Black soybean thua nao with Bacillus subtilis

What is black soybean thua nao?

Thua nao is a fermented soybean food closely associated with northern Thailand and the neighbouring Shan and Tai cultural regions. It is made by cooking soybeans until tender, allowing selected bacteria to break down the beans, then eating the fermented product fresh or drying it into thin discs.

The two broad commercial and culinary forms are thua nao sod, the moist fermented preparation, and thua nao khaep, the flattened, dried form. A related preparation, thua nao moe, is soft fermented soybean paste that can be seasoned and grilled.

The black-soybean preparation described here is a controlled research adaptation. It uses black soybeans, Glycine max (L.) Merr., and a commercially prepared culture of Bacillus subtilis rather than relying on microorganisms naturally present in the environment.

The defining character of the food is its savoury, nutty and sometimes pungent flavour, created through the breakdown of soybean proteins and other compounds. Drying then concentrates those flavours and creates an ingredient that can be crumbled, roasted or ground into seasoning.

This is an alkaline fermentation, not the lactic-acid fermentation familiar from sauerkraut. Increasing acidity is not its principal preserving mechanism.

Recipe at a glance

Starting material — 100 g cooked black soybeans

Starter — 0.3 g commercial lyophilized B. subtilis

Reported starter viable count — 6.35 × 10^6 CFU/g

Soaking — 24 hours at 4°C

Steaming — 4 hours at 100°C

Fermentation — 72 hours at 37°C

Fermentation humidity — 80% relative humidity

Disc portion — 25 g fermented paste before drying

Dried-disc yield — Not reported

Food-safety status — Research method; not validated for home production

The original study's precise culture identity, inoculum quality and controlled incubation conditions matter. Substituting ordinary natto, an unidentified fermentation starter or a different Bacillus product does not reproduce the experimental method.

History and cultural background

Thua nao belongs to a family of Asian fermented soybean foods that includes Japanese natto, Korean cheonggukjang and Himalayan kinema.

These foods share important microbiological characteristics, but they are not identical in flavour, production or cultural use.

Northern Thailand's thua nao is especially associated with Lanna cuisine, where fermented soybeans provide seasoning in dishes that may otherwise obtain savouriness from fermented fish or other animal ingredients.

Shan or Tai Yai communities in Myanmar and northern Thailand also maintain related preparations. Tai Lue traditions provide further regional connections.

The exact date thua nao was invented is not known. Its association with particular communities does not establish a single birthplace or an uninterrupted recipe traceable to a particular historical century.

Written scientific descriptions and documentation from northern Thai food researchers establish the continued use of both fresh and dried forms.

Chiang Mai University records local preparations including thua nao moe and khua thua nao. Its Lanna food collection includes documented recipes and identifies Tai Yai influence in some preparations.

The commercial-starter version is considerably more recent. Research published in 2024 investigated how different drying methods influence aroma compounds in thua nao manufactured from black soybeans.

The black soybeans used in that work came from a Royal Project Foundation source in Chiang Mai province. This establishes the provenance of the research material, not the origin of all black-soybean thua nao.

Black soybeans — 100 g cooked weight

Black soybeans are pigmented varieties of Glycine max. Their dark seed coats contain pigments including anthocyanins, while the cotyledons supply protein, fat and carbohydrates.

Protein provides an important substrate for the proteolytic enzymes produced during fermentation.

Soybean fats contribute to mouthfeel and provide precursors for aroma compounds, although oxidation can also produce unwanted flavours.

The recipe's 100 g quantity means cooked beans, not 100 g dry beans. The original dry weight is not reported, so it should not be reconstructed from an assumed water absorption ratio.

Commercial Bacillus subtilis — 0.3 g

The specified starter is a lyophilized preparation with a reported initial viable count of 6.35 × 10^6 CFU/g.

Lyophilization is freeze drying, used here to preserve a commercial bacterial culture.

The organisms produce enzymes that transform soybean components during fermentation.

The strain and manufacturing quality are important. The name B. subtilis alone does not guarantee that an arbitrary commercial or environmental isolate is suitable for this particular food.

Potable water — quantity not stated

Water hydrates the dry soybeans during soaking.

The amount must be sufficient for the soaking process, but the published preparation does not specify a water-to-bean ratio.

A numerical volume should therefore not be invented.

Optional seasonings

The research formulation does not add garlic, chilli, herbs or salt during fermentation.

Such ingredients appear in regional culinary preparations, but they are not part of this controlled experimental batch.

They belong to subsequent cooking or documented variants, not to the published inoculation formula.

Equipment

The published process requires refrigeration capable of maintaining the soaking water at 4°C, steaming equipment, a controlled fermentation chamber, accurate weighing equipment and a grinder.

Its dried form additionally involves nonstick sheets and drying equipment.

An ordinary warm cupboard cannot be assumed equivalent to a fermentation chamber maintaining both 37°C and 80% relative humidity.

The starter quantity is sufficiently small that an appropriate laboratory balance is needed for accurate measurement.

Method: documented controlled preparation

The following stages reproduce the research process for reference. They do not constitute a validated consumer manufacturing procedure.

Stage 1 — Clean and soak the beans

Clean the black soybeans and place them in potable water.

Hold the beans at 4°C for 24 hours.

Soaking hydrates the tissues and helps prepare the beans for subsequent softening. Refrigeration limits microbial growth during this prolonged wet stage.

The published trial does not specify the exact mass of dry soybeans or the volume of soaking water used to produce each 100 g cooked portion.

Stage 2 — Steam

Drain the hydrated beans and steam at 100°C for four hours.

The long steaming period softens the soybeans and changes the structure of their proteins and carbohydrates.

Heating also reduces the number of competing microorganisms, although it cannot be assumed to eliminate every bacterial spore.

The consistency of the cooked beans influences the later fermentation because the bacteria must be able to access nutrients.

Stage 3 — Cool

Allow the cooked beans to cool to room temperature before inoculation.

The published trial does not provide an exact numerical endpoint for this cooling stage.

Uncontrolled prolonged cooling of cooked legumes creates food-safety risks; the research method should not be interpreted as permission to leave cooked beans unrefrigerated indefinitely.

Stage 4 — Inoculate

Combine:

100 g cooked black soybeans

0.3 g specified commercial lyophilized Bacillus subtilis

Distribute the culture through the cooked beans.

The reported viable count of the inoculum is a characteristic of the commercial preparation, not a finished-food microbial count.

The process depends on using an appropriate identified culture. Substituting an unknown starter changes the experiment.

Stage 5 — Ferment

Place the inoculated beans in a controlled fermentation chamber.

Maintain 37°C and 80% relative humidity for three days.

This is a 72-hour fermentation.

During this period, bacterial growth and enzyme activity transform the softened soybeans.

The recorded method does not provide a consumer-facing end-point test demonstrating that this incubation procedure reliably excludes hazardous organisms in an ordinary home environment.

Stage 6 — Grind

After fermentation, homogenize the beans using a household grinder.

The resulting paste is used to manufacture the experimental dried discs.

Grinding distributes the transformed beans into a relatively uniform material and makes it possible to form flat portions.

It also exposes additional surfaces to air, making hygiene and subsequent handling important.

Stage 7 — Form discs

Weigh 25 g portions of fermented paste and form flat discs on nonstick sheets.

The published trial describes discs approximately 10 cm in diameter.

It does not report a universal thickness in the original recipe record.

The disc geometry influences drying by creating a relatively large exposed surface compared with an unflattened mass.

Stage 8 — Drying treatments

The research compared separate drying processes:

Sun drying — Exposed for 8 hours or until constant weight

Hot-air drying — 50°C for 8 hours or until constant weight

Microwave-vacuum drying — Two 800 W magnetrons, 800 mbar vacuum, 5 minutes; observed product temperature 45–46°C

Vacuum drying — 40°C; full parameters require the original study's method specification

These are separate experimental treatments, not consecutive operations.

A fixed time and constant weight are also not equivalent safety criteria. Constant weight can show that mass loss has substantially stopped under particular drying conditions, but it does not independently establish freedom from pathogens or toxins.

The finished dry mass is not reported in the original recipe record.

What happens during fermentation?

The central biochemical process is the action of enzymes produced by Bacillus subtilis.

Proteolysis

Soybean proteins are large molecules made from chains of amino acids.

Bacterial proteases cut these proteins into smaller peptides. Other peptidases release free amino acids.

Among them, glutamate contributes savoury taste.

As the reaction progresses, the beans develop a flavour that is substantially different from boiled soybeans.

Alkaline metabolism

During alkaline fermentation, microorganisms can metabolize amino acids and release nitrogen-containing compounds, including ammonia.

This helps explain the characteristic pungency of some Bacillus-fermented soybean foods.

The process should not be confused with lactic-acid fermentation, where increasing acidity often serves as a preservation barrier.

An alkaline environment does not automatically inhibit foodborne pathogens.

Changes in carbohydrates

Microbial enzymes can alter soybean carbohydrates, making smaller molecules available for further reactions.

These compounds help shape the sensory character of the fermented material.

Drying chemistry

The drying study identified major differences in volatile aroma compounds among its treatments.

Sun drying encouraged compounds associated with lipid oxidation.

Hot-air drying produced different proportions of nutty, roasted, sweet and other aromas associated with oxidation and heat-driven chemistry.

The Maillard reaction also contributes to aroma formation when suitable compounds react under heat.

Vacuum-based drying limited some oxidative changes but did not create the same aroma profile as hot-air treatment.

This is why drying cannot be described merely as water removal. It also changes flavour.

Fresh and dried forms

Fresh thua nao retains substantial moisture and has a softer texture.

Dried thua nao is commonly formed into thin sheets or discs that can be stored, roasted and crumbled as seasoning.

Traditional sheets are often made from whole fermented soybeans processed into paste, sometimes with additional ingredients.

The research formulation uses black soybeans and a commercial starter, giving it a narrower identity.

The most appropriate label is therefore controlled black-soybean thua nao, rather than a universal traditional recipe.

Regional variations

In northern Thai cooking, fermented soybean products appear in wet and dry preparations.

A soft form may be seasoned and grilled in banana leaves.

A dried disc may be roasted over heat before being crushed into a curry or chilli paste.

Lanna recipes can include shallots, galangal, lemongrass, makrut lime leaves and chilli.

These additions create different finished dishes rather than changing the identity of the underlying fermented soybean.

Shan-associated dried discs may be used as a concentrated savoury seasoning.

The use of black rather than pale soybeans changes the raw material, but the available evidence does not establish that every regional thua nao producer uses a particular seed colour.

How to eat it

Thua nao is commonly used in northern Thai soups, chilli pastes and vegetable dishes.

Dried discs can be toasted and crushed into seasonings, contributing a strong fermented soybean flavour.

Related preparations appear in nam ngiao, a northern Thai noodle soup, and in different forms of nam phrik, or chilli relish.

Soft fermented beans may be cooked with egg, herbs, chilli or other aromatics.

The best serving method depends on which form has been prepared.

A moist experimental batch should not automatically be used raw as a ready-to-eat condiment merely because traditional dried thua nao is used in finished dishes.

How to judge the process

The research describes controlled fermentation and measured changes in aroma and composition.

A practical warning is that there is no validated household sensory test establishing that the finished experimental product is microbiologically safe.

Expected changes may include softening, increased savouriness, stronger fermented aromas and changes in surface texture.

But those characteristics are not exclusive to safe fermentations.

Unexpected coloured growth, visible mould, slime inconsistent with the intended preparation, or putrid odours are reasons to reject a batch.

Equally, a visually normal batch could still contain unwanted bacterial contaminants.

For dried discs, excessive moisture retention can undermine storage stability. Rancid aromas may indicate lipid oxidation.

Safety and storage

The principal concern is uncontrolled growth of spore-forming bacteria, including Bacillus cereus.

Some bacterial spores can survive cooking and subsequently grow in suitable moist, warm food.

The toxins associated with B. cereus food poisoning cannot be reliably excluded through appearance or smell.

Although B. subtilis is a recognized food-fermentation organism, using a named species does not make every process safe.

The recorded account method does not establish a validated home food-safety plan, final acceptable microbial counts or a universally safe storage interval.

The drying research measured changes in water activity and aroma, but its experimental results should not be turned into a generalized commercial shelf-life claim.

The correct classification for this recipe is research preparation requiring process validation.

Why this recipe is important

This preparation demonstrates how contemporary food science can study a traditional food without replacing the traditional food itself.

A standardized culture helps researchers distinguish the effects of drying from the uncontrolled differences introduced by spontaneous fermentation.

The precise temperatures, inoculum quantity and humidity make the experiment reproducible within an appropriately equipped laboratory.

The aroma analysis then reveals why two dried thua nao products can taste different even when their starting ingredients and fermentation are similar.

It is a useful bridge between northern Thai culinary heritage, microbial fermentation and modern food-processing research.