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HYDROLYSIS OF WHEAT STRAW HEMICELLULOSE AND DETOXIFICATION OF THE HYDROLYSATE FOR XYLITOL PRODUCTION
Junping Zhuang, Ying Liu, Zhen Wu, Yong Sun, and Lu Lin *
Xylitol can be obtained from wheat straw hemicellulose containing a high content of xylan. This study describes a new system of hydrolysis, utilizing a mixed solution of formic acid andhydrochloric acid in which xylan can be hydrolyzed effectively. The hydrolysate contains a high content of formic acid, which markedly inhibits the fermentation. One of the most efficient methods for removing inhibiting compounds is treatment of the hydrolysate with ion-exchange resins. Formate can be removed by a factor of 77.78%, and furfural, acetic acid, phenolic compounds can be removed by 90.36%,96.29%, and 77.44%, respectively after the hydrolysate has been treated with excess Ca(OH)2 and D311 ion-exchange resin. The xylose from the hydrolysis process can be fermented by Candida tropicalis strain (AS2.1776) to produce xylitol with a yield of 41.88 % (xylitol/xylose). Keywords: Straw hydrolysate; Formic acid; Xylose; Xylitol; Fermentation Contact information: Department of ResourcesScience and Engineering, State Key Lab of Pulp and Paper Engineering, South China University of Technology, Guangzhou,guangdong, 510640, P R. China. * Corresponding author: lclulin@scut.edu.cn

INTRODUCTION Xylitol, a natural five-carbon sugar alcohol with sweetness comparable with that of sucrose has found increasing use in the food and medical–pharmaceutical industries (Mitchell 2006). It hasanticariogenic properties, it is appropriate to replace glucose in diets of diabetics, and it has been recommended for parenteral nutrition and to prevent otitis, osteoporosis, and lung infections (Choi et al. 2000; Villarreal et al. 2006). Xylitol is currently manufactured by the chemical reduction of the five-carbon sugar xylose, which is one of the major components present in hemicellulosehydrolysates. As the hemicellulose hydrolysate fraction also contains other sugars, the chemical process includes expensive separation and purification steps to remove these by-products from xylose or xylitol. Xylitol can also be produced by microbiological methods with xyloseutilizing yeasts, especially species of the genus Candida (Oh and Kim 1998). In order to obtain a better bioconversion yield offermentation from xylose to xylitol, benign pretreatment methods of lignocelluloses for production of higher quality of xylose have been investigated, including acid, alkali, ray, steam explosion, and combinations of different methods. Among these pretreatments, formic acid is a novel agent utilized for hydrolysis of lignocellulose into fermentable sugars and exhibits the potential prospects ofindustrial application because the reaction conditions employed are

Zhuang et al. (2009). “Xylitol from wheat straw,” BioResources 4(2), 674-686.

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mild, such as temperatures below 70oC (Sun et al. 2007). Moreover, the corrosion of formic acid to the reaction apparatus is much less than that by the concentrated inorganic acid, such as higherconcentration of sulphuric acid. Formic acid together with the lower concentration of hydrochloric acid can be effectively recovered and reused. However, it was found that formic acid hydrolysate of lignocellulosics such as wheat straw cannot be directly applied as a fermentation substrate owing to the presence of various fermentation inhibitors in it. Acidic hydrolysate of hemicellulose comprises aconsiderably complex mixture of components. These compounds can be divided into three major groups: organic acids (acetic, formic and levulinic acids), furan derivatives, for example, furfural and 5hydroxymethylfurfural (5-HMF), and phenolic compounds derived from sugar degradation or released from degradation of lignin polymer (López et al. 2004). Among these fermentation inhibitors, especially...