Robak Katarzyna, Balcerek Maria
Lodz University of Technology, Faculty of Biotechnology and Food Sciences, Institute of Fermentation Technology and Microbiology, Department of Spirit and Yeast Technology, Wolczanska 171/173, PL 90-924 Lodz, Poland.
Food Technol Biotechnol. 2018 Jun;56(2):174-187. doi: 10.17113/ftb.56.02.18.5428.
In the context of climate change and the depletion of fossil fuels, there is a great need for alternatives to petroleum in the transport sector. This review provides an overview of the production of second generation bioethanol, which is distinguished from the first generation and subsequent generations of biofuels by its use of lignocellulosic biomass as raw material. The structural components of the lignocellulosic biomass such as cellulose, hemicellulose and lignin, are presented along with technological unit steps including pretreatment, enzymatic hydrolysis, fermentation, distillation and dehydration. The purpose of the pretreatment step is to increase the surface area of carbohydrate available for enzymatic saccharification, while minimizing the content of inhibitors. Performing the enzymatic hydrolysis releases fermentable sugars, which are converted by microbial catalysts into ethanol. The hydrolysates obtained after the pretreatment and enzymatic hydrolysis contain a wide spectrum of sugars, predominantly glucose and xylose. Genetically engineered microorganisms are therefore needed to carry out co-fermentation. The excess of harmful inhibitors in the hydrolysate, such as weak organic acids, furan derivatives and phenol components, can be removed by detoxification before fermentation. Effective saccharification further requires using exogenous hemicellulases and cellulolytic enzymes. Conventional species of distiller's yeast are unable to ferment pentoses into ethanol, and only a very few natural microorganisms, including yeast species like , () and , metabolize xylose to ethanol. Enzymatic hydrolysis and fermentation can be performed in a number of ways: by separate saccharification and fermentation, simultaneous saccharification and fermentation or consolidated bioprocessing. Pentose-fermenting microorganisms can be obtained through genetic engineering, by introducing xylose-encoding genes into metabolism of a selected microorganism to optimize its use of xylose accumulated in the hydrolysate.
在气候变化和化石燃料枯竭的背景下，交通运输部门迫切需要石油的替代品。本综述概述了第二代生物乙醇的生产，它与第一代及后续几代生物燃料的区别在于使用木质纤维素生物质作为原料。介绍了木质纤维素生物质的结构成分，如纤维素、半纤维素和木质素，以及包括预处理、酶水解、发酵、蒸馏和脱水在内的工艺单元步骤。预处理步骤的目的是增加可用于酶促糖化的碳水化合物表面积，同时尽量减少抑制剂的含量。进行酶水解会释放出可发酵糖，这些糖被微生物催化剂转化为乙醇。预处理和酶水解后得到的水解产物含有多种糖类，主要是葡萄糖和木糖。因此需要基因工程改造的微生物来进行共发酵。水解产物中过量的有害抑制剂，如弱酸、呋喃衍生物和酚类成分，可在发酵前通过解毒去除。有效的糖化还需要使用外源半纤维素酶和纤维素分解酶。传统的酿酒酵母无法将戊糖发酵成乙醇，只有极少数天然微生物，包括如 、 （） 和 等酵母菌种，能将木糖代谢为乙醇。酶水解和发酵可以通过多种方式进行：单独糖化发酵、同步糖化发酵或联合生物加工。可以通过基因工程获得戊糖发酵微生物，即将木糖编码基因引入选定微生物的代谢中，以优化其对水解产物中积累的木糖的利用。
