Production of ethanol from wood hemicellulose hydrolyzates by a xylose-fermenting yeast mutant, Candida sp. XF 217

Conversion of pentoses to ethanol by yeasts and fungi

Fermentation of D-xylose is of interest in enhancing the yield of ethanol obtainable from lignocellulosic hydrolysates. Such hydrolysates can contain both pentoses and hexoses, and while technology to convert hexoses to ethanol is well established, the fermentation of pentoses had been problematical. To overcome the difficulty, yeasts and fungi have been sought and identified in recent years that can convert D-xylose into ethanol. However, operation of their cultures in the presence of the pentose to obtain rapid and efficient ethanol production is somewhat more complex than in the archetype alcoholic fermentation, Saccharomyces cerevisiae on D-glucose. The complexity stems, in part, from the association of ethanol accumulation in cultures where D-xylose is the sole carbon source with conditions that limit growth, by oxygen in particular, although limitation by other nutrients might also be implicated. Aspects of screening for appropriate organisms and of the parameters that play a role in determining culture variables, especially those associated with ethanol productivity, are reviewed. Performance with D-xylose as sole carbon source, in sugar mixtures, and in lignocellulosic hydrolysates is discussed. A model that involves biochemical considerations of D-xylose metabolism is presented that rationalizes the effects of oxygen on cultures where D-xylose is the sole carbon source, notably effects of the specific rate of oxygen use on the rate and extent of ethanol accumulation. Alternate methods to direct fermentation of D-xylose have been developed that depend on its prior isomerization to D-xylose, followed by fermentation of the pentulose by certain yeasts and fungi. Factors involved in the biochemistry, use, and performance of these methods, which with some organisms involves sensitivity to oxygen, are reviewed.

Fermentation of xylose and hemicellulose hydrolysates by an ethanol-adapted culture of Bacteroides polypragmatus

Bacteroides polypragmatus type strain GP4 was adapted to grow in the presence of 3.5% (w/v) ethanol by successive transfers into 1% (w/v) D-xylose media supplemented with increasing concentrations of ethanol. The maximum specific growth rate of the ethanol-adapted culture (mu = 0.30 h-1) was not affected by up to 2% (w/v) ethanol but that of the non-adapted strain declined by about 50%. The growth rate of both cultures was limited by nutrient(s) contained in yeast extract. The ethanol yield of the adapted culture (1.01 mol/mol xylose) was higher than that (0.80 mol/mol xylose) of the non-adapted strain. The adapted culture retained the ability to simultaneously ferment pentose and hexose sugars, and moreover it was not inhibited by xylose concentrations of 7-9% (w/v). This culture also readily fermented hemicellulose hydrolysates obtained by mild acid hydrolysis of either hydrogen fluoride treated or steam exploded Aspen wood. The ethanol yield from the fermentation of the hydrolysates was comparable to that obtained from xylose.

Aerobic Fermentation of D-Xylose to Ethanol by Clavispora sp

Eleven strains of an undescribed species of Clavispora fermented D-xylose directly to ethanol under aerobic conditions. Strain UWO(PS)83-877-1 was grown in a medium containing 2% D-xylose and 0.5% yeast extract, and the following results were obtained: ethanol yield coefficient (ethanol/D-xylose), 0.29 g g −1 (57.4% of theoretical); cell yield coefficient (dry biomass/D-xylose), 0.25 g g −1 ; maximum ethanol concentration, 5.9 g liter −1 ; maximum volumetric ethanol productivity, 0.11 g liter −1 h −1 . With initial D-xylose concentrations of 40, 60, and 80 g liter −1 , maximum ethanol concentrations of 8.8, 10.9, and 9.8 g liter −1 were obtained, respectively (57.2, 57.1, and 48.3% of theoretical). Ethanol was found to inhibit the fermentation of D-xylose ( K p = 0.58 g liter −1 ) more than the fermentation of glucose ( K p = 6.5 g liter −1 ). The performance of this yeast compared favorably with that reported for some other D-xylose-fermenting yeasts.