Yeast are single-celled microorganisms found globally in diverse environments. These organisms have played a significant role in human activities for thousands of years, contributing to various food and beverage productions.
The Gas and Its Production
The gas yeast produces during its metabolic processes is carbon dioxide (CO2), a byproduct of alcoholic fermentation. During this process, yeast cells break down sugars, such as glucose, fructose, and sucrose, in the absence of oxygen. Yeast converts these sugars into ethanol (alcohol) and carbon dioxide. This chemical transformation yields energy for the yeast. The process is anaerobic, meaning it takes place without the presence of air or oxygen.
Yeast is a facultative anaerobe, meaning it can adapt its metabolism to either use oxygen (aerobic respiration) or function without it (anaerobic respiration or fermentation). When oxygen is present, yeast produces carbon dioxide and water. However, when oxygen is limited or absent, yeast switches to fermentation, producing carbon dioxide and ethanol.
Why Yeast Releases Gas
Yeast produces carbon dioxide to generate energy for its survival and growth. In environments where oxygen is scarce, such as inside bread dough or a sealed fermentation tank, yeast relies on fermentation to produce adenosine triphosphate (ATP), which is the primary energy currency of cells. While carbon dioxide is expelled as a waste product, its formation is intrinsically linked to the energy-generating reactions.
This metabolic pathway allows yeast to continue functioning and multiplying even in oxygen-deprived conditions. Although anaerobic respiration yields less energy per sugar molecule compared to aerobic respiration, it enables the yeast to rapidly process sugars, which can be advantageous for its growth rate under certain circumstances.
Everyday Uses of Yeast’s Gas Production
The carbon dioxide produced by yeast has several practical applications in human society, most notably in the food and beverage industries. In baking, the gas is responsible for making bread and other dough-based products rise. As yeast ferments sugars in dough, the carbon dioxide gas becomes trapped within the elastic network of gluten proteins, causing the dough to expand and create a light, airy texture. During baking, the heat further expands these gas bubbles, contributing to the final structure of the bread, while any alcohol produced largely evaporates.
In brewing, the carbon dioxide generated by yeast plays a significant role in carbonating beverages like beer and sparkling wine. During the fermentation of wort (the liquid extracted from malted grains), yeast converts sugars into alcohol and CO2. While the alcohol is the primary desired product, the naturally produced carbon dioxide gives the beverages their characteristic fizz, contributes to mouthfeel, and influences their aroma. Brewers can either allow this CO2 to naturally carbonate the beverage or add additional CO2 later to achieve a desired level of carbonation.