Physiological characterization of thermotolerant yeast for cellulosic ethanol production.

The conversion of lignocellulose into fermentable sugars is considered a promising alternative for increasing ethanol production. Higher fermentation yield has been achieved through the process of simultaneous saccharification and fermentation (SSF). In this study, a comparison was performed between the yeast species Saccharomyces cerevisiae and Kluyveromyces marxianus for their potential use in SSF process. Three strains of S. cerevisiae were evaluated: two are widely used in the Brazilian ethanol industry (CAT-1 and PE-2), and one has been isolated based on its capacity to grow and ferment at 42 °C (LBM-1). In addition, we used thermotolerant strains of K. marxianus. Two strains were obtained from biological collections, ATCC 8554 and CCT 4086, and one strain was isolated based on its fermentative capacity (UFV-3). SSF experiments revealed that S. cerevisiae industrial strains (CAT-1 and PE-2) have the potential to produce cellulosic ethanol once ethanol had presented yields similar to yields from thermotolerant strains. The industrial strains are more tolerant to ethanol and had already been adapted to industrial conditions. Moreover, the study shows that although the K. marxianus strains have fermentative capacities similar to strains of S. cerevisiae, they have low tolerance to ethanol. This characteristic is an important target for enhancing the performance of this yeast in ethanol production.

Production and characterization of ß-glucanase secreted by the yeast Kluyveromyces marxianus.

An extracellular ß-glucanase secreted by Kluyveromyces marxianus was identified for the first time. The optimal conditions for the production of this enzyme were evaluated by response surface methodology. The optimal conditions to produce ß-glucanase were a glucose concentration of 4% (w/v), a pH of 5.5, and an incubation temperature of 35 °C. Response surface methodology was also used to determine the pH and temperature required for the optimal enzymatic activity. The highest enzyme activity was obtained at a pH of 5.5 and a temperature of 55 °C. Furthermore, the enzyme was partially purified and sequenced, and its specificity for different substrates was evaluated. The results suggest that the enzyme is an endo-ß-1,3(4)-glucanase. After optimizing the conditions for ß-glucanase production, the culture supernatant was found to be effective in digesting the cell wall of the yeast Saccharomyces cerevisiae, showing the great potential of ß-glucanase in the biotechnological production of soluble ß-glucan.

The influence of presaccharification, fermentation temperature and yeast strain on ethanol production from sugarcane bagasse.

Ethanol can be produced from cellulosic biomass in a process known as simultaneous saccharification and fermentation (SSF). The presence of yeast together with the cellulolytic enzyme complex reduces the accumulation of sugars within the reactor, increasing the ethanol yield and saccharification rate. This paper reports the isolation of Saccharomyces cerevisiae LBM-1, a strain capable of growth at 42 °C. In addition, S. cerevisiae LBM-1 and Kluyveromyces marxianus UFV-3 were able to ferment sugar cane bagasse in SSF processes at 37 and 42 °C. Higher ethanol yields were observed when fermentation was initiated after presaccharification at 50°C than at 37 or 42° C. Furthermore, the volumetric productivity of fermentation increased with presaccharification time, from 0.43 g/L/h at 0 h to 1.79 g/L/h after 72 h of presaccharification. The results suggest that the use of thermotolerant yeasts and a presaccharification stage are key to increasing yields in this process.