Modification of yeast metabolism by immobilization onto porous glass

Ethanol production from wheat straw by Saccharomyces cerevisiae and Scheffersomyces stipitis co-culture in batch and continuous system

In this research, Scheffersomyces stipitis and Saccharomyces cerevisiae in immobilized and suspended state were used to convert pentose and hexose sugars to ethanol. In batch and continuous systems, S. stipitis and S. cerevisiae co-culture performance was better than S. cerevisiae. Continuous ethanol production was performed in packed bed immobilized cell reactor (ICR). In ICR, S. stipitis cells were found to be more sensitive to oxygen concentration and other possible mass transfer limitations as compared to S. cerevisiae. Use of co-immobilized S. stipitis and S. cerevisiae resulted in maximum xylose consumption (73.92%) and 41.68 g/L day ethanol was produced at HRT (hydraulic retention time) of 6h with wheat straw hydrolysate. At HRT of 0.75 h, the highest amount of ethanol with the values of 356.21 and 235.43 g/L day was produced when synthetic medium and wheat straw hydrolysate were used as feeding medium in ICR, respectively.

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.

Enhanced production of bioethanol and ultrastructural characteristics of reused Saccharomyces cerevisiae immobilized calcium alginate beads

Yeast immobilized on alginate beads produced a higher ethanol yield more rapidly than did free yeast cells under the same batch-fermentation conditions. The optimal fermentation conditions were 30°C, pH 5.0, and 10% initial glucose concentration with 2% sodium alginate beads. The fermentation time using reused alginate beads was 10-14 h, whereas fresh beads took 24h, and free cells took 36 h. All bead samples resulted in nearly a 100% ethanol yield, whereas the free cells resulted in an 88% yield. Transmission electron microscopy (TEM) showed that the shortened time and higher yield with the reused beads was due to a higher yeast population per bead as well as a higher porosity. The ultrastructure of calcium alginate beads and the alginate matrix structure known as the "egg-box" model were observed using TEM.

Independent prediction of naphthalene transport and biodegradation in soil with a mathematical model

Experiments were performed to test the ability of a mathematical model to predict naphthalene transport and biodegradation. Pseudomonas putida G7, a model bacterial strain capable of degrading naphthalene, was added to a column packed with the soil that had been pre-equilibrated with naphthalene. Model prediction for transport and degradation were based on predetermined parameters that described naphthalene desorption kinetics and the utilization of naphthalene by the test bacterium. However, initial prediction for naphthalene biodegradation was high, and the formation of cell aggregates is advanced as a plausible explanation. Access of substrate to cells in the interior of an aggregate would be restricted. When the numerical simulation was conducted with a factor to account for cell aggregation, it successfully described the experimental data. Thus, with a single adjustable parameter (an average effectiveness factor), the model predicted macroscopic responses of naphthalene in soil-columns where naphthalene was subject to transport and biodegradation.

Factors affecting the production of L-phenylacetylcarbinol by yeast: a case study

L-Phenylacetylcarbinol (L-PAC) is the precursor for L-ephedrine and D-pseudoephedrine, alkaloids possessing alpha- and beta-adrenergic activity. The most commonly used method for production of L-PAC is a biological method whereby the enzyme pyruvate decarboxylase (PDC) decarboxylates pyruvate and then condenses the product with added benzaldehyde. The process may be undertaken by either whole cells or purified PDC. If whole cells are used, the biomass may be grown and allowed to synthesize endogenous pyruvate, or the cells may be used as a catalyst only, with both pyruvate and benzaldehyde being added. Several yeast species have been investigated with regard to L-PAC-producing potential; the most commonly used organisms are strains of Saccharomyces cerevisiae and Candida utilis. It was found that initial high production rates did not necessarily result in the highest final yields. Researchers then examined ways of improving the productivity of the process. The substrate, benzaldehyde, and the product, L-PAC, as well as the by-products, were found to be toxic to the biomass. Methods examined to reduce toxicity include modification of benzaldehyde dosing regimes, immobilization of biomass or purified enzymes, modification of benzaldehyde solubility and the use of two-phase reaction systems. Various means of modifying metabolism to enhance enzyme activity, relevant metabolic pathways and yield have been examined. Methods investigated include the use of respiratory quotient to influence pyruvate production and induce fermentative activity, reduced aeration to increase PDC activity, and carbohydrate feeding to modify glycolytic enzyme activity. The effect of temperature on L-PAC yield has been examined to identify conditions which provide the optimal balance between L-PAC and benzyl alcohol production, and L-PAC inactivation. However, relatively little work has been undertaken on the effect of medium composition on L-PAC yield.

Phosphorus-31 and carbon-13 nuclear magnetic resonance study of glucose and xylose metabolism in agarose-immobilized Candida tropicalis

Candida tropicalis can ferment both hexose and pentose sugars. Here, we have used 31P and 13C nuclear magnetic resonance spectroscopy to study the capacity of this yeast species to metabolize glucose or xylose when immobilized in small (< 1-mm-diameter) agarose beads. Immobilized C. tropicalis metabolizing glucose showed rapid initial growth within the beads. A corresponding drop in the intracellular pH (from 7.8 to 7.25) and hydrolysis of intracellular polyphosphate stores were observed. Although the initial rate of glucose metabolism with immobilized C. tropicalis was similar to the rate observed previously in cell suspensions, a decrease by a factor of 2.5 occurred over 24 h. In addition to ethanol, a significant amount of glycerol was also produced. When immobilized C. tropicalis consumed xylose, cell growth within the beads was minimal. The intracellular pH dropped rapidly by 1.05 pH units to 6.4. Intracellular ATP levels were lower and intracellular Pi levels were higher than observed with glucose-perfused cells. Consumption of xylose by immobilized C. tropicalis was slower than was previously observed for oxygen-limited cell suspensions, and xylitol was the only fermentation product.

Immobilization of cells for application in the food industry

Immobilization of cells offers advantages to the food process industries, including enhanced fermentation productivity and cell stability and reduced downstream processing costs due to facilitated cell recovery and recycle. This article summarizes the varied immobilization methodologies, including adsorption, entrapment, covalent binding, and microencapsulation. Examples of interest to the food industry are provided, together with a review of the physiological effects of immobilization. Topics in process engineering include immobilized cell bioreactor configurations and the scale-up potential of the various immobilization techniques.

Immobilization of microbial cells by adsorption

Immobilized cells cover a wide area of applications and are essential components of many biotechnological processes. In general it can be distinguished between two immobilization methods: (1) entrapment into polymers and (2) natural adsorption onto porous and inert support materials. The immobilization by adsorption is discussed by the following criteria: biomass loading, strength of adhesion, enzymatic stability/specific activity of the biocatalyst, effectivity/reaction engineering and operational stability.

On the merits of viable-cell immobilisation

Many advantages have been claimed over the years for the use of immobilised cells, both as enzyme systems and as whole viable cell systems for complete fermentation reactions. However, few of the claims have been fully substantiated, and may not even be entirely justified. Most research is involved with single applications and the best that can be hoped for is some evidence that immobilised cells in each of these individual cases display some advantage over the equivalent free cell system. The purpose of this review is to assess the merits of viable cell immobilisation in the light of published literature and to elucidate the underlying mechanisms. Particular attention is paid to the generally unanticipated, but widely observed enhanced stability of immobilised cell fermentation processes.

Application of immobilized cells for biotransformations of steroids

A survey is given of possible solutions and open-ended questions in the biotransformation of steroids (without side chain degradation of sterols) by using immobilized cells. The data of literature between 1975 and 1990 and results of preliminary experiments from the microbial, biochemical, biophysical, physiological, as well as the biotechnological point of view are summarized and discussed.

Metabolic characteristics of bacterial cells entrapped in beaded calcium alginate and/or pectate gels

A mixture of heterotrophic bacteria and collection strains of Escherichia coli and Pseudomonas fluorescens were immobilized in calcium alginate or pectate gels. Comparison of respiratory activity, substrate uptake and biosynthetic capacity of immobilized cells showed that both types of carriers permit a prolonged preservation of metabolic activity but the transfer of substances through the gel is faster in the pectate. Morphological changes include some intracellular structures, partial shrinkage of the plasma membrane of immobilized cells, and transformation of a rod-like cell shape to an oval one.

Maximizing productivity in an immobilized cell reactor

A vertical immobilized cell reactor employing Saccharomyces cerevisiae cross-linked to a gelatin support with glutaraldehyde has proven to be an effective system to achieve high cell concentrations and high dilution rates. The reactor is very stable over long periods of time, during which the cell concentration increases continuously without achieving steady state. Therefore, periodic regeneration by gas purging is required to remove excess biomass from the interstitial spaces. The glucose concentrations along the reactor follow an exponential profile when plotted as a function of the true residence time. Such profiles are a function of the initial glucose concentration fed to the system. The overall productivity of the reactor is a function of the flow rate and the inlet glucose concentration. For a 99% conversion, the maximum overall productivity is obtained at a substrate concentration of between 15% and 20%. Theoretical cell profiles were obtained and they indicate that mass transfer is promoted at high substrate concentrations and flow rates. The performance of a variety of packing materials having different shapes and materials of construction were compared in a vertical packed bed immobilized cell reactor. With other parameters being constant, the performance of the reactor is dominated by the quantity of cells present. The packing that has the highest surface area per volume of bed yields the most extensive monolayer and gives faster reactor start-up. Packing materials having high biomass loading rates are desirable at prolonged operating periods when growth beyond the monolayer occurs. Ultimately, the packing with the highest initial porosity would be capable of loading the highest cell volume, provided that sufficient interstitial spaces were provided for cell entrapment. When the immobilized cell reactor is operated in the horizontal position, CO2 holdup is decreased, as is evidenced by an increase in liquid holdup. However, the horizontal and vertical reactors showed almost identical substrate profiles at constant cell densities. In addition, under prolonged operation, the performance of the horizontal reactor decayed after several days, while the vertical reactor remained stable for over 40 days. The improved operation of this type of column in the vertical position is attributed to the necessary promotion of mass transfer with CO2 evolution and better liquid distribution. The addition of fatty acids to the media for an ICR results in limiting growth and increasing productivity. Overgrowth can be minimized, thus allowing longer periods of operation without regeneration.(ABSTRACT TRUNCATED AT 400 WORDS)