Publication

Optimal Control of Fed-Batch Fermenters

Abstract

Optimal control of fed-batch fermenters S. Valentinotti† C. Cannizzaro‡ M.Rhiel‡ U. Holmberg† U. von Stockar‡ D. Bonvin† †Institut d’Automatique, EPFL, 1015 Lausanne, Switzerland ‡Institut de Genie Chimique, EPFL, 1015 Lausanne, Switzerland Fermentors are often run in a fed-batch manner to avoid the formation of overflow metabolites. At a high growth rate, the most efficient metabolic pathway(s) of certain microorganisms become saturated resulting in overflow metabolite production. These byproducts are undesirable since their accumulation in the reactor may be inhibitory and the productivity of biomass and growth-associated products is reduced. The ideal way to run such fed-batch fermentation is to grow the cells in the reactor at the critical growth rate, i.e., the point at which overflow metabolite production begins. However, since this value changes from run to run, or even during a given fermentation, its identification is not trivial. A simple way to overcome this difficulty is to maintain a very small, but constant overflow metabolite concentration in the reactor, ensuring that most of the substrate is consumed efficiently. However due to exponential cell growth, standard controllers can maintain a constant concentration only for a limited time period. In this work an adaptive control strategy to maintain a constant overflow metabolite concentration in fed-batch fermentation is presented. The proposed approach requires the knowledge of only two system parameters: the yield coefficient, expressing the relation between overflow metabolite and substrate, and the instantaneous concentration of the overflow metabolite. Baker’s yeast fed-batch experiments were performed with the ob jective of maximizing biomass productivity and minimizing ethanol production. Mid-infrared spectroscopy was used to measure the ethanol concentration that was provided on-line to the controller. The results from numerous experiments have demonstrated the effectiveness of the proposed control strategy. The specific growth rate was maintained constant, at a value close to the critical point, until oxygen transfer limitation occurred. Then, the controller automatically reduced the feed rate to prevent excess ethanol production. The biomass increased from 0.5 to 65 grams per liter during the exponential growth phase. Simulation results based on this control strategy show its applicability to other overflow metabolite organisms, such as Escherichia coli.

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Related concepts (40)
Fermentation
Fermentation is a metabolic process that produces chemical changes in organic substances through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.
Metabolism
Metabolism (məˈtæbəlɪzəm, from μεταβολή metabolē, "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the conversion of food to building blocks for proteins, lipids, nucleic acids, and some carbohydrates; and the elimination of metabolic wastes. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments.
Ethanol fermentation
Ethanol fermentation, also called alcoholic fermentation, is a biological process which converts sugars such as glucose, fructose, and sucrose into cellular energy, producing ethanol and carbon dioxide as by-products. Because yeasts perform this conversion in the absence of oxygen, alcoholic fermentation is considered an anaerobic process. It also takes place in some species of fish (including goldfish and carp) where (along with lactic acid fermentation) it provides energy when oxygen is scarce.
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