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Emerging Technology for Fermenting D-Xylose

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Emerging Technology for Fermenting D-Xylose Trends in Biotechnology, Vol. 3, No. 8, 1985 stimulated under aerobic conditions, Emerging technology for fermenting and the classical methods do not pro- vide for aeration during screens for fer- D-xylose mentative activity13. Current efforts to ferment D-xylose Thomas W. Jeffries largely began with the discovery by Wang, Shopsis and Schneider in In the past four years, numerous yeasts which convert D-xylose to 198014 that Schizosaccharomyces pombe ethanol have been reported. The conversion occurs most readily under and various other yeasts would’ aerobic conditions. Various aspects of this conversion have provided ferment D-xylulose, the keto-isomer of new insight into the mechanisms and metabolic regulation of ethanol D-xylose, to ethanol. This finding was fermentation in yeasts. Although specific fermentation rates, product significant because D-xylose can be yields and product concentrations are significantly lower with D-xylose readily converted to D-xylulose by than with D-glucose, technology is emerging which may prove to be D-xylose isomerase (= glucose iso- feasible for commercial fermentation of D-xylose-containing waste merase), Since this enzyme is so readily streams. available and produced on such a large scale, technology for the conversion of D-Xylose is the second most abundant by Karczewska4 in 1951. This D-xylose to ethanol by a two-stage sugar in nature, comprising up to 25% observation, however, went largely un- isomerization and fermentation was of the total dry weight of woody angio- recognized - a major review in 19765 rapidly developed 15 18. Interest in the sperms1 and an even larger fraction of recorded that roughly half of all yeast two-stage process waned, however, some agricultural residues2 where it species listed would assimilate when the direct fermentation exists as the polymer xylan. Even D-xylose for aerobic growth but none of D-xylose to ethanol by though it is not yet cheap nor com- would ferment it anaerobically. More- Pa. tannophilus19,20 and other mercially abundant, D-xylose, along over, in a recent taxonomic treatise on yeasts 9,21,23 was discovered. Of these with other hemicellulosic sugars can yeasts6, 64% of the species listed are species only a few, most notably be obtained in good yield (80-90% or cited as capable of assimilating xylose Pa. tannophilus and C. shehatae23, will more) through acid or enzymatic aerobically, but none are cited as carry out the fermentation at rates of hydrolysis of the hemicellulosic frac- capable of fermenting this sugar. practical interest (Table 1). tion. Moreover, D-xylose (or oligo- Another recent taxonomic synopsis7, Several conclusions are immediately meric xylan) is present in many waste however, notes that a few yeast evident from Table 1. First con- streams from sulfite and dissolving species - most conspicuously Pichia ventional D-glucose fermentations by pulp mills, fiberboard and hardboard stipitis, Candida shehatae, Pachysolen S. cerevisiae are 6-35 times faster than manufacturing plants3. Combined use tannophilus and Brettanomyces naar- even the best D-xylose fermentations of D-xylose and D-glucose during denensis - ferment D-xylose to ethanol by C. shehatae or Pa. tannophilus. In production of chemicals or fuel at various rates. In the last four years at this same regard, the D-glucose specific (ethanol) from angiosperm feedstocks least 41 yeast species, including 23 of fermentation rate (g ethanol (g cell dry could improve the overall process the genus Candida and eight of the weight) -1 h-1) of S. cerevisiae is 8-10 economics and using D-xylose from genus Pichia, have been shown times faster than the D-glucose specific waste streams could reduce disposal to produce some ethanol from fermentation rate of these two other costs and provide alternative by- D-xylose8 12. In most instances, the species. Second, reported ethanol product credits for existing processes. conversion to ethanol occurs yields (g product produced/g substrate aerobically. consumed) with D-xylose are only Xylose-fermenting organisms The discrepancy between results of about 56-82% of those reported for The anaerobic production of ethanol classical taxonomic methods for deter- D-glucose. Third, maximum attainable from D-xylose was first demonstrated mining fermentative activity and ethanol concentrations from D-xylose current findings stems largely from the are only about 23-46% of those attain- fact that in many instances, production able from D-glucose (Table 1). T. W. Jeffries is at the USDA, Forest Products Laboratory, 1 Gifford Pinchot of ethanol from D-xylose by yeasts is Taken together, these facts present a Drive, Madison, WI 53705, USA. either obligately aerobic or is greatly dismal picture for the D-xylose fer- Trends in Biotechnology, Vol. 3, No. 8, 1985 209 mentation. Ultimately, D-xylose must anaerobic conditions. Anaerobically, colonies on agar plates as a con- compete economically with D-glucose the cells do not grow; hence the sequence of their diminished res- as a feedstock. In view of the relative apparent increase in the rate is lower piratory capacity. Since they do not ages of the two technologies, however, than when observed aerobically31. possess a functional electron transport there is room for some optimism. There is general agreement that, pathway for ATP generation, they Moreover, it is possible that by using at least under aerobic conditions, must derive all their ATP from D-xylose from waste streams, a plant C. shehatae and Pi. stipitis strains are substrate-level phosphorylation and operator could avoid disposal charges superior to all other known yeast the cell yield is consequently smaller. and thereby offset some of the higher species in their rates of D-xylose fer- Grande strains can respire and hence process costs incurred in the D-xylose mentation. Similarities observed form large colonies. fermentation. between Pi. stipitis and C. shehatae Respiration-deficient mutants might have led to the suggestion that these be particularly useful in the D-xylose Selection of improved strains might be the teleomorphic and ana- fermentation where aeration plays a Improvements in the D-xylose morphic forms (the sexually perfect conspicuousrole in reducing the specific fermentation rate and final and asexual stages) of the same ethanol yield. To date, however, no ethanol concentration have been organism6. However, significant dif- petite mutants have been described for obtained principally by isolating better ferences exist among the various Pa. tannophilus, and it appears to be a strains from nature, and by mutating named strains25,and some strains petite-negative yeast (i. e. petite mutants and selecting strains in the laboratory. exhibit considerable instability in both do not survive more than a few genera- Genetics and strain selection are just their morphological and fermentative tions). In the case of C. shehatae, beginning with D-xylose-fermenting activities 32.Strains of C. shehatae however, unstable petite and grande yeasts. More progress has been made exhibiting high respiratory- and low strains have been demonstrated and with Pa.tarrnophilusthan with fermentative-activities have been iso- their fermentative abilities assessed32. C. sheharae or Pi. stipitis. Methods for lated on xylitol agar. As might be expected, strains with crossing strains have been developed A recent quantitative screening of 56 diminished respiratory capacity for Pa. tannophilus28; various yeast isolates identified as Candida (determined by the tetrazolium agar aneuploid and polyploid strains have species, C. tenuis, C. shehatae and Pi. overlay method39) generally show. been constructed29 and their capacities stipitis showed Pi. stipitis CSIR Y633 greater fermentative activity than for ethanol production have been to give the greatest yield of ethanol those exhibiting higher respiratory assessed30. Increasing the chromosome (0.45 g ethanol (g xylose)-1 consumed), capacity. It is worth noting that on number from the haploid to the diploid at the highest volumetric rate (0.92 g yeast malt agar, the strain exhibiting level resulted in a significant increase ethanol (g cells)-1 h-1 )33. most conspicuous petite/grande transi- in the ethanol yield. Further increases tions, C. shehatae ATCC 22984, in ploidy enhanced the ethanol yield Respiration deficiency reverts to a heterogeneous mixture of and D-xylose specific fermentation rate Selection for respiration deficiency, phenotypes with petite-like cells pre- to a lesser extent. Selection of has been proposed as a method to dominating”. The relationships Pa. tannophilus strains capable of rapid improve the efficiency of ethanol pro- between respiration capacity and growth on xylitol-plus-nitrate medium duction 34 37. Normal, respiration- ability to grow on a particular carbon also results in stable isolates of sufficient cells can oxidize ethanol source are complex and incompletely Pa. tannophilus exhibiting up to a two- after fermentation. Ethanol oxidation understood, and because these strains fold increase in the volumetric rates can also occur during fermentation if of C. shehatae are unstable, analysis is (g ethanol 1-1 h-1) of D-xylose (and respiration is not repressed by the difficult. D-glucose) fermentations under aerobic carbon source used38.Respiration- If a yeast strain does not possess the conditions, and a 1.5-fold increase in deficient (petite, rho-) mutants are biochemical machinery enabling it to the specific fermentation rate under strains of yeasts which form small carry out a balanced fermentation of 210 Trends in Biotechnology, Vol. 3, No. 8, 1985 xylose (see later), then the net accumu- mechanisms seems to have relevance to nitrate as a nitrogen source did not lation of NAD(P)H under anaerobic the D-xylose fermentation. show anaerobic fermentation in the conditions will stop metabolism. This The aerobic fermentation of presence of nitrate, whereas cells effect is particularly apparent when a D-xylose by C. tropicalis is similar in grown on ammonium did52. Our reduced carbon source such as xylitol is some ways to the Custers or Kluyver current studies show that shifting used. effects.
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