The Open Systems Biology Journal, 2010, 3, 9-20 9 1876-3928/10 2010 Bentham Open Open Access Kinetic Studies on Alcoholic Fermentation Under Substrate Inhibition Conditions Using a Bioreactor with Stirred Bed of Immobilized Yeast Cells Anca-Irina Galaction 1 , Anca-Marcela Lupteanu 2 and Dan Cacaval *,2 1 University of Medicine and Pharmacy “Gr.T. Popa” of Iasi, Faculty of Medical Bioengineering, Dept. of Medical Biotechnologies, M. Kogalniceanu 9-13, 700454, Iasi, Romania 2 Technical University “Gheorghe Asachi” of Iasi, Faculty of Chemical and Environmental Engineering, Dept. of Biochemical Engineering, D. Mangeron 71, 700050, Romania Abstract: The studies on the alcoholic fermentation with immobilized S. cerevisiae cells on alginate using a bioreactor with stirred beds of biocatalysts indicated that the diffusion inside the biocatalyst particles attenuates or avoids the inhibitory effects, preserving the microbial activity. Using a mathematical model adequate for ethanol formation in heterogeneous systems the kinetic parameters V, K I and K’ M have been estimated. Therefore, the obtained values of V and K’ M were found to be higher than those corresponding to the homogeneous media under ethanol inhibition, but lower than those for homogeneous media without inhibitory fenomena. Keywords: Stirred bed, immobilized cells, yeasts, alcoholic fermentation, kinetic. INTRODUCTION The “white biotechnology”, the new concept promoted at the International Conference “European Bioperspectives - En Route to a Knowledge-Based Bio-Economy” (Cologne, Germany, May 31 - June 1 2007), proclaims that the use of renewable raw materials for biofuels production by low- expensive and ecofriendly biotechnologies constitutes one of the priority of industrial activities [1]. Among the tested and currently used biofuels, the bioethanol represents an attractive alternative to the conventional fossil fuels, its production by converting various substrates by free or immobilized cells of bacteria (Clostridium sp.) or yeasts (Saccharomyces sp., Zymomonas sp.) being intensively studied in the last two decades [2-5]. The fermentation with immobilized cells avoids the substrate inhibition, in these systems the use of higher concentration of carbohydrates becoming poosible, allows the easier recovery of biocatalysts and attenuates the reduction of the microbial activity. Therefore, the biocatalysts can be reused for many fermentation cycles [1,3,5]. Among the bioreactors with immobilized biocatalysts, those with stirred/mobile beds are the most attractive ones, owing to their constructive and operational similitude to the well-known stirred bioreactors. But, as it can be observed from literature, most of the experiments on alcoholic fermentation with immobilized cells have been carried out in fixed bed bioreactors in continuous, semicontinuous or fed- batch systems [2-13]. The fixed beds of immobilized cells have been preferred due to the higher sensitivity of the *Address correspondence to this author at the Technical University “Gheorghe Asachi” of Iasi D. Mangeron 71, 700050 Iasi, Romania; Fax: 0040232271311; E-mail: dancasca@ch.tuiasi.ro, dancasca@from.ro immobilized cells or enzymes to the forces generated by the impellers. Our previous studies on alcoholic fermentation performed in a bioreactor with stirred beds of immobilized S. cerevisiae cells in alginate indicated that it is possible to reach very efficient mixing without or with minimum loss of biocatalyst physical integrity by selecting the optimum impellers combination [14-16]. These studies have been dedicated to the analysis of the medium circulation for different biocatalysts particles sizes and volumetric fractions using seven types of radial impellers. The results have been directly correlated with the shear effects on the immobilized cells particles [17]. The previous experiments are continued by investigating the kinetics of alcoholic fermentation of glucose under substrate inhibition limitation and by developing a mathematical model describing the internal diffusion of the substrate and, implicitly, the distribution of substrate concentration inside the biocatalysts particle. In this paper, the kinetic study of glucose conversion into ethanol is presented. MATERIALS AND METHODOLOGY The experiments have been carried out in 5 l (4 l working volume) laboratory bioreactor (Biostat A, B. Braun Biotech International), with computer-controlled and recorded parameters [18]. The mixing equipment consists of two pitched bladed turbines of 64 mm diameter and three baffles. The inferior impeller has been placed at 64 mm from the bioreactor bottom. The superior impeller was placed on the same shaft at a distance of 32 mm from the inferior one. The rotation speed was maintained at 250 rpm, thus value avoiding the “cave” formation at the broths surface, solid phase deposition at the bioreactor bottom and mechanical