Lychee-Derived, Thermotolerant Yeasts for Second-Generation Bioethanol Production

Thermotolerant yeasts are widely considered to be alternative strains to traditional yeasts for bioethanol production at high temperatures. In this study, thirty-two yeasts isolated from lychees were screened for thermotolerance, and seven selected isolates were identified as Candida tropicalis (isolates H8, H19, and H23), Meyerozyma guilliermondii (isolates H1 and H12) and Saccharomyces cerevisiae (isolates H10 and H18). They tolerated up to 45 °C, 12% (v/v) ethanol concentration, 10 g/L acetic acid, and 5 g/L furfural, respectively, and produced 47.96 to 70.18 g/L of ethanol from 160 g/L glucose at 40 °C during 48 h of fermentation. Among the evaluated yeasts, M. guilliermondii H1 showed great potential for second-generation bioethanol fermentation with its ability to ferment xylose and arabinose. Under the optimal conditions resulting from a Plackett Burman design and a Box Behnken design, the highest ethanol concentration of 11.12 g/L was produced from 40 g/L substrate-based sugarcane bagasse hydrolysate (non-detoxified hydrolysate) at 40 °C by M. guilliermondii H1. These findings suggested that the newly isolated thermotolerant yeast M. guilliermondii H1 is a good candidate for ethanol production from agricultural wastes.