J. C. du Preez

Fermentation of D-xylose to ethanol by a strain ofCandida shehatae

SummaryA strain ofCandida shehatae fermented 9% D-xylose directly to ethanol within 40 hours with a yield coefficient of 0.29. The specific rate of ethanol production attained a maximum value of 0.28 g ethanol (g cells h)−1. The aeration rate greatly influenced the fermentation parameters. The performance of this yeast compared favourably with the published data for other xylose-fermenting microorganisms.

A quantitative screening of some xylose-fermenting yeast isolates

SummaryA quantitative screening procedure for xylose fermentation was conducted on 56 yeast isolates. Several of the isolates were found to be markedly superior toC. shehatae CSIR-Y492, one of the better xylose-fermenting yeasts identified thus far. The outstanding isolate was a strain ofPichia stipitis which had an ethanol yield coefficient of 0.45 from xylose and which produced no detectable amounts of xylitol.

Astaxanthin production by a Phaffia rhodozyma mutant on grape juice

During fermenter cultivation of Phaffia rhodozyma on a grape juice medium, the presence of glucose initially delayed fructose utilization, although fructose was consumed before glucose depletion. Total pigment and astaxanthin production were growth associated and reached maximum values of 15.9 μg/ml and 9.8 μg/ml, respectively, after depletion of the carbon source. The total cellular pigment and astaxanthin content increased during the stationary growth phase due to a decrease in biomass, reaching final values of 2120 μg/g and 1350 μg/g, respectively, without the volumetric concentration in the culture changing. The final cell yield was 0.33 g/g sugar utilized. High sugar concentrations in shake-flasks as well as O2 limitation decreased the astaxanthin content of the cells. Addition of yeast extract to a grape juice minimal medium markedly increased the maximum specific growth rate, total pigment and astaxanthin content of the cells. An excess of ammonia decreased the intracellular astaxanthin content, which reached a maximal value in cultures with no residual glucose or ammonia.

Selection and evaluation of astaxanthin-overproducing mutants of Phaffia rhodozyma.

Mutagenesis of Phaffia rhodozyma with NTG yielded a mutant with an astaxanthin content of 1688 μg (g dry biomass)-1, a cell yield coefficient of 0.47 on glucose and a maximum specific growth rate of 0.12 h-1. Re-mutation of the mutant decreased the cell yield and maximum specific growth rate but increased the astaxanthin content. The use of mannitol or succinate as carbon sources enhanced pigmentation, yielding astaxanthin contents of 1973 μg g-1 and 1926 μg g-1, respectively. The use of valine as sole nitrogen source also increased astaxanthin production, but severely decreased the maximum specific growth rate and cell yield coefficient. The optimum pH for growth of P. rhodozyma was between pH 4.5 and 5.5, whereas the astaxanthin content remained constant above pH 3.

Ethanol tolerance of Pichia stipitis and Candida shehatae strains in fed-batch cultures at controlled low dissolved oxygen levels

SummaryFed-batch cultivations of Pichia stipitis and strains of Candida shehatae with d-xylose or d-glucose were conducted at controlled low dissolved oxygen tension (DOT) levels. There were some marked differences between the strains. In general growth was inhibited at lower ethanol concentrations than fermentation, and ethanol levels of up to 47 g·l-1 were produced at 30°C. Ethanol production was mainly growth associated. The yeast strains formed small amounts of monocarboxylic acids and higher alcohols, which apparently did not enhance the ethanol toxicity. The maximum ethanol concentration obtained on d-xylose could not be increased by using a high cell density culture, nor by using d-glucose as substrate. The latter observation suggested that the low ethanol tolerance of these xylose-fermenting yeast strains was not a consequence of the metabolic pathway used during pentose fermentation. In contrast with the C. shehatae strains, it was apparent with P. stipitis CSIR-Y633 that when the ethanol concentration reached about 28 g·l-1, ethanol assimilation exceeded ethanol production, despite cultivation at a low DOT of 0.2% of air saturation. Discontinuing the aeration enabled ethanol accumulation to proceed, but with concomitant xylitol production and cessation of growth.

Production of gamma-linolenic acid by Mucor circinelloides and Mucor rouxii with acetic acid as carbon substrate

SummaryThe production of gamma-linolenic acid (GLA) by Mucor circinelloides CBS 203.28 and M. rouxii CBS 416.77 in fed-batch cultures operated in pH-stat mode with acetic acid as carbon substrate and titrant compared favourably with the performance of M. circinelloides in batch culture on glucose. On acetic acid M. circinelloides accumulated up to 39.8 mg GLA/g biomass, with a crude oil content of 28% containing 91% neutral lipids. The GLA content of the neutral lipid fraction was 15.6%.

Mucor-a source of cocoa butter and gamma-linolenic acid.

Lipid analyses were performed on 28 strains of various species of the genus Mucor. In shake flasks with glucose as carbon source, the gamma-linolenic acid (GLA) content in the neutral lipid (NL) fraction of some Mucor species was up to 38 mg GLA/g dry biomass. Some Mucor species produced more than 20% (w/w) stearic acid (18:0) and more than 60% of their NL content as symmetrical triacylglycerols (SUS-TAGs) which corresponded to those of cocoa butter. Three Mucor species were evaluated in terms of the production of SUS-TAGs and GLA in pH-stat, fed-batch cultures in an air-lift fermenter with acetic acid as titrant and carbon source. Mucor circinelloides f. circinelloides CBS 108.16 accumulated 27% 18:0 in the NL fraction, which constituted approximately 40% of the dry biomass. In this case, the NL fraction contained more than 70% (w/w) SUS-TAGs.

D-xylose fermentation by Candida shehatae and pichia stipitis at low dissolved oxygen levels in fed-batch cultures

SummaryThe fermentation of D-xylose byCandida shehatae andPichia stipitis was studied in fed-batch fermentations using dissolved oxygen tension (DOT) control in the range of 0.2 to 1.4% air saturation. The response of these two yeasts to DOT was significantly different. Whereas the ethanol yield withC. shehatae was 0.35 to 0.38 g.g−1 at all DOT levels, that ofP. stipitis decreased from 0.44 at a zero DOT reading to 0.19 g.g−1 at 1.4% DOT.

The production of hemicellulases by Thermomyces lanuginosus strain SSBP: influence of agitation and dissolved oxygen tension

Abstract Shake-flask cultivation of T.u2009lanuginosus strain SSBP on coarse corn cobs yielded β-xylanase levels of 56,500u2009nkat/ml at 50u2009°C, whereas other hemicellulases (β-xylosidase, β-glucosidase, and α-l-arabinofuranosidase) were produced at levels less than 7u2009nkat/ml. Cultivation on d-xylose yielded much lower levels of xylanase (350u2009nkat/ml), although other hemicellulase levels were similar to those produced on corn cobs. The influence of agitation rate and dissolved oxygen tension (DOT) on hemicellulase production was studied further in a bioreactor. On xylose, xylanase activities of 4,330u2009nkat/ml and 4,900u2009nkat/ml were obtained at stirrer speeds up to 1,400u2009rpm to control DOT. At a constant stirrer speed of 400u2009rpm, xylanase activities of 10,930u2009nkat/ml and 15,630u2009nkat/ml were obtained when cultivated on xylose and beechwood xylan respectively, despite DOT levels below 5% for the duration of fermentation. The results indicate that there is an interaction between agitation rate and DOT, impacting on xylanase and accessory enzyme production. Higher agitation rates favoured the production of xylosidase, arabinofuranosidase and glucosidase by T. lanuginosus strain SSBP, whereas the lower agitation rates favoured xylanase production. Rheological difficulties precluded cultivation on corn cobs in the bioreactor. Volumetric xylanase productivities of 1,060,000u2009nkat/lu2009·u2009h and 589,000u2009nkat/lu2009·u2009h obtained on beechwood xylan and xylose indicate that T. lanuginosus strain SSBP is a hyper-xylanase producer with considerable industrial potential.

Protein enrichment of sweet potato residue by solid-state cultivation with mono- and co-cultures of amylolytic fungi.

The degree of protein enrichment of sweet potato residue by different amylolytic moulds in solid-state cultivation was much greater than that obtained using amylolytic yeasts. The optimum initial moisture content for protein enrichment was about 65% (w/w). Adding nitrogen sources to the culture twice (at the start of the incubation and after 24 h) considerably improved the final protein content. A co-culture of amylolytic mycelial fungi yielded a product with 32% (w/w) crude protein after 4 days incubation at 30°C. In a column reactor, the highest temperatures reached were 42°C and 40°C and the minimum O2 concentrations were 1.5% and 2.5% of full saturation in the central and bottom layers, respectively.