Production of 2,3-butanediol and coproducts by Klebsiella oxytoca: Energetics and bioreactor optimization
Abstract
Production of 2,3-butanediol, which has potential commercial applications as a precursor of methyl-ethyl-ketone and butadiene, or as a fuel additive, by Klebsiella oxytoca occurs only under conditions of an oxygen limitation. If the fermentation is too aerobic, cell mass is predominantly formed; whereas, if it is anaerobic, the yield of 2,3-butanediol is significantly lowered by the production of ethanol. The degree of oxygen limitation is defined as the actual oxygen supply rate relative to the supply rate required to maintain completely aerobic conditions, and is the most influential variable effecting system kinetics. Many of these effects are energetic in nature--i.e., the degree to which substrate is oxidized to 2,3-butanediol and its co-products, acetic acid, acetoin and ethanol, and the relative flow rates of substrate to energetic and biosynthetic pathways. An energetic framework has been developed with which to analyze such metabolic behavior. As part of this research project, we have added to this framework two relationships which further describe the response of K. oxytoca to an oxygen limitation. The first relationship describes the strong coupling between growth and energy production observed under oxygen-limited conditions. This has allowed us to calculate energetic parameters, to model the cell mass and substrate profiles, and to describe system behavior in terms of only the degree of oxygen limitation. The second relationship describes the average degree of oxidation of the end-product flow. This represents a generalization of previous product formation modelling efforts and has directed us in obtaining optimal operating conditions in batch culture. Batch and continuous culture data have supported this formulation. During these kinetic studies, two distinct phases of growth have been observed: the energy-coupled growth regime which was described above, and an energy-uncoupled growth regime, which arises when the degree of oxygen limitation reaches a critical value. We have concluded that optimal culture performance with respect to 2,3-butanediol productivity occurs during energy-coupled growth and have therefore designed a control system which maintains the batch culture at a constant level of oxygen limitation. This control scheme, based on a continually increasing partial pressure of oxygen in the feed gas which in turn continually increases the oxygen transfer rate, has resulted in (1) a balanced state of growth, (2) a repression of ethanol formation, and (3) an increase in 2,3-butanediol productivity of 30%, relative to uncontrolled culture.
Degree
Ph.D.
Advisors
Tsao, Purdue University.
Subject Area
Chemical engineering
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