Sunday, April 29, 2007

Xylitol production from wheat straw hemicellulose: A complete assessment

Larissa Canilha1, Walter Carvalho1, João Batista Almeida e Silva1, and Marco Giullietti2. (1) Department of Biotechnology, Engineering College of Lorena-University of São Paulo, Lorena, Brazil, (2) Center of Process and Products Technology, Technology Research Institute (IPT), São Paulo, Brazil

Wheat straw represents a renewable and widely available source of carbohydrates that can be used as substrates in a variety of bioconversion processes. Dilute-acid hydrolysis of its hemicellulose results in a hydrolysate rich in fermentable sugars, mainly xylose, which can be used for xylitol production. Xylitol is a sugar-alcohol with anticariogenic properties and clinical applications commonly used by pharmaceutical and food industries. In this study, the best conditions to promote the xylose-to-xylitol bioconversion, by the Candida guilliermondii FTI 20037 in wheat straw hemicellulosic hydrolysate, were defined by using screening design and response surface methodologies: Three-fold concentrated hydrolysate, supplemented with ammonium sulphate (1.0 g/L) and rice bran extract (5.0 g/L), 6.0 initial pH, 300 rpm agitation speed and 0.2 vvm air flowrate (kLa = 15 h-1). The kLa was successfully used as an upscale criterion to carry out the bioconversion in a 15-L pilot reactor, resulting in 30.7 g/L xylitol production after 82 h fermentation (0.7 g/g bioconversion yield; 0.4 g/Lh productivity). After centrifugation, the fermented medium was clarified with anion-exchange resins (A-860S, followed by A-500PS). This treatment reduced in 99.5% color and removed 93% total phenols and 97% acetic acid of the fermented medium. The clarified medium was concentrated up to 726.5 g/L xylitol and finally submitted to crystallization (50°C saturation temperature and 0.2-0.4°C/min linear cooling rates) using ethanol as solvent. Xylitol crystals with 95.9 and 95.3% purity were obtained when the fermented medium was mixed with 70 and 50% pure xylitol solution, respectively. Acknowledgements: CNPq and FAPESP