Bioethanol is an alcohol produced through the fermentation of biomass, primarily serving as a renewable fuel additive to gasoline. The United States leads global production, relying overwhelmingly on corn kernels as the feedstock due to their high starch content. Starch is a complex carbohydrate that converts into fermentable sugars. The industrial process is a controlled sequence of biological and chemical steps designed to break down the starch and transform the resulting sugar into fuel. This process involves mechanical preparation, enzymatic conversion, biological fermentation, and intensive purification to yield fuel-grade ethanol.
Preparing the Feedstock and Converting Starch to Sugar
The process begins with the physical preparation of the raw corn, typically using a method called dry-milling. Whole corn kernels are first ground into a fine powder, or meal, using a hammer mill to maximize the surface area for subsequent chemical reactions. This corn meal is then mixed with water and recycled process liquid, known as stillage, to create a slurry or mash.
The next step is liquefaction, which involves cooking the mash and adding the first enzyme. The mixture is heated to high temperatures (90 to 165 degrees Celsius) to gelatinize the starch, causing the granules to swell. An enzyme called alpha-amylase is then introduced to the hot mash. Alpha-amylase cleaves the long starch molecules into shorter chains called dextrins.
The mash is cooled to a temperature suitable for the second enzymatic reaction, typically around 30 degrees Celsius. This is the saccharification phase, where glucoamylase is added. Glucoamylase continues the breakdown, targeting the dextrins to release simple, fermentable sugars, primarily glucose. This sugar-rich liquid is now ready for the biological stage.
The Fermentation Phase
The prepared mash, now containing a high concentration of glucose, is transferred into large fermentation tanks. This is where the core biological conversion occurs, driven by a specific type of yeast, most commonly Saccharomyces cerevisiae. The yeast is introduced into the mash to begin consuming the glucose in a controlled, oxygen-free environment.
The yeast metabolizes the simple sugars through anaerobic respiration. The chemical reaction converts glucose into ethanol and carbon dioxide. This transformation provides the yeast with energy while generating the desired alcohol product.
Fermentation is typically a batch process taking 40 to 72 hours. Temperature control is monitored, maintaining the optimal range for the yeast between 30 and 32 degrees Celsius. Carbon dioxide gas is continuously released, and the resulting liquid mixture, known as “beer,” contains approximately 10 to 15 percent ethanol by weight.
Separating and Purifying the Ethanol
Once fermentation is complete, the “beer mash” undergoes separation to isolate the ethanol from water and remaining solids. The first purification step is distillation, which uses the difference in boiling points between ethanol and water. The mash is fed into distillation columns and heated, causing the ethanol to vaporize first.
The ethanol vapor is collected and condensed back into a liquid, separating it from non-fermentable components and most of the water. Conventional distillation can only concentrate the ethanol to a maximum purity of about 95 percent by volume. This limitation is due to the formation of an azeotrope, preventing further separation by simple boiling.
To achieve the 99.5 percent purity required for fuel-grade ethanol, a final step called dehydration is necessary. This is primarily accomplished using molecular sieves, which are beds of synthetic zeolite material with precisely controlled pore sizes. The small water molecules are adsorbed and trapped within the pores of the sieve material, while the larger ethanol molecules pass through unimpeded. This pressure-swing adsorption process effectively removes the final residual water, yielding anhydrous, or water-free, ethanol.
Valuable Co-Products of Ethanol Production
The industrial process utilizes the entire corn kernel, leading to several important co-products. After distillation, the remaining material is whole stillage, containing non-fermentable solids, spent yeast, and water. This stillage is processed to separate the liquid and solid components.
The separated solids are combined with concentrated liquids recovered from the stillage and dried, resulting in a high-value animal feed known as Distillers Dried Grains with Solubles (DDGS). Since the starch is removed, remaining nutrients like proteins, fibers, and fats are concentrated approximately three times compared to the original corn. DDGS is a nutrient-dense ingredient used for cattle, poultry, and swine.
The carbon dioxide released during fermentation is often captured and repurposed. This concentrated CO2 stream can be purified and sold for use in various industries, including carbonating beverages or food freezing. Extracting corn oil from the stillage is also a common practice, contributing to the overall efficiency of the dry-mill plant.