Yeast, a single-celled microorganism, is primarily known for its role in fermentation. This metabolic process converts sugars into other compounds, a technique humans have used for millennia in food and beverage production. Yeast’s ability to transform simple ingredients underpins its wide application.
The Gas Yeast Produces
During fermentation, yeast primarily produces carbon dioxide (CO2). Carbon dioxide is a colorless, odorless, non-flammable gas, about 1.5 times denser than air. This gas is important for many applications of yeast fermentation.
The carbon dioxide produced by yeast is a primary factor in creating desired textures in baked goods and effervescence in beverages. While other gases like ethanol vapor and nitrogen can contribute, carbon dioxide is the main gas responsible for leavening. It can be trapped within various matrices.
The Fermentation Process
Yeast produces carbon dioxide through a process called alcoholic fermentation, which occurs in the absence of oxygen. Yeast cells metabolize carbohydrates, such as sugars in flour or grape juice. Specifically, a glucose molecule is converted into two molecules of ethanol and two molecules of carbon dioxide. This conversion is how yeast derives energy for its survival and growth.
The process can be simply described as yeast “eating” sugar and “excreting” alcohol and carbon dioxide. While yeast can also perform aerobic respiration in the presence of oxygen, producing carbon dioxide and water, it switches to anaerobic fermentation when oxygen is limited. This metabolic flexibility allows yeast to thrive in diverse environments and is fundamental to its industrial use.
Everyday Uses of Yeast Fermentation
The carbon dioxide produced by yeast fermentation has several practical applications in everyday life, particularly in the food and beverage industries. In baking, especially with bread, the gas is responsible for the dough rising. As yeast ferments sugars in the dough, it generates carbon dioxide bubbles that become trapped within the gluten network, causing the dough to expand and achieve a light, airy texture. The heat during baking further expands these gas bubbles, and the alcohol produced evaporates, contributing to the bread’s final structure and flavor.
In brewing, yeast fermentation is essential for producing alcoholic beverages like beer and sparkling wine. The carbon dioxide created during fermentation contributes to the effervescence, or fizziness, of these drinks. Brewers often control the amount of dissolved carbon dioxide to achieve specific levels of carbonation, which impacts the mouthfeel and overall drinking experience. Sometimes, additional sugar is added before bottling to allow for a secondary fermentation, naturally carbonating the beverage within the sealed container.