Isolation and evaluation of xylose-fermenting thermotolerant yeasts for bioethanol production

Abstract

Three thermotolerant yeasts capable of producing more than 7 g/L of ethanol from 30 g/L xylose were isolated. Molecular identification using ribosomal DNA internal transcribed spacer showed that isolates Pa27 and Pi131 were strains of Pichia kudriavzevii while Ma9 was a strain of Candida tropicalis. Under optimal conditions, Pa27, Ma9 and Pi131 produced maximum ethanol yields (g ethanol/g xylose) of 0.35 ± 0.002, 0.35 ± 0.007 and 0.39 ± 0.002, respectively. At a fermentation temperature of 42 °C, the maximum ethanol yields (g ethanol/g xylose) were 0.21 ± 0.005, 0.22 ± 0.005 and 0.23 ± 0.006 after 96 hours for Pa27, Ma9 and Pi131, respectively. The isolates co-fermented glucose and xylose to ethanol, and the presence of a small amount of glucose improved the xylose utilization rate. Pa27 was the best strain for fermentation of a mixture of xylose and glucose. At the optimum ratio of glucose to xylose (4:6 to 5:5), ethanol yield by Pa27 decreased from 0.43 to 0.35 when the temperature was increased from 30 to 42 °C. It is notable that even at 42 °C, all the three isolates fermented a mixture of xylose and glucose with ethanol yields higher than 0.33 g ethanol/g sugar.