Effect of controlled oxygen limitation on Candida shehatae physiology for ethanol production from xylose and glucose

Abstract

Carbon distribution and kinetics of Candida shehatae were studied in fed-batch fermentation with xylose or glucose (separately) as the carbon source in mineral medium. The fermentations were carried out in two phases, an aerobic phase dedicated to growth followed by an oxygen limitation phase dedicated to ethanol production. Oxygen limitation was quantified with an average specific oxygen uptake rate (OUR) varying between 0.30 and 2.48 mmolO(2) g dry cell weight (DCW)(-1) h(-1), the maximum value before the aerobic shift. The relations among respiration, growth, ethanol production and polyol production were investigated. It appeared that ethanol was produced to provide energy, and polyols (arabitol, ribitol, glycerol and xylitol) were produced to reoxidize NADH from assimilatory reactions and from the co-factor imbalance of the two-first enzymatic steps of xylose uptake. Hence, to manage carbon flux to ethanol production, oxygen limitation was a major controlled parameter; an oxygen limitation corresponding to an average specific OUR of 1.19 mmolO(2) g DCW(-1) h(-1) allowed maximization of the ethanol yield over xylose (0.327 g g(-1)), the average productivity (2.2 g l(-1) h(-1)) and the ethanol final titer (48.81 g l(-1)). For glucose fermentation, the ethanol yield over glucose was the highest (0.411 g g(-1)) when the specific OUR was low, corresponding to an average specific OUR of 0.30 mmolO(2) g DCW(-1) h(-1), whereas the average ethanol productivity and ethanol final titer reached the maximum values of 1.81 g l(-1) h(-1) and 54.19 g l(-1) when the specific OUR was the highest.

Assessment of Candida shehatae viability by flow cytometry and fluorescent probes

“…The maximum yield of ethanol production on xylose was 0.45 g·g −1 (Slininger et al, 1985) whereas maximum ethanol titre reached was 48 g/L (Fromanger et al, 2010) which is much lower than that observed with Saccharomyces cerevisiae (140 g/L). The physiological behaviour of yeast towards high ethanol concentrations needs further investigation to identify the biological bottlenecks of ethanol production (Kastner et al, 1999).…”

“…Strain and culture C. shehatae strain ATCC 22984, a xylose fermenting yeast, was maintained on Plate Count Agar (PCA) (AES Chemunex), and its long-term storage at −80°C used Yeast Peptone Dextrose (YPD) medium with 45% glycerol. Mineral medium for growth was supplemented with vitamins and oligo-elements as described by (Fromanger et al, 2010). Pre-culture grown on YPD (without glycerol) for 24 h at 30°C, stirred at 100 rpm was transferred to a flask containing mineral medium complemented with 20 g/L xylose.…”

Bioconversion of lignocellulosic waste to bioethanol by Trichoderma and yeast fermentation

“…Similarly, the present study attained maximum bioethanol production of 85.6 % under the optimized conditions of temperature of 36.5 °C, incubation time of 102 h and enzyme-treated sawdust of 45.14 ml l −1 and agitation of 330 rpm; little variations of the optimized conditions were attributed due to the higher availability of monosugars in the Trichoderma -derived enzyme-treated sawdust hydrolysis. Thus, the temperature has a significant role in the bioethanol production by increasing the enzyme hydrolysis of the sawdust concentration and also the bioethanol yield due to increased production of fermentable monosugars (Fromanger et al 2010; Zuroff and Curtis 2012). The present work suggested that the maximum production of the bioethanol from the sawdust could be achieved by proper method of acid and enzyme pretreatment and yeast fermentation employing H. estonica and S. cerevisiae .…”

Using microorganisms to brew biofuels

“…One drawback of C. shehatae is its inability to produce ethanol in the presence of oxygen. Current studies are focused on measuring the optimal oxygen uptake rate required for maximum ethanol production from xylose (Fromanger et al 2010). These studies reveal that while C. shehatae is able to produce ethanol from xylose, the yield is rather low.…”