Kinetics, modeling and optimization of recombinant yeast fermentations
Abstract
The objective of this research is to optimize the performance of fed-batch fermentations involving recombinant organisms. This goal is achieved by using, as a model system, the production of invertase from Saccharomyces cerevisiae containing the recombinant plasmid pRB58. The plasmid contains a copy of the yeast SUC2 gene which codes for invertase. The expression of the SUC2 gene is repressed at high glucose levels in the media. Batch and fed-batch fermentations were performed with this strain in selective media. It was observed that the cell yield decreases with when glucose concentration in the medium is high. Also, the specific invertase activity is very tightly regulated by glucose levels. An unstructured mathematical model was developed to describe the experimental results. The model explains the variation in cell yield by postulating a cybernetic principle, which regulates the fluxes in the respiratory and fermentative pathway. The invertase production was modeled by a simple substrate-inhibition form. A conjugate gradient algorithm was developed and tested for a variety of singular systems. This technique was used to determine the optimum substrate feed rate profiles in a fed-batch mode of fermentation for the model system. The optimum feed rates result in an initial cell growth phase followed by an invertase production phase. Aside from this main research theme, two other optimization problems were solved. First, a simple model was developed to describe anaerobic fermentations involving recombinant yeast. This model was then used to determine the optimum glucose feed rate profiles in a fed-batch mode of operation. Second, a mathematical model was developed to describe a recombinant E. coli system. Using singular control theory the problem of determining the best feed rate in a fed-batch fermentation was reduced to a simple one-parameter optimization problem. This simpler problem was then solved using a one-dimensional search. In both these systems, optimal feed rates resulted in an initial high cell growth phase followed by high product formation phase.
Degree
Ph.D.
Advisors
Seo, Purdue University.
Subject Area
Chemical engineering
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