Ethanol can be made from any crop or plant that contains a large amount of sugar or components that can be converted into sugar, such as starch or cellulose.

Starch vs Cellulose

Sugar beets and sugar cane can be have their sugars extracted and processed. Crops such as corn, wheat and barley contain starch that can be easily converted to sugar, then made into ethanol. Most of the US production of ethanol is from starch, and almost all of the starch-based ethanol is made from corn grown in Midwest states. 

Trees and grasses have much of their sugars locked up in a fibrous material called cellulose, which can be broken down into sugars and made into ethanol. By-products of forestry operations can be used for cellulosic ethanol: sawdust, wood chips, branches. Crop residues can also be used, such as corn cobs, corn leaves, or rice stems. Some crops can be grown specifically to make cellulosic ethanol, most notably switchgrass. Sources of cellulosic ethanol are not edible, which means that the production of ethanol does not come into direct competition with the use of crops for food or livestock feed.

The Milling Process

Most ethanol is produced using a four-step process:

.          The ethanol feedstock (crops or plants) are ground up for easier processing;

.          Sugar is dissolved from the ground material, or the starch or cellulose is converted into sugar. This is done through a cooking process.

.          Microbes such as yeast or bacteria feed on the sugar, producing ethanol in a process called fermentation, essentially the same way beer and wine are made. Carbon dioxide is a byproduct of this fermentation;

.          The ethanol is distilled to achieve a high concentration. Gasoline or another additive is added so it cannot be consumed by humans - a process called denaturation. This way, the ethanol also avoids a tax on beverage alcohol.

The spent corn is a waste product called distiller's grain. Fortunately it is valuable as feed for livestock such as cattle, hogs, and poultry.

It is also possible to produce ethanol through a wet-milling process, which is used by many large producers. This process involves a steeping period after which the grain germ, oil, starch, and gluten are all separated and further processed into many useful byproducts. High-fructose corn syrup is one of them and is used as a sweetener in many prepared foods. Corn oil is refined and sold. Gluten is also extracted during the wet milling process and is sold as a feed additive for cattle, hogs, and poultry.