Fermentation is an anaerobic metabolic process that organisms use to convert sugars into energy in the absence of oxygen. This biochemical pathway allows microorganisms to generate adenosine triphosphate (ATP) by breaking down glucose. The process results in various end products, and for many forms of fermentation, carbon dioxide (\(text{CO}_2\)) is produced.
How Carbon Dioxide is Generated
The formation of carbon dioxide (\(text{CO}_2\)) is an intrinsic part of the chemical process known as alcoholic fermentation. This process begins with glycolysis, where a six-carbon glucose molecule is split into two three-carbon molecules of pyruvate. Pyruvate is then directed through a subsequent two-step reaction because oxygen is not available to complete the breakdown.
The first step of this anaerobic conversion involves the enzyme pyruvate decarboxylase, which acts on the pyruvate. This enzyme removes a carboxyl group from the three-carbon pyruvate structure, releasing a molecule of \(text{CO}_2\) as a byproduct. The remaining two-carbon compound is acetaldehyde. Microorganisms, such as yeast, produce two molecules of \(text{CO}_2\) for every molecule of glucose processed.
Different Pathways: When \(text{CO}_2\) is Not Produced
Carbon dioxide production is not universal across all forms of fermentation, as the pathway depends on the organism and the available enzymes. The major distinction lies between alcoholic fermentation and lactic acid fermentation, which uses a different chemical route. Lactic acid fermentation is carried out by certain bacteria, and also occurs in human muscle cells during intense exercise.
In lactic acid fermentation, the three-carbon pyruvate molecule is converted directly into lactic acid, or lactate. This conversion is handled by the enzyme lactate dehydrogenase. Because no carbon atom is removed from the pyruvate structure during this reaction, no carbon dioxide is released as a byproduct. This explains why products created through lactic acid fermentation, such as yogurt and sauerkraut, do not generate gas for leavening or carbonation.
Practical Applications of the Gas
The carbon dioxide gas produced during fermentation has several practical applications in food science. One recognizable use is leavening, the process of causing dough to rise. As yeast ferments sugars in the dough, the \(text{CO}_2\) gas is released and becomes trapped within the elastic network of proteins, primarily gluten. This trapped gas expands, forming tiny pockets, causing the dough to increase in volume and develop a light, airy texture after baking.
The gas is also responsible for the effervescence, or fizz, in many fermented beverages. When alcoholic fermentation occurs in a sealed environment, such as a bottle or pressurized tank, the \(text{CO}_2\) cannot escape and dissolves into the liquid. This process creates the characteristic bubbles and sharp mouthfeel associated with carbonated drinks. Brewers and winemakers control fermentation conditions, including temperature and pressure, to achieve the desired level of carbonation in products like beer, champagne, and kombucha.