Wu Jia, Collins Samuel R A, Elliston Adam, Wellner Nikolaus, Dicks Jo, Roberts Ian N, Waldron Keith W
1The Biorefinery Centre, Quadram Institute Bioscience, Norwich Research Park, Colney, Norwich, NR4 7UA UK.
2Quadram Institute Bioscience, Norwich Research Park, Colney, Norwich, NR4 7UA UK.
Biotechnol Biofuels. 2018 Jun 11;11:162. doi: 10.1186/s13068-018-1157-1. eCollection 2018.
Rice husk and rice straw represent promising sources of biomass for production of renewable fuels and chemicals. For efficient utilisation, lignocellulosic components must first be pretreated to enable efficient enzymatic saccharification and subsequent fermentation. Existing pretreatments create breakdown products such as sugar-derived furans, and lignin-derived phenolics that inhibit enzymes and fermenting organisms. Alkali pretreatments have also been shown to release significant levels of simple, free phenolics such as ferulic acid that are normally esterified to cell wall polysaccharides in the intact plant. These phenolics have recently been found to have considerable inhibitory properties. The aim of this research has been to establish the extent to which such free phenolic acids are also released during hydrothermal pretreatment of rice straw (RS) and rice husk (RH).
RS and RH were subjected to hydrothermal pretreatments over a wide range of severities (1.57-5.45). FTIR analysis showed that the pretreatments hydrolysed and solubilised hemicellulosic moieties, leading to an enrichment of lignin and crystalline cellulose in the insoluble residue. The residues also lost the capacity for UV autofluorescence at pH 7 or pH 10, indicating the breakdown or release of cell wall phenolics. Saponification of raw RS and RH enabled identification and quantification of substantial levels of simple phenolics including ferulic acid (tFA), coumaric acid (pCA) and several diferulic acids (DiFAs) including 8--4'-DiFA, 8,5'-DiFA and 5,5'-DiFA. RH had higher levels of pCA and lower levels of tFA and DiFAs compared with RS. Assessment of the pretreatment liquors revealed that pretreatment-liberated phenolics present were not free but remained as phenolic esters (at mM concentrations) that could be readily freed by saponification. Many were lost, presumably through degradation, at the higher severities.
Differences in lignin, tFA, DiFAs and pCA between RS and RH reflect differences in cell wall physiology, and probably contribute to the higher recalcitrance of RH compared with RS. Hydrothermal pretreatments, unlike alkali pretreatments, release cinnamic acid components as esters. The potential for pretreatment-liberated phenolic esters to be inhibitory to fermenting microorganisms is not known. However, the present study shows that they are found at concentrations that could be significantly inhibitory if released as free forms by enzyme activity.
稻壳和稻草是生产可再生燃料和化学品的有前景的生物质来源。为了有效利用，木质纤维素成分必须首先进行预处理，以实现高效的酶促糖化和后续发酵。现有的预处理会产生诸如糖衍生的呋喃和木质素衍生的酚类等分解产物，这些产物会抑制酶和发酵生物。碱预处理也已显示会释放大量简单的游离酚类，如阿魏酸，这些酚类在完整植物中通常与细胞壁多糖酯化。最近发现这些酚类具有相当大的抑制特性。本研究的目的是确定在稻草（RS）和稻壳（RH）的水热预处理过程中，此类游离酚酸的释放程度。
RS和RH在广泛的强度范围（1.57 - 5.45）内进行水热预处理。傅里叶变换红外光谱（FTIR）分析表明，预处理使半纤维素部分水解并溶解，导致不溶性残渣中木质素和结晶纤维素富集。残渣在pH 7或pH 10时也失去了紫外自发荧光能力，表明细胞壁酚类物质的分解或释放。对未处理的RS和RH进行皂化处理，能够鉴定和定量大量的简单酚类，包括阿魏酸（tFA）、香豆酸（pCA）和几种二阿魏酸（DiFAs），包括8 - 4'-二阿魏酸、8,5'-二阿魏酸和5,5'-二阿魏酸。与RS相比，RH的pCA含量较高，tFA和DiFAs含量较低。对预处理液的评估表明，预处理释放出的酚类并非游离状态，而是以酚酯形式存在（毫摩尔浓度），可通过皂化轻易释放出来。在较高强度下，许多酚类可能因降解而损失。
RS和RH在木质素、tFA、DiFAs和pCA方面的差异反映了细胞壁生理的差异，可能导致RH比RS具有更高的抗降解性。与碱预处理不同，水热预处理以酯的形式释放肉桂酸成分。预处理释放的酚酯对发酵微生物的抑制潜力尚不清楚。然而，本研究表明，它们的浓度如果因酶活性而以游离形式释放，可能具有显著的抑制作用。
