Ethanologenic yeasts from Ethiopian fermented beverages and optimization of fermentation conditions

Abstract

The rising global demand of renewable energy sources such as bioethanol is pushing stakeholders to search for efficient ethanologenic microbes. This study aimed to isolate yeasts from Ethiopian traditional fermented beverages, evaluate their ethanol yield using banana peel as a substrate, and identify best-performing isolates. Moreover, this study attempted to statistically optimize fermentation conditions through Placket Burman Design and Central Composite Design based Response Surface Methodology (RSM). A progressive screening and selection approach was followed to select best performing isolates. Factors that significantly influence fermentation efficiency were selected using Plackett-Burman Design (PBD) and then optimized via Central Composite Design (CCD). A total of 11 isolates were obtained. Three isolates (DMTD2, GB1D5 and RTj3D3) were selected based on their tolerance to stress conditions, and carbohydrate fermentation and assimilation performances. GB1D5 and RTj3D3 were highly tolerant whereas DMTD2 was moderately tolerant to 10 % ethanol concentration with survival percentages ranging from 40.8 ± 5.6 to 65.7 ± 10.5 %. However, they were slightly tolerant to 15 and 20 %, with an insignificant survival percentage rate. They yield 11.6 ± 0.9 to 12.9 ± 0.3 g/L ethanol from 8 % (w/v) banana peel waste. D1/D2 genotyping identified them as Saccharomyces cerevisiae strains. Glucose concentration and incubation period significantly affect ethanol production. The maximum actual ethanol yield produced by isolates at optimum condition was the same i.e. 31.4 g/L, whereas the ethanol yield predicted by the model ranged between 31.8 and 32.5 g/L. Results demonstrate these strains are promising candidates for bioethanol production. Optimization of fermentation conditions using banana peel waste as substrate is recommended.