Elsevier

Bioresource Technology

Volume 221, December 2016, Pages 188-194
Bioresource Technology

Optimization of pretreatment, enzymatic hydrolysis and fermentation for more efficient ethanol production by Jerusalem artichoke stalk

https://doi.org/10.1016/j.biortech.2016.09.021Get rights and content

Highlights

Abstract

Jerusalem artichoke (JA) is a potential energy crop for biorefinery due to its unique agronomic traits such as resistance to environmental stresses and high biomass yield in marginal lands. Although JA tubers have been explored for inulin extraction and biofuels production, there is little concern on its stalk (JAS). In this article, the pretreatment of JAS by alkaline hydrogen peroxide was optimized using the response surface methodology to improve sugars yield and reduce chemicals usage. Scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis were applied to characterize the structures of the pretreated JAS to evaluate the effectiveness of the pretreatment. Furthermore, the feeding of the pretreated JAS and cellulase was performed for high solid uploading (up to 30%) to increase ethanol titer, and simultaneous saccharification and fermentation with 55.6 g/L ethanol produced, 36.5% more than that produced through separate hydrolysis and fermentation, was validated to be more efficient.

Introduction

At present, starch- and sugar-based biomass is still major resources for the production of biofuels and bio-based chemicals, which is not sustainable, particularly in developing countries. Therefore, non-food related feedstock has to be developed. Jerusalem artichoke (JA) is tolerant to environmental stresses such as drought, salinity and plant diseases and pests, which can thus grow well in marginal lands with high biomass yield, making it an alternative energy crop (Long et al., 2016). The major biomass of JA is from its tubers (JAT) and stalk (JAS). While JAT has been explored for inulin extraction (Li et al., 2012, Li et al., 2015) and biofuel production as well (Matías et al., 2015, Sarchami and Rehmann, 2014, Gunnarsson et al., 2014), less concerns have been focused on how to utilize JAS to credit the JAT biorefinery.

Kim et al. (2013) and Kim and Kim (2014) converted the mixture of JAS and JAT into ethanol through simultaneous saccharification and fermentation (SSF) by the inulinase-producing yeast Kluyveromyces marxianus, but apparently such a process compromised the advantage for JAT to be used for producing value-added inulin and other products. Most recently, Khatun et al. (2015) explored lignocellulosic ethanol production solely from JAS by an engineered inulinase-producing yeast Saccharomyces cerevisiae. However, almost no research on the integration and optimization of the major unit operations has been reported.

Pretreatment is the first step of lignocellulose bioconversion, which aims to destroy the rigid structure of the feedstock and separate major components cellulose, hemicelluloses and lignin from each other for more efficient hydrolysis of the cellulose component. Among various pretreatment technologies, alkaline hydrogen peroxide (AHP) pretreatment could effectively remove lignin at moderate temperature (Banerjee et al., 2012, Correia et al., 2013, Mou et al., 2013), leading to high cellulose recovery and low inhibitor formation.

The second unit operation is the enzymatic hydrolysis of the cellulose component in the pretreated JAS to release glucose for microbial fermentation. Although the development of more efficient cellulase is essential, it is vital to explore the process engineering strategy to maximize the enzyme’s potential. In order to meet the industrial requirement for more than 5% ethanol produced during the fermentation, glucose released from the enzymatic hydrolysis should be higher than 12%, resulting the uploading of the pretreatment biomass at least 20% (Chu et al., 2012). But the high solids uploading significantly deteriorates mixing and mass transfer performance of the hydrolysis and fermentation system, making it a necessities for optimizing the feeding of both the feedstock and enzyme (Chu et al., 2012, Yang et al., 2010, Olofsson et al., 2010, Liu et al., 2015, Unrean et al., 2015).

Fermentation can be coupled with the saccharification (simultaneous saccharification and fermentation, SSF) or separated from the unit operation (separate hydrolysis and fermentation, SHF). Compared to SHF, SSF enables the fermentation system to maintain at low sugar levels, which consequently alleviate substrate inhibition, particularly the inhibition of glucose in the cellulase activity, and in the meantime decrease the contamination risk for ethanol fermentation. However, temperature for ethanol fermentation by yeast is much lower than that for cellulase to efficiently hydrolyze the cellulose component. On the other hand, hydrolysate of the pretreated biomass presents various environmental stresses on yeast growth and ethanol fermentation, and the self-flocculating yeast exhibits better tolerance to these stresses (Liu et al., 2012).

In this study, AHP pretreatment was optimized by the response surface methodology (RSM) analysis, followed by the studies on uploading strategies for the pretreated biomass and enzyme and ethanol fermentation by the SSF or SHF processes using the self-flocculating S. cerevisiae SPSC01. At the end, the integrate process for ethanol production from JAS was assessed.

Section snippets

Feedstock, strain and culture medium

JAS harvested from Dongying (Shandong Province), Yancheng (Jiangsu Province) and Yinchuan (Ningxia Province), China (Table 1), was dried naturally and milled to a size range of 1–10 mm and rinsed by water to remove dust and other impurities for reliable results, then dried at 50 °C for 48 h in an oven. The self-flocculating yeast Saccharomyces cerevisiae SPSC01 developed at the authors’ laboratory and deposited at China General Microbiological Culture Collection Center (CGMCC) with the reference

JAS composition

Jerusalem artichoke fits varied environments under which food crops grow poorly, thus the component of JAS should be dependent on the conditions of soils and climates. As listed in Table 1, the lowest cellulose and the highest lignin were found in JAS harvested from the saline land in Yancheng, Jiangsu province, but JAS grown in saline land in Dongying, Shandong province contained 37.06% cellulose, the highest among all the JAS, but less hemicelluloses and lignin than JAS from Yancheng. The JAS

Conclusions

The optimal conditions for JAS pretreatment by AHP were 2% NaOH and 4% H2O2, which were validated by SEM, XRD and TGA. The enzymatic hydrolysis of the AHP-JAS indicated significant improvement in glucose yield under high biomass loading up to 30%, with 93.8 g/L glucose released, and 55.6 g/L ethanol produced by the SSF process, 36.5% more than that produced through the SHF process. These results provide insights for utilizing JAS for ethanol production.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC) with grant numbers of 21406030 and 51561145014, and China Postdoctoral Science Foundation Special Project (2015T80240).

References (29)

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