Cabbage, salt and two days produce the same process that makes bread, cheese, kimchi and kombucha. What fermentation is biologically, why almost every culture on earth arrived at it independently, and how a live drink behaves in the bottle.
Shred a cabbage, weigh out salt at roughly two per cent of its weight, pack it into a jar and press it under its own brine. Within a day or two, fine bubbles start to climb through the liquid. Nobody added them. Nothing was heated, stirred or inoculated. The cabbage has begun to ferment, and it will keep going without any further help.
That jar is a working model of one of the oldest pieces of technology our species has. Fermentation, in one sentence, is food being changed by living microorganisms that feed on its sugars and leave acid, gas and aroma behind. Long before anyone could see a bacterium, people almost everywhere had worked out that certain foods, left in certain conditions, would turn sour, fizzy, aromatic and durable — and that the change could be steered.
What is fermentation, exactly?
Fermentation is the transformation of food by microorganisms — mostly bacteria and yeasts — that feed on sugars and leave behind acids, gases, alcohols and a long list of aroma compounds. The raw material goes in as one thing and comes out as another. Milk becomes yoghurt. Flour and water become sourdough. Cabbage becomes sauerkraut. Sweetened tea becomes kombucha.
Biologists and cooks use the word slightly differently, which is worth knowing. In strict biochemistry, fermentation means releasing energy from sugar without using oxygen: the cell breaks glucose down part of the way and dumps the leftover carbon into a waste product it can excrete. In the kitchen, the word is broader and covers several processes that a biochemist would separate, including vinegar-making, which needs plenty of air.
How fermentation works: sugar into acid, gas and alcohol
Sugar is the fuel in almost every ferment: the fructose in fruit, the lactose in milk, the maltose released from starch in grain. The microorganisms do not swallow it whole. They secrete enzymes, which are proteins built for cutting particular molecules into smaller pieces, and it is those smaller pieces that get taken up and metabolised. Enzymes are a subject in their own right, and we cover them separately in what enzymes are and how they work.
Where the sugar goes next depends on which organisms are in charge. Three routes account for most of the world's fermented food.
Lactic acid fermentation
Lactic acid bacteria convert sugars mainly into lactic acid. The acid accumulates, the pH falls, and the food turns sour and firm. This is the pathway behind sauerkraut, kimchi, yoghurt, most cheeses, salami, sourdough's tang, brined olives and the sour vegetable ferments of nearly every cuisine. Some strains produce only acid; others also make carbon dioxide and a little alcohol, which is why some jars fizz and others do not.
Alcoholic fermentation
Yeasts, most famously Saccharomyces cerevisiae, convert sugar into ethanol and carbon dioxide. Two products, two applications: brewers and winemakers keep the ethanol and let the gas escape, while bakers keep the gas — the bubbles that lift a loaf — and let almost all the ethanol bake off. Same organism, same reaction, different half of the output valued.
Acetic acid fermentation
Acetobacter bacteria take ethanol and, using oxygen, oxidise it to acetic acid. This is the second act rather than the first: something alcoholic has to exist before it can be made into vinegar. It is also part of what happens in a kombucha vessel, where yeasts and acetic acid bacteria work in sequence in the same liquid.
Real ferments are rarely one pathway alone. A sourdough starter is a partnership of wild yeasts and lactic acid bacteria. A kombucha culture is a community. And not everything of interest is a lactic acid bacterium: Bacillus subtilis, the hay bacillus, is the organism behind Japanese nattō and several West African seed ferments, and it is one of the cultures we work with.
Why fermentation is not the same as rotting
Fermentation and spoilage are both microbial breakdown. The difference is which organisms establish themselves first and what they leave behind.
A fermenting jar is a competition that has been rigged. Salt draws water out of the vegetables and suits salt-tolerant bacteria better than most spoilage organisms. Submerging everything under brine removes the oxygen that moulds need. Once lactic acid bacteria take hold, the acid they produce drops the pH far enough that the jar becomes an inhospitable place for most of the competition.
That is the practical genius of the technique. The food is not sterilised; it is colonised, deliberately, by organisms whose by-products defend the territory they have taken.
Why people have fermented food for thousands of years
Chemical residues on Neolithic pottery from Jiahu, in central China, carry the signature of a fermented drink of rice, honey and fruit made around nine thousand years ago. Fermentation is older than writing, older than the wheel, and it arose independently almost everywhere people settled.
Four reasons recur across those cultures. Preservation came first: before refrigeration, a sour crock of vegetables was how a household ate through winter. Then portability — cheese is milk that survives a journey. Then edibility, because fermentation makes usable food out of raw materials that are harsh or unpleasant otherwise: bitter olives, cassava, soybeans, cacao beans and coffee cherries are all fermented before they become anything you would want. And finally flavour, which is why we still do it now that the fridge exists. Fermentation manufactures complexity — the savoury depth of aged cheese, the sourness of rye bread, the cellar note in a kvass-style grain drink from the Baltics, the sharp finish of kombucha. None of that can be added from a bottle.
Wild fermentation and cultured fermentation
Wild, or spontaneous, fermentation relies on the microorganisms already present — on the vegetable skins, in the flour, on the equipment, in the air of the room. Sauerkraut, traditional sourdough, many farmhouse cheeses and the spontaneously fermented beers of Belgium all begin this way. The result carries a strong sense of place, and it is genuinely unpredictable: the same recipe in two kitchens gives two outcomes.
Cultured fermentation begins with a deliberate introduction. The maker adds a known culture — a defined starter, or a ladle of a previous successful batch, a practice called backslopping. The chosen organisms start with an overwhelming numerical advantage and get to the sugar first, which makes the outcome consistent and much easier to keep safe. Almost all commercial yoghurt, cheese, beer and bread works this way.
Neither is superior. Wild fermentation trades control for character; cultured fermentation trades some unpredictability for repeatability. Deciding the culture in advance is what makes a batch reproducible, which is why a bottled starter such as our Fermentum Concentrate exists at all: it lets a fermentation begin from a known population rather than whatever happens to be in the room.
How a live drink behaves in the bottle
A drink is live when the microorganisms that made it are still present and viable when you open it. The fermentation was never stopped — it was slowed by chilling, and it continues quietly in the bottle.
Many fermented products on a shelf have been stopped, usually by heat, sometimes by fine filtration. There are sound reasons for that: stopped products are stable, keep for months without refrigeration, taste identical from first bottle to last, and will not build pressure in transit. Pasteurised beer, shelf-stable sauerkraut and any kombucha sold from an ambient shelf are all stopped ferments.
A live drink behaves differently, and it is fair to know what you are getting.
- It has to be kept cold.
- It carries dissolved gas, so it needs opening carefully.
- It throws a natural sediment, which settles and is normal.
- It drifts in taste over its life — the same batch is a little softer in week three than in week one.
- It contains a trace of naturally occurring alcohol from the fermentation itself, minimal enough that ours are classed as non-alcoholic.
These are the ordinary consequences of a food still being alive, which is why our own drinks are shipped chilled, and why how we make them is a page of its own. What those cultures actually are, organism by organism, is set out in live cultures in fermented food.
Where to read next
Fermentation is a hub subject. It branches into what enzymes actually are, which bacterial cultures do what, how the world's fermented drinks differ from one another, and how to run a first ferment at home.