Journal of Energy Bioscience 2025, Vol.16, No.3, 117-127 http://bioscipublisher.com/index.php/jeb 121 5 Microbial Fermentation for Ethanol Production 5.1 Microorganisms used: yeast (e.g., Saccharomyces cerevisiae), engineered strains There are two types of microorganisms commonly used in the process of converting sorghum biomass into ethanol. One is traditional yeast, such as Saccharomyces cerevisiae; the other is engineered bacteria. Studies have found that if the conditions are well controlled, S. cerevisiae can produce about 68 grams per liter of ethanol from sorghum straw. Another yeast, Pachysolen tannophilus, can also produce about 56 grams per liter of ethanol (Sathesh-Prabu and Murugesan, 2011). In addition to yeast, scientists have also used some engineered strains of Escherichia coli. These bacteria can process both hexose and pentose sugars, which increases ethanol production and allows for more efficient use of raw materials (van Rijn et al., 2018). 5.2 Inhibitors from sorghum processing: fermentation challenges due to phenolics and acids During the process of processing and decomposing sorghum biomass, some byproducts, such as phenols and organic acids, are produced. These substances interfere with the fermentation process, hinder the normal growth of microorganisms, and reduce ethanol production (Batog and Wawro, 2022; Joy and Krishnan, 2022). Some pretreatment methods slow down yeast metabolism, which in turn affects ethanol production. To improve the yield, it is necessary to study better treatment methods to remove these inhibitors. 5.3 Strategies for optimization: co-fermentation, fed-batch, immobilization In order to reduce the impact of inhibitors and to produce more ethanol, researchers have tried many improvement methods. One is co-fermentation, such as "simultaneous saccharification and fermentation" (SSCF). This method can utilize hexose and pentose sugars at the same time, making the utilization rate of sugar higher and increasing ethanol production (van Rijn et al., 2018; Joy and Krishnan, 2022). Another is fed-batch fermentation. This method is suitable for use when there are a lot of solid materials. It can help microorganisms maintain their vitality and increase the ethanol concentration. For example, under the condition of 20% solid loading, the yield can reach 36 grams per liter (Joy and Krishnan, 2022). There is also cell immobilization technology. This method can make microorganisms more resistant to inhibitors, and the fermentation process is more stable and efficient (Sathesh-Prabu and Murugesan, 2011; Joy and Krishnan, 2022). 6 Environmental and Economic Considerations 6.1 Life cycle assessment (LCA): GHG reduction potential, water and land use Sorghum has performed well in reducing greenhouse gas emissions when used to produce fuel ethanol. Many LCA studies have found that using sweet sorghum or grain sorghum to make ethanol can reduce greenhouse gas emissions by more than half compared to gasoline. If combined with a combined heat and power (CHP) system and recycling by-products (such as distiller's grains and liquid fertilizers), the emission reduction effect can be increased to more than 70% (Cai et al., 2013; Spatari et al., 2018). Compared with traditional raw materials such as corn, sorghum has similar emission reduction capabilities, and in some places even better (Cai et al., 2013; Spatari et al., 2018; Kent et al., 2020). Sorghum is also more drought-tolerant and has strong adaptability. It can be planted in marginal land and will not compete with food crops for good land and water, which helps alleviate the problem of "food and energy competing for land" (Rivera-Burgos et al., 2019; Batog et al., 2020). 6.2 Economic feasibility: cost breakdown: cultivation, processing, infrastructure For sorghum ethanol to be profitable, it depends on several key costs. For example, planting, harvesting, transportation, and subsequent pretreatment, enzymatic hydrolysis, fermentation, and the cost of building a factory (van Rijn et al., 2018; Wirawan et al., 2024). In 2018, Vermerris' team conducted a study and analysis and found that if the sweet sorghum residue is properly handled, the price of the ethanol it produces can be close to the energy equivalent price of gasoline. If the cost of enzymes is reduced in the future, or the by-products are better developed, the economic benefits will be even higher (van Rijn et al., 2018). In some countries such as Indonesia, there are some small sorghum ethanol plants. These factories use the entire sorghum plant, including the grains, leaves and residues, to produce not only ethanol but also many other products. The return on investment of these factories can reach 28%, and the cost can be recovered in 4 years, which shows that this model is also quite cost-effective (Wirawan et al., 2024).
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