Cheonggukjang: The Fermented Power Behind the Smell

The first time I made cheonggukjang at home, my neighbor knocked on the door. “What is that smell?” I was embarrassed. But later, I realized that smell was telling me something. This wasn’t just soybeans going bad. It was soybeans transforming into something else entirely — in just a few days. The smell was proof that it was working.

I’d been eating cheonggukjang my whole life, but I didn’t really understand it until I started making it myself. One ingredient: soybeans. Two to three days. And yet what happens inside that container is far more complex than it looks. This article covers why cheonggukjang smells the way it does, what fermentation is actually happening beneath that smell, and how to make and eat it properly at home.

What’s That Smell? It’s a Signal, Not a Warning

Most people who encounter cheonggukjang for the first time call it a “rotten smell.” That’s half right. The core of the cheonggukjang aroma comes from two things: ammonia and pyrazine-class compounds. Both are byproducts of Bacillus subtilis (the hay bacillus, or gochokgyun in Korean) breaking down soy protein.

Bacillus subtilis occurs naturally on rice straw and in the air. When it multiplies in a warm environment around 40°C (104°F), it aggressively breaks soy protein into smaller and smaller pieces. That process generates a large amount of amino acids. Some of those amino acids break down further and release ammonia as a gas. That’s the sharp smell that hits your nose.

But here’s the thing — that breakdown process is exactly the point. When protein gets split into amino acids, your body doesn’t have to do that work anymore. Raw soybeans are notoriously hard to digest. Cheonggukjang arrives already “pre-digested” by the bacteria. According to research published in the Journal of Microbiology and Biotechnology, fermentation by B. subtilis significantly increases the bioavailability of amino acids compared to unfermented soybeans. The smell is unpleasant, but it’s also a sign the fermentation is doing exactly what it should.

Short Fermentation, Big Changes — How Is That Possible?

Doenjang takes months. Ganjang (soy sauce) takes years. Cheonggukjang takes two to three days. That might make you skeptical. But Bacillus subtilis reproduces at a pace that other fermentation microbes simply can’t match. Under the right conditions, it divides roughly every 20 minutes. Within 48 hours, an enormous population of bacteria covers every soybean in the batch.

During that short window, several notable compounds are produced:

Nattokinase: An enzyme generated by B. subtilis. Some studies suggest it may support healthy circulation. A study in Scientific Reports (2015) found that nattokinase may help break down fibrin, a protein involved in blood clot formation. It’s worth noting this research is ongoing and not conclusive.
Poly-gamma-glutamic acid (PGA): The sticky, stringy substance you see when you stir cheonggukjang. Those stretchy threads are PGA. It’s produced entirely by the bacteria, not the soybeans themselves.
Isoflavones: Soybeans already contain isoflavones, but fermentation may convert them into more bioavailable forms. Research from the National Institutes of Health (NIH) notes that fermented soy products may offer higher isoflavone absorption than unfermented soy, though more human trials are needed.

Doenjang and ganjang develop their own complex flavor compounds through long aging. Cheonggukjang takes a different path — concentrated, highly active bacterial fermentation over a short burst of time. Different direction, not a lesser product.

How to Make Cheonggukjang at Home — Full Recipe

Store-bought versions are widely available, but making it yourself lets you watch the fermentation happen in real time. The ingredient list is almost nothing. And if something goes wrong, you haven’t lost much.

Ingredients

500g dried meju soybeans (yellow soybeans)
A small handful of rice straw (or 1 tablespoon of store-bought cheonggukjang as a starter)
An insulated container (a cooler box or dedicated fermentation appliance)
Warm water, cheesecloth or a thin cotton cloth

Steps

Step 1. Wash the soybeans thoroughly and soak them in water for at least 12 hours. They should more than double in size.
Step 2. Drain and cook the soaked beans in a pressure cooker. Bring to high pressure, then reduce to low and cook for 40 minutes. The beans are ready when they crush easily between two fingers.
Step 3. Drain the cooked beans in a colander. Do not let them cool completely. You want them around 45°C (113°F) when you move to the next step.
Step 4. Blanch the rice straw in boiling water for 30 seconds to sanitize it. Lay the straw on a cotton cloth, place the warm beans on top, and fold the straw around them. If you don’t have rice straw, toss the warm beans directly with 1 tablespoon of store-bought cheonggukjang instead.
Step 5. Wrap everything in the cloth and place inside your cooler box. Add a sealed plastic bottle filled with hot water to maintain temperature. Target: 40–45°C (104–113°F).
Step 6. Replace the hot water bottle every 12 hours. If using a fermentation appliance, set it to 42°C (108°F).
Step 7. After 48 to 72 hours, check for white mycelium (fuzzy white coating) on the beans and visible stringy threads when stirred. That means it’s done.
Step 8. Store finished cheonggukjang in the refrigerator for up to 5–7 days, or freeze for up to 3 months.

Tips That Actually Matter

Temperature at inoculation is everything. If the beans cool down too much before they contact the straw or starter, the bacteria won’t take hold. Keep the beans above 45°C when introducing the culture.
Don’t peek constantly. Opening the container drops the temperature. Check every 12 hours, not more.
Watch for over-fermentation. If the mycelium turns thick and yellowish and the smell is sharply bitter, the batch has gone too far. For the best flavor balance, pull it at 48 hours rather than stretching to 72.

How to Cook Cheonggukjang Jjigae the Right Way

A lot of people avoid it because of the smell. But the way you cook it has a big effect on how strong that smell ends up being.

Add the cheonggukjang last. If you drop it into the pot at the start, all the bacteria die instantly and the sharp raw smell intensifies. Add it 2–3 minutes before turning off the heat. This also preserves some of the heat-sensitive enzymes like nattokinase.

Mix in a spoonful of doenjang. Pure cheonggukjang stew can taste flat and the raw edge of the smell comes through more strongly. A single tablespoon of doenjang as a base rounds out the flavor and softens the aroma.

Use a proper stock. Cheonggukjang needs something underneath it. A light broth built from dried anchovies and kelp gives the stew structure. Without it, the whole thing tastes thin.

If the smell during cooking genuinely bothers you, open a window. It’s intense while it’s on the stove. By the time the bowl hits the table, it’s noticeably calmer.

What to Know Before You Eat It Regularly

Cheonggukjang is on the saltier end of fermented foods. Store-bought versions often contain 700–1,000mg of sodium per 100g depending on the brand. If you’re eating it as a stew and drinking the broth, your daily sodium intake adds up fast. People with high blood pressure or reduced kidney function should be mindful of portion size. The U.S. FDA recommends keeping daily sodium intake below 2,300mg for the general adult population.

Homemade cheonggukjang made without added salt has almost no sodium. If you want the fermented benefits without the sodium load, try it raw — mixed with tofu, eaten over rice, or as a simple side. No cooking required, and nothing is lost.

If you take anticoagulant medications such as warfarin, talk to your doctor before eating cheonggukjang regularly. Some research suggests nattokinase may interact with blood-thinning drugs. A 2012 review in Nutrition Research and Practice flagged potential interactions between fermented soy products and anticoagulant therapy, and recommended medical guidance for patients on such medications.

Cheonggukjang may offer benefits as part of a balanced diet, but it is not a treatment or cure for any condition. Eating more of it will not fix a health problem on its own.

This content is for informational purposes only and is not medical advice.

Cheonggukjang: The Fermented Power Behind the Smell