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Sweet sorghum: a potential resource for bioenergy production      229


           (73%), soluble sugars (13%), cellulose (5.3%), hemicellulose (3.7%), and lignin
           (2.7%). After the first cut of sweet sorghum around 50 120 t of stalks can be
           produced. Maximum amount juice around 70% can be extracted from sweet sor-
           ghum stalks and the residues left over after extraction, that is, bagasse, constitu-
           tes 15.33 MT/ha (Sipos et al., 2009). Sucrose, glucose, and fructose are the main
           constituents of the juice and can be directly used for ethanol fermentation
           (Guiying et al., 2000; Kim and Day, 2011). The solid residues left over after
           extraction of juice from sweet sorghum stalks can be used for ethanol produc-
           tion. The grain in sweet sorghum mainly contains starch, which can also be used
           for ethanol fermentation. Even though sweet sorghum can be considered as a
           potential feedstock for ethanol production, its yield varies with genotype, tem-
           perature, geographical location, and fertility of soil (Umakanth et al., 2019).
           Fig. 10.6 reveals the ethanol production efficiency using various parts of sweet
           sorghum biomass.
              Cellulose, hemicellulose, and lignin are the main constituents of sweet sorghum
           bagasse and can be considered as lignocellulosic waste material for ethanol fermen-
           tation. As sweet sorghum juice present in its stalks is fermented directly to ethanol,
           bagasse requires pretreatment by physical, chemical, or biological methods to partly
           remove lignin and further hydrolyzed using enzymes, namely, cellulases and hemi-
           cellulases, to produce fermentable sugars. Then, yeast addition results in ethanol
           production and different microorganisms required for both pentoses and hexoses
           (Ray et al., 2019).
              Matsakas used dried sweet sorghum stalks and liquefied using commercial cellu-
           lases (Celluclast 1.5 L) for ethanol production (62.5 g/L) at high-solid concentration
           using submerged fermentation (Matsakas and Christakopoulos, 2013b). To achieve
           high hydrolysis yield, hydrothermal pretreatment was employed prior to liquefac-
           tion. The results revealed that saccharification at high solids load of 18% dry matter
           yielded ethanol of 41.4 g/L at 12 h of hydrolysis time (Matsakas and
           Christakopoulos, 2013a). Khalil et al. (2015) evaluated five varieties of pretreated
           sweet sorghum biomass with H 2 SO 4 at 120 C for 1 h for ethanol production.

           Fermentation was carried out using Saccharomyces cerevisiae, Zymomonas mobilis,
           or mixed culture of both. SS-301 variety yielded 160 mL ethanol/kg sweet sorghum
           juice and bagasse. Zhang et al. (2019a) carried out simultaneous saccharification
           and fermentation (SSCF) using sweet sorghum juice and acid pretreated (2%
           H 2 SO 4 ) bagasse to produce 67.5 g ethanol/kg of fresh sweet sorghum stalk. Direct
           cofermentation of microwave pretreated sweet sorghum bagasse liquid was carried
           out using S. cerevisiae and Z. mobilis in acid conditions for 24 h and maximum eth-
           anol yield of 480 g/kg with 94% conversion efficiency (Marx et al., 2014). Yu et al.
           (2016) studied the influence of alkali pretreated sweet sorghum bagasse without
           washing on enzymatic hydrolysis using Ctec-3 and Htec-3 and production of etha-
           nol using Z. mobilis TSH-ZM-01. Goshadrou et al. found a reliable fungus Mucor
           hiemalis, an alternative to baker’s yeast for ethanol production from hydrolysates of
           NaOH-ultrasound pretreatment followed by enzymatic hydrolysis (commercial cel-
           lulase and β-glucosidase) yielded 81% of theoretical ethanol yield in 24 h
           (Antonopoulou et al., 2012).
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