The conversion of sugar into alcohol is a fundamental biological process performed by yeast. This process, known as alcoholic fermentation, is a metabolic pathway that allows the organism to extract energy from sugars like glucose and fructose. The yeast consumes the sugar and converts it into two primary byproducts: ethyl alcohol, or ethanol, and carbon dioxide gas. This ancient biochemical reaction forms the basis for the production of beverages like beer and wine, as well as the leavening of bread.
Understanding the Environment for Fermentation
Yeast performs this sugar conversion as an adaptation to environments where oxygen is limited or entirely absent. When oxygen is unavailable, the organism cannot use aerobic respiration to generate energy. Instead, it relies on a less efficient pathway to produce the necessary energy molecule, adenosine triphosphate ($\text{ATP}$).
This survival strategy is centered on keeping the initial energy-extraction step, called glycolysis, running continuously. Glycolysis breaks down glucose and produces a minimal amount of $\text{ATP}$ to sustain the cell. A necessary component for this initial step is the molecule nicotinamide adenine dinucleotide ($\text{NAD}^+$), which acts as an electron acceptor.
During glycolysis, $\text{NAD}^+$ is consumed and converted into its reduced form, $\text{NADH}$. If $\text{NADH}$ is not converted back into $\text{NAD}^+$, the energy-producing process will quickly stop. Since oxygen is not present to act as the final electron acceptor, the yeast must find an internal, organic molecule to regenerate the $\text{NAD}^+$ supply, which is the purpose of fermentation.
The Molecular Steps of Alcohol Production
The process of converting sugar into ethanol begins with glycolysis, where a six-carbon glucose molecule is broken down within the yeast cell’s cytoplasm. Glucose is systematically split into two molecules of a three-carbon compound called pyruvate. This initial breakdown yields a net gain of two $\text{ATP}$ molecules for the yeast.
Glycolysis also involves the reduction of two $\text{NAD}^+$ molecules to two $\text{NADH}$ molecules, which must be recycled. The pyruvate molecules then enter the second stage of alcoholic fermentation, which is the first step dedicated to regenerating the $\text{NAD}^+$ supply. The enzyme pyruvate decarboxylase removes a carboxyl group from each pyruvate molecule.
This action releases a molecule of carbon dioxide ($\text{CO}_2$) gas from each pyruvate, leaving behind a two-carbon compound known as acetaldehyde. The release of $\text{CO}_2$ causes bread dough to rise and beer to foam. The final step involves the acetaldehyde molecule serving as the electron acceptor the yeast needs.
The acetaldehyde accepts electrons from the $\text{NADH}$ molecules, a reaction catalyzed by the enzyme alcohol dehydrogenase. This transfer of electrons accomplishes two things simultaneously: it reduces the acetaldehyde to form the final product, ethanol, and it oxidizes the $\text{NADH}$ back into $\text{NAD}^+$. The regenerated $\text{NAD}^+$ is now available to re-enter the glycolysis pathway, allowing the cycle of energy extraction and fermentation to continue.
The Final Products: Ethanol and Carbon Dioxide
The two molecules produced in the final stages of fermentation are ethanol and carbon dioxide. Ethanol ($\text{C}_2\text{H}_5\text{OH}$) is a simple alcohol that acts as a central nervous system depressant, making it the intoxicating component in alcoholic beverages. It is also used industrially as an effective solvent, antiseptic, and fuel source.
The accumulation of ethanol in the surrounding environment can eventually become toxic to the yeast itself. Fermentation typically ceases when the alcohol concentration reaches between 14\% and 18\% by volume, which is why most naturally fermented wines do not exceed this range.
The other major byproduct, carbon dioxide ($\text{CO}_2$), is a non-toxic gas. In brewing, $\text{CO}_2$ is responsible for the carbonation that gives beer its effervescence. In baking, the gas becomes trapped within the elastic structure of dough, causing it to inflate or “leaven” during proofing.