Bioprocess Engineering 11 (1994) 1.35 144 :~ Springer-Verlag 1994 A robust fed-batch feeding strategy for optimal parameter estimation for baker's yeast production A. O. Ejiofor, C. H. Posten, B. O. Solomon, W.-D. Deckwer Abstract Parameter identification of structured models is often a problem in biotechnology, because the poor data situation and the number of unknown parameters only allow for inaccurate estimates. But often only a subset of all kinetic parameters of the model are of interest for production purposes, e.g. for fed-batch cultivation. These parameters should be estimated with a given accuracy. In addition, the experiments for information acquisition with respect to these parameters should be as simple as possible and should consider some practical restrictions. In this contribution a fed-batch feeding strategy is proposed to allow for an accurate estimation of yield and of critical growth rate of baker's yeast. The feeding also allows for economic and stereotyped use of staff and equipment and is therefore suitable for routine use in screening of strains and media. The overall pattern is similar to that one, usually used in production scale to minimize errors by limited model validity. After an initial phase for achieving a reproducible state three different growth rates are adjusted to cover the range of possible critical growth rates. From biomass and ethanol measurements yield and critical growth rate can be estimated with an accuracy of about 2.1%. The fermentation pattern ends up with a constant feeding rate to simulate a limited oxygen transfer rate and to allow for an uptake of residual sugar and ethanol before a dough test can be carried out. Beside experimental results simulations and sensitivity analyses are shown. List of symbols P ethanol concentration S substrate concentration Received Jo ]anuary I994 A. O. Ejiofor ~, C. H. Posten, B. O. Solomon 2, W.-D. Deckwer GBF - Gesellschaft f/ir Biotechnologische Forschung, Mascheroder Weg 1, D - 38124 Braunschweig, Germany Present addresses: ~Department of Applied Microbiology& Brewing, Nnamdi Azikiwe University, Awka,Anambra State, Nigeria 2Department of Chemical Engineering, Obafemi AwolowoUniversity,Ile-Ife, Nigeria Correspondence to: C. Posten Dedicated to the 65th birthday of Professor Fritz Wagner. A. O. Ejiofor and B. O. Solomon are grateful to the Alexander yon Humboldt Stiftung for granting them fellowships and to GBF for providing all the materials necessary for their successful research stay in Germany. Sf substrate concentration in feed T fermentation time V fermenter volume X biomass concentration C measurement error covariance matrix F Fisher information matrix X state variables Y output variables Xp state sensitivity functions with respect to parameters Yp output sensitivity functions e eigenvectors k vector of limitation and inhibition parameters n number of observations qin feeding stream qb stream for samples and ammonia feed r vector of specific turnover rates y vector of yields p specific weight 3. eigenvalues # specific growth rate /~set exponent in exponential feeding a standard deviation 1 Introduction One of the major problems militating against the achievement of high yield and high volumetric productivity during fermentation of Saccharomyces cerevisiae for the production of baker's yeast at both laboratory and industrial scales is the conversion of a lot of the limiting substrate into ethanol even under aerobic conditions as soon as a so called critical growth rate is exceeded. Consequently, most control strategies in baker's yeast fermentations are designed to eliminate ethanol production or at least minimize its production rate. In practice, while various modes of fermentation have been employed at laboratory scale, many yeast plants utilize a combination of batch and fed-batch fermentations. By this practice ethanol is produced maximally in the batch phase following the availability of the carbon source in excess. This ethanol serves as the new carbon source on the exhaustion of the first prior to the commencement of the fed-batch phase. This strategy demands that the yeast strain in use be highly ethanol tolerant otherwise the attainment of growth tolerance level (6-9% w/v) will cause cessation of growth and even death of the cells. An entirely fed-batch operation serves the purpose of controlling the specific growth rate on a value below the critical 135