Alcoholic fermentation is a metabolic pathway that allows certain microorganisms to generate cellular energy from sugar when oxygen is lacking. Simple sugars, such as glucose, are broken down to produce a minimal amount of energy, yielding ethanol and carbon dioxide as waste products. The organism’s fundamental purpose is to sustain its energy-producing mechanism, not to create alcohol. This chemical transformation is the basis for countless food and beverage products consumed globally.
The Chemical Transformation
The process begins with glycolysis, a sequence of reactions that occurs in the cell’s cytoplasm, where a six-carbon glucose molecule is split into two three-carbon molecules of pyruvate. Glycolysis generates a small net amount of adenosine triphosphate (ATP), the cell’s energy currency, but it also produces a compound called NADH. In anaerobic conditions, the cell must recycle the NADH back into its oxidized form, NAD+, since oxygen is not available to process it for more energy.
To regenerate the necessary NAD+ so glycolysis can continue, the three-carbon pyruvate molecule is first converted into acetaldehyde. This conversion removes a single carbon atom, which is released as carbon dioxide gas, and is catalyzed by the enzyme pyruvate decarboxylase.
Next, acetaldehyde acts as the final electron acceptor, receiving hydrogen atoms and electrons from NADH. This chemical reduction transforms acetaldehyde into ethanol, oxidizing the spent NADH back into NAD+. The regenerated NAD+ then cycles back to the start of glycolysis, allowing the organism to continue producing energy.
Yeast The Essential Microbe
The primary organism responsible for alcoholic fermentation is Saccharomyces cerevisiae, a single-celled fungus classified as a facultative anaerobe. This means it can survive with or without oxygen. In the presence of oxygen, this yeast performs aerobic respiration, a much more efficient process for generating energy. When oxygen is depleted, it switches its metabolism to the less efficient fermentation pathway to survive.
Controlling the environment optimizes the yeast’s activity and the resulting product. Temperature plays a strong role, with different strains having distinct preferences. For example, Saccharomyces strains used for brewing ales prefer warmer temperatures (15 to 25 degrees Celsius), which encourages the production of flavor compounds called esters. Conversely, strains like Saccharomyces pastorianus used for lagers operate better at cooler temperatures (around 7 to 12 degrees Celsius), resulting in a cleaner taste profile.
The final concentration of ethanol also dictates the survival of the organism, as high levels can be toxic to the yeast itself. Most strains of S. cerevisiae will cease their metabolic activity and die when the alcohol concentration reaches approximately 13 to 15 percent by volume. This natural limit is why fortified beverages require the addition of distilled spirits to achieve higher alcohol content.
Fermentation in Food and Drink Production
The practical application of fermentation relies on selecting the appropriate sugar source, or substrate. Winemaking uses simple sugars naturally present in crushed grapes. Brewing beer involves first malting grains, such as barley, to convert complex starches into fermentable sugars. The distinct flavors and characteristics of these beverages are influenced by the initial substrate and the specific yeast strain used.
Cider and mead use fermentable sugars derived from apples and honey, respectively. Although these applications focus on ethanol production, the simultaneous release of carbon dioxide is also harnessed. In sparkling wines and some beers, the dissolved carbon dioxide creates the characteristic fizziness.
Baking bread uses this reaction, but the primary desired product is carbon dioxide, not ethanol. Baker’s yeast consumes the sugar or starches in the flour and releases carbon dioxide gas. These gas bubbles become trapped within the dough’s gluten structure, causing it to rise and creating the light, airy texture of baked goods. The small amount of ethanol produced is volatile and completely evaporates when the bread is exposed to the heat of the oven.