Adapted Substrate Delivery Strategies to Enhance Production in Ethanol Fermentation
Creators: Hung, Yueh-Hao
Publisher: name: University of Alberta Library
Year: 2024

<p>Bioethanol, a renewable liquid biofuel, has been widely used to reduce carbon emissions from road transportation. Lignocellulosic biomass, such as forestry wood biomass or agricultural residues, is a sustainable resource for fuel ethanol production which does not compete with food supply. However, bioethanol production from lignocellulosic biomass still faces technological and economic challenges; hence the production steps – pretreatment, hydrolysis, fermentation, and purification – require further advancements. Despite fermentation being extensively applied to lignocellulosic ethanol production, studies on the management of substrate delivery to improve fermentation performance are limited. For instance, self-cycling fermentation (SCF), a semi-continuous fermentation approach, enables sustained yeast growth in the exponential phase and enhanced ethanol productivity through cycling operations. However, the final ethanol titer is restricted in this system by low substrate loading. The primary objective of this thesis is to advance efficient control of substrate delivery in the fermentation process to enhance ethanol production. <br>
In the first study two adapted feeding strategies, in which feed medium addition was adjusted to increase the supply of fermentable sugar, were investigated in pulsing fed-batch fermentation with Saccharomyces cerevisiae. Specifically, a linear adapted feeding strategy was established based on changes in cell biomass, and an exponential adapted feeding strategy was developed based on cell biomass accumulation. These two adapted feeding strategies led to increases in overall ethanol productivity of ~20% compared to fixed feeding operations. The results suggest that adjusting sugar feedings based on cell biomass in pulsing fed-batch fermentation leads to higher ethanol productivity than the fixed feeding modes. As the pulsed feeding operation spiked the feed medium into the 5-L bioreactor, glucose concentration increased dramatically followed by depletion until the next feeding.<br>
To maintain lower glucose content during the feeding period, a second study explored adapted feeding strategies using a continuous feeding approach. Evolved gas production, which positively correlated with glucose consumption, was used to adjust the sugar feed rate in fed-batch fermentations. As the sugar feed rate gradually increased, the residual glucose content remained near zero until the yeast was metabolically impacted by the ethanol accumulated. The results showed that the adapted feeding strategy enhanced ethanol productivity by 21% compared to the fixed continuous feeding strategy, in which the sugar feed rate was stable. In addition, the adapted continuous feeding strategy maintained the same ethanol productivity even under low-nitrogen feeding conditions. This study suggests that evolved gas production, a metabolic monitoring parameter, can be used to guide the continuous feeding approach for effectively delivering sugar in yeast ethanol fermentation. <br>
Finally, in the third study, the continuous adapted feeding strategy was integrated into SCF operation. Additionally, a single pulse feed second stage was implemented to further enhance ethanol titer and productivity. The two-stage integrated high-cell density SCF system was successfully applied to the fermentation of hydrolysate from steam-exploded poplar, achieving ~11% (v/v) ethanol, which is considered economically favorable for subsequent recovery in the bioethanol industry. <br>
Overall, this thesis demonstrates that adapted feeding strategies, based on the yeast’s metabolic responses, can efficiently manage sugar delivery through either pulsing or continuous feeding approaches in ethanol fermentation, thereby enhancing ethanol production. Furthermore, this thesis acts as a foundation to promote lignocellulosic ethanol production, especially when developing an integrated fermentation approach for large-scale production. </p>