Elsevier

Biomass and Bioenergy

Volume 32, Issue 12, December 2008, Pages 1353-1358
Biomass and Bioenergy

Removal of fermentation inhibitors from alkaline peroxide pretreated and enzymatically hydrolyzed wheat straw: Production of butanol from hydrolysate using Clostridium beijerinckii in batch reactors

https://doi.org/10.1016/j.biombioe.2008.04.009Get rights and content

Abstract

In these studies, alkaline peroxide pretreatment of wheat straw was investigated. Pretreated wheat straw was hydrolyzed using cellulolytic and xylanolytic enzymes, and the hydrolysate was used to produce butanol using Clostridium beijerinckii P260. The culture produced less than 2.59 g L−1 acetone–butanol–ethanol (ABE) from alkaline peroxide wheat straw hydrolysate (APWSH) that had not been treated to reduce salt concentration (a neutralization product). However, fermentation was successful after inhibitors (salts) were removed from the hydrolysate by electrodialysis. A control glucose fermentation resulted in the production of 21.37 g L−1 ABE, while salt removed APWSH resulted in the production of 22.17 g L−1 ABE. In the two fermentations, reactor productivities were 0.30 and 0.55 g L−1 h−1, respectively. A comparison of use of different substrates (corn fiber, wheat straw) and different pretreatment techniques (dilute sulfuric acid, alkaline peroxide) suggests that generation of inhibitors is substrate and pretreatment specific.

Introduction

Currently, 24.96 hm3 (6.50 billion gallons) of ethanol is produced in the United States from corn annually [1]. The capacity of ethanol production is 30.30 hm3 (7.89 billion gallons). With another 21.27 hm3 (5.54 billion gallons) of ethanol production facilities under construction, the total production capacity will be over 51.57 hm3 (13.43 billion gallons) per year in the near future [1]. Further increase in ethanol production will necessitate use of agricultural residues or dedicated energy crops. The use of agricultural residues or energy crops will require hydrolysis of these residues to simple sugars prior to fermentation as there are no cultures that can efficiently hydrolyze biomass and simultaneously ferment to ethanol or butanol (a biofuel). Hydrolysis of biomass requires physical and chemical pretreatment and hydrolysis using enzymes. During pretreatment, biomass is subjected to severe conditions such as a combination of high temperature and reaction with chemicals such as (i) dilute sulfuric acid, (ii) dilute alkali, (iii) ammonia, (iv) hot water, and (v) alkaline peroxide. As a result of pretreatment conditions, fermentation inhibitors such as salts, phenolic acids (ρ-coumaric and ferulic), and aldehydes are produced [2]. It has been observed that salts are potent fermentation inhibitors [3], [4], [5], [6]. Removal of these inhibitors prior to fermentation is essential for successful biofuel fermentation.

Another problem that exists with the use of biomass hydrolysates for ethanol fermentation is inefficient uptake and metabolism of biomass component sugars such as pentoses. Production of acetone–butanol–ethanol (ABE) by Clostridium acetobutylicum/Clostridium beijerinckii is a process where pentoses along with hexoses are efficiently used by the cultures. Use of both sugars may make ABE fermentation more attractive than ethanol or other solvent production processes. Additionally, butanol has higher energy content than ethanol, can be used in existing pipelines, mixed with gasoline in any proportion, and is less hazardous to handle [7]. In our previous studies, we have demonstrated that ABE can be produced from dilute sulfuric acid pretreated wheat straw (WS) without any inhibition to the producing organism (C. beijerinckii P260) [8], [9], [10]. During those studies, it was also observed that the rate of fermentation was faster than control fermentations where glucose was used. In recent studies, it has been demonstrated that alkaline peroxide pretreatment is an effective technique to pretreat WS [11]. Hence, the objective of these studies was to produce ABE from alkaline peroxide pretreated WS hydrolysate (WSH) and compare fermentation performance of the two hydrolysates (dilute sulfuric acid and alkaline peroxide).

Section snippets

Culture and inoculum development

C. beijerinckii P260 was a generous gift from Professor David Jones, University of Otago (Dunedin, New Zealand). Methods for culture maintenance and inoculum development have been documented elsewhere [9], [12]. The inoculums were developed in 125 mL screw-capped bottles containing 100 mL medium as detailed previously [9].

Wheat straw (WS) pretreatment and hydrolysis

Details of WS including cultivar, harvest, and storage have been given in our previous report [10]. WS was pretreated with alkaline peroxide and hydrolyzed using enzymes. In

Results and discussion

In order to compare results of this investigation, a control batch fermentation was run with 60 g L−1 initial glucose concentration in the medium. The fermentation was run for 72 h and during this time period the culture produced 21.37 g L−1 total ABE resulting in a productivity of 0.30 g L−1 h−1. During the fermentation, ABE yield of 0.36 was achieved [9]. Following this, butanol was produced using APWSH as the substrate. During 72 h of fermentation, 2.59 g L−1 ABE (acetone 0.98, butanol 1.26, and

Conclusions

It has been observed that the alkaline peroxide pretreatment technique generates salts that are inhibitory to C. beijerinckii P260. However, fermentation was successful after these inhibitors (salts) were removed from the fermentation medium. A control fermentation resulted in the production of 21.37 g L−1 ABE while salt removed APWSH resulted in the production of 22.17 g L−1 ABE. In the two fermentations, reactor productivities were 0.30 and 0.55 g L−1 h−1, respectively. Fermentation of APWSH prior

Acknowledgments

N. Qureshi would like to thank Professor David Jones (University of Otago, Dunedin, New Zealand) for his generous gift of C. beijerinckii P260. He would also like to thank John Michael Henderson, Greg Kennedy, and Mark Maroon for preparing APWSH and conducting some of the experiments.

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