Fermentação de xaropes artificiais constituídos de glicose e xilose e de hidrolisados enzimáticos de bagaço de cana-de-açúcar utilizando leveduras não convencionais e cocultivos

Abstract

The agro-industrial lignocellulosic residues such as sugarcane bagasse can be used as feedstock in biotechnological production of value-added compounds such as ethanol and xylitol, due to its high cellulose and hemicellulose content. In the present work it was observed the ability of non-conventional yeasts to convert mixtures of glucose and xylose to second-generation ethanol and xylitol. Among them, one strain of Spathaspora hagerdaliae UFMG-CM-Y303 showed good bioconversion of xylose. This strain was grown in 5 L bioreactors under aerobiose and microaerophilic conditions, with the yields of ethanol (Y P/S) of 0.25 and 0.39 g.g-1 respectively. Microaerophilic conditions were tested with sugarcane bagasse enzymatic hydrolyzate and was obtained a Y P/S of 0.47 g.g-1 , near theoretical yields. This strain was used in co-cultures with Saccharomyces cerevisiae CAT-1 to evaluate the yeast growth dynamics, using a differential medium to distinguish the colonies of the two species. This methodology was efficient to evaluate the growth kinetics and which strain is predominant in fermentation. The study was initially performed on synthetic medium using flasks for pH and inoculum optimization and the best results were reproduced in 5 L bioreactors. Co-cultures were evaluated in sugarcane bagasse pretreated by steam explosion at 200 ºC for 8 min, under Separate Hydrolysis and co-Fermentation (SHF) and Hydrolysis Simultaneous Saccharification and Fermentation (SSF). The co-cultures were able to metabolize the xylose and glucose present in pretreated sugarcane bagasse to ethanol, reaching a final concentration of 15 g L -1 in bioreactor with 5 L of working volume in SSF conditions. Since S. hagerdaliae UFMG-CM-Y303 showed potential for producing ethanol, genetic variants of this strain that were capable of growing in synthetic medium containing xylose and 2-deoxyglucose were selected after being treated with ultraviolet (UV) light. One variant selected was subjected to selection pressure and studied for fermentation of mixtures of glucose and xylose in synthetic medium and medium containing sugarcane bagasse hydrolyzates, and xylose and glucose consumption and ethanol production were evaluated. The variant M13 showed ethanol production 36% and 68% higher than the parental strain in synthetic medium and medium containing sugarcane bagasse hydrolyzates, respectively.