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Sweet sorghum: a potential resource for bioenergy production      227


           sorghum bagasse (Chen et al., 2012; Choudhary et al., 2012; Marx et al., 2014).
           Chen et al. (2012) investigated the batch microwave-assisted ammonia heating sys-
           tem for the production of ethanol and observed that glucose and ethanol (from glu-
           cose only) yields were 42/100 g dry biomass and 21/100 g dry biomass,
           respectively. Similarly, 480 g/kg of ethanol was obtained from microwave-assisted
           H 2 SO 4 pretreated sorghum bagasse (Marx et al., 2014). Choudhary et al. (2012)
           reported that when sweet sorghum was subjected to microwave radiation to 1000 W
           for 4 min in presence of 0.01 g/mL lime, it resulted into release of 65.1% of total
           sugars.
              Ultrasound are sound waves with higher frequency of audible limit, which is uti-
           lized for chemical processing, medical, and diagnostic applications. Ultrasound
           from 20 kHz to 1 MHz is used in case of biomass pretreatment and other chemical
           processing. This pretreatment alters the surface structure and produces oxidizing
           radicals, which attacks the lignocellulose biomass. It also disrupts the lignin and
           polysaccharide structure, which forms cavitation bubbles and represents an efficient
           and eco-friendly pretreatment method for biomass (Hassan et al., 2018). In
           ultrasound-assisted dilute ammonia treatment, 2.0% (w/v) ammonium was subjected
           to sorghum stalk using ultrasound at 90 W and 50 kHz, wherein the highest enzy-
           matic hydrolysis sugar yield was approximately 57% (Xu et al., 2017).
              Gamma ray is obtained from radioisotopes of Cobalt-60 or Cesium-137. γ-Ray
           radiation successfully tested on different lignocellulose biomass, resulted in lignin
           modification and cellulose crystallinity reduction (Hassan et al., 2018). Another
           technique that is efficient for the pretreatment of lignocellulosic biomass and which
           has been used for decades in the food industry is the high hydrostatic pressure
           method. It is successfully applied in the United States, Europe, and Japan for pas-
           teurization of food products. In this treatment, biomass was subjected to a high
           hydrostatic pressure which is around 100 600 MPa. For instance, a combination of
           high hydrostatic pressure and fungal pretreatment was utilized to improve hydroly-
           sis of biomass (Albuquerque et al., 2016). However, this technology has higher ini-
           tial capital and operating costs (Hassan et al., 2018). Electron beam ionizing
           radiation is a type of energy in which accelerated beams of electrons are used to
           irradiate the biomass by disrupting the structural components (Hassan et al., 2018).

           10.3.6 Enzymatic hydrolysis

           Enzymes (cellulase and hemicellulase) play a significant role in the conversion of
           cellulose and hemicellulose to sugars. Reis et al. (2016) used commercial cellulases
           for saccharification of alkaline hydrogen peroxide pretreated sweet sorghum
           bagasse without supplementation of β-glucosidase, wherein the efficiency of enzy-
           matic hydrolysis was estimated to be 88.07%. Xu et al. (2017) observed enzymatic
           hydrolysis sugar yield at 56.9% with 4 h using cellulases in aqueous ammonia com-
           bined with ultrasonic sound pretreated sweet sorghum stalks. Wang et al. found that
           liquid hot water pretreatment on sweet sorghum bagasse removes 90% hemicellu-
           lose and enhances the cellulose conversion after addition of cellulase for hydrolysis.
           The authors also found that at high-solid loading of 15% and 20% and by
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