Every loaf of bread, every glass of wine, every can of beer involves the same basic process: yeast consuming sugar and excreting alcohol and carbon dioxide as byproducts. The organisms doing this have been doing it for billions of years. Humans have been exploiting it for at least 9,000.

What Fermentation Is

Fermentation is anaerobic metabolism. The breakdown of organic compounds without oxygen. In the context of sugar chemistry, the relevant type is alcoholic fermentation by yeast (Saccharomyces cerevisiae being the primary species in food production), though lactic acid fermentation by bacteria is equally important for yogurt, cheese, sourdough, sauerkraut, and kimchi.

In aerobic respiration, what happens when oxygen is available, glucose is fully oxidized to CO₂ and water, yielding 36-38 ATP molecules per glucose. In fermentation, the same glucose yields only 2 ATP. Yeast switch to fermentation when oxygen is depleted, not because it’s efficient, but because it’s what’s available.

The Chemical Equation

Alcoholic fermentation:

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

Glucose breaks down into ethanol and carbon dioxide. By mass, that puts the theoretical ceiling at 51.1% ethanol and 48.9% CO₂, a figure worked through in detail in the equation and pathway for glucose fermentation.

Mass balance of glucose fermenting to ethanol and carbon dioxide One mole of glucose weighing 180.16 grams yields two moles of ethanol weighing 92.14 grams and two moles of carbon dioxide weighing 88.02 grams. Ethanol is therefore 51.1 percent of the starting mass and carbon dioxide 48.9 percent. Real fermentations reach about 90 to 95 percent of this ceiling because yeast diverts carbon into glycerol and new cells. C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ Every gram of glucose fermented has to come out as one of two things. Starting mass 1 mol glucose = 180.16 g Products, bar drawn to scale Ethanol · 92.14 g 51.1% of starting mass Carbon dioxide · 88.02 g 48.9% — lost as gas 92.14 + 88.02 = 180.16 g. Nothing is unaccounted for — this is the Gay-Lussac equation. In practice you never hit 51.1%. Yeast spends 4-6% of the sugar building new cells and making glycerol, so real fermentations land at roughly 90-95% of the theoretical ceiling.
This is why a fermenting vessel loses weight. Just under half the sugar leaves as carbon dioxide, which is also why an airlock bubbles and why a sealed vessel is dangerous.

This is the simplified equation. The actual pathway runs through glycolysis: glucose is converted to two molecules of pyruvate via a 10-step enzymatic process. In the absence of oxygen, pyruvate is converted to acetaldehyde (by the enzyme pyruvate decarboxylase), which is then reduced to ethanol (by alcohol dehydrogenase), regenerating NAD⁺ in the process. The regeneration of NAD⁺ is what allows glycolysis to continue, without it, the process would stall.

The alcoholic fermentation pathway from glucose to ethanol Glucose is converted by glycolysis into two pyruvate, yielding a net two ATP and reducing two NAD plus to two NADH. Pyruvate decarboxylase removes two carbon dioxide to give two acetaldehyde. Alcohol dehydrogenase then reduces acetaldehyde to two ethanol, oxidising the two NADH back to NAD plus. That regeneration of NAD plus is what allows glycolysis to keep running without oxygen. Glucose C₆H₁₂O₆ · one 6-carbon sugar 2 × Pyruvate C₃H₄O₃ each · 3 carbons 2 × Acetaldehyde C₂H₄O each · 2 carbons 2 × Ethanol C₂H₅OH each · the alcohol Glycolysis 10 enzymatic steps net +2 ATP Pyruvate decarboxylase releases 2 CO₂ — the gas that raises bread and carbonates beer Alcohol dehydrogenase reduces acetaldehyde to ethanol 2 NAD⁺ → 2 NADH 2 NADH → 2 NAD⁺ NAD⁺ regenerated
Yeast does not make ethanol because ethanol is useful to it. The last step exists to turn NADH back into NAD⁺. Without that regeneration, glycolysis runs out of NAD⁺ within seconds and the cell's only anaerobic source of ATP stops. Ethanol and CO₂ are the waste.

For lactic acid fermentation:

C₆H₁₂O₆ → 2 CH₃CH(OH)COOH

The pathway diverges after glycolysis: pyruvate is directly reduced to lactic acid rather than going through acetaldehyde. Lactobacillus species do this. The lactic acid accumulates, lowering pH, which is what preserves food and gives fermented dairy products their characteristic sourness.

Sugar Type Matters

Not all sugars ferment equally. Saccharomyces cerevisiae ferments glucose and fructose readily. Sucrose (table sugar) is quickly split into glucose and fructose by the enzyme invertase on the yeast cell wall, so it ferments efficiently too.

Maltose, two glucose units linked by an α-1,4 glycosidic bond, is the primary sugar in malted barley and is the main fermentable sugar in beer production. Some yeast strains are better at metabolizing maltose than others, which is why different yeast strains are used for beer versus wine versus bread.

Lactose (milk sugar) is not fermented by standard bakers’ yeast, which lacks the enzyme lactase. This is why milk doesn’t naturally ferment into alcohol with S. cerevisiae — but certain bacterial strains and some specialized yeasts can ferment lactose, which is how kefir and koumiss are produced.

Yeast also works through the fermentable sugars in a set order rather than all at once, and several common sugars are left untouched entirely. The full list, and the transporter and enzyme requirements behind it, is covered in which sugars yeast can ferment.

Beyond Ethanol and CO₂

The two-product equation is accurate for stoichiometry and misleading about flavor. Fermentation also generates glycerol, higher alcohols formed from amino acid breakdown, esters that carry fruit aromas, acetaldehyde, organic acids, new yeast cells, and a substantial amount of heat. Those minor products account for most of the sensory difference between an ale and a lager, or between two wines made from the same grape.

Temperature is the main lever. Warmer fermentation accelerates ester and fusel alcohol production, which is why ale strains run at 18-24°C and lager strains at 8-14°C. Alcoholic fermentation in yeast covers the byproducts and the conditions that govern them.

Bread

In bread, the CO₂ from fermentation is what creates the rise. The ethanol evaporates during baking (bread contains less than 0.5% ethanol after baking). The yeast consumes sugars naturally present in the flour (mostly glucose and maltose from enzyme activity on starch) along with any added sugar.

Sourdough bread uses a mixed culture of wild yeast and lactic acid bacteria. The bacteria produce lactic and acetic acid, which gives sourdough its characteristic sourness and also inhibits mold growth, extending shelf life. The wild yeast in sourdough starters are typically Kazachstania humilis (formerly Candida humilis) rather than S. cerevisiae, and they ferment more slowly, which is why sourdough requires longer proofing times.

Beer and Wine

In beer, barley grain is malted, allowed to germinate, then dried. Germination activates amylase enzymes that break starch down into fermentable sugars, primarily maltose. The malt is then mashed in hot water to extract those sugars. Hops are added as a bittering and preserving agent. Yeast is pitched and fermentation proceeds.

Lager yeasts (S. pastorianus) ferment at low temperatures and settle to the bottom of the vessel. Ale yeasts (S. cerevisiae) ferment warmer and rise to the top. This distinction, bottom-fermenting vs. top-fermenting, is the fundamental biological difference between lagers and ales.

Wine fermentation uses the natural sugars in grape juice. A mix of glucose and fructose. Wild yeast on grape skins can start fermentation spontaneously, though commercial winemakers generally inoculate with known strains for consistency. The sugar content of the grapes at harvest determines the potential alcohol content. In years with more sunlight, grapes develop more sugar, producing higher-alcohol wines.

CO₂: Bread Rise, Bubbles, and Carbonation

Carbon dioxide from fermentation has applications beyond bread rise. In champagne and other sparkling wines, a secondary fermentation is induced inside the bottle, trapping CO₂ under pressure. When the bottle is opened, the pressure releases and the dissolved CO₂ comes out of solution as bubbles.

In the production of beer, CO₂ can be collected during fermentation and later added back to carbonate the final product. The CO₂ bubbles in a bottle of beer are either the product of fermentation or added carbonation, large commercial brewers typically use the latter.

Sugar Fermentation: Key Numbers

  • Alcoholic fermentation: 1 molecule glucose → 2 ethanol + 2 CO₂
  • Net ATP yield: 2 per glucose (vs. 36-38 via aerobic respiration)
  • Bread rises from CO₂; ethanol evaporates at ~78°C during baking
  • S. cerevisiae ferments glucose, fructose, sucrose, and maltose; not lactose
  • Most wine grapes contain 150-250g of sugar per liter of juice at harvest

Explore Further

Sources:

Frequently Asked Questions

What is fermentation in simple terms? Fermentation is anaerobic metabolism — microorganisms breaking down sugars without oxygen, producing alcohol and CO₂ (yeast) or lactic acid (bacteria) as byproducts. It’s the process behind bread rising, wine and beer alcoholic content, and the sourness of yogurt, cheese, and sourdough.

What sugars can yeast ferment? Saccharomyces cerevisiae (the yeast in bread, beer, and most wine) ferments glucose, fructose, sucrose, and maltose. It cannot ferment lactose (milk sugar) because it lacks the enzyme lactase. Specialist yeasts and bacteria are used for lactose-fermenting products like kefir and koumiss.

Why does fermentation produce only 2 ATP instead of 36? Glycolysis (the fermentation pathway) only partially oxidizes glucose, stopping at pyruvate and then converting it to ethanol or lactic acid. The full energy release from glucose requires oxidative phosphorylation in the mitochondria, which requires oxygen. In the absence of oxygen, cells settle for 2 ATP per glucose because that’s what anaerobic conditions allow.

What’s the difference between alcoholic and lactic acid fermentation? Both start with glycolysis breaking glucose into pyruvate. Alcoholic fermentation (by yeast) converts pyruvate to acetaldehyde then to ethanol, releasing CO₂. Lactic acid fermentation (by Lactobacillus and similar bacteria) converts pyruvate directly to lactic acid. Both regenerate NAD⁺, which allows glycolysis to continue.