Abstract
Yeasts are essential microorganisms for the ethanol production, and their efficiency depends on the physicochemical characteristics of the fermenting substrate. These microorganisms can be found in various soil and plant sources, such as leaves, bark, and litter of natural forests. The goal of the current study was to assess ethanol production using stress‐tolerant yeasts isolated from leaves, litters, rhizosphere soils, and tree barks collected from three natural forests (Belete‐Gera, Boter‐Becho, and Yayu Biosphere Reserve) in Southwest Ethiopia. Matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) was used to identify the isolates. Design‐Expert 11.1 (response surface methodology) was used to optimize the fermentation condition for effective ethanol production. A total of 406 yeasts were isolated, of which 67% were capable of ethanol production while 33% were not producers. The strongly fermentative isolates belonged to three genera: Saccharomyces cerevisiae, Meyerozyma guilliermondii, and Candida pelliculosa. S. cerevisiae 9Li2 and 35L1 were excellent candidates for ethanol production, with S. cerevisiae 9Li2 producing the highest ethanol (19.05 g/L) at optimal conditions (pH 5.09, 61.19 hours, and 30°C), while S. cerevisiae 35L1 produced maximum ethanol (19.03 g/L) at pH 4.92 and 30.03°C, within 71.55 hours. On the other hand, S. cerevisiae 9Li2 was able to ferment 40 g/L of glucose to produce 15 g/L of ethanol at 40°C with 73% efficiency at the ideal temperature (40°C), pH (5.167), and duration (72 hours), with a desirability of 0.819. The highest ethanol production (99.5 g/L) was achieved by S. cerevisiae 9Li2 after 48 hours at pH 5.0 and 30°C using 200 g/L of glucose. This work shows that multi‐stress‐tolerant S. cerevisiae isolates can thrive in harsh environments and should be enhanced for industrial bioethanol production exploiting their promising fermentation capacity.