Adaptation of xylose fermenting yeasts, isolated from various sources in the Limpopo Province, to improve ethanol production in the biofuel industry

Abstract

The recent oil crisis and environmental concerns over fossil fuels has led to the development of biofuels from lignocellulosic materials. Two main sugars from lignocellulose that can be used for bioethanol production are glucose and xylose. Xylose is problematic, because there are few yeasts that can utilise and ferment it. Xylose fermentation is not as efficient compared to glucose fermentation. Some of the factors that affect xylose fermentation include rate of xylose consumption, aeration, temperature and inhibitors. To improve ethanol production and fermentations and to make the process economically viable at industrial scale, there is a need to find a robust microorganism that can ferment efficiently in harsh industrial conditions. Therefore, the aim of this study was to investigate by means of evolutionary engineering (adaptation), the adaptability of seven locally isolated yeasts in terms of growth on high xylose concentration, in the presence of acetic acid as well as at elevated temperatures. Seven yeast strains (Candida guilliermondii MBI2, Candida sp. Kp6.2ey, Candida tropicalis Kp21ey, Candida tropicalis Kp42ey, Candida tropicalis Kp43ey, Ogatea methanolica Kp2ey and Pichia kudriavzevii Kp34ey) were adapted to ferment 60 g/L xylose as sole carbon source in the presence of 3 g/L acetic acid at 37°C. P. kudriavzevii Kp34ey was the only yeast to adapt to these conditions. The adapted P. kudriavzevii Kp34ey was compared with the parental strain (unadapted) and a reference strain, Scheffersomyces stipitis NRRLY-7124, using different volumetric oxygen transfer coefficient (KLa) rates. P. kudriavzevii Kp34ey (adapted and parental strain) and S. stipitis NRRLY-7124 produced the highest ethanol concentrations at a KLa value of 3.3. Overall, for all KLa values tested, the adapted strain performed better than the parental strain and S. stipitis NRRLY-7124. The adapted P. kudriavzevii Kp34ey yielded 4.03 g/L ethanol on 60 g/L xylose with 3 g/l acetic acid at 37°C at a KLa value of 3.3 and was the only yeast tested to grow under these conditions.