Release of cell wall phenolic esters during hydrothermal pretreatment of rice husk and rice straw

Abstract

BackgroundRice 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).ResultsRS 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-O-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.ConclusionDifferences 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.

Comparison of different lignocellulosic biomass for Solvent production by Clostridium beijerinckii strains

“…The fermentation results are consistent with the result of the compositional analysis and SEM analysis which showed less disruption in rice husk biomass on pretreatment in comparison to rice straw and sugarcane bagasse. These results suggest that sugarcane bagasse and rice straw are superior fermentation lignocellulosic substrates than rice husk and were found to be comparable to the results reported by [55,56].…”

Concentration of xylooligosaccharides with a low degree of polymerization using membranes and their effect on bacterial fermentation

“…It has been pointed out that autohydrolysis pretreatment can cause extensive disruption of lignin bonds to produce low molecular weight fragments 18 . The phenolics are mostly composed of ferulic acid (194 g mol −1 ), coumaric acid (164 g mol −1 ), or their ester forms 19 . The molecular weights of the phenolics are similar to the MWCO of the membrane, which might have led to the concentration of the phenolics and thus the increase in color intensity.…”

Advances and Prospects of Phenolic Acids Production, Biorefinery and Analysis

“…Agro-industrial and agricultural waste contains mostly bound phenolic acids, which can be released after alkaline hydrolysis [ 148 , 149 ]. For the straws, techniques such as hydrothermal pretreatment are applied for enhanced release of bounded phenolic acids [ 150 ]. The amount of phenolic acids in an agro-industrial waste can reach up to a few percent of the raw material.…”

“…However, agro-industrial waste contains more variety of phenolic acids, including isoferulic, isovanillic ( 5 ), hypogallic, and γ-resorcylic ( 15 ) acids [ 151 , 152 , 153 ]. In the extracts, hydroxycinnamic acids ( p -, o -coumaric, ferulic acids) can be found in both configuration as cis and trans stereoisomers, with the latter being the predominant form [ 148 , 150 ].…”