Traditions
Natto, Thua Nao, Tempeh and Doenjang: The Two Ways Legumes Ferment
Bacillus and mould take soybeans in opposite chemical directions. Which organism runs which ferment, and what that means for smell, texture and use.
9 min read ·
What is thua nao?
Thua nao (ถั่วเน่า, literally 'rotten bean') is a fermented soybean product of northern Thailand and the neighbouring Shan areas of Myanmar. Cooked soybeans are wrapped — traditionally in banana leaves — and left to ferment for two to three days, during which Bacillus species colonise them and break down the protein. The result is used fresh as a paste, or mashed, spread thin, sun-dried into brittle discs and stored, then toasted or fried and crumbled into curries and relishes as a savoury base.
It belongs to a broad Asian family of Bacillus-fermented soybeans that includes Japanese natto, Nepali and Sikkimese kinema, Korean cheonggukjang, Manipuri hawaijar and Meghalayan tungrymbai. The shared mechanism is confidently documented; the species-level microbiology of the individual regional products is less thoroughly studied than natto's, and FRMNT records those attributions at medium confidence rather than presenting them as settled.
Why does natto smell of ammonia and miso does not?
Because they run in opposite directions on the pH scale. Bacillus subtilis is a proteolytic organism: it cleaves soy protein aggressively into peptides, amino acids and, at the end of the chain, ammonia. Ammonia is alkaline, so the substrate's pH rises during fermentation. That rise is the defining feature of this whole family — it is why they are called alkaline ferments, and it is the source of the pungent, cheese-adjacent aroma.
Miso and doenjang go the other way. There the primary organism is a mould, Aspergillus oryzae, which supplies enzymes rather than doing the fermenting itself; salt-tolerant lactic acid bacteria and yeasts then work in the salted mash, producing acid and esters. The pH falls, and the product is savoury and mildly sour rather than ammoniacal. Same bean, two organisms, two chemistries, two entirely different foods.
How is tempeh different from natto?
Tempeh is a mould ferment, not a bacterial one. Rhizopus oligosporus grows as a filamentous mycelium through a bed of dehulled, cooked, acidified soybeans held at 30–32 °C for roughly a day or two, binding them into a firm white cake that can be sliced and fried. The mould partially digests the beans, but the texture it produces is dry and knit rather than sticky.
Natto's Bacillus, by contrast, secretes polyglutamic acid, the polymer responsible for the long viscous threads that are the point of the dish. Two more practical differences follow: tempeh needs an acidified soak — a splash of vinegar — to keep bacteria out while the mould establishes, and tempeh is essentially always cooked before eating, whereas natto is not.
Which legume ferments need salt, and why?
The fast ferments do not. Natto, thua nao, kinema, cheonggukjang and tempeh all finish within one to three days and are protected by speed and by a dominant, deliberately introduced organism. Salt would only slow the organism you want.
The slow ferments cannot exist without it. Miso, doenjang, soy sauce and douchi run for months or years, and over that timescale salt is the only thing standing between the mash and everything that might otherwise grow in it. It also sets the clock directly: sweet white miso at around 5% salt is ready in weeks, while a red miso at 12–13% may age for two or three years, darkening through Maillard reactions between free amino acids and reducing sugars. Choosing the salt level is choosing the timeline.