# Pretreatments Applied to Wheat Straw to Obtain Bioethanol
## 2 Structure of Lignocellulosic Biomass  
Lignocellulosic biomass consists of non-fermentable and fermentable sugars, and it contains cellulose (used for hydrolysis processes [27]), hemicellulose, lignin, and silica [28].  
Hemicelluloses, the most abundant polysaccharides in nature, are found in the cell wall of agricultural straws. They are connected by hydrogen bonds to cellulose and to lignin by chemical bonds [29,30].  
Figure 2 shows the role of pretreatments applied to wheat straw [30]. As can be seen, lignin is one of the three major compounds in biomass lignocellulosic, along with cellulose and hemicellulose, being an amorphous polymer three-dimensional structure consisting of methoxylated phenylpropane structures. In the cell walls in plants, lignin fills the spaces between cellulose and hemicellulose, acting as a resin that holds the lignocellulosic matrix together. Lignin is a secondary product of wheat straw. It is degradable in nature and creates an impermeable barrier in the biomass of wheat straw, connecting cellulose and hemicellulose. Due to the presence of lignin in the wall of wheat straw, they have good durability and resistance to microbial attacks.  
Figure 3 shows schematically the process of obtaining bioethanol from pretreated wheat straw, as well as the effect of cellulase on lignocellulosic biomass. Lignocellulosic biomass represents an unlimited and advantageous source of  
Figure 2: The role of pretreatments applied to wheat straw (adapted from [31]).  
lulose and hemicellulose (Figure 4). Ethanol is obtained from lignocellulosic materials through cellulolysis (hydrolysis and fermentation). Cellulose is a linear, crystalline homopolymer that is formed by repeating units of glucose joined together by \(\beta\)-D-glycosidic bonds. The structure is rigid, and therefore a rough pretreatment is necessary to break it. Hemicellulose is made up of short, linear and branched chains of sugars. Unlike cellulose, which is a polymer made only of glucose, hemicellulose is a heteropolymer made of D-xylose, D-glucose, D-galactose, D-mannose, and L-arabinose.  
spaces between cellulose and hemicellulose in the cell wall. It provides rigidity and hydrophobicity to the biomass. Lignin is composed of phenolic compounds and has a highly branched structure, which makes it difficult to degrade. Heat pretreatment is necessary to further crush the straw to make the cellulose and hemicellulose more accessible to the enzymes in the saccharification process. Due to the heat, pressure, and retention time, the shredded straw is broken and the so-called pre-treated \"substrate\" is obtained. In the vapor phase, furfural--a byproduct of thermal reactions--is removed from the reactor to a wet scrubber, where the furfural is absorbed into the water. The heat contained in the steam can be recovered and used in other process units [29].  
The process of breaking down carbohydrates into simple sugars, most of which can later be easily fermented by microorganisms such as _Saccharomyces cerevisiae_, is called hydrolysis and can be achieved by (thermo)chemical and/or enzymatic pretreatment. Pretreatment is an expensive and decisive stage for the efficiency of subsequent stages in the technological process of obtaining bioethanol. During pretreatment, products can be formed that can inhibit the subsequent enzymatic hydrolysis and/or fermentation processes. The main products that can result from this stage are formic, levulinic and acetic acid, furfural, and hydroxymethylfurfural. Glucose, xylose, arabinose, galactose and mannose are part of the simple sugars that must be obtained by hydrolysis in order for fermentation to occur.