化学预处理提高酒糟生物质酶解糖化效果

任海伟; 徐娜; 李金平; 张轶; 李志忠; 王永刚; 刘晓风; 张涛; 孙永明

摘要

为促进酒糟生物质的酶解糖化，筛选适宜的预处理方法，以脱除木质素，提高综纤维素（纤维素和半纤维素之和）保留率为目标，研究比较了酸-超声波耦合（ultrasound-assisted acid pretreatment，UAAP）、液氨（pretreatment by soaking in aqueous ammonia，PSAA）、碱性双氧水（alkaline hydrogen peroxide pretreatment，AHPP）和酸性亚硫酸氢盐（bisulfite pretreatment，BP）4种预处理法对酒糟化学组分、结构特性和酶解得率的影响。结果表明，与其余3种方法相比，BP法处理后酒糟的纤维素和半纤维素保留率最高，分别为84.59%和84.87%，即综纤维素保留率为84.68%。与未处理酒糟（unpretreatment，UP）相比，4种方法预处理后酒糟的综纤维素酶解得率分别提高了49.12%（酸-超声波，UAAP）、55.48%（液氨，PASS）、92.79%（碱性双氧水，AHPP）和99.15%（酸性亚硫酸氢盐，BP），其中BP法对酒糟酶解糖化的促进作用最有效。扫描电镜（scanning electron microscope， SEM）和X-衍射（X-ray differaction，XRD）结果显示，酒糟经不同方法预处理后表观结构发生了明显变化，木质纤维网络结构遭到破坏，表面呈现无规则或形状各异的膨松状态，沟壑明显，孔隙率增加，比表面积增大，有利于提高水解酶的可及性。化学组分和结构特性的变化说明酒糟的酶解得率与综纤维素的保留、木质素的去除、表面微观形貌变化以及纤维素结晶度等因素直接相关。总之，酸性亚硫酸氢盐（BP）法是适用于酒糟生物质糖化预处理的一种有效可行方法。%Fuel ethanol production from lignocellulosic biomass hydrolysates by microbial fermentation was of great economic and environmental significance. Distillers grains, which are composed of unhydrolyzed and unfermented polymeric sugars, are the co-products of white spirit industries, their high polysaccharides portion of cellulose and hemicellulose make them attractive feedstock for conversion to fuel ethanol. However, biomass recalcitrance that limit the availability of polysaccharides for biological conversion by enzymatic digestibility necessitates a appropriate pretreatment, which is in favor of hydrolytic enzyme penetration to enhance enzymatic digestibility. The aim of this study is to increase accessibility of cellulolytic enzymes (xylanase and cellulase) to the feedstock and evaluate the effects of different pretreatment methods on enzymatic saccharification of distillers grains biomass. Four pretreatment processes including ultrasound-assisted acid pretreatment (UAAP), soaking in aqueous ammonia pretreatment (SAAP), alkaline hydrogen peroxide pretreatment (AHPP) and bisulfite pretreatment (BP) were employed in this study to determine how each method affected the digestibility of distillers grains during enzymatic hydrolysis. The effective mechanisms of four different pretreatment methods on the chemical composition and structural characteristics were studied and analyzed with the goal of removing lignin and reserving of holocelluloses (cellulose and hemicellulose). Then the effects of four different pretreatment methods on degradation efficiency of cellulose and hemicellulose to produce sugars were evaluated with the index of the enzymatic hydrolysis yield. SEM and XRD were applied to characterize structural changes associated with improved cellulolytic enzyme digestibility. The results showed that the retention rate of cellulose and hemicellulose by BP pretreatment was higher than that of other three pretreatment methods, which were 84.59% and 84.87%, respectively. Four pretreatment methods effectively enhanced enzymatic hydrolysis efficiency by disrupting lingocellulosic structure and increasing accessibility of cellulolytic enzymes to distillers grains over un-pretreated feedstock. Compared with the un-pretreated distillers grains, the enzymatic hydrolysis yield of holocellulose were increased by 49.12% (UAAP), 55.48% (PASS), 92.79% (AHPP) and 99.15% (BP), respectively. Compared with other three pretreatment approaches, BP pretreatment showed the greatest improvement on enzymatic hydrolysis of the distillers grains. The maximum conversion of holocellulose was 32.88%, which were 1 times of the control (un-pretreated 16.51%). SEM data showed that the structural modifications of 4 pretreatments were pronounced in comparison to that of feedstock, which presented a rigid and compact morphology. Remarkably, most of the large irregular pores with different sizes and shapes were formed and the lignocellulosic structure was also severely disrupted as a result of the removal of lignin, leading to the increasing surface exposure and porosity. Furthermore, 4 pretreatments disrupted the internal structure of the holocelluloses, increased the reaction surface area and the porosity of the distillers grains. The results based on the analysis of XRD suggested that these pretreatments disrupted the crystal structure of cellulose and increased the available surface area, which made the cellulose better accessible for enzymatic hydrolysis. The changes of physical structure and chemical composition declare that the enzymatic digestibility is related with the removal of lignin, the retention or disorder structure of holocelluloses and the crystallinity of cellulose. Therefore bisulfite pretreatment (BP) is effective and appropriate for the pretreatment of distillers grains to increase the enzyme saccharification yield.

化学预处理提高酒糟生物质酶解糖化效果

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