Fermentation is an anaerobic process (occurs in the absence of oxygen) that breaks down the glucose within organic materials. It is a series of chemical reactions that convert sugars to ethanol.

The basic fermentation process involves the conversion of a plant’s glucose (or carbohydrate) into an alcohol or acid. Yeast or bacteria are added to the biomass material, which feed on the sugars to produce ethanol (an alcohol) and carbon dioxide. The ethanol is distilled and dehydrated to obtain a higher concentration of alcohol to achieve the required purity for the use as automotive fuel. The solid residue from the fermentation process can be used as cattle-feed and in the case of sugar cane; the bagasse can be used as a fuel for boilers or for subsequent gasification.

The most common forms of biomass that are used in the production of bio-ethanol are high in sugar and include sugarcane, corn and sweet potatoes. Other forms of biomass that are used in fermentation processes are starchy materials such as wheat, barley, oat and rice along with lignocellulosic materials such as agricultural wastes and woody materials.

Lignocellulosic biomass is slightly different because, along with consisting of cellulose and hemicellulose that can be converted to sugars, it also contains a non-fermentable fraction called lignin. Lignin, however, is high in energy content and can be utilized for the production of electricity and/or heat. Although the decomposition of the material into fermentable sugars is more complicated, the fermentation, distillation and dehydration process steps are basically identical for bio-ethanol from either agricultural crops or lignocellulosic biomass.

Additional Resources:

Fermentation BIC Factsheet
Fermentation of Lignocellulosic Biomass