Diffusion coefficients of glucose and ethanol in cell-free and cell-occupied calcium alginate membranes

The diffusivities of glucose and ethanol in cell-free and cell-occupied membranes of calcium alginate were measured in a diffusion cell. The lag time analysis was used. Diffusivities decreased with increasing alginate concentration and were comparable with those in water for a 2% alginate membrane. Glucose and ethanol concentrations had no effect on the respective diffusion coefficients. The ratio of ethanol diffusivity to glucose diffusivity in 2 and 4% alginate agreed closely with the inverse ratio of the hydrodynamic raii for the two molecules in water, indicating that the hydrodynamic theory of diffusion in liquids may be applicable to diffusion in dilute alginate gels. Also, the presence of 20% dead yeast cells had no effect on the diffusivities. The data reported can be used to study reaction and diffusion in immobilized cell reactors and cell physiology under immobilized conditions.

Theoretical and methodological studies of continuous microbial bioreactors

This article reviews most of the author's studies on process development and reactor design for continuous microbial reactions. (1) Enzyme reactions of growing and non-growing microbial cells immobilized in agar gel beads were analyzed pertaining to the effects of external and internal diffusion of substrate on reaction kinetics. (2) Experimental correlations of production rates of beta-fructosidase and acid phosphatase with dilution rate of continuous culture were simulated based on an operon model for enzyme regulation. (3) Population dynamics of an amylase-producing bacteria and their mutant were discussed in relation to enzyme productivity in a continuous culture of spore-forming bacteria. (4) Plasmid mobilization in a mixed population of donor, recipient, and helper cells was investigated in a continuous culture as a model study of accidental release of a genetically modified plasmid into a natural environment. (5) A production rate increase of up to 100-fold was achieved by cell-recycle culturing of continuous acetic acid fermentation using a filter module with a hollow fiber membrane. (6) The feasibility of a continuous surface culture for the biooxidation of organic substances was ascribed to an enhanced oxygen absorption rate in the presence of a microbial film on a liquid surface. (7) Simultaneous separation of inhibitory products using an electrodialysis module during some organic acid fermentations was effective for increasing production in a continuous culture.

Development of bioreaction engineering

In addition to summarizing the early investigations in bioreaction engineering, the present short review covers the development of the field in the last 50 years. A brief overview of the progress of the fundamentals is presented in the first part of this article and the key issues of bioreaction engineering are advanced in its second part.

Investigation of oxygen transfer through the membrane of polymer hollowspheres by oxygen micro-electrodes

Estimation of oxygen transfer from an aqueous solution into polymer hollow spheres, as used for encapsulation of microorganisms, requires detection of the dissolved oxygen concentration inside the hollow sphere. Using microcoaxial needle electrodes, oxygen kinetics in different penetration depths within single cellulose-sulfate hollow spheres loaded with and without living yeast cells could be measured. Based on the reaction kinetics the diffusion coefficient for oxygen and the oxygen-uptake rate have been calculated. The diffusion coefficient DO2 within the thin membrane of cell-free spheres was in the order of 10(-10)m2s-1. Due to the liquid core of the hollow sphere the corresponding diffusion coefficient DO2 is in the range of the value for water. The oxygen-uptake rate QO2 in cell containing spheres could be estimated to 90 mmol l-1 h-1, which corresponds to a specific oxygen-uptake rate qO2 of 10 mmol g-1 h-1. It is to conclude that oxygen supply of the microorganisms inside the hollow spheres was, in this case, not critically influenced by the thin polymeric wall of the capsules.

Diffusion of sucrose and dextran through agar gel membranes

Mass transfer limitations severely impede the performance of bioreactions involving large molecules by gel-entrapped microorganisms. This paper describes a quantitative investigation of such diffusional limitations in agar gel membranes. Sucrose and commercial dextran fractions with (weight-average) molecular weights ranging from 10,000 to 2,000,000 Da were used as standard diffusants. For all tested solutes but sucrose, the values of the agar/water partition coefficients highlighted steric hindrance at the entrance of the membrane pores. The effective diffusivity of sucrose in agar was similar to that in water. All dextran fractions, however, displayed restricted diffusion in the agar membranes. Their effective diffusivities were a decreasing function of the agar content of the gel membrane (0.5, 1.0, or 1.5% w/v). The effective diffusivity in a given membrane decreased as the molecular weight of the diffusing molecule increased. T500 (Mw = 470,000 Da) and T2000 (Mw = 1,950,000 Da) fractions were unable to diffuse through 1.0 or 1.5% agar membranes. The diffusion data did not agree with the classical (Renkin) model for a hard sphere diffusing through a cylindrical pore. These results are discussed in terms of gel and diffusant characteristics.

A theoretical model for immobilized whole cell enzyme

A method of estimating effectiveness factor for immobilized whole cells is developed by considering microbial cells as microspheres containing enzyme activity dispersed in the gel phase of the support matrix. The proper model equations describing the system are solved and the corresponding effectiveness factors calculated for various bead sizes, and numbers and activities of cells. The cell wall resistance (permeability) is found to be one of most important variables in the system. The model is applied in predicting the experimental data of other investigators.

Immobilized cells

Three basic types of immobilization (i.e. without carrier, entrapment and immobilization on the carrier surface) of microbial cells, nonmicrobial cell populations and subcellular organelles are reviewed. These are further developed into a number of combined and less frequently used techniques of immobilization and application of cell biocatalysts for industrial biotransformation in pharmacy, food industry and agriculture, including novel approached and some unpublished authors' results.

[Steroid transformation with immobilized microorganisms. I. Transformation of cholesterol to cholestenone in organic solvents]

The transformation of cholesterol to cholest-4-ene-3-one has been investigated in the presence of toluene and carbontetrachloride using cells of Nocardia erythropolis (IMET 7185) immobilized by different methods. The adsorption on DEAE-cellulose has been observed to be the most effective method. The stability of immobilized cells relating to cholesteroloxydase activity, transformation capacity, and the influence of water have been investigated.

A new method for cell immobilization

A new method for producing particles and membranes containing immobilized bacteria is presented. These immobilized bacteria display good stability over time making them well suited for use in a packed-bed reactor. Such a reactor is tested as a function of the different parameters of the system. The results are qualitatively similar to those obtained with purified enzyme reactors, but some discrepancies with the plug-flow model are noted. It is necessary to use a more sophisticated model in order to fit the experimental data.

Pore diffusion model for a two-substrate enzymatic reaction: application to galactose oxidase immobilized on porous glass particles

An analysis of the pore diffusion model involving a two-substrate enzymatic reaction is presented. The resulting equations have been applied to the case of galactose oxidase catalyzed oxidation of galactose when the enzyme is immobilized on porous glass particles. The physical constants of the system were obtained by theoretical predictions and the enzyme concentration in the porous medium was derived from the experimental results. The calculations were performed with the assumption that the kinetic parameters of the enzyme remain unchanged upon immobilization. The theoretically calculated effectiveness factors were compared with the experimental effectiveness factors determined from the batch kinetic experiments and were found to be in agreement. The results are presented as effectiveness factor plots graphed as functions of bulk galactose and oxygen concentrations. The model was extended in order to study the effect of external mass transfer coefficients and pore enzyme concentrations on the effectiveness factors.