Akira Ando

A UV-induced mutant of Pichia stipitis with increased ethanol production from xylose and selection of a spontaneous mutant with increased ethanol tolerance

In the fermentation process of lignocellulosic biomass (such as wood and rice straw), efficient conversion of pentose (mainly xylose) into ethanol is important. Mutants of Pichia stipitis NBRC1687 were obtained after UV mutagenesis and selection of large colonies on ethanol-containing medium. One mutant, PXF58, produced 4.3% ethanol from 11.4% xylose while the parent strain only produced 3.1%. The ethanol productivities of PXF58 from glucose and fructose were about were about 1.4-fold higher than those of the parent strain. After continuous cultivation of PXF58 in YNB (yeast nitrogen base) medium containing 2% xylose and 5-7% ethanol, an ethanol-tolerant mutant, PET41, was obtained. Strain PET41 was able to produce 4.4% ethanol when first supplied with xylose then with glucose. This isolate might be thus useful for two-phase fermentation in which xylan is saccharified by xylanase to produce xylose, and glucan is saccharified later by cellulase and β-glucosidase to produce glucose.

Selection of stress-tolerant yeasts for simultaneous saccharification and fermentation (SSF) of very high gravity (VHG) potato mash to ethanol.

Highly concentrated bioethanol production requires less volume in fermentation tanks and conserves distillery energy. We screened osmotolerant yeasts from a collection of 1699 yeast strains at our institute and found that three strains, NFRI3062, NFRI3213, and NFRI3225, were candidates for use in bioethanol production. All of these strains belonged to Saccharomyces cerevisiae. NFRI3062 produced 15.0% (w/v) of ethanol from YPD medium containing 35% glucose cultivated at 30 degrees C for 60 h, while S. cerevisiae NBRC0224, which has previously been reported suitable for ethanol production, only produced 13.0% (w/v). The thermotolerances of NFRI3213 and NFRI3225 were also superior to those of NBRC0224 and NFRI3062. We also demonstrated the simultaneous saccharification and fermentation (SSF) of very high gravity (VHG) potato mash and sweet-potato mash. NFRI3225 produced ethanol from potato mash at the fastest rate and in the highest volume (13.7% (w/v)) among the tested strains. The maximum productivity and ethanol yields were 9.1g/L/h and 92.3%, respectively. Although the potato mash was not sterilized, bacterial contamination was not observed. This may have been due to the growth inhibition of bacteria by the rapid glucose consumption and ethanol production of NFRI3225 during the VHG-SSF process.

Ethanol production by repeated-batch simultaneous saccharification and fermentation (SSF) of alkali-treated rice straw using immobilized Saccharomyces cerevisiae cells.

Repeated-batch simultaneous saccharification and fermentation (SSF) of alkali-treated rice straw using immobilized yeast was developed to produce ethanol. Saccharomyces cerevisiae cells were immobilized by entrapping in photocrosslinkable resin beads, and we evaluated the possibility of its reuse and ethanol production ability. In batch SSF of 20% (w/w) rice straw, the ethanol yields based on the glucan content of the immobilized cells were slightly low (76.9% of the theoretical yield) compared to free cells (85.2% of the theoretical yield). In repeated-batch SSF of 20% (w/w) rice straw, stable ethanol production of approx. 38gL(-1) and an ethanol yield of 84.7% were obtained. The immobilizing carrier could be reused without disintegration or any negative effect on ethanol production ability.