Koikuchi Soy Sauce (Dark Soy Sauce)

Koikuchi soy sauce is produced by inoculating cooked soybeans (whole or defatted) and roasted, crushed wheat with Aspergillus oryzae to form koji, mixing the koji with brine to form moromi (mash), and fermenting the moromi over an extended period (commonly 6–18 months for honjozō natural-fermentation production). After pressing, the raw shōyu is pasteurized (hi-ire) and bottled.

The roughly 50:50 soybean-to-wheat ratio distinguishes koikuchi from tamari (almost all soybean) and shiro (mostly wheat). The wheat contribution is responsible for koikuchi's distinctive aromatic profile and moderate sweetness; the soybean contribution drives the umami and color depth.

Industrial production uses two main ingredient grades — 丸大豆 (whole soybeans) and 脱脂加工大豆 (defatted processed soybeans). Both are JAS-permitted; whole-soybean products tend to be priced higher and marketed as premium. Production is concentrated historically in Chiba (Noda, Chōshi), Hyōgo, Kagawa, and several other regions.

Koikuchi shōyu is the standard cooking and table soy sauce in Japan. It serves as the base for nearly every Japanese savory preparation — from simmered dishes (nimono), grilled and braised dishes (yakimono, terimono), and stir-fries to dipping sauces, marinades, and dressings.

In OEM seasoning manufacturing, koikuchi shōyu is the principal liquid base for tare formulations (yakitori-tare, yakiniku-tare, ramen-tare), mentsuyu (noodle soup base), and many bottled finishing sauces. It also serves as the seasoning component in retort and frozen prepared meals.

For export, koikuchi is the default Japanese soy sauce category and is generally the format used unless a recipe or label specifies one of the other four varieties.

Koikuchi shōyu is a JAS-standardized variety under the Japanese Agricultural Standard for soy sauce (しょうゆの日本農林規格). The standard sets compositional requirements including total nitrogen content, color, and salt level.

JAS distinguishes 本醸造 (honjozō, naturally fermented) from 混合 (kongō, mixed-preparation) production methods. Honjozō must use only soybeans, wheat, salt, and water as raw materials with full fermentation; kongō methods include hydrolyzed protein additions.

Allergens: soy and wheat must be declared on labels under Japanese food labeling regulations. Some regional koikuchi-style products with reduced wheat content (closer to tamari) are sold but classified separately.