Abstract

A yeast strain with higher rates and yields in the fermentation of glucose, mannose and galactose in semiaerobic conditions than Pichia stipitis and Sacharomyces cerevisiaeand ethanol tolerance than P. stipitis, was isolated from sugarcane baggase from Iranian resources. This strain that can ferment xylose with lower rates and yields than P. stipitisis characterized as Kluyveromyces marxianus. The ability of K. marxianus to ferment mixed sugars comprised of 30 g/l glucose, 30 g/l xylose, 12 g/l mannose and 8 g/l galactose (total sugar 80 g/l), as a model of many hydrolysates, were compared to P. stipitis and S. cerevisiae and then a coculture of P. stipitis and S. cerevisiae was compared with a coculture of P. stipitis and K. marxianus. In mixed sugars fermentation with individual yeasts P. stipitis shows the highest yield (0.40 gg-1) and maximum ethanol (30.23 gl-1), but K. marxianus shows the highest Qpmax (1.09 gl-1h-1) and substrate utilization efficiency (E,>99%). P. stipiti and K. marxianuscoculture shows the best results with high yield (0.42 gg-1) and maximum ethanol (31.87 gl-1), Qpmax (1.09 gl-1h-1) and substrate utilization efficiency (E,>99%). Because of the higher rates and yields of K. marxianus to ferment hexoses than P. stipitis and its higher ethanol tolerance, K. marxianus helps the P. stipitis to reach the concentration higher than 30 g/l ethanol concentration.

Key words: Ethanol, Pichia stipitis, Kluyveromyces marxianus, xylose.