Fermentation: What It Is and How It Transforms Food

The cell’s backup generator

When the power goes out at home, a backup generator does not run everything: it keeps the essential lights on. Fermentation is the backup generator of the cell: a way of extracting energy from sugars when oxygen is absent, or simply not enough. It yields very little compared with respiration, but it runs anywhere — at the bottom of a vat of grape must, inside a pot of yogurt, in our muscles during a sprint.

And like any self-respecting generator, it produces waste. Except that here the waste is ethanol, carbon dioxide and lactic acid: in other words wine, beer, bread, yogurt and vinegar. A good part of what we call “flavour” is the waste product of a microbe at work.

How it works: it all starts with glycolysis

The first step is the same for every living thing. Through glycolysis, a molecule of glucose (6 carbon atoms) is split into two molecules of pyruvate (3 atoms each). The reward is modest: 2 molecules of ATP, the cell’s “energy currency”. Glycolysis needs no oxygen, but it has a problem: it consumes a shuttle molecule called NAD⁺, which picks up the electrons released as glucose is broken down. When the NAD⁺ runs out, everything grinds to a halt.

This is where the roads fork.

• With oxygen, pyruvate enters the mitochondria and is dismantled completely by cellular respiration: the haul rises to roughly 38 ATP per glucose, and NAD⁺ is regenerated by the chain that uses oxygen.
• Without oxygen, a plan B is needed. Fermentation dumps the electrons back onto the pyruvate itself (or one of its derivatives): NAD⁺ becomes available again, glycolysis restarts, and the cell keeps collecting its 2 ATP at a time. The leftover ethanol or lactic acid is the price to pay.

In other words: fermentation is not there to make alcohol or acid — those are side effects. It is there to regenerate NAD⁺ and keep the engine from stalling.

The three fermentations worth knowing

	Alcoholic	Lactic	Acetic
Who does it	Yeasts (Saccharomyces cerevisiae)	Lactic acid bacteria, muscle cells	Acetic acid bacteria (Acetobacter)
What it makes	Ethanol + CO₂	Lactic acid	Acetic acid
Needs oxygen?	No	No	Yes (obligate aerobes)
Where you meet it	Wine, beer, bread	Yogurt, sauerkraut, cheese, muscles	Vinegar

Alcoholic: yeasts at work

The star is a microscopic fungus, the yeast Saccharomyces cerevisiae. In the absence of oxygen it turns pyruvate into ethanol, releasing carbon dioxide. The ethanol is what we are after in wine and beer; in bread it is the gas that counts, puffing up the dough as it rises, while the alcohol almost entirely evaporates in the oven.

Yeasts, however, are victims of their own waste: most strains die once the alcohol passes 10-15%. That is why commercial fermented drinks top out between 5 and 21% alcohol: going beyond that takes distillation, which is not a fermentation but a physical separation.

Practical example: in grape must, the sugars were put there by the vine through photosynthesis. The yeasts naturally present on the skins and in the environment find them and start fermenting: the must “seethes” with rising CO₂, and within days the sugars become alcohol. The wine is done when the yeasts run out of sugar, or when their own alcohol puts them out of action.

Lactic: from sauerkraut to your muscles

Here pyruvate is converted straight into lactic acid, in a single step. It is the work of lactic acid bacteria (genera such as Lactobacillus, Streptococcus and Leuconostoc), which turn milk into yogurt and cheese, cabbage into sauerkraut, cucumbers into pickles. Some of these bacteria turn up again in probiotic supplements.

But it is not a microbes-only affair: our own cells can do it too. During intense effort, when the blood cannot deliver enough oxygen, muscles switch on the backup generator and make ATP by lactic acid fermentation. The accumulated lactate is then carried to the liver, which converts it back into pyruvate. Red blood cells, which have no mitochondria, ferment their glucose all the time.

Practical example: to make yogurt, lactic acid bacteria are added to milk and the mixture is kept warm for a few hours. The bacteria consume the lactose (the sugar in milk) and release lactic acid: the pH drops, the milk proteins coagulate, and the liquid thickens into a cream. The tangy taste is, quite literally, the taste of the acid they produced.

Acetic: the “fermentation” that breathes

Vinegar’s name is misleading. Acetic acid bacteria (such as Acetobacter) turn ethanol into acetic acid in two steps, but theirs is not an anaerobic process: they are obligate aerobes, and without oxygen they stop working. Technically they perform an oxidation, not a fermentation; the traditional name simply stuck. This is why a bottle of wine left open to the air turns to vinegar, while a well-corked one does not.

From mystery to science: the history of fermentation

Humanity fermented for millennia without knowing what it was doing: the oldest wine jars found by archaeologists are roughly 7,000 years old. For all that time fermentation remained a craft: people knew how to make wine, but not why the must seethed.

Science took two centuries to catch up. In the 1600s Antoni van Leeuwenhoek observed yeast under the microscope, with no inkling of its role. In 1815 Gay-Lussac described the chemistry that turns sugar into alcohol. Between 1835 and 1840 several researchers recognized that yeast is a living organism that reproduces, against the view of the most influential chemists of the day, convinced that fermentation was a mere reaction between molecules.

The turning point came with Louis Pasteur, who had begun studying fermentations in Lille in 1854. From 1857 onwards he proved with rigorous experiments that fermentation proceeds only when the microorganisms are alive and multiplying: first the lactic kind, then the alcoholic. He defined it as “life without air”, and watching certain microbes work only away from oxygen, he coined the concept of the anaerobic organism. His studies on the diseases of wine led in 1865 to pasteurization (patented that year): heating the drink to between 60 and 100 °C to kill unwanted microbes, a technique later extended to the milk we drink today.

One last question remained: is the living cell itself required, or just something it contains? In 1897 Eduard Buchner made sugar ferment using a yeast extract with no living cells in it: the work is done by zymase, a complex of enzymes, proteins whose instructions are written in the yeast’s DNA. The discovery, rewarded with the Nobel Prize in Chemistry in 1907, closed the circle: Pasteur was right about the who (the microbes), the chemists about the what (the molecules). And it opened a new field: biochemistry.

Why fermented foods keep

Before refrigerators, fermenting was one of the few ways to make food last. The mechanism is chemical: the acids and alcohol produced by the “friendly” microbes create an environment the dangerous ones cannot live in. Acetic acid concentrations above 5 grams per litre are enough to stop most microorganisms from growing, and the low pH of yogurt and sauerkraut has the same effect. In practice, the first colonizers claim the territory and make it uninhabitable for anyone arriving later.

Beyond food

The same chemistry works outside the kitchen too. Bacterial fermentations produce industrial solvents such as acetone and butanol, along with compounds used in drugs and vitamins. In the lab, fermentation even helps identify bacteria: the dangerous strain E. coli O157:H7, for instance, is recognized precisely because, unlike its harmless relatives, it cannot ferment a particular sugar, sorbitol. A process born to survive without oxygen has become a microbial fingerprint.