Adaptation of Scheffersomyces Stipitis Cells as a Strategy to the Improvement of Ethanol Production from Sugarcane Bagasse Hemicellulosic Hydrolysate

Abstract

Sugarcane bagasse, a co-product from sugar mills in Brazil, is a biomass constituted by cellulose and hemicellulose, rich in carbohydrates like pentoses (xylose and arabinose) and hexose (glucose, manose and galactose). Acid hydrolysis with diluted H₂SO₄ has been used to the release of sugars, resulting also in the generation of by-products such as acetic acid, furfural, hydroxymethylfurfural, phenols that are potential fermentation inhibitors and affect the growth rate of Scheffersomyces stipitis. The inhibition may occur by the action of multiple factors, such as interference in enzymatic activities, since these compounds can act synergistically or alone. Detoxification strategies have been evaluated to remove these inhibitory compounds, but they usually result in sugar and hydrolysate volume loss besides adding costs to the process. Therefore, the adaptation techniques of microorganisms could be used as a strategy to increase the fermentability of hydrolysates. In this context, the current study aims to evaluate the adaptation of Scheffersomyces stipitis cells in different ratios (25 %, 50 %, 75 % and 100 %) of non-detoxified sugarcane bagasse hemicellulosic hydrolysate and subsequently to use this adapted cells in detoxified hydrolysate for ethanol production. Inoculum adaptation was accomplished by sequential transfer of culture samples to adaptation media containing concentrations of non-detoxified hydrolysate from 25 to 100 % (25, 50, 75 and 100 %). The adapted and non-adapted cells were cultured in hydrolysate detoxified by flocculation with vegetal polymer and supplemented at 30 °C, 200 rpm for 72h in a 125 mL Erlenmeyer flasks containing 50 mL medium. The cell adaptation technique improved the bioconversion of xylose to ethanol. During the fermentation of detoxified hydrolysate using adapted cells with 50% of non-detoxified hydrolysate it can be observed a xylose consumption (98 %) and ethanol production (14.97 g L^-1) with values of yield (Y_P/S), productivity (Q_P) and conversion efficiency of 0.33 g g^-1, 0.21 g L^-1h^-1 and 64.38 %, respectively. These values of Y_P/S and Q_P were about 22 and 49% higher when compared with not- adapted cells. The adapted S. stipitis cells in 50 % of non-detoxified hydrolysate was able to ferment the detoxified sugarcane bagasse hemicellulosic hydrolysate improving ethanol production and presenting a good strategy to overcome the problems caused by the presence of toxic compounds in the hydrolysate.