Introduction

Agricultural waste is one of the most abundant lignocellulosic biomasses available in India and an attractive alternative to renewable energy generation. Renewable energy generated from agricultural biomasses has the possibility of substituting fossil fuel generation [1]. But due to a lack of awareness, these are burned and dumped in the open environment leading to high greenhouse gas (GHG) emissions. Many countries have planned to reduce GHG emissions by switching to cleantech sources, i.e., ethanol. Ethanol is produced from lignocellulosic waste, the most abandoned renewable biomass, derived from agricultural feedstock such as wheat husk, rice straw, sugarcane bagasse, and corn stover. This organic waste is derived from biological sources, primarily plant biomass, being the most readily available global source of renewable materials, with an estimated annual worldwide production of 1010 MT. On evaluating the total production of various agricultural residues globally, the sugarcane bagasse (SCB) biomass is considered one of the abundant agricultural residues that hold the key to solving the global energy problem and environmental concern [2]. According to recent research, the potential for lignocellulose biomass contributed from SCB worldwide annually is 243 million tonnes, which translates to producing 4.3 EJ of energy, which covers 6.8% of the present global supply of bioenergy [3]. Similarly, corn stover production in 2021–22 was approximately 120 MT globally and has the potential to produce 23–53 billion tons of bioethanol in the United States alone [4]. Approximately 512.8 MT of rice is produced globally every year [5] and according to IRRI, the typical rice grain to straw production ratio is 0.7:1.4 [6]. Thus, it is estimated to produce 1025.6 MT of straw that is burned by the local farmers which if utilized rationally could add up to the global bioethanol production [7]. Among these agricultural waste obtained from food crops, peanut shell is considered bulky waste, producing about 230–300 gm/kg of peanuts with 50.34 MMT of peanut produced worldwide in the 2021–2022 season which can also add up to the cause [8]. It could be easily comprehended that utilizing agricultural residue for ethanol production could be one of the most promising sustainable energy processes due to unending supplies of available lignocellulosic biomass wastes. Global biofuel production relative to different countries in the year 2021 is illustrated in Fig. 1, taking reference from Statista report on world biofuel production by various countries [9] and bioethanol production influence with both positive and negative impact is illustrated in Fig. 2.

Fig. 1
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Illustration of global biofuel production relative to different countries in the year 2021

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Fig. 2
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Illustration of positive and negative impact of bioethanol production from feedstock

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Lignocellulosic biomass has complex biochemical and highly heterogeneous structures, characterized by using both chemical and physical structural properties. The structural properties of biomass which include chemical composition, fiber characterization, and cell proposition show a significant effect on the further saccharification process [10]. It is concluded that feedstock with higher cellulose, hemicellulose, lower lignin, and silica content is suitable for bioethanol production. It is estimated that the biochemical structure with the compositional analysis of cellulose (32–47%), hemicellulose (19–27%), and lignin (5–24%) are suitable feedstock for bioethanol production. Since most agricultural wastes contain ≥ 50% fermentable sugars but due to their recalcitrant structure, it is not used further for any chemical and biological process to ferment sugar [11]. However, to make it suitable for further process of hydrolysis and fermentation, a prerequisite step, i.e., pretreatment is required. An ideal pretreatment step dwindles the connective link between lignocellulosic recalcitrant structure and makes feedstock available for further process, i.e., enzymatic accessibility and saccharification process with less inhibitor formation and increase in the recovery rate of cellulose and hemicellulose [12]. The process can be cost-effective by deploying advanced techniques of pretreatment. According to various reports, effective pretreatment reduces the size of the biomass, minimizes sugar loss, and maximizes lignin removal along with a reduction in the formation of inhibitors, thereby making the process economical.

Pretreatment is required to disintegrate the lignin structure and to make the cellulosic complex more accessible for hydrolysis by enhancing enzyme accessibility. Pretreatment is used to reinforce the accessibility of biomass for the conversion of cellulose to glucose, thus making it more accessible to the enzymatic action by hydrolysis of hemicellulosic content and by solubilization of lignin content in the biomass [13]. Figure 3 depicts the diagrammatic representation of the production of bioethanol through various processes along with the cellulase effect on lignocellulosic biomass.

Fig. 3
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Diagrammatic representation of the production of bioethanol through various processes along with cellulase enzyme effect on lignocellulosic biomass

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This review paper covers various pretreatment techniques with an integrated approach toward the degradation of the recalcitrant structure of biomass. Various distinct pretreatment methods are examined in this review along with integrated pretreatment approaches with emphasis on the effect of pretreatment on numerous lignocellulosic biomass. There is a huge need to produce bioethanol in a cost-efficient manner and make it available for commercial purposes. It has been earlier formulated that pretreatment and hydrolysis are relatively costlier processes. The main concern remains regarding the strategy that should be adopted to make biomass affordable for further processing towards ethanol production, which can act as a replacement for petroleum-based fuel that will further solve issues related to dependency on petroleum fuels and provide flexibility in the operation.

Lignocellulosic biomass structure

Lignocellulosic biomass is a complex structure consisting of fermentable and non-fermentable sugar. Cellulose is the most abundant LCB (lignocellulosic biomass) with compositional analysis of 33–47% that is utilized for further process of hydrolysis [14]. Another copious compound in the LCB is hemicellulose (19–27%) in composition. Non-fermentable part of biomass are lignin (5–24%) and silica (18.3%) component which forms lignin–carbohydrates complex and hinders the further process of hydrolysis by binding with cellulose, reducing the exposed surface area for enzymatic action [15] as well as forms a hindrance against external encroachment and prevents degradation. Both hemicellulose and lignin form cover over a cellulosic portion of biomass and reduce the efficiency of enzymatic hydrolysis and fermentation which ultimately lowers the product yield. It is a prerequisite to have the region-wise analysis of biomass as LCB (lignocellulosic biomass), as a versatile resource not only used for biofuel production, but also turned out to account for the production of varied profit-based industrial products. With its high economic value, it is required to estimate the economic viability of the biofuel industry [16].

Cellulose

The largest carbohydrate constituent of LCB (lignocellulosic biomass) is a polymer of anhydrous-D-glucose with a lengthy structural chain constituent of β-glucose monomers having an affinity with β-(1,4)-glycosidic bond and gathered together into microfibril bundle [17]. The linear cellulosic chain is associated together with inter- and intramolecular H-bonds presenting a different degree of polymerization. This H-bond forms a highly ordered crystalline region that makes it accessible for the activity of the hydrolytic enzyme [18]. Some regions in the cellulosic structure are less crystalline–amorphous regions that make it resistant to biodegradation and the enzyme can easily bind to cellulose in these regions to start the hydrolysis process. It has been visualized that feedstock with more cellulosic content is accessible for bioethanol production.

Hemicellulose

Hemicellulose is the group of polysaccharides consisting of a short-branched chain of sugars such as arabino-glucouronoxylan, arabino-4-O-methyl-glucuronic- xylan, glucurono-xylan, arabino-xylan, and galactic-arabino-glucurono-xylan. In another word, it is the polymer edifice of both hexose sugars (D-glucose, D-mannose, and D-galactose) and pentose sugars (D-xylose and L-arabinose) and acetylated sugars [17]. It is a random structure containing five or six-carbon sugar. It is the second most abandoned polymer located in the secondary cell wall of plants.

The main hemicellulose in the plant cell wall is in the form of xylan, which gets converted into its by-product xylose in the hydrolysis process utilized for strain development in biomass [19]. Thus, acetylation frequently takes place during the biosynthesis of galactose residue and another by-product such as acetic acid formed by hydrolysis of hemicellulose which inhibits microbial growth and ethanol fermentation [15]. Thus, to inhibit the formation of by-products, required to maintain the temperature and retention time of hemicellulose degradation. Due to its branched-chain structure with a short lateral chain and low molecular weight, hemicellulose can easily be hydrolyzed [20].

Lignin

It is a complex and large molecular structure, mainly formed by three types of monomers such as p-coumarin, sinapyl alcohols, and coniferyl, which are combined to form integrated and highly interlinked structure, has a high ambivalence, which is responsible for the hardness of structure [21]. It is barren of a sugar-based edifice having a 3D structure that possesses an alkyl-aryl bond among cellulose and hemicellulose moiety embedded in it and acts like an adhesive between them. Typically, the presence of lignin reduces the efficiency of enzymatic hydrolysis. Through electrostatic, hydrophobic, and H-bond interactions, lignin may bind enzymes, and the discharge of chemicals that are soluble lignin-derived may serve as harmful enzyme inhibitors [22].

Pretreatment processes often break down the hemicellulose polymer that links the cellulose molecules into fibres. A portion of the cellulose fibres may also be broken by pretreatment, especially in the amorphous areas. In the ensuing hydrolysis processes, the elimination of the lignin and hemicellulose improves the access of the hydrolytic reagents to the cellulose molecules. However, several physical, biological, chemical, and physiochemical pretreatment are enacted to loosen the strong interactions among these LCB (lignocellulosic biomass) and remove lignin for increasing accessibility of carbohydrates for further process of ethanol production [23]. Figure 4 depicts the widely varying composition of commonly available lignocellulosic sources such as rice straw, wheat straw, and SCB. The first step towards the utilization of LCBs is the disruption of the natural boundaries to extract the cellulose and hemicellulose, which become the substrate for the further process of saccharification. At present, this approach is to break the barrier of LCB degradation through pretreatment that can eliminate lignin and hemicellulose along with rupturing linkage with cellulose to destruct its crystalline structure and degree of polymerization [24]. It was shown that using 2% NaOH (sodium hydroxide) at 121 °C for 1 h removed the lignin content with a slight effect on cellulose and hemicellulose as compared with increasing concentration of H2SO4 in which cellulose and hemicellulose content increased while reversed with lignin content. Thus, using acid pretreatment, hemicellulose can easily be hydrolyzed [25], and further, it is required to evaluate the correct compositional analysis of lignocellulosic biomass for maximum conversion yield and to determine the economic process of bioethanol conversion. There are some methods for compositional analysis of LCBs, these are sulfuric acid hydrolysis method, kinetic analysis methods, and near-infrared spectroscopy methods [23].

Fig. 4
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Representation of varying composition of commonly available lignocellulosic sources such as rice straw, wheat straw and sugarcane bagasse

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The pretreatment methods show the following effect on the lignocellulosic biomass by comparing its pretreatment efficiency both before and after the pretreatment process. The pretreatment is considered to disrupt the compositional analysis of the biomass and enhances the adaptation towards available biomass with the main emphasis on particle size, and degradation of lignin, hemicellulose, and cellulose for subsequent processing. This will enhance the formation of reducing sugar and compatibility towards fermentation, further morphological analysis using XRD, TGA, FESEM, and FTIR spectra show the variation in the structural composition of biomass both before and after the pretreatment process. The efficient pretreatment has minimum sugar degradation with a slight formation of toxic compounds. The pretreatment is the pre-requisite step towards bioethanol production and its effect on feedstock are size reduction, and cellulose disruption along with hemicellulose and lignin depolarization are illustrated in the supplementary file (Additional file 1: Figure S1).

It is quite impossible to follow the strict criteria of each pretreatment, some of the compromises can be made by associating various merits of unusual pretreatment and employing hybrid pretreatment techniques with a maximum yield of desired products. While combining these processes has increased the production cost as well as the complexity of the methods. Nevertheless, to overcome these effects, some pretreatment methods with their mechanism along with some of the hybrid forms of pretreatment methods have been discussed. The main available pretreatment techniques performed on lignocellulosic biomass, a path towards the conversion of affordable biomass available for further processing of saccharification and fermentation is depicted in Fig. 5.

Fig. 5
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Illustration of various available pretreatment techniques performed on lignocellulosic biomass

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Physical pretreatment processes

Physical pretreatment mainly centres on energy and strength by disrupting the lignin barrier in the lignocellulosic complex and making sugar available for conversion to biofuels [26]. This method creates variations in temperature and pressure simultaneously requiring high energy consumption and power that results in high production costs. This pretreatment involves microwave radiation, milling, extrusion, pyrolysis, and mechanical process [27]. It is carried out through mechanized size reduction, surface area, and crystallinity index of biomass which improves further downstream processing which is an energy and cost-intensive process.

Mechanical process

The mechanical process is the most vital step in the pretreatment process as it is meant for size reduction of the biomass along with disrupting biomass surface configuration by breaking the physical structure of feedstock. The cellulose crystallinity of agricultural waste is reduced through milling methods, which may be in the form of ball milling, wet disk milling, roll milling, grinding, and chip** processes [28]. This process enables the complete conversion of cellulose into its amorphous form and makes it available for hydrolysis so that it can easily be attacked by the hydrolytic enzyme. Ball milling pretreatment (BMP) decreases the crystallinity and size of biomass, which slackens the interior structure of biomass. It increases the internal energy of the pretreated rice straw and decreases the stability of the hydrogen bond between lignocellulosic biomasses. One of the significant importance of ball milling is that no weight loss and no inhibitor formation during the fermentation process [29]. On the other hand, high energy consumption and low effectiveness of the process have hindered its further application over other processes. Moreover, different mechanical methods have a comparable effect on biomass based on its impact, compression, friction, and shear force. It is estimated that the vibratory milling process is more effective in reducing the size of biomass as well as the crystallinity of cellulose obtained from LCBs [30]. Water absorption at 400% (w/w) during ball milling at 80 °C for 30 min on corn stover biomass enhances the glucose yield up to 66.69% than at 100 °C along with ball milling process, thus, water intake during the milling process has increased its sustainability and made the process efficient at a commercial scale [31]. Using suitable conditions for the pretreatment process by chemo-mechanical method reduces energy consumption by up to 20–80%. Thus, combining the milling process with alkaline pretreatment, i.e., NaOH proves to be the best alternative for combined process by enhancing glucose yield up to 300 mg/g of SCB biomass, the highest among various alkaline and acidic pretreatment methods. By rupturing the ether linkages in lignin/phenolics carbohydrates complexes, alkaline pretreatment can effectively remove hemicelluloses without dissolving lignin [32]. Even little dosages of dilute alkali of 4% (w/w) NaOH assisted with ball milling and then hydrothermal pretreatment at 100 ℃ for 40 min, yielded 40.75% of reducing sugar with 20.08% of xylose from pretreated SCB biomass. In this instance, NaOH had a more significant role in the deacetylation process than that of the alkaline reagent, which led to the creation of enzyme inhibitors [33]. Similarly, the combined pretreatment method of wet disk milling and liquid hot water compression at 160 ℃ for 30 min along with autoclaving at 135 ℃ for 60 min of residence time have led to 90% of glucose yield while 79% of xylose yield. The significance of using liquid hot water compression is to dissolve a portion of hemicellulose and make a portion of rice straw available for the respective action of cellulase on the surface of biomass [34]. The critical drawback of using a mechanical process for lignocellulosic conversion is that it is an energy-intensive process to break the LCB structure [35]. Due to its high energy usage in large-scale manufacturing, LCB milling increases biofuel yield but is not cost-effective. A recent study found that mechanical pretreatment improved the disintegration of structural parts in two distinct stages by decreasing the trailing duration throughout anaerobic digestion (AD) thus increasing the biofuel output by up to 22%. So, to surmount this limitation, it is required to combine both chemical and mechanical pretreatment that has been proven to lower the energy consumption of milling and ultimately increase the efficiency of glucose yield.

Extrusion process

Nowadays, the combined process of pretreatment has increased attention. One such process is the extrusion process, which mainly includes a combination of mechanical, thermal, and chemical techniques simultaneously, leading to structural alteration due to force generated by high-power rotation. The rotation process generates shearing force among different components associated with the process, i.e., biomass, screw, and barrel, that lead to an increase in temperature and pressure of the barrel [24]. This rotation reduces biomass, efficient heat transfer, and ultimately leads to high sugar recovery from the biomass. The pretreatment process is performed radically at three reaction zone, namely conveying, reverse, and kneading. The conveying zone automatically squeezes the biomass and dispatches it to the kneading zone, where the catalyst gets mixed and forwards it to the next zone, i.e., the reverse, conveying zone where the reactor maintains the pressure required for the process. There is the continuous movement of biomass from the kneading zone to conveying zone for the pretreatment process [36]. It may be either single-gear or twin-gear extrusion-mediated pretreatment required for effective sugar recovery from various types of biomasses available. The process such as a twin-gear extruder is a promising way of pre-treating rice straw with high solid lignocellulose as it removes the amorphous region, leading to a rise in crystallinity index (CrI) up to 50% [37]. After this process, cellulose peaks become sharper, revealing an increase in glucan content ranging from 40.83% to 63.16% as well as the removal of lignin, i.e., 64.51%. Twin gear extrusion is a viable pretreatment method for lignocellulose with a high solid content in particular biomass [30]. The major optimum condition for the extrusion process is the material diameter (60 mesh), extrusion temperature (143 ℃), screw speed of 350 rpm, the reaction time of around 1.5 h; around 77.5% of cellulose and hemicellulose conversion. The findings suggest that cellulose and hemicellulose both were broken down as a result of extrusion, with hemicellulose losing more of its structural integrity than cellulose. This further suggests that cellulose is more challenging to disintegrate than hemicellulose. Additionally, it was discovered that extrusion pretreatment can result in a notable reduction in dietary fiber that is insoluble. The mechanical interruption of cell wall assembly caused by extrusion was likely caused by the combined impact of heat and shearing forces, which resulted in the breakdown of the lignocellulosic biomass structure [24]. The advantage of using the extrusion process is high continuous output, and economic feasibility, the product is obtained with no sugar degradation, under enhanced monitoring and control process with higher yield in a cost-effective manner. The aforementioned studies show that when the extrusion process combines with other pretreatment methods, it has a significant effect on the breakdown of cellulose and hemicellulose structure and enhances the total output of reducing sugars.

Irradiation

Microwave irradiation alters the complex structure of cellulose as well as degrades hemicellulose and lignin in LCBs (lignocellulosic biomass) material and increases the enzymatic vulnerability of biomass for ethanol production. It persuades the breakdown of LCB through molecular collisions such as blending and stretching by dielectric polarization on covalent bonds between cellulose and hemicellulose. This dielectric polar movement leads to rapid heating with an elevated frequency of approximately up to a million times per second which depresses the operation time. Next, the electron beam irradiation process proved to be significant for enzyme digestibility and increases the crystalline portion of available feedstock. It is estimated that glucose yield is theoretically 52.1% higher than the untreated rice straw obtained after 132 h of hydrolysis. Due to the bombardment of electron irradiation during EBI, the interior surface of the biomass was more exposed to enzymatic hydrolysis [29]. Similarly, the researcher comprehensively studied the consequence of microwaves on chemically pretreated feedstock. This type of radiation implies 1% (w/v) NaOH pretreated rice straw yielding 31.3% ethanol in a limited time as contrasted with the traditional alkali pretreatment method [29]. The chemical treatment anticipated before microwave pretreatment disrupts the crystalline cellulose and lignin solubilization. The maximum reducing sugar yield obtained is 246.34 mg/g, cellulose content is 17.53% when pretreated with 1% (w/v) NaOH at a frequency of around 2450 MHz for 5 min at 850 W and around 150 ℃ temperature. This showed the stretching of aromatic rings (C = O bond) corresponding to acetyl groups of hemicellulose that lead to the reduction of hemicellulose content in pretreated rice straw [15]. It has been found that yield from the irradiation process increases with the decrease in the size of biomass. Similarly, the application of microwave irradiation is considered the alternative to conventional barometers as it provides a shorter heating duration with better performance, along with its immediate stop-and-start application over the feedstock. Lower dosages were probably insufficient to significantly alter glucose production. The constituents of the lignocellulose are likely to break down with the increase in irradiation doses, leading to lower glucose production [29]. The optimum condition for performing microwave irradiation is 300 W of constant power generation for 5, 10, and 15 min at 372 kPa pressure. This produces 75% cellulose hydrolysis from rye and wheat stillages. It also shows that intensive microwave pretreatment, i.e., at 372 kPa for 10 min, increases the dehydration of reducing sugar produced as well as inhibits the fermentation process, and subsequently leads to a decrease in ethanol production [38]. Thus, it is necessary to scale up the biorefinery method for the economically efficient synthesis of ethanol from microwave–alkali–acid pretreated biomass.

Another effective ϒ-irradiation of 891 kGy was appropriate for the conversion of microcrystalline cellulose with the highest degree of crack and swelling in the biomass. This is an effective pretreatment technique as the increase in the doses of irradiation has increased the glucose from 0.01% to 0.65% as well as oligosaccharide from 0.04% to 26.78%. It showed that the cellulose underwent gamma irradiation, producing carbonyl and carboxy groups at the locations of bond breaking [39]. Thus, during the microwave irradiation process, thermal "hot spots" are created that lead microwave radiation to penetrate deep into the biomass, accelerating the interaction of ions with nearby molecules of LCBs. The silicified waxy surface was disrupted and the lignin–hemicellulose matrix was broken down as a result of the dipole molecules' rapid rotation, which generated a sharp rise in temperature [33].

Sonification

Ultrasound (sonification) is the advanced technique of pretreatment of LCBs (lignocellulosic biomass). Ultrasound-assisted pretreatment alters both the physical and chemical properties of biomass. This pretreatment process recognizes itself as an efficient and eco-friendly technique. This helps in the formation of subsequent bubbles that lead to the disruption of cellulose and hemicellulose recalcitrant structure along with an increase in the porosity of cellulosic content leading to its breakage into simple reducing sugar [15]. The deformation process of ultrasonic pretreatment reduces the lignin percentage, disrupts the biomass functional groups, and increases the crystallinity index along with an increase in the surface porosity and area of biomass. The sonification process was performed mainly at a frequency around 24 kHz and at an operating power of 400 W. Ultrasonic-assisted alkali pretreatment had been used desirably to boost the efficiency of alkali-treated biomass that can increase the lignin removal up to 80–85% and is performed with 0.5% (w/v) NaOH alkaline solution. This would consequently release more carbohydrates with fewer fermentation-inhibitory residues through the creation of Ultrasound induced cavitation [40]. Likewise, ultrasonic-assisted acid pretreatment has made its way adjacent to bioethanol production from waste potato mass. Further, it was assumed that the increase in sonification time from 5 to 10 min consequently, increases bioethanol yield up to 65.8 mg/l. Higher ultrasonic time, i.e., > 10 min disintegrates starch particles and releases the lignin in the cell wall that forbids the saccharification process and disfavors bioethanol output at a subsequent stage [41]. This ultrasonic wave of 20 kHz creates a disturbance, cavitation, or agitation in the chemistry of the lignocellulosic biomass structure. Cavitation in the biomass structure increases the mass transfer and reduction in the particle size of the exposed biomass. This process can be applied even at mild concentrations and showed regular cleavage of rice straw structure resembling rough and irregular surfaces. The microporous structure of interior biomass enables easy penetration of enzymes [42].

Chemical pretreatment processes

This pretreatment is mainly based on the use of chemicals in the procedure which transforms the crystalline structure of lignocellulosic mass into an amorphous form in need of energy requirement. For chemical pretreatment, it is required to maintain ambient temperature during the process which subsequently enhances the glucose yield for further process. Various chemicals have different abilities to break down the compact structure of lignocellulosic biomass. These are meant to disturb and break the hydrogen bond along with the covalent bond between cellulose, hemicellulose, and lignin. Thus, it is required to understand the mechanism of the process along with the advantages and disadvantages of the particular chemical pretreatment method (Table 1).

Table 1 Various physical and chemical pretreatments with associated advantages and disadvantage
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Alkaline pretreatment

Among chemical pretreatment methods, alkaline pretreatment was widely accepted as being a simple process and having a strong pretreatment effect over some time. This method selectively removes lignin from its carbohydrate counterparts and expands the surface area as well as the porosity of the biomass, decrease in the polymerization degree and crystallinity, resulting in enhancing enzymatic hydrolysis [21]. The chemicals used under alkaline pretreatment are non-corrosive and non-pollutants such as sodium hydroxide (NaOH), ammonia and lime (calcium hydroxide, Ca(OH)2), sulfite, and ammonium hydroxide [43]. The common effect regarding alkaline pretreatment is that it raises the digestibility of lignocellulosic biomass, which can be obtained by transforming the intricate lignin–hemicellulose network enhancing lignin removal. With rupturing cellulose's H-bonds and enabling amorphous cellulose more soluble at higher temperatures or longer residence times, high-severity conditions cause the thermochemical alterations [44]. Experimental analysis reveals that NaOH is the most effective pretreatment which breaks the intercellular bond between cellulose and another component (lignin and hemicellulose). NaOH pre-treated biomass causes the lignin to break down by solubilizing the lignin carbohydrate bond and increases the surface area of cellulose structure while minimizing the degree of crystallinity and polymerization, carried out at low temperature and pressure [45]. In alkaline conditions, lignin's alkyl-aryl bonds are easily disrupted for enzymatic activity.

In a comparison of various alkaline pretreatment processes, alkaline peroxide is best as it increases the fermentation yield by solubilizing lignin effectively and increases the digestibility of feedstock required for further process. Alkaline peroxide used for the process was 5% H2O2 solid concentration at 50 ℃ for 1 h performed on rape seed straw which results in higher enzyme digestibility [46]. This alkaline peroxide treatment before alkaline pretreatment is performed at mild conditions effectively destructing the structure of biomass while facilitating isolation of lignin from the complex recalcitrant structure of the macromolecules. The peroxide loading carried out at 80 mg/g of pretreated wheat straw results in 59.9% lignin removal due to the degradation of lignin during the process [47].

Alkaline peroxide pretreatment is combined with other two oxidizing reagents namely NaOH and HCL (37%) with H2O2 (33%) employing different thermo and thermochemical reactions by autoclaving both reagents used in the process. This shows around 74% hemicellulose solubilization with the release of 2.6% of glucose as compared with pretreatment methods performed alone. Since NaOH shows strong lignin removal and hemicellulose solubilization by breaking ester linkages, it is frequently used for lignocellulose pretreatment. This increases biomass porosity and deteriorates the polysaccharide chain and cellulosic content [48]. Similarly, pretreatment with 1% NaOH assisted with 1% of H2O2 treatment for 24 h results in total sugar of 0.17 g/g of dry biomass while reducing sugar is about 0.024 g/g of dry biomass after 48 h at ambient temperature. Furfural and other inhibitors of hemicellulose breakdown were not taken into consideration with most alkaline solutions perturbing and disintegrating the association of LCBs structure but do not destroy hemicellulose like acid pretreatment. Following alkaline pretreatment, the diversity of sugars found in the liquid fraction shows that NaOH can absorb a wide range of soluble compounds under moderate circumstances [49]. Likewise, NaOH catalyzed organosolvent pretreatment with 10% loading of NaOH along with 150 ml of ethanol/water at 60/40 (%v/v) ratio at 180 ℃ for 30 min resulted in the solid recovery of 81.2% along with lignin removal of 40.7% from SCB biomass. However, when the alkali concentration rises, more cellulose can be converted into oligosaccharides and subunits, reducing the amount of solid recovered and the amount of sugar produced [129].

Bacterial pretreatment

Biological pretreatment assisted with bacterial treatment showed higher lignin degradation than fungal pretreatment due to its easier genetic manipulation and its tolerance towards environmental conditions. Various bacteria such as Sphingobium sp. SYK-6, Rhodococcus sp., Ceriporiopsis sp., Pandoraea sp., galactomyces sp., and Mycobacterium sp. used for bacterial pretreatment show efficient lignin and polycyclic aromatic hydrocarbon degradation prior to cellulose. The bacterial strain Mycobacterium smegmatis L2-K2 grows in the medium containing glucose as the carbon source required during the process [

Conclusion

Second-generation (lignocellulosic) bioethanol production appears to be the most promising renewable feedstock for meeting Sustainable Development Goals. Several feedstock pretreatments have revealed process challenges in terms of yield and product inhibitors. However, the pretreatment of lignocellulosic biomass is a crucial step towards bioethanol production from available biomass due to the recalcitrant structure of LCBs. It is required for the delignification of biomass, i.e., the removal of lignin to make the availability of cellulose and hemicellulose for further processes of saccharification. Till now, the known pretreatment methods, i.e., physical, chemical, biological, and physiochemical approaches are enacted. Further advancement in these processes is required to develop the combined pretreatment for economically feasible processes. The main focus is to develop an efficient pretreatment to remove the non-fermentable part of lignocellulosic biomass to get fermentable sugar. Subsequently, the combined process of pretreatment will lessen the incubation time for the process with more efficient desired outcomes. Thus, this will shorten the pretreatment time as well as it will help in develo** various new combined pretreatment processes at the required temperature, pH and retention time. This review article is based on current and future aspects of various combined pretreatment processes performed on available feedstocks. This will help the researcher in further planning, selection and development of an effective pretreatment process that will disintegrate the recalcitrant structure of lignocellulosic biomass.