Bioprospecting thermotolerant yeasts from distillery effluent and molasses for high-temperature ethanol production

Aims Isolation, characterization and assessment of inhibitor tolerance of thermotolerant yeasts associated with distillery effluent and molasses, and their use in high-temperature ethanol production from alkali-treated rice straw. Methods and Results A total of 92 thermotolerant yeasts were isolated from seven different distillery effluent and molasses samples. Based on MSP-PCR, 34 yeasts were selected and identified by sequencing the D1/D2 domain of LSU rDNA. These yeasts belonged to eight genera and nine different species. We assessed the inhibitor tolerance of these 34 well-characterized yeasts against various pre-treatment-generated inhibitors (furfural, 5-hydroxymethyl furfural and acetic acid) and also evaluated their ethanol yields at 40, 45 and 50celcius. Among selected strains, Pichia kudriavzevii DSA3.2 exhibited the highest ethanol production (24.5 g l(-1)) with an efficiency of 95.7% at 40celcius using 5% glucose. At 45celcius, P. kudriavzevii DSA3.2 and Kluyveromyces marxianus MSS6.3 yielded maximum ethanol titres; 22.3 and 23 g l(-1) with 87.4% and 90% efficiency, respectively. While using alkali-treated RS at 45celcius, K. marxianus MSS6.3 produced 10.5 g l(-1) of ethanol with 84.5% fermentation efficiency via separate hydrolysis and fermentation, and 10.9 g l(-1) of ethanol with 85% efficiency via simultaneous saccharification and fermentation. Pichia kudriavzevii DSA3.2, DSA3.1 and K. marxianus MSS6.3 also exhibited significant tolerance against multiple inhibitors. Conclusions Yeast isolates P. kudriavzevii DSA3.2 and K. marxianus MSS6.3 exhibited significant inhibitor tolerance and proved to be suitable for high-temperature ethanol fermentation. After additional optimization and scale-up experiments, these isolates can be exemplary candidates for industrial-scale ethanol production from lignocellulosic biomass. Significance and Impact of the Study Our study recognizes distillery effluents and molasses as specialized niches for yeasts with a broad substrate range, capable of tolerating multiple inhibitors and yielding high levels of ethanol at elevated temperatures. These yeasts can further be exploited for bioethanol production through SSF/SHF at a larger scale.

Automated summary

The study isolated thermotolerant yeasts from distillery effluent and molasses, characterized their inhibitor tolerance, and conducted ethanol production experiments at high temperatures. Findings reveal significant potential of Pichia kudriavzevii and Kluyveromyces marxianus yeasts for high-temperature ethanol fermentation.