Open access peer-reviewed chapter
Abstract
Augmenting value-added products generation with the biorefinery process of sugar cane by utilizing the by-products helps to achieve a more sustainable model of the sugarcane industry and in turn, contributes to the circular economy. Among the value-added products produced from sugarcane waste, functional foods offer additional health benefits besides their nutritional and calorific value. In recent years non-digestible sugars gained interest as potential prebiotic functional foods which benefit the host without increasing calorific value. These sugars are produced by the breakdown of carbohydrate polymers like cellulose and xylan, by thermochemical treatment or by enzymatic hydrolysis, or a combination of both. Sugar cane bagasse (SB) is an economical source of xylan which can serve as the substrate for xylooligosaccharides (XOS), xylobiose, xylitol, and ethanol. Cellulases, xylanases, and ligninases have wide applications in food processing, agro-fiber, pharmaceutical, and the paper and pulp industries including nutraceuticals production, where these enzymes provide eco-friendly alternatives to some chemical processes and help to reduce environmental impact. Conventional thermochemical methods for nutraceuticals production require chemicals that result in the release of toxic byproducts thus requiring additional steps for refining. In this context, the sustainable and eco-friendly processes for the production of nutraceuticals require employing biocatalysts like microbial enzymes or microbes as a whole, where in addition to averting the toxic byproducts the refining process requires lesser steps. The present chapter discusses the current research and challenges in the production of value-added products from sugarcane byproducts and their contribution to the sustainability of the sugarcane industry.
Keywords
- sugar cane industry
- bagasse
- value added products
- sustainable processing
1. Introduction
The quest for environmentally friendly and economically feasible alternatives for conventional fuel resources was necessitated by the increase in price hikes and climate change caused by fossil fuels for many years [1]. An increase in the production cost of ethanol due to an increase in prices of food/feed-derived substrates (food grains, sugar molasses) poses a challenge to sustainability in a long run. These challenges have forced researchers to look into other approaches for a biofuel supply that is both affordable and sustainable. Sugarcane straw, sugarcane pulp/bagasse, and sugarcane residues may be great options of feedstock for the production of second-generation (2G) biofuel (Figure 1) [2].
Figure 1.
Procedural flow diagram for the bioconversion of cane biomass into 2G ethanol. These renewable, high-carbohydrate raw material sources do not compete with the demand for food or animal feed. It is still difficult to market cellulosic ethanol due to the limitations in efficient bioconversion of SB/SS (efficient pre processing techniques, deconstruction of cellulosic fiber, and bioconversion of generated sugars). Extensive pre-treatment and the emergence of the effective biological conversion process, which incorporates strains of ethanol-producing bacteria that can convert pentose and hexose sugars, are among the technological hurdles that must be addressed [
The significant productivity gains, product diversification, and a decrease in the environmental effect of fields of sugarcane crop, sugar industries, distillation facilities, and traditional sugar manufacturing by evaporation, resulted in the global sugar sector currently exhibiting inertia in production and weak sustainability [6]. The inclusion of numerous factors, which include quantity and quality-related aspects, estimated by various measurements, or at least the creating standard units for benchmarking, highlights the complexity of sustainability monitoring in the sugar market [7]. Sustainability success can be assessed by the improvements in productivity of sugar within the existing plant area simultaneously reducing ecological impact with socially acceptable and effective coordination between stakeholders, markets, and policies made for the public across nations which will positively impact the environmental, societal, and financial benefits [7].
Sugarcane yields one of the highest amount of sugars per hectare as a commercial crop grown in tropical and sub-tropical regions that is highly productive in harvesting sun energy, atmospheric carbon dioxide, water, and nutrients, basically Nitrogen, into biomass and simple pentoses like sucrose [2]. The production of food for human consumption, animal feed, biofuels, value added products, and highly specialized commercial products can be attained with potentially and commercially profitable raw materials. Employing various agricultural by-products and sugarcane by-products such bagasse, sugarcane tops and molasses, can lead to the production of a number of value-added products. This comprises a variety of enzymes, organic acids, amino acids, pigments, animal feed, composite, chelating agents, alkaloids, bioethanol, biodiesel, biobutanol, 2, 3-butanediol, biohydrogen, bioelectricity, and biopolymer. In addition to catering the needs of more than 80% of the sugar consumed globally in excess to 100 countries [7]. The largest producer of sugarcane in the world is Brazil (40 percent of world production). India, Thailand, China, Mexico, and Pakistan, round out the list of nations that produce sugar [2, 7].
The negative perception of refined sugar being identified as a potent causative agent in lifestyle diseases like hypertension, overweight, and insulin resistance, the lack of stability in the market of refined sugar as a commodity, the impact of increasingly popular use of alternative sweeteners like high fructose corn syrup HFCS, Stevia (
Nevertheless, sugarcane farming, which is a subject of agro-economic research, is the most pursued research topic in the conventional linear industrial economy in cane agriculture [3]. The sugar agroindustry stakeholders are working together to fully exploit the mechanical and environmental features of biorefineries, sugar mills, and distilleries in spite of concentrating on the effects of field practices on the environment when taking the Sustainable Developmental Goals (SDGs) into account [9, 10].
2. Chemical composition and properties of sugarcane
The sugar cane plant’s variety and the type of cultural management are some of the elements that impact the stem’s length, which can range from less than 2 m to over 4 m in size in an adult stem; the stem’s diameter changes as well, fluctuating between 250 and 350 m in the middle [4]. The color is influenced by agronomic factors, anthocyanin content, and chlorophyll content; the sugarcane material has a wide range of moisture contents, ranging from as high as 82.3 percent (tops) to as low as 13.5 percent (dry leaves); the three components of the straw have similar values for fixed carbon, volatile matterand ash with cane bagasse having a lower value for ash; virtually all substances have the same percentages of carbon (45%), nitrogen (0.5–1%), hydrogen (6%), oxygen (43%), and sulfur (0.1%) [8]. The three parts of the SS’s mineral composition differ slightly for alkalis and phosphorus, showing that the SS’s content grows from the tops of the dried leaves and is significantly higher than SB [4, 11].
As per estimates, sugar cane bagasse comprises 38.9% glucan, 23.9% Lignin, 20.6% xylan, 5.6% arabinan, and 11% others. The relative compositions of major lignocellulosic fractions i.e., cellulose, hemicellulose, and lignin depends on numerous aspects, which includes plant genetics, growing environments, processing conditions, and methodologies used for the compositional analysis [12], therefore the fact that relative composition of plant polymers varies within the same type of plant material is not surprising. The composition of samples from various origins, taken by various laboratories using various methodologies, cannot be compared (Figure 2) [11].
Figure 2.
General chemical composition of Sugarcane.
3. Pre-treatments for conversion of sugarcane bagasse
3.1 Physical pre-treatments
3.1.1 Milling
A mechanical preparation called milling reduces the crystallinity of cellulose and disrupts the structure of lignocellulosic materials [13]. The most used technique is ball milling, which involves reducing the size of the particles by contacting the biomass with balls. Because this process does not require the addition of chemicals and does not produce inhibitors, it can be regarded as environmentally friendly. The high-power requirements of the equipment and ensuing high energy expenditures are a drawback of milling. For sugarcane bagasse pre-treatment, numerous cycles and miller passes are required, and the cycles often last for a longtime [13, 14].
3.1.2 Pyrolysis
High temperatures (greater than 300°C) are used during the pyrolysis process. Rapid cellulose degradation into H2, CO, and leftover char results from this mechanism [14]. Following the removal of the char, the recovered solution is predominantly made up of glucose, which can subsequently be fermented to
produce ethanol. Operating temperature, residence time, rate of heating, reaction time, type of sweeping gas, reactor type, type and amount of catalyst, and flow rate are some of the variables that affect the quality and yield of products following pyrolysis and feedstock characteristics.
3.1.3 Microwave
Pre-treatment using microwave technology is regarded as a substitute for conventional heating. Microwave pre-treatment involves application of electromagnetic field producing high heating efficiency and simple operation, in contrast to conventional heating methods that use surface heat transfer [15]. The quick times of reaction and uniform heating of the reaction mixture are this technique’s key benefits.
3.2 Physicochemical pre-processing
3.2.1 Hydrothermal treatment or steam explosion
This is among the most used pre-treatment techniques for pre-treatment of substrates (also known as hydrothermal). When it comes to environmental problems, this pre-treatment involves little or no chemical input, making it a promising technology. With low lignin solubility, steam explosion treatment results in high hemicellulose solubility (generating primarily oligosaccharides). A possible method to increase the amount of fermentable sugars is the steam explosion procedure followed by enzymatic saccharification [16].
3.2.2 Ammonia fiber explosion (AFEX)
Combining steam explosion and ammonia in liquid form make up the AFEX process. It is an alkaline thermal treatment that involves rapidly releasing pressure after rapidly exposing the lignocellulosic substrate to high pressure and temperatures simultaneously. This pre-treatment can considerably increase the rate of fermentation of different grasses and herbaceous plants. The key benefits of AFEX are its effective lignin removal, low production of inhibitors, and significant carbohydrate retention in the substrates. Additionally, it is a quick and easy method [17]. The material’s structure is altered during the AFEX, increasing its capacity to store water and its susceptibility to enzyme digestion of substrates (hemicellulose and cellulose), resulting in a high sugar recovery rate.
3.2.3 CO2 explosion
In accordance to the idea that CO2 can generate carbonic acid and speed up the pretreated material’s hydrolysis, the CO2 explosion happens similarly to the ammonia explosion. Because CO2 explosion uses mild temperatures throughout the process and prevents any significant monosaccharide breakdown, it has relatively higher conversion efficiency than a steam explosion, is more economically feasible than ammonia explosion, and does not produce xenobiotic inhibitors. This process is safe for the environment, non-flammable, and nontoxic. It is a method with complicated process and challenging operation, nevertheless [13].
3.2.4 Treatment with hot water
With this technique, a significant portion of the hemicellulose fraction is removed while under high pressure conditions hot water hydrates the cellulose in the lignocellulosic biomass. The absence of chemicals in this process makes it unnecessary to utilize materials that resist corrosion in the hydrolysis reactor, which is one of its key advantages. Additionally, it’s not necessary to reduce the raw material’s size [18].
3.2.5 Acid pre-treatment
The most widely utilized acid, H2SO4, encourages hemicellulose hydrolysisto xylose and other sugars when it comes into contact with biomass [16, 18]. Other acids, such as HCl, nitric acid, phosphoric acid, and oxalic acid also demonstrated positive outcomes. Typically, the process can be carried out at temperatures between 120 and 180°C and residence periods between 15 and 60 minutes [19]. The acid pre-treatment method has the benefit of operating at low and medium temperatures, which reduces energy expenditures.
3.2.6 Alkali pre-treatment
Alkali pre-treatment is a delignification method that also significantly solubilizes hemicellulose. It uses a variety of bases, such as sodium hydroxide, calcium hydroxide (lime), potassium hydroxide, ammonia hydroxide, and sodium hydroxide combined with hydrogen peroxide or other substances. Compared to other pre-treatment technologies, this method uses lower temperatures and pressures, but pre-treatment takes hours or days. Alkali pre-treatment’s efficacy varies with the substrate and pre-treatment circumstances in addition to severity of chemicals used. In general, lignocellulosic substrates with low lignin content such as gramineous crops, herbaceous crops and hardwoods, gives better yields with alkaline pre-treatment compared to softwoods with high lignin content [20].
3.2.7 Oxidative delignification
In this process, the peroxidase enzyme catalyzes the breakdown of the lignin in the presence of H2O2. A wide range of biomass, including corn stover, barley straw, sugarcane bagasse, wheat straw, rice straw, and bamboo have all undergone this pre-treatment process [21].
3.2.8 Ozonolysis
The hemicellulose and lignin fractions of lignocellulosic substrates like wheat straw, pine, bagasse, cotton straw, peanut, and sawdust of poplar can be broken down using ozone. The advantages of ozonolysis pre-treatment include the efficient removal of lignin, the lack of harmful residues for the subsequent procedures, and the fact that the reactions take place at room temperature and pressure [22]. However, the process is costly because a lot of ozone is needed.
3.2.9 Organo solvent treatment
One of the most promising approaches for pre-treating lignocellulosic materials is the Organo solvent procedure. When compared to other processes of a similar nature, the organo solvent procedure produces less waste and consumes less chemical energy to neutralize the hydrolysate. As a catalyst, substances like NaOH or Na2SO3 may be utilized [23]. This technique has been shown to remove lignin with great efficiency and high carbon dioxide pressure.
3.2.10 Wet oxidation
The sodium carbonate catalyst is the most common one employed in the wet oxidation process, which takes place in the presence of oxygen or catalysed air. High quantities of biomass can be converted into monosaccharides through wet oxidation, with little to no furan and phenolic aldehyde production. With an increase in aliphatic acids during the wet oxidation process, delignification is reportedly documented. This preliminary care is deemed pricey [24]. The primary benefit of this pre-treatment is its combination with alkalis, which allows released sugars to be obtained without the production of furfural and 5-hydroxymethylfurfural, two molecules that are unfavourable for fermentations.
3.2.11 Evaporation (concentration)
A physical method of detoxification that minimizes the concentration of volatile chemicals like acetic acid, vanillin and furfural, is the concentration of hydrolysate by evaporation by applying vacuum [25]. However, this procedure has the drawback of making extractives’ non-volatile hazardous chemicals more prevalent.
3.2.12 Use of membranes
Membranes have a number of benefits over traditional extraction. Membrane adsorption inhibits the mixing of the organic solvent (solvent phase), which is likely to inhibit microbial growth and survival, with the aqueous phase (hydrolysate) [26]. The membranes’ internal pores have functionally active surface groups linked to them, which can help eliminating the common metabolic inhibitors like acetic acid, furfural, 5-hydroxymethylfurfural (HMF), formic acid, levulinic acid, and sulfuric acid (Figure 3) [27].
Figure 3.
Pre-treatment of sugarcane for the production of eyhanol/Biofuel [
3.3 Biological treatment
The biological technique makes use of particular enzymes or microorganisms that modify the inhibitory chemicals in the hydrolysate. Enzyme usage is a well-researched and promising technique. White rot fungi-derived laccase and peroxidase enzymes have been discovered to be efficient at removing phenolic chemicals from lignocellulosic hydrolysates. Inhibitor chemicals from lignocellulosic hydro-lysates have also been removed using microorganisms [29]. Yeasts, fungus, and bacteria are among the microorganisms that can ingest inhibitory chemicals naturally. While keeping cellulose and hemicellulose in the substrate, these bacteria can efficiently break down lignin. Another name for this process is in situ microbial delignification (ISMD). Another intriguing replacement for the detoxification process is the adaption of a bacterium to a non detoxified hydrolysate. This approach relies on a series of fermentation where the microbe from one experiment serves as the inoculum for the subsequent one [12]. Utilizing specialized microbes lowers the cost of detoxification and prevents the loss of fermentable sugars.
3.4 Bioconversion of hexose sugars into ethanol
Hexose sugars are bio transformed into ethanol. In the biological process of ethanol fermentation, carbohydrates are transformed by microorganisms into ethanol and CO2. Even though there are numerous techniques and procedures for using lignocellulosic materials to produce ethanol, it is still challenging to produce economically viable amounts of ethanol from lignocellulosic wastes. Yeasts are the microorganism most frequently employed in the fermentation process, and Saccharomyces cerevisiae is the yeast most commonly utilized for ethanol fermentation. This yeast can thrive on the disaccharide sucrose as well as simple sugars like glucose.
Figure 4.
Bioconversion steps for the production of ethanol from Sugarcane [
3.5 Bioconversion of pentose sugars into ethanol
Pentose sugars are bio-converted into ethanol. In order to develop an affordable and practical conversion technique for the manufacture of bioethanol from sugarcane bagasse (SB) and sugarcane straw, the maximum utilization of all sugar fractions is necessary (SS). The hemicellulose fraction must be fermented at the same conversion rates as the cellulose fraction in order to get the necessary ethanol yields from Sugarcane bagasse or sugarcane straw hydrolysates. In order to absorb pentose sugars, the enzyme D-xylose reductase (EC 1.1.1.21) converts xylose to xylitol, which is then immediately oxidized by the enzyme xylitol dehydrogenase (EC 1.1.1.9), yielding D-xylose-5-phosphate. Transaldolase, transketolase, and ribulose phosphate-3-epimerase progressively transform D-xylose-5-phosphate into fructose-6-phosphate and glyceraldehyde-3-phosphate via non-oxidative rearrangement, leading to the synthesis of ethanol by the Emden reaction -Meyerhoff Route [30, 33].
3.6 Distillation of ethanol
It is required to recover the ethanol from the fermenting broth. Ethanol (5–12 weight percent) and water make up the final medium [34]. Conventional distillation methods cannot separate the ethanol and water because they create an unfavourable mixed system. Distillation, rectification, and dehydration are the three processes in the ethanol purification process. In the first two processes, a highly concentrated ethanol solution is produced (approximately 92.4% wt), and the mixture is subsequently dehydrated to produce anhydrous ethanol. Azeotropic distillation, liquid–liquid extraction, extractive distillation, adsorption, or more sophisticated hybrid separation techniques can all be used to achieve the dehydration.
4. Sustainability in economizing the biorefinery of the sugarcane industry
The majority of sugarcane mills operating now are plants that also produce ethanol; the other plants only produce ethanol (autonomous distilleries). Through innovative framework, the so called Virtual Sugarcane Biorefinery, technical, environmental, and economic effects of these first-generation sugarcane processing facilities can be examined [35]. Optimization methods have the ability to improve economic outcomes and lessen environmental consequences for both autonomous and annexed facilities when compared to basic scenarios. Further while taking into account the average pricing over the previous 10 years, annexed facilities that diverted more sugarcane juice for the manufacturing of sugar were more lucrative. Additionally, findings show that scenarios involving more flexibility in an annexed plant are more economic. If price hikes were to occur, this alternative would be more profitable than the typical annexed one (diverting 50% of the sugarcane juice to sugar and 50% to ethanol production) [35, 36]. This helps to comprehend the true advantages of the sugarcane plant’s adaptability by quantitatively demonstrating the advantages of optimization strategies.
As a consequence of the sugar beet processing, sugar beet pulp has a lot of potential as a bioeconomy component because it can be transformed into a number of useful products and by-products. The pulp is manufactured in vast quantities, it is inexpensive, underutilized, and has a desirable chemical make-up. High polysaccharides fraction is one of the beneficial components that are associated with the latter. However, it should be emphasized that the composition of sugar beet pulp might vary, depending on things like the weather. The conversion of beet pulp into bioethanol is one method for creating products with added value [37]. Comparing the use of sugar beet pulp to selling it as feed may boost its economic feasibility. Lower energy prices could have positive effects because pulp used to make biofuels does not need to be dried. Simultaneous saccharification and fermentation were employed to produce ethanol from sugar beet pulp [28].
An opportunity for both the provision of sustainable energy and the reduction of greenhouse gas emissions is provided by the biorefining of sugar beet pulp biomass for bioenergy generation. Life cycle assessment (LCA) of numerous energy-focused biorefinery scenarios and a few conversion routes were carried out to evaluate the significance of the alternative use of biomass leftovers, including sugar beet pulp (two involving bioethanol and two biogas). As an input to the LCA, specialized biochemical models are designed to establish precise mass, energy, and substance balances for each biomass conversion route [28, 32]. The results of this study generally confirmed the findings of other studies that highlighted the environmental benefits of conversion pathways including electricity and heat provision.
The standard sugar beet business was to be restructured into a revolutionary biorefinery where the sugar beet pulp can be successfully fractionated into pectins, phenolic compounds, and a sugar-rich hydrolysate, which can then be used as a fermentation feedstock for the production of succinic acid, an essential platform chemical for the growth of sustainable chemical industry and a precursor for the creation of numerous bulk chemicals, polymers, and resins [37].
Sugarcane bagasse, a waste by-product from the sugar processing industry can be used for the production of a solid fuel that has a high calorific value of around 28.2 MJ/kg using torrefaction technology [28, 32]. Due to its high fixed carbon content of 76 percent and high heating value, bio coal can be utilized in place of coal. A high calorific value bio coal with the potential to be produced from sugarcane bagasse was looked into as a potential coal substitute for the sugar industry [36]. Bagasse, a lignocellulose waste product of the sugar-processing industry, has the energy potential to make environmentally beneficial bio coal. The yield of biochar during torrefaction was 70%. The biochar can be ground to a 300-μm sand size, compressed at 2–8 MPa using molasses in a 70:30 ratio, and finally turned into bio coal. The bio coal had a calorific value of 28.2 MJ/kg, 6.3 percent moisture, 74.6 percent fixed carbon, and 1.4 percent ash. Due to the characteristics of bio coal, it can be used as an alternative to coal and reintegrated back into the sugar manufacturing sector [32, 37].
The sugarcane bio coal has a high burning period of 30 minutes and a low ignition time of 10 s. Biocoal is an eco-friendly fuel due to the low pollutants emitted during burning.
5. Conclusions
In numerous nations sugarcane, straw and bagasse (SB) are appealing second-generation renewable feedstock options. If used wisely, this feedstock might offer a steady supply of single-cell proteins, organic acids, drop-in ethanol, industrial enzymes, and other products. However, a sizeable portion of this biomass is used by businesses to generate steam and power. The remaining portion is the appropriate source of raw materials for producing high-value commodities. Due to the rapid advancements in downstream processing technologies, pre-treatment strategies, and advanced microbial biotechnology over the past three decades, it is now possible to use sugarcane wastes for the large-scale manufacturing of a variety of products without endangering the needs for food and feed. The greatest obstacle to the efficient use of these leftovers is recalcitrance/resistance of biomass to bioconversion by enzymatic fermentation. Pre-treatment is a necessary technique to improve the accessibility of carbohydrate polymers to enzymes for the subsequent hydrolysis reactions to produce fermentable sugars in order to overcome the biomass recalcitrance. There are a number of effective pre-treatment techniques available; however, the final decision regarding the pre-treatment process will depend on the efficiency of delignification or hemicellulose removal, economic viability, the minimal amount of inhibitor generation, time savings, low sugar loss, and the ability to cause the least amount of environmental pollution. Following enzymatic degradation and hemicellulose depolymerization, the sugars that were liberated are transformed into ethanol by the appropriate ethnologic strain. The ethnologic strains should be able to use pentose and hexose sugars, be resistant to inhibitors, and have good osmotic tolerance in order to produce the appropriate ethanol yields. The following six requirements are pivotal in order to establish a long-term sustainable second-generation ethanol production.
Choosing the best pre-treatment and detoxification plan
Development of ethanol-producing strains and in-house cellulase enzymes
Production from pentoses like xylose, arabinose and hexose sugars that exhibit inhibitor tolerance, ethanol resistance, and quicker sugar conversion rates.
Process optimization and intensification: combining fermentation and hydrolysis in one location.
Quick, affordable, and efficient distillation of ethanol.
Combining sugar/distilleries with bioethanol production facilities to share machinery, reactors, and other equipment.
Acknowledgments
Author K. Ravichandra is greatful for funding support from DBT, Govt of India and JNTU Kakinada, Andhra Pradesh.
Authors Mr. K. Rosangzuala, Ms. Rveena Gajjala, and Mr. Patlolla Ravinder Reddy acknowledge the support from AcSIR, CSIR-IICT, Hyderabad.
Dr. Linga Banoth acknowledges the support from DST-SERB EMEQ.
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