferment
• 40 g salt, non-iodised; divided for salting fish and layering
• Vegetables
• 1 medium carrot, peeled and cut into fine matchsticks
• 30 g yuzu peel, fresh or dried; cut into thin strips
• 2 dried persimmons (hoshigaki), sliced thin
• 100 g konnyaku, sliced thin; blanched briefly in boiling water and drained

Method

1. 1
   Salt the fish fillets liberally on both sides using a portion of the measured salt. Lay them on a rack set over a tray and refrigerate overnight. The following day, rinse lightly and pat dry. The surface should feel firm and tacky, not wet — excess moisture would dilute the fermentation environment and invite spoilage.
2. 2
   Combine the cooled cooked rice with the koji, working the two together with clean hands until the koji is evenly distributed through the rice. The mixture should smell faintly sweet and floral from the koji's amylase activity — no sour or off notes at this stage.
3. 3
   Select a clean, odour-free wooden barrel (oke) or a food-grade ceramic crock. Any residual off-aromas in the vessel will transfer directly into the ferment. Scatter a thin layer of the rice-koji mixture across the base, then dust lightly with salt.
4. 4
   This is the step on which the dish lives or dies: layer fish pieces over the rice base in a single, closely fitted layer; distribute carrot, yuzu peel, dried persimmon, and konnyaku evenly across the fish; cover with another layer of rice-koji mixture and a dusting of salt. Repeat the sequence — fish, vegetables, rice-koji, salt — until all materials are used, finishing with a rice-koji layer. Under-salting at each stratum allows undesirable organisms to compete with lactobacillus; over-packing without full salt coverage creates anaerobic pockets hostile to controlled fermentation. Each layer must be pressed firmly and evenly salted.
5. 5
   Set a clean flat weight (otoshibuta drop-lid, or a plate weighted with a stone) directly on the surface. The weight should press the layers into contact without crushing. Cover the barrel mouth with cloth and secure. Place the barrel in a cold environment — a traditional late-autumn unheated room, a cold cellar, or a refrigerator set above 4 °C. Fermentation must proceed at low temperature; warmth at this stage produces uncontrolled fermentation and spoilage rather than beneficial lactic activity.
6. 6
   Check the barrel after seven days. A faint sour aroma — lactic and clean, not putrid — should be building. The surface brine, drawn out by the salt and the weight, should be visible around the edges. If the smell is sharp, clean, and pleasantly fermented at 10–14 days, the izushi is ready. Bring a small portion to room temperature before tasting: the koji sweetness and lactic complexity emerge as it warms from cold.