Corn stover is the residue of the corn plant after harvesting which usually consists of the following fractions: leaves, cobs and stock. Corn stover is a good lignocellulosic biomass resource for biofuel (cellulosic ethanol) production. However, each fraction (leaves, corn, stalk) are considered to have different compositions of the following components: cellulose, hemicellulose and lignin. Cellulose and hemicellulose are the polymers which are broken down (i.e., "hydrolyzed" chemically or enzymatically) into simple (ethanol fermentable) sugars, while lignin is the tight wrapping around the cellulose and hemicellulose molecules, which must be removed by thermo-chemical treatment. The relative amounts of these three components affect the intensity of pretreatment conditions (for example, the temperature and dosage of chemicals needed to achieve lignin breakage and the conversion of cellulose/hemicellulose into simple sugars). Furthermore, the composition of the fractions may also vary with the period at which they were harvested (early or late harvest). Scientists from Michigan State University's Biomass Conversion Laboaratory (Department of Chemical Engineering and Materials Science) (United States) attempted to determine whether individual corn stover fractions (both early and late harvests) have different optimal pretreatment conditions. They used ammonia fiber explosion (AFEX), followed by enzymatic hydrolysis as the pretreatment method. Their results showed that the optimal AFEX pretreatment conditions for all corn stover fractions were generally the same, regardless of the harvest period. Details of the study can be accessed in the open access journal, Biotechnology for Biofuels (URL above)..