What Is Fermentation? The Science Behind Living Food
What Is Fermentation? The Science Behind Living Food
Stirring a pot of doenjang jjigae one day, I caught myself wondering: where does that smell come from? How does a soybean turn into something so complex and deeply savory? The word “fermentation” feels familiar, but try explaining it and the words don’t come easily. It’s good for you. Leave it long enough. Microbes do something. That’s about as far as most of us get.
So I decided to work it out properly. What fermentation actually does, why food gets more delicious because of it, and what most people are still missing.
Fermentation Is Microbes Digesting Their Food
Put simply: microbes break down organic matter to get energy. The byproducts of that process are the flavors, aromas, and textures we’re after.
Go one layer deeper, and fermentation is a metabolic reaction that happens without oxygen. With oxygen, microbes respire aerobically. Without it, they ferment. That’s why doenjang crocks are sealed tight, and why you press kimchi down to force out the air. You’re creating anaerobic — oxygen-free — conditions on purpose.
Three main categories of microbes drive fermentation:
- Bacteria: Lactic acid bacteria are the standout example. They’re responsible for kimchi, doenjang, and yogurt.
- Yeast: Breaks sugar down into alcohol and carbon dioxide. The engine behind makgeolli, bread, and wine.
- Mold: Aspergillus species convert starches into sugars. They’re central to ganjang, doenjang, and cheonggukjang.
These three work alone or together depending on the food. Kimchi is led almost entirely by lactic acid bacteria. Makgeolli is more of a relay race — mold converts the starch to sugar, then yeast converts the sugar to alcohol.
Why Fermented Food Tastes Better
This is the real point. The reason fermentation is more than just a preservation trick.
Boiled soybeans are hard to digest and not particularly interesting to eat. Doenjang is a different story entirely. As proteins break down into amino acids, glutamic acid forms — and that’s the source of umami, that deep savory richness. Think of it this way: the microbes have already chewed the soybeans for you. They’ve done the work your digestive system would otherwise have to do.
In kimchi, lactic acid bacteria break down the cell walls of the cabbage, which shifts the texture. That distinctive bite — crisp but yielding — comes from that process. The bacteria produce lactic acid, pH drops, and that gentle sourness pulls the whole flavor profile into balance.
In makgeolli, yeast converting sugar to alcohol also releases carbon dioxide. That’s the fizz, the lightness. Organic acids and ester compounds form alongside it, layering in a complexity that’s hard to pin down but easy to enjoy.
Fermentation doesn’t just transform ingredients. It creates entirely new compounds — flavors and aromas that didn’t exist in the raw material. That’s what separates fermented food from food that’s simply cooked.
Fermentation vs. Spoilage: What’s the Difference?
This is where people get confused, and understandably so. Both involve microbes breaking down organic matter. The line between them feels thin.
The difference is whether the outcome is beneficial or harmful to humans. Fermentation is what happens when you guide microbes in the direction you want. Spoilage is what happens when unwanted microbes take over and produce harmful substances.
In practice, that line can feel surprisingly blurry. Someone smelling cheonggukjang for the first time might genuinely think it’s gone bad. It hasn’t — it’s the work of Bacillus subtilis, producing a completely safe fermented product. On the other side, food that looks perfectly fine can harbor harmful bacteria quietly producing toxins.
The key to successful fermentation is stacking the environment in favor of the microbes you want. Salt does this job. Add salt to kimchi and you suppress competing bacteria while salt-tolerant lactic acid bacteria thrive. The high salinity in doenjang works on the same principle.
Practical Tips for Working With Fermented Foods
The things people who’ve been doing this for years take for granted. The things nobody tells you at the start.
1. Temperature Controls Speed
Leave kimchi out at room temperature and it’ll turn sour in a day or two. Refrigerate it and that same process takes weeks. Lactic acid bacteria are highly sensitive to temperature. The standard approach: ferment at room temperature for a day, then move to the refrigerator. Want faster fermentation? Raise the temperature. Want a slower, more complex result? Keep it cool and give it time. That’s the whole logic behind kimchi refrigerators.
2. Keeping Out Air Matters More Than You Think
Lactic acid bacteria are anaerobic. Let air in and other, unwanted microbes will move in with it. Pressing kimchi down firmly so the brine rises to the surface isn’t just habit — it’s keeping the bacteria working in the right conditions. If the surface is exposed to air, mold can form. Keep the brine above the vegetables. Small detail, significant consequence.
3. Salt Quantity Changes the Microbial Community
Less salt means faster fermentation with a wider variety of microbes. More salt means a slower process dominated by specific strains. When making kimchi, the salt used to brine the cabbage and the salt in the seasoning paste serve different purposes and should be thought about separately. Low-sodium kimchi ferments faster and needs to be eaten sooner. Once you understand the principle, you can tune it to match how you actually eat.
4. Container Material Affects the Result
There’s a reason traditional onggi (earthenware) crocks have a good reputation. The clay is slightly porous — a small amount of air moves through it, but it’s not open to the environment. Not fully sealed, not fully open. That middle ground suits fermentation: it allows gas to escape and supports healthy microbial activity. Glass and plastic containers seal completely, so you need to manage gas buildup actively. Leave the lid slightly ajar in the early stages, or use an airlock designed for fermentation.
5. Starting Temperature and Storage Temperature Are Different Things
A lot of people put fresh kimchi straight into the refrigerator. That’s too cold, too soon — fermentation barely gets started. Lactic acid bacteria need roughly 15–20°C (59–68°F) to multiply actively in the early stage. Once fermentation is underway, move it to the refrigerator to slow things down. Getting that timing right is what separates good kimchi from flat, underwhelming kimchi.
What to Know Before Eating Fermented Foods
Fermented foods offer real benefits. Bioavailability of nutrients may improve, some studies suggest fermented foods can support gut microbiome diversity, and the flavors are genuinely irreplaceable. Research published in Cell (Wastyk et al., 2021) found that a high-fermented-food diet may increase microbiome diversity and reduce certain markers of inflammation — though the authors note that more research is needed to fully understand the effects.
That said, a few things are worth keeping in mind.
Traditional Korean fermented foods — kimchi, doenjang, ganjang, jeotgal — are high in sodium. The daily recommended intake is 2,000mg (per the World Health Organization), and it’s easy to exceed that through these foods alone. People managing high blood pressure or kidney conditions in particular should monitor their intake. More of a good thing is not always better.
Some commercially produced “fermented” products haven’t actually been fermented — they’re flavored to taste like it. When buying packaged fermented foods, look for a label that specifies fermentation took place and, where possible, lists a live culture count.
Finally, fermented foods are foods. They may support overall health as part of a balanced diet, but they are not treatments for disease and should not be used in place of medical care. If you have an existing health condition or are currently being treated for one, speak with a healthcare professional before making significant changes to your diet.
Fermentation is one of humanity’s oldest technologies, and researchers are still finding new things to say about it. A single spoonful of doenjang contains hundreds of millions of living microbes. Once you understand a little of what’s happening in there, an ordinary meal starts to look like something else entirely.
This content is for informational purposes only and is not medical advice.
