Elsevier

Bioresource Technology

Volume 101, Issue 18, September 2010, Pages 6994-6999
Bioresource Technology

Xylose and cellulose fractionation from corncob with three different strategies and separate fermentation of them to bioethanol

https://doi.org/10.1016/j.biortech.2010.03.132Get rights and content

Abstract

To the aim of efficient utilization of both of xylose and cellulose, a laboratory xylose/cellulose fractionation and separate fermentation (XCFSF) bioethanol process was performed. Three xylose/cellulose fractionation strategies: (A) dilute sulfur acid hydrolysis and detoxification, (B) lime pretreatment and xylanase hydrolysis, (C) bio-treatment with Phanerochaete chrysosporium and xylanase hydrolysis were applied to corn cobs. As a result, the maximum xylose yields obtained from A, B and C fractionation methods were 78.47%, 57.84% and 42.54%, respectively, and 96.81%, 92.14% and 80.34% of cellulose were preserved in the corresponding solid residues. The xylose dissolved in acid and enzymatic hydrolysates was fermented to ethanol by Candida shahatae and the cellulose remaining in solid residues was converted to ethanol by simultaneous saccharification and fermentation (SSF) with Saccharomyces cerevisiae. Finally, for A, B, C fractionation methods, 70.40%, 52.87%, 39.22% of hemicellulose and 89.77%, 84.30%, 71.90% of cellulose in corn cobs was converted to ethanol, respectively.

Introduction

d-Xylose is the second most abundant sugar in lignocellulosic materials and its efficient conversion is one of the prerequisites for lignocellulosic ethanol industrialization (Hahn-Hägerdal et al., 2007). In the presence of glucose, xylose could not be converted into ethanol at an acceptable efficiency for industrial level because of preferential glucose utilization (Yomano et al., 2009, Panchal et al., 1988). Efforts have been taken to explore the regulatory mechanism of preferential glucose utilization, but it is very complex and no clear solution has been achieved yet (Matsushika et al., 2009).

Since xylose in the C5 and C6 sugars mixture is difficult to be converted into ethanol, it is a good option to separate xylose from cellulose and ferment xylose and cellulose to ethanol separately. This leads to another lignocellulosic ethanol production strategy which we called here XCFSF process.

Lignocellulose fractionation is not a novel idea. Previously, some strategies (Ibrahim and Glasser, 1999, Kim and Lee, 2005, Kim and Lee, 2006, Hongzhang and Liying, 2007, Persson et al., 2009) have been adopted to different lignocellulosic materials to fractionate the three main components (hemicellulose, cellulose and lignin) for high value-added materials production.

In our opinion, XCFSF process for ethanol production contains the following steps: pretreatment, hydrolysis of hemicellulose, hemicellulose hydrolyzate/cellulosic residues fractionation, xylose ethanol fermentation and cellulosic residues ethanol fermentation. Lignin has significantly negative effect on the hydrolysis of hemicellulose and cellulose (Hendriks and Zeeman, 2009). For efficient hemicellulose and cellulose hydrolysis, certain level of pretreatment for decomposing the lignin is necessary in XCFSF process. Hemicellulose can be easily hydrolyzed into dissolved sugars (mainly xylose) with enzyme or chemicals, and then separated from cellulosic residues by proper liquid–solid separation method. Cellulose remaining in residues can be easily converted into ethanol by successful glucose-fermenting yeast Sacchromyces cerevisiae after enzymatic hydrolysis to glucose. For xylose fermentation, some xylose-fermenting yeasts, such as Pichia stipitis, Candida shahatae, and Pachysolen tannophilus, could be used. These yeasts possess relatively high xylose fermentation efficiency, and have the potential for further engineering (Jeffries et al., 2007).

In this paper, we performed a laboratory XCFSF bioethanol process using corn cobs as raw materials. Three leading pretreatment methods, dilute acid hydrolysis, lime pretreatment, and bio-degradation, were applied in xylose/cellulose fractionation. The hemicellulose remaining in pretreated corn cobs were hydrolyzed to xylose with commercial xylanase. Xylose-containing hydrolysate was fermented to ethanol by C. shahatae. The cellulose remaining in solid residues was converted to ethanol employing simultaneous saccharification and fermentation (SSF) method with S. cerevisiae.

Section snippets

Raw material

Corn cob samples were obtained from local farmer in the suburb of Tianjin, China, and grinded to particle size of 1–2 mm with a Straw stalk knife mill (Shandong, China). It contained 40.67% cellulose, 31.1% hemicellulose and 11.7% lignin and 4.43% ash. The processed substrate was washed thoroughly and dried overnight at 60 °C.

Commercial cellulase (XWS-G-1) from Trichoderma reesei (10 FPU/mg), xylanase (JT-G-M) from Aspergillus niger (18 U/mg) were purchased from Noao Sci & Tech Development Co.,

Pretreatment

Different pretreatment methods have been developed during the past a few decades for effective enzymatic hydrolysis of lignocellulosic materials (Chandra et al., 2007). In our study, three strategies were adopted to fractionate xylose and cellulose. The first one was to treat corn cob with dilute sulfur acid and hydrolyze the hemicellulose into dissolved sugar directly. Second one was to pretreat corncob with lime to increase enzymatic accessibility and then hydrolyze hemicellulose into sugars

Conclusions

In this study, we applied three leading pretreatment methods for XCFSF purpose. Dilute acid hydrolysis was most effective for xylose recovery, but with inhibitors generation. Lime pretreatment was found very effective to remove lignin, but quite amount of hemicellulose were lost at the same time. Biodelignification with P. chrysosporium was friendly to environment. However, lots of hemicellulose and cellulose were consumed during the processing. Based on analysis of data, the contribution to

Acknowledgements

The authors thank Dr. Baojun Xu and Dr. Jianhua Yan for the lively discussion of and valuable comments on the manuscript.

References (27)

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