Li Wenqi, Amos Kirtley, Li Mi, Pu Yunqiao, Debolt Seth, Ragauskas Arthur J, Shi Jian
1Biosystems and Agricultural Engineering, University of Kentucky, Lexington, KY 40506 USA.
2Department of Horticulture, University of Kentucky, Lexington, KY 40506 USA.
Biotechnol Biofuels. 2018 Nov 8;11:304. doi: 10.1186/s13068-018-1305-7. eCollection 2018.
Lignin is a promising source of building blocks for upgrading to valuable aromatic chemicals and materials. Endocarp biomass represents a non-edible crop residue in an existing agricultural setting which cannot be used as animal feed nor soil amendment. With significantly higher lignin content and bulk energy density, endocarps have significant advantages to be converted into both biofuel and bioproducts as compared to other biomass resources. Deep eutectic solvent (DES) is highly effective in fractionating lignin from a variety of biomass feedstocks with high yield and purity while at lower cost comparing to certain ionic liquids.
In the present study, the structural and compositional features of peach and walnut endocarp cells were characterized. Compared to typical woody and herbaceous biomass, endocarp biomass exhibits significantly higher bulk density and hardness due to its high cellular density. The sugar yields of DES (1:2 choline chloride: lactic acid) pretreated peach pit () and walnut shell () were determined and the impacts of DES pretreatment on the physical and chemical properties of extracted lignin were characterized. Enzymatic saccharification of DES pretreated walnut and peach endocarps gave high glucose yields (over 90%); meanwhile, compared with dilute acid and alkaline pretreatment, DES pretreatment led to significantly higher lignin removal (64.3% and 70.2% for walnut and peach endocarps, respectively). The molecular weights of the extracted lignin from DES pretreated endocarp biomass were significantly reduced. H-C HSQC NMR results demonstrate that the native endocarp lignins were SGH type lignins with dominant G-unit (86.7% and 80.5% for walnut and peach endocarps lignins, respectively). DES pretreatment decreased the S and H-unit while led to an increase in condensed G-units, which may contribute to a higher thermal stability of the isolated lignin. Nearly all β--4' and a large portion of β-5' linkages were removed during DES pretreatment.
The high lignin content endocarps have unique cell wall characteristics when compared to the other lignocellulosic biomass feedstocks. DES pretreatment was highly effective in fractionating high lignin content endocarps to produce both sugar and lignin streams while the DES extracted lignins underwent significant changes in SGH ratio, interunit linkages, and molecular sizes.
木质素是用于升级转化为有价值的芳香族化学品和材料的有前景的原料来源。内果皮生物质是现有农业环境中的一种非食用作物残渣，既不能用作动物饲料，也不能用作土壤改良剂。与其他生物质资源相比，内果皮具有显著更高的木质素含量和体积能量密度，在转化为生物燃料和生物产品方面具有显著优势。与某些离子液体相比，深共熔溶剂（DES）在从各种生物质原料中以高产率和高纯度分离木质素方面非常有效，且成本较低。
在本研究中，对桃和核桃内果皮细胞的结构和组成特征进行了表征。与典型的木质和草本生物质相比，内果皮生物质由于其高细胞密度而表现出显著更高的堆积密度和硬度。测定了DES（1:2氯化胆碱：乳酸）预处理的桃核（）和核桃壳（）的糖产率，并表征了DES预处理对提取的木质素物理和化学性质的影响。DES预处理的核桃和桃内果皮的酶促糖化产生了高葡萄糖产率（超过90%）；同时，与稀酸和碱性预处理相比，DES预处理导致显著更高的木质素去除率（核桃和桃内果皮分别为64.3%和70.2%）。从DES预处理的内果皮生物质中提取的木质素的分子量显著降低。H-C HSQC NMR结果表明，天然内果皮木质素为SGH型木质素，以G单元为主（核桃和桃内果皮木质素分别为86.7%和80.5%）。DES预处理减少了S单元和H单元，同时导致缩合G单元增加，这可能有助于提高分离出的木质素的热稳定性。在DES预处理过程中，几乎所有的β-O-4'和大部分β-5'键都被去除。
与其他木质纤维素生物质原料相比，高木质素含量的内果皮具有独特的细胞壁特征。DES预处理在分离高木质素含量的内果皮以产生糖和木质素流方面非常有效，而DES提取的木质素在SGH比例、单元间连接和分子大小方面发生了显著变化。
