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ABSTRACT
The policy of the Government of India to reduce pollution and fuel-import cost by blending up to 20% ethanol and 80% gasoline as an automotive fuel which came into force 8 March 2021 (Nair, 2021). In India there is continuous demand of alternative fuels to produce bioethanol from various feedstock’s. Currently, alcohol is produced only with molasses as the only feedstock. In addition, molasses also costs from Rs. 2000 to Rs. 5000 ton-1 excluding the cost of producing ethanol from molasses, which also varies. In addition, the ethanol production process is also not good for the environment because molasses also comes from the sugar industry as a byproduct. Like grain sorghum, the sweet sorghum can accumulate up to 20% of sugars in its stalks (Hunter and Anderson 1997) and can be a great alternative as a feedstock with minimal crop duration and water requirement as compared to sugarcane which is used to extract molasses. After extracting sweet sorghum juice, the bagasse contains higher calorific content. Hence, it can generate around 3.25 MW ha-1 of electricity (Hunsigi et al. 2007). In addition, several countries are looking for the steps to boost sweet sorghum’s production. Since stalk juice will only be used for producing ethanol, it will not affect food security. According to the Planning Commission of India, sweet sorghum is the best alternative to molasses for producing ethanol without affecting the environment in India (Ratnavathi et al. 2003., Planning Commission, GOI. 2006). This article is aimed to explore the environmental and economical benefits of sweet sorghum and some challenges in ethanol production by conducting research on various studies and to find out the best steps that the Government of India can take to save the environment while fulfilling energy needs.
INTRODUCTION
Like grain sorghum, accepting its sweet and juicy stalk, sweet sorghum is being grown for fodder in Africa and for syrup in USA over the centuries and is widely expected to meet the demands for fiber, fuel, feed, and food and is known as the bioethanol feedstock for the future. Some yields of sweet sorghum can achieve 78% of juice off total biomass of the plant with up to 23% of soluble fermentable sugar (Rao et al. 2009). The sugar consists of up to 80% of sucrose, glucose, and fructose. Sugars are mostly distributed evenly in the stalk with up to 2% in the inflorescences and leaves (Vietor and Miller 1990), making crop amenable to direct the extraction of fermentable sugar. On the other side, sweet sorghum is a plant that comes from C4 species with an elliptical and curved head and wide and flat leaves. At the time when it matures, it is full of grain. Like grain sorghum, it has been cultivated for around 3000 years and it can be grown easily in semiarid tropics, where it is not possible to grow other crops. Barren dryland areas are highly suitable for this crop.
Irrigation can produce truly high yields. Sweet sorghum can be latent in very dry months and continue their growth when they get proper moisture back (Gnansounou et al. 2005). It can easily thrive in sub-tropical, tropical, semi-arid, temperate, and even in poor quality soils on all continents. It is well regarded as the “camel of crops” and “sugarcane of the desert” as it can tolerate heavy drought. It also has higher water use efficiency than maize and sugarcane with minimal inputs of water and fertilizer on low-quality soils (Vinutha et al. 2014).
Fig 1: Comparison of sweet sorghum with other bioethanol feedstocks
Refrences: K.S. Vinutha et al. 2014 and M.E Melobane et al. 2018
International Crops Research Institute for Semi-Arid Tropics (ICRISAT) was the first behind a revolutionary attempt to identify and assess high biomass sweet sorghum germplasm in India (Seetharama and Prasad 1987). The All India Coordinated Sorghum Improvement Project (AICSIP) and National Research Center for Sorghum (NRCS) were the leaders in sweet sorghum improvement initiative that ran for over two decades and helped in developing several breeding lines to release various varieties of sweet sorghums like SSV 84, CSH 22 SS (NSSH 104), etc. ICRISAT brought a lot of breeding materials, experimental hybrids with higher stalk sugar, varieties, and biomass yields of sweet sorghum (Reddy et al. 2005). In 2007, ICRISAT introduced an international BioPower project to bring various opportunities for the dryland poor population in renewable energies. The collaboration of NARS with the Philippines, Mali, and private partnerships in the USA, Brazil, Mexico, and Germany has brought many opportunities. ICRISAT is aimed to produce cultivars with hybrids parent development to deal with abiotic and biotic stresses while promoting the value chains of sweet sorghums (Saharan et al. 2016).
DISCUSSION
Water Saving BioEnergy Crop – The millennium development goals (MDGs) of UNs provide a blueprint for preservation of natural resources, improvement of livelihoods, and environment till 2015. The leading development institutions in the world and UN member states came into contract under MDGs. However, none of them had any plan for power conservation as it is the fuel for economic growth and helpful in controlling poverty. This way, Reddy et al. (2007) propose the idea of growing water-saving bio-energy crops of sweet sorghum to eradicate extreme poverty in rural areas, improve farmers’ incomes, fulfill their energy needs, and help 75% of the 2.5 billion poor population of the world. Energy is important for social needs like healthcare and education, consumption purposes like lighting, cooking, entertainment, heating, etc., industries, transportation like rail, road, and air, and agriculture sectors. In most developing nations, agricultural practices mostly rely on human and animal energy to a great extent. It is important to provide easy access to renewable resources like wind, solar, and biofuel to the agricultural sector for farm productivity. Relying on fossil fuels is not going to provide economic, environment, and social benefits due to carbon footprint and high prices related to their use. On the other side, the biofuels based on agriculture biomass provide eco-friendly and sustainable energy solutions (Reddy et al. 2007).
Sweet sorghum as biofuel feedstock – Burning fossil fuels leave a lot of carbon footprint to the environment in the form of global CO2 levels which, as a result, causes global warming with significant changes to the weather and climate across the world. In addition, there is a limited availability of fossil fuels on this planet. Discovery of recent fossil fuel reserves and fuel recovery technologies may extend the timeline of fossil fuel depletion but the planet would no longer be able to sustain the catastrophic effects of combustion. So, it is very vital to look for sustainable sources of energy which can mitigate catastrophic effects on Planet Earth while fulfilling energy needs (Mathur et al. 2017).
Sweet sorghum is a C4 crop which requires minimal input and yields high volume of sugar in stalks and is an ideal crop for producing biofuel. However, large varieties of optimized traits related to biofuel and tolerance to abiotic and biotic factors are required for extensive plantation on trivial lands. A lot of studies have been conducted to generate genomic and genetic resources for the production of sweet sorghum. Mathur et al. (2017) discuss several qualities of sweet sorghum making it best for producing biofuel feedstock and present genetic diversity, resources, and tools provided for marker-based breeding and/or engineering. There has been a lot of progress in identifying quantitative trait loci/genes required for molecular breeding and agronomic traits for producing more varieties.
Sweet sorghum as a bioenergy crop – Sweet sorghum is well regarded for its great potential to produce ethanol and biofuel. This sugar crop can produce more ethanol per unit of land than several other crops, while requiring minimal input. Sweet sorghum can easily thrive in unfavorable growing conditions like salinity, water logging, water deficiency, alkalinity and other limitations. The sweet sorghum can yield 6000 L ha-1 of ethanol and can attain three units of energy per unit invested.
Sweet sorghum has been produced for centuries as a syrup crop and its production is not unknown. It is available in different varieties and diversities like juice sucrose concentration, Brix % (13e24), fresh stalk yield, etc. Producing sweet sorghum for biofuel is affected by insufficient technology that can help in large-scale transport, harvest, and storage of such a huge amount of juice and biomass produced, especially in terms of a short harvest window. Regassa and Wortmann (2014) recommend fermenting chopped stalks directly or fermentation of juice extracted from stems to convert sweet sorghum into ethanol. However, many developing countries have yet to integrate distillation and fermentation of the juice extracted from sweet sorghum into ethanol plants.
Production of bioethanol from sweet sorghum – It is evident that using biofuel like bioethanol may help in reducing gasoline imports and carbon footprint in the environment. It can also serve as an alternative to Methyl tert-butyl ether (MTBE) and lead which contaminate the air and underground water. Plants can meet the expected demands for bioethanol. A comparative study is needed about technology solutions with various feedstocks. This way, Almodares and Hadi (2009) and other studies explain that it is possible to use sweet sorghum as feedstock for producing ethanol under dry and hot climates.
This crop can easily withstand drought and salty regions as compared to corn and sugarcane that are being used for producing biofuel worldwide. In addition, sweet sorghum stalks contain a similar amount of carbohydrates to that of sugarcane with minimal fertilizer and water input requirements. Additionally, sugarcane cannot tolerate salt. The stalk of sweet sorghum contains highly fermentable sugar which is more ideal for fermentation. So, the authors recommend sweet sorghum for plantation and biofuel production in dry and hot climates to reduce gasoline imports, greenhouse gases, and solve high gasoline octane issues.
Regulation of Sugar in Sweet Sorghum Crop – Sweet sorghum contains high amounts of juice and sucrose in its stem. Hence, there is a great research opportunity in its biochemistry, molecular value of sucrose, and physiology. The association between accumulation of sucrose in its stem and photosynthetic property in its leaf is yet to be studied. Communication between sink and source may be important to control synthesis of sucrose. Siddique et al. (2018) recommend the process of assimilate partitioning to export photosynthesis extracted from leaves via small veins for the storage of tissues. The researchers are aimed to analyze how to improve partitioning of carbohydrates by expanding plant or source capacity or by expanding silk quality. Expanding the production of carbohydrate may possibly raise up light interception either by rising total leaf area or number of leaves, enhancing photosynthesis capacity of plants, and evergreen traits. The authors explore various qualities of sweet sorghum making it the best crop for producing syrup, ethanol, and biofuel and also discuss available resources for breeding and engineering of sweet sorghum and genetic diversity.
Sweet sorghum-a potential alternate raw material for bio-ethanol and bio-energy – Sweet sorghum is a special crop which needs minimal input and yields output much like sugarcane with its sugary stalk. This crop is adaptable to a huge scale along with high sugar accumulation, significant growth, and potential for biomass production. Since the availability of water becomes a major limitation for agricultural produce for several years, it is not easy to cultivate sugarcane everywhere. On the other side, sweet sorghum has become a sustainable crop instead of sugarcane, especially in areas receiving dry climates. Sweet sorghum requires less rainfall and irrigation along with inputs than sugarcane. This crop may yield sugar content up to 1623% Brix. It can produce syrup, jaggery, and even ethanol. According to Reddy et al. (2005), the stillage can be used for power generation after extracting juice from this crop.
Ethanol production from sweet sorghum – The widespread use of fossil fuels across the world leads to global warming and greenhouse gases and there is an urgent need to switch to renewable and eco-friendly resources. Water scarcity and food security are the major limitations in using crops for biofuel production. Hence, it is important to grow crops that can be grown with minimal input of water and that don’t risk food security, while yielding high volume of fermented sugars. This way, sweet sorghum is an ideal crop that can yield both energy (in the form of sugary juice extracted from stems) and food grains. In addition, it is possible to use sorghum bagasse for producing ethanol. This way, Mutepe (2012) is aimed to determine the amount of sugar in different varieties of sorghum at various harvest periods along with cultivars that can yield the highest volume of ethanol at ideal fermentation environment.
Sweet Sorghum and Nitrogen Fertilizer Application – In every aspect, sweet sorghum is much like its grain variant with accumulated sugar in the stalk like sugarcane. Hence, sweet sorghum is well regarded as a power crop. Renewable energy sources have widely become a global interest and emerging trend because of increasing concern on the environment. It is very vital to improve productivity of energy crops to meet the rising demand, especially by practicing proper agronomic management. According to Olugbemi et al. (2017), nitrogen fertilizer has come as a vital nutrient to improve productivity of crops which can also influence the production of sweet sorghum. The application rates required are affected by soil type, genotype, and climate along with location and seasons. However, studies suggest that the level of fertilizer from 60 to 120 kg N ha-1 would yield great productivity as per the soil type, location, and N value of the soil according to soil test.
Nutrient Requirements and Use Efficiency by Sweet Sorghum – Sweet sorghum can be the source of renewable energy for ethanol production. According to R.P. Wiedenfeld (1984), sweet sorghum viability can be influenced with nutrient use efficiency and nutrient requirements as renewable energy resource as fertilizer is the major input for non-renewable energy. There is a growing need to come up with alternative energy sources due to declining fossil fuel reserves. With a great potential being a chemical feedstock, phytomass has been a renewable resource. There is a possibility to obtain fermentable carbohydrates from several crops, such as sugarcane and grains to produce ethanol. It is not feasible to divert those crops for production of alcohol as these are important sources of feedstock and food supplies. Currently, sweet sorghum is produced for livestock feed and table syrups, irrespective of improper growth. Sweet sorghum can easily withstand different ecological conditions and it has minimal input requirements, high growth potential, and great carbohydrate content. There has been an increased focus in research on its usage as a supplement of sucrose for producing crystalline sugar and when milling capacity has been affected for processing sugarcane. Cultivar is well regarded to have more favorable aspects of disease residence, sugar content, purity, and vigor.
Sweet Sorghum: A Potential Alternate Raw Material for Bio-ethanol and Bioenergy – Reddy et al. (2005) explains that there is a great adaptability of sweet sorghum with high sugar content, biomass production, and rapid growth potential. It has a sugary stalk almost like sugarcane. Since the availability of water is all set to become a major hurdle in agricultural production in a few years to come, sugarcane cultivation has become a challenge. Hence, sweet sorghum is much more likely to be a smart crop option instead of sugarcane. It is possible to cultivate sorghum with less rainfall and irrigation requirements and inputs than sugarcane. Sweet sorghum also has high sugar content from 16% to 23% Brix. It can also be used for syrup, jaggery, and fuel alcohol production. It is possible to use stillage after juice extraction for power generation.
Features of sweet sorghum juice and their performance in ethanol fermentation: With the growing production of fuel ethanol and its demand to unexpected levels, there would be a great shortage of feedstock for the production of ethanol. This way, sweet sorghum has come as the best feedstock for production of ethanol in the Midwest and Southeast. Around 18% of fermentable sugar can be extracted from sweet sorghum juice and it can be fermented directly as yeast into fuel ethanol. But to achieve the highest sugar value, the harvest period is too short and it cannot be stored for a long time as it degrades its sugar content rapidly. These are some of the technical issues of producing biofuels with sweet sorghum. According to Wu et al. (2010), sweet sorghum juice may lose around 20% of fermentable sugar within 3 days at room temperature due to contaminated bacteria and it can also decline pH values and increase bacterial count. This study observed no major changes in sugar contents, pH value, and sugar content when sweet sorghum juices are stored in the refrigerator. The fermentation qualities of autoclaved juice, fresh juice, and concentrated juice having 20% sugar were known to be higher as compared to 93% in the batch process of shake flask.
Structure and composition of sweet sorghum stalk components – Billa et al. (1997) analyzed the pith and stem bark of sweet sorghum for production of cellulose, sucrose, hemicelluloses, reducing sugars, phenolic acids, and lignin contents. In addition, syringyl and guaiacyl are monomeric units in non-condensed structures exemplified with thioacidolysis and cell wall was determined with alkaline hydrolysis at the temperature of 170C. According to the results, it was found that the pith and bark are heterogeneous and structure of chemical compositions is focused. In addition, the pith value is almost double in water-soluble sugars as compared to the one in bark.
Photosynthetic Regulation Under Salt Stress and Salt-Tolerance Mechanism of Sweet Sorghum – Sweet sorghum (Sorghum bicolor (L.) Moench) is a C4 crop with high yields and fastest growth, making it an ideal source of feedstock, food, fuel and fiber. Sweet sorghum survives and raises its sugar level on salty land. Hence, it is well known as the rich source of salty genes. Yang et al. (2020) review the biochemical and physiological responses of this ethanol-source to salt stress like hormonal regulation, sucrose synthesis, photosynthesis, and ion homeostasis, along with their salt-resistance properties. The researchers found some benefits of salt-resistant sweet sorghum – (1) Maintains high sugar content in saline lands by covering the photosystem structures, inhibiting sucrose degradation, and enhancing sucrose synthetase activity, and photosynthetic performance, and (2) improve Na+ exclusion to retain ion homeostasis in salty regions in roots and resulting in low Na+ value in shoots. Such genes can help raise the salt tolerance with genetic engineering and breeding.
Sweet sorghum – A six-year field experiment on low and high input production in north-eastern Poland – Sweet sorghum is well regarded as the power crop with low input requirements due to great tolerance to saline soils and can easily battle with maize monocultures. Jankowski et al. (2020) present a field experiment for six years on the energy efficiency and biomass production of sweet sorghum at various levels of agricultural inputs in Northeastern Poland. In high-input production, there was higher dry matter yield, i.e. 15.4?Mg ha?1. In low-input technology, biomass yield was 16% lower. The authors compared production technologies to determine the energy needs in sweet sorghum biomass production with energy inputs of farming operations. The lower power consumption (14.9-15.8?GJ ha?1) resulted in low-input technology. High-input technology increased energy inputs by 46% in production of sweet sorghum. The sweet sorghum biomass gains energy from 170 to 226 GJ ha-1from low-input to high-input technologies respectively. The energy efficiency in low-input technology was found higher in sweet sorghum biomass in North-Eastern Poland.
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