H. van As

Cluster Structure of Anaerobic Aggregates of an Expanded Granular Sludge Bed Reactor

ABSTRACT The metabolic properties and ultrastructure of mesophilic aggregates from a full-scale expanded granular sludge bed reactor treating brewery wastewater are described. The aggregates had a very high methanogenic activity on acetate (17.19 mmol of CH4/g of volatile suspended solids [VSS]·day or 1.1 g of CH4 chemical oxygen demand/g of VSS·day). Fluorescent in situ hybridization using 16S rRNA probes of crushed granules showed that 70 and 30% of the cells belonged to the archaebacterial and eubacterial domains, respectively. The spherical aggregates were black but contained numerous whitish spots on their surfaces. Cross-sectioning these aggregates revealed that the white spots appeared to be white clusters embedded in a black matrix. The white clusters were found to develop simultaneously with the increase in diameter. Energy-dispersed X-ray analysis and back-scattered electron microscopy showed that the whitish clusters contained mainly organic matter and no inorganic calcium precipitates. The white clusters had a higher density than the black matrix, as evidenced by the denser cell arrangement observed by high-magnification electron microscopy and the significantly higher effective diffusion coefficient determined by nuclear magnetic resonance imaging. High-magnification electron microscopy indicated a segregation of acetate-utilizing methanogens (Methanosaeta spp.) in the white clusters from syntrophic species and hydrogenotrophic methanogens (Methanobacterium-like andMethanospirillum-like organisms) in the black matrix. A number of physical and microbial ecology reasons for the observed structure are proposed, including the advantage of segregation for high-rate degradation of syntrophic substrates.

Time-Domain NMR Applied to Food Products

Abstract Time-domain NMR is being used throughout all areas of food science and technology. A wide range of one- and two-dimensional relaxometric and diffusometric applications have been implemented on cost-effective, robust and easy-to-use benchtop NMR equipment. Time-domain NMR applications do not only cover research and development but also quality and process control in the food supply chain. Here the opportunity to further downsize and tailor equipment has allowed for “mobile” sensor applications as well as online quality inspection. The structural and compositional information produced by time-domain NMR experiments requires adequate data-analysis techniques. Here one can distinguish model-driven approaches for hypothesis testing, as well as explorative multi-variate approaches for hypothesis generation. Developments in hardware and software will further enhance measurement speed and reveal more detailed structural features in complex food systems.

Characterization of the diffusive properties of biofilms using pulsed field gradient-nuclear magnetic resonance

The mobility of water in intact biofilms was measured with pulsed field gradient nuclear magnetic resonance (PFG-NMR) and used to characterise their diffusive properties. The results obtained with several well-defined systems, viz. pure water, agar, and agar containing inert particles or active bacteria were compared to glucose diffusion coefficients measured with micro-electrodes and those calculated utilising theoretical diffusion models. A good correspondence was observed indicating that PFG-NMR should also enable the measurement of diffusion coefficients in heterogeneous biological systems. Diffusion coefficients of several types of natural biofilms were measured as well and these results were related to the physical biofilm characteristics. The values had a high accuracy and reflected the properties of a sample of ca. 100 biofilms, while non-uniformity or non-geometrical shapes did not negatively influence the results. The monitored PFG-NMR signal contains supplementary information on e.g. cell fraction or spatial organisation but quantitative analysis was not yet possible. Copyright 1998 John Wiley & Sons, Inc.

NMR methods for imaging of transport processes in micro-porous systems.

Abstract Magnetic resonance imaging (MRI) allows non-destructive and non-invasive measurement and visualisation of both static and dynamic water phenomena. Flow and transport processes can either be measured by following the local intensity in time-controlled sequential images, by mapping the effect of contrast agents or labelled molecules, or by mapping the (proton) displacement in a well known time interval directly. By a proper choice of methods, a time window ranging from milliseconds to weeks (or even longer) can be covered. Combining transport measurements with relaxation time information allows the discrimination of transport processes in different environments or of different fluids, even within a single picture element within an image. Here we present an overview of the principles of NMR imaging techniques to visualise and unravel complex, heterogeneous transport processes in porous systems. Applications and limitations will be discussed, based on results obtained in model and artificial soil systems.

Gas and liquid phase distribution and their effect on reactor performance in the monolith film flow reactor

Nuclear magnetic resonance imaging (MRI) has been applied to study the phase distribution in the monolith film flow reactor. The accumulation of the liquid in the corners of the square channel with an arc-shaped gas–liquid interface has been determined. The average liquid saturation is in good agreement with model calculations. Non-uniformities of the liquid distribution over the four corners of the square channel were apparent, besides the maldistribution over the cross-section of the monolith. Computational fluid dynamics (CFD) calculations applying the measured liquid distribution predict a broadening of the residence time distribution and a shorter break-through time, due to the maldistribution, which is in very good agreement with experimental results. The impact on the modeled gas–liquid mass transfer performance seems to be negligible, due to the nearly linear relation between kGLaV and uLs.

Quantitative 1H-NMR imaging of water in white button mushrooms (Agaricus bisporus)

MRI represents a valuable tool for studying the amount and physical status of water in plants and agricultural products, for example, mushrooms (Agaricus bisporus). Contrast in NMR images originates from the mixed influence of the fundamental NMR parameters, amongst others, spin-density, T2- and T1 relaxation processes. Maps of these parameters contain valuable anatomical and physiological information. They can, however, be severely distorted, depending on the combination of parameter settings and anatomy of the object under study. The influence of the tissue structure of mushrooms, for example, tissue density (susceptibility inhomogeneity) and cell shape on the amplitude, T2, and T1 images is analyzed. This is achieved by vacuum infiltration of the cavities in the mushrooms spongy structure with Gd-DTPA solutions and acquiring Saturation Recovery-Multispin Echo images. It is demonstrated that the intrinsic long T2 values in the cap and outer stipe tissue strongly relate to the size and geometry of the highly vacuolated cells in these spongy tissues. All observed T2 values are strongly affected by susceptibility effects. The T2 of gill tissue is shorter than T2 of the cap and outer stipe, probably because these cells are less vacuolized and smaller in size. The calculated amplitude images are not directly influenced by susceptibility inhomogeneities as long as the observed relaxation times remained sufficient long. They reflect the water distribution in mushrooms best if short echo times are applied in a multispin echo imaging sequence at low magnetic field strength.

Use of 1H NMR to study transport processes in porous biosystems

The operation of bioreactors and the metabolism of microorganisms in biofilms or soil/sediment systems are strongly dictated by the transport processes therein. Nuclear magnetic resonance (NMR) spectroscopy or magnetic resonance imaging (MRI) allow nondestructive and noninvasive quantification and visualisation (in case of MRI) of both static and dynamic water transport phenomena. Flow, mass transfer and transport processes can be measured by mapping the (proton) displacement in a defined time interval directly in a so-called pulsed field gradient (PFG) experiment. Other methods follow the local intensity in time-controlled sequential images of water or labelled molecules, or map the effect of contrast agents. Combining transport measurements with relaxation-time information allows the discrimination of transport processes in different environments or of different fluids, even within a single picture element in an image of the porous biosystem under study. By proper choice of the applied NMR method, a time window ranging from milliseconds to weeks (or longer) can be covered. In this paper, we present an overview of the principles of NMR and MRI techniques to visualise and unravel complex, heterogeneous transport processes in porous biological systems. Applications and limitations will be discussed, based on results obtained in (model) biofilms, bioreactors, microbial mats and sediments. Journal of Industrial Microbiology & Biotechnology (2001) 26, 43–52.

Quantitative measurement and imaging of transport processes in plants and porous media by 1H NMR

NMR and MRI have been applied to transport processes, that is, net flow and diffusion/perfusion, of water in whole plants, cells, and porous materials. By choosing proper time windows and pulse sequences, magnetic resonance imaging can be made selective for each of the two transport processes. For porous media and plant cells the evolution of the spatial distribution of excited spins has been determined by q-space imaging, using a 20 MHz pulsed 1H NMR imager. The results of these experiments are explained by including spin-relaxation and exchange at boundaries. A 10 MHz portable 1H NMR spectrometer is described, particularly suitable to study the response of net flow in plants and canopies to changing external conditions.

NMR imaging of white button mushroom (Agaricus bisporis) at various magnetic fields

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) have been applied to visualize physiological phenomena in plants and agricultural crops. Imaging sequences that result in contrast of a combination of parameters (e.g., proton density, T1, T2, T2*) cannot be used for a correct and unique interpretation of the results. In this study multiecho imaging together with monoexponential T2 decay fitting was applied to determine reliable proton density and T2 distributions over a mushroom. This was done at three magnetic field strengths (9.4, 4.7, and 0.47 T) because susceptibility inhomogeneities were suspected to influence the T2 relaxation times negatively, and because the influences of susceptibility inhomogeneities increase with a rise in magnetic field strength. Electron microscopy was used to understand the different T2s for the various tissue types in mushrooms. Large influences of the tissue ultrastructure on the observed T2 relaxation times were found and explained. Based on the results, it is concluded that imaging mushrooms at low fields (around or below 0.47 T) and short echo times has strong advantages over its high-field counterpart, especially with respect to quantitative imaging of the water balance of mushrooms. These conclusions indicate general validity whenever NMR imaging contrast is influenced by susceptibility inhomogeneities.

A method for the simultaneous measurement of NMR spin-lattice and spin-spin relaxation times in compartmentalized systems.

Abstract A method for simultaneous measurement of spin-lattice and spin-spin relaxation times of water in compartmentalized systems is presented. The method is based on a combination of the saturation-recovery and the Carr-Purcell-Meiboom-Gill sequences (SR-CPMG). The result is a two-dimensional data set. The data set can be analyzed in different ways, i.e., in a conventional way yielding the values of T1 and T2 present in the system, or in a way yielding correlated T1 and T2 values for each compartment. The observed relaxation times are the intrinsic relaxation times of the compartments if exchange between the compartments is absent, or additionally affected by exchange, as is demonstrated using two phantoms: (i) two concentric tubes containing nonexchanging MnCl2 solutions of different concentration and (ii) apple fruit tissue, containing water in compartments exhibiting mutual exchange. For (i), the SR-CPMG sequence results in a better S N ratio for the fit of T1 and in the corresponding intrinsic T1 and T2 values of the MnCl2 solutions in each tube. For (ii), the observed relaxation times reflect the exchange kinetics of water between the compartments in addition to the intrinsic T1 and T2 relaxation times in those compartments.