Oxygen transfer and uptake rates during xanthan gum production F. Garcı ´a-Ochoa*, E. Go ´mez Castro, V.E. Santos Dept. Ingenierı ´a Quı ´mica, Fac. CC. Quı ´micas, Universidad Complutense, 28040 Madrid, Spain Received 14 October 1999; received in revised form 13 July 2000; accepted 18 July 2000 Abstract Oxygen uptake rate and oxygen mass transfer rate have been studied during xanthan gum production process in stirred tank bioreactor. Empirical equations for the oxygen mass transfer coefficient have been obtained taking into account several variables such as air flow rate, stirrer speed and apparent viscosity. Oxygen uptake rate evolution in the course fermentation has been measured, obtaining an equation as a function of biomass concentration, including overall growth and non growth-associated oxygen uptake. A metabolic kinetic model has been employed for xanthan gum production description including oxygen mass transfer and uptake rates. The results point out that this model is able to describe adequately not only oxygen dissolved evolution, but also of the production of xanthan and substrate consumption. Also, the influence of several parameters (k L a, air flow rate and dissolved oxygen) in the evolution of the key compounds of the system have been studied. The results of the simulation shown that an increasing of dissolved oxygen concentration favor the xanthan gum production. © 2000 Elsevier Science Inc. All rights reserved. Keywords: Xanthan gum production; Oxygen transfer rate; Oxygen uptake rate; Kinetic model 1. Introduction Xanthan gum is an anionic polysaccharide widely used in many different industries [1] due to its interesting rheo- logical properties [2]. Xanthomonas campestris is a bacte- rium able to produce xanthan gum, as bacteria capsule [3], using different hydrocarbon sources. It is a strictly aerobic microorganism, oxygen being an essential nutrient both for X. campestris growth and for xanthan production. Due to that fact, xanthan fermentation is accompanied by a dra- matic increase of the viscosity of the broth, because of the extracellular accumulation of the biopolymer, which pro- duces a significant decrease of oxygen mass transfer rate. The dissolved oxygen concentration becomes the limiting nutrient and the oxygen mass transfer rate (OTR) can be- come the controlling step rate for the overall process. The oxygen mass transfer rate in a fermentor depends on several factors, such as the geometry of the bioreactor (type of bioreactor, distributor or stirrer design, etc.), the liquid properties (viscosity, superficial tension, etc.), and the dis- sipated energy in the fluid, which depends on the air flow rate, the stirrer speed, etc. Therefore, composition and rheo- logical properties of the medium changing with time have an important effect on mass transfer rate. The characteristic parameter for this transfer rate is the volumetric oxygen mass transfer coefficient, k L a. This coefficient plays an important role towards carry out the design, scale-up and economy of the process. Therefore, k L a values have to be obtained and correlated using empirical equations in order to obtain the appropriate design tools. Most of the oxygen transfer coefficient correlations in the literature belong to two categories. In the first kind k L a is correlated to stirrer speed (or power input per unit volume) and superficial gas velocity, and in the case of the non-Newtonian fluids the broth viscosity must be also taken into account [4 –7]. The second category includes empirical correlation based on dimensionless numbers [7–9], i.e., expressing the depen- dence of the Sherwood number as a function of the Reyn- olds, Weber and Aeration numbers [7]. Most of the works in the literature dealing with OTR in microbial systems are carried out without microbial trans- formation by means of the simulation of the rheological behaviour of the fermentation broth and the correlation of the volumetric mass transfer obtained in such conditions are employed during microbial transformation [4,10 –13]. The available literature contains a few references about studies made on the influence of operational variables on oxygen * Corresponding author. Tel: +34-91-394-4176; fax: +34-91-394- 4171. E-mail address: fgochoa@eucmos.sim.ucm.es (F. Garcı ´a-Ochoa) www.elsevier.com/locate/enzmictec Enzyme and Microbial Technology 27 (2000) 680 – 690 0141-0229/00/$ – see front matter © 2000 Elsevier Science Inc. All rights reserved. PII: S0141-0229(00)00272-6