Yeast produces alcohol through a metabolic process fundamental to human food production for millennia. Yeast is a single-celled fungus whose ability to convert sugars into other compounds forms the basis of several ancient and modern industries. Alcohol production is a survival strategy yeast employs when its environment changes. This biological action forms the foundation of all alcoholic beverages and influences the texture of baked goods.
The Process of Fermentation
Yeast generates alcohol through its energy-generation mechanism when oxygen is scarce. This process, known as anaerobic respiration, allows the yeast to continue metabolizing sugars when efficient oxygen-based respiration is not possible. The starting material is a simple sugar, such as glucose, which the yeast consumes for growth and reproduction.
During this metabolic pathway, yeast breaks down glucose into two molecules of pyruvate. In the absence of oxygen, the yeast converts pyruvate into two final products: ethanol (alcohol) and carbon dioxide (CO2). This sequence is summarized by the chemical reaction: Sugar $\rightarrow$ Ethanol + Carbon Dioxide. Producing ethanol allows the yeast to regenerate a molecule necessary to keep its initial sugar-breaking pathway running.
Essential Role in Food and Drink Production
The byproducts of yeast’s sugar metabolism, ethanol and carbon dioxide, are harnessed for two distinct applications. In alcoholic beverages like beer and wine, the primary goal is the accumulation of ethanol. The species Saccharomyces cerevisiae, often called brewer’s yeast, consumes sugars in grape juice or grain wort to produce the desired alcohol content.
Conversely, in baking, carbon dioxide is the most sought-after product. When Saccharomyces cerevisiae (baker’s yeast) is mixed with flour and water, the gas becomes trapped in the dough. This gas creates small bubbles that cause the dough to expand and rise, a process called leavening. Although ethanol is produced during bread-making, it evaporates almost completely when the dough is exposed to the heat of the oven.
Why Alcohol Production Eventually Stops
The yeast’s own product, ethanol, creates a self-limiting environment that halts fermentation. As the alcohol concentration increases, ethanol acts as a toxic agent to the yeast cells. This toxicity damages the yeast’s cell membranes, disrupting their function and eventually killing the colony.
The specific alcohol concentration that causes this cessation varies depending on the yeast strain, but it falls within a range of 12% to 18% alcohol by volume. Beyond this point, the environment is too harsh for the yeast to survive and continue metabolic activity. Depletion of the sugar source is a secondary factor that stops production, starving the yeast of the fuel needed to generate energy.