Keywords

8.1 Introduction

Continuously rising demand and speedy exhaustion of conventional fuels raise the requirement of alternative sustainable energy are actually very urgent (Allen 2017). Speedy exhaustion of petroleum fuels causes depletion in oil reservoir levels while elevating the CO2 and CO in the air (Uzair Ali et al. 2020). Air pollution, the greenhouse effect, and global warming are significant impacts of unlimited fuel consumption, alarming the earth’s situation (Uzair Ali et al. 2020). Nowadays, the world’s energy requirement relies on conventional fossil fuels, which will not withstand future energy demand. Continuous depletion in fossil fuel reservoir gives rise to its price hiking and the requirement of nonconventional future alternative of petroleum oil. So, environmental concerns bring many opportunities and build a substantial market for biofuel. Climate change concerns generate the need to reduce GHG emissions and encourage bioethanol demand to replace conventional fuels (Arto et al. 2016). One more issue is the continuous rise of waste dumping in an open area, harming the natural habitat and the dumpsite’s nearby environment (Esparza et al. 2020). Producing energy from waste is reasonable, affordable, and effective. A vast range of renewable energy sources and technologies is available, including biogases, solid biomass, and liquid fuels. A biofuel is a biomass-generated fuel that involves several biochemical treatments and not originated from geological processes like fossil fuels. Biomass with complex or simple carbohydrates undergoes some pretreatment; later, it is converted into some soluble saccharides that are used to produce bioethanol (Demirbas 2007; Hafid et al. 2017). The biomass is classified into two main categories: starchy biomass (sugarcane and other sugar crops, by-products of sugar mills) and lignocellulosic (agricultural waste) feedstock. Previously, first-generation biofuels reported utilization of much starchy biomass like sugarcane, corn, and potatoes for bioethanol production. Yet, it has economic and socio-ethical barriers. Currently, second-generation biofuels are gaining more attention to utilize residual biomass for bioethanol production. Lignocellulosic feedstock involves agricultural residues such as bagasse, stalks, corn stover, straws, leaves, and switchgrass for the second-generation biofuels. Any agricultural waste biomass with a considerable amount of saccharide could be used as a raw material for bioethanol production. Pineapple, potato, and sugarcane are significant plant sources that resulted in an excellent yield of bioethanol as by-products due to a high amount of saccharide in it. Bioethanol is an alternative biofuel for existing engines with a better octane rating with reliable production processes and easy adaptability (Bhuvaneswari and Sivakumar 2019).

8.1.1 Fruit and Vegetable Wastes (FVW) as a Raw Feedstock for Bioethanol Production

Food processing and nutrition sector founds a vital connection between agriculture and industry. Food industries generate abundant fruit peel waste by processing fruits and juices. Most of fruits contain 15–50% of peel. After consumption of mesocarp (fleshy part), peels are rejected as waste. For several production units, quantity of waste generated is greater than the product obtained (Alarcón García et al. 2015; Wadhwa and Bakshi 2013; Pathak et al. 2015). It is expected that, in upcoming years, majority of organic chemical compounds will be generated through bio-based processing using agricultural, forestry, municipal, and food waste feedstocks (Gnansounou and Pandey 2016). Fresh fruits or juice consumed is mainly in the ready form of consumption or concentrated form. Many studies stated that FVW biomass is rich in carbohydrates. Saccharides are one of the potential substrates for sustainable energy production. Research on FVW biomass utilization for biofuel manufacture caught much attention in various countries. Vegetable waste could be fresh or cooked parts generated during cultivation, harvesting, storage, marketing, and consumption. Rotten vegetables and vegetable peels are biodegradable waste generated in huge quantities every day, usually dumped on the open household area or landfills. This act emits a nasty odor and a big attraction for pigs, rodents, or scavengers and possibly transmitting various human diseases. The waste FVW biomass goes to landfills where the waste biomass spread nasty smell and generates methane (a GHG), and a massive quantity of hazardous leachate pollutes soil and groundwater. When we talk about waste management, there is some hierarchy to follow: (1) waste reduction or reuse, (2) recycling, (3) energy recovery or composting, and (4) treatment and disposal. Applying the identical waste management method for fruit and vegetable peels with a different biorefinery approach could be helpful for the valorization of FVW. The economic viability of the biorefinery approach for FVW could be attained by producing a various range of high-volume, low-value products such as animal feed and compost or less-volume high-value compounds such as pectin, essential oils, phenolic compounds, etc. (Joglekar et al. 2019). Figure 8.1 demonstrates the general representation of a biorefinery approach to process FVW. FVW is used either directly fresh (having moisture content) or dried, followed by size reduction. Based on the various varieties of FVW feedstock, different solvent extraction procedures are selected for the extraction of phenolic compounds (antioxidants), whereas further steam processing is performed for essential oil extraction. Remaining lignocellulosic biomass is kept in aqueous acidic solution for its conversion into simple saccharides (reducing sugar). Hydrolysis step is significant to boost the fermentation and yield of products. Filtered solid hydrolysate residues could be used for either anaerobic digestion or gasification to obtain biogas and syngas, whereas fermentation is carried out with filtered liquid hydrolysate. Fermentation products are passed to distillation column to obtain pure ethanol (Joglekar et al. 2019).

Fig. 8.1
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General schematic of a biorefinery approach to process FVW

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After the fruit juice extraction, the leftover parts of the fruits are a rich source of lignocellulose, and they could be utilized as a fresh substrate for bioethanol fermentation (Bhuvaneswari and Sivakumar 2019). In a study, banana, pineapple, and plantain peels were used for simultaneous saccharification and fermentation at different temperatures (20–50 °C) by co-culture of S. cerevisiae and A. niger for a week (Itelima et al. 2013). It was detected that the highest pH and temperature for the banana peels fermentation were 6 and 30 °C. The optimal bioethanol yields were 3.98% v/v, 7.45% v/v, and 8.34% v/v for plantain, banana, and pineapple peels, respectively (Itelima et al. 2013). For citrus peel waste-based hydrolysates, the most effective conditions for bioethanol fermentation were observed (Patsalou et al. 2019). Pichia kudriavzevii KVMP10 at 42 °C results in maximum bioethanol production of 30.7 g/L (Patsalou et al. 2019). Pineapple peels were utilized as a cheap and affordable raw material for bioethanol fermentation by S. cerevisiae (Casabar et al. 2019). NaOH was used in the pretreatment of pineapple peels for the high production of simple saccharides. T. harzianum inoculum was used for the hydrolysis process, which can efficiently raise the reducing sugar content of the pineapple peels after hydrolysis (Casabar et al. 2019). The maximum ethanol generation obtained was 5.98 g/L (at 48 h of incubation) which was followed by 5.31 g/L (at 24 h) and 4.5 g/L (at 72 h) of fermentation. Table 8.1 reports the use of FVW biomass for bioethanol production.

Table 8.1 FVW biomass utilized for bioethanol production
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Promon et al. (2018) reported that vegetable waste biomass is rich in lignocellulose content and use nonedible vegetable waste to hydrolyze it to obtain d-xylose and glucose (Promon et al. 2018). Lignocellulose is mainly the composition of 10–15% lignin, 20–40% hemicellulose, and 30–50% cellulose (Wilson and Lee 2014). Cellulose is a homologous glucose polymer linked by the β-1,4 glycosidic bond (Zhao et al. 2011).

8.1.2 Role of Microorganisms

Bioethanol production from FVW biomass includes several steps: pretreatment of waste biomass, saccharification of lignocellulose using enzymatic action, and ultimately fermentation. Each step somehow deals with the involvement of several microbial steps. Lignocellulosic biomass usually has a stiff texture, and it is made up of cellulose, lignin, and hemicellulose, making the raw feedstock unmanageable for complete enzymatic digestion (Zhao et al. 2011). However, commercial enzymatic conversion of lignocellulosic waste is limited by the cost of cellulase enzyme. Amid the various microbial sources of the cellulolytic enzyme, fungal strains are major cellulase producers compared to bacteria. A fungal species, Trichoderma reesei, is widely used for commercial cellulase production. There are three main types of cellulolytic enzymes produced by T. reesei are (1) b-glucosidases (EC 3.2.1.21), (2) cellobiohydrolases (EC 3.2.1.91), and (3) endoglucanases (EC 3.2.1.4), which are used to convert cellulose into glucose. Lesser b-glucosidase activity is the primary drawback of T. reesei (Zhang et al. 2010). Some novel fungal strains have been explored for cost-effective bioconversion of lignocellulosic waste. A fungus, Chrysoporthe cubensis (plant pathogen), has been examined as another source of enzyme cellulases (Falkoski et al. 2013). Another study signifies the role of the Phoma exigua ITCC 2049 (a fungus), which is usually a potato pathogen that could be utilized for cellulase production (Tiwari et al. 2013). A thermophile fungi M. cinnamomea and one more fungus A. strictum were discovered as a possible bioresource for cellulase production (Goldbeck et al. 2013; Mahajan et al. 2016). A unique b-glucosidase was obtained from P. piceum, which achieves maximum trans-glycosylation activity to yield cellulase inducers (Gao et al. 2013). Glucose xylose is another most abundant saccharide obtained after the hydrolysis and saccharification of lignocellulosic waste. Application of xylose conversion into fermentable saccharides has excessive significance for greater bioethanol yield. Then again, the native strain of S. cerevisiae is unable to ferment xylose into ethanol. Using synthetic biology, a novel strain design was developed for simultaneous utilization of acetic acid, xylose, and cellobiose (Wei et al. 2015). In this design to make possible the use of cellobiose, intracellular β-glucosidase encoding gene gh1–1 and cellodextrin transporter encoding gene cdt-1 from the fungi Neurospora crassa were expressed and amplified within S. cerevisiae (Wei et al. 2015). Additionally, the xylose-metabolizing genes XDH and XR from a yeast Scheffersomyces stipitis were expressed and amplified within the S. cerevisiae. To aid the reduction of acetate into ethanol, adhE gene from E. coli was expressed in the S. cerevisiae. The ultimate strain obtained exhibit all the three pathways for acetic acid, cellobiose, and xylose assimilation and its conversion into bioethanol, which significantly enhance ethanol yield compared to control strain (Wei et al. 2015). S. cerevisiae is generally admired for bioethanol production due to its extensive pH tolerance and less infection susceptibility.

8.1.3 Pretreatment and Detoxification of FVW

Household and pulp mill refuse fruit and vegetable biomass is an extensive range of lignocellulose-rich feedstock material. Pretreatment involves various procedures for converting lignocellulosic feedstock into its essential components such as lignin, cellulose, and hemicellulose. Pretreatment procedures mainly concern with lignin elimination, hemicellulose preservation, reducing the cellulose rigidity, and enhancing the porosity of the biomass (Chiaramonti et al. 2012). An economical way of pretreatment procedure should assist in increasing the availability of carbohydrates in the enzymatic hydrolysis step while reducing the loss of simple saccharides for hydrolysis and fermentation (Chiaramonti et al. 2012). Figure 8.2 lists different pretreatment methods for FVW biomass.

Fig. 8.2
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Different pretreatment methods for fruit and vegetable biomass

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The primary purpose of an efficient pretreatment process is (Kumari and Singh 2018) (1) to obtain simple saccharides by hydrolysis, (2) to avoid loss of simple saccharides formed, (3) to limit the production of inhibitors, (4) to minimize energy requirements, and (5) to reduce bioethanol production cost. Biomass pretreatment procedures are categorized as (Kumari and Singh 2018) (1) physical/mechanical pretreatment, (2) chemical pretreatment, (3) physicochemical pretreatment, and (4) biological/enzymatic pretreatment. Table 8.2 represents various pretreatment methods utilized in waste biomass processing.

Table 8.2 Pretreatment methods
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The initial stage of pretreatment involves the size reduction of lignocellulosic feedstock, but this size drop should not be too big or too little. Pretreatment approaches are chosen on the basis of the type and composition of feedstock used for bioethanol fermentation (Kumari and Singh 2018). Among different physical/mechanical pretreatment practices defined in Table 8.2, extrusion is supposed to be cost-effective and easy to process when combined with mixing, shearing, and heating. It assists in the release of a considerable quantity of simple saccharides. Microwave heating is a heating pretreatment approach that should be carried out at appropriate temperatures based on biomass composition (Li et al. 2012). Acid pretreatment is a well-recognized chemical approach generally used for lignocellulosic biomass and covert hemicellulose into its monomers and ultimately enhances bioethanol fermentation. Acids such as hydrochloric acid, acetic acid, formic acid, nitrous acid, nitric acid, maleic acid, phosphoric acid, and sulfuric acid have been utilized for acid pretreatment of lignocellulosic feedstock (Bagudo et al. 2014; Ajayi and Adefila 2012; Bai et al. 2016; Sen et al. 2016; Ranjan et al. 2013; Han et al. 2013). Acid pretreatment gets used in two ways: (1) high acid concentration at significantly lower temperatures and (2) low acid concentration at high temperatures (Sen et al. 2016). The typical downside of acid treatment is the simultaneous production of different inhibitors like furfural, acetic acid, and 5-hydroxymethyl furfural, which prevent the growth of microbial biomass (Taherzadeh and Karimi 2008). However, concentrated acid pretreatment is exceptionally efficient for cellulose hydrolysis, owning to higher solubilization of cellulose and hemicellulose while the simultaneous exclusion of lignin from the feedstock (Kumari and Singh 2018). Acid and alkali pretreatment was more broadly applicable for the lignocellulosic feedstock. NH3, Ca(OH)2, KOH, and NaOH are primarily used alkalis for pretreatment (Wan et al. 2011). The most promising microbes for biotic treatment are different white-rot fungi related to the Basidiomycetes class. P. chrysosporium is capable of lignin biodegradation and has extreme competence compared to the different acknowledged species of white-rot fungi as of its excessive growth rate (Sindhu et al. 2016). For valorization of FVW a combined way is much effective that include multiple pretreatment methods. In this fractionation method, FVW feedstocks are converted into its component at low cost, and simultaneously it will offer fractions of several valuable by-products. However, cellulose is the final product of FVW pretreatment which is further used for bioethanol fermentation. In this strategy FVW was washed in hot water and then kept in several solutions having acid, alkali, and oxidative agent sequentially. Such combined pretreatment methods are advantageous to obtain different fractions of sugar, lignin, pectin, cellulose, hemicellulose, and other bioactive components (Szymańska-Chargot et al. 2017). Figure 8.3 represents fractionation process for the pretreatment of FVW waste.

Fig. 8.3
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Fractionation process for the pretreatment of FVW waste (Szymańska-Chargot et al. 2017)

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In a study, the consequences of different parameters were analyzed on the pretreatment efficiency using C. subvermispora for bioethanol fermentation and obtained up to 94% of cellulose degradation with 31.59% lignin digestion (Wan and Li 2010). Pretreatment is a significant part of bioethanol production from lignocellulosic feedstock. Pretreatment methods substantially impact the whole production process, and it directs the production of lignocellulose derivatives like acetic acid, 5-hydroxymethylfurfural, formic acid, furfural, and levulinic acid. If the accumulation of pretreatment derivatives is sufficiently high, it will act as an enzyme inhibitor for consequent stages of microbial fermentation (Cavka and Jönsson 2013). The presence of inhibitors can be confirmed by (1) by adding some alkali like NH4OH, Ca(OH)2, and NaOH, (2) using enzymatic action of peroxidase and laccase, (3) thermal treatment or vaporization, (4) using liquid-liquid extraction or supercritical extraction, and (5) microbial treatment using Trichoderma reesei (Zabed et al. 2016).

8.1.4 Bioethanol Production

Bioethanol production from FVW could be achieved in three steps: (1) saccharification/hydrolysis, (2) fermentation, and (3) ethanol separation (distillation). Generally, bioethanol fermentation has been accomplished by using some bacteria such as Zymomonas mobilis or yeast (Ma et al. 2008). Yeast like S. cerevisiae can utilize carbohydrates and is effective for glucose to ethanol conversion (Chen 2011). Using yeast cell culture for anaerobic digestion of leftover biomass can reduce up to 30–50% of COD (Suwannarat and Ritchie 2015). Yeasts cell did not hold the whole range of amylolytic enzymes like glucoamylase, α-amylase, and β-amylase, which is essential for breaking down complex saccharides into glucose completely. YIR019C (FLO11, MUC1, STA4) and YIL099W (SGA1) are two genes in yeast cells that encode for α-glucoamylases only. Ethanol production can be performed two ways: either saccharification performed with fermentation simultaneously (SSF) or separated hydrolysis processing, followed by the fermentation step (SHF). In SSF, saccharification is carried out with fermentation, together in a single-chamber bioreactor; hence SSF is economically practical compared to SHF. In spite of that, the optimal parameters and conditions for saccharification and fermentation processes are diverse (Vohra et al. 2014). Ethanol production or fermentation appears to be a biochemical redox reaction inside the yeast cells that required an appropriate range of oxidation-reduction potential (Ma et al. 2016b). Since xylose makes a significant constituent in the hydrolyzed biomass, S. cerevisiae (widely utilized for ethanol production) performs glucose fermentation but could not metabolize xylose. An ascomycetous yeast, P. stipitis, was found able to ferment xylose, so the co-culture of P. stipitis and S. cerevisiae seems effective for elevating the fermentation of pretreated feedstock consisting of both glucose and xylose (Kordowska-Wiater and Targonski 2001). Studies disclosed the relevance of the co-culture method for bioethanol fermentation and generally include the following co-culture: (1) immobilization of Z. mobilis and unbound P. stipites co-culture, (2) Z. mobilis with Candida tropicalis, (3) S. cerevisiae with P. tannophilis, and (4) co-culture of S. cerevisiae with E. coli strain KO11. However, the best combination for xylose and glucose fermentation is the co-culture of P. stipitis with immobilized Z. mobilis (Chen 2011). The efficiency of the co-culture method is based on the growth rate of microorganisms on the diverse feedstock and the fermentation conditions such as pH and temperature (Cardona and Sánchez 2007). Figure 8.4 represents a generalized process involved in ethanol fermentation from FVW.

Fig. 8.4
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The process involved in bioethanol fermentation from FVW

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Bioethanol production mainly differs in the following three methods that are SSCF, SSF, and SHF. Fermentation is wholly detached from lignocellulose hydrolysis in SHF. Separate processing of enzymatic hydrolysis and fermentation enabled the enzyme operation at a high temperature and showed exceptional performance. Microbial culture needs a lesser temperature for sugar consumption and optimization during fermentation. Figure 8.5 depicts differences in processing of these three methods. In SSCF and SSF methods, hydrolysis and fermentation happen together to maintain lower glucose concentration, so the entire process occurs quickly. In the SSF, glucose is isolated from pentose, whereas the SSCF procedure handles glucose and pentose in the same bioreactor (Canilha et al. 2012). SSF and SSCF processes are highly efficient and need a single bioreactor for production; hence, it is chosen over the SHF (Chandel et al. 2007). Ohgren et al. (2007) compared SSF and SHF process configurations for bioethanol production using pretreated corn stover with 8% water-insoluble solids. During SHF, pretreated corn stover slurry has significant concentration of inhibitors that affect enzyme hydrolysis negatively, whereas SSF minimized the negative impact of inhibitors. SSF minimize glucose inhibition (during hydrolysis). Hence SSF was determined as improved configuration in comparison to SHF (Ohgren et al. 2007).

Fig. 8.5
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Processing strategy in SHF, SCF, and SSCF

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Continuous or batch, repeated batch, and fed-batch mode are standard practices used for bioethanol production. In the batch mode of production, the substrate is supplied at the initial phase of the fermentation without adding or eliminating further substrate into the medium (Hadiyanto et al. 2013). Batch mode is recognized as minimal, easy, and flexible to run with control systems for fermentation. A closed-loop arrangement maintains an elevated sugar and inhibitor concentration during the initial and terminal phases of the fermentation, whereas the process deals with high product concentration (Thatoi et al. 2016). Batch mode of fermentation includes several processing benefits such as overall sterilization, no need for advanced labor skills, easy to control, easy handling of feedstocks, and flexibility for several product specifications (Jain and Chaurasia 2014), but the productivity is significantly less and requires high labor cost. The presence of a significant concentration of saccharides inside the bioreactor may cause inhibition of microbial biomass growth and ethanol production (Cheng et al. 2009). To overcome this inhibition for enhanced ethanol production, fed-batch mode of fermentation is preferred. Fed-batch fermentation involves feeding substrate to the reactor and avoiding the effluent removal; hence it is considered as a combination of batch and continuous mode. In a fed-batch, the substrate must be fed at a specific rate, whereas culture size varies. For higher ethanol yield in the fed-batch mode, it is essential to maintain lower substrate concentration because lower substrate concentration is suitable for converting fermentable sugar into ethanol (Jain and Chaurasia 2014). Fed-batch mode has several advantages: better availability of dissolved oxygen, lower toxicity, quick fermentation, and high ethanol yield (Cheng et al. 2009). Fed-batch has been effectively run in the SSF system by repetitively fed pretreated feedstock to attain high fermentation and ethanol yield (Azhar et al. 2017). The bioreactor is continuously loading with the substrate, culture medium, and nutrient media in a continuous processing method, whereas the culture quantity remains the same, and the final effluent (products) continuously drains off from the reactor. Some desired particular products such as microbial biomass, remaining sugar, and ethanol could be recovered from the bioreactor (Azhar et al. 2017). Small-size fermenters, greater ethanol yield, and economic effectiveness are the benefits of the continuous mode that make it chosen over batch and fed-batch modes. The ability of S. cerevisiae for fermentation and ethanol production is considerably falling with long cultivation time. The risk of contamination is more in continuous mode than any other fermentation modes (Chandel et al. 2007).

8.1.5 Ethanol Recovery by Distillation

After fermentation, downstream processing began with several unit operations which are performed for bioethanol recovery from the fermentation broth. At first, liquid-solid separation is performed to separate solid fractions (containing residual saccharides) and bioethanol from fermentation broth. Definitely, filtration and centrifugation are the best choice for liquid-solid separation. To minimize the water content of hydrolysate, supernatant is driven to the rotary evaporator. Serial evaporation helps to attain pure condensate with the concentrated syrup. Evaporation is followed by distillation. Condensate consisting bioethanol will be circulated to the distillation unit. Separation of ethanol from condensate is based on the differences in the boiling points of water (100 °C) and bioethanol (~78 °C) mixture. If water and ethanol solution is very dilute, repeated distillation is preferred to attain >95% of ethanol concentration. Bioethanol recovery using distillation attains 99.6% efficiency to minimize the losses of evaporated portion of bioethanol (Avilés Martínez et al. 2012; Balat 2011).

8.2 Factors Affecting Fermentation

FVW has sufficient chemical nutrients for microbial growth and fermentation. Studies show no significant impact on ethanol production after adding external inorganic additives such as KH2PO4 and (NH4)2SO4 to the production medium (Thongdumyu et al. 2014; Tang et al. 2008; Ma et al. 2008). However, when Ca2+ was added in ethanol fermentation by S. cerevisiae KRM-1, using kitchen waste, it has been found that Ca2+ can enhance the flocculation rate of yeast cells significantly (Ma et al. 2009). Besides nutrients, bioethanol fermentation depends on several fermentation conditions such as inoculation size, temperature for saccharification/fermentation, pH, moisture content, and fermentation time. Table 8.3 shows factors affecting bioethanol production and optimal condition for fermentation.

Table 8.3 Factors affecting bioethanol fermentation
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pH can alter the nature of proteins. Multiple enzymes are involved in the metabolic processes which occur during fermentation. A very low pH may alter the nature and structure of enzymes by causing the dislocation of chemical bonds. It is found that at higher pH, yeast tends to produce acids in place of bioethanol (Tahir et al. 2010). Microbial metabolic activities are basically enzyme-catalyzed reactions that further relied on several external factors such as temperature and pH. Temperature change deeply affects metabolic pathways by denaturing enzyme structure. Conventionally, saccharification is achieved at 95–105 °C (high temperature) using thermophilic α-amylase (Xu et al. 2016), whereas fermentation temperature is based on feedstock composition, and, generally, fermentation temperature varies from 25 to 30 °C (Vohra et al. 2014). The size of the initial inoculum significantly affects microbial cell density during fermentation. Small inoculum size leads to a more extended lag phase by slowing the cell growth and lowering ethanol production, whereas large inoculum yields overgrowth of cells, which further causes substrate competition for microbial population (Ma et al. 2008, 2016b). FVW has a high water content that significantly affects microbial growth and activity. A high solid to liquid ratio results in greater ethanol concentration, affecting microbial activity negatively (Ma et al. 2008). Whereas a low solid to liquid ratio does not affect microbial activity, however during the distillation process, the high moisture content in waste biomass demands enormous energy and ultimately enhances the production cost of ethanol. Fermentation efficiency is hugely affected by incubation time. Extended fermentation time requires additional energy consumption that will raise the production cost of ethanol. Adequate fermentation time avoids the accumulation of by-products (glycerol, organic acids) responsible for yeast activity inhibition (Ma et al. 2016a).

8.3 Ethanol as Biofuel

Bioethanol has high octane number, which measures performance. Higher octane number tends to the more significant compression that the fuel could endure prior to ignition. Lower octane numbers cause premature ignition and engine/cylinder knocking, so the gasoline engines require high compression ratios that can be achieved by a fuel having a higher octane number. Bioethanol is selected to be a fuel that can be utilized in high-performance engines due to its high octane number (Dabelstein et al. 2000). High oxygen content increases combustion efficiency and reduced hydrocarbon emissions. Table 8.4 represents the effect of ethanol blending on various performance measuring parameters of spark ignition engine (Thangavelu et al. 2016).

Table 8.4 Effect of ethanol blending on different performance measuring parameters of engines
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While counting on GHG emission, significant reduction of unburnt hydrocarbon and CO was observed (Karavalakis et al. 2014). However, there is no remarkable decline in NOX, CO2 emissions, and other unregulated emissions like carbonyls, aromatics, particulate matter, etc. (Thangavelu et al. 2016). Bioethanol utilization does not require any alteration in the motor engine, and it does not emit greenhouse gases and eco-friendly and affordable production cost (Ritslaid et al. 2010; Sutjahjo 2018).

8.4 Future of Bioethanol in India

At current, the Indian population is highly dependent on conventional fuel resources. The Indian energy requirement is primarily relying on imported crude oil to satisfy its domestic consumption requirements. However, according to “National Policy on Biofuels 2018,” bioethanol produced from sugar cane, sweet sorghum, sugar beet, corn, forestry/agricultural waste residues, rotten potatoes, cassava, bagasse, etc., are used for transportation or stationary fuel requirements. As per the policy, the government will also take some crucial steps for the adoption of biofuels. Indian government starts a 5–18% Ethanol Blended Petrol Program (EBPP), in which ethanol produced from various biomass feedstocks will be blended with petrol. Similarly, the commercialization and development of second-generation ethanol technologies have been promoted (Das 2020). Indian ethanol market is categorized as solvent, beverages, fuel and fuel additive, disinfectant, flavoring, and fragrance on the basis of its application. The government is emphasizing the biofuel production methods using waste biomass for ethanol production in the future. To reduce dependence on crude oil imports, the Indian government incentivizes sugar manufacturers to produce bioethanol for oil marketing companies (OMCs). Predictably, ethanol production will increase by three to fivefolds by 2030 to meet its 20% Ethanol Blended Petrol Program requirement. Bajaj Hindustan Sugar, Triveni Engineering & Industries Ltd., HPCL Biofuels Limited, India Glycols, Balrampur Chini Mills Ltd., Shree Renuka Sugars Ltd., Jeypore Sugar Company Ltd., Mawana Sugars Ltd., E.I.D Parry India Ltd., and Simbhaoli Sugars Ltd. are some of the key manufacturers in the Indian ethanol market. TATA Projects (an infrastructure company) got a project from BPCL (Bharat Petroleum Corporation Limited) for a bioethanol production plant with a capacity of 100,000 L/day in Bargarh, Odisha, India (Web Resource 1 2020). As the government of India is promoting cellulosic/agricultural feedstock for bioethanol production, it will be a boon for farmers to gain additional income from their agricultural waste. The burning of agricultural waste biomass causes air pollution, so the utilization of lignocellulosic feedstock sustains the country’s Ethanol Blended Petrol Program as well as reduces environmental stress of greenhouse gas emissions.

8.5 Conclusion

The future of bioethanol is knotted with a greater extent on metabolic and genetic engineering of microorganisms and nonfood crops used during fermentation. The main aim of such bioengineering practices is to evolve such microbes that are able to perform efficient saccharification of lignocellulosic waste biomass or nonfood crops. Fruit and vegetable residues contain a high amount of simple and complex carbohydrates, and these sugars can be used as a raw and fresh material for the production of bioethanol using microbial culture. Currently, 80% of bioethanol is produced from edible grains supplies such as sugar and starch. Recently, lignocellulosic biomass is considered for bioethanol production. More attention is needed toward cellulolytic enzyme production, as enzyme production charges more than 50% of the biomass saccharification cost. The enzyme used for cellulose hydrolysis can be improved by molecular engineering of enzymes themselves or genetic engineering of enzyme-producing microbes; so the production cost can be reduced. The second-generation bioethanol production uses two separate steps for saccharification and fermentation. SHF is favorable for these two steps to be carried out under their optimal conditions separately. For SSF, a microbe should be engineered or isolated to perform cellulose hydrolysis and ethanol fermentation simultaneously. Otherwise, co-cultures of two or more microbes could be utilized for combined saccharification and fermentation. Bioethanol produced from agricultural waste, fruit, and vegetable waste biomass is a sustainable fuel that obliging the engine to produce less greenhouse gas emissions. Fruit and vegetable waste has a lower cost and a wide range of availability, making it an excellent economical choice for bioethanol production.