A comparison of methods of ethanol production from sweet sorghum bagasse

Evaluation of free and immobilized cellulase on chitosan-modified magnetic nanoparticles for saccharification of sorghum residue

Enzymatic hydrolysis plays a pivotal role in transforming lignocellulosic biomass. Addressing alternate techniques to optimize the utilization of cellulolytic enzymes is one strategy to improve its efficiency and lower process costs. Cellulases are highly specific and environmentally benign biocatalysts that break down intricate polysaccharides into simple forms of sugars. In contrast to the most difficult and time-consuming enzyme immobilization processes, in this research, we studied simple, mild, and successful techniques for immobilization of pure cellulase on magnetic nanocomposites using glutaraldehyde as a linker and used in the application of sorghum residue biomass. Fe3O4 nanoparticles were coated with chitosan from the co-precipitation method, which served as an enzyme carrier. The nanoparticles were observed under XRD, Zeta Potential, FESEM, VSM, and FTIR. The size morphology results presented that the Cs@Fe3O4 have 42.2 nm, while bare nanoparticles (Fe3O4) have 31.2 nm in size. The pure cellulase reaches to 98.07% of loading efficiency and 71.67% of recovery activity at optimal conditions. Moreover, immobilized enzyme's pH stability, thermostability, and temperature tolerance were investigated at suitable conditions. The kinetic parameters of free and immobilized enzyme were estimated as Vmax; 29 ± 1.51 and 27.03 ± 2.02 µmol min-1 mg-1, Km; 4.7 ± 0.49 mM and 2.569 ± 0.522 mM and Kcat; 0.13 s-1, and 0.89 s-1. Sorghum residue was subjected to 2% NaOH pre-treatment at 50 ℃. Pre-treated biomass contains cellulose of 64.8%, used as a raw material to evaluate the efficiency of reducing sugar during hydrolysis and saccharification of free and immobilized cellulase, which found maximum concentration of glucose 5.42 g/L and 5.12 g/L on 72 h. Thus, our study verifies the use of immobilized pure cellulase to successfully hydrolyze raw material, which is a significant advancement in lignocellulosic biorefineries and the reusability of enzymes.

Simultaneous Saccharification and Fermentation of Empty Fruit Bunches of Palm for Bioethanol Production Using a Microbial Consortium of S. cerevisiae and T. harzianum

A simultaneous saccharification and fermentation (SSF) optimization process was carried out on pretreated empty fruit bunches (EFBs) by employing the Response Surface Methodology (RSM). EFBs were treated using sequential acid-alkali pretreatment and analyzed physically by a scanning electron microscope (SEM). The findings revealed that the pretreatment had changed the morphology and the EFBs’ structure. Then, the optimum combination of enzymes and microbes for bioethanol production was screened. Results showed that the combination of S. cerevisiae and T. harzianum and enzymes (cellulase and β-glucosidase) produced the highest bioethanol concentration with 11.76 g/L and a bioethanol yield of 0.29 g/g EFB using 4% (w/v) treated EFBs at 30 °C for 72 h. Next, the central composite design (CCD) of RSM was employed to optimize the SSF parameters of fermentation time, temperature, pH, and inoculum concentration for higher yield. The analysis of optimization by CCD predicted that 9.72 g/L of bioethanol (0.46 g/g ethanol yield, 90.63% conversion efficiency) could be obtained at 72 h, 30 °C, pH 4.8, and 6.79% (v/v) of inoculum concentration using 2% (w/v) treated EFBs. Results showed that the fermentation process conducted using the optimized conditions produced 9.65 g/L of bioethanol, 0.46 g/g ethanol yield, and 89.56% conversion efficiency, which was in close proximity to the predicted CCD model.

Sweet sorghum for phytoremediation and bioethanol production

As an energy crop, sweet sorghum (Sorghum bicolor (L.) Moench) receives increasing attention for phytoremediation and biofuels production due to its good stress tolerance and high biomass with low input requirements. Sweet sorghum possesses wide adaptability, which also has high tolerances to poor soil conditions and drought. Its rapid growth with the large storage of fermentable saccharides in the stalks offers considerable scope for bioethanol production. Additionally, sweet sorghum has heavy metal tolerance and the ability to remove cadmium (Cd) in particular. Therefore, sweet sorghum has great potential to build a sustainable phytoremediation system for Cd-polluted soil remediation and simultaneous ethanol production. To implement this strategy, further efforts are in demand for sweet sorghum in terms of screening superior varieties, improving phytoremediation capacity, and efficient bioethanol production. In this review, current research advances of sweet sorghum including agronomic requirements, phytoremediation of Cd pollution, bioethanol production, and breeding are discussed. Furthermore, crucial problems for future utilization of sweet sorghum stalks after phytoremediation are combed.

Graphical Abstract

High cell density culture of Paracoccus sp. LL1 in membrane bioreactor for enhanced co-production of polyhydroxyalkanoates and astaxanthin

A cell retention culture of Paracoccus sp. LL1 was performed in a membrane bioreactor equipped with an internal ceramic filter module to reach high cell density and thus enhance the co-production of polyhydroxyalkanoates (PHA) and astaxanthin as growth-associated products. Cell retention culture results showed that PHA accumulation increased with increasing dry cell weight (DCW), giving rise to a maximum of 113 ± 0.92 g/L of DCW with 43.9 ± 0.91 g/L of PHA (38.8% of DCW) at 48 h. A significant increase in both intracellular and extracellular astaxanthin concentrations was also recorded during fermentation process achieving a maximum of 8.51 ± 0.20 and 10.2 ± 0.24 mg/L, respectively. Amounts of PHA and total astaxanthin produced by cell retention culture were 6.29 and 19.7-folds higher, respectively, than those recorded under batch cultivation. PHA and total astaxanthin productivities by cell retention culture also increased up to 0.914 g/L/h and 0.781 mg/L/h, respectively, which were 3.54 and 11.1-folds higher than those of batch culture. Based on gas chromatography, Fourier transform infrared spectroscopy, and ¹H nuclear magnetic resonance spectroscopy, the extracted PHA was identified as a copolymer of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with a 3-hydroxyvalerate content of 3.78 mol%.

Bioconversion of Sweet Sorghum Residues by Trichoderma citrinoviride C1 Enzymes Cocktail for Effective Bioethanol Production

Improved cost-effective bioethanol production using inexpensive enzymes preparation was investigated. Three types of waste lignocellulosic materials were converted—for the production of enzyme preparation, a mixture of sugar beet pulp and wheat bran, while the source of sugars in hydrolysates was sweet sorghum biomass. A novel enzyme cocktail of Trichoderma citrinoviride C1 is presented. The one-step ultrafiltration process of crude enzyme extract resulted in a threefold increase of cellulolytic and xylanolytic activities. The effectiveness of enzyme preparation, compared to Cellic® CTec2, was tested in an optimized enzymatic hydrolysis process. Depending on the test conditions, hydrolysates with different glucose concentrations were obtained—from 6.3 g L−1 to 14.6 g L−1 (representing from 90% to 79% of the CTec2 enzyme yield, respectively). Furthermore, ethanol production by Saccharomyces cerevisiae SIHA Active Yeast 6 strain DF 639 in optimal conditions reached about 120 mL kg d.m.−1 (75% compared with the CTec2 process). The achieved yields suggested that the produced enzyme cocktail C1 could be potentially used to reduce the cost of bioethanol production from sweet sorghum biomass.

Simultaneous nitrogen fixation and ethanol production by Zymomonas mobilis

This work reports on production of ethanol with simultaneous fixing of nitrogen (N₂) using the anaerobic bacterium Zymomonas mobilis DSM 473. A batch fermentation with an initial glucose concentration of 50 g L^-1, an initial pH of ∼5.5, an inoculum size of 10% by volume and a N₂ feeding rate of 50 mL min^-1 without mechanical agitation was found to provide the highest ethanol productivity (0.401 g L^-1 h^-1). Ethanol yield on glucose exceeded 97% of the theoretical maximum. The nitrogen content of the microbial biomass was 10.4% w/w at 65 h and all of it was derived by fixation of dinitrogen. Repeated-batch fermentations were investigated for ethanol production using simultaneous nitrogen fixation. A 2-cycle repeated-batch fermentation lasting 71 h gave a maximum ethanol yield on glucose of 0.475 g g^-1 and an ethanol productivity of 0.675 g L^-1 h^-1. The yield (0.415 g g^-1) and productivity (0.638 g L^-1 h^-1) were reduced in a 3-cycle repeated batch operation lasting 94 h. The need to fix nitrogen did not reduce the final achievable ethanol concentration, or the ethanol yield on glucose, relative to fermentations provided with fixed nitrogen, but did reduce the ethanol productivity by ∼82% because less cell mass was produced.