The Effects Of Oxidoreduction Potential On Very High Gravity Ethanol Fermentation And Studies Of Underlying Mechanisms

Very high gravity (VHG) fermentation can save energy consumption, not only for ethanol distillation but also for distillage treatment due to a significant reduction of the distillage discharged from the distillation system. On the other hand, oxidoreduction potential (ORP) that can effectively monitor physiological changes of yeast cells under ethanol fermentation conditions. In this project, the ORP control was incorporated into VHG ethanol fermentations, with an objective to improving ethanol concentration as well as ethanol yield.1. Optimizing the ORP set point for VHG ethanol fermentationWhen Saccharomyces cerevisiae was grown under three glucose concentrations (ca.200,250 and 300 g/L) and controlled at two redox potentials -150 and -100 mV, we observed the highest ethanol yield and ethanol concentration were achieved at -150 mV, and the effect of the ORP control was enhanced with the increase of initial glucose concentrations, making this strategy more suitable for VHG fermentations. Compared with experiment without ORP control, the fermentation efficiency under -150 mV ORP control increased by 0.09%,6.09% and 7.46%, respectively.2. Development of kinetic models for ORP-controlled VHG fermentationsA growth kinetic model was developed for VHG fermentations under ORP control conditions, which, together with the kinetic equations of ethanol production and glucose consumption developed from mass balance principles, was used to predict ethanol fermentation performance, with the following results:the maximum ethanol concentration and efficiency of 133.2 g/L and 97.2% could be achived for the VHG fermentations with the media containing glucose of 275 and 235 g/L, and the ORP at -140 mV and -145 mV, respectively.3. Establishment of the ORP-controlled strategies for VHG fermentationsThree ORP control schemes, aeration-controlled scheme (ACS), glucose-controlled feeding scheme (GCS) and combined Chemostat and aeration-controlled scheme (CCS), were proposed to maximize its effect on VHG fermentations. Experimental results indicated that they were significant in improving ethanol fermentation performance, compared to the control without ORP control.4. Designing ageing vessels for the Chemostat and ACS systemThe application of continuous VHG fermentation is hindered by incomplete sugar consumption, which correspondingly compromises ethaol yield that is calculated based on the total sugars fed into the fermentation system without the deduction of residual sugars. To overcome this disadvantage, a continuous ORP control fermentation configuration with a Chemostat vessel was connected to two ageing vessels arranged in parallel. The Chemostat vessel is subjected to ORP control to maintain yeast viability, and the ageing vessels are used to deplete glucose before discharging to next process unit. When two aging vessels are switched alternatively, the steady state operation of the down stream process can be guaranteed.5. Yeast gene expression profiles under VHG fermentation and ORP control conditionsYeast cells adjust gene expression to fulfill VHG fermentation, where various environmental stresses such as ethanol inhibition and osmotic pressure exist. The global gene expression profiles of the industrial strains of S. cerevisiae were analyzed by microarrays with～6300 yeast open reading frames. Genomics analysis showed that the time course gene expression dramatically changed during two periods including 6-12 h (from lag phase to log phase) and 30-36 h (from stationary phase to death phase). Gene expression in the first period involed many pathways, while gene expression in the second period was mainly for stress response. Three Gene Ontology (GO) classes, response to stress, cellular carbohydrate metabolic and cellular respiration, were further given details by clustering. The genes GPD2 and NDI1 might have tight relationships with the ORP control.6. Applying the ORP control scheme into ethanol fermentation with self-flocculating yeastThe brewing industry often takes the advantage of the flocculation of yeast as a cost-effective way to separate biomass from fermentation products. When introducing the ORP control scheme into ethanol fermentation with self-flocculating yeast, not only should the ethanol production performance be investigated, but also the size distribution of yeast flocs. The results showed that the ORP control duration was prolonged than that for regular yeast systems, indicating this strategy is more effective. The size distributions of flocs were influenced by ORP levels, but its mechanism is still not clear.In conclusion, the performance of VHG ethanol fermentation can be improved when the ORP control strategy is applied. With the kinetic models established, the fermentation system can be further optimized and competitive advantage on economy will help the world reduce dependence on fossil fuel.