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      The Influence of Controlling Redox Potential on Plasma Membrane Fatty Acid Composition during Very High Gravity Fermentation

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      Date
      2016-01-08
      Author
      Bai, Yun
      Type
      Thesis
      Degree Level
      Masters
      Metadata
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      Abstract
      Fatty acid components on yeast plasma membrane were critical in maintaining proper cell activity during bioethanol fermentation. The alteration of fatty acid composition on yeast plasma membrane was recognized as an adaptive response to several environmental stress including osmotic pressure, ethanol inhibition and nutrients limit. These stresses were exacerbated under very-high-gravity condition in which excessive fermentable sugar was provided in feedstock. Controlling redox potential was proved beneficial in improving yeast performance under very-high-gravity condition. Fatty acid synthesis and desaturation pathways involved dissolved oxygen as well as balance between NAD+/NADH and NADP+/NADPH which could be influenced by the regulation of redox potential in media. In this study, fatty acid composition profiles under different glucose concentrations and different redox potential control level were examined. Its connection with yeast cell growth, ethanol productivity and other metabolites’ concentrations were studied as well to reveal any causal correlation between redox potential control, membrane fatty acid composition and yeast activity. Two glucose concentrations used in this study were 200 g/L and 300 g/L which represented normal and very high gravity respectively in bioethanol fermentation. In 300 g/L fermentation, three redox conditions were adopted while two different redox conditions were used in 200 g/L fermentation. Biomass concentration, ethanol productivity and fatty acid composition were observed to be affected by both gravity and ORP control strategy. Final biomass concentrations were 4.302 g/L in 200 g/L glucose with no ORP control condition and 7.658 in 200 g/L glucose with ORP controlled at -100 mV condition. In 300 g/L glucose fermentation, final biomass concentrations were 3.400 g/L for no ORP control, 4.953 g/L for -150 mV ORP control and 5.260 for -100 mV ORP control. Ethanol productivities were 2.574 g/Lh for 200 g/L glucose without ORP control and 3.780 g/Lh for 200 g/L glucose with -100 mV ORP control. In 300 g/L glucose fermentation, ethanol productivity decreased to 1.584 g/Lh when no ORP control was imposed. ORP control at -150 mV could improve the ethanol productivity to 1.693 g/Lh while -100 mV ORP control was able to further enhance the ethanol productivity to 1.829 g/Lh. Fatty acid composition was observed to shift to more saturated components when no ORP control was applied. Such trend of saturation was increased by higher gravity condition. ORP control was shown to change this tendency to saturation and help restore fatty acid components on plasma membrane to a more balanced distribution.
      Degree
      Master of Science (M.Sc.)
      Department
      Chemical and Biological Engineering
      Program
      Chemical Engineering
      Supervisor
      Lin, Yen-Han
      Committee
      Niu, Catherine; Nemati, Mehdi
      Copyright Date
      December 2015
      URI
      http://hdl.handle.net/10388/ETD-2015-12-2362
      Subject
      very-high-gravity fermentation
      redox potential control
      fatty acid composition
      membrane
      biomass
      ethanol productivity
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