Production of second-generation ethanol from coffee farming residue by fermentative bioprocesses with different wild yeast strains

Bioethanol is a renewable energy alternative and can be produced from biomass rich in organic matter such as coffee straw, an abundant agro-industrial waste in Brazil. This study evaluated fermentative bioprocesses with two strains of wild yeasts and using hydrolysate obtained in the treatment of coffee straw by thermochemical hydrolysis in batch systems for obtaining second-generation ethanol. The molecular identification of 15 strains of wild yeasts isolated from brewery wastewater and decaying fruit was performed. A screening of those strains revealed Pichia kudriavzevii (LMP-Y 10) and Saccharomyces cerevisiae (LMP-Y 14) showed the best ethanol production (1.64 and 1.91 g L⁻¹, respectively). According to the optimization of biomass production, LMP-Y 10 at 38 °C, 20 g L⁻¹ of glucose, and pH 6.0 and LMP-Y 14, at 34 °C, 30 g L⁻¹ of glucose, and pH 7.0 provided better biomass yield. Fermentation bioprocess assays in a batch system from hydrolyzed coffee husk pretreated with H₂SO₄ 1 % and 2 %, with and without agitation, led to better ethanol production with H₂SO₄ 2 %, obtaining 0.58 g L⁻¹ with LMP-Y 10, under agitation, and 0.43 g L⁻¹ with LMP-Y 14, in static condition for both strains (in 20 mL of cultures at an F/M ratio = 1). The results reveal two previously unstudied wild strains with strong potential for second-generation bioethanol production through the valorization of coffee husk that can contribute to future research and advances in biotechnology, offering sustainable alternatives to produce bioenergy from agro-industrial waste.