Biomedical subjects

J C van den Heuvel

Publications and source records attributed to J C van den Heuvel.

6 recordsLinked to original sources

PubMed2001

Intra-particle oxygen diffusion limitation in solid-state fermentation.

Oxygen limitation in solid-state fermentation (SSF) has been the topic of modeling studies, but thus far, there has been no experimental elucidation on oxygen-transfer limitation at the particle level. Therefore, intra-particle oxygen transfer was experimentally studied in cultures of Rhizopus oligosporus grown on the surface of solid, nutritionally defined, glucose and starch media. The fungal mat consisted of two layers--an upper layer with sparse aerial hyphae and gas-filled interstitial pores, and a dense bottom layer with liquid-filled pores. During the course of cultivation ethanol was detected in the medium indicating that oxygen was depleted in part of the fungal mat. Direct measurement of the oxygen concentrations in the fungal mat during cultivation, using oxygen microelectrodes, showed no oxygen depletion in the upper aerial layer, but revealed development of steep oxygen concentration gradients in the wet bottom layer. Initially, the fungal mat was fully oxygenated, but after 36.5 hours oxygen was undetectable at 100 microm below the gas-liquid interface. This was consistent with the calculated oxygen penetration depth using a reaction-diffusion model. Comparison of the overall oxygen consumption rate from the gas phase to the oxygen flux at the gas-liquid interface showed that oxygen consumption of the microorganisms occurred mainly in the wet part of the fungal mat. The contribution of the aerial hyphae to overall oxygen consumption was negligible. It can be concluded that optimal oxygen transfer in SSF depends on the available interfacial gas-liquid surface area and the thickness of the wet fungal layer. It is suggested that the moisture content of the matrix affects both parameters and, therefore, plays an important role in optimizing oxygen transfer in SSF cultures.

Aerobiosis↗

PubMed1999

Kinetics and performance of a co-immobilised system of amyloglucosidase and Zymomonas mobilis.

High operational stability and productivity of co-immobilised systems are important aspects for their successful application in industrial processes. A dynamic model is required to describe artificially co-immobilised systems because the time needed to reach steady state normally exceeds the operational life span of these systems. Time dependent intraparticle concentration profiles and macroscopic conversion were modelled to study the operational stability and productivity of these systems theoretically. The model was used to describe experimental results of ethanol production from maltose by a co-immobilised system of amyloglucosidase and Zymomonas mobilis. Furthermore, the influence of the immobilisation procedure with glutaraldehyde and polyethyleneimine could also be studied with and incorporated in the model. From the model it could be derived that co-immobilised systems performing a consecutive reaction evolve towards a steady state, characterised by a constant concentration of the intermediate in the particle if product inhibition is neglected. Such a situation develops independently of the biomass concentration and the radial position, and has important consequences for co-immobilised systems. When the concentration of the intermediate in the bulk liquid is lower than this constant value in the biocatalyst particle, two regions may be distinguished in the particle: an inactive peripheral region without biomass and an active core with a biomass concentration depending on the substrate and immobilised enzyme concentration. Unlike immobilised single cell systems, it is possible to obtain a real steady state and therefore a stable situation for co-immobilised systems. However, a high operational life time could only be achieved at the expense of the productivity of the biocatalyst particle. A stability criterion is derived which agrees very well with the simulation results.

Biotechnology↗

PubMed1999

Microscale distribution of populations and activities of Nitrosospira and Nitrospira spp. along a macroscale gradient in a nitrifying bioreactor: quantification by in situ hybridization and the use of microsensors.

The change of activity and abundance of Nitrosospira and Nitrospira spp. along a bulk water gradient in a nitrifying fluidized bed reactor was analyzed by a combination of microsensor measurements and fluorescence in situ hybridization. Nitrifying bacteria were immobilized in bacterial aggregates that remained in fixed positions within the reactor column due to the flow regimen. Nitrification occurred in a narrow zone of 100 to 150 microm on the surface of these aggregates, the same layer that contained an extremely dense community of nitrifying bacteria. The central part of the aggregates was inactive, and significantly fewer nitrifiers were found there. Under conditions prevailing in the reactor, i.e., when ammonium was limiting, ammonium was completely oxidized to nitrate within the active layer of the aggregates, the rates decreasing with increasing reactor height. To analyze the nitrification potential, profiles were also recorded in aggregates subjected to a short-term incubation under elevated substrate concentrations. This led to a shift in activity from ammonium to nitrite oxidation along the reactor and correlated well with the distribution of the nitrifying population. Along the whole reactor, the numbers of ammonia-oxidizing bacteria decreased, while the numbers of nitrite-oxidizing bacteria increased. Finally, volumetric reaction rates were calculated from microprofiles and related to cell numbers of nitrifying bacteria in the active shell. Therefore, it was possible for the first time to estimate the cell-specific activity of Nitrosospira spp. and hitherto-uncultured Nitrospira-like bacteria in situ.

Bacteria↗

PubMed1993

Microelectrode measurements of the activity distribution in nitrifying bacterial aggregates.

Microelectrodes for ammonium, oxygen, nitrate, and pH were used to study nitrifying aggregates grown in a fluidized-bed reactor. Local reactant fluxes and distribution of microbial activity could be determined from the microprofiles. The interfacial fluxes of the reactants closely reflected the stoichiometry of bacterial nitrification. Both ammonium consumption and nitrate production were localized in the outer shells, with a thickness of approximately 100 to 120 mum, of the aggregates. Under conditions in which ammonium and oxygen penetrated the whole aggregate, nitrification was restricted to this zone; oxygen was consumed in the central parts of the aggregates as well, probably because of oxidation of dead biomass. A sudden increase of the oxygen concentration to saturation (pure oxygen) was inhibitory to nitrification. The pH profiles showed acidification in the aggregates, but not to an inhibitory level. The distribution of activity was determined by the penetration depth of oxygen during aggregate development in the reactor. Mass transfer was significantly limited by the boundary layer surrounding the aggregates. Microelectrode measurements showed that the thickness of this layer was correlated with the diffusion coefficient of the species. Determination of the distribution of nitrifying activity required the use of ammonium or nitrate microelectrodes, whereas the use of oxygen microelectrodes alone would lead to erroneous results.

Journal Article↗

PubMed1993

pH and Glucose Profiles in Aggregates of Bacillus laevolacticus.

Size distributions and glucose and pH profiles of aggregates of the d-(-)-lactic acid-producing organism Bacillus laevolacticus were measured. The organisms were grown in continuous culture with a medium glucose concentration of either 280 or 110 mM. A maximal aggregate diameter of 2.2 mm, with a Sauter mean of 1.46 mm, was determined for the former culture condition, whereas aggregates from a culture with 110 mM glucose input had a maximal diameter of 1.9 mm (Sauter mean of 1.07 mm). A pH gradient of approximately 2 U was observed for large aggregates (above 1.5 mm). In smaller aggregates (0.75 mm), the pH value in the interior part was approximately 0.4 U lower than that in the culture fluid. It could be concluded that, in cultures with the high glucose input, lactic acid accumulated within the aggregates to such an extent that metabolism in the central region of the larger aggregates could not proceed further. In these cultures, approximately 90% of the total biomass was active. In aggregates from cultures with a low glucose input, glucose only partly penetrated the larger-sized aggregates, and the activity of this culture was reduced to approximately 70% of the biomass. These aggregates were found to decrease in size after prolonged periods of cultivation. It is suggested that this is caused by glucose depletion in the interior of the aggregates. It is concluded that the availability of glucose is an important factor in determining the size of aggregates of B. laevolacticus.

Journal Article↗

PubMed1991

Determination of glucose diffusion coefficients in biofilms with micro-electrodes.

A glucose micro-electrode was developed for direct measurements inside biofilms, and applied for the determination of effective diffusion coefficients in a model system of agar beads containing immobilized yeast cells. Two methods were used, one based on concentration gradients present at the liquid/solid interface of an active biofilm under steady-state conditions, the other based on the rate of glucose redistribution in an inactivated biofilm under transient-state conditions. Additional measurements with pH and oxygen micro-electrodes were performed and thus allowed for in-situ correction of the glucose electrode signal. From the micro-electrode measurements in the model system it was concluded that the glucose micro-sensor is a useful tool with which to obtain effective diffusion coefficients in biofilms.

Agar↗