Barley koji
Steamed pearled barley grown into a white koji cake with Aspergillus oryzae over about 36 hours in a 30 °C chamber, turned three times under a damp cloth.
Equipment · Baskets, trays and beds
Shallow breathable koji tray
麹蓋 · koji-buta · morobuta
A koji-buta is a shallow wooden tray for spreading grain as it grows into koji. Japanese sources document a traditional form, while modern methods also use cloth-lined or steel trays; size and construction vary.
The Japanese koji-buta
The Japanese name 麹蓋 is read koji-buta. The National Research Institute of Brewing defines it as a traditional rectangular, stackable container for making koji and describes futa-koji as the traditional method of using shallow wooden trays. A 2024 BC Centre for Disease Control guideline likewise says traditional koji methods use wooden trays. These sources establish a Japanese tray tradition, not one ancient date or a single design used everywhere.
A current Japanese retailer calls its cedar example kojibuta or morobuta. Its catalog photographs show a simple low-sided wooden tray; the specific model is a present-day product, not archaeological evidence. The general item here describes the older tray form while keeping modern model details separate.
A shallow bed, with cloth and air
The tray divides steamed grain into a thin bed while the koji grows. The BCCDC guideline describes a manual tray sequence of loading, kneading, piling, mixing, rearranging and finishing, and notes that mixing releases heat. Those operations matter because the tray is passive: it does not hold a fixed temperature or move heat out of the grain by itself.
Cloth arrangements vary. Fermentation Culture's account describes lining a tray to keep grain from sitting in pooled water, and also describes cloth laid over grain. Its photograph shows a modern wooden tray with a white cloth, while the exact contents are not visible. The wood in that writer's tray is a local choice; the article separately notes cedar's traditional use in Japan.
Modern tray variants
Not every koji tray is wooden. Nordic Food Lab describes its barley method in a stainless-steel gastronorm tray with a roughly two-centimetre grain layer under a warm, damp cloth. The team loosens the grain into furrows during the process to shed heat. Fermentation Culture also reports using perforated stainless steel with thick cloth, while warning against water pooling below the grain.
These are documented alternatives, not evidence that stainless steel is the traditional koji-buta. A tray made for one batch or cabinet may differ from the low wooden tray used in manual koji methods; the sources give no universal dimensions or capacity.
Sources for this section
Places, scale and care
The sources locate the traditional wooden tray method in Japan. A fermentation writer in Austria describes a modern setup and says spruce was more practical there than cedar. That Austrian example is a documented adaptation, not a claim about the Japanese tray's origin. The retailer's measurements and stated rice capacity apply to its named size only; NRIB's glossary also gives an approximate load for its reference tray, not a standard for all koji-buta.
One retailer advises hand-washing its wooden tray with water, wiping it dry and leaving it in a ventilated room. The other sources do not set a shared cleaning or repair routine. None establishes that every tray is cedar, every cloth has the same role, or that the tray by itself controls humidity, temperature or safety.
Equipment details
CapacityNo universal capacity is established. NRIB gives an approximate load for its glossary example; individual product listings give model-specific quantities.
SizeSources describe rectangular shallow trays but provide no universal dimensions; listed measurements belong to particular products.
PowerNo power The tray itself is passive; heat and humidity are supplied by its surroundings or separate equipment.
How you get oneNot established A Japanese retailer lists a cedar koji-buta model, but source availability and specifications are model-specific; the sources do not establish a general acquisition route.
CareOne retailer advises washing its cedar tray by hand with water, wiping it dry and leaving it in a well-ventilated room. This is the advice for that product, not a universal care rule for every tray.
Made of
- Wood, traditionally cedar in Japan — Tray body
NRIB defines the traditional tray as shallow wood; a fermentation writer says cedar is traditionally used in Japan. Contemporary materials vary. - Cloth — Liner or cover in documented methods
Sources describe cloth in different positions; it is not a fixed part of every tray.
Limits
A tray does not control temperature or remove heat on its own; sources describe mixing, rearranging or furrowing the grain during koji growth.
Source →Water pooling beneath grain can leave the lower layer soggy; a cloth or tray arrangement is used to avoid pooled moisture in the described methods.
Source →Places
Japan
Traditional preparation · Country evidenceThis names the documented Japanese tray tradition; it does not claim every koji practice uses this form.
Austria
Documented use · Country evidenceA modern adaptation documented by one writer, distinct from the Japanese traditional form.
Related
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Sources
National Research Institute of Brewing, JapanName · Form · Traditional use
Brewing TermsThe NRIB glossary maps 麹蓋 to koji-buta and describes a traditional rectangular stackable koji container.
It defines futa-koji as a traditional koji-making method using shallow wooden trays.
Cited in: The Japanese koji-buta; Places, scale and care
BC Centre for Disease Control, Environmental Health Services; updated December 2024Traditional use · Process · Limits
Safety of Fermented Foods: Section 3.10 Koji & MisoThe guideline states that traditional methods use wooden trays for koji, while modern methods use automated ventilated equipment.
It names manual loading, kneading, piling, mixing, rearranging and finishing stages; mixing releases heat during koji growth.
Cited in: The Japanese koji-buta; A shallow bed, with cloth and air
Nordic Food LabModern method · Tray variant
Koji – history and processThe 2013 process article describes a thin barley layer in a stainless gastronorm tray beneath a warm damp cloth and describes furrowing it to release heat.
This is a modern steel-tray method and is not the same form as a traditional wooden koji-buta.
Cited in: Modern tray variants
Fermentation CultureMaterials · Cloth · Modern adaptation
What Koji needs to grow and how to provide for its needsThe writer says wooden trays absorb excess moisture and help regulate humidity, and describes cloth-lined baking trays and perforated steel as alternatives.
The article says cedar is traditionally used for trays in Japan; its author in Austria chose spruce because cedar was difficult and expensive there.
The article's photograph titled “The tray I use” shows a current cloth-and-wood setup, not a traditional Japanese specimen.
Cited in: A shallow bed, with cloth and air; Modern tray variants; Places, scale and care
Kawashimaya the JapanstoreName · Modern model · Care · Materials
Koji Tray (Morobuta/Kojibuta) Size S – Locally Made, 100% Natural Japanese Cedar WoodThe retailer calls its cedar product kojibuta or morobuta and describes hand-made Japanese cedar construction.
The retailer provides dimensions, capacity and care for this particular size; these are not universal tray specifications.
Cited in: The Japanese koji-buta; Places, scale and care