Fermentation of lignocellulosic hydrolysates. I: inhibition and detoxification

Furfural was reduced to furfuryl alcohol by Saccaromyces cerevisiae with a yield of 0.97 +/- 0.01 mmol/ mmol causing a lag phase in cell growth. In the presence of 29 mM, furfural the cell-mass yield on glucose decreased from 11.0 +/- 0.1 mg/mmol (0.06 +/- 0.0006 g/g) in a reference fermentation without furfural to 9.7 +/- 0.07 mg/ mmol (0.05 +/- 0.0004 g/g), whereas the ethanol yield on glucose increased from 1.6 +/- 0.03 to 1.7 +/- 0.03 mmol/mmol. No glycerol was excreted during furfural reduction, and the lag phase in acetate production was extended from 1 h in the reference fermentation to 5 h in the presence of furfural. Acetaldehyde and pyruvate were excreted during the furfural reduction phase. Cell growth and cell maintenance were proportional to glucose consumption during the entire fermentation, whereas the cell-mass yield on ATP produced was low during furfural reduction. These observations indicate that furfural addition to a batch culture decreased cell replication without inhibiting cell activity (designated as replicative inactivation). The absence of glycerol production during furfural reduction suggests that furfural acted as an alternative redox sink oxidizing excess NADH formed in biosynthesis. A mechanistic mathematical model was developed that described accurately the fermentation in the absence and presence of furfural. The model was based on the assumptions that: (i) furfural reduction to furfuryl alcohol by NADH-dependent dehydrogenases had a higher priority than reduction of dihydroxyacetone phosphate to glycerol; and (ii) furfural caused inactivation of cell replication. The effect of furfural on cell replication is discussed in relation to acetaldehyde formation. Inactivation of cell replication was modeled by considering two populations within the cell culture, both metabolically active, but only one replicating. The kinetic description was developed as a tool to estimate transient fluxes of carbon, NADH/NAD+ and ATP/ADP.

Influence of Furfural on anaerobic glycolytic kinetics of Saccharomyces cerevisiae in batch culture

Recent concerns on the radiotoxicity and longevity of nuclides have prompted the development of new technologies for their removal from wastes produced from nuclear power programs and nuclear fuel reprocessing activities. Alongside developments from traditional chemical treatment processes, interest has also centered on the application of biotechnology for efficient waste treatment. Many biological techniques have relied on empirical approaches in simple model systems, with scant regard to the nature and volume of actual target wastes; such considerations may limit the application of the new technologies in practice. This review aims to identify some of the likely problems, to discuss the various approaches under current consideration, and to evaluate ways in which either the target waste or the detoxifying biomass may be modified or presented for the most efficient treatment.

The application of biotechnology to the treatment of wastes produced from the nuclear fuel cycle: biodegradation and bioaccumulation as a means of treating radionuclide-containing streams.

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 Cells for Application in the Food Industry

A strategy for the optimization of an expanded bed adsorption process has been developed by studying a model system involving the adsorption of lysozyme onto the adsorbent STREAMLINE SP. The hydrodynamic and adsorption properties of this ion exchange adsorbent in a variety of viscosities of feedstocks have been compared by analyzing bed expansion characteristics, liquid phase dispersion characteristics, equilibrium adsorption isotherms, and mass transfer characteristics. Additionally, the influences of the degree of bed expansion on adsorption performance have been investigated by frontal analysis. In these experiments, viscous feedstocks were simulated by the inclusion of glycerol in the adsorption buffers. Breakthrough curves for lysozyme were characterized and compared in terms of overall purification processing time and productivity. On the basis of these results, the relative productivities of different operating modes with the same process liquid were found to be almost the same. However, the processing time for each purification cycle decreased with increasing velocity of process liquid. It is demonstrated that an adsorption process carried out at a constant degree of bed expansion (twice its settled bed height, corresponding to bed voidage of 0.7) is more efficient, when characterized by the apparent dynamic binding capacity, than operation at a constant liquid velocity of 300 cm/h. These results have significant implications on the design and operation of the expanded bed adsorption procedures. The advantages and problems encountered in the use of expanded bed techniques for the direct extraction of proteins from unclarified feedstocks are also discussed.

Development of operating conditions for protein purification using expanded bed techniques: the effect of the degree of bed expansion on adsorption performance.

The use of residence time distribution analysis to characterize the liquid phase hydrodynamics in bioreactors is reviewed, with special emphasis on packed bed bioreactors. The importance of tracer selection, the method of data analysis and other uses for the data collected in RTD measurements are discussed.

Review of Liquid Mixing in Packed Bed Biological Reactors

Kinetic expressions for the fermentative production of relatively high concentrations [12% (w/v)] of ethanol have been examined Several expressions which account for both substrate and product inhibition have been formulated, and have been applied to suspended cell and immobilized cell reactors Experimental data have been used to validate the kinetic expressions used, and the impact of combined inhibition on optimal reactor configuration has been assessed The process implications of combined substrate and product inhibition for suspended and immobilized cell systems have been discussed

Examination of substrate and product inhibition kinetics on the production of ethanol by suspended and immobilized cell reactors.

Previous work has demonstrated that high ethanol productivities can be achieved using yeast or bacterial cells adsorbed onto the surface of ion exchange resin in vertical packed bed bioreactors The present work quantitatively characterizes the overall degree of backmixing in such reactors at two scales of operation: 20 and 80 L Stimulus-response experiments, using two solvents (2,3-butanediol and 2-ethoxyethanol) as tracers, were performed to measure the liquid phase residence time distribution (RTD) during continuous ethanol fermentations using the yeast Saccharomyces cerevisiae and the bacterium Zymomonas mobilis at the 2-L scale, and with S cerevisiae at the 8-L scale In order to separately determine the effects of liquid flow rate and gas evolution on the degree of mixing, stimulus-response experiments were also performed in the systems without microbial cells present The evolution of CO(2) was found to dramatically increase the extent of mixing; however, the tanks-in-series model for non-ideal flow represented the systems adequately The packed beds were equivalent to over 70 tanks-in-series during abiotic operation while during fermentations, with similar liquid flow rates, they ranged in equivalence from 35 to 15 tanks-in-series This increased knowledge of the overall degree of mixing in packed bed, immobilized cell bioreactors will allow for more accurate kinetic modelling and efficient scale up of the process

Donald E. Swaine

Papers

Review of Liquid Mixing in Packed Bed Biological Reactors

Examination of substrate and product inhibition kinetics on the production of ethanol by suspended and immobilized cell reactors.

Liquid residence time distributions in immobilized cell bioreactors.