De Bari Isabella, Cuna Daniela, Di Matteo Vincenzo, Liuzzi Federico
ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Laboratory of Technology and Engineering for Biomass, S.S. 106 Jonica, 75026 Rotondella, MT, Italy.
ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Laboratory of Technology and Engineering for Biomass, S.S. 106 Jonica, 75026 Rotondella, MT, Italy.
N Biotechnol. 2014 Mar 25;31(2):185-95. doi: 10.1016/j.nbt.2013.12.003. Epub 2013 Dec 28.
Agricultural by-products such as corn stover are considered strategic raw materials for the production of second-generation bioethanol from renewable and non-food sources. This paper describes the conversion of steam-pretreated corn stover to ethanol utilising a multi-step process including enzymatic hydrolysis, isomerisation, and fermentation of mixed hydrolysates with native Saccharomyces cerevisiae. An immobilised isomerase enzyme was used for the xylose isomerisation along with high concentrations of S. cerevisiae. The objective was to assess the extent of simultaneity of the various conversion steps, through a detailed analysis of process time courses, and to test this process scheme for the conversion of lignocellulosic hydrolysates containing several inhibitors of the isomerase enzyme (e.g. metal ions, xylitol and glycerol). The process was tested on two types of hydrolysate after acid-catalysed steam pretreatment: (a) the water soluble fraction (WSF) in which xylose was the largest carbon source and (b) the entire slurry, containing both cellulose and hemicellulose carbohydrates, in which glucose predominated. The results indicated that the ethanol concentration rose when the inoculum concentration was increased in the range 10-75 g/L. However, when xylose was the largest carbon source, the metabolic yields were higher than 0.51g(ethanol)/g(consumed) sugars probably due to the use of yeast internal cellular resources. This phenomenon was not observed in the fermentation of mixed hydrolysates obtained from the entire pretreated product and in which glucose was the largest carbon source. The ethanol yield from biomass suspensions with dry matter (DM) concentrations of 11-12% (w/v) was 70% based on total sugars (glucose, xylose, galactose). The results suggest that xylulose uptake was more effective in mixed hydrolysates containing glucose levels similar to, or higher than, xylose. Analysis of the factors that limit isomerase activity in lignocellulosic hydrolysates excluded any inhibition due to residual calcium ions after the detoxification of the hemicellulose hydrolysates with Ca(OH)2. By contrast, most of the enzyme activity ceased during the fermentation of the entire slurry after steam explosion, probably due to synergistic inhibition effects of various fermentation co-products.
玉米秸秆等农业副产品被视为从可再生和非粮食来源生产第二代生物乙醇的战略原料。本文描述了利用多步工艺将蒸汽预处理的玉米秸秆转化为乙醇的过程，该工艺包括酶水解、异构化以及用天然酿酒酵母对混合水解产物进行发酵。使用固定化异构酶进行木糖异构化，并加入高浓度的酿酒酵母。目的是通过对工艺时间进程的详细分析来评估各个转化步骤的同步程度，并测试该工艺方案用于转化含有几种异构酶抑制剂（如金属离子、木糖醇和甘油）的木质纤维素水解产物。该工艺在酸催化蒸汽预处理后的两种水解产物上进行了测试：(a) 木糖是最大碳源的水溶性部分 (WSF)，以及 (b) 含有纤维素和半纤维素碳水化合物且葡萄糖占主导的整个浆料。结果表明，当接种物浓度在10 - 75 g/L范围内增加时，乙醇浓度会升高。然而，当木糖是最大碳源时，代谢产率高于0.51 g(乙醇)/g(消耗)糖，这可能是由于利用了酵母内部细胞资源。在从整个预处理产物获得的混合水解产物的发酵中未观察到这种现象，其中葡萄糖是最大碳源。基于总糖（葡萄糖、木糖、半乳糖），干物质 (DM) 浓度为11 - 12% (w/v) 的生物质悬浮液的乙醇产率为70%。结果表明，在葡萄糖水平与木糖相似或高于木糖的混合水解产物中，木酮糖的摄取更有效。对木质纤维素水解产物中限制异构酶活性的因素进行分析后发现，在用Ca(OH)₂对半纤维素水解产物进行解毒后，不存在因残留钙离子而产生的任何抑制作用。相比之下，蒸汽爆破后整个浆料发酵过程中大部分酶活性停止，这可能是由于各种发酵副产物的协同抑制作用。
