Fermentation
Ferment means letting bacteria and yeast transform food. They turn sugars into acids, alcohol and gas, which preserves the food and gives it sour, savory flavor.
Say it: fer-MENT
Also called: Fermentation Process, Microbial fermentation, Fermantation, Fermintation
By Andrew Cooks · Updated · 11 min read
At a glance
Fermentation is the ancient art of letting friendly microbes (bacteria and yeast) transform food, both to preserve it and to give it deep, tangy, savory flavor. The microbes eat the sugars and starches in the food and produce acids, alcohol and gas, which sour the food, drop its acidity and crowd out the microbes that cause spoilage. It is one of the oldest ways humans have kept food from rotting, and it gives us an enormous family of everyday foods: sour vegetables like sauerkraut, kimchi and the salt-brined fermented pickle; fizzy or sour drinks like kombucha, kvass, tepache and water kefir; and, further afield, bread risen with sourdough, tangy yogurt and kefir, salty miso, soy sauce and fish sauce, and of course beer, wine and cheese. Most home fermentation is a form of lacto-fermentation, where salt and lactic-acid bacteria do the work in a brine. Read fermentation as controlled, delicious, largely safe spoilage: rot steered toward flavor instead of ruin.
What it does for you
Fermentation does two jobs at once: it preserves food and it transforms its flavor. Microbes consume the sugars and starches and excrete acids (mainly lactic and acetic acid), alcohol and carbon dioxide.
Cook's tip
Keep all vegetables submerged under the brine, since anything exposed to air can grow mold. Ferment somewhere cool rather than warm, because a cool, slow ferment avoids soft vegetables and off-flavors.
Kitchen trivia
Did you know people fermented food for thousands of years before anyone knew microbes existed, and that it took Louis Pasteur, in the 19th century, to prove that living organisms cause fermentation?
See the answerKey facts
- Method
- No heat
- Skill level
- Intermediate
- Origin
- Prehistoric, worldwide: one of the oldest food technologies
- Key ingredients
- Cabbage, cucumbers, milk, grains, soybeans, grapes, tea; salt and time
Common uses
Fermentation is used to preserve and flavor an astonishing range of foods. Vegetables are salted and fermented into sauerkraut, kimchi, curtido, salt-brined fermented pickles and countless regional sours; citrus is salt-fermented into preserved lemon. Grains and their sugars are fermented into bread (via sourdough and yeast), into beer, and into low-alcohol drinks like kvass; fruit and sweetened liquids ferment into wine, cider, tepache, kombucha and water kefir.
Milk is fermented into yogurt, kefir and cheese. Soybeans and other legumes are fermented into miso, soy sauce, tempeh, natto, doenjang and gochujang; fish into fish sauce. Cooks reach for fermentation whenever they want to keep a glut of vegetables, add sour and savory depth to a dish, make a probiotic drink, or build the backbone flavors (miso, soy, fish sauce, sourdough tang), that define whole cuisines.
It is at once a preservation method and a flavor engine.
The simplest way into fermentation is a jar of salted vegetables, and the rules that keep it safe are few but firm. Weigh your vegetables and add salt at roughly 2 percent of their weight (the NCHFP recommendation for sauerkraut works out to about 3 tablespoons of salt per 5 pounds of cabbage); salt draws out juice, seasons the ferment and keeps unwanted microbes in check. Pack the salted, bruised vegetables tightly into a clean jar or crock so their own juices (or an added salt brine) cover them completely, then weigh them down so nothing pokes above the liquid: submerged vegetables ferment cleanly, exposed ones grow mold.
Cover loosely so gas can escape, and leave it somewhere cool (a cool room is better than a warm one) for days to weeks, tasting as it sours. A white, filmy skin of surface yeast is harmless and can be skimmed; fuzzy, colored, fluffy mold, sliminess or a putrid smell means throw it out. When it tastes sour enough, move it to the fridge to slow the ferment right down.
Keep everything clean, keep the vegetables under the brine, and trust your nose.
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Save your recipes in Cooked.it, freeMore about ferment
Fermentation is the controlled transformation of food by microorganisms, chiefly bacteria and yeast, which convert sugars and starches into acids, alcohol and carbon dioxide, both preserving the food and developing sour, savory flavor. In the kitchen it covers lactic-acid vegetable ferments (sauerkraut, kimchi, curtido, fermented pickle), salt-cured ferments like preserved lemon, fermented drinks (kombucha, kvass, tepache, water kefir, beer, wine), leavened bread and sourdough, cultured dairy (yogurt, kefir, cheese) and savory pastes and sauces (miso, soy sauce, fish sauce, gochujang). Most home vegetable fermentation is lacto-fermentation, driven by lactic-acid bacteria in a salt brine.
It is distinguished from vinegar pickleing (which adds pre-made acid rather than growing it) and from curing or canning (which preserve by salt/sugar or by heat and a sealed jar), though fermentation often overlaps with all of them.
Fermentation is a process rather than a single food, but it has a recognizable look and feel. A vegetable ferment sits submerged in a cloudy, salty brine in a jar or crock, slowly bubbling as gas is released; over days and weeks the vegetables soften, dull in color and grow pleasantly sour-smelling. A drink ferment like kombucha or water kefir fizzes gently and may carry a living culture: a rubbery beige kombucha pancake (the SCOBY, a symbiotic culture of bacteria and yeast) or translucent water-kefir grains.
A sourdough starter is a bubbling, sour-smelling batter of flour and water. Across all of them you see the signs of live microbial life: bubbles, a tangy or yeasty aroma, a drop in clarity, and a sour taste. A harmless film of white surface yeast (kahm) sometimes forms on brines; fuzzy, colored, fluffy mold is the danger sign and means the batch should be discarded.
Properly fermented food smells sharp, sour and appetizing, never putrid.
Those acids drop the food's pH, make it sour, and a sufficiently acidic, salty, oxygen-poor environment is hostile to the bacteria and molds that cause spoilage and food poisoning, so the food keeps for weeks, months or longer without refrigeration or canning. That is the preservation half. The flavor half is just as important: fermentation breaks proteins into savory, umami-rich compounds, builds acidity and complexity, produces fizz and alcohol, and mellows harsh raw flavors into something rounded and deep: the difference between a raw cabbage and sauerkraut, raw soybeans and miso, grape juice and wine.
Many ferments are also credited with feeding beneficial gut bacteria (they are often rich in live cultures), though the food-safety value (keeping food edible) is the older and more certain benefit. In short, fermentation turns perishable raw ingredients into sour, savory, shelf-stable, living foods.
How to tell it's done
Tang/sourness develops, bubbling (active fermentation), pH drops; taste and smell guide it.
Common confusions
Fermenting (microbial transformation) is NOT the same as vinegar pickling (adding acid); fermentation CREATES the acid.
Safety notes
General kitchen safety tips. Always follow the manufacturer's instructions and current food safety guidance.
Properly done lacto-fermentation is very safe: the acid environment suppresses pathogens; but some ferments (fermented fish, sausages, certain conditions) carry real risks and need correct technique/salt/temperature.
Substitutions
| Use instead | How much | What changes |
|---|---|---|
| lacto-fermentation (the core method) | The salt-brine, lactic-acid method behind most home vegetable ferments, start here | |
| sauerkraut (the beginner's ferment) | Salted fermented cabbage: the simplest, most forgiving vegetable ferment to learn on |
Swaps that don't work
- pickle (the vinegar route): Quick or vinegar pickling adds ready-made acid instead of growing it, not fermentation
Common mistakes
- Not using enough salt, or none at all: salt is what keeps a vegetable ferment safe and clean while the lactic-acid bacteria take over; too little invites spoilage and mush.
- Letting vegetables float above the brine: anything exposed to air can grow mold, so everything must stay submerged.
- Fermenting too warm, which rushes the ferment, softens vegetables and encourages off-flavors; cool and slow is better.
- Sealing a jar airtight so building gas has nowhere to go (a burst jar, or worse); ferments need to vent.
- Confusing harmless white surface yeast (kahm) with dangerous mold, and either panicking over the former or ignoring the latter: fuzzy, colored, fluffy growth means discard the batch.
- Using reactive metal or lead-glazed vessels, which the acid can leach; ferment in glass, crock or food-grade plastic.
- Relying on guesswork for higher-risk ferments (fermented fish, low-salt or airtight ferments) instead of a tested recipe.
- And, generally, trusting sight and smell: properly fermented food smells pleasantly sour, never putrid; when in doubt, throw it out.
In short
- Bad sanitation, wrong salt %, confusing with vinegar pickling, impatience
Product picks
- Fermentation crock with weightsSee it on Amazon: fermentation crock with weights
- Wide-mouth mason jars with airlock lidsSee it on Amazon: wide-mouth mason jars with airlock lids
- Kitchen scale for measuring saltSee it on Amazon: kitchen scale for measuring salt
History
Fermentation is one of humanity's oldest technologies, older than written history and probably older than agriculture in some forms. Long before anyone understood microbes, people across the world discovered that salting vegetables, letting grape juice sit, souring milk or leaving grain and water to bubble produced food that kept longer and tasted better, and every region developed its own ferments from what it grew: sauerkraut and sour rye in central and eastern Europe, kimchi and fermented bean pastes in Korea, miso, soy sauce and sake in Japan, fish sauce around the ancient Mediterranean and Southeast Asia, injera, ogi and countless others in Africa, kvass among the Slavs, pulque and tepache in Mexico. For most of history these ferments were vital preservation, letting people store a harvest through winter and carry protein and vitamins on long journeys.
Only in the 19th century did Louis Pasteur show that living microorganisms (not some vital force in the food itself) drive fermentation, which finally explained the age-old craft and opened the way to controlling it. The traditions, though, long predate the science.
Is it still used today?
Fermentation is both a global food industry and a booming home craft. Industrially, fermentation underpins an enormous share of what we eat and drink: all beer, wine and spirits, all bread and vinegar, all cheese and yogurt, soy sauce, miso and every commercial sauerkraut, kimchi and pickle. At home, fermentation has enjoyed a huge revival, driven by interest in gut health, in traditional foodways, and in flavor: home cooks ferment their own sauerkraut, kimchi, hot sauces, kombucha, water kefir, sourdough and more, and books, courses and starter cultures abound.
Modern chefs treat fermentation as a flavor laboratory, making everything from fermented chili sauces to house misos and garums. Alongside the enthusiasm, food-safety bodies like the NCHFP, USDA and FDA publish tested guidelines for fermenting safely at home, because while fermentation is remarkably safe when done right, it must be done with enough salt, clean equipment and submerged food. Ancient, industrial and fashionable all at once, fermentation is more alive today than ever.
Questions people ask
What is fermentation?
Fermentation is the process of letting microbes, mainly bacteria and yeast, transform food. The microbes eat the sugars and starches and produce acids, alcohol and carbon dioxide, which sour and preserve the food and give it deep, savory, tangy flavor. It is one of the oldest ways of preserving food, and it gives us a huge range of everyday foods and drinks: sauerkraut, kimchi, fermented pickles, kombucha, bread and sourdough, yogurt, cheese, miso, soy sauce, beer, wine and more.
Most home vegetable fermentation is lacto-fermentation, where salt and lactic-acid bacteria do the work in a brine.
Is fermented food safe to eat?
Yes: fermentation is remarkably safe when done properly, and it has kept people fed for thousands of years. The acid and salt the process creates make the food hostile to spoilage and disease microbes. The keys are using enough salt (about 2 percent of the vegetable weight for a veg ferment), keeping the food submerged under its brine so nothing grows mold on the surface, using clean glass or crock vessels, and fermenting somewhere cool.
Harmless white surface yeast can be skimmed; fuzzy, colored, fluffy mold, sliminess or a rotten smell means discard the batch. For higher-risk ferments, follow a tested recipe from a source like the NCHFP or USDA. Properly fermented food smells pleasantly sour, never putrid.
What is the difference between fermenting and pickling?
They overlap, and a fermented pickle is both, but the words point to two different ways of souring food. In true fermentation, microbes GROW the acid: you salt vegetables and let lactic-acid bacteria turn their sugars into lactic acid over days or weeks, as in sauerkraut, kimchi and salt-brined sour pickles. In quick or vinegar pickleing, you ADD ready-made acid: you soak the food in vinegar, so no fermentation happens and the sourness is immediate.
Fermented foods are alive with cultures and taste complex and tangy; vinegar pickles taste sharply of vinegar and are not fermented. Confusingly, both can be called 'pickles'.
Are all fermented foods full of probiotics?
Many raw, unpasteurized ferments are rich in live bacteria and are often eaten for their probiotic cultures: think unpasteurized sauerkraut, kimchi, kombucha, kefir and live yogurt. But not all fermented foods deliver live cultures to your gut. Anything that is cooked, baked or pasteurized after fermenting (bread and sourdough, miso soup that has been boiled, shelf-stable canned sauerkraut, beer and wine) has had its live microbes killed or removed.
Those foods still gained the flavor and preservation benefits of fermentation, but they are not a source of live probiotics. If live cultures matter to you, look for raw, refrigerated, unpasteurized ferments.
Do I need special equipment to ferment food at home?
Not much. A clean glass jar or a stoneware crock, some way to keep the vegetables submerged under their brine (a smaller jar, a water-filled bag, a glass weight or a clean stone), and a loose cover to let gas escape are enough to make sauerkraut or a fermented pickle. Purpose-made fermentation crocks, airlock lids and weights make it tidier and more foolproof, and a kitchen scale helps you get the salt right.
Drinks like kombucha or water kefir need their living cultures (a SCOBY or the grains) but otherwise just clean jars. The two things that matter most are cleanliness and keeping the food under the liquid; fancy gear is optional.
Why does fermented food taste sour and savory?
Because of what the microbes make. Lactic-acid and acetic-acid bacteria turn the food's sugars into acids, and that acidity is the sour, tangy taste of sauerkraut, yogurt or a fermented pickle. At the same time, fermentation breaks down proteins into savory, umami-rich building blocks, which is why fermented soybean foods like miso and soy sauce, and aged cheese, taste so deeply savory.
Fermentation also develops fizz (from carbon dioxide), a little alcohol, and a complexity that raw ingredients lack. The result is food that tastes sour, savory, complex and alive: the transformation is the whole point.
Trivia answer
Fermentation is one of the oldest food technologies on Earth, far older than any understanding of how it works. For thousands of years, across every inhabited continent, people salted vegetables, soured milk, leavened bread and let grain and fruit bubble into beer and wine, and they knew, from hard-won experience, exactly how to do it and that the results kept longer and tasted better. But WHY it happened was a complete mystery.
For most of history it was assumed that fermentation was a purely chemical process, or the work of some 'vital force' within the food itself. It was Louis Pasteur, in the 1850s and 1860s, who demonstrated through careful experiments that fermentation is caused by living microorganisms (yeasts and bacteria) feeding and multiplying, and that different microbes produce different results (and that unwanted ones cause spoilage and disease). This was a revolutionary idea: it connected fermentation, spoilage and infection to the invisible world of microbes, helped launch the science of microbiology, and led directly to pasteurization, named after him.
So the humble jar of sauerkraut on your counter is the product of a craft that is prehistoric in its practice but was only understood scientifically about 150 years ago. Every fermented food is a collaboration with living things that our ancestors harnessed for millennia without ever seeing them.
Origin and background
- Why it was developed
Preserve food, create flavor and nutrition, make foods safe and digestible before refrigeration.
Related
- Lacto-fermentation
- Sauerkraut
- Kimchi
- Kombucha
- Sourdough
- Pickle
- Brine
- Miso
- Brine
Brine is simply salt dissolved in water, and 'brining' is the technique of soaking food in it.
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