The reactants of alcoholic fermentation are sugar (glucose) and two molecules involved in energy transfer within the cell: ADP and NAD⁺. In its simplest and most commonly referenced form, the overall equation shows one molecule of glucose (C₆H₁₂O₆) being converted into two molecules of ethanol and two molecules of carbon dioxide. But the full picture involves a few more moving parts than that single equation suggests.
The Core Equation
The balanced summary equation for alcoholic fermentation is:
C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
One molecule of glucose, a six-carbon sugar, yields two molecules of ethanol (the alcohol) and two molecules of carbon dioxide (the gas that makes bread rise and beer fizzy). From a mass standpoint, the theoretical maximum yield is about 0.51 kilograms of ethanol for every kilogram of sugar consumed. The remaining mass leaves as carbon dioxide.
This equation is a neat summary, but it hides the intermediate steps and the cellular machinery that makes the process work. Glucose doesn’t simply break apart into ethanol on its own. It first goes through glycolysis, the same sugar-splitting pathway used by nearly every living cell, and then two additional reaction steps that are unique to alcoholic fermentation.
What Happens Step by Step
Glycolysis breaks one molecule of glucose into two molecules of pyruvate (pyruvic acid). This stage consumes NAD⁺, converting it to NADH, and produces a small net gain of ATP, the cell’s energy currency. So far, this is identical to what happens in your own muscle cells.
The fermentation-specific reactions come next, in two steps. First, an enzyme called pyruvate decarboxylase strips a carbon atom off each pyruvate molecule, releasing it as CO₂ gas and leaving behind a two-carbon compound called acetaldehyde. Second, another enzyme called alcohol dehydrogenase transfers electrons from NADH to that acetaldehyde, converting it into ethanol and recycling the NADH back into NAD⁺.
That NAD⁺ recycling is the entire point of fermentation from the yeast’s perspective. Without it, glycolysis would grind to a halt because NAD⁺ is needed to keep breaking down sugar. The ethanol is essentially a waste product the cell dumps to keep its energy metabolism running. The net energy yield is just 2 ATP per glucose molecule, compared to roughly 18 ATP when yeast fully burns glucose using oxygen.
Which Sugars Can Serve as Reactants
Glucose gets top billing in textbooks, but it’s not the only sugar yeast can ferment. Fructose works just as well and enters the same glycolysis pathway. Sucrose (table sugar) and maltose (a sugar released when starch breaks down) are also fermentable, though yeast must first split these disaccharides into their simpler components before processing them. In bread making, for example, yeast works through the glucose and fructose in flour first, then tackles maltose released as enzymes break down starch.
Pentoses, five-carbon sugars like xylose, are a different story. Standard brewer’s and baker’s yeast (Saccharomyces cerevisiae) cannot ferment them without genetic modification, which is one of the major challenges in producing ethanol from woody plant material.
Why Oxygen Isn’t in the Equation
Alcoholic fermentation is an anaerobic process, meaning it does not require oxygen. That’s what makes it useful to yeast in oxygen-poor environments like the inside of a sealed wine barrel or deep within a lump of bread dough. When oxygen is available, yeast can instead fully oxidize glucose through respiration, extracting far more energy per molecule.
There is a notable exception. S. cerevisiae will ferment sugar even when oxygen is present, as long as glucose concentrations are high enough. This behavior, called the Crabtree effect, is why wine fermentation begins almost immediately after yeast is added to sugar-rich grape juice, regardless of how much air is around. The yeast essentially prioritizes speed of energy production over efficiency when sugar is abundant.
Nutrients Yeast Needs Beyond Sugar
Sugar is the headline reactant, but yeast cells are living organisms that need more than just a carbon source. The most important supplementary nutrient is nitrogen, which yeast uses to build proteins and grow. In winemaking, this is tracked as “yeast assimilable nitrogen,” a combination of ammonia and amino acids naturally present in grape juice. When nitrogen runs low, fermentation can stall before all the sugar is consumed, a problem winemakers call a “stuck fermentation.”
Yeast also requires small amounts of B vitamins, specifically biotin, pantothenic acid, and thiamin, which function as helpers for the enzymes driving fermentation. In most food and beverage applications, these are present in sufficient quantities in the raw ingredients, but industrial ethanol production sometimes requires supplementation.
Byproducts Beyond Ethanol and CO₂
The simple equation lists only ethanol and carbon dioxide as products, but real fermentation generates a constellation of minor byproducts that matter enormously for flavor. Glycerol is the most abundant of these, serving as a carbon competitor against ethanol. It contributes body and slight sweetness to wine and beer.
Other byproducts include organic acids (acetic, succinic, lactic), higher alcohols sometimes called fusel alcohols, acetaldehyde, and various esters. These compounds are responsible for much of what you taste in fermented beverages: the banana note in wheat beer comes from an ester called isoamyl acetate, the rosy aroma in some wines from phenylethyl alcohol. Higher temperatures and higher sugar concentrations tend to push yeast toward producing more glycerol, acetic acid, acetaldehyde, and fusel alcohols, which is one reason fermentation temperature is so carefully controlled in brewing and winemaking.
When Fermentation Slows or Stops
Even with plenty of sugar remaining, fermentation eventually slows as ethanol accumulates. Standard strains of S. cerevisiae begin to struggle at ethanol concentrations of about 4 to 6 percent, with growth seriously inhibited above 6 percent. Some strains, particularly those selected for wine production, can tolerate 12 to 15 percent before stopping. Engineered strains in research settings have pushed that threshold to around 10 percent or higher by producing protective compounds like trehalose inside the cell.
Temperature also plays a role. Fermentation rates peak around 30 to 35°C, with an optimal zone near 32°C at a slightly acidic pH of about 5. Below 15°C, fermentation slows dramatically. Above 40°C, the yeast’s enzymes begin to denature and the cells die.