Li Jing, Zhang Yu, Shi Suan, Tu Maobing
1Alabama Center for Paper & Bioresource Engineering, Auburn University, Auburn, AL 36849 USA.
3Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221 USA.
Biotechnol Biofuels. 2020 Apr 10;13:67. doi: 10.1186/s13068-020-01710-2. eCollection 2020.
Lignin plays an important role in biochemical conversion of biomass to biofuels. A significant amount of lignin is precipitated on the surface of pretreated substrates after organosolv pretreatment. The effect of this residual lignin on enzymatic hydrolysis has been well understood, however, their effect on subsequent ABE fermentation is still unknown.
To determine the effect of residual extractable lignin on acetone-butanol-ethanol (ABE) fermentation in separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) processes, we compared ABE production from ethanol-washed and unwashed substrates. The ethanol organosolv pretreated loblolly pine (OPLP) was used as the substrate. It was observed that butanol production from OPLP-UW (unwashed) and OPLP-W (washed) reached 8.16 and 1.69 g/L, respectively, in SHF. The results showed that ABE production in SHF from OPLP-UW prevents an "acid crash" as compared the OPLP-W. In SSF process, the "acid crash" occurred for both OPLP-W and OPLP-UW. The inhibitory extractable lignin intensified the "acid crash" for OPLP-UW and resulted in less ABE production than OPLP-W. The addition of detoxified prehydrolysates in SSF processes shortened the fermentation time and could potentially prevent the "acid crash".
The results suggested that the residual extractable lignin in high sugar concentration could help ABE production by lowering the metabolic rate and preventing "acid crash" in SHF processes. However, it became unfavorable in SSF due to its inhibition of both enzymatic hydrolysis and ABE fermentation with low initial sugar concentration. It is essential to remove extractable lignin of substrates for ABE production in SSF processes. Also, a higher initial sugar concentration is needed to prevent the "acid crash" in SSF processes.
木质素在生物质转化为生物燃料的生化过程中起着重要作用。在有机溶剂预处理后，大量木质素沉淀在预处理底物表面。这种残留木质素对酶水解的影响已得到充分了解，然而，它们对后续丙酮-丁醇-乙醇（ABE）发酵的影响仍不清楚。
为了确定残留可提取木质素在单独水解和发酵（SHF）以及同步糖化发酵（SSF）过程中对丙酮-丁醇-乙醇（ABE）发酵的影响，我们比较了乙醇洗涤和未洗涤底物的ABE产量。以乙醇有机溶剂预处理的火炬松（OPLP）为底物。观察到在SHF中，来自未洗涤的OPLP（OPLP-UW）和洗涤后的OPLP（OPLP-W）的丁醇产量分别达到8.16和1.69 g/L。结果表明，与OPLP-W相比，SHF中OPLP-UW的ABE生产可防止“酸崩溃”。在SSF过程中，OPLP-W和OPLP-UW均发生了“酸崩溃”。抑制性可提取木质素加剧了OPLP-UW的“酸崩溃”，导致ABE产量低于OPLP-W。在SSF过程中添加解毒的预水解物缩短了发酵时间，并有可能防止“酸崩溃”。
结果表明，高糖浓度下的残留可提取木质素可通过降低代谢速率并防止SHF过程中的“酸崩溃”来促进ABE生产。然而，由于其在低初始糖浓度下对酶水解和ABE发酵均有抑制作用，在SSF中变得不利。在SSF过程中生产ABE时，去除底物中的可提取木质素至关重要。此外，需要更高的初始糖浓度来防止SSF过程中的“酸崩溃”。
