Big Chemistry: Fuel Ethanol

If legend is to be believed, three disparate social forces in early 20th-century America – the temperance movement, the rise of car culture, and the Scots-Irish culture of the South …read more

May 21, 2025 - 23:00

If legend is to be believed, three disparate social forces in early 20th-century America – the temperance movement, the rise of car culture, and the Scots-Irish culture of the South – collided with unexpected results. The temperance movement managed to get Prohibition written into the Constitution, which rankled the rebellious spirit of the descendants of the Scots-Irish who settled the South. In response, some of them took to the backwoods with stills and sacks of corn, creating moonshine by the barrel for personal use and profit. And to avoid the consequences of this, they used their mechanical ingenuity to modify their Fords, Chevrolets, and Dodges to provide the speed needed to outrun the law.

Though that story may be somewhat apocryphal, at least one of those threads is still woven into the American story. The moonshiner’s hotrod morphed into NASCAR, one of the nation’s most-watched spectator sports, and informed much of the car culture of the 20th century in general. Unfortunately, that led in part to our current fossil fuel predicament and its attendant environmental consequences, which are now being addressed by replacing at least some of the gasoline we burn with the same “white lightning” those old moonshiners made. The cost-benefit analysis of ethanol as a fuel is open to debate, as is the wisdom of using food for motor fuel, but one thing’s for sure: turning corn into ethanol in industrially useful quantities isn’t easy, and it requires some Big Chemistry to get it done.

Heavy on the Starch

As with fossil fuels, manufacturing ethanol for motor fuel starts with a steady supply of an appropriate feedstock. But unlike the drilling rigs and pump jacks that pull the geochemically modified remains of half-billion-year-old phytoplankton from deep within the Earth, ethanol’s feedstock is almost entirely harvested from the vast swathes of corn that carpet the Midwest US (Other grains and even non-grain plants are used as feedstock in other parts of the world, but we’re going to stick with corn for this discussion. Also, other parts of the world refer to any grain crop as corn, but in this case, corn refers specifically to maize.)

Don’t try to eat it — you’ll break your teeth. Yellow dent corn is harvested when full of starch and hard as a rock. Credit: Marjhan Ramboyong.

The corn used for ethanol production is not the same as the corn-on-the-cob at a summer barbecue or that comes in plastic bags of frozen Niblets. Those products use sweet corn bred specifically to pack extra simple sugars and less starch into their kernels, which is harvested while the corn plant is still alive and the kernels are still tender. Field corn, on the other hand, is bred to produce as much starch as possible, and is left in the field until the stalks are dead and the kernels have converted almost all of their sugar into starch. This leaves the kernels dry and hard as a rock, and often with a dimple in their top face that gives them their other name, dent corn.

Each kernel of corn is a fruit, at least botanically, with all the genetic information needed to create a new corn plant. That’s carried in the germ of the kernel, a relatively small part of the kernel that contains the embryo, a bit of oil, and some enzymes. The bulk of the kernel is taken up by the endosperm, the energy reserve used by the embryo to germinate, and as a food source until photosynthesis kicks in. That energy reserve is mainly composed of starch, which will power the fermentation process to come.

Starch is mainly composed of two different but related polysaccharides, amylose and amylopectin. Both are polymers of the simple six-carbon sugar glucose, but with slightly different arrangements. Amylose is composed of long, straight chains of glucose molecules bound together in what’s called an α-1,4 glycosidic bond, which just means that the hydroxyl group on the first carbon of the first glucose is bound to the hydroxyl on the fourth carbon of the second glucose through an oxygen atom:

Amylose, one of the main polysaccharides in starch. The glucose subunits are connected in long, unbranched chains up to 500 or so residues long. The oxygen atom binding each glucose together comes from a reaction between the OH radicals on the 1 and 4 carbons, with one oxygen and two hydrogens leaving in the form of water.

Amylose chains can be up to about 500 or so glucose subunits long. Amylopectin, on the other hand, has shorter straight chains but also branches formed between the number one and number six carbon, an α-1,6 glycosidic bond. The branches appear about every 25 residues or so, making amylopectin much more tangled and complex than amylose. Amylopectin makes up about 75% of the starch in a kernel.

Slurry Time

Ethanol production begins with harvesting corn using combine harvesters. These massive machines cut down dozens of rows of corn at a time, separating the ears from the stalks and feeding them into a threshing drum, where the kernels are freed from the cob. Winnowing fans and sieves separate the chaff and debris from the kernels, which are stored in a tank onboard the combine until they can be transferred to a grain truck for transport to a grain bin for storage and further drying.

Corn harvest in progress. You’ve got to burn a lot of diesel to make ethanol. Credit: dvande – stock.adobe.com

Once the corn is properly dried, open-top hopper trucks or train cars transport it to the distillery. The first stop is the scale house, where the cargo is weighed and a small sample of grain is taken from deep within the hopper by a remote-controlled vacuum arm. The sample is transported directly to the scale house for a quick quality assessment, mainly based on moisture content but also the physical state of the kernels. Loads that are too wet, too dirty, or have too many fractured kernels are rejected.

Loads that pass QC are dumped through gates at the bottom of the hoppers into a pit that connects to storage silos via a series of augers and conveyors. Most ethanol plants keep a substantial stock of corn, enough to run the plant for several days in case of any supply disruption. Ethanol plants operate mainly in batch mode, with each batch taking several days to complete, so a large stock ensures the efficiency of continuous operation.

The Lakota Green Plains ethanol plant in Iowa. Ethanol plants look a lot like small petroleum refineries and share some of the same equipment. Source: MsEuphonic, CC BY-SA 3.0.

To start a batch of ethanol, corn kernels need to be milled into a fine flour. Corn is fed to a hammer mill, where large steel weights swinging on a flywheel smash the tough pericarp that protects the endosperm and the germ. The starch granules are also smashed to bits, exposing as much surface area as possible. The milled corn is then mixed with clean water to form a slurry, which can be pumped around the plant easily.

The first stop for the slurry is large cooking vats, which use steam to gently heat the mixture and break the starch into smaller chains. The heat also gelatinizes the starch, in a process that’s similar to what happens when a sauce is thickened with a corn starch slurry in the kitchen. The gelatinized starch undergoes liquefaction under heat and mildly acidic conditions, maintained by injecting sulfuric acid or ammonia as needed. These conditions begin hydrolysis of some of the α-1,4 glycosidic bonds, breaking the amylose and amylopectin chains down into shorter fragments called dextrin. An enzyme, α-amylase, is also added at this point to catalyze the α-1,4 bonds to create free glucose monomers. The α-1,6 bonds are cleaved by another enzyme, α-amyloglucosidase.

The Yeast Get Busy

The result of all this chemical and enzymatic action is a glucose-rich mixture ready for fermentation. The slurry is pumped to large reactor vessels where a combination of yeasts is added. Saccharomyces cerevisiae, or brewer’s yeast, is the most common, but other organisms can be used too. The culture is supplemented with ammonia sulfate or urea to provide the nitrogen the growing yeast requires, along with antibiotics to prevent bacterial overgrowth of the culture.

Fermentation occurs at around 30 degrees C over two to three days, while the yeast gorge themselves on the glucose-rich slurry. The glucose is transported into the yeast, where each glucose molecule is enzymatically split into two three-carbon pyruvate molecules. The pyruvates are then broken down into two molecules of acetaldehyde and two of CO₂. The two acetaldehyde molecules then undergo a reduction reaction that creates two ethanol molecules. The yeast benefits from all this work by converting two molecules of ADP into two molecules of ATP, which captures the chemical energy in the glucose molecule into a form that can be used to power its metabolic processes, including making more yeast to take advantage of the bounty of glucose.

Anaerobic fermentation of one mole of glucose yields two moles of ethanol and two moles of CO₂.

After the population of yeast grows to the point where they use up all the glucose, the mix in the reactors, which contains about 12-15% ethanol and is referred to as beer, is pumped into a series of three distillation towers. The beer is carefully heated to the boiling point of ethanol, 78 °C. The ethanol vapors rise through the tower to a condenser, where they change back into the liquid phase and trickle down into collecting trays lining the tower. The liquid distillate is piped to the next two towers, where the same process occurs and the distillate becomes increasingly purer. At the end of the final distillation, the mixture is about 95% pure ethanol, or 190 proof. That’s the limit of purity for fractional distillation, thanks to the tendency of water and ethanol to form an azeotrope, a mixture of two or more liquids that boils at a constant temperature. To drive off the rest of the water, the distillate is pumped into large tanks containing zeolite, a molecular sieve. The zeolite beads have pores large enough to admit water molecules, but too small to admit ethanol. The water partitions into the zeolite, leaving 99% to 100% pure (198 to 200 proof) ethanol behind. The ethanol is mixed with a denaturant, usually 5% gasoline, to make it undrinkable, and pumped into storage tanks to await shipping.

Nothing Goes to Waste

The muck at the bottom of the distillation towers, referred to as whole stillage, still has a lot of valuable material and does not go to waste. The liquid is first pumped into centrifuges to separate the remaining grain solids from the liquid. The solids, called wet distiller’s grain or WDG, go to a rotary dryer, where hot air drives off most of the remaining moisture. The final product is dried distiller’s grain with solubles, or DDGS, a high-protein product used to enrich animal feed. The liquid phase from the centrifuge is called thin stillage, which contains the valuable corn oil from the germ. That’s recovered and sold as an animal feed additive, too.

Ethanol fermentation produces mountains of DDGS, or dried distiller’s grain solubles. This valuable byproduct can account for 20% of an ethanol plant’s income. Source: Inside an Ethanol Plant (YouTube).

The final valuable product that’s recovered is the carbon dioxide. Fermentation produces a lot of CO₂, about 17 pounds per bushel of feedstock. The gas is tapped off the tops of the fermentation vessels by CO₂ scrubbers and run through a series of compressors and coolers, which turn it into liquid carbon dioxide. This is sold off by the tanker-full to chemical companies, food and beverage manufacturers, who use it to carbonate soft drinks, and municipal water treatment plants, where it’s used to balance the pH of wastewater.

There are currently 187 fuel ethanol plants in the United States, most of which are located in the Midwest’s corn belt, for obvious reasons. Together, these plants produced more than 16 billion gallons of ethanol in 2024. Since each bushel of corn yields about 3 gallons of ethanol, that translates to an astonishing 5 billion bushels of corn used for fuel production, or about a third of the total US corn production.