How Does Fermentation Work?

In 1876, Louis Pasteur published a paper describing something he called la vie sans air (life without air). He had spent years studying what happened to wine when it spoiled, and he noticed that the transformation from grape juice to wine continued perfectly even when he sealed the barrels airtight. Something living was doing chemistry without oxygen. Pasteur called this process fermentation, and he was right to find it remarkable: he had stumbled onto one of the oldest metabolic strategies on Earth, and one that humans have been exploiting, intentionally or not, for roughly 13,000 years.

The short answer

Fermentation is a type of anaerobic metabolism (meaning it generates energy without oxygen). Microorganisms, primarily yeast and bacteria, break down sugars and release the electrons onto other organic molecules within the same reaction. The two most common outcomes are alcoholic fermentation (yeast converting glucose to ethanol and carbon dioxide) and lactic acid fermentation (bacteria converting glucose to lactic acid). These processes preserve food, create distinctive flavors, and produce the carbon dioxide that makes bread rise.

The full picture

Why organisms ferment

At its core, fermentation is a fallback system. Respiration (the process that powers most living cells when oxygen is available) is extraordinarily efficient. A single glucose molecule can yield up to 32 ATP molecules, the energy currency of cells. Fermentation, by contrast, squeezes out only 2 to 5 ATP per glucose molecule. That is a significant disadvantage, so why does it exist at all?

The answer is that fermentation requires no external electron acceptor. In aerobic respiration, oxygen acts as the final destination for electrons stripped from glucose. When oxygen is absent or insufficient, cells must route those electrons somewhere else fast. Fermentation solves this by having organic molecules serve as both the electron donor and the electron acceptor within the same reaction. The cell can keep generating ATP through substrate-level phosphorylation (a direct transfer of a phosphate group to ADP) even without a functioning respiratory chain.

This matters practically. During intense exercise, your muscle cells demand ATP faster than your bloodstream can deliver oxygen. The cells switch partially to lactic acid fermentation, converting glucose to lactate to keep the ATP supply flowing in the short term. This is why your muscles burn during a hard sprint.

Over 25% of all bacteria and archaea are fermenters, making it one of the most widespread metabolic strategies in the microbial world. Fermentation end products number nearly 300 different compounds across different organisms, from ethanol and lactic acid to propionic acid, butyric acid, and acetone.

The two main types

Alcoholic fermentation is carried out primarily by yeast, especially Saccharomyces cerevisiae, the workhorse of bread, beer, and wine production. The chemistry is straightforward: glucose is broken down through a series of steps called glycolysis, producing pyruvate. An enzyme called pyruvate decarboxylase removes carbon dioxide from pyruvate, yielding acetaldehyde. Another enzyme, alcohol dehydrogenase, then transfers electrons from NADH to acetaldehyde, producing ethanol.

The net reaction is: glucose → 2 ethanol + 2 carbon dioxide + 2 ATP

The carbon dioxide is what makes bread dough rise and what gives beer its natural carbonation. The ethanol accumulates in the liquid, whether that is wort for beer or grape juice for wine. In bread making, the ethanol evaporates during baking but contributes to the complex flavor development of the crust.

Lactic acid fermentation is performed by bacteria, particularly species of Lactobacillus. These bacteria import glucose and convert it to pyruvate through glycolysis, then reduce pyruvate directly using NADH to produce lactic acid. No carbon dioxide is released, which is why yogurt and sourdough starter have a thick, dense texture rather than a bubbly one.

Lactic acid fermentation is the reason yogurt is thick and tangy, why sauerkraut stays preserved without refrigeration, and why your sourdough starter becomes acidic over time. The lactic acid lowers the pH of the environment, inhibiting the growth of harmful bacteria and preserving the food. This antimicrobial effect is why fermentation evolved as a preservation technique long before refrigeration existed.

Fermentation through human history

The archaeological record suggests humans were fermenting beverages by roughly 13,000 years ago. Residues found in pottery shards from the Fertile Crescent contain chemical traces consistent with a fermented beverage made from wheat, barley, honey, and fruit. These were not accidental discoveries: the consistent recipe across multiple sites suggests intentional experimentation with fermentation as a way to preserve caloric energy and create something socially significant.

Wine appears in Egyptian records by 3,000 BCE. Beer was so nutritionally important in ancient Mesopotamia that it was used as payment for laborers building the great ziggurats. The same was true across the ancient world: fermented foods provided calories that kept longer than fresh food, and fermented beverages offered both nutrition and a form of social cohesion that water alone could not.

What nobody understood for thousands of years was the mechanism. Fermentation seemed almost magical: grape juice becoming wine, milk becoming cheese, cabbage becoming sauerkraut. It was not until Pasteur’s work in the 1870s that scientists established the biological cause. Even then, the full biochemical pathway was not mapped until the twentieth century, culminating in our current understanding of glycolysis and its regulation.

The role of fermentation today

Today, fermentation is both ancient craft and modern industrial process. Craft breweries and natural wineries use fermentation as their central tool, controlling temperature, yeast strains, and sugar availability to shape flavor in precise ways. Artisanal cheese makers manipulate lactic acid bacteria to develop specific textures and tastes over months or years of aging. Kombucha producers ferment sweetened tea with a symbiotic culture of bacteria and yeast, producing a mildly acidic, faintly effervescent drink.

At industrial scale, fermentation produces commodity chemicals including ethanol for fuel, lactic acid for biodegradable plastics, and citric acid for food additives. The pharmaceutical industry relies on fermentation to produce antibiotics, insulin, and vaccines. The engineered yeast strains used in these facilities are often genetically optimized versions of natural fermenters, carrying modifications that increase yield, tolerate higher alcohol concentrations, or produce specific compounds.

Even within your own body, fermentation matters. The human gut contains trillions of bacteria, many of which ferment dietary fiber into short-chain fatty acids that your colon cells use for energy. This gut fermentation influences immune function, inflammation levels, and possibly even mood through the gut-brain axis. A 2019 study published in Cell found that the composition of an individual’s gut microbiota significantly affects how they metabolize certain foods and medications.

Why it matters

Understanding fermentation changes how you see everyday objects. Every loaf of sandwich bread you eat exists because of a metabolic accident in yeast: the carbon dioxide they release as waste gets trapped in dough, expanding it into a light, airy structure. Every glass of wine represents a controlled decomposition of fruit sugars by a microorganism that humans learned to direct rather than merely observe. The sour tang of yogurt, the sharpness of aged cheese, the depth of soy sauce, the fizz in your pint: all are products of fermentation’s chemistry.

For food security, fermentation remains one of the most accessible preservation technologies available. It requires no electricity, no sophisticated equipment, and no scientific training. Communities worldwide use fermentation to extend the shelf life of perishable foods, to neutralize antinutrients in seeds and grains, and to create micronutrient-dense foods from inexpensive ingredients. As climate pressures challenge global food supply chains, these traditional techniques gain renewed relevance.

In biotechnology, engineered fermentation is a cornerstone of the bioeconomy. Ethanol from fermented corn and sugarcane displaces petroleum derivatives in fuel markets. Bacterial fermentation produces biodegradable polymers that could replace plastics. Laboratory-grown proteins, including novel therapeutics, depend on cellular fermentation systems. The global industrial fermentation market was valued at over $40 billion in 2025 and continues to grow.

Common misconceptions

“Fermentation is just rotting.” Both processes involve microbial decomposition, but rotting typically refers to unwanted decay by multiple competing organisms, producing foul-smelling and sometimes toxic end products. Fermentation, by contrast, involves specific microbial strains acting on specific substrates under controlled conditions, yielding predictable and desirable results. The difference is intention and control, not the underlying biology.

“Alcoholic fermentation produces mostly alcohol.” The ethanol in a 5% beer represents only about 2% of the glucose consumed by the yeast. The majority of the sugar goes toward producing carbon dioxide, new yeast cells, and dozens of minor flavor compounds including esters, fusel alcohols, and aldehydes. These trace compounds are what distinguish a lager from an ale, a Chianti from a Chardonnay.

“Sourdough is just bread made without yeast.” Sourdough is fermented with wild yeast captured from the flour and environment, combined with lactic acid bacteria that acidify the dough. The acid is what gives sourdough its distinctive tang, and the wild yeast strains produce a different flavor profile than commercial baker’s yeast. Sourdough fermentation also changes the dough’s gluten structure in ways that improve digestibility for some people, as documented in a 2019 study in Applied and Environmental Microbiology.

Key terms

Glycolysis: The metabolic pathway that breaks down glucose into pyruvate, producing a small amount of ATP and NADH. Glycolysis occurs in nearly all living cells, both fermenting and respiring.

Ethanol: The alcohol produced by yeast during alcoholic fermentation. In bread making, it evaporates during baking. In beverages, it accumulates to the concentrations you drink.

Lactic acid: The end product of lactic acid fermentation, produced by bacteria such as Lactobacillus. It is what gives yogurt, sourdough, and sauerkraut their tangy flavor and preservative acidity.

Pyruvate: The three-carbon molecule produced at the end of glycolysis. Depending on conditions and organism type, pyruvate is then converted to ethanol, lactic acid, or fed into the citric acid cycle.

Substrate-level phosphorylation: The direct transfer of a phosphate group from a high-energy molecule to ADP, producing ATP. This occurs during glycolysis and is the source of ATP in fermentation, unlike respiration which generates most ATP via the electron transport chain.

Acetic acid fermentation: The process by which acetic acid bacteria convert ethanol to acetic acid, producing vinegar. Acetobacter species are the primary actors, oxidizing ethanol stepwise to vinegar.