|dc.description.abstract||Currently, fermentation research's primary objective is to develop economic and sustainable processes by increasing the product yield and reducing the operating cost. A tremendous effort had been made in the past few decades by discovering and developing various fermentative strains. The fermentation process measurement, control, and supervision are the next battlefield for further fermentation technology development. Fermentation manipulation is a complex process from both biological and engineering points of view. Different factors, including substrates type, substrates concentration, strain type, and operating modes, need to be taken into account. These aspects encourage the scientific community to find a robust, sophisticated, and versatile measure for controlling fermentation.
Redox potential, also known as the oxidation-reduction potential (ORP), reflects the overall biochemical reactions, electron transfer, and redox balance within the fermentation broth. The biological significance of ORP includes indirectly explicating metabolic activities during fermentation and regulating the metabolic network, affecting the metabolic pathway and gene expression. The monitoring and controlling environmental ORP provides a thoughtful understanding to help control the intracellular metabolic activities and fermentation process. This approach has been proven and considered as a real-time approach to increase fermentation efficiency in scientific communities and industrial sectors. In particular, the ORP measurement can provide an online and consistent signal during fermentation. It can be used at any stage of fermentation, providing both high signal-integrity and measurement reliability. Although effective, the relatively high fabrication cost of these sensors has so far made it impractical for extensive applications in large-scale contaminated soil monitoring, in particular.
Cost-effectiveness is an endless effort in engineering, an economically optimized ORP monitor tool is urgently needed. With the development of bio-electrochemical research, the microbial fuel cell (MFC) as an old technology has been adopted as a biosensor to produce power and electricity by bacteria catalyzation. In this work, an MFC-based biosensor device was designed and developed as a fermentation biosensor using an indigenous microorganism and modified Nernst equation to integrate among the MFC voltage output, fermentation ORP, and fermentation stages.
This study investigated different factors on the performance of an MFC-based biosensor. These factors include strain types (Bacillus subtilis and Pseudomonas fluorescens) in the presence or absence of methylene blue mediator, cathodic treatments (sparging, aerated cathode, and potassium ferricyanide solution), and anodic aeration rates (0, 11.32, and 22.64 vvm). After optimal conditions were established, this study used turbidimetric measurement as the indicator for microbial growth. The correlational between microbial growth and ORP, voltage, potential parameter (X) were investigated. Results showed that B. subtilis exhibited superior performance under MFC condition. Sparging cathodic treatment provided a feasible and sustainable supply of electron acceptor. Keeping anodic aeration rate at 11.32 vvm constructed a suitable anodic environment not only to support B. subtilis growth but also to sustain the voltage generation from MFC device. After all conditions had been settled, the voltage signal was projected to a linear increase, and the ORP signal was likely to generate a bathtub-shaped curve. Two peaks occurred on the curve by integrating both signals into the potential parameter X and plotting the potential parameter X over time. Three distinct growth phases were revealed by comparing the potential parameter X with microbial growth information and ORP profile. Potential parameter X indicated the endpoint of lag phase, mid-point of exponential phase, and the starting point of stationary phase. Such a result proved that the potential parameters could provide fruitful and high-resolution information that enables precious and real-time fermentation monitoring and controlling. To conclude, this thesis demonstrated the development of a novel fermentation biosensor by utilizing an MFC device as a biosensor. By applying the unitless parameter (i.e., potential parameter) derived from the modified Nernst equation, an MFC device equipped with an ORP sensor could successfully unveil the hidden internal information during the course of fermentation and explicate microbial growth dynamics.
Although successful, many questions were also raised during the course of this research. One main limiting factor the choice of the microorganism. In this research, Bacillus subtilis was selected as it could generate extracellular electrons which then pick up by the carbon electrode in anodic chamber, resulting in voltage flow. When different microbes were chosen, one needs to investigate whether such a microbial strain could export electron in MFC device. If not, one could attempt to supplement electron mediator to assist electron movement from microbial surface to carbon electron. The operating condition for this developed device needs to be optimized as different strains possess different growth requirements.||