UMYU Journal of Microbiology Research

E-ISSN: 2814 – 1822; P-ISSN: 2616 – 0668

REVIEW ARTICLE

Review on Bioethanol Production using Diverse Substrates and Fungal Strains

¹Yusuf Munir Aliyu, ¹Bahauddeen Salisu Dandashire*, and ¹Kamaluddeen Kabir

Department of Microbiology, Umaru Musa Yar’adua University, PMB 2218, Katsina, Nigeria

*Corresponding author: bahauddeen.salisu@umyu.edu.ng

Abstract

The scarcity and unsustainable supply of fossil fuels in reservoirs prompt researchers to explore several alternative and sustainable energy sources from renewable feedstocks. Given the significance of bioethanol being produced in order to meet the energy demand, the available data is scattered, with little effort to condense the findings, which will be imperative to comprehend. This review highlights and summarizes various findings on bioethanol production. Published studies from 2000 to 2024 were reviewed. A total of 3,650 records were collected from various databases and sorted based on the title. Bioethanol has recently seen growing commercialization due to its market stability, low cost, sustainability alternative fuel energy composition, greener output and massive fossil fuel depletion but the major challenges that hindered bioethanol production are due to a lack of optimization which results in a lower yield of bioethanol produced and as a result, it cannot be applied for large scale production. The enzymatic capabilities of fungal strains are essential for Bioethanol production and can be enhanced through modern technologies such as synthetic biology and genome editing. Future research should concentrate on harnessing the capabilities of fungal strains to improve enzymatic hydrolysis and fermentation, particularly emphasizing strain engineering strategies that enhance sugar utilization and resistance to fermentation inhibitors.

Keywords: Fossil-Fuel, Bioethanol, Biomass, Renewable-feedstocks, Fungal strains

1.0 INTRODUCTION

The growing consumption of fossil fuels derived from petroleum resources raises questions about environmental impact and energy security (Falano et al., 2014). Researchers are looking for other methods to produce fuels from sustainable bioresources because of a number of issues, such as the global climate change caused by greenhouse gas emissions (Bezerra and Ragauskas, 2016). Socioeconomic progress and development in industrialized and emerging countries rely heavily on fossil fuels for power generation, yet this has various negative consequences (Dogru et al. 2020). Aside from these health and environmental concerns, the use of fossil fuels in energy generation systems exacerbates the problem of low power generation, which not only widens the gap between demand and supply but also decreases people's living standards (Gautam et al., 2019). More over 1.1 billion people, or 17% of the world's population, live in poverty and without access to power (Nabipour et al., 2020). Access to clean energy remains a luxury in many parts of the world (Douf et al., 2024). This has led to a surge in research, investment, and innovation in renewable energy technologies like solar, wind, and biofuels (Elia et al., 2021). Recently, there has been an increase in interest in producing bioethanol from biomass materials, which is one of the most cost-effective liquid fuel alternatives to non-renewable fossil fuels (Tekaligne and Dinku, 2019).

Biofuels are believed to have a lower "carbon footprint" than fossil fuels and contribute less to greenhouse gas emissions due to their CO₂-neutral conversion (Osman et al., 2024). However, there are various concerns with biofuels, including the chance that biofuel plants will displace food crops. This has a negative influence on food security, especially in developing nations (Vassilev et al., 2015). Biofuel has gained acceptability as a fuel alternative to fossil fuels as people become more aware of environmental issues. (EPA, 2023). To avoid conflicts between edible resources for human consumption and industrial uses, researchers have investigated the production of bioethanol from lignocellulosic material (Dahnum et al. 2015). Additionally, value was added by utilizing lignocellulosic biomass from weeds, agricultural waste, and micro- and macroalgal biomass (Ambaye et al., 2021).

Bioethanol is a renewable, colorless, less harmful, and rapidly biodegradable fuel derived from biological sources that can be used for heating, electricity, and fuel (Kida et al., 2023). It is now used as an alternative fuel since it is biodegradable, derived from renewable sources, has a high-octane number, and is less harmful than traditional petroleum-based fuels (Akhabue et al., 2018). Biomass energy has the potential to drastically reduce greenhouse gas emissions (Osman et al., 2024). According to Sebayang et al. (2016), bioethanol can be manufactured from a variety of raw materials that are divided into three groups based on their chemical composition: sucrose-containing feedstocks, starch materials, and lignocellulosic materials. Although cellulosic materials are more easily available and less expensive, the process of converting them into ethanol is costly due to the several steps required. In these circumstances, employing renewable substrates such as starchy byproducts, lignocellulosic biomass, and agricultural waste necessitates a novel approach (Broda et al., 2024). Many research studies have been conducted on waste items, especially fruit waste. Coconut trash and starchy bio waste (Bello et al., 2014; Hossain et al., 2017; Hashem et al., 2021)

The United States and Brazil are the world's major ethanol producers, accounting for 85% of the total (Alternative Fuels Data Center, 2016). The vast majority of ethanol in the United States is made from corn starch, whereas Brazil generally uses sugarcane (Pattanathu and Rahman, 2017). Thailand and China create bioethanol from cassava (Manihot esculenta), which is also an edible feedstock (Deesuth et al., 2015). However, because of their primary importance as food and feed, these traditional crops cannot match the global need for bioethanol production. To reduce human dependence on fossil fuels, efforts are being made to produce bioethanol from non-edible feedstocks such as lignocellulosic and starchy agricultural feedstocks (Aditiya et al., 2016).

Given the importance of bioethanol production around the world in meeting energy demand, data remains scattered, with little effort made to condense the findings, which will be critical to comprehend (Toor et al.,2020) in order to identify knowledge gaps and provide a roadmap for future directions. This review summarizes previous research on bioethanol production, including its physicochemical properties, various feedstocks, the role of fungal strains in bioethanol production, common waste biomass, pretreatment methods, and various fermentation conditions for bioethanol production.

2.1 Overview of bioethanol production

Bioethanol is widely produced through a variety of chemical and biological methods (Fan et al. 2012). The biological approach entails fermentation of biomass with ethanologenic microbes in anaerobic or semi-anaerobic conditions (Clain et al., 2016; Kumar et al., 2016). Fermentation is an ancient technology that refers to the bioconversion of carbohydrates into acid or alcohol via glycolytic intermediates. The bioprocessing of carbohydrate-containing feedstock is primarily done in two steps (Carvalheiro et al., 2024). The first step is the hydrolysis of polysaccharides into fermentable sugars, which are then converted to bioethanol using appropriate microorganisms (Dave et al., 2019). Furthermore, downstream processing includes bioethanol purification and concentration through the distillation process. A significant limitation of the production process is the lower concentration of bioethanol in the fermentation broth (Lassmann et al., 2014).

Bioethanol outperforms gasoline due to its high compression ratio, shorter burn time, and lean burn engine (Splitter et al., 2016; Carrillo-Nieves et al., 2019; Elshenawy et al., 2023). Octane number measures engine performance, with a higher number indicating better combustion (Ilves et al., 2019), because of its 35% oxygen content, ethanol emits fewer particulates, hydrocarbons, and NOx after combustion (Toor et al., 2020). Furthermore, bioethanol has a higher-octane number and combustion efficiency than gasoline, a small flame luminosity, corrosive nature, lower vapor pressure (making cold starts difficult), water miscibility, and ecosystem toxicity" (MacLean and Lave, 2003). The properties of ethanol are given in Table 1.

Table 1: Typical physicochemical properties of ethanol NCBI, (2025)

2.2 Bioethanol feedstock

A variety of biomass can be used to produce bioethanol, and these feedstocks fall into one of three categories (Table 2). Feedstocks that contain sucrose, such as sugarcane, sugar beet, and sweet sorghum; starchy substances, such as rice, wheat, corn, and barley; and cellulosic biomass, such as wood, forestry residue, straw, and grasses, are examples of the first three. (Toor et al., 2020). First generation: Biofuels are produced by fermenting sugar-based raw substrates or edible substrates.A refined fuel requires only a few basic processing steps and is typically made from grains, sugars, or seeds of which only a specific (usually edible) portion is used (Azhar et al., 2017; Derman et al., 2018). Second generation: Bioethanol is made from lignocellulosic biomass. The second-generation bioethanol processes use sugars released from cellulose, necessitating the use of enzymes to hydrolyze cellulose (Carrillo-Nieves et al., 2019; Rocha-Meneses et al., 2019). Under the second generation, various agricultural byproducts such as corn stalks or rice husks, wheat straw, rice straw, and non-edible plants such as trees or grasses grown specifically for energy, wood trimmings, sawdust, bamboo, cotton stocks, and other cellulose-containing biomass can be used to produce bioethanol (Derman et al., 2018; Carrillo-Nieves et al., 2019). Third-generation use algae as substrate for the production of bioethanol. It's still in its early stages (Jambo et al., 2019). Algal fuels' appealing characteristics is that, they can be grown with minimal impact on freshwater resources and can produce up to 300 times more oil than conventional crops (Yang et al., 2010). Fourth-generation biofuels use metabolic engineering or systems biology strategies in feedstock modification, such as E. coli gene modifications, which are more efficient than yeasts (Azhar et al., 2017; Jambo et al., 2019). The fourth-generation fuels include solar fuels or those that capture carbon from the process (Rastogi and Shrivastava, 2017).

Table 2: Bioethanol production from various feedstock.

2.3 Common waste biomass

2.3.1 Agricultural waste

Agricultural waste, among others, has become a major source of pollution in Nigeria. The use of agricultural waste as a renewable feedstock for bioethanol production has the potential to generate clean energy (Salisu and Umar, 2023). The agricultural waste contains a lot of carbohydrates that can be converted into bioethanol (Sahman et al., 2020). Agricultural waste is inexpensive, renewable, and abundant. Rice straw is one of the most widely used and abundant lignocellulosic feedstocks worldwide, particularly in Asia and Africa (Singh et al., 2024). Each year, approximately 667.6 million tons of biomass are post-harvested in Asia (Hossain et al., 2017). Along with rice straw, rice husk is being considered as a potential source for bioethanol production via yeast fermentation (Chavan et al., 2024). Bioethanol production from rice husk can reach 3.20 ± 0.36 g/l, with an ethanol yield of 0.27 g/g total sugar (Srivastava & Agrawal, 2014). Coconut waste biomass has been identified as another remarkable source. The maximum bioethanol yield of coconut waste was 90.09% and productivity was 0.21 g/L.h, derived solely from green coconut shell by Saccharomyces cerevisiae (yeast) fermentation (Hossain et al., 2017). Commercial bioethanol experiments using coconut waste are being carried out in the Northeast region of Brazil (Goncalves et al., 2015). Bioconversion of sorghum crop residues to ethanol has great potential for improving ethanol yield for sustainable bioethanol production. Sweet sorghum bagasse and juice yielded 157 and 121 L/tons of bioethanol, respectively, based on industrial production output (Nasidi et al., 2016). Sweet potato residue fermented with an amylolytic industrial yeast strain 1974-GA-temA produced 27.27 g/L ethanol (Wang et al., 2024). Steam-exploded corn Stover hydrolysate (SECSH) yielded 0.454 g/g and an ethanol concentration of 22.96 g/L (Wu et al., 2023). Other residues, such as wheat straw, corn straw, and cereal straw, can also be viable candidates for bioethanol production through fermentation (Swain et al., 2019)

2.3.2 Municipal Plant-based Waste Biomass

In terms of environmental cleanliness and public health safety, the R&D sector is currently focused on recycling and utilizing waste from municipal drainage (Hossain et al., 2017). Korea has already started a bioethanol production project using municipal waste and sludge from a local industrial complex (Park et al., 2010). Meanwhile, Sweden began producing bioethanol through fermentation from starch plants obtained from slurries and streams (Linde et al., 2008). Apart from industrial waste, bioethanol can also be produced from kitchen waste through a fermentation process. The sugars produced after hydrolysis of kitchen waste were mainly attributed to the monosaccharides, glucose (80%) and fructose (20%). The fermentable sugars obtained were subsequently used as a carbon source for bioethanol production by locally isolated yeasts, Saccharomyces cerevisiae, Candida parasilosis, and Lachancea fermentati. The yeasts successfully consumed the sugar hydrolysate and produced the highest ethanol yield, ranging from 0.45 g/g to 0.5 g/g and productivity between 0.44 gL−1h−1 - 0.47 gL−1h−1 after 24 hours of incubation, which was equivalent to 82.06 - 98.19% of conversion based on theoretical yield (Hafid et al., 2016)

2.3 3 Lignocellulosic biomasses

Lignin, hemicellulose, and cellulose are the primary components of biomass cell walls. Lignin is made up of a wide variety of phenolic polymers. Hemicellulose is a polysaccharide that consists of arabinose, acetic acid, and xylose linked together. According to Chundawat (2011), cellulose is a macromolecule composed of β-linked glucose molecules. All plant cell walls contain these components; the amount of each component varies only slightly, so any plant material can be used as a feedstock for sugar production. Table 3 shows the composition of lignocellulose biomass from various sources

Table 3. Composition of Lignocellulose Biomass from various Sources

2. 4 Role of fungal strains in bioethanol production

Fungal strains play an important role in bioethanol production. The fungus Aspergillus niger can degrade cellulose and convert paper waste into bioethanol, providing the required carbon, nitrogen, vitamins, and amino acids (Darwesh et al., 2020; Bellaouchi et al., 2021). The secretion of fungal amylase by yeast strains has enabled the conversion of raw substrate into ethEthanolowering production costs (Favaro et al., 2015). Furthermore, phlebioid fungal species enabled the bioconversion of lignocellulose waste, demonstrating the feasibility of single-step bioethanol production (Mattila et al., 2017). Fungi play an important role in both biomass pretreatment and sugar conversion to bioethanol, making them essential for efficient and sustainable bioethanol production. Cellulases and lignocellulolytic enzymes are known to be produced by Trichoderma, Penicillium, and Fusarium, which break down plant cellulose and hemicellulose into fermentable sugars. Hydrolytic enzymes enable a diverse range of fungi to break down carbon compounds (Lange et al., 2017)

Previous research (Table 4), has shown that fungal strains play an important role in bioethanol production, from biological pretreatment to the fermentation process. These strains harness the conversion of substrate biomass into ethanol. Oji et al. (2024) found that fermenting yeast (g/L) with 6% banana peel yielded 44.68±0.82% bioethanol after 3 days at 5.5 pH and 35°C. Shitophyta et al. (2023) reported that Banana peel was fermented with Saccharomyces cerevisiae and Rhizopus oryzae at room temperature for 120 hours, with yeast concentrations of 2, 3, and 5 g/L. R. oryzae produced more ethanol than S. cerevisiae. Water Hyacinth (Eichhornia crassipes) co-cultured with A. oryzae, A. niger, and S. cerevisiae produced approximately 56% more ethanol than S. cerevisiae - single culture and S. stipitis - single culture (Shakila Begam et al., 2024). Cassava peel prepared with Saccharomyces cerevisiae and Zymomonas mobilis. Cassava peels produced a high percentage yield of 30% in 45 mL of ethanol (Behingbe et al., 2021). Sweet potato residue fermented with an amylolytic industrial yeast strain named 1974-GA-temA yielded 27.27 g/L ethanol over 8 days (Wang et al., 2024). Steam-exploded corn stover hydrolysate (SECSH) produced with Saccharomyces cerevisiae had an ethanol concentration of 22.96 g/L and a yield of 0.454 g/g (Wu et al., 2023). Ethanol was optimally produced at 12% substrate concentration using rice chaff, at a temperature of 35 °C and pH of 5.0 (Adeyemo et al., 2021). Ragi husk as a substrate for Aspergillus fumigatus JCM 10253 cellulase production demonstrates potential for value-added industrial products and lignocellulosic bioethanol production (Saroj and Narasimhulu, 2020). A new strain of Trametes villosa from the Paranaense rainforest efficiently hydrolyzes barley straw to produce bioethanol, potentially lowering the overall cost of bioethanol production (Coniglio et al., 2020). Yeast co-culturing of Saccharomyces cerevisiae, Pichia barkeri, and Candida in pairs or triples significantly increases bioethanol production from starchy biowastes, reaching 167.80 0.49 g/kg of biowaste during experiments in a 7-L fermenter (Hashem et al., 2021). The maximum ethanol concentration and ethanol productivity values for cellulose-rich corncob (CRC) residence with S. cerevisiae were 31.96 g/L and 0.222 g/L/h, respectively (Boonchuay et al., 2021). Lignocellulosic hydrolysate produced by Aspergillusniger, Zymomonas mobilis, and Trichoderma longibrachiatum yields the highest bioethanol yield from lignocellulosic biomass, indicating promising pathways for sustainable biofuel technologies (Bendaoud et al., 2024).

Table 4: Summary of bioethanol production from different fungal strains

2.5 Biological conversion of lignocellulosic biomass to bioethanol

2.5.1 Pretreatment process

There are various pretreatment methods (Table 5) increase cellulose reactivity and the potential yield of fermentable sugars (Edeh, 2021). These could be either traditional or advanced pretreatments. Traditional pretreatments are divided into four categories: chemical, physical, physicochemical, and biological, whereas advanced pretreatment methods can be acid-based fractionation or ionic liquid-based fractionation (ILF) (Maurya et al., 2015). Mechanical pretreatment is the process of reducing the size of biomass particles to reduce the crystallinity of the lignocellulose and increase the accessible surfaces, thereby promoting subsequent hydrolysis. According to Abo et al. (2019), lignocellulosic material is typically ground to less than 2 mm fragment size. Biological pretreatment is the use of microorganisms to break down lignocellulosic biomass before further enzymatic hydrolysis by organisms which include white-rot, brown-rot, and soft-rot fungi, as well as bacteria (Hassan et al., 2018). Chemical pretreatment uses a variety of chemical reagents, including acids, bases, and oxidizing agents. The impact on lignocellulosic material varies according to the chemical reagent used (Abo et al., 2019). The primary challenge of these pre-treatment processes is to make cellulose easily accessible while avoiding harsh conditions that could lead to sugar degradation.

Table 5: Review of various pretreatment processes for bioethanol production

2.5.2 Hydrolysis

The hydrolysis process is the first and limited step in converting insoluble biopolymers into soluble organic complexes like oligomers and monomers, depending on the microorganisms used in anaerobic digestion, the hydrolysis step of the process may be rate-limiting (Ma et al., 2013). During the hydrolysis reaction, proteins are degraded into amino acids, carbohydrates are hydrolyzed into monosaccharides, and fatty acids are obtained by hydrolysis of lipids by the enzyme’s proteases, cellulases, or amylases, and lipases, respectively (Kumar and Anand, 2019), it is the most important fungus used in biotechnological applications worldwide. It has been discovered that Aspergillus strains may produce a range of enzymes (Mostafa et al.; 2016; Sattar et al., 2019), including cellulase, and amylase (Ahmad et al., 2024; Saeed et al., 2025).

2.5.3 Fermentation

Fermentation is a biological process in which microorganisms such as yeast, fungi, or bacteria convert the monomeric sugar units obtained during the hydrolysis step into ethanol, and gases (Sharma and Lorrache, 2020). After the biomass has been digested by enzymes, microorganisms such as yeasts and bacteria ferment sugars such as galactose, fructose, glucose, and mannose to produce ethanol (Gonzalez et al. 2024). Yeast species can make bioethanol from sugar fermentation, despite Saccharomyces cerevisiae being the most common sugar fermenter (Walker and Walker, 2018). While Scheffersomyces stipitis uses lignocellulose substrates (Liang et al., 2013) or algal biomass (Obata et al., 2016), According to Parapouli et al. (2020), S. cerevisiae's distinct biological characteristics, such as its capacity to ferment and create alcohol and CO2, as well as its tolerance to adverse osmolarity and low pH, make it ideal for biotechnological applications. Biomass with high lignocellulose content is used as feedstock, providing an alternative fermentation method (Mishra et al., 2019). Until recently, combinations of bacteria and yeast (Mishra et al., 2019; Wang et al., 2019), yeast and yeast (Ntaikou et al., 2018; Singh et al., 2014), or fungi and yeast (Paschos et al., 2015; Izmirlioglu et al., 2017) were used in co-cultures for simultaneous saccharification and co-fermentation. Co-cultures have also been investigated as a technique to increase ethanol yield (Mishra et al., 2019; Izmirlioglu et al., 2017). The Table 6 shows some instances of different microorganisms employed in simple sugar fermentation, as well as their corresponding ethanol yields at varying operating conditions.

Table 6: Various fermentation condition for bioethanol production

2.6 Types of fermentation techniques

2.6.1 Simultaneous saccharification and fermentation (SSF)

The simultaneous saccharification and fermentation (SSF) design consists of a single reactor in which both hydrolysis and fermentation take place. Adopting this type of solution overcomes the inhibition problem observed in separate hydrolysis and fermentation (SHF), as glucose and cellobiose are gradually used during their manufacture (Mazzeo and Piemonte et al., 2020). Recently, the simultaneous saccharification and fermentation method has been used, which combines biomass saccharification with simultaneous sugar fermentation in a single reactor (Rastogi and Shrivastava, 2018). Kumagai et al. (2014) also reported that the development of an SSF process was ideal for producing ethanol from Hinoki cypress and Eucalyptus after fibrillation via steam pretreatment and subsequent wet-disk milling.

2.6.2 Batch processing or culture

This system involves inoculating a batch of culture medium with microorganisms. After a certain amount of time, the fermentation process is complete, and the product is harvested. At the start of the stationary phase, the culture is disbanded to recover its biomass (cells, organisms) or the compounds that accumulated in the medium (alcohol, amino acids), and a new batch is established (Behl et al., 2023). Due to these inherent disadvantages and lower yields, the commercial market believes in shifting to other fermentation techniques (Puligundla et al., 2018; De Araujo Guilherme et al., 2019; Liu et al., 2019).

2.6.3 Fed-Batch culture

In fed-batch fermentation, the feed rate is limited, so the cell mass density is not increased excessively (Azhar et al., 2017). As a result, the cell mass concentration must be maintained at a specific level to ensure the highest ethanol productivity (Ariyanti et al., 2014; Moshi et al., 2014; Phukoetphim et al., 2018). The fed-batch system adds a fresh aliquot of medium on a continuous or periodic basis, without removing the culture fluid. The fermenter is designed to handle increasing volumes. The system is always in quasi-steady state.

2.6.4 Continuous fermentation

Continuous fermentation produces more ethanol than batch fermentation (Phwan et al. 2018). Continuous culture is an open system in which nutrients are added to the bioreactor aseptically and continuously while the culture broth (containing cells and metabolites) is removed at the same time. The volume of the culture broth remains constant due to a constant feed-in and feed-out rate (Kuene, 2019). Continuous operations are generally easier to control and less laborious than batch operations, but there is a serious contamination issue with this operating method (Mahboubi et al., 2017; Carrillo-Nieves et al., 2019).

2.6.5 Solid state fermentation

Solid state fermentation conditions are ideal for growing microbes such as bacteria, yeasts, and filamentous fungi on solid substrates, increasing their potential for use in bioprocesses (Ortiz et al., 2016; Marín et al., 2019; Salom~ao et al., 2019). Solid State Fermentation is the controlled growth of microorganisms in the absence of free water. Solid State Fermentation products include industrial enzymes, fuels, and nutrient-rich animal feeds. The use of modern biotechnical knowledge and process control technologies can result in significant productivity gains from this ancient process. Solid state fermentation reduces the risk of bacterial contamination by eliminating free water; more concentrated enzymes are produced, which can be extracted with a small amount of water (Kapilan, 2015).

2.7 Factors affecting bioethanol production

Temperature, sugar concentration, pH, fermentation time, rate of agitation, and inoculum size are all factors that influence bioethanol production (Zabed et al., 2014). However, one of the most important factors influencing the amount of ethanol produced is the temperature during fermentation. Previous studies (Piarpuzan et al., 2014; Garcia et al., 2015) found that the ideal fermentation temperature ranges from 30 to 38ºC. Thus, the temperature is precisely controlled throughout the fermentation process. The temperature is precisely controlled. Furthermore, high temperatures can denature the tertiary structure of enzymes that regulate microbial activity and the fermentation process, making them inactive (Lopez-Trujillo et al., 2023). There have also been reports of using enzymatic hydrolysis to accelerate sugar release (Piarpuzan et al., 2014). Several conditions, including steam purging, microwave and ultrasonic wave treatment, have been proposed for acid or alkaline prepretreatmentth hydrochloric acid or aqueous ammonia (Garcia et al., 2014; Gabhane et al., 2015).

2.8 Ethanol Recovery

Bioethanol is produced in a diluted state (Saini et al., 2020), thus water and other contaminants must be removed to obtain a fuel-grade ethanol product (Aditiya et al., 2016). Bioethanol can be recovered at a variety of temperatures: (i) at or near fermentation temperature; (ii) slightly higher than fermentation temperature that does not harm microorganisms or hinder enzyme activity (Saini et al., 2020). Ethanol recovery begins with a standard distillation process, which produces azeotropic ethanol. Furthermore, dehydration and purification stages are used to produce fuel-grade ethanol, which uses a large amount of energy and has high operational costs, limiting the economic feasibility of lignocellulosic ethanol on a commercial scale (Saini et al., 2020). Ethanol separation is the most expensive and energy-intensive phase in the ethanol production process (Zentou et al. 2019). The energy required for ethanol recovery and purification varies with the concentration of ethEthanol the feed stream (Saini et al., 2020). The energy required for ethanol separation has been calculated to be between 12-15% and 35% of combustion energy for input streams containing 12 and 4 wt% ethEthanolespectively (Granjo et al., 2020). Membrane technology employs semi-permeable barriers that exploit the principle of selective permeability, which is widely used in the purification of bio-based products (Méireleset al., 2016). Azeotropic distillation helps separate azeotropic mixtures into their pure constituents (Saini et al., 2020). Ethanol purification involves two primary steps: ethanol pre-concentration and ethanol dehydration (Chandra et al., 2018). The mechanism of the separation into these two distinct phases is that ethanol-water mixtures exhibit azeotropic behavior by mass fraction (Habaki et al., 2016).

CONCLUSION AND FUTURE PERSPECTIVE

The world is experiencing significant global warming due to the widespread use of fossil fuels. Bioethanol has recently seen increased commercialization due to its market stability, low cost, sustainability, and greener output, as well as its potential to reduce fossil fuel depletion. However, the major challenges that have hampered bioethanol production are a lack of optimization, which results in a lower yield of bioethanol produced and, as a result, it cannot be used for large-scale production. This review has offered a complete understanding of physicochemical features, diverse feedstocks, the role of fungal strains in bioethanol production, common waste biomass, pretreatment procedures, and various fermentation settings for bioethanol production. The steps necessary for making bioethanol as the cost-effective, reliable, and widely available biofuel for a growing global population. The invention of bioethanol was hailed as a great breakthrough in converting waste biomass to fuel energy, hence lowering the widespread usage of fossil fuels. The production efficiency of bioethanol from diverse substrates, including sugar-based, starchy by-products, cellulosic biomass, and agricultural waste, will necessitate an innovative method. The enzymatic capabilities of fungal strains are critical, and can be further improved by implementing novel technologies such as synthetic biology and genome editing to develop superior microorganisms. Further research should investigate the potential of fungal strains for improved enzymatic hydrolysis and fermentation, with an emphasis on strain engineering to improve sugar usage and inhibitor tolerance.

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