Isolation and Characterization of Wild Yeasts from Selected Natural Forests of Southwest Ethiopia and evaluation for bioethanol production

Abstract

Yeasts are widely used for the production of bioethanol from biomasses rich in sugar ensure and retrieve rising energy demand. The present study was aimed at isolating, screening, and characterizing fermentative wild yeast isolated from natural forests of south west Ethiopia for bioethanol production using fruit peels of Banana, Papaya ,Mango, and Pineapple as a substrate. The wild yeasts were characterized according to their morphology and physiological activities. Matrix-Assisted Laser Desorption/Ionization-Time of Flight Mass Spectrometry (MALDI-TOF/MS) was used for identification. Estimation of sugar and ethanol content were done by spectrophotometer and refractometer, respectively. In this study, 260 wild yeast species belonged to 11 genera were isolated from 160 samples through enrichment method. Four strains such as Meyerozyma guilliermondii, Candida pelliculosa, Pichia norvegensis, and Saccharomyces cerevisiae were screened as tolerant to 16-18% (v/v) ethanol. These isolates also showed tolerance to 45°C, 60% of sugar, and pH2 and they showed as fast gas producer from glucose with in 12hour. The highest Reduce sugar yield produced from 10% substrate Load of Banana peel, Papaya, Mango and Pineapple were 8.5 g/L, 9.1 g/L, 8.6 g/L and 8.3 g/L, respectively at the optimized hydrolysis conditions were 2% H2SO4, 90℃ and 48hour. The optimized fermentation condition for 10% pretreated and hydrolyzed Banana, Papaya ,Mango and Pineapple peel hydrolysates fermentation mediated by single culture of wild yeasts were 1% inoculum size active yeast culture, pH4, 30℃, 72hr, 200rpm with 14.3%v/v, 13%v/v, 9.7%v/v and 11%v/v bioethanol yield . The highest Bioethanol yield produced by Pichia norvegensis, and Saccharomyces cerevisiae. The optimized fermentation condition for 10% pretreated and hydrolyzed Banana ,Papaya, Mango and Pineapple peel hydrolysate fermentation mediated by co-culture of wild yeast were 0.5% inoculum size, pH5 , 30℃, 72hr and 200rpm with 22.9%v/v, 19.4%v/v 21.3%v/v and 16.5%v/v bioethanol yield were produced by co-culture of Pentose sugar fermenter Pichia norvegensis, and hexose sugar fermenter Saccharomyces cerevisiae. Co-culture (xylose and glucose-fermenting yeasts) proved to be more efficient and productive than single culture in producing bioethanol from selected substrates hydrolysate. The purity of bioethanol from Banana peel, Papaya peel, Mango peel, and peel by co-culture were 97.4%, 95.9%, 96.2% and 86.6% respectively. Thus, the newly isolated stress tolerant wild yeasts exhibit a great potential for commercial-scale bioethanol production in the future.