The following processes for converting sugar and starch crops are used commercially today.

Size reduction
Biomass such as agricultural residues go through a grinding process and wood goes through a chipping process to achieve a uniform particle size to make the ethanol production process more efficient.

Pretreatment
In this step, a chemical reaction called hydrolysis occurs when dilute sulfuric acid is mixed with the biomass feedstock in which the complex chains of sugars that make up the hemicellulose are broken, releasing simple sugars - a mixture of soluble five-carbon sugars, xylose and arabinose, and soluble six-carbon sugars, mannose and galactose. A small portion of the cellulose is also converted to glucose in this step.

Enzyme Production
The cellulase enzymes that are used to hydrolyze the cellulose fraction of the biomass are grown in this stage. Alternatively the enzymes might be purchased from commercial enzyme companies.

Cellulose Hydrolysis
The remaining cellulose is hydrolyzed to glucose and cellulase enzymes are used to break the chains of sugars that make up the cellulose, releasing glucose. Cellulose hydrolysis is also called cellulose saccharification because it produces sugars.

Fermentation of Glucose
Fermentation is a series of chemical reactions that convert sugars to ethanol which is caused by yeast or bacteria, which feed on the sugars producing ethanol and carbon dioxide.

Fermentation of Pentose
Xylose, which is the most prevalent pentose released by the hemicellulose hydrolysis reaction, is fermented in this step using Zymomonas mobilis or other genetically engineered bacteria.

Recovery of Ethanol
In this step the ethanol is separated from the other components in the broth - The fermentation product from the glucose and pentose fermentation is. The remaining water, if any, is removed from the ethanol by a final dehydration step.

Utilization of Lignin
Lignin and other byproducts of the biomass-to-ethanol process can be burnt to produce the electricity required for the ethanol production process. Burning lignin actually creates more energy than needed and selling electricity may help reduce the cost of production of ethanol.

Converting cellulosic biomass to ethanol is currently too expensive to be used on a commercial scale. So researchers are working to improve the efficiency and economics of the ethanol production process by focusing their efforts on the two most challenging steps:

Cellulose hydrolysis
Researchers are developing enzymes that work together to efficiently break down the crystalline structure of cellulose to help with the hydrolysis of cellulose to simple sugars.

Pentose fermentation
While it is difficult to ferment five-carbon sugars and thus limits ethanol production from cellulosic biomass, Researchers are using genetic engineering to design microorganisms that can efficiently ferment both five- and six-carbon sugars to ethanol at the same time. In response to this challenge, Biomass Program researchers metabolically engineered the bacterium Zymomonas mobilis to add ability to ferment xylose and arabinose to its natural ability to ferment glucose.