Japan-wide — production concentrated in Chiba, Hyogo, Aichi prefectures
Japanese soy sauce (shoyu) is a fermented condiment of profound complexity produced from wheat and soybeans (standard koikuchi), soybeans and wheat at higher wheat ratio (tamari, which is thicker), or special regional variations. The primary varieties: Koikuchi (dark, standard, 80% of Japanese production — from Chiba and Hyogo prefectures); Usukuchi (light-coloured but saltier, from Hyogo's Tatsuno — used in Kansai cooking); Tamari (thick, wheat-free or low-wheat, from Aichi and Mie — used for dipping and finishing); Shiro shoyu (white, almost colourless, from Aichi — maximum wheat, minimum colour); Saishikomi (double-brewed, very dark and rich — from Yamaguchi). Production: soybeans are boiled, mixed with toasted wheat, inoculated with Aspergillus sojae or A. oryzae to make koji, combined with brine to make moromi mash, fermented in cedar barrels 6–36 months, pressed, and pasteurised. Barrel-aged shoyu (kioke jukusei) from traditional producers is increasingly rare and precious.
Koikuchi: rich, savoury, balanced umami; usukuchi: salty, aromatic, light colour; tamari: concentrated, almost molasses-like depth; shiro: delicate salt with minimal colour
Koikuchi is the default Japanese soy — use for general seasoning, marinades, and dipping; usukuchi seasons without colouring — essential for pale soups and Kansai-style dishes; tamari concentrates depth without wheat flavour — superior for reducing to glazes; shiro shoyu adds saltiness with minimal colour — used in premium dashi and pale sauces; heat affects soy differently — raw soy has bright aromatics that dissipate rapidly; adding soy early seasons protein, late preserves aroma.
The benchmark traditional producers: Yamasa, Kikkoman (standard); Yuasa Shoyu (Japan's oldest soy sauce town in Wakayama); Takesan Kishibori (artisan Shodoshima Island); the revival of kioke (cedar barrel) fermentation is producing Japan's most complex soy sauces; finishing dishes with a few drops of good tamari (not cooking with it) adds profound depth; in sashimi soy, diluting premium koikuchi with a small amount of dashi creates 'sashimi shoyu' that doesn't overpower delicate fish.
Using one soy variety for all purposes (losing the expressive range of Japanese seasoning); treating cheap mass-produced soy as equivalent to traditionally brewed soy (no comparison in depth and complexity); adding soy at the wrong cooking stage for the desired effect; using tamari interchangeably with koikuchi in recipes requiring coating/glazing (tamari is too thick and reduces differently).
Japanese Cooking: A Simple Art — Shizuo Tsuji