The purpose of this study was to isolate, identify, and characterize the thermotolerant yeasts for use in high-temperature ethanol fermentation. Thermotolerant yeasts were isolated and screened from soil samples collected from the Mekong Delta, Vietnam, using the enrichment method. Classification and identification of the selected thermotolerant yeasts were performed using matrixassisted laser desorption ionization/time-of-fight mass spectrometry (MALDI-TOF/MS) and nucleotide sequencing of the D1/D2 domain of the 26S rDNA and the internal transcribed spacer (ITS) 1 and 2 regions. The ethanol production by the selected thermotolerant yeast was carried out using pineapple waste hydrolysate (PWH) as feedstock. A total of 174 yeast isolates were obtained from 80 soil samples collected from 13 provinces in the Mekong Delta, Vietnam. Using MALDI-TOF/MS and nucleotide sequencing of the D1/D2 domain and the ITS 1 and 2 regions, six different yeast species were identified, including Meyerozyma caribbica, Saccharomyces cerevisiae, Candida tropicalis, Torulaspora globosa, Pichia manshurica, and Pichia kudriavzevii. Among the isolated thermotolerant yeasts, P. kudriavzevii CM4.2 displayed great potential for high-temperature ethanol fermentation. The maximum ethanol concentration (36.91 g/L) and volumetric ethanol productivity (4.10 g/L h) produced at 45°C by P. kudriavzevii CM4.2 were achieved using PWH containing 103.08 g/L of total sugars as a feedstock. These findings clearly demonstrate that the newly isolated thermotolerant yeast P. kudriavzevii CM4.2 has a high potential for secondgeneration bioethanol production at high temperature.

High-temperature ethanol fermentation by thermotolerant yeast is considered a promising technology for ethanol production, especially in tropical and subtropical regions. In this study, optimization conditions for high-temperature ethanol fermentation of pineapple waste hydrolysate (PWH) using a newly isolated thermotolerant yeast, Saccharomyces cerevisiae HG1.1, and the expression of genes during ethanol fermentation at 40 °C were carried out. Three independent variables, including cell concentration, pH, and yeast extract, positively affected ethanol production from PWH at 40 °C. The optimum levels of these significant factors evaluated using response surface methodology (RSM) based on central composite design (CCD) were a cell concentration of 8.0 × 107 cells/mL, a pH of 5.5, and a yeast extract concentration of 4.95 g/L, yielding a maximum ethanol concentration of 36.85 g/L and productivity of 3.07 g/L. Gene expression analysis during high-temperature ethanol fermentation using RT–qPCR revealed that the acquisition of thermotolerance ability and ethanol fermentation efficiency of S. cerevisiae HG1.1 are associated with genes responsible for growth and ethanol stress, oxidative stress, acetic acid stress, DNA repair, the pyruvate-to-tricarboxylic acid (TCA) pathway, and the pyruvate-to-ethanol pathway.

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