The United States uses corn-derived ethanol as a renewable fuel to augment the gasoline supply. This bioethanol is ethyl alcohol produced from the starch found in field corn, not the sweet corn people typically eat. It is primarily blended with gasoline, most commonly as E10 (10% ethanol and 90% gasoline), to meet federal requirements for cleaner-burning fuel. Corn ethanol is important to the agricultural economy and the nation’s energy portfolio, making the efficiency of its production a topic of interest.
The Quantitative Answer
On average, one bushel of field corn yields approximately 2.8 to 2.9 gallons of denatured fuel ethanol. This figure represents the industry standard for modern dry-mill ethanol plants, which account for the vast majority of U.S. ethanol production.
A bushel of corn is standardized to weigh 56 pounds. Using the industry average, a single gallon of ethanol requires about 19.3 to 20 pounds of corn. This high conversion rate is due to the fact that corn kernels are about two-thirds starch, which is the component converted into alcohol.
The Conversion Process
The conversion of corn starch into ethanol is a multi-step biochemical process primarily occurring through the dry milling method. The process begins with milling, where the entire corn kernel is ground into a fine powder or meal. This maximizes the starch’s surface area, preparing it for subsequent chemical reactions.
The corn meal is mixed with water to create a slurry or mash, and heat-stable enzymes are introduced. This mixture is cooked at high temperatures in a process called liquefaction, which gelatinizes the starch and initiates its breakdown into shorter chains.
A second enzyme, glucoamylase, is added during saccharification, completing the breakdown of starch chains into simple glucose sugars.
The mash is then cooled and transferred to fermentation tanks where yeast is added. The yeast metabolizes the glucose, converting the sugar into ethanol and carbon dioxide over 40 to 50 hours. The resulting liquid, containing 12% to 15% ethanol, is called “beer.”
Purification separates the fuel-grade alcohol from water and remaining solids. Distillation heats the mixture to separate the ethanol vapor, resulting in a product that is about 95% pure. Dehydration removes the remaining water to achieve nearly 100% purity. A small amount of denaturant, like gasoline, is then added before the fuel is transported.
Factors Affecting Production Efficiency
The final ethanol yield is not a fixed number and fluctuates based on several factors, primarily the quality of the corn feedstock. Corn with a higher percentage of starch provides more material for conversion into sugar and subsequently, more ethanol. Producers continually seek corn strains developed for high starch content to maximize the output per bushel.
Processing Technology
The type of processing facility also influences efficiency. Dry-mill facilities, which process the entire kernel, are the most common. Advancements in milling and fractionation technologies allow for better separation and utilization of the starch component, leading to incremental yield improvements.
Enzyme Advancements
Ongoing advancements in enzyme technology play a significant role. New, more effective enzymes can break down complex starches into fermentable sugars more quickly and completely. This directly increases the amount of ethanol recovered from the same amount of corn.
Beyond Ethanol: Co-products
The corn ethanol process generates several valuable secondary products from the non-fermentable components of the corn kernel. The most abundant co-product is Dried Distillers Grains with Solubles (DDGS), which consists of the protein, fiber, fat, and minerals left over after the starch is converted to ethanol. DDGS is a nutrient-dense feed ingredient widely used in the livestock industry as a substitute for corn and soybean meal. Approximately 15 to 18 pounds of distillers grains are produced from every bushel of corn processed for ethanol.
The extraction of corn oil is another significant co-product, with about 0.9 pounds recovered per bushel. This corn oil is used in the production of biodiesel and as a high-fat additive in animal feed. Additionally, carbon dioxide, a natural byproduct of the fermentation step, is often captured and sold for use in beverage carbonation, food processing, and dry ice production. The revenue generated from these co-products is important to the economic viability of ethanol plants, effectively lowering the net cost of the corn feedstock.