How Ethanol is Made

Dry mill production of ethanol is composed of eight steps.

1. Milling The feedstock (usually corn, but can be a variety of other materials, including milo, wheat, sugar cane, beets, etc.) is passed through a hammer mill, which pulverizes it into fine particles, called meal.

2. Liquefaction The meal is mixed with water and an enzyme called alpha-amylase, which helps break down the meal further, into individual molecules. It is then passed through cook tanks, where the starch molecules from the meal are liquefied from the actions of the enzyme and from heat. The cook temperature usually ranges from 250° F to 300° F. The liquefied meal is now called "mash", and is held at around 200° F, which reduces bacterial buildup while the mash is in holding.

3. Saccharification The mash from the cook tanks is then cooled, and a second enzyme called gluco-amylase is added. This enzyme breaks down the starches in the mash into simpler molecules of sugars. The type of sugar created from this process is called dextrose.

4. Fermentations Yeast is now added to the mash to ferment the sugars. Fermentation breaks down the sugar molecules into ethanol, a liquid, and carbon dioxide, a gas. When the fermentation process is complete, the "mash" is now referred to as "beer." It is stored in a beer well before transfer to the next stage.

5. Distillation The beer is about 10% alcohol by volume, and is not completely liquid. It also contains all the solids from the original feedstock (corn, milo, etc.) and from the added yeast. It is pumped from the beer well into a multi-column distillation system, which removes the alcohol from the beer by distillation. Basically, distillation utilizes the differences in the evaporating points of ethanol and water. This causes the gas to condense back to liquid form, and contains a much higher percentage of ethanol than the original beer. This liquid condensate is then passed to the next distillation column in the series, where the process is repeated. By the time the product reaches the final distillation column, it is 96% ethanol, or 190 proof. The residue from distillation, called stillage, is pumped from the bottom of these distillation columns to the co-product processing area. (See 8, below.)

6. Dehydration The 190 proof ethanol is then passed through a molecular sieve, which removes remaining water that was not eliminated in distillation. Following dehydration, the ethanol is 200 proof and is referred to as anhydrous ethanol, which means "ethanol without water".

7. Denaturation The Bureau of Alcohol, Tobacco, and Firearms (The ATF), requires any alcohol used for fuel be denatured, or un-drinkable. To render the ethanol unfit for human consumption, 2-5% gasoline is added to the ethanol.

8. Co-Product Processing There are two main co-products created during the ethanol production process. The first is carbon dioxide. During fermentation, as the yeast "eat" the sugar, they not only create ethanol, but they also release large amounts of carbon dioxide gas. This gas can be captured, purified, compressed, and sold. It is used primarily in the food processing industry for carbonated beverages, and also for use in the flash-freezing of meat.

The stillage (the solids that remain after distillation) is sent through a centrifuge to remove excess liquid. This works much like the spin cycle of your washing machine. The liquid that is separated out is sent back into the process, and the remaining solids are referred to as "distillers grains". Wet distillers grains (WDG) are transferred directly from the centrifuge to a wet cake pad, where they are transferred primarily to local feedlots and dairies for use as a cattle ration. While the shelf life of this product is rather limited, several companies are marketing preservatives proven to significantly extend the product.