常压甘油自催化预处理麦草浓醪发酵纤维素乙醇

High titer ethanol production from an atmospheric glycerol autocatalytic organosolv pretreated wheat straw

摘要:

目前纤维素乙醇成本偏高的根本原因在于没有达到淀粉质乙醇发酵水平的“三高” (高浓度、高转化率和高效率) 指标，提高水解糖液浓度和避免发酵抑制物来实现浓醪发酵，是解决问题的关键。文中以常压甘油自催化预处理麦草为底物，尝试采用不同发酵策略，探讨其浓醪发酵产纤维素乙醇的可行性。在优化培养条件 (15%底物浓度，加酶量30 FPU/g干底物，温度37 ℃，接种量10%) 下同步糖化发酵72 h，纤维素乙醇产量为31.2 g/L，转化率为73%，发酵效率0.43 g/(L·h)；采用半同步 (预酶解24 h) 糖化发酵72 h，纤维素乙醇浓度达到33.7 g/L，转化率为79%，发酵效率为0.47 g/(L·h)，其中 (半) 同步糖化发酵中90%以上纤维素已被糖化水解用于发酵；采用分批补料式半同步糖化发酵，补料到基质浓度相当于30%，发酵72 h时纤维素乙醇产量达到51.2 g/L，转化率为62%，发酵效率为0.71 g/(L·h)。在所有浓醪发酵中乙酸不足3 g/L，无糠醛和羟甲基糠醛等发酵抑制物。以上结果表明，常压甘油自催化预处理木质纤维素基质适用于纤维素乙醇发酵；分批补料式半同步糖化发酵策略可用来进行浓醪纤维素乙醇发酵；未来工作中提高基质纯度和强化酶解产糖是浓醪纤维素乙醇达到“三高”指标的关键。

Abstract:

The expensive production of bioethanol is because it has not yet reached the ‘THREE-HIGH’ (High-titer, high-conversion and high-productivity) technical levels of starchy ethanol production. To cope with it, it is necessary to implement a high-gravity mash bioethanol production (HMBP), in which sugar hydrolysates are thick and fermentation-inhibitive compounds are negligible. In this work, HMBP from an atmospheric glycerol autocatalytic organosolv pretreated wheat straw was carried out with different fermentation strategies. Under an optimized condition (15% substrate concentration, 10 g/L (NH4)2SO4, 30 FPU/g dry matter, 10% (V/V) inoculum ratio), HMBP was at 31.2 g/L with a shaking simultaneous saccharification and fermentation (SSF) at 37 °C for 72 h, and achieved with a conversion of 73% and a productivity of 0.43 g/(L·h). Further by a semi-SFF with pre-hydrolysis time of 24 h, HMBP reached 33.7 g/L, the conversion and productivity of which was 79% and 0.47 g/(L·h), respectively. During the SSF and semi-SSF, more than 90% of the cellulose in both substrates were hydrolyzed into fermentable sugars. Finally, a fed-batch semi-SFF was developed with an initial substrate concentration of 15%, in which dried substrate (= the weight of the initial substrate) was divided into three portions and added into the conical flask once each 8 h during the first 24 h. HMBP achieved at 51.2 g/L for 72 h with a high productivity of 0.71 g/(L·h) while a low cellulose conversion of 62%. Interestingly, the fermentation inhibitive compound was mainly acetic acid, less than 3.0 g/L, and there were no other inhibitors detected, commonly furfural and hydroxymethyl furfural existing in the slurry. The data indicate that the lignocellulosic substrate subjected to the atmospheric glycerol autocatalytic organosolv pretreatment is very applicable for HMBP. The fed-batch semi-SFF is effective and desirable to realize an HMBP.