Optimization of alkaline extraction of hemicellulose from sweet sorghum bagasse and its direct application for the production of acidic xylooligosaccharides by Bacillus subtilis strain MR44.

Lusha Wei,Tongjing Yan,Yifei Wu,Baoshan Zhang,Hui Chen,Rafael Vazquez-Duhalt

doi:10.1371/journal.pone.0195616

Abstract

As predominant components of hemicelluloses in grasses, methylglucuroarabinoxylans (MeGAXn) are sources for the production of acidic xylooligosaccharides (U-XOS). Bacillus subtilis MR44, an engineered biocatalyst to secrete only the XynC xylanase and Axh43 arabinoxylan hydrolase is capable of processing MeGAXn to exclusively U-XOS. The present studies are directed at the explosion on direct alkaline extraction serving for production of U-XOS. Response Surface Methodology was used to optimize xylan extraction conditions on the sweet sorghum bagasse to achieve maximum hemicelluloses yield. The optimized condition was as follows: extraction time of 3.91 h, extraction temperature of 86.1°C, and NaOH concentration (w/w) of 12.33%. Crude xylan extracted with NaOH revealed a compositional analysis of xylose (79.0%), arabinose (5.3%), glucose (1.7%), lignin and ash (5.6%). After neutralization this xylan preparation supported growth of MR44, processing MeGAXn from sweet sorghum and accumulating U-XOS. The quality of U-XOS produced by MR44 using alkaline-treated sweet sorghum bagasse was comparable to that obtained from purified MeGAXn. Overall, the present study demonstrates that direct alkaline treatment of sweet sorghum bagasse is useful to improve the bioavailability of MeGAXn for MR44-mediated conversion to U-XOS with average degrees of polymerization of 11–12, providing alternative resources with applications in nutrition and human and veterinary medicine.

Highlights

Glucuronoarabinoxylans (MeGAXn) are heteroxylans which have a backbone of β-(1–4)-xylosyl residues variably modified with α-1, 2 methylglucuronate or α-1, 3 linked L-arabinofuranose residues
The results showed that U-XOS with average degree of polymerization 11–12 with 4-Omethyl-glucuronic acid α-1,2-linked on average to one of 11–12 xylose residues which was agreed closely with U-XOS produced on purified xylan in previous study [7]
The present study demonstrates that crude xylan extracted directly from plant tissue using NaOH is potential to provide substrate for the production of acidic oligosaccharides by B. subtilis strain MR44

Summary

Introduction

Glucuronoarabinoxylans (MeGAXn) are heteroxylans which have a backbone of β-(1–4)-xylosyl residues variably modified with α-1, 2 methylglucuronate or α-1, 3 linked L-arabinofuranose residues. It is necessary to evaluate those methods of extracting hemicelluloses from sweet sorghum bagasse for growth of B. subtilis stains and production of U-XOS. Strain MR44 was engineered to secrete a XynC xylanase and a Axh arabinoxylan hydrolase as the only xylanolytic enzymes accumulated U-XOS with an average DP of 11–12 and a single 4-Omethyl-glucuronic acid α-1,2-linked on average to one of 11–12 xylose residues each penultimate to the reducing terminal xylose in purified sweet sorghum xylan. The objective of this research work was to explore on direct substrate extraction method serving for MR44 growth and production of U-XOS For this purpose, the Response Surface Methodology (RSM) technique was applied to optimize the process for extraction of crude xylan from sweet sorghum bagasse. We evaluated the bioconversion of extracted crude xylan substrate to U-XOS by MR44 strain for the production of U-XOS

Materials and methods

Results

Discussion