Ibaraki Natto Heritage and Fermentation Culture
Ibaraki Prefecture — Mito City as symbolic capital; production distributed across Kanto and Tohoku
Ibaraki Prefecture is Japan's largest natto-producing region, accounting for approximately 40% of national production, with Mito City as the symbolic capital of natto culture. Natto — fermented soybeans with Bacillus subtilis natto — is one of Japan's most polarising foods: intensely aromatic (ammonia and isovaleric acid), viscously stringy (the poly-gamma-glutamic acid strands), and deeply savoury with an earthy funk. The Mito natto origin legend attributes its creation to Minamoto no Yoshiie (11th century), whose army supposedly discovered fermented soybeans in rice straw after leaving cooked beans wrapped in straw bundles — Bacillus subtilis natto is naturally abundant in rice straw and warm fermentation occurs spontaneously. Traditional production: small (kotsubu, small bean) or large soybeans are washed, soaked 12 hours, steamed until fully cooked but intact, then inoculated with Bacillus subtilis natto starter, placed in straw rope or modern polystyrene trays, and fermented at 40–42°C for 16–24 hours. The fermentation time and temperature precisely determine the stringiness level, ammonia intensity, and surface texture. Premium artisan natto (Azuma Foods, Takayanagi Foods) is sold in straw bindings (wara natto) — the straw imparts earthy, grassy notes absent from tray-packed versions. The correct eating technique involves vigorous stirring (100 stirs is the traditional number cited) to develop the maximum poly-glutamic acid strings before adding tare (soy sauce and mustard packet) and green onion.