Empty fruit bunch of oil palm tree (EFBOPT), solid waste of palm oil industries, is potential for raw materials of biofuel especially bioethanol production because of its cellulose and hemicellulose contents. There are four steps to produce bioethanol, called the second generation bioethanol, from EFBOPT or other lignocellulosic materials. The steps are (a) pretreatment of lignocellulose biomass into cellulose/hemicellulose, (b) hydrolysis of cellulose/hemicellulose into monosaccharides, (c) fermentation of monosaccharides into bioethanol, and (d) recovery of bioethanol from medium fermentation broth. Pretreatment steps are the key success factor to convert lignocellulosic materials into bioethanol. This paper will review EFBOPT and pretreatment steps, including physical pretreatments, physicochemical pretreatments, and biological pretreatments.
- empty fruit bunch of oil palm tree
Depletion of fossil fuels, increasing of climate changes, and improvement of world energy consumption  have directed towards the development of biofuel production . Biofuels are able to replace petroleum-based fuels, decrease greenhouse gas emission , and have significant potential sustainability . Biofuels can be as gaseous (methane or hydrogen) or liquid (biodiesel, biobutanol, or bioethanol) forms and are commonly generated from agricultural materials. Either agricultural commodities or agricultural waste materials can be used for developing biofuels; for example, vegetable oil [5, 6] is used for biodiesel and agricultural wastes [7, 8] were processed for developing methane or hydrogen gases. Meanwhile, bioethanol can be produced either from agricultural high starch/sugar-containing commodities such as corn  and sugarcane  or from agricultural solid wastes such as non-edible parts of cassava , banana peels , cocoa-pod waste , rice straw [14, 15], sugarcane bagasse [16, 17, 18, 19], sorghum bagasse , and empty fruit bunch of oil palm tree (EFBOPT) [21, 22, 23, 24]. Bioethanol produced from starch/sugar containing commodities is called the first generation bioethanol, and one produced from agricultural solid wastes or lignocelluloses is called the second generation bioethanol.
Among agricultural materials, oil palm tree (Elaeis guineensis Jacq.) is considered as a major source of biofuel. In the world, oil palm tree is planted in about 43 developing countries and plantation area increased eight time in the past four decades to over 12 million ha in 2009 . In a good condition, oil palm tree can yield about 4.5 ton oil and 7–8 ton EFBOPT/ha/year. Unlike palm oil which can be processed directly into biodiesel, EFBOPT has to be pretreated before converted into bioethanol .
There are four steps to produce bioethanol, called the second generation bioethanol, from lignocellulosic materials [27, 28, 29] such as EFBOPT. They are (a) pretreatment of lignocellulose biomass into cellulose/hemicellulose, (b) cellulose/hemicellulose hydrolysis into monosaccharides, (c) monosaccharides fermentation into bioethanol, and (d) bioethanol recovery (Figure 1) . The key success factor to convert EFBOPT into bioethanol is pretreatment step ; therefore, this paper will review the EFBOPT pretreatment and discuss recent research results which significantly enhanced enzymatic saccharifications.
2. Empty fruit bunch of oil palm tree
EFBOPT is solid waste residue generated from palm oil factories. Fresh fruit bunches are harvested from oil palm tree (Figure 2) and then sterilized in a steam sterilizer for inactivating enzymes that present in pericarp and loosening fruits from bunches. The sterilized bunches are then fed into a rotary drum thresher to separate the sterilized fruit from bunches without fruit, which are called as empty fruit bunch of oil palm tree (EFBOPT). The sterilized fruits are further processed for palm oil production and the EFBOPT (Figure 3) are conveyed to the damping ground and left unused. It was reported that each 100 ton of fresh fruit bunches yielded 14 tons oil-rich fiber and 20–22 tons of EFBOPT . EFBOPT is dry and brown; its shape is not uniform with low bulk density; and its length and width can vary from 17 to 30 cm long and 25 to 35 cm wide depending on the size of fresh fruit bunch .
Like other lignocellulose materials, native EFBOPT fiber is mainly constructed from a complex matrix of three main polymers (Figure 4) , namely cellulose, hemicellulose, and lignin. The EFBOPT fiber consists of 44.2–50.0% cellulose, 22.0–33.5% hemicellulose, and 20.4–21.0% lignin [34, 35].
Cellulose, a major constitutes of FEBOPT, is closely associated with hemicelluloses and lignin, and the separation of the cellulose from hemicelluloses and lignin requires intensive physical and chemical treatments. Cellulose is a linear polymer of D-glucopyranose units bound by β-1-4-glycosidic linkages (Figure 5). The successive glucose units are rotated by 180o relative to each other to form a cellobiose unit as a repeating unit of cellulose chains ended by a hydroxyl group. The hydroxyl (OH) groups in the cellulose act as functional groups which are able to interact each other or with O-, N-, and S-groups forming hydrogen bonds. Hydrogen bonds also occur between the OH-group of cellulose and water. Through hydrogen bond, cellulose chains are packed together to set up highly crystalline microfibrils. An individual microfibril contains ten of glucan chains in a parallel orientation . This microfibril fraction can be hydrolyzed into glucose either by enzymatic or by chemical methods .
Hemicellulose is a polymer consisting of heteropolymers of D-xylose, D-mannose, D-glucose, D-galactose, and L-arabinose, in the form of linear and branched (Figure 6). Its structure is not crystalline and is, therefore, easier to hydrolyze than cellulose . Hemicelluloses usually form cross-linked to other polysaccharides, proteins, or lignin. Xylans are considered to be the main interface between lignin and other carbohydrates .
Lignin is a cross-linked aromatic, hydrophobic, and complex polymer consisting of three different phenyl-propane precursor monomer units which are very difficult to biodegrade (Figure 7). Lignin is mostly observed as an integral part of the plan cell wall, embedded in a polymer matrix of cellulose and hemicellulose. Thus, lignin is the most non-biodegradable component of the plant cell wall .
Pretreatment is actions given to biomass materials such as EFBOPT for enhancing the cellulose reactivity with cellulase enzymes and for increasing the yield of fermentable sugars. There are eight requirements for effective and economical pretreatment; they are (a) producing higher reactive cellulose fibers for enzymatic attachment, (b) producing less residues, (c) avoiding formation of compound inhibitors for hydrolytic enzymes and fermenting microorganisms, (d) avoiding destruction of celluloses and hemicelluloses, (e) reducing of material cost for setting upper-treatment reactors, (f) minimizing the energy demand, (g) reducing the cost of size reduction for feedstock, (h) consuming little or no chemical, and (i) using a cheap or no chemical . The goal of pretreatment is to disrupt the crystallinity of cellulose, to open lignin and hemicellulose protection, to increase EFBOPT surface accessibility, and to decrease the degree of hemicellulose acetylation . Pretreatment can increase significantly bioethanol or biogas yield and productivity. Effects of pretreatment on the degrading enzyme accessibility, bioethanol or biogas yield, and productivity from lignocellulosic materials were diagrammatically (Figure 8) shown by Taherzadeh and Karimi .
Based on actions given to biomass materials, pretreatment methods are classified into three groups, namely physical, chemical and physicochemical, and biological pretreatment methods . The physical methods include milling (ball milling, colloid milling, hammer milling, two-roll milling, and vibrant energy milling), irradiation (electron-beam irradiation, gamma-ray irradiation, and microwave irradiation), and expansion, extrusion, high pressure, hydrothermal, and pyrolysis. The chemical and physicochemical pretreatments consist of acid (hydrochloric acid, phosphoric acid, and sulfuric acid), alkali (ammonia and sodium hydroxide), explosion (ammonia fiber explosion/AFEX, CO2 explosion, SO2 explosion, and steam explosion), gas (chlorine dioxide, nitrogen dioxide, and sulfur dioxide), oxidizing agents (hydrogen peroxide, ozone, and wet oxidation), and solvent extraction of lignin (benzene-water extraction, butanol-water extraction, ethanol-water extraction, ethylene glycol extraction, and swelling agents), and biological. Biological pretreatments commonly utilize fungi and actinomycetes as microbial producing enzymes which can degrade lignin compounds. Advantages and disadvantages of some different pretreatments are well tabulated (Table 1) by Brodeur et al. and Maurya et al. [42, 43].
|Ammonia recycle percolation|
|Liquid hot water|
3.1. Acid pretreatments
The main objective of the acid pretreatment is to solubilize lignin and hemicelluloses chemically so that the cellulose is more accessible to enzymes. Either diluted or concentrated acid can be utilized to perform this acid pretreatment. The diluted acid pretreatment method is more attractive because of less inhibitor compound formation, such as furfural, 5-hydroxymethylfurfural, phenolic acids, and aldehydes. In addition, diluted acids are less toxic, corrosive, hazardous, and corrosive, as well as more feasible for industrial scale. The diluted acid pretreatment methods have been developed in different types of reactors including percolation, plug flow, shrinking-bed, batch, flow-through reactor and countercurrent reactors . There are two approach processes of dilute acid pretreatment methods, namely (a) high temperature (e.g., 180°C) during a short period of time and (b) lower temperature (e.g., 120°C) for longer retention time (30–90 min). The most widely used acid is dilute H2SO4 which provides high hydrolysis yields [37, 44]. Other acids used for cellulosic material pretreatments are acetic acid, C2H4O3, formic acid, hydrochloric acid (HCl), maleic acid, oxalic acid, phosphoric acid, and nitric acid [45, 46, 47, 48, 49].
3.2. AFEX pretreatment
AFEX is one of the physicochemical pretreatments which treated lignocellulosic biomass with liquid ammonia at relatively moderate temperature (90–100°C) for about 30–60 min, then followed by a rapid pressure release . A rapid expansion of the liquid ammonia causes swelling and physical disruption of lignocellulosic fibers and partial reduction of cellulose crystallinity. AFEX process is able to either modify or effectively decrystallization of cellulose and lignin fractions . AFEX removes the least acetyl groups of biomass by deacetylation process, so that the digestibility of lignocellulosic biomass increases [52, 53]. The main advantage of the AFEX is that it does not produce inhibitors for the downstream biological processes, so water wash is not needed. AFEX is more effective for agricultural residues . The AFEX process conditions (ammonia loading, temperature, blowdown pressure, moisture content of biomass, and residence time) have been optimized . At the optimal conditions, AFEX can convert over 90% cellulose and hemicellulose to fermentable sugars for a broad variety of biomass materials including EFBOPT. Due to high volatility, ammonia is easy to be recovered and recycled , and leaving the dried biomass ready for enzymatic hydrolysis . After pre-pretreatment, ammonia must be recycled in order to reduce the cost and protect the environment .
3.3. Alkali pretreatment
Alkali pretreatment is commonly utilized ammonium, calcium, sodium, and potassium hydroxides at certain temperature and pressure. The main advantage of this pretreatment is that lignin is efficiently removed from the biomass (Table 1). This process eliminates acetyl and uronic acid groups at hemicelluloses; as a result, the accessibility of enzyme that degrades hemicellulose increases . Xylan ester linkages on hemicellulose residues are also hydrolyzed . The advantages of alkali pretreatments are able to largely improve the cellulose digestibility, solubilize lignin more effectively, exhibit less cellulose, and hemicellulose solubilization compared to the acid pretreatments . Alkali pretreatments can also be performed at lower temperature, pressure, and time ranging from hours to days. NaOH solution is more effective than other alkalis [60, 61]. Alkali pretreatments were shown to be more effective on decreasing the degree of polymerization and crystallinity, increasing the internal surface area of cellulose, and disrupting the lignin structure .
Besides NaOH, Ca(OH)2 (lime) is another alkali widely used. It also eliminates lignin-carbohydrate ester and acetyl groups, and enhances cellulose digestibility . Lime pretreatment has been proven successful for several biomass pretreatments, such as wheat straw, poplar wood, switchgrass, and corn stover [61, 62]. This pretreatment has lower reagent cost and less safety requirements compared to NaOH or KOH pretreatments. In addition, lime can be easier recovered from hydrolysate by reaction with CO2 . The air/oxygen addition to the alkaline pretreatments (NaOH or lime) can increase lignin removal .
3.4. Ammonia recycle percolation
Ammonia recycled percolation (ARP) is an another type of ammonia-based pretreatment in which aqueous ammonia (5–15 wt %) passes through a packed of bed reactor along with biomass materials at high temperature (140–210°C) for 90 min and the rate of percolation is maintained at 5 mL/min [56, 64]. ARP can remove hemicellulose and lignin from the biomass as the liquid phase  although requires high liquid loading or process temperature. To reduce energy cost, soaking in aqueous ammonia (SAA) at lower temperatures (40–90°C) for longer reaction times has been used. This approach can preserve most of the glucan and xylan in the biomass samples which are then fermented using the simultaneous saccharification and cofermentation (SSCF) method .
3.5. Biological pretreatment
Biological pretreatments are treatments to biomass materials with microbes such as white rot fungi. Like conventional physicochemical methods, the objectives of biological pretreatments are to degrade lignin. The biological pretreatment is considered as a cheap, ecofriendly, and efficient method . This method is carried out using cellulolytic and hemicellulolytic microbes, such as filamentous fungi which are ubiquitous and can be isolated from soil, living plants or lignocellulosic waste material [65, 66]. The most effective microorganisms for the pretreatment of most of the biomass materials are white-rot fungi , such as Ceriporia lacerata, Ceriporiopsis subvermispora, Cyathus stercoreus, Phanerochaete chrysosporium, Pleurotus ostreatus, Pycnoporus cinnabarinus, and P. chrysosporium. These fungi produce lignin peroxidases—lignin-degrading enzymes and manganese-dependent peroxidases—which show high delignification efficiency on various biomass materials [52, 67]. Some advantages of biological pretreatments include low-capital cost, low energy requirement, no chemical requirement, and mild environmental conditions (Table 1). However, the main drawback of the biological methods is that hydrolysis rate is very low . To solve this drawback, some researches have to perform to find out isolates which have ability to delignify the biomass materials more quickly and efficiently.
3.6. CO2 explosion pretreatment
CO2 explosion pretreatment utilizes CO2 as a supercritical fluid. The fluid displays gas like mass transfer properties and a liquid-like solvating power. This method can remove lignin effectively so that enzymes can digest biomass materials effectively . Supercritical CO2 has been mostly used as an extraction solvent because of its several advantages including easy recovery after extraction, environmental acceptability, non-toxicity, non-flammability, and relatively low cost (Table 1) . In aqueous solution, CO2 reacts with H2O to form carbonic acid and increases hydrolysis rate. Because of their small size, CO2 molecules are able to penetrate small pores accessible to ammonia and water molecules. In this pretreatment, cellulose and hemicellulose structures disrupt so that the surface area of the substrate increases and can easily attack by the digestive enzymes. For several substrates, the CO2 explosion pretreatment is more cost-effective than ammonia expansion and produces lower inhibitors than steam explosion .
3.7. Liquid hot water pretreatment
Liquid hot water (LHW) is one of the hydrothermal pretreatment without rapid decompression and any catalyst or chemical additions and performs under high pressure in order to maintain the water in the liquid state at high temperatures. It is usually carried out at temperature range between 170 and 230°C and pressure (5 MPa) . LHW eliminates hemicellulose from biomass materials so that the cellulose is more accessible to enzymatic attack (Table 1). After pretreatment, the obtained slurry is able to be filtered to yield two fractions, namely a solid cellulose-enriched fraction and a liquid fraction containing high hemicellulose derived sugars. Better pH (4–7) of this pretreatment can be controlled in order to minimize the non-specific degradation of polysaccharides and also to avoid the formation of inhibitors . To promote more effective contact between the biomass materials and the liquid water, three methods have been developed, namely co-current, countercurrent, and flow-through methods. In co-current method, water and biomass slurry are heated to the desired temperature and held at the pretreatment conditions for a certain residence time before being cooled. Countercurrent method is designed to move water opposite to biomass through the pretreatment system. Hot water flows through passage system over a stationary bed of biomass which hydrolyzes and dissolves biomass components and brings them out of the system [72, 73]. LHW pretreatments are generally preferred because it is required lower costs due to no need chemicals and corrosion-resistant materials for hydrolysis reactors. In addition, the LHW pretreatments produce lower concentration of the solubilized hemicellulose and lignin products due to high water input (Table 1). Lower formation of inhibitory components and higher pentose recovery can be achieved in this LHW pretreatment compared to the steam explosion. However, this method has not yet developed at a commercial scale due to higher water demand and high energy input.
3.8. Milling pretreatment
Milling pretreatment is commonly used for reducing size biomass and altering the inherent ultrastructure of biomass and degree of crystallinity so that the biomass can be easier accessed by cellulase enzymes . This pretreatment is performed prior to enzymatic hydrolysis or even other chemical pretreatment processes [74, 75, 76, 77]. There are several kinds of milling, such as ball, tow roll, hammer, colloid, and vibro energy millings . Milling is able to improve susceptibility to enzymatic hydrolysis because of reducing the biomass size , decreasing the biomass crystallinity,  and increasing the biomass area.
3.9. Steam explosion pretreatment
Steam explosion pretreatment is a treatment with high pressure saturated steam for few seconds (30 s) to several minutes (20 min), and then pressure is suddenly reduced. These pretreatments are the most commonly used for treating biomass materials [80, 81]. Steam explosion—typically a combination of mechanical forces and chemical—is able to hydrolyze (autohydrolyze) acetyl groups of hemicellulose. At high temperatures (160–260°C), autohydrolysis occurs and produces acetic acid from acetyl groups of the biomass materials[82, 83]. In addition, water is also able to act as an acid at high temperatures. Reducing pressure suddenly produces explosive decompression so that biomass fibers separate each other. This process is able to degrade hemicellulose and lignin because of explosive decompression, and thus increase the potential of cellulose hydrolysis .
Steam explosion processes have several advantages compared to other pretreatment methods (Table 1). The advantages include high sugar recovery, less hazardous process chemicals and conditions, lower environmental impact, lower capital investment, possibility of using larger chip size, no acid catalyst additions except for softwoods, more efficient in energy usage, and significant improvement in enzymatic hydrolysis as well as its feasibility at industrial scale . The main drawbacks of steam explosion pretreatment are the partial degradation of hemicelluloses producing inhibitor compounds that can affect the enzymatic hydrolysis and fermentation process [85, 86]. Thus, an inhibitor compound separation becomes necessary (e.g., addition of activated charcoal, over liming, and ion exchange) and will increase the overall process cost [31, 87].
3.10. Wet oxidation pretreatment
Wet oxidation is treated biomass materials with water and air/oxygen at temperatures higher than 120°C for 30 min . This wet oxidation pretreatment is suitable for pretreatment of biomass materials containing high lignin content. The most effective parameters in the wet oxidation process are temperature, reaction time, and oxygen pressure . This pretreatment yields acid compounds from the hydrolytic processes and oxidative reactions of biomass materials.
Wet oxidation pretreatments affect all three main components of lignocellulosic materials, namely, cellulose, hemicellulose, and lignin. The hemicelluloses are extensively hydrolyzed to low molecular weight sugars that become soluble in water. Lignin is cleaved and oxidized, and cellulose is partly degraded and becomes highly susceptible to enzymatic hydrolysis. Addition of some alkaline compounds, such as sodium carbonate make easier to hydrolyze hemicellulose components and also minimizes the formation inhibitor compounds, such as furan-based degradation products .
The main advantage of wet oxidation is efficient lignin removal and less inhibitor formations (Table 1). The main disadvantage of this method is requirements for high temperature and pressure maintenances and the presence of strong oxidizing agents such as H2O2. These requirements cause to high costs of maintenance and also require large-scale reactors. Therefore, application of this process is limited in large-scale pretreatment of biomass materials.
4. Conclusions and future perspectives
Empty fruit bunch of oil palm trees have been described in terms of their physical and chemical characteristics. As solid waste products of palm oil factories, physical EFBOPT characteristics are brown, not uniform, low bulk density, varying in length from 17 to 30 cm, and varying in width from 25 to 35 cm depending on the size of fresh fruit bunch. EFBOPT fiber is mainly constructed from a complex matrix of three main polymers, namely, cellulose, hemicellulose, and lignin. The main chemical component of EFBOPT is cellulose (44.2–50.0%) and the others are hemicellulose (22.0–33.5%) and lignin (20.4–21.0%) which should be removed through pretreatments before the cellulose is hydrolyzed enzymatically.
There are three kinds of pretreatment methods, namely, physical, chemical, and physicochemical and biological pretreatment methods. The various pretreatment methods for biomass materials have been described to improve enzymatic saccharifications and ethanol production. Each method has its advantages and disadvantages. There is no one treatment method yielding 100% conversion of biomass into fermentable sugars. There is always a loss of biomass materials, which affects the final yield and increases the cost of finished product, i.e., bioethanol. Although a combination of two or more pretreatment methods has indicated promising results, we still feel that there is a need for extensive researches in this area so that either a new efficient treatment method is discovered or an existing method is upgraded to yield better results. Predictive models will enable the selection, design, optimization, and process control of pretreatment methods that are suitable for biomass materials.
The authors are thankful to: (a) The Minister of Research, Technology, and Higher Education (Kementerian Riset, Teknologi, dan Pendidikan Tinggi), Indonesia, (b) Lampung University (Universitas Lampung), Lampung, Indonesia, and (c) Millennium Challenge Account Indonesia (MCA-Indonesia) for their supports.