Abstract

BackgroundFor bioethanol production from lignocellulosic biomass, phenolics derived from pretreatment have been generally considered as highly inhibitory towards enzymatic hydrolysis and fermentation. As phenolics are produced from lignin degradation during pretreatment, it is likely that the pretreatment will exert a strong impact on the structure of phenolics, resulting in varied levels of inhibition of the bioconversion process. Despite the extensive studies on pretreatment, it remains unclear how pretreatment process affects the properties of generated phenolics and how the inhibitory effect of phenolics from different pretreatment varies on enzymatic hydrolysis and fermentation.ResultsIn this study, the structural properties of phenolic compounds derived from four typical pretreatment [dilute acid (DA), liquid hot water pretreatment (LHW), ammonia fiber expansion (AFEX) and alkaline pretreatment (AL)] were characterized, and their effect on both enzymatic hydrolysis and fermentation were evaluated. The inhibitory effect of phenolics on enzymatic hydrolysis followed the order: AFEX > LHW > DA > AL, while the inhibitory effect of phenolics on Zymomonas mobilis 8b strain fermentation followed the order: AL > LHW > DA > AFEX. Interestingly, this study revealed that phenolics derived from AFEX showed more severe inhibitory effect on enzymatic hydrolysis than those from the other pretreatments at the same phenolics concentrations (note: AFEX produced much less amount of phenolics compared to AL and DA), while they exhibited the lowest inhibitory effect on fermentation. The composition of phenolics from different pretreatments was analyzed and model phenolics were applied to explore the reason for this difference. The results suggested that the amide group in phenolics might account for this difference.ConclusionsPretreatment process greatly affects the properties of generated phenolics and the inhibitory effects of phenolics on enzymatic hydrolysis and fermentation. This study provides new insight for further pretreatment modification and hydrolysate detoxification to minimize phenolics-caused inhibition and enhance the efficiency of enzymatic hydrolysis and fermentation.

Highlights

  • For bioethanol production from lignocellulosic biomass, phenolics derived from pretreatment have been generally considered as highly inhibitory towards enzymatic hydrolysis and fermentation

  • Mass balance of water‐soluble phenolic compounds (WPC) During pretreatment, various phenolics were generated from lignin and reported to be inhibitory toward enzymatic hydrolysis and fermentation [19, 21, 22]

  • Results showed that the inhibitory effect of WPC on enzymatic hydrolysis and fermentation was greatly dependent on the WPC concentration and the pretreatment employed

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Summary

Introduction

For bioethanol production from lignocellulosic biomass, phenolics derived from pretreatment have been generally considered as highly inhibitory towards enzymatic hydrolysis and fermentation. The pretreatment-derived degradation products include phenolics (lignin derivatives), furfural, acetic acid, hydroxymethyl furfural (HMF), etc. At relatively high concentrations (2–6 g/L), xylo-oligosaccharides reduced the hydrolytic activities of key cellulases and significantly inhibited cellulose hydrolysis [8, 9]. Different from these degradation products, the water-soluble phenolic compounds mainly derived from lignin have been considered as highly inhibitory compounds, which could strongly inhibit both enzymatic hydrolysis and fermentation at very low concentrations [10, 12]. The studies on phenolics inhibition studies were quite limited; it remains unclear that how the inhibitory effect of phenolics varies dependent on pretreatments and how they may affect both enzymatic hydrolysis and fermentation

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