How Is Ethanol Made?

How is a bushel of corn transformed into ethanol? It is a long process that spans many days and includes the following steps:

1. Grain Delivery
Corn is delivered to the production plant by truck or by rail.

2. Grain Handling
The corn is loaded into storage silos built to hold enough corn to supply the plant for ten days.

3. Milling
The corn is processed in a hammer mill which grinds it into a flour, or powder, called meal that exposes the cornstarch, which is central to the fermentation process.

4. Slurry Tanks
The corn meal is mixed with water and the primary enzyme (alpha-amylase), an enzyme that helps break down the starch, creating a slurry, or mash.

5. Liquefaction
The corn mash is held in liquefaction tanks and heat is applied to enable the enzymes time to break down the starch into fermentable sugars and reduce bacteria levels.

6. Saccharification
The mash is removed from the tanks and cooled and a secondary enzyme (gluco-amylase), an enzyme that helps break down the mash by converting starch into fermentable sugars (dextrose).

7. Fermentation
The corn mash is transferred to fermentation tanks and is mixed with yeast and allowed to ferment for 40 to 50 hours, which changes the sugar to ethyl alcohol and carbon dioxide. The fermented mash contains about ten percent alcohol. The rest of the mixture is water and corn/yeast solids that couldn't be fermented.

8. Distillation
To separate the ethyl alcohol from the fermented mash, the mixture is heated once again, this time to a temperature at which the alcohol vaporizes. The alcohol, or ethanol, vapor is collected and cooled, where it condenses to its liquid form, which is about 190 proof. The water and corn/yeast solids that can't be fermented (distiller's grains) are removed.

9. Dehydration
At this point the ethanol contains about 5% water. To purify the ethanol and remove the remaining water, it's passed through a dehydration system of molecular membranes. This results in anhydrous ethanol (anhydrous means "without water"), which is about 200 proof.

10. Ethanol Holding and Denaturing
Ethanol that is used for fuel must be "denatured" to make it unfit for human consumption. This is achieved by adding a small amount (2 - 5 %) of gasoline.