The production of xylitol from pentose-fermenting yeast is closely related to the amount of oxygen present in the culture medium. This study evaluated the effects of agitation and aeration in xylitol production using the yeast Starmerella meliponinorum FRP.09, previously isolated and selected due to its capacity to produce ethanol and xylitol from xylose. Fermentations were conducted in 125 mL flasks with 15 mL of culture medium and shaken at 150 rpm and 200 rpm without forced aeration and compared to fermentation in a static system. To assess the forced aeration, bioreactor was used with an air flow of 0.1 vvm and agitation of 100 rpm, 0.25 vvm / 150 rpm and 0.5 vvm / 200 rpm. Higher agitation and airflow significantly increased the xylose consumption. The yeast S. meliponinorum FRP.09 achieved the best xylitol yield of 0.6 g of xylitol per g of xylose consumed, when the fermentation was conducted with an air flow/rotation speed of 0.5 vvm / 200 rpm in bioreactor and 150 rpm in the fermentation flasks.

ABSTRACT In Brazil, ethanol is obtained by fermentat of sugar cane juice using Saccharomyces cerevisiae. The cane juice extraction generates the bagasse that has been used for obtaining generation biofuel. However, the sugarcane bagasse has 30% pentose that cannot be fermented to ethanol by S. cerevisiae. Thus the aim of this study was to isolate a yeast able to ferment xylose to ethanol. Samples of cane juice and flowers were used for the isolation of 165 strains that were then screened for ethanol production using plate testing. Among them, the ethanol positive strains Wickerhamomyces anomalus, Schizosaccharomyces pombe and Starmerella meliponinorum were selected for a xylose fermentation assay, using a semi-synthetic and bagasse hydrolysate as must. S. meliponinorum and S. pombe produced 0.63 and 2.7 gL-1 of ethanol, respectively, from xylose in a semisynthetic medium. In the medium consisting of bagasse hydrolysate must, 0.67 and 1.1 gL-1 of ethanol were obtained from S. meliponinorum and S. pombe, respectively. All the yeasts produced xylitol from xylose in the semisynthetic medium and S. meliponinorum was that which produced the highest quantity (14.5 g L-1).

This study aimed to explore the variability in the metabolism of nine wild yeasts isolated from the sugarcane juice from a distillery in the Brazilian State of Mato Grosso. Cell viability under the stress conditions was evaluated. The yeasts were inoculated in the test tubes containing sugarcane juice adjusted from 12 to 21º Brix, ethanol from 6 to 12% in volume and temperature at 30, 35 and 40ºC. The viability was established by the growth in Petri dishes and visually by the CO2 production in the test tubes. None of the evaluated yeasts showed simultaneous resistance to the three stress conditions. The potential of yeast BB.09 could be emphasized due to its ability to ferment up to12% ethanol at 30°C.