Impact of Alkali Pretreatment Methods of Bagasse from Different Sweet Sorghum Varieties for Bioethanol Production

The uniqueness and relevance of the current work involve the production of bioethanol from the lignocellulosic biomass of two popular sweet sorghum varieties, viz., CSV19 and CSV24, using a yeast hybrid strain SP2-18, generated through genome shuffling. Bioethanol production from sweet sorghum bagasse varieties -CSV19 and CSV24 were examined utilizing three different alkali pretreatment techniques (PT1, PT2, and PT3). To determine the influence of the pretreatment procedure, a combination of alkali and hydrogen peroxide, with or without steam, was evaluated on the biomass of two promising varieties CSV19 and CSV24. To deconstruct the biomass, pretreatment procedures such as PT1 (2% NaOH, steam, and H2O2), PT2 (2% NaOH and H2O2), and PT3 (2% NaOH and steam) were used. The results revealed that bagasse processed with PT1 produced more bioethanol than the other two pretreatments, PT2 and PT3. Furthermore, Fourier-Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) were used to investigate the structural changes in biomass following pretreatment. The FTIR and SEM characterizations demonstrated that the microstructural changes of the pretreated sweet sorghum bagasse significantly differed from those of the untreated biomass. Enzymatic hydrolysis studies were also carried out on the pretreated sweet sorghum biomass (PT1, PT2, and PT3). Enzymatic hydrolysis yielded total reducing sugar concentrations of 58.62 g/L and 52.89 g/L in CSV19 and CSV24 sweet sorghum varieties respectively. Finally, we demonstrated that the resulting hydrolysate was used as a substrate and fermented with hybrid yeast strain SP2-18 derived from the parental strain Saccharomyces cerevisiae to convert the reducing sugars into bioethanol, a promising intermediate to biofuels and bioproducts. Genome shuffled yeast hybrid SP2-18 produced higher bioethanol productivity of 0.91 gL−1 h−1 and 0.85 gL−1 h−1 for CSV19 and CSV24 varieties of sweet sorghum respectively, as compared to parental strain S. cerevisiae which had bioethanol productivity of 0.56 gL−1 h−1 and 0.47 gL−1 h−1 respectively for sweet sorghum varieties of CSV19 and CSV24.