Japan — natto tradition from at least the early Heian period (10th century); wara (rice straw) natural fermentation method predates commercial culture production; Mito City (Ibaraki) is the historical capital of natto culture
While natto's cultural role in Japanese cuisine is broadly documented, its microbiology represents one of the most fascinating chapters in fermented food science. Natto is produced by inoculating cooked soybeans with Bacillus subtilis var natto—a specific strain of the ubiquitous soil bacterium, selected for its ability to produce nattokinase (an enzyme with documented fibrinolytic properties that has generated significant pharmaceutical research) and the characteristic poly-gamma-glutamic acid polymer that creates natto's defining sticky, thread-pulling mucilage. The fermentation occurs at precisely 40–42°C for 16–24 hours in shallow trays or straw containers—B. subtilis var natto is an aerobic bacterium requiring oxygen access, which is why natto is never made in sealed containers. The straw fermentation tradition (wara natto) allowed natural B. subtilis inoculation from rice straw's inherent bacterial flora—the bacteria lived on the straw surface and colonised the beans naturally during the fermentation period. Modern commercial natto uses laboratory-isolated B. subtilis var natto strains in specific concentrations for consistent results, but the underlying biochemistry is identical to the straw tradition: the bacteria produce amylase, protease, and lipase enzymes that partially digest the soybean's protein and fat, creating the characteristic amino-acid-rich flavour compounds and the nattokinase polymer. Regional natto varieties—from Mito (Ibaraki's natto capital), Kyoto, and Hokkaido—use different soybean varieties, fermentation times, and post-fermentation maturation to produce distinct flavour profiles.
Pungent, ammonia-adjacent, deeply umami-savoury with bitter notes; mustard and soy sauce condiments bridge the flavour gap for less experienced palates; the mucilaginous polymer creates a unique texture that many Japanese consider as pleasurable as the flavour is challenging
{"B. subtilis var natto inoculation: commercial starter cultures applied at 0.1–0.5g per kg soybeans; temperature must reach 40–42°C within 2 hours to ensure dominant colonisation before contamination organisms establish","Aerobic fermentation requirement: B. subtilis is obligate aerobe; trays or straw wrappers must allow oxygen access; sealed containers produce anaerobic fermentation by different organisms—not natto","Nattokinase and poly-gamma-glutamic acid: nattokinase is the fibrinolytic enzyme (studied for anticoagulant potential); poly-γ-PGA is the biopolymer creating the characteristic mucilaginous 'strings'—these are distinct molecules with different functions","Soybean variety: small soybeans (hikiwari or small round varieties) are considered superior for natto due to higher surface area-to-volume ratio allowing better bacterial penetration; large soybeans are less common","Maturation period: freshly fermented natto is sharper and more pungent; 2–3 days of refrigeration post-fermentation allows flavours to round and the bacterial activity to partially subside—most commercial natto is sold after this brief aging","Stirring technique: the traditional 10, 50, or 100 stirs before eating develop the mucilaginous polymer into a foam that softens the texture and distributes the flavour compounds more evenly"}
{"Natto home fermentation: purchase B. subtilis var natto starter culture, inoculate cooked soybeans, maintain in yoghurt maker or insulated box at 40°C for 22 hours—produces genuinely superior natto to most commercial products","Natto condiment refinement: the standard packet includes soy sauce and mustard; upgrade with a small amount of dashi, wasabi instead of mustard, and finely chopped takuan (pickled daikon) for a more complex preparation","Natto toast: natto on toasted shokupan with soy sauce, green onion, and optional melted cheese—a yōshoku-style bridge preparation that introduces natto to first-timers more gently than rice service","The nattokinase research angle: there is legitimate peer-reviewed research on nattokinase's fibrinolytic (clot-dissolving) properties—this scientific depth provides compelling content for health-conscious guests without making medical claims","Regional natto sourcing: Mito (Ibaraki) hikiwari natto (lightly pounded broken soybeans) has a more refined texture and less aggressive appearance for first-time encounters; consider sourcing specifically for introductory presentations"}
{"Fermenting at incorrect temperature—below 38°C allows contamination organisms to compete; above 44°C denatures the enzymes; 40–42°C is the narrow optimal window","Using sealed containers—B. subtilis requires oxygen; sealed fermentation produces off-flavours from anaerobic organisms instead of natto's characteristic complexity","Not stirring before eating—the traditional stirring develops the polymer into a foam that fundamentally changes the texture and flavour integration of natto with its seasonings","Adding cold condiments to refrigerator-cold natto—natto's flavour is fuller at room temperature; allowing to sit 5 minutes before seasoning and eating improves the experience","Treating all commercial natto as equivalent—soybean variety, fermentation time, and post-fermentation maturation create measurably different products; Mito hikiwari natto versus large-bean commercial natto are distinct eating experiences"}
The Art of Fermentation — Sandor Katz; Koji Alchemy — Jeremy Umansky and Rich Shih