The improvement of bioethanol production by pentose fermenting yeasts previously isolated from herbal preparations, dung beetles and marula wine

Thesis (M.Sc. (Microbiology)) -- University of Limpopo, 2020

Production of bioethanol from lignocellulosic biomass has gained significant attention worldwide as an alternative fuel source for the transportation sector without affecting food supply. Efficient conversion of pentose sugars (L-arabinose and D-xylose) produced during hydrolysis of hemicellulose to ethanol can enhance the economic viability. In this study, a total of 390 yeasts isolated from Marula wine, the gut of dung beetles, herbal concoctions and banana residues were screened for the ability to ferment L-arabinose and D-xylose. Fourteen yeasts were able to ferment both pentose sugars and ten strains were subjected to an adaptation process in the presence of acetic acid using L-arabinose as carbon source. Four adapted strains of Meyerozyma caribbica were able to ferment L-arabinose to ethanol and arabitol in the presence of 3 g/L acetic acid at 35 °C. Meyerozyma caribbica Mu 2.2f fermented D-xylose, L-arabinose and a mixture of D-xylose and L-arabinose to produce 1.7, 3.0 and 1.9 g/L ethanol, respectively, compared to the parental strain with 1.5, 1.0 and 1.8 g/L ethanol, respectively, in the absence of acetic acid. The adapted strain of M. caribbica Mu 2.2f produced 3.6 and 0.8 g/L ethanol from L-arabinose and D-xylose, respectively in the presence of acetic acid while the parental strain failed to grow. In the bioreactor, the adapted strain of M. caribbica Mu 2.2f produced 5.7 g/L ethanol in the presence of 3 g/L acetic acid with an ethanol yield and productivity of 0.338 g/g and 0.158 g/L/h, respectively at a KLa value of 3.3 h-1. The adapted strain produced 26.7 g/L arabitol with a yield of 0.900 g/g at a KLa value of 4.9 h-1. Meyerozyma caribbica Mu 2.2f could potentially be used to produce ethanol and arabitol under stressed conditions.

National Research Foundation (NRF)