*4.1.1. Gamma-ray irradiation*

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

Among various physical methods, irradiation is considered an attractive method for biomass and biofiber pretreatment. In biomass irradiation process, biomass and biofiber is exposed to high- energy radiations such as ultrasonic waves, microwaves, γ-rays, and electron beam. The irradiation effect on the biomass and process mechanisms varies according to the method applied. Generally, radiation processing technology is defined as: a radiolysis reaction, which uses γ-rays from radioisotopes such as cobalt-60 or cesium-137, or an electron beam produced by an electron accelerator to induce degradation of cellulose. In this process, the high energy radiation generated could change the characteristic of cellulosic biomass including: enhance specific surface area, reduce the degree of polymerization and crystallinity of cellulose, hydrolysis of hemicellulose, and partial depolymerization of lignin [26–29]. Typically, the irradiation pretreatment mechanism mode significantly depends on the technology applied during the pretreatment process. The effect of irradiation pretreatment is assessed base on the reducing sugar production during enzymatic saccharification and the solid residues left after pretreatment. The effectiveness of the treatment depends on several factors such as frequency of radiations, time of exposure, composition of the biomass, and resistance to the radiations by medium between radiations and biomass [30, 31]. Besides, the pretreatment combination used of the irradiation pretreatment and chemical treatment also gives a significant effect on the reducing sugar production during the enzymatic saccharification process [32, 33]. The detailed explanation on the effect of irradiation pretreatment on the biomass and biofiber

There are four different irradiation pretreatment methods that is commonly being used to pretreat biomass prior to enzymatic saccharification process. The irradiation methods are gamma-ray irradiation, electron-beam irradiation, microwave, and ultrasonication. Afore‐ mentioned in the previous section, the pretreatment mechanism of each process is different

on the irradiation pretreatment is described in the next section.

336 Radiation Effects in Materials

**4. Irradiation pretreatment and its mechanisms**

structure and functional group is described in the next section.

**4.1. Type of irradiation pretreatments**

according to the method applied.

Gamma ray is a high-energy ionizing radiation in electromagnetic spectrum that easily penetrates most materials. This irradiation is extremely large high frequency waves and largely depends on the radiation source. This technology is commonly applied in radiotherapy as a tracer in food and medical apparatus sterilization. Recently, the utilization of this technology has gain great attention especially in a biomass and biofiber pretreatment for liquid biofuel production. Radioactive nuclides such as cobalt-60 and cesium-137 are the common radioac‐ tive used in this pretreatment [28]. The main goal of this irradiation is to decrease intra and intermolecular order in cellulose due to the breakdown of the intermolecular hydrogen bonds. In this process, the radiation will travel from the seal source and penetrates (bombard) the biomass and biofiber. The energy carried by gamma radiation is transferred to the biomass component by collision of radiation, resulting to the loss of electron by the atom and lead to the ionization. Under exposure to radiation, the biomass component mainly cellulose macro‐ molecules undergo scission, and various short and long-lived radicals are formed [34]. Also, the content of fragments with a low degree of polymerization generated from the process gradually increases, leading to the alteration of biomass structure, thus, providing ease of access for subsequent process such as enzymatic saccharification process.

The potential of gamma irradiation technology in biomass and biofiber pretreatment has been studied on various types of biomass for instance, jute fiber, poplar sawdust, wheat straw, and cotton-cellulose [33, 35]. There were only scanty studies on gamma irradiation pretreatment on tropical biomass and biofiber that has also been reported. A study on gamma irradiation of empty fruit bunches (EFB) indicated that the pretreatment has reduced the lignin and increased the cellulose content in the EFB [36]. Scanning electron microscopy – EDX (SEM-EDX) analysis showed that there is a significant change on the carbon and oxygen content in the EFB biomass. Typically, untreated EFB contains high carbon and low oxygen content, while the study found a decrease of carbon (9% increment) and decrease of oxygen content (16% decrease), indicating the reduction of lignin content in the EFB.

A comparison on gamma ray irradiation pretreatment on soft and hardwood has also been carried out using different level of dosage ranges between 10–100 kGy [37]. The study found that the most suitable condition for softwood was at 40 kGy, while higher dosage is required to pretreat hardwood (90 kGy). The study also concluded that gamma ray pretreatment process is species-dependent, wherein higher dosage is needed to disrupt hardwood cell structure compared to softwood.

#### *4.1.2. Electron-beam irradiation*

Electron-beam is one of the irradiation pretreatment used to pretreat biomass prior to enzy‐ matic saccharification. This technology has been widely used in various applications such as welding, drilling, and surface treatment [38]. For commercial use, the most important charac‐ teristics of an accelerator are its electron energy and average beam power. Therefore, industrial electron accelerators are usually classified according to their energy ranges, which are divided into low (80–300 keV), medium (300 keV–5 MeV), and high-energy ranges (above 5 MeV). In the electron beam pretreatment, the biomass and biofiber is exposed to a highly charged stream electron. The electron is emitted from an electron beam gun and accelerated by accelerator (**Figure 5**). In this pretreatment process, the electron energy can be controlled and modulated by varying the irradiation dose. The high-energy electrons emitted travel into biomass and biofiber component and transfer the energy within the materials. The heating process initiates chemical and thermal reaction in the biomass including cellulose depolymerization, and production of carbonyl group, resulting from the oxidation of the biomass. Crosslinking of biomass component has also been reported to occur when the biomass is exposed to irradiation beam [39]. Also, reduction of the biomass mechanical strength has been observed from the biomass exposed to electron beam. This could be due to the disruption of hydrogen bond between cellulose chains making it less crystalline and more amorphous [40].

**Figure 5.** Experimental set-up for electron beam irradiation.

Recently various research groups have studied the potential of electron beam radiation on various type of biomass including tropical biomass and biofiber such as bamboo, rice straw, oil palm, fruit bunch, and kenaf [30, 41, 42]. Overall, most of the EBI pretreatment indicated that a significant cellulose degradation was observed after the process [39]. Moreover, the study also showed that this pretreatment has enhanced enzymatic saccharification and reduce sugar production from biomass [31, 41]. A study on EBI pretreatment of hybrid grass biomass indicated that the pretreatment could enhance 59% of glucose yield from the biomass com‐ pared to untreated sample. This is similar to a study by Bak et al. [43] who reported that EBI pretreatment on rice straw could increase enzyme digestibility and energy during the pre‐ treatment process.

Similar to other pretreatment process, EBI pretreatment process could be influenced by several factors. EBI dosage is one of the factors that play a major role in the EBI pretreatment of biomass process [39, 42]. A study on the EBI pretreatment of bamboo chips at various EBI dosage range 0.5–50 kGy, indicated that significant cellulose degradation was attained from the pretreat‐ ment dosage between 0–50 kGy. Furthermore, the study showed no significant changes on the hemicellulose content. This indicates that EBI pretreatment process is a selective process and the degradation level can be controlled by the EBI dosage [39].
