Emilio Miraflores Gómez

Xanthan gum: production, recovery, and properties

Xanthan gum is a microbial polysaccharide of great commercial significance. This review focuses on various aspects of xanthan production, including the producing organism Xanthomonas campestris, the kinetics of growth and production, the downstream recovery of the polysaccharide, and the solution properties of xanthan.

Bioreactor scale-up and oxygen transfer rate in microbial processes: An overview

In aerobic bioprocesses, oxygen is a key substrate; due to its low solubility in broths (aqueous solutions), a continuous supply is needed. The oxygen transfer rate (OTR) must be known, and if possible predicted to achieve an optimum design operation and scale-up of bioreactors. Many studies have been conducted to enhance the efficiency of oxygen transfer. The dissolved oxygen concentration in a suspension of aerobic microorganisms depends on the rate of oxygen transfer from the gas phase to the liquid, on the rate at which oxygen is transported into the cells (where it is consumed), and on the oxygen uptake rate (OUR) by the microorganism for growth, maintenance and production. The gas-liquid mass transfer in a bioprocess is strongly influenced by the hydrodynamic conditions in the bioreactors. These conditions are known to be a function of energy dissipation that depends on the operational conditions, the physicochemical properties of the culture, the geometrical parameters of the bioreactor and also on the presence of oxygen consuming cells. Stirred tank and bubble column (of various types) bioreactors are widely used in a large variety of bioprocesses (such as aerobic fermentation and biological wastewater treatments, among others). Stirred tanks bioreactors provide high values of mass and heat transfer rates and excellent mixing. In these systems, a high number of variables affect the mass transfer and mixing, but the most important among them are stirrer speed, type and number of stirrers and gas flow rate used. In bubble columns and airlifts, the low-shear environment compared to the stirred tanks has enabled successful cultivation of shear sensitive and filamentous cells. Oxygen transfer is often the rate-limiting step in the aerobic bioprocess due to the low solubility of oxygen in the medium. The correct measurement and/or prediction of the volumetric mass transfer coefficient, (k(L)a), is a crucial step in the design, operation and scale-up of bioreactors. The present work is aimed at the reviewing of the oxygen transfer rate (OTR) in bioprocesses to provide a better knowledge about the selection, design, scale-up and development of bioreactors. First, the most used measuring methods are revised; then the main empirical equations, including those using dimensionless numbers, are considered. The possible increasing on OTR due to the oxygen consumption by the cells is taken into account through the use of the biological enhancement factor. Theoretical predictions of both the volumetric mass transfer coefficient and the enhancement factor that have been recently proposed are described; finally, different criteria for bioreactor scale-up are considered in the light of the influence of OTR and OUR affecting the dissolved oxygen concentration in real bioprocess.

Mass transfer coefficient in stirred tank reactors for xanthan gum solutions

Abstract The oxygen transfer rate in Newtonian and non-Newtonian fluids in a stirred tank reactor (STR) of 20 l of operating volume has been studied. The volumetric mass transfer coefficient, kLa, has been measured by a dynamic technique in solutions of xanthan gum for a wide interval of operational conditions. This coefficient has been determined changing many variables, such as the superficial gas velocity (Vs), the stirred speed (N) or the power input by unit of volume ( P V ). Also, the properties of the liquid phase, mainly the rheologic properties of the liquid, as well as the geometry of the stirrer, have been studied considering the effect of the number and type of stirrers (paddle or turbine), the number of blades of the stirrers and the sparger type (ring and disk). The kLa values are dramatically affected by the viscosity of the liquid; to take into account this effect, the rheology of the system has been described using both the Ostwald-de Waele and the Casson models. Dimensional correlations — as a function of the operational conditions—have been obtained, and also dimensionless equations — expressing the Sherwood number as a potential function of the Reynolds, Aireation and Weber numbers—have been determined for several stirrer types.

Desulfurization of dibenzothiophene using the 4S enzymatic route: Influence of operational conditions on initial reaction rates

The influence of operational conditions (pH, temperature and oxygen transfer rate) on the initial reaction rates of the four reactions involved in the 4S biodesulfurization route of dibenzothiophenes (DBT) has been studied. The bioprocess was carried out using a genetically modified organism, Pseudomonas putida CECT 5279. The rates of the four reactions were calculated from the rates of production of different compounds involved in the 4S pathway, by matrix manipulation. The initial (zero time) reaction rates showed a slight dependence on oxygen transfer rate. Temperature and pH were optimal at 30°C and 9, respectively, temperature being the most important variable. This study also identifies the last reaction as the limiting step in the pathway.

Behavior of several pseudomonas putida strains growth under different agitation and oxygen supply conditions

The growth rate of four strains of Pseudomonas putida, KT2440, KT2442, KTH2, and KTH2 (pESOX3), under different fluid dynamic conditions has been studied. The cultures were conducted in a stirred tank bioreactor by changing the stirrer speed. Several process variables, such as biomass concentration, dissolved oxygen concentration, oxygen mass transfer rate and oxygen uptake rate, have been measured or calculated. Also cell viability was determined by viable colony counting in Petri dishes and culture samples were subjected into a transmission electron microscopy analysis, in order to describe the integrity of the individual cells. The experimental results show that the genetically modified organisms, the strains KTH2 and KTH2 (pESOX3), present a different growth under low agitation conditions, and low oxygen supply level, while the growth of the wild type strains, KT2440 and KT2442, followed the typical sigmoidal evolution that could be described by the logistic equation. The presence of outer membrane vesicles has been observed in the GMO strains. When the cultures were conducted at low stirrer speed, and so at low oxygen transfer rate, these vesicles were detected, indicating the bacterial response to oxidative stress, caused by the catalytic activity of the HpaC enzyme. For all of the strains tested, no hydrodynamic stress has been detected, even at very high agitation levels.

Stirred Tank Bioreactors

Stirred tank bioreactors (STBRs) are the reactors most widely employed for culturing of biological agents such as cells, enzymes, or antibodies. They are contactors where the well-mixed among phases is obtained mainly by internal mechanical agitation. The impeller must provide sufficiently rapid agitation to disperse all compounds and achieve an effectively homogeneous concentration inside the bioreactor. The technical design of an STBR for a bioprocess must determine the required volume, which depends on the production. Therefore, it is necessary to know the rates of the different phenomena involved (mass transport, heat transfer, complex network of reactions, etc.) and combine the characteristic variables and parameters of each one of these phenomena into a kinetic model that adequately describes the complex evolution of the biological system. For this purpose, the balances of mass and energy have to be formulated. This work is focused on design of STBR in batch and continuous operation. As a means of illustrating typical design equations for both way operations, the xanthan gum production has been used. The simulation and discussion of results with different kinetics model proposed in the literature are presented.