Produção de xilitol a partir de leveduras isoladas de madeira em decomposição do arquipélago das Ilhas Galápagos (Equador)

Abstract

Xylitol is a polyol with high added value due to the sweeteness similar to sucrose with anticariogenic properties and insulin metabolism independent that guarantee its application in food and pharmaceutical industries. In addition, it is mainly used in clinic treatment of osteoporoses and respiratory diseases. It is currently produced by chemical catalysis of the xylose from lignocellulosic residues. However, this process needs expensive purification steps to obtain pure xylitol. Alternatively, it can be produced by biotechnological methods, using micro-organisms or enzymes, which allowing the release of D-xylose from hemicellulosic hydrolysates. A major lignocellulosic material that generates value-added products, such as xylitol, is the sugarcane bagasse, raw material available in abundance in Brazil and other countries. Bioconversion of D-xylose in this polyalcohol by the use of yeasts is possible, since the xylitol is an intermediate product of the metabolic pathway of D -xylose. In this context, the present study aimed the screening and selection of yeast species to fermentation processes using sugarcane bagasse hemicellulosic hydrolysates for xylitol production. A total of 140 yeast strains were isolated from 35 rotting wood samples collected in the islands Florena, Santa Cruz and Isabela, belonging to the Galápagos Archipelago, Equador. These samples were cultured in Yeast Nitrogen Base (YNB)-D-xilose, carboxymethyl cellulose and YNB-xylan media. Species of the genus Candida (including isolates related to the clades Yamadazyma, Kazachstania, Kurtzmaniella, Lodderomyces/Spathaspora, Metschnikowia and Saturnispora) were predominant in this study. Candida (Lodderomyces/Spathaspora) tropicalis followed by Kazachstania unispora, Candida (Kurtzmaniella) natalensis, Zygowilliopsis californica and Candida sinolaborantium were the most frequently isolated yeasts. In semi-synthetic fermentation assays using D-xylose (50 g.L-1) culture medium, C. tropicalis CLQCA-24SC-125 show the highest xylitol production yield (Yp/s: 0.67 g.g-1) and a productivity (Qp: 0.34 g.L-1.h-1). Furthermore, in fermentations with sugarcane bagasse hydrolysates as a substrate, this yeast showed the highest production of xylitol with the same yield of 0.67 g.g-1 and a productivity of 0.38 g.L-1.h -1. Whereas, the second species had better results for xylitol production in sugarcane hydrolysates it was a new Lindnera species, represented by strain CLQCA-24SC-025, with Yp/sof 0.64 g.g -1 and Qp 0.33 g.L -1.h-1. In 2.4 L bioreactor using sugarcane bagasse hemicellulosic hydrolysates, C. tropicalis CLQCA-24SC-125 showed the best results, with a production of 37.15 g.L -1 in xylitol in 96 h of fermentation, with an Yp/s of 0.83 g.g -1, Qp of 0.39 g.L -1 .h-1, total efficiency of 90.9% and Yp/x of 12,53 U.g-1 cells. This study demonstrates the biotechnological potential of yeasts isolated from decaying wood of the Galapagos Archipelago for xylitol production using sugarcane bagasse hemicellulosic hydrolysates.