Rice-koji doenjang from a controlled study

A modern approach to an old Korean soybean paste

Doenjang is a Korean fermented soybean paste prized for its rich, savoury flavour and its ability to provide depth to soups, stews and sauces.

Its Korean name is 된장, romanised doenjang. The term combines doen, referring to thickness or firmness, with jang, the broad Korean category of fermented sauces and pastes.

The main biological ingredient is soybean, Glycine max (L.) Merr.

Traditional doenjang is commonly associated with fermented soybean blocks called meju (메주), brine and prolonged maturation.

This recipe is different. It uses rice koji, prepared using the mould Aspergillus oryzae, alongside cooked soybeans, salt and water.

The controlled study was designed to examine how changing the amount of rice koji affects the chemical and enzymatic properties of doenjang during 30 days of fermentation.

It is therefore a modern experimental formulation of a Korean paste, not a reconstruction of a historical household process.

The distinction is important because traditional doenjang, rice-koji doenjang and Japanese miso may share ingredients and microorganisms while differing in production and culinary identity.

Historical context

Soybean fermentation is an established part of Korean culinary history.

The family of fermented condiments known as jang includes doenjang, ganjang and gochujang.

Ganjang is a liquid soy-based seasoning. Doenjang is a thicker soybean paste. Gochujang incorporates chilli and starch-based ingredients to produce a different flavour and texture.

Traditional Korean soybean processing often involves cooking soybeans, crushing or shaping them into blocks and allowing microbial activity to develop before brining and maturation.

The blocks are called meju.

Historical evidence for Korean soybean fermentation reaches back well before the development of contemporary laboratory koji systems.

However, the current formula cannot be assigned a traditional invention date.

It was documented in a scientific investigation published in 2021, undertaken at Jeonbuk National University.

The researchers used a controlled koji process and compared three formulations.

This makes the recipe particularly valuable for understanding the role of koji concentration in Korean soybean-paste chemistry.

It also makes it unsuitable as evidence that all traditional doenjang has historically been prepared using rice inoculated with pure Aspergillus oryzae.

The role of rice koji

Koji is a substrate, commonly rice or another cereal, cultivated with a selected food-fermentation mould.

In this preparation, the mould is Aspergillus oryzae.

As the mould develops on the cooked rice, it produces enzymes capable of breaking down starches and proteins.

Amylases act on starch molecules, producing smaller carbohydrates.

Proteases act on proteins, producing peptides and amino acids.

These enzymatic transformations are fundamental to the development of fermented soybean paste.

The sugars contribute sweetness and may become substrates for other microorganisms.

The amino acids, including glutamate, contribute savoury flavour.

Because soybeans are relatively rich in protein, the proteolytic part of the process is especially important.

The koji is not simply a flavouring sprinkled into the paste. It is an enzyme-bearing ingredient that changes the chemistry of the mixture during maturation.

Why rice koji differs from meju

Traditional meju is usually made from soybeans themselves.

Its microbial population can be diverse, involving moulds, bacteria and yeasts.

Rice koji begins with rice prepared under controlled conditions and deliberately inoculated with a selected mould.

The resulting enzyme profile may be more predictable, although actual activity depends on the culture and growth conditions.

Using rice koji also adds cereal-derived carbohydrates to the finished paste.

This can affect sweetness and the balance between carbohydrate breakdown and protein breakdown.

Consequently, rice-koji doenjang may develop characteristics different from a paste made predominantly with traditional soybean meju.

The modern experiment investigates this interaction rather than treating the two fermentation systems as automatically equivalent.

The three experimental formulations

The study compared three recipes, labelled K1, K2 and K3.

Each totals 2,000 g of listed ingredients before accounting for any subsequent processing changes.

The present recipe follows K1.

The soybean and salt quantities remain constant across all three formulations.

Increasing koji is balanced by reducing water.

This means the study changes both koji percentage and moisture-related formulation conditions.

The K1 mixture contains 20% koji and 12.5% added salt by initial total ingredient weight.

K2 contains 22.5% koji, and K3 contains 25%.

These percentages are formulation values, not final analytical concentrations after fermentation.

Soybeans — 800 g — 800 g — 800 g

Rice koji — 400 g — 450 g — 500 g

Salt — 250 g — 250 g — 250 g

Water — 550 g — 500 g — 450 g

Ingredients for K1

Main paste formulation

Separate from this, the study prepares rice koji from:

Not all the finished koji is incorporated into K1.

That is the central correction to the original record, which listed 500 g koji in the ingredients while its method used 400 g.

Soybeans — 800 g — Protein-rich substrate

Finished rice koji — 400 g — Source of enzymes and cereal material

Salt — 250 g — Flavour and water-activity control

Water — 550 g — Adjusts paste consistency and processing conditions

Koji ingredient — Quantity

Rice — 500 g

Water for soaking — 750 mL

Aspergillus oryzae starter — 0.2% by weight

Preparing the rice koji

The rice stage is a controlled fungal cultivation rather than a general instruction to let rice grow mould.

The study uses selected Aspergillus oryzae starter material.

The culture's identity matters because the genus Aspergillus contains species with substantially different properties, including organisms that can produce undesirable mycotoxins.

Unidentified environmental mould is not an acceptable substitute for a food-grade koji starter.

The method includes high-temperature treatment in an autoclave, cooling under controlled conditions and subsequent mould incubation.

The process must provide suitable oxygen and moisture for growth.

Mixing during incubation redistributes the material and changes the conditions experienced by the developing mould.

The experiment specifies both the temperature and the relative humidity.

These parameters should be retained when documenting the research.

Method

Rice-koji cultivation 72 hours · Soybean soaking 9 hours · Paste fermentation 30 days at 35°C

These steps reproduce the scientific methodology. They do not constitute a validated domestic food-production procedure.

Prepare the rice. Wash 500 g rice and soak in 750 mL water at 20°C for 30 minutes. Drain the soaking liquid.

Sterilise the rice. The experiment uses an autoclave at 121°C for 40 minutes, under the documented pressure condition of 0.1 MPa. Cool the material promptly to 25°C.

Inoculate the rice. Introduce the selected Aspergillus oryzae starter at 0.2% by weight. The study uses a named commercial culture; this is not an instruction to collect an unidentified mould.

Cultivate the koji. Incubate the rice for 72 hours at 35°C and 85% relative humidity. Mix the culture using sterilised utensils at 24-hour intervals.

Prepare the soybeans. Soak 800 g soybeans in 1,200 mL water for 9 hours at 20°C. Drain the soaking water.

Heat-treat the soybeans. Sterilise in the laboratory autoclave at 121°C for 40 minutes. Cool promptly to 25°C.

Mix K1. Combine the prepared soybeans with 400 g finished rice koji, 250 g salt and 550 g water. Blend until the ingredients are distributed through a consistent paste.

Ferment the paste. Incubate for 30 days at 35°C and 85% relative humidity. The experiment mixed the paste evenly with sterilised utensils every 24 hours.

Assess the finished research samples. The study collected samples at five-day intervals, measuring chemical and enzymatic changes. It did not establish a general household release criterion or extended storage life.

What changes during maturation?

Doenjang fermentation involves a combination of enzyme activity and microbial metabolism.

Koji-derived proteases break soybean proteins into progressively smaller molecules.

Amino acids accumulate as protein breakdown proceeds.

This can increase the savoury quality of the paste.

Amylases break down accessible carbohydrates, changing the distribution of sugars.

Yeasts and other microorganisms may use some of these sugars to produce further metabolites.

Organic acids can develop, contributing to acidity.

At the same time, reactions involving amino compounds and other substances contribute to changes in flavour and aroma.

The process is not governed by one fermentation organism in the same way as the deliberately inoculated natto study.

Koji supplies the selected mould system, but the subsequent paste fermentation may involve additional microbial activity.

Measured pH changes

The pH declined during fermentation.

The K1 pH was reported as decreasing from approximately 6.15 to 5.47 in the investigation's summary analysis.

The researchers also recorded pH differences between formulations during fermentation.

These results help show that acidification occurs, even though proteolysis is an important part of the product's flavour development.

Doenjang is not simply a paste in which acidity is driven as low as possible.

Its final balance depends on salt, enzyme activity, microbial succession and maturation.

The measured pH values describe experimental outcomes.

They are not independent safety thresholds.

Enzyme measurements

A central result of the study was the relationship between koji percentage and enzyme activity.

The researchers measured α-amylase, β-amylase, acidic protease and neutral protease.

By day 30, acidic protease activity in K1 had increased from approximately 7.20 to 24.33 units/g.

Neutral protease activity rose from 7.28 to 31.12 units/g.

The K3 treatment, containing the greatest proportion of koji, generally showed higher final protease activity.

This is consistent with the idea that more enzyme-bearing koji can increase the capacity of the paste to break down proteins.

However, it does not establish that K3 is universally the best-tasting or safest formulation.

Higher enzyme activity may change flavour, but consumer preference and microbiological stability require separate assessment.

Traditional and modern variations

Traditional doenjang is often produced through meju fermentation followed by brining and maturation.

Regional household methods can vary according to season, soybean preparation and vessel use.

Some forms are matured in earthenware jars called onggi (옹기).

Rice-koji-assisted doenjang represents a more controlled modern variant.

The use of rice changes the carbohydrate contribution.

The chosen koji strain can also affect the enzyme system.

The three experimental formulas differ in their koji percentage and water allocation.

They should remain variants on one page, rather than separate foods.

Japanese miso is technologically related but belongs to a different culinary tradition and includes its own range of formulations.

Similarity in ingredients is not sufficient reason to merge the product identities.

How doenjang is eaten

Doenjang is central to numerous Korean dishes.

Doenjang-jjigae (된장찌개) is a hearty soybean-paste stew.

It may contain tofu, vegetables and other ingredients.

Doenjang-guk (된장국) is a soup incorporating the paste.

Doenjang can also be used in dipping sauces and vegetable preparations.

The paste contributes saltiness, fermented aroma and umami.

Its intensity means it is typically used as a seasoning component rather than consumed in large quantities alone.

The current K1 formula may differ in flavour from a traditionally matured meju-based doenjang.

A scientific formulation should not be assumed to reproduce every flavour associated with a long-aged household paste.

Signs of successful fermentation

A developing paste may change colour, aroma, acidity and texture.

The research measured these changes rather than relying entirely on appearance.

Variations in colour can reflect enzymatic and chemical reactions.

However, darkening alone is not proof that fermentation is progressing safely.

An unusual fuzzy growth, unexpected discolouration or evidence of deterioration should prompt rejection or professional investigation.

The presence of mould cannot be judged safe merely because the recipe includes Aspergillus oryzae during the earlier koji stage.

The intended mould was deliberately selected and grown under controlled conditions.

An unknown organism appearing later in the paste is a separate issue.

Safety and storage

This recipe uses laboratory sterilisation and controlled incubation.

It should not be casually rewritten for an ordinary pressure cooker.

The starter strain must be suitable for food fermentation.

Salt plays an important role in the fermentation environment, but its initial percentage alone does not validate the final product.

The BC Centre for Disease Control has published process-specific guidance for koji and miso-type fermentations, including the importance of culture control and appropriate process conditions.

The original doenjang study investigated physicochemical and enzymatic changes rather than providing a complete microbiological safety validation for home use.

It does not establish a general shelf life for the K1 product.

Any commercial or household adaptation must recognise these limits.