Avaliação da produção de xilitol em hidrolisado hemicelulósico de bagaço de cana de açúcar por linhagens de Cyberlindnera xylosilytica e Wickerhamomyces rabaulensis

Abstract

In Brazil the agroindustrial sector generates byproducts, such as sugarcane bagasse which after correct processing and destination arouse great economic and social interest. The sugarcane is a lignocellulosic material, wich after pretreatment with dilute sulfuric acid, can release fermentable sugars from its hemicellulosic fraction, such as D-xylose that can be bioconverted to xylitol, a high added value product with pharmaceutical and food use. Candida tropicalis is the most studied species in this bioconversion for producing xylitol as the main product of xylose metabolism. However, the isolation and obtaining of new xylitol-producing strains or species have been highlighted in recent years. In this context, this work proposes the investigation of xylitol production by strains of the yeast species Cyberlindnera xylosilytica (five strains) and Wickerhamomyces rabaulensis (eleven strains) using sugarcane bagasse hemicellulosic hydrolysate (HHBC). Initially, the best xylitol producers were cultured in 125 mL Erlenmeyer flasks containing 50 mL of complex medium YPX (D-xylose 60 gL-1, 20 gL-1 peptone, 10 gL-1 yeast extract). The best producers of xylitol were Cyberlindnera xylosilytica: UFMG-CM-Y309 and UFMG-CM-Y409, and Wickerhamomyces rabaulensis: UFMG-CM-Y3716 and UFMG-CM-Y3747). These yeasts were selected for cultures in 125 mL Erlenmeyer flasks containing 50 mL of HHBC medium, in which carried out the influence of supplementation in the xylitol production using complete factorial design (factors studied and their levels: ammonium sulphate (0 -2 gL-1), rice bran extract (5-10 gL-1), and yeast extract (1-2 gL-1). C. xylosilytica UFMG-CM-Y-309 and UFMG-CM-Y-409 strains were the best xylitol producers in cultures supplemented with an alternative source of nitrogen (rice bran extract, 10 gL-1) and for the reduction of the more expensive nitrogen source (yeast extract, 1.0 gL-1). In this case, C. xylosilytica UFMG-CM-Y-409 stands out as the largest producer of xylitol (P 14.06 gL-1, Yp/s 0.63 gg-1, Qp 0.20 gL-1h-1). Afterwards, cultures were carried out with two strains C. xylosilytica to evaluate the influence cell concentrations and oxygen availability in 125 mL Erlenmeyer flasks varying HHBC volume (25, 50 or 75 mL) using complete factorial design 22. The strain UFMG-CM-Y309 showed maximum yield of xylitol (22.13 gL-1, Yp/s 0.75 g/g-1 and Qp 0.18 gL-1h-1) with the use of 1.0 gL-1 of inoculum and 25 mL of HHBC medium and was selected for cultures in bench bioreactor (2L). The performance of Cyberlindnera xylosilytica (UFMG-CM-Y309) under pre-selected conditions in sugarcane bagasse hemicellulosic hydrolysate was evaluated in bench bioreactor and it was found that under conditions of lower oxygen transfer (kLa 21.60 h-1) the production of xylitol was similar to the assays in Erlenmeyer flasks. The conduction of the process in lower oxygen transfer favored the maintenance of the maximum specific rate of cell growth (μx = 0.050 h-1) at lower levels, than in condition of greater oxygen transfer dissolved in the medium (kLa of 35.60 h-1) and with this, the production of xylitol (19.56 gL-1) by Cyberlindnera xylosilytica (UFMG-CM-Y309) in a bench bioreactor was favored.