Felix Garcia-Ochoa

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.

Xanthan gum production under several operational conditions: molecular structure and rheological properties☆☆

Xanthan gum production under several operational conditions has been studied. Temperature, initial nitrogen concentration and oxygen mass transfer rate have been changed and average molecular weight, pyruvilation and acetylation degree of xanthan produced have been measured in order to know the influence of these variables on the synthesised xanthan molecular structure. Also, xanthan gum solution viscosity has been measured, and rheological properties of the solutions have been related to molecular structure and operational conditions. The Casson model has been employed to describe the rheological behaviour. The parameter values of the Casson model, tau(0) and K(c), have been obtained for each polysaccharide synthesised under different operational conditions. Both pyruvilation and acetylation degrees and average molecular weight of xanthan increase with fermentation time at any operating conditions. Xanthan molecules with the highest average molecular weight have been obtained at 25 degrees C. Nevertheless, at this temperature acetate and pyruvate radical concentration are lowest. Nitrogen concentration in broth does not show any clear influence over xanthan average molecular weight, although with high nitrogen source concentration xanthan with low pyruvilation degree is produced.

Kinetic modeling of lactose hydrolysis with an immobilized β-galactosidase from Kluyveromyces fragilis.

The kinetic model of the hydrolysis of lactose with a beta-galactosidase from Kluyveromyces fragilis immobilized on a commercial silica-alumina (KA-3, from Südchemie) has been determined. A wide experimental range of the main variables has been employed: temperature, concentrations of substrate, and products and concentration of enzyme. The runs were performed in a complex buffer with the salt composition of milk. The effect of pH and temperature on the stability and the activity of the enzyme have been studied. The optimum pH for the enzyme activity was, approximately, seven. The immobilized enzyme was more stable than the free one at acidic pH, but more instable at basic pH. The maximum temperature used for the hydrolysis runs performed to select the kinetic model was 40 degrees C, so inactivation of the enzyme during the kinetic runs has been avoided. Agitation, concentration of enzyme in the solid and particle size were selected to ensure that the overall rate was that of the chemical reaction. Eleven kinetic models were proposed to fit experimental data, from first order to more complex ones, such as those taking into account inhibition by one of the compounds involved in the hydrolysis reaction. Applying statistical and physical criteria, a Michaelis-Menten model with a competitive inhibition by galactose has been selected. The model is able to fit the experimental data correctly in the wide experimental range studied. Finally, the model obtained is compared to the one selected in a previous work for the hydrolysis of lactose with the free enzyme.

Sophorolipid production by Candida bombicola: Medium composition and culture methods

Candida bombicola is able to produce sophorolipid molecules with surfactant properties when grown in a medium composed of two different carbon sources (usually sugar and oil) and a nitrogen source (frequently yeast extract). In this work, the composition of the medium and the culture method employed have been studied. The influences of glucose concentration, properties of oil and yeast extract concentration have been taken into account. Accordingly, a production medium composition is proposed (100 g/l glucose, 100 g/l sunflower oil and 1 g/l yeast extract). The most frequent culture methods reported in literature, i.e. batch, medium pulse mode and resting-cell methods, have been tested. The resting-cell method was found to produce the highest final concentration of sophorolipid, obtaining a good yield of carbon sources in a relatively short time. Under the best operational conditions and using the resting-cell method, 120 g/l sophorolipid was obtained in 8 d, with a carbon source yield of 0.60. Product distribution has also been investigated and the sophorolipid molecular structure of opened or cycled molecules has been determined under different operational conditions. Low yeast extract concentration and long fermentation time enhance the production of cycled structures by all the production methods studied.

Kinetic Modeling of Lactose Hydrolysis by a β-Galactosidase from Kluyveromices Fragilis

Abstract The kinetic model of lactose hydrolysis by means of a commercial β-galactosidase from Kluyveromices fragilis provided by Novo nordisk has been determined using a wide range of the main variables: enzyme, substrate, and product concentrations and temperature. Lactose hydrolysis, which is of great interest due to physiological, nutritional, technological, and environmental reasons, has been performed in a buffer whose salt composition is similar to that of milk. The effect of pH and temperature on enzyme activity and stability has been studied and it has been found that the optimal pH was 6.5. Temperatures over 45°C cause a significant deactivation in few hours; thus, a pH of 6.5 and a range of temperatures from 5–40°C have been employed to accomplish the kinetic model discrimination. Five kinetic models described in the literature using different β-galactosidases and reaction media have been considered. Substrate and product inhibition have been taken into account. Runs with different initial amounts of monosaccharides have been performed in order to discriminate among different kinetic models. Applying statistical and physical criteria, a Michaelis-Menten model with a competitive inhibition by galactose has been finally chosen, yielding a good fitting of the experimental data in the wide interval of variables studied.

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.

A kinetic model for beer production under industrial operational conditions

A kinetic model for beer production is proposed. The model takes into account five responses: biomass, sugar, ethanol, diacetyl and ethyl acetate. In contrast with previously published models, this model segregates biomass into three components: lag, active and dead cells and considers the active cells as the only fermentation agent. Experiments were first performed at laboratory scale and isothermal runs were carried out at five temperatures (8°C, 12°C, 16°C, 20°C and 24°C). Fitting of experimental data was made by non-linear regression. Parameter values calculated were similar to those given in the literature. The kinetic model was able to fit experimental data with a very good agreement. Afterwards, experiments were conducted at pilot plant scale and runs were now carried out changing temperature with time, in the industrial way. The kinetic model, with the parameter values calculated as a function of temperature, was able to predict with a very high accuracy the non-isothermal experimental data achieved. This model can be used for simulation of the industrial process under different operational conditions and for faults detection. It can also be utilized for the optimization and even for the supervised control of the process and its automatization.

Unstructured kinetic model for sophorolipid production by Candida bombicola

Abstract A kinetic model for sophorolipid production by Candida bombicola is proposed. The unstructured nonsegregated kinetic model takes into account three responses: biomass, sugar, and sophorolipid. In the kinetic model, the nitrogen—from yeast extract—is assumed to be the limiting nutrient of the yeast growth, the sugar—glucose—is assumed to be used in both growth and sophorolipid production, and oil is supposed to be spent in product formation and maintenance of biomass is attributed to glycerol eased by the catabolism of the oil. The parameters of the kinetic model are calculated by fitting model to experimental data. These data have been obtained in experiments in which a batch stirred and an aerated tank bioreactor have been used. Fitting has been carried out according to a nonlinear regression algorithm, coupled with a Runge–Kutta method for the interpretation of integral data (evolution of system composition with time). The proposed kinetic model is able to explain both our experimental data and data given by other authors in previous works in literature in which nutrient and product concentrations evolution are given.

Kinetic model for anaerobic digestion of livestock manure

Two unstructured segregated kinetic models to describe the anaerobic digestion of livestock manure are developed and experimental batch data obtained from beef cattle in a 2.0-l work volume digestor fitted to both proposed kinetic models to obtain the values of the parameters. The results obtained by fitting show that the second model proposed has both statistical and physical meaning in the parameter values obtained. The model takes into account a simplified reaction scheme formed by six reactions. Several simplifications have been made (lumping, pseudo-steady state for one compound, first order kinetics, etc.) yielding four key compounds to be analysed and fitted to the model as production-rates expressions (total biomass, chemical oxygen demand (COD), volatile acids, and methane). The model considers three main stages in the process: enzymatic hydrolysis of the waste, growth of ‘acetogenic’ microorganisms (production of acids nongrowth associated), and growth of ‘methanogenic’ microorganisms associated with methane production; the two last processes are accompanied by substrate consumption for maintenance. A non-linear multiple-response regression technique coupled to a fourth-order Runge‐Kutta algorithm has been employed to obtain the values of the ten parameters. The model is able to reproduce the experimental data obtained for beef manure anaerobic digestion with more accuracy than experimental error.