Yeast, most commonly Saccharomyces cerevisiae, is a single-celled fungus used for thousands of years in food and beverage production. This microorganism sustains itself through fermentation, extracting energy from sugars when oxygen is absent. When yeast cells encounter a sugary environment, they convert carbohydrates into two primary byproducts, one of which is a gas that transforms the materials. This metabolic process is the foundation of many products we consume daily.
Identifying the Primary Gas
The main gaseous byproduct released by yeast during its metabolic activity is carbon dioxide. This gas is the source of the visible activity seen in fermentation. The rapid release of carbon dioxide creates the foam and bubbles that appear at the surface of a liquid medium or the frothing observed when yeast is mixed with sugar and water. In a closed system, the accumulating gas can create significant pressure, demonstrating the volume of carbon dioxide produced by these microscopic organisms.
The Chemical Pathway of Production
The production of carbon dioxide is part of the yeast cell’s energy generation through alcoholic fermentation. This metabolic pathway begins with glycolysis, where a six-carbon sugar, such as glucose, is broken down into two three-carbon molecules of pyruvate. Since sufficient oxygen is absent, the yeast cannot proceed with aerobic respiration. It must find an alternative way to regenerate necessary cofactors to keep glycolysis running.
The gas is released when each pyruvate molecule undergoes decarboxylation. During this reaction, an enzyme removes a carboxyl group from the pyruvate, resulting in the release of a single carbon dioxide molecule. This leaves behind a two-carbon compound called acetaldehyde.
Finally, the acetaldehyde is converted into ethanol, a two-carbon alcohol, completing the fermentation process. The overall reaction yields two molecules of carbon dioxide and two molecules of ethanol for every molecule of glucose consumed. This process successfully regenerates the cofactors the yeast needs to continue generating energy from sugar, and the carbon dioxide is expelled as a waste product.
Practical Applications of Gas Production
The continuous expulsion of carbon dioxide gas is harnessed across several industries, providing effects central to the final quality of the products. In baking, the gas is responsible for leavening, causing the dough to rise. As the yeast ferments the sugars in the flour, the carbon dioxide is trapped within the elastic gluten network of the dough.
These microscopic gas bubbles expand, causing the dough to swell and increase in volume, creating the light, airy texture of baked goods like bread. During baking, the heat causes the trapped gas to expand further, setting the loaf’s structure, while the alcohol produced evaporates completely. The amount of carbon dioxide produced correlates with the final volume and crumb structure of the bread.
In the production of beer and sparkling wine, the gas serves both practical and aesthetic purposes. The carbon dioxide produced during fermentation is either allowed to escape and then reintroduced later, or it is captured directly to create carbonation. This dissolved gas gives these beverages their effervescence and mouthfeel.
Beyond carbonation, the release of carbon dioxide bubbles during initial fermentation serves a practical function by helping to push unwanted oxygen out of the liquid. This natural purging action ensures the anaerobic conditions yeast requires to produce ethanol. The gas is an active component in shaping the final sensory profile and stability of various fermented drinks.