**3. Pretreatment of biomass and biofiber for liquid biofuel**

**Biomass Extractive (%) Hemicellulose (%) Cellulose (%) Lignin (%) Ash (%) References**

Straw nd 28 40 17 15 [2] Hemp nd 18 74 4 4 [2] Jute nd 13 72 13 2 [2] Sugarcane bagasse nd 24.5 35.2 22.2 20.9 [3] Corn stover nd 24.18 37.12 18.20 20.5 [4] Eucalyptus saligna nd 48.07 12.69 26.9 12.3 [4] Montery pine nd 41.70 20.50 25.90 11.9 [4] Palm EFB 3.21 29.6 50.49 17.84 3.4 [5] Palm trunk 5.35 32.04 41.02 24.51 2.2 [5] Sago hampas nd 40.5 26.0 7.5 26 [6] Sago pith nd 14.5 44.0 4.9 36.6 [6] Banana stem 10.6 2.0 63.9 18.6 15.5 [7] Kenaf bast 15.9 9.8 69.8 9.2 1.1 [8] Kenaf core 7.5 32.3 45.3 19.0 1.4 [8]

**Table 1.** Chemical composition of various types of renewable biomass and biofiber.

Production of liquid fuel and value-added chemicals from biomass is believed to be one of the approaches to increase the value of biomass and biofiber generated. However, one of the main huddles to ensure the success of this process is the pretreatment process. Pretreatment process has been reported to contribute substantial portion in biofuel production cost. Thus, selecting

the most efficient and low cost production could reduce biofuel production cost.

**Figure 2.** Process flow diagram for biofuel production from biomass and biofiber through biochemical conversion.

nd, Not determined.

332 Radiation Effects in Materials

Pretreatment is one of the most important processed involved in liquid biofuel production through a biochemical conversion pathway. The main goal of the pretreatment is to increase the enzyme accessibility and improve the digestibility of polysaccharides or carbohydrate available in the biomass [9]. The highly organized structure makes plant biomass recalcitrant to physical, chemical, and microbial attack [10]. Thus, the challenge of using lignocellulosic biomass is to have a fast and economical process by integrating variety of pretreatment during the conversion of biofuel. Appropriate selection of pretreatment method must be taken into consideration accordingly to the type of biomass [11]. The pretreatment step involves reduc‐ tion in biomass size, depolymerization, fractionation, and solubilization of the major compo‐ nents in the biomass, such as hemicellulose, cellulose, lignin, and extractives, making the remaining solid biomass more accessible for further subsequent process. Cellulose is a linear polymer composed of D-anhydroglucopyranose unit which is linked together by β-(1–4) glucosidic bond. This cellulose chains are packed into microfibrils that are attached to each other by hemicelluloses and amorphous polymer. These structures are attached together and covered by lignin. Lignin is an amorphous polymer that provides rigidity to the plant cell wall and protect against microbial attack.

**Figure 3.** Pretreatment of biomass and biofiber for sugar production.

In order to provide better access for enzymatic saccharification, the lignin, and hemicellulose needs to be separated from the cellulose through pretreatment process (**Figure 3**). Generally, pretreatment of biomass is totally dependent on the chemical composition of the biomass. The key factors for the effectiveness of the pretreatment of biomass and biofiber are: highly digestible, less sugar degradation, and produce less inhibitors that could reduce fermentation performance [12]. Biomass and biofiber that possesses a high recalcitrant component requires a harsh pretreatment condition to disrupt the cell structure.

The pretreatment of biomass and biofiber can be categorized into four different methods, namely thermal, physical, chemical, and biological (**Figure 4**). Thermal pretreatment is a treatment used to solubilize the biomass by applying heat in the pretreatment system. This method is one of the most common method used for the pretreatment of biomass and biofiber. Generally, the thermal pretreatment is sub-divided into three categories: (1) thermal treatment (temperature = < 100°C under atmospheric pressure); (2) hydrothermal treatment (temperature = > 100°C with gradual pressure release after treatment); and (3) thermal treatment with steam explosion (temperature > 100°C with sudden pressure drop after pretreatment). Temperature and reaction time are the most important factor that plays a major role in this pretreatment process [13]. This method proved to display a significant effect on the disruption of biomass and biofiber such as pelletized corn stover, rice hulls, kenaf, Tahoe mix, and switch grass [14– 16]. Although this method was reported to display a positive effect on enzymatic saccharifi‐ cation process, this method is not selective and less effective for the biomass with less lignin content. Thermal pretreatment at high temperature would partially degrade hemicellulose and produce more inhibitors that could influence fermentation process [12, 17]. The major fermen‐ tation inhibitors such as hydroxymethylfurfural (HMF) and furfural are one of the major products made from the thermal pretreatment process [13].

**Figure 4.** Pretreatment methods of biomass and biofiber for biofuel production.

Chemical pretreatment is one of the most promising methods used to pretreat biomass and biofiber. Generally, this process has been proven successful, particularly when combined with heat [18, 19]. The chemical most commonly applied in this process is either an acid or alkali reagent. The main goal of chemical pretreatment is to solubilize polymers, favoring the availability of carbohydrate in the biomass for enzymatic saccharification. The most common acids used for biomass pretreatment are hydrochloric acid (HCl) and sulfuric acid (H2SO4). In this process, the acid will catalyzed the linkage bond and solubilizes hemicellulose. Unlike acid pretreatment, the alkaline pretreatment method is considered very mild and environment friendly as this method uses low concentration of alkali [20]. Pretreatments with alkali such as sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2), hydrazine, and anhydrous ammonia cause swelling of biomass, disrupts the lignin structure, and breaks the linkage between lignin and the other carbohydrate fractions. Chemical pretreatment using acid and alkaline has been reported to be a promising approach to pretreat biomass and biofiber prior to enzymatic saccharification due to its capability to remove lignin and hemicellulose from the biomass. However, this pretreatment has few disadvantages such as, generation of inhibitors during the pretreatment and chemical used, which could affect subsequent fermentation process and is not environment friendly [21]. Thus, it has encouraged more exploration on other alternative pretreatment process that is sustainable and could be beneficial for the whole production line.

In order to provide better access for enzymatic saccharification, the lignin, and hemicellulose needs to be separated from the cellulose through pretreatment process (**Figure 3**). Generally, pretreatment of biomass is totally dependent on the chemical composition of the biomass. The key factors for the effectiveness of the pretreatment of biomass and biofiber are: highly digestible, less sugar degradation, and produce less inhibitors that could reduce fermentation performance [12]. Biomass and biofiber that possesses a high recalcitrant component requires

The pretreatment of biomass and biofiber can be categorized into four different methods, namely thermal, physical, chemical, and biological (**Figure 4**). Thermal pretreatment is a treatment used to solubilize the biomass by applying heat in the pretreatment system. This method is one of the most common method used for the pretreatment of biomass and biofiber. Generally, the thermal pretreatment is sub-divided into three categories: (1) thermal treatment (temperature = < 100°C under atmospheric pressure); (2) hydrothermal treatment (temperature = > 100°C with gradual pressure release after treatment); and (3) thermal treatment with steam explosion (temperature > 100°C with sudden pressure drop after pretreatment). Temperature and reaction time are the most important factor that plays a major role in this pretreatment process [13]. This method proved to display a significant effect on the disruption of biomass and biofiber such as pelletized corn stover, rice hulls, kenaf, Tahoe mix, and switch grass [14– 16]. Although this method was reported to display a positive effect on enzymatic saccharifi‐ cation process, this method is not selective and less effective for the biomass with less lignin content. Thermal pretreatment at high temperature would partially degrade hemicellulose and produce more inhibitors that could influence fermentation process [12, 17]. The major fermen‐ tation inhibitors such as hydroxymethylfurfural (HMF) and furfural are one of the major

a harsh pretreatment condition to disrupt the cell structure.

334 Radiation Effects in Materials

products made from the thermal pretreatment process [13].

**Figure 4.** Pretreatment methods of biomass and biofiber for biofuel production.

Another pretreatment that is commonly used to pretreat biomass and biofiber is biological pretreatment. This pretreatment involves microbes and enzymes to degrade the chemical compound and release fermentable sugar from the biomass and biofiber. In this method, microorganisms such as brown-, white-, and soft-rot fungi are used to degrade the biomass and biofiber cell wall. White rot fungi such as *Phanerocheate chrysosprorium*, *Cleriponopsis subremospera*, *Phlebia subserialisis,* and *Pleuroisu ostriosis* are commonly used in biological pretreatment [22]. While, brown rot fungi for instance *Gleophylium sepiarum*, *Fomitopsis pinicola,* and *Laetiporus suiphureus* are among the common brown rot fungi used to pretreat biomass and biofiber via this process [23]. During the biological pretreatment, hydrolytic enzyme such as lignin peroxidase (LiP) is produced by the bacteria or fungi and it will attack biomass and biofiber cell wall to a small compound with a low molecular weight, which subsequently, can be used in anaerobic fermentation for biofuel production. Currently, research on the direct enzymatic saccharification of biomass is still scarce.

This method appears to have a few advantages, for example, it requires low energy input and this process is mildly environment friendly. However, the large diversity of chemical compo‐ sition among different types of biomass, enzyme production, and low hydrolysis rate are among the drawbacks that needs to be considered before the method is applied in a large-scale biofuel production. In spite of the many pretreatment methods tested, currently available pretreatment techniques can hardly meet the requirements of commercial application due to long processing times, chemical recycle problems, or high operational costs [9, 24]. Therefore, more works are required to understand and generate more information on the pretreatment of biomass and biofiber.

Physical pretreatment is a process that acts directly at breaking the cells through physical force. This method is widely used as a preliminary step for biomass pretreatment process. Physical pretreatment will reduce biomass size and increase the accessible surface area and pore size. Besides, it could also decrease the cellulose crystallinity and polymerization degrees. Various types of physical pretreatment have been introduced to pretreat biomass including commi‐ notium, milling (ball milling, colloid milling, and vibro energy milling), extrusion, and irradiation. The biomass pretreatment using irradiation has been reported to require less energy compared to other approaches mentioned. Furthermore, this approach is selective and easy to control, thus it is more efficient for production of the desired product [25]. The details on the irradiation pretreatment is described in the next section.
