Douchi: The Salted Fermented Black Soybean Probiotic from China

Douchi is a traditional Chinese fermented soybean product made from black soybeans, known for its pungent aroma, salty bitter taste, and soft semi dry texture . Dating back to the Han dynasty (165 BCE) as the oldest known soybean food, douchi is used primarily as a seasoning rather than a standalone food . It forms the foundation of black bean sauce and is a key ingredient in dishes like mapo tofu, steamed spare ribs, and stir fried bitter melon . Unlike natto or tempeh, douchi is intensely salty and consumed in small quantities.

Cultural Roots, Regional Names, and Classification

Historical Origins

The earliest physical evidence of fermented black soybeans comes from Han Tomb No. 1 at Mawangdui, sealed around 165 BCE . The Records of the Grand Historian (90 BCE) mentions shì (fermented soybeans) as an important commodity, noting that exiled nobility were provisioned with it alongside rice and salt . This establishes douchi as a food with over two thousand years of continuous history.

Regional Names Across Asia

Douchi is known by various names across different cultures:

· Japan: Daitokuji natto, hamanatto, or shiokara natto

· Korea: Chunjang (a roasted black bean sauce derivative used in jjajangmyeon)

· Philippines: Tausi (from Hokkien tāu-sīⁿ)

· Thailand: Tausi

· Vietnam: Tàu xì or đậu xị

· Cambodia: Seang (fermented salted bean)

· Chinese Indonesian: Tausi (used in kakap tahu tausi)

· Latin America: Tausí or tau-sí

Similar African fermented products include ogiri and iru .

Four Types of Douchi Based on Fermentation Microbes

Douchi is classified into four categories according to the primary microorganisms used:

1. Aspergillus type: Uses Aspergillus oryzae; most common commercial variety

2. Mucor type: Uses Mucor species; traditional household method

3. Rhizopus type: Uses Rhizopus species; less common

4. Bacterial type: Relies on Bacillus species; similar to natto in some respects

Mucor type and Rhizopus type are considered more traditional artisanal varieties .

Microbiology and Probiotic Profile

Microbial Community Dynamics

The microbial composition of douchi changes dramatically across fermentation stages. Using high throughput sequencing technology, researchers have documented a structured succession of bacterial communities .

Early Fermentation Stage (Day 0 to Day 5)

Dominant bacterial genera include:

· Acinetobacter

· Myroides

· Proteus

· Klebsiella

· Lactobacillus species (including Lactobacillus spp.)

· Staphylococcus species

During this stage, bacterial richness and diversity increase, reaching their highest point on day 5 of fermentation . The pH drops continuously during this period as lactic acid bacteria produce organic acids . Metabolic function gene expression is at its highest, indicating vigorous microbial activity .

Middle to Late Fermentation Stage (Day 9 to Day 19)

A significant shift occurs where Bacillus species become the predominant bacteria, belonging to the phylum Firmicutes . Key species identified include:

· Bacillus subtilis (the dominant Bacillus species)

· Other Bacillus spp. (various)

Environmental parameters driving this shift include rising temperature peaking near 45 degrees Celsius and changing pH conditions . Functional gene expression for environmental information processing and genetic information processing increases during this harsh fermentation environment .

Final Product Characteristics

Finished douchi has pH ranging from 4.7 to 5.9, salt content between 4.4 percent and 14.0 percent, and water content from 6.8 percent to 51.6 percent . Aerobic plate counts average 5.2 to 9.2 log CFU per gram . Total coliform and Escherichia coli are typically absent in properly fermented products .

Bacterial Counts in Bacterial Type Douchi

For bacterial fermented douchi specifically, research shows:

· Pure culture fermentation (starter inoculated): Bacillus counts reach 1.62 x 10⁷ to 1.89 x 10⁹ CFU per gram, representing 87 to 101 percent of total bacteria

· Natural fermentation: Bacillus counts reach 1.74 x 10⁵ to 3.80 x 10⁷ CFU per gram, representing 67 to 83 percent of total bacteria

Pure culture fermentation produces higher and more stable Bacillus counts compared to natural fermentation .

Peak Probiotic Diversity and Count Stage

The peak of both probiotic diversity and microbial count occurs at two distinct points depending on the parameter measured:

Maximum bacterial diversity and richness: Day 5 of fermentation, when the transition from early stage bacteria to Bacillus species is underway and multiple genera coexist .

Maximum Bacillus count: Day 9 to Day 19 of fermentation, after the bacterial community has shifted to Firmicutes dominance. At this stage, Bacillus species constitute the vast majority of viable organisms .

For bacterial type douchi, the optimal consumption point for maximum probiotic benefit would be after the Bacillus population has fully established, approximately 36 to 48 hours into fermentation when counts reach 10⁹ CFU per gram range and before prolonged storage reduces viability .

Preparation Guidelines

Traditional Two Stage Fermentation Process

Stage 1: Mold Fermentation (Aspergillus type)

Raw materials for 1 kilogram finished product

Black soybeans (Glycine max)

Quantity: 1 kilogram, whole dried black soybeans

Wheat flour

Quantity: As needed for inoculation carrier

Aspergillus oryzae starter

Quantity: Commercial starter culture

Filtered water

Quantity: For soaking and cooking

Rock salt

Quantity: For stage 2 brine

Pre processing guidelines

Bean preparation

Select whole black soybeans. Do not use black turtle beans which are a different species . Wash thoroughly. Soak in filtered water for 8 to 12 hours until beans double in size.

Cooking

Steam or boil the soaked beans until they become soft enough to mash easily between fingers. Drain completely.

Inoculation

Mix the warm cooked beans with parched wheat flour inoculated with Aspergillus oryzae spores. Spread the mixture in shallow trays.

Incubation

Incubate at 27 to 32 degrees Celsius for approximately 72 hours. The mold will grow and eventually turn green, indicating sporulation .

Mold removal (critical step)

Unlike some other fermented soybean products, the moldy beans are washed to remove the green spores, which would otherwise impart bitter flavor to the final product .

Stage 2: Brine Fermentation

Brine preparation

Prepare a brine solution with rock salt, typically around 10 to 14 percent salt concentration. Some recipes include spices, wine, or chili paste.

Salting and aging

Pack the washed, mold fermented beans into clean glass jars or earthenware vessels. Cover completely with brine. Ferment for 4 to 6 months. The aging process develops the characteristic salty, slightly bitter, umami flavor profile .

Drying (optional)

After brine fermentation, the beans may be dried for longer shelf life. Dried douchi can be stored for extended periods.

Alternative Simplified Method (Traditional Household)

Some traditional households use a simpler approach:

· Rinse dry black soybeans without fully drying them

· Spread in a single layer on a tray

· Sprinkle with salt approximately 0.5 tablespoon per pound

· Transfer to glass jars (odor will never leave plastic)

· Seal tightly and store in a dry place with steady temperature

· Gently shake or roll jars occasionally to distribute microbes

· Allow to ferment for approximately 6 months

This method relies on naturally present environmental microbes rather than commercial starter cultures .

Signs of Readiness

Properly fermented douchi appears blackish in color with a soft, semi dry texture. The smell is sharp, pungent, and spicy. The taste is salty, somewhat bitter, and sweet . Finished douchi is not meant to be consumed in large quantities but used as a seasoning .

Storage

Store finished douchi in airtight glass jars in a cool, dry place. Properly fermented and salted douchi has excellent keeping qualities. Some traditional practitioners believe it improves with age similar to soy sauce .

Medicinal and Nutraceutical Benefits

Douchi offers a range of bioactive compounds and health benefits beyond its culinary applications. Recent research has focused on its potential as a functional food .

Bioactive Compounds Identified

Polyphenols

Fermentation releases and transforms phenolic compounds from soybeans, increasing antioxidant capacity.

Peptides

Bioactive peptides with various physiological functions are generated during fermentation, including angiotensin converting enzyme (ACE) inhibitory peptides that may help manage blood pressure.

Fibrinolytic enzymes

Douchi contains natural enzymes capable of breaking down fibrin, suggesting potential cardiovascular benefits similar to nattokinase.

Gamma aminobutyric acid (GABA)

Produced by certain microbes during fermentation, GABA acts as a neurotransmitter modulator with potential anti anxiety and sleep promoting effects.

These bioactive compounds contribute to the prevention and management of various diseases, positioning douchi as a candidate for functional food development .

Anti Inflammatory Properties

Recent studies have demonstrated anti inflammatory effects. A 2025 study on atopic dermatitis induced mice found that a complex extract containing douchi, when administered over three weeks, showed significant results:

· Increased positive reactions for cannabinoid receptors CBR1, CBR2, and GPR55 (involved in endocannabinoid system regulation)

· Reduced markers of oxidative damage including 8 hydroxydeoxyguanosine (8 OHdG)

· Decreased CD68 positive cells (macrophage activity marker)

· Reduced matrix metalloproteinase 9 (MMP 9) expression

· Lowered Fc ε receptor and substance P levels

The research concluded that douchi containing extracts can reduce skin inflammation by restoring structural damage to the skin lipid barrier through endocannabinoid system activity .

Safety Considerations: Histamine Content

Histamine is a biogenic amine that can cause adverse effects in sensitive individuals. Douchi has been documented to contain variable levels of histamine depending on production methods.

Reported histamine levels in douchi products

Black bean douchi

· Average histamine: 29.0 mg per 100 grams

· Range: Some samples contained 56.3, 62.1, 80.2, and 80.8 mg per 100 grams

· Percentage exceeding 5 mg per 100 g (USFDA guideline for fish): 18 of 19 samples

Soybean douchi (white soybeans)

· Average histamine: Lower than black bean variety

· Percentage exceeding 5 mg per 100 g: 4 of 7 samples

The US Food and Drug Administration (USFDA) sets an allowable level of 5 mg per 100 grams for scombroid fish, with a hazard action level of 50 mg per 100 grams .

Histamine forming bacteria identified in douchi

· Bacillus subtilis (4 strains) capable of producing 11.7 to 601 ppm histamine

· Staphylococcus pasteuri (1 strain)

· Staphylococcus capitis (3 strains) described as halotolerant and capable of producing more than 500 ppm histamine in the presence of 0.5 to 10 percent sodium chloride

S. capitis is particularly notable as a potent histamine former that thrives in the salty environment of douchi fermentation .

Comparison of fermentation methods and histamine

Research comparing pure culture fermentation versus natural fermentation for bacterial type douchi found:

· Pure culture fermentation: Produced lower thiobarbituric acid (TBA) values, lower peroxide values (POV), and lower total volatile basic nitrogen (TVB N) compared to natural fermentation

· Pure culture fermentation: Resulted in higher protease activity (27.44 to 91.95 U/g versus 9.99 to 92.26 U/g) and higher viscous substance content (13.32 to 16.48 percent dry basis versus 5.92 to 9.65 percent)

· Food safety indicators: TBA, POV, and TVB N values were lower in pure culture fermentation, suggesting better control of undesirable compounds

· Nitrite content: Pure culture fermentation produced slightly higher nitrite levels (2.66 to 10.08 μg/g) compared to natural fermentation (1.79 to 9.14 μg/g), though both remained below safety limits

These findings indicate that pure culture fermentation using defined starter organisms may produce douchi with more consistent quality and potentially lower levels of certain safety concerns .

Additional Nutraceutical Highlights

Nutritional enhancement during fermentation

Free amino acids, total acid, and reducing sugar content increase progressively throughout the fermentation period, improving both nutritional value and flavor complexity .

Digestive benefits

The fermentation process breaks down complex soybean proteins and carbohydrates, making douchi more digestible than unfermented soybeans despite its intense flavor.

Traditional medicinal use

Douchi has been incorporated into Chinese traditional medicines since before the Han dynasty, continuing to be added to certain herbal formulations today .

Comparative advantage

Unlike many other fermented soybean products, douchi does not require refrigeration during storage due to its high salt content, making it historically significant as a preservation method.

Usage Note

Douchi is intensely salty and should be used as a seasoning rather than consumed directly in large quantities. Rinse before use to reduce saltiness if desired. Individuals with histamine intolerance, mast cell disorders, or severe small intestinal bacterial overgrowth (SIBO) should exercise caution due to potentially high histamine content. Those sensitive to tyramine or other biogenic amines should introduce douchi gradually. The salt content may be problematic for individuals on sodium restricted diets.

Enjoy douchi as a flavoring agent in stir fried vegetables, steamed fish dishes, mapo tofu, or as a component of black bean sauce served with noodles or rice.