**4. Hemicellulosic fraction**

#### **4.1. Structure of hemicellulose**

Hemicellulose differs substantially from cellulose to be amorphous, which makes it more easily hydrolyzed than cellulose [49]. The hemicellulosic fraction reaches 40% of lignocellu‐ losic material and acts as substance of reserve and support. This fraction presents branched structure composed by pentoses (D-xylose and L-arabinose), hexoses (D-galactose, D-man‐ nose and D-glucose) and small amounts of acetic and uronic (D-glucuronic, D-4-O-methyl‐ glucuronic and D-galacturonic acids) acids [8, 21]. Other sugars such as L-rhamnose and L- fucose may also be present in small amounts. Xylose is the main carbohydrate present in the hemicellulosic fraction, representing about 80% of total sugars [35, 50].

The heterogeneous structure of hemicellulose with a low polymerization degree makes it in‐ teresting fraction for fermentation process. The open three-dimensional conformation of hemicellulose favors the diffusion of the catalyst in the molecule, providing a better yield of hydrolysis in mild conditions [51, 52].

The hemicellulosic fraction can be removed of lignocellulosic materials by some type of pre‐ treatments, summarized in the Table 3, liberating sugars, mainly xylose, that subsequently can be fermented to sustainable products such as xylitol, butanediol, single cell protein and ethanol [24, 27].
