The creation of alcohol, specifically ethanol (C2H5OH), is a process of applied biochemistry that converts a sugar compound into a different chemical compound. This transformation is driven by microorganisms and is harnessed for industrial and consumable purposes. The overall goal is a controlled chemical conversion, moving from complex carbohydrates to a purified alcohol solution. This process involves three distinct stages: preparing the starting material, the core conversion reaction, and the final purification step.
Preparing the Carbon Source
The microorganism responsible for alcohol production, yeast, cannot directly consume complex carbohydrates like starch or cellulose found in grains and potatoes. Yeast requires simple sugars, or monosaccharides, such as glucose or fructose, to begin conversion. Therefore, the long chains of starches (polysaccharides) must first be broken down chemically into these smaller, fermentable units.
This preparatory step is called saccharification, meaning “making sugar.” It is a hydrolysis reaction where water is used to cleave the glycosidic linkages that hold the starch molecules together. This breakdown is facilitated by specialized enzymes, primarily amylase, which can be sourced from malted grains or added separately.
The amylase enzyme works most effectively under specific conditions, operating at an optimal temperature around 50°C and a slightly acidic pH of about 5.0. Achieving these conditions ensures that complex starch is efficiently converted into a wort or mash rich in glucose (C6H12O6) and other simple sugars. This sugar-rich liquid becomes the chemical feedstock, primed for the next stage of biological conversion.
The Core Reaction: Alcoholic Fermentation
Once the simple sugars are available, the core chemical reaction of alcoholic fermentation begins, catalyzed by the yeast Saccharomyces cerevisiae. This biological process is an anaerobic pathway, meaning it must occur in the absence of oxygen to produce ethanol efficiently. If oxygen is present, the yeast undergoes aerobic respiration, converting the sugar entirely into water and carbon dioxide for maximum energy yield instead of producing alcohol.
The reaction starts with glycolysis, a metabolic sequence that splits one six-carbon glucose molecule into two molecules of pyruvate. In the absence of oxygen, the yeast converts each pyruvate molecule into acetaldehyde, releasing carbon dioxide (CO2) gas as a byproduct. The release of this gas causes bubbling during the reaction.
Finally, acetaldehyde is reduced to ethanol (C2H5OH) in a step that regenerates the molecules needed to continue glycolysis. The overall simplified chemical equation for the conversion of glucose is C6H12O6 \(\rightarrow\) 2C2H5OH + 2CO2. This sequence extracts a small amount of energy for the yeast while producing the two primary end products: ethanol and carbon dioxide.
Controlling the Chemistry
To maximize the yield and quality of the finished product, the chemistry of the fermenting mixture must be carefully managed. Temperature is a significant factor because yeast enzymes have a narrow operating range. Optimal fermentation temperatures fall between 20°C and 30°C, depending on the yeast strain.
Temperatures outside this range can slow the reaction or cause stress, leading to the production of undesirable secondary compounds. If the temperature becomes too high, the yeast’s protein structure can denature, causing a rapid and irreversible cessation of the reaction. Similarly, the acidity (pH) of the mixture is maintained in a slightly acidic range, often between pH 4.0 and 5.5, which favors yeast activity and inhibits the growth of competing bacteria.
The reaction is self-limiting due to the accumulation of its primary product. As the concentration of ethanol increases, it becomes toxic to the yeast cells by disrupting their cell membranes. Most strains of Saccharomyces cerevisiae can tolerate an alcohol concentration up to about 10% to 15% by volume before toxicity stops their activity and concludes fermentation.
Increasing Purity Through Distillation
Following fermentation, the liquid (wash or mash) is a dilute mixture of water, ethanol, and various byproducts. To achieve a higher concentration of alcohol, distillation is used, relying on the differing boiling points of the components. Water boils at 100°C (212°F), while ethanol boils at a lower temperature of 78.3°C (173.1°F).
When the mixture is heated, the more volatile ethanol vaporizes preferentially and travels up the still column. It is then cooled and condensed back into a liquid with a higher alcohol content. This process also separates other volatile compounds based on their boiling points, such as highly toxic methanol (boiling point 64.7°C) and heavier, oily compounds known as fusel alcohols (higher boiling points than ethanol).
Simple distillation cannot achieve an ethanol concentration higher than approximately 95.6% by weight. At this point, ethanol and water form an azeotrope, a mixture that boils at a constant temperature and composition, making further separation by conventional boiling impossible. Achieving 100% pure, or anhydrous, ethanol requires specialized techniques like azeotropic or extractive distillation, which introduce a third chemical agent to break the constant-boiling-point relationship.