Persson Michael, Galbe Mats, Wallberg Ola
Department of Chemical Engineering, Lund University, P.O. Box 124, 221 00 Lund, Sweden.
Biotechnol Biofuels. 2020 Sep 5;13:156. doi: 10.1186/s13068-020-01791-z. eCollection 2020.
The integration of first- and second-generation bioethanol processes has the potential to accelerate the establishment of second-generation bioethanol on the market. Cofermenting pretreated wheat straw with a glucose-rich process stream, such as wheat grain hydrolysate, in a simultaneous saccharification and fermentation process could address the technical issues faced during the biological conversion of lignocellulose to ethanol. For example, doing so can increase the final ethanol concentration in the broth and mitigate the effects of inhibitors formed during the pretreatment. Previous research has indicated that blends of first- and second-generation substrates during simultaneous saccharification and fermentation have synergistic effects on the final ethanol yield, an important parameter in the process economy. In this study, enzymatic hydrolysis and simultaneous saccharification and fermentation were examined using blends of pretreated wheat straw and saccharified wheat grain at various ratios. The aim of this study was to determine the underlying mechanisms of the synergy of blending with regard to the yield and volumetric productivity of ethanol.
Replacing 25% of the pretreated wheat straw with wheat grain hydrolysate during simultaneous saccharification and fermentation was sufficient to decrease the residence time needed to deplete soluble glucose from 96 to 24 h and shift the rate-limiting step from ethanol production to the rate of enzymatic hydrolysis. Further, a synergistic effect on ethanol yield was observed with blended substrates, coinciding with lower glycerol production. Also, blending substrates had no effect on the yield of enzymatic hydrolysis.
The effects of substrate blending on the volumetric productivity of ethanol were attributed to changes in the relative rates of cell growth and cell death due to alterations in the concentrations of substrate and pretreatment-derived inhibitors. The synergistic effect of substrate blending on ethanol yield was attributed in part to the decreased production of cell mass and glycerol. Thus, it is preferable to perform simultaneous saccharification and fermentation with substrate blends rather than pure substrates with regard to yield, productivity, and the robustness of the process.
第一代和第二代生物乙醇工艺的整合有可能加速第二代生物乙醇在市场上的建立。在同步糖化发酵过程中，将预处理的小麦秸秆与富含葡萄糖的工艺流（如小麦籽粒水解液）共同发酵，可以解决木质纤维素生物转化为乙醇过程中面临的技术问题。例如，这样做可以提高发酵液中的最终乙醇浓度，并减轻预处理过程中形成的抑制剂的影响。先前的研究表明，在同步糖化发酵过程中，第一代和第二代底物的混合对最终乙醇产量具有协同作用，这是工艺经济性的一个重要参数。在本研究中，使用不同比例的预处理小麦秸秆和糖化小麦籽粒混合物进行了酶水解以及同步糖化发酵实验。本研究的目的是确定混合底物协同作用在乙醇产量和体积生产率方面的潜在机制。
在同步糖化发酵过程中，用小麦籽粒水解液替代25%的预处理小麦秸秆足以将耗尽可溶性葡萄糖所需的停留时间从96小时缩短至24小时，并将限速步骤从乙醇生产转变为酶水解速率。此外，观察到混合底物对乙醇产量有协同作用，同时甘油产量降低。而且，混合底物对酶水解产量没有影响。
底物混合对乙醇体积生产率的影响归因于底物浓度和预处理衍生抑制剂浓度的变化导致细胞生长和细胞死亡相对速率的改变。底物混合对乙醇产量的协同作用部分归因于细胞质量和甘油产量的降低。因此，就产量、生产率和工艺稳健性而言，使用底物混合物进行同步糖化发酵比使用纯底物更可取。
