2.1. Alkali pretreatment of lignocellulosic biomass

Alkaline pretreatment improves cellulose digestibility, the ability to saponify intermolecular ester bonds, cross-linking xylan hemicelluloses, and other components. The effect of alkali pretreatment on lignocellulosic biomass is dependent on lignin content [20]. Alkaline reagents suitable for alkali pretreatment are NaOH, KOH, Ca(OH)2, and NH4OH. The sugar yield of alkali pretreatment is dependent on the feedstock used [3]. Some of the alkalis cause swelling, an increase in cellulose internal surface area, decreasing the degree of polymerization and crystallinity [38], while some disrupt the lignin structure of the material and remove acetyl groups from hemicellulose, thereby enhancing cellulose digestibility and increasing the reactivity of the remaining polysaccharides during delignification [12, 15, 22]. The advantages of alkali pretreatment are no washing of samples after pretreatment, no corrosion problem in the equipment used for the treatment as compared to acid, and the use of lower temperatures and pressures compared with other pretreatment techniques [39, 40].

#### 2.2. Microwave-assisted alkali pretreatment technology and enzymatic saccharification

Ethanol from cellulose-based biomass is one of the most attractive alternatives to replace fossil fuels because using non-edible material as feedstock to produce ethanol and corresponding bioproducts minimizes environmental challenges [7, 22]. Microwave pretreatment method is a physico-chemical process involving thermal and non-thermal effects. The early discoveries of microwave pretreatment on lignocellulosic biomass were reported by Ooshima et al. [41] and Azuma et al. [42]. Since then, the technology has shown efficient applications in various ways [43–45]. Recently, many research studies have used MW heating as a pretreatment technique to assist in converting lignocellulosic biomass into useful bioproducts [46]. MW pretreatment combines both thermal and non-thermal effects within the aqueous environment of physical, chemical, or biological reactions [47], and its thermal heating may considerably decrease the time and efficiency of the pretreatment [22]. The pretreatment of lignocellulosic biomass using MW heating is done selectively especially at the polar parts, resulting in an increase in the disruption of the recalcitrant structures of the biomass [48]. To date, different pretreatment techniques to make lignocellulose accessible to enzymes for enhancing bioethanol conversion have been widely studied [20]. On the other hand, energy utilization in the pretreatment process raises the overall cost of producing bioethanol considerably, and this is a critical factor to consider before investing in biorefinery processes [20]. Darji et al. [49] and Aguilar-Reynosa et al. [18] reported the different studies on MW heating process describing MW heating as a better technology with energy efficiency to reduce energy consumption during pretreatment.

Afzal [32] and Xu [31] described three ways in which MW irradiation and materials can interact, namely (1) MW-transparent material (insulator) where microwaves pass through without losses like Teflon™ or quartz, (2) conducting material which cannot allow microwave penetration but reflected like metals, and (3) absorbing materials such as oil and water. The electromagnetic radiation in MW heating is shaped like energy propagating in a vacuum without any material in motion, can be observed as light, and used as waves and non-ionizing

In 1949, Spencer Percy discovered that electromagnetic frequency radiation could be used in dielectric heating via microwave for heating application in food and other process requiring the use of heat. Von Hippel in 1954 provided further elaboration based on understanding of theories on macroscopic interactions of microwave and matters explaining his theory with dissipated power, electric field intensity, and propagation constant [18]. MW heating is directly from inside the material—wave interactions, leading to heat transfer and basically has a higher energy yield in comparison with conventional oven techniques which transmit heat by conduction-convention mechanism [34]. Xu [31] stated that conventional heating is transferred from the surface toward the center of the material by conduction, convention, and radiation;

Numerous research studies have reported advantages and disadvantages of MW relative to conventional heating [18]. Advantages of MW include shorter residence time, faster heat transfer, selective, instantaneous on and off operation, precise and controlled heating, rapid and efficient, and environmental friendly process [4]. Due to its efficient process, MW heating has limited disadvantages such as (1) poor distribution of MW power within the material because of non-homogeneous material, (2) non-uniform heating, and (3) low penetration of

Alkaline pretreatment improves cellulose digestibility, the ability to saponify intermolecular ester bonds, cross-linking xylan hemicelluloses, and other components. The effect of alkali pretreatment on lignocellulosic biomass is dependent on lignin content [20]. Alkaline reagents suitable for alkali pretreatment are NaOH, KOH, Ca(OH)2, and NH4OH. The sugar yield of alkali pretreatment is dependent on the feedstock used [3]. Some of the alkalis cause swelling, an increase in cellulose internal surface area, decreasing the degree of polymerization and crystallinity [38], while some disrupt the lignin structure of the material and remove acetyl groups from hemicellulose, thereby enhancing cellulose digestibility and increasing the reactivity of the remaining polysaccharides during delignification [12, 15, 22]. The advantages of alkali pretreatment are no washing of samples after pretreatment, no corrosion problem in the equipment used for the treatment as compared to acid, and the use of lower temperatures and

2.2. Microwave-assisted alkali pretreatment technology and enzymatic saccharification

Ethanol from cellulose-based biomass is one of the most attractive alternatives to replace fossil fuels because using non-edible material as feedstock to produce ethanol and corresponding

however, MW heating converts electromagnetic energy into thermal energy.

waves in mobile cell phones and infrared [33].

50 Renewable Resources and Biorefineries

radiation in bulk materials [35–37].

2.1. Alkali pretreatment of lignocellulosic biomass

pressures compared with other pretreatment techniques [39, 40].

Enzymatic saccharification is a biochemical conversion preceded by pretreatment and followed by microbial catalyst conversion [50]. This is a microbial degradation process, accomplished by using enzymes and the result is usually a decrease in sugar [12]. Converting lignocellulosic biomass to ethanol involves disintegrating the biomass cell wall structure, thereby releasing the simple sugars which are fermented by yeast to produce ethanol [51]. Maitan-Alfenas et al. [52] reported that microorganisms are essential in enzyme production for lignocellulosic biomass saccharification. The saccharification process in the ethanol conversion requires less energy and is done in mild conditions at pH of 5.2–6.2 and a temperature range of 45–50 C [53, 54]. There are three distinct major types of cellulase enzymes used in the process: (1) endoglucanases (E C 3.2.1.4) hydrolyze at random internal β-1, 4-glucosidic linkages in the cellulose chain producing oligosaccharides of different lengths and with a shorter chain appearance; (2) exoglucanases of cellobiohydrolases (E C 3.2.1.91) progress along cellulose chain ends and release major products as cellulose or glucose; and (3) β-glucosidases known as β-glucoside glucohydrolases (E C 3.2.1.21) hydrolyze cellulose to glucose, liberate cellobiose, soluble cellodextrins to glucose [12, 55]. For hemicellulases, hydrolysis of the hemicellulose fraction requires more complex group of enzymes, and endo-β-1, 4-xylanase enzyme is needed for the hydrolysis of xylana, the major polymer component in hemicelluloses [56, 57]. Cellulases and hemicellulases production involve many microorganisms such as filamentous fungi (Trichoderma spp. and Aspergillus spp. native or genetically modified). During saccharification process, one of the fungi lacks β-glucosidase activity (Trichoderma), and it is supplemented with Aspergillus spp. in enzymes blending to improve the conversion of lignocellulose to simple sugars [9, 52, 58, 59].

Related research investigations have reported different activities of enzymatic saccharification process with limiting factors on the lignocellulosic biomass such as moisture, available surface area, crystallinity of cellulose, degree of polymerization, and lignin content [8, 20, 60, 61]. Biomass formed in a complex network of lignocellulose contents has indicated that most enzymes used in process can be absorbed by resultant condensed lignin to reduced yield by nonspecific linkages of the enzymes [52]. In addition, Palonen et al. [62] reported that the hemicellulose removal increases the mean pore size of the biomass, thereby increasing the chances of cellulose to get hydrolyzed. Consequently, lignin content reduces enzymatic saccharification by forming a shield and blocking substrate digestible parts from hydrolyzing [60]. Janker-Obermeier et al. [63] studied solubilization of hemicellulose and lignin from wheat straw through MW-assisted alkali treatment. The result suggested that more than 80% hemicellulose and 90% lignin could be removed from the solid wheat straw substrate without excessive saccharide solubilizing high amount of cellulose.

Biomass MW power (W) MW

Oil palm empty fruit bunch (EFB)

Switchgrass and Coastal Bermudagrass

Corn straw and rice husk

Sugarcane bagasse

Sweet sorghum bagasse (SSB)

Sweet sorghum juice

Wheat straw ear

Switchgrass 1000 (Setting #1 to #4)

> 100, 180, 300, 450, 600, and 850

400, 700, 1000 5–15

Switchgrass 250 5–20

100–160 C

C interval)

(15

Pineapple 170–510 (170 W interval)

Cashew apple bagasse

time (min)

250 5–20 NaOH,

1300 2 Glycerol-

Alkali solution (%w/v)

Na2CO3 and Ca (OH)2

water and glycerol-NaOH

30–120 s NaOH and water

1–30 NaOH and H2SO4

1000 2, 4, 6 Lime Cellulase

(5 min interval)

(5 min interval)

Rice straw 70–700 1–5 NaOH E-CLEAN, endo-

5, 10, 20, 40, 60, 120, 180 s

60 Dilute

600 or 900 15 or 30 NaOH Commercial

NaOH and H2SO4

ammonia

180 3–21 NaOH Trichoderma reesei

Enzymes Sugar

Pretreatment of Crop Residues by Application of Microwave Heating and Alkaline Solution for Biofuel…

and βglucosidase

Trichoderma reesei and βglucosidase

M. heterothallica and cellulase Celluclast

Celluclast 1.5-L and Novozyme

Commercial cellulase

(ACCELLERASE

Trichoderma reesei and βglucosidase

Spezyme CP and Saccharomyces cerevisiae (D5A).

1, 4-β-glucanase and EBLUC and β-glucosidase

NaOH Cellulase HPAEC-

celluclast and βglucosidase

PAD

NaOH Cellubrix L NREL 1000 W/15 min:

1500)

188

analysis method

Sugar yield (dry biomass)

http://dx.doi.org/10.5772/intechopen.79103

DNS 178 mg [11]

1% NaOH/10 min: 87 and 59% glucose and

highest sugar yield for both samples

30 min: 58.7 g/100 g Water/MW: 34.5 g/

SSB/MW/lime: 52.6%

10 min: highest yield

low temperature and short MW time

33.5% total sugar yield at 6.375 W/g

DNS 2% NaOH/10 min: 82 and 63% glucose and

xylose

xylose

DNS Glycerol-NaOH:

NREL 0.1 g/g NaOH/

100 g

DNS MW-alkali/600 W/ 4 min: 0.665 g/g MW-acid/100 W/ 30 min: 0.249 g/g MW-alkali (1%)-acid (1%): 0.83 g/g

DNS SSB/MW/no-lime: 65.1%

> 148.93 g/kg Untreated: 26.78 g/kg

NREL 4.2 g glucose/10 g at

DNS MW-assisted alkali: 1334.79 μg/ml

for 5 s

NREL 0.2 and 1.0 mol/L NaOH: 372 and 355 mg/g

DNS NaOH/250 W/

Reference

53

[44]

[71]

[66]

[72]

[73]

[74]

[67]

[25]

[75]

[76]

[77]

The combination of MW-assisted pretreatment and chemical pretreatment on different biomass as reported by several research studies indicated a higher sugar recovery, and various chemicals used in this process are dilute ammonia, iron-chloride and the common ones, alkaline and acid. All these chemicals assist MW pretreatment technology in removing lignin (alkali solution) and hemicellulose (acid solution) for cellulose accessibility [47]. The combined process separates lignocellulosic biomass components by disrupting the biomass structure, reducing the crystallinity of cellulose, improving the formation of fermentable sugars, and reducing the degradation of carbohydrates [64]. At lower temperatures, the combined pretreatment of lignocellulosic biomass improves enzymatic saccharification by accelerating the pretreatment reaction [65–67]. A combination of acid (H2SO4, 2% w/v) and steam (140 C, 30 min) is reported to have efficiently solubilized the hemicellulose, resulting in 96% yield of pentose in pretreatment and enzymatic hydrolysis of soybean hull [68]. Consequently, more research studies on MW pretreatment technique are still ongoing using different feedstocks and chemical combinations.
