Durbha et al.

Consequently, glucose utilization was also almost complete and nearly same

at 30 and 35°C temperatures whereas considerable amount of glucose was left

unutilized at 40°C.

Hacking et al., (1984) reported that six among the 55 isolated yeast strains were

able to grow at 45°C. Szczodrak and Targonski 1988 and D’Amore et al., 1989

reported thermotolerant strains of Saccharomyces that could grow at above 40°C

and ferment sugars at 40°C. Sree et al., 2000 reported Saccharomyces strains that

tolerate 44°C but growth and ethanol yields were low. Optimal range of 30–35°C

is required for fuel ethanol production (Bollók et al., 2000; Lin and Tanaka 2006)

but 40-50°C is desirable to decrease operation costs and moreover, ethanol was

efficiently extracted at 40°C when compared to at 35°C (Babiker et al., 2010).

Unlike the statement of Cantarel et al., 2009 that yeast ferments below 35°C, the

above cited reports and the two strains of the present study are able to ferment at

40°C.

Though thermotolerant strain (Babiker et al., 2010) or a metabolically altered

thermophillic bacterium can be used (Shaw et al., 2008) in simultaneous

saccharification and fermentation (SSF), their ethanol tolerance is around 2% v/v

which is very less compared to that of yeast. Kluyveromyces marxianus is thermo

tolerant and natural xylose-fermenting yeast that could grow up to 52°C with 98%

theoretical yield (Banat et al., 1992) and could ferment at elevated temperatures

above 40°C, however, like recombinant xylose-fermenting S. cerevisiae strains, it

also accumulates xylitol during fermentation (Cantarel et al., 2009). Hansenula

polymorpha is also a naturally fermenting yeast which can ferment xylose at 48°C

but produces low amount of ethanol.

Minor changes in physical or chemical parameters can affect fermentation

efficiency of yeast strains leading to decreased ethanol yield. As the present two

strains are showing nearly similar performance, they can be used for large-scale

ethanol production. Among these two, R3DSC5 is better than R3DPMP due its

ability to grow and produce ethanol at higher glucose level (30% g/v) with good

yield and higher temperature tolerance. The optimum conditions identified are 7 to

9 × 10 8 cells/ml of inoculum density, 30 to 35 °C temperature and from 5-7 initial

pH for both the strains whereas 20% and 25% (g/v) glucose concentrations for

R3DPMP and R3DSC5 strains respectively.

76

Int. J. Bioinformatics and Biological Sci.: v.2 n.1&2, p. 71-83. March & June 2014