Fermentation ofd-xylose,d-glucose andl-arabinose mixture byPichia stipitis Y 7124: sugar tolerance

Ethanol production from eucalyptus wood hemicellulose hydrolysate by Pichia stipitis

Ethanol production was evaluated from eucalyptus wood hemicellulose acid hydrolysate using Pichia stipitis NRRL Y-7124. An initial lag phase characterized by flocculation and viability loss of the yeast inoculated was observed. Subsequently, cell regrowth occurred with sequential consumption of sugars and production of ethanol. Polyol formation was detected. Acetic acid present in the hydrolysate was an important inhibitor of the fermentation, reducing the rate and the yield. Its toxic effect was due essentially to its undissociated form. The fermentation was more effective at an oxygen transfer rate between 1.2 and 2.4 mmol/L h and an initial pH of 6.5. The hydrolysate used in the experiences had the following composition (expressed in grams per liter): xylose 30, arabinose 2.8, glucose 1.5, galactose 3.7, mannose 1.0, cellobiose 0.5, acetic acid 10, glucuronic acid 1.5, and galacturonic acid 1.0. The best values obtained were maximum ethanol concentration 12.6 g/L, fermentation time 75 h, fermentable sugar consumption 99% ethanol yield 0.35 g/g sugars consumed, and volumetric ethanol productivity 4 g/L day. (

Chemical and biochemical generation of carbohydrates from lignocellulose-feedstock (Lupinus nootkatensis)--quantification of glucose

Different chemical and enzymatic methods were applied for the hydrolysis of main stems from Lupinus nootkatensis (harvest November 2002). The whole process (all steps) is based on the lignocellulose-feedstock biorefinery regime. The acid hydrolysis of L. was performed with concentrated hydrochloric acid; advantages in this process are exothermic hydrolysis and the possibility of acid recovery. Enzymatic hydrolysis achieved high yields of fermentable carbohydrates (regarding to input cellulose) with high selectivity. However, this way requires the generation of cellulose from L. by chemical pulping. Monosaccharide derivatives thus obtained were identified by their GC retention times and the corresponding MS fragmentation. Hexamethyldisilazane was used as derivatization reagent to prepare the trimethylsilyl derivatives of the carbohydrates and of the degradations products of cellulose from the different fractions. The glucose content was quantified by GC peak integration with respect to an internal standard.

Peculiarities of the regulation of fermentation and respiration in the crabtree-negative, xylose-fermenting yeast Pichia stipitis

The respiration of Pichia stipitis was not repressed by either high concentrations of fermentable sugars or oxygen limitation. Fermentation was not induced by high sugar concentrations, but was inactivated by aerobic conditions. The activity of pyruvate dehydrogenase was constitutive. In contrast, pyruvate decarboxylase, alcohol dehydrogenase, and aldehyde dehydrogenase were induced by a reduction in the oxygen tension. It was demonstrated that in P. stipitis, the pyruvate decarboxylase is not induced by a signal from glycolysis. Contrary to Saccharomyces cerevisiae, the pyruvate decarboxylase was not inhibited by phosphate.

Arbinose utilization by xylose-fermenting yeasts and fungi

Various wild-type yeasts and fungi were screened to evaluate their ability to ferment L-arabinose under oxygen-limited conditions when grown in defined minimal media containing mixtures of L-arabinose, D-xylose, and D-glucose. Although all of the yeasts and some of the fungi consumed arabinose, arabinose was not fermented to ethanol by any of the strains tested. Arabitol was the only major product other than cell mass formed from L-arabinose; yeasts converted arabinose to arabitol at high yield. The inability to ferment L-arabinose appears to be a consequence of inefficient or incomplete assimilation pathways for this pentose sugar.

Regulation of d-xylose utilization by hexoses in pentose-fermenting yeasts

The aldopentose D-xylose is one of the most abundant sugars in plant biomass and its efficient microbial utilization is of fundamental importance in the overall bioconversion of lignocellulosic materials into liquid fuels and chemicals. The discovery of pentose-fermenting yeasts in the early 1980's led to world wide interest because of the perceived potential for improved D-xylose fermentation to enhance the prospect of biomass conversions. However, the utilization of D-xylose by pentose-fermenting yeasts can be adversely affected by the hexoses, mainly D-glucose and D-mannose, which are usually present in high amounts in lignocellulosic hydrolysates. Research in the past several years has uncovered some of the regulatory effects of D-glucose on D-xylose utilization. However, much remains unknown about the mechanisms responsible for these effects. This review summarizes the current state of knowledge on the induction, repression and inactivation of D-xylose utilization in pentose-fermenting yeasts.