**3. Pre-treatment of plant biomass**

Cellulose, the most abundant natural polymer, is predominantly found in plant cell wall. However, besides cellulose, plant cell wall also comprised of hemicellulose and lignin, which make an interwoven structure in plant cell wall to make it recalcitrant for microbial and/or enzymatic attack [9]. **Table 1** illustrate how the compositional analysis of different type of biomass differs.

The conversion of biomass into value added chemicals involves the hydrolysis of plant biomass to fermentable sugars. However, to access cellulose a pre-treatment step to remove hemicellulose and lignin is required [20]. Biomass recalcitrance is due to various factors such as the degree of lignification, crystalline structure of cellulose, and the structural heterogeneity and complexity of cell-wall elements and it needs to be overcome for valuable exploitation of lignocellulosic feedstocks [21]. The recalcitrant structure of lignocellulose is disrupted during the pre-treatment process, resulting in lignin sheath breakage, hemicellulose degradation, and a reduction in crystallinity and degree of polymerization of cellulose [22, 23].

There are various types of pre-treatments such as physical methods (such as milling, grinding, irradiation, and sonication); chemical methods (such as alkali, acid, oxidizing agents, organic solvents, ionic liquids, and deep-eutectic solvents); physico-chemical methods (such as steam pretreatment with/without a catalyst, wet-oxidation, and hydrothermolysis); or biological methods (**Table 2**).

The alkali pre-treatment involves the addition of bases to biomass, which increase the internal surface by swelling, and it decreases the polymerization degree and crystallinity, destruction of links between lignin and other polymers, and lignin breakdown [27, 28]. While, acid treatments are known to extract hemicellulose by disrupting the Van der waals force, hydrogen bonds, and covalent bonds that hold the biomass components, which consequently causes the solubilization of hemicellulose and the reduction of cellulose [27]. Steam explosion is another important pretreatment method. Steam explosion is carried out by treating biomass with highpressure saturated steam, and then the pressure is abruptly lowered, causing the


#### **Table 1.**

*Various feedstock composition analysis.*


#### **Table 2.**

*Various pre-treatments methods used in biorefineries [24–26].*

materials to decompress explosively. Steam explosion is generally initiated at a temperature of 160–260°C (corresponding pressure, 0.69–4.83 MPa) for few seconds to a few minutes before the material is subjected to air pressure [29]. It facilitates the removal of hemicelluloses from the microfibrils exposing the cellulose surface and increasing the enzyme accessibility. Lignin is only partially eliminated during the pretreatment, but it is redistributed on the fiber surfaces as a result of melting and depolymerization/repolymerization processes. Liquid hot water (LHW) treatment is similar to steam explosion, except instead of steam, LHW uses water at extreme temperatures (170–230°C) and pressure (up to 5 MPa). LHW hydrolyzes hemicellulose by releasing its acetyl groups and eliminates lignin, exposing cellulose fibers [30]. **Figure 3** illustrates the necessity of pre-treatments in biomass [31].

These pre-treatments methods can be combined depending on the requirement of the pre-treated material. The pre-treatment method selection is not only based on the yields of the cellulose, hemicellulose, or lignin it was able to extract but there are other factors also to consider, such as the effectiveness of the pre-treatment for a certain post-processing step. For instance, for ethanol production besides sugars, low level of toxic inhibitor compounds such as aldehydes, organic acid,

*Conversion of Cellulose into Value-Added Products DOI: http://dx.doi.org/10.5772/intechopen.100022*

**Figure 3.** *Illustrates how the pre-treatments detach hemicellulose, cellulose, and lignin from biomass [31].*

ketones, phenolic compounds, in the hydrolysate is one of the prerequisite factors [32]. While in case of production of polymers, gels or binders from plant biomass, the mechanical strength and swelling properties of the polymers should be kept in consideration while selecting the appropriate pretreatment method.
