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Current research indicates the ethanol fuel production from lignocellulosic materials, such as residual wood chips from the cellulose industry, as new emerging technology. This work aimed at evaluating the ethanol production from hemicellulose of eucalyptus chips by diluted acid pretreatment and the subsequent fermentation of the generated hydrolysate by a flocculating strain of Pichia stipitis. The remaining solid fraction generated after pretreatment was subjected to enzymatic hydrolysis, which was carried out simultaneously with glucose fermentation [saccharification and fermentation (SSF) process] using a strain of Saccharomyces cerevisiae. The acid pretreatment was evaluated using a central composite design for sulfuric acid concentration (1.0-4.0 v/v) and solid to liquid ratio (1:2-1:4, grams to milliliter) as independent variables. A maximum xylose concentration of 50 g/L was obtained in the hemicellulosic hydrolysate. The fermentation of hemicellulosic hydrolysate and the SSF process were performed in bioreactors and the final ethanol concentrations of 15.3 g/L and 28.7 g/L were obtained, respectively.

The influence of potassium on ethanol production by Saccharomyces cerevisiae wild type and AR5 cells carrying the plasmid pCYG4 was investigated. This plasmid carries the glutamate dehydrogenase gene conferring an 11-fold higher level of expressed enzyme activity over the wild type cells. All experiments were carried out in batch culture with medium supplemented to different potassium concentrations up to 180 mM. Maximum ethanol production rate was observed in the AR5 cells grown in medium supplemented with 3.5 mM of potassium ions. Glucose uptake rate increased with increasing potassium up to 60 mM, but higher concentrations depressed glucose uptake rate in both strains. Furthermore, the wild type cells showed higher growth rate, ethanol production, and glucose consumption rate than the AR5 cells. These lower rates in the AR5 cells could be explained by repression of potassium uptake by an enhancement of ammonium feeding, and greater energy requirements by these cells due the presence of the plasmid.