A total of 500 thermotolerant fermenting yeast isolates (100 from palm-wine and 400 from spoilt fruits) were screened for ethanol production at high temperatures. The best isolate that produced up to 4% ethanol from 10% glucose at 45˚C was selected for further experiments. The optimum pH for ethanol production by the isolate was pH 6 at both 30˚C and 42˚C. The isolate was identified as Pichia kudriavzevii base on the 18s ribosomal DNA. Ethanol production from 200 g/L cassava pulp using Simultaneous Saccharification and Fermentation (SSF) method at 30˚C and 42˚C by the isolate was investigated. At 30˚C, an ethanol concentration of 30 g/L was produced. This represents an ethanol yield of 0.15 g/g of cassava pulp and 58.8% of the theoretical yield. However at 42˚C, the concentration of ethanol produced increased to 42 g/L representing an ethanol yield of 0.21 g/g of cassava pulp and 82.4% of the theoretical yield. The isolate produced slightly higher ethanol than the two typed strains NCYC 587 and NCYC 2791 at 42˚C. This isolate has a good potential to be used for commercial bioethanol production since it can produce ethanol at 45˚C without a significant drop in ethanol yield.
Background
Although bioethanol production has been gaining worldwide attention as an alternative to fossil fuel, ethanol productivities and yields are still limited due to the susceptibility of fermentation microorganisms to various stress and inhibitory substances. There is therefore an unmet need to search for multi-stress-tolerant organisms to improve ethanol productivity and reduce production cost, particularly when lignocellulosic hydrolysates are used as the feedstock.
Results
Here, we have characterized a previously isolated Pichia kudriavzevii LC375240 strain which is thermotolerant to high temperatures of 37 °C and 42 °C. More excitingly, growth and ethanol productivity of this strain exhibit strong tolerance to multiple stresses such as acetic acid, furfural, formic acid, H2O2 and high concentration of ethanol at 42 °C. In addition, simple immobilization of LC375240 on corncobs resulted to a more stable and higher efficient ethanol production for successive four cycles of repeated batch fermentation at 42 °C.
Conclusion
The feature of being thermotolerant and multi-stress-tolerant is unique to P. kudriavzevii LC375240 and makes it a good candidate for second-generation bioethanol fermentation as well as for investigating the molecular basis underlying the robust stress tolerance. Immobilization of P. kudriavzevii LC375240 on corncobs is another option for cheap and high ethanol productivity.
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