Glucose fermentation is the reference case for the whole subject. Every other fermentable sugar is either converted into glucose first or joins the same pathway a step or two downstream. Understanding what happens to one glucose molecule accounts for most of what happens in a fermenter.

The Balanced Equation

C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂

One molecule of glucose yields two of ethanol and two of carbon dioxide. The formulation is often credited to Gay-Lussac, who worked out the stoichiometry in the early nineteenth century, well before anyone understood that a living organism was responsible.

Checking the mass balance:

Molar mass Quantity Total
Glucose 180.16 g/mol 1 180.16 g
Ethanol 46.07 g/mol 2 92.14 g
Carbon dioxide 44.01 g/mol 2 88.02 g

92.14 + 88.02 = 180.16. The equation balances exactly, which means the theoretical maximum yield is 51.1% ethanol and 48.9% CO₂ by mass.

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.

Real fermentations fall short of this. Roughly 90-95% of the theoretical figure is typical, because a portion of the glucose is diverted into building new yeast cells and into glycerol, which the cell produces to manage its internal redox balance. Pasteur established this discrepancy experimentally in the 1860s, and it remains the practical benchmark.

Including yeast above the arrow is a common convention in teaching, and it is a useful reminder that the reaction does not occur spontaneously:

C₆H₁₂O₆ –[yeast]→ 2 C₂H₅OH + 2 CO₂

The Pathway, Step by Step

The single-line equation compresses a sequence of roughly twelve enzyme-catalyzed reactions.

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.

Stage one: glycolysis

Glucose is converted to two molecules of pyruvate through ten steps in the cytoplasm. The first half of the sequence spends ATP, phosphorylating glucose and rearranging it. The second half recovers more than it spent.

The accounting across the whole of glycolysis:

  • 2 ATP invested
  • 4 ATP produced
  • Net gain: 2 ATP
  • 2 NAD⁺ reduced to NADH

The rate-limiting step is the phosphorylation of fructose-6-phosphate by phosphofructokinase, which is the cell’s main control point over how fast glucose is processed.

Stage two: pyruvate to acetaldehyde

Pyruvate decarboxylase removes a carboxyl group from each pyruvate, releasing it as carbon dioxide and leaving acetaldehyde. This enzyme requires thiamine pyrophosphate, a derivative of vitamin B1, as a cofactor. This step is the source of the CO₂ that raises bread and carbonates beer.

Stage three: acetaldehyde to ethanol

Alcohol dehydrogenase reduces acetaldehyde to ethanol, consuming the NADH generated back in glycolysis and regenerating NAD⁺.

That regeneration is the entire point. Glycolysis cannot proceed without a supply of NAD⁺, and with no oxygen available there is no electron transport chain to recycle it. Converting acetaldehyde to ethanol is how the cell keeps its NAD⁺ pool replenished. Ethanol is not the goal of fermentation. It is the disposal mechanism that allows the ATP-generating steps to continue running.

Conditions Required

Absence of oxygen, or an excess of glucose. Fermentation is the anaerobic route, but Saccharomyces cerevisiae will ferment even in aerated conditions when glucose concentration is high, an effect named after Herbert Crabtree. Above roughly 0.1 g/L glucose, fermentation dominates regardless of available oxygen.

A living, viable culture. No yeast, no reaction. Glucose solutions do not convert themselves to ethanol.

Temperature within the strain’s range. Typically 18-24°C for ale strains, 8-14°C for lager strains. Below range, fermentation stalls. Above it, the yeast is stressed and generates off-flavors, and sustained heat kills the culture.

Acidic pH. Yeast performs well between about pH 4 and 6, and fermentation lowers pH further as organic acids accumulate.

Assimilable nitrogen and micronutrients. The yeast is building cells as well as producing ethanol, and it cannot do so from sugar alone.

The Classroom Demonstration

The standard laboratory version: combine glucose solution with a suspension of bakers’ yeast in a flask, seal it with a stopper and delivery tube, and run the tube into limewater. Bubbles appear in the limewater within minutes and it turns cloudy, confirming CO₂. A control flask with boiled yeast produces nothing, which demonstrates that a living organism is required rather than a chemical property of the sugar itself.

Measuring the mass lost by the flask over time gives the CO₂ evolution rate, which serves as a proxy for fermentation rate and is the usual basis for experiments comparing temperature or sugar type.

Fermentation Compared with Aerobic Respiration

Both begin with the same ten steps of glycolysis. They diverge at pyruvate.

With oxygen present, pyruvate enters the mitochondrion, feeds the citric acid cycle, and its electrons pass down the electron transport chain, producing roughly 30-32 ATP per glucose. Older textbooks give 36-38, a figure calculated before the energetic cost of transporting the intermediates was accounted for.

Without oxygen, pyruvate is converted to ethanol and the process stops at 2 ATP. The remaining energy stays chemically bound in the ethanol, which is why ethanol is combustible and useful as a fuel.

Lactic acid fermentation is the third variant, taking the same glycolytic route and then reducing pyruvate directly to lactate without releasing CO₂:

C₆H₁₂O₆ → 2 CH₃CH(OH)COOH

Glucose Fermentation: Key Numbers

  • Balanced equation: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
  • Molar masses: glucose 180.16, ethanol 46.07, CO₂ 44.01 g/mol
  • Theoretical yield: 51.1% ethanol, 48.9% CO₂ by mass
  • Practical yield: roughly 90-95% of theoretical, the remainder going to cell growth and glycerol
  • Net ATP: 2 per glucose, against roughly 30-32 for aerobic respiration
  • CO₂ is released at the pyruvate decarboxylase step, not during glycolysis

Further Reading:

Frequently Asked Questions

What is the equation for the fermentation of glucose? C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. One glucose molecule produces two ethanol molecules and two carbon dioxide molecules. Yeast is written above the arrow by convention, since it catalyzes the reaction without being consumed by it.

How much ethanol does glucose produce? The theoretical maximum is 51.1% by mass, meaning 180 g of glucose yields 92 g of ethanol along with 88 g of CO₂. Actual fermentations reach roughly 90-95% of that, because some glucose is used to build yeast cells and to produce glycerol.

What conditions are needed for glucose fermentation? A viable yeast culture, an absence of oxygen or a high enough glucose concentration to trigger the Crabtree effect, a temperature within the strain’s range (commonly 18-24°C for ale yeast), acidic pH between roughly 4 and 6, and adequate assimilable nitrogen.

Where does the CO₂ come from in fermentation? From the conversion of pyruvate to acetaldehyde by pyruvate decarboxylase, after glycolysis is complete. Glycolysis itself releases no carbon dioxide.

Why does fermentation produce only 2 ATP? Glycolysis alone yields a net 2 ATP, and fermentation adds none beyond that. The subsequent steps exist to regenerate NAD⁺ so glycolysis can keep running, not to capture energy. Extracting the remaining energy from glucose requires the citric acid cycle and oxidative phosphorylation, both of which need oxygen.