Methods for peak assignment in low-resolution multidimensional NMR cross-correlation relaxometry.

Several NMR protocols are presented for assigning peaks in complex T1-T2 spectra, including the effects of varying the spectrometer frequency and the CPMG pulsing rate. Extensions into a third dimension based on chemical-shift; diffusion- and field-cycled weighted T1-T2 cross-correlation methods are also explored as a means of peak assignment. We illustrate the power of these novel techniques with reference to simple aqueous sucrose solutions, but the methodology should be generally applicable.

Motional relativity and industrial NMR sensors.

We explore the implications of motional relativity in NMR and show that sample translation can be used to acquire NMR signals without the need for pulsed RF excitation or pulsed magnetic field gradients. Novel single-shot, on-line NMR acquisition protocols for samples being conveyed at high speed are discussed and preliminary results using a low-cost, on-line prototype NMR sensor are presented.

A low-field, low-cost Halbach magnet array for open-access NMR.

A working prototype of a novel low-cost Halbach-array-based NMR system is described. The new design provides open access to the sample relative to conventional NMR magnet designs and this facilitates the simultaneous use of multi-sensor techniques on the same sample, in which NMR/MRI can potentially be combined with other spectroscopies such as impedance spectroscopy, laser scattering and rheological experiments.

Microstructural effects on microbial survival: phase-separating dextran solutions.

Evolving microstructure in a model dextran solution is shown to exert a major influence on the survival of Escherichia coli K-12 frag 1 and Salmonella typhimurium LT2. The microstructure results from microscopic phase separation, which develops over several hours resulting in hardening of the solution into a glassy state. The microstructure is characterized by an array of physical methods including image analysis, electron spin resonance and bulk rheology, and it is shown that bacterial survival depends on the formation of microscopic. water-rich domains and not primarily on bulk water activity or hardness.

NMR diffusometry of oil-in-water emulsions.

The diffusion propagator of the continuous aqueous phase of concentrated oil-in-water emulsions is used to probe the relationship between emulsion microstructure and bulk rheological properties. This is done by expanding the stimulated echo amplitude, S(q,Delta), as a multiple exponential time series expansion in Delta, with wavevector dependent expansion coefficients. These coefficients are compared with predictions from several theoretical models for three types of stable emulsion, each differing in microstructure. Empiric relationships between the wavevector expansion coefficients and bulk rheology are established.

Microstructural factors controlling the survival of food-borne pathogens in porous media.

The survival of Salmonella typhimurium LT2, Escherichia coli K-12 and Pseudomonas putida in several model porous media poised at a water activity of 0.94 is shown to depend critically on the microstructure of the particulate matrix and the microscopic water distribution. The porous media were made by randomly dispersing a liquid inoculum containing ca. 10(7) cells/ml throughout the pores and interparticle spaces of packed beds of silica particles and Sephadex microspheres. The purely "microstructural stress" effects were isolated by comparison with a homogeneous liquid growth medium having the same water activity. The possibility of exploiting similar microstructural stress effects in food preservation is discussed.

NMR Q-space microscopy of concentrated oil-in-water emulsions.

The pulsed field gradient stimulated echo technique with selective excitation is used to probe the diffusion of water in the continuous phase of concentrated oil-in-water emulsions. The dependence of the echo amplitude, S(q,Delta), on wavevector, q, and diffusion time, Delta, shows that the water diffusion propagator is sensitive to emulsion microstructure. This is analyzed using a multiple exponential time series expansion of S(q,Delta) in Delta, with wavevector dependent expansion coefficients. These coefficients are compared with predictions from several theoretical models for three types of stable emulsion, each differing in microstructure. The relationship between the nuclear magnetic resonance q-space measurements and bulk rheology for all three types of emulsion is also explored.

An NMR relaxation study of the state of water in gelatin gels.

A combination of 1H and 23Na NMR is used to probe the dynamic state of water in gelatine gels as the water content is lowered from 70% to dryness. A sharp increase in the proton and sodium transverse relaxation rates is observed as the water content falls from 20 to 15% while the proton longitudinal and dipolar cross relaxation rates show a maximum at ca. 15%. We show that these observations can be understood if monolayer coverage occurs at 15% and multilayers of less strongly interacting hydration water are formed between 15 and 20%. Above 20% the water appears to behave as an unperturbed bulk phase.

Microstructural characterization of starch systems by NMR relaxation and Q-space microscopy.

Nuclear magnetic resonance (NMR) relaxation and q-space diffusion measurements have been used to probe the microscopic water distribution in a variety of starch-based systems, including packed beds of native starch granules with varying water contents, starch gels, and freeze-dried starch gels. The q-space data for the granular beds is compared with a variety of theoretical models and conforms best to unbounded diffusion in a lower dimensional space. In contrast to some earlier reports, the data for the gelatinized samples are not anomalous and conform to simple unrestricted diffusion in a three-dimensional space. The paper concludes with a novel method for probing pore size distribution in freeze-dried starch gels by infusion of acetone.

Echo-planar imaging relaxometry to measure the viscosity of a model meal.

A method for measuring noninvasively the viscosity of a polysaccharide model meal using the known relationship between relaxation times and polysaccharide concentration in solution in vitro is presented. The aim is to develop a method for monitoring digesta viscosity in vivo, using EPI to capture the motion of the gastrointestinal lumen. The transverse relaxation rate T-12 of locust bean gum solutions was calibrated against the zero-shear viscosity at 37 degreesC. Differences in viscosity were distinguished significantly using T-12 measurements. T-12 and viscosity were insensitive to exposure to gastric juice and changes in pH, and the model meal was well received by volunteers and provided good contrast in vivo in EPI images. Therefore it would be possible to use this method to monitor the changes in meal viscosity within the gastric lumen in vivo.

Water availability and the survival of Salmonella typhimurium in porous systems.

The survival of Salmonella Typhimurium LT2 in randomly packed beds of glass beads, microporous silica particles and Sephadex microspheres is examined. It is shown that the decrease in the percentage cell recovery in these porous materials at reduced water content is not correlated with the global water activity as determined by conventional vapour pressure measurements but rather with the osmotic shock induced by the sudden redistribution of water and air among the microscopic pores in the matrix surrounding the cells. For this reason the bacterial survival and growth data correlates best with physical measurements, such as NMR and electrical conductivity, which are sensitive to the microscopic air-water distribution. The implications of this observation in food safety and preservation are discussed.

Constant gradient stimulated echo studies of diffusion in porous materials at high spectrometer fields.

A high field strength, constant gradient stimulated-echo pulse sequence is applied to a model heterogeneous system consisting of randomly packed beds of glass microspheres. A multiple exponential analysis of the dependence of the stimulated echo amplitude on diffusion time, delta, yields coefficients that depend explicitly on both the wavevector, q and on the time delay, delta. The wavevector and delta- dependence of the coefficients is analyzed both theoretically and experimentally and shown to be sensitive to the effects of coupled relaxation and diffusion. It is proposed that these effects could be exploited as a new probe of microstructure.

Dynamic NMR Q-space studies of microstructure with the multigrade CPMG sequence.

A new approach to q-space studies of microstructure is proposed, which exploits the combined information contained in the water proton transverse relaxation time distribution and the frequency dependence of the apparent water diffusivity in heterogeneous systems. Using an automated two-dimensional multigrade CPMG sequence, both the pulse spacing and the amplitude of the applied field gradient are varied systematically and used to measure the frequency and wave vector dependence of the multiple exponential echo decay constants and amplitudes. Undesirable crossterms in the applied and background field gradients are eliminated by a simple procedure involving a sign reversal in the applied gradient. Nonlinear, local susceptibility-induced field gradients are shown to lead to enhanced, frequency-dependent apparent water diffusivities that are sensitive to the local microstructure.

Combined relaxation and diffusion studies of porous media using the multigrade CPMG sequence.

A new approach to q-space NMR studies of microstructure is proposed that exploits the combined information contained in the water proton transverse relaxation time distribution and the frequency dependence of the apparent water diffusivity. A simple protocol is used to eliminate undesirable crossterms in background susceptibility gradients. Local, microscopic nonlinearity in the applied field gradients is shown to lead to enhancement of the apparent water diffusivity in small pores at high frequencies. The relationships between NMR relaxation studies of pore emptying with sorption isotherms, electrical conductivity, and microbiological survival are indicated.

A new model for bacterial growth in heterogeneous systems.

A general theory of spatially dependent bacterial growth in heterogeneous systems is developed by combining a structured-cell kinetic model of bacterial metabolism with reaction-diffusion equations describing the transport of nutrients in the growth medium. Rate constants in the theory are determined for Listeria monocytogenes by fitting the viable count batch growth data acquired in homogeneous, shake-flask cultures. The rate constants are then used to calculate the form of bacterial growth when it occurs as localized microcolonies in structured systems. The same theory is used to analyse the effects of transient variations in environmental variables.

A quantitative study of water proton relaxation in packed beds of porous particles with varying water content.

A quantitative analysis of the dependence of water proton relaxation on water content in randomly packed beds of Sephadex is undertaken. A combination of Osmotic and Capillary theory is used to describe the morphological changes occurring in the packed beds as the water content is lowered. At each water content the water proton relaxation is calculated using a "proton exchange-diffusion" model which takes into account fast chemical exchange between water and dextran hydroxyl protons and the diffusion of water molecules between the water compartments inside and outside the Sephadex beads. The relaxation time distribution are shown to provide a sensitive probe of the air-water distribution in the bed and of the shrinkage of the Sephadex beads at lower water contents. The theoretical models provide an accurate, quantitative description of the relaxation behavior except for the largest beads at high water contents when there is slow diffusion between the water compartments. In this case, a more realistic three-dimensional description of the air-water distribution in a randomly packed bed is required.

Water proton relaxation in dilute and unsaturated suspensions of non-porous particles.

NMR water proton relaxation times are reported for suspensions of silica powder of varying silica/water ratios. Pore size distributions and pore connectivities are derived from the relaxation time distributions for the water-saturated suspension. Capillary theory appears to explain the relaxation behaviour of the unsaturated, packed suspensions. The relaxation data in suspensions that have lower solid/liquid ratios than the saturated, packed suspension are sensitive to the particular radial distribution function. This is analysed with a simple cluster model.

An MRI study of drying in granular beds of nonporous particles.

MRI is used to study the role of capillary and gravitational forces in controlling mass transport of water during isothermal drying of granular beds of nonporous particles. A new model is presented that shows how capillary and gravitational forces relate to Fickian diffusion driven by a chemical potential gradient. In granular beds, where capillary and gravitational forces dominate, the image profiles give direct information on the dependence of the degree of saturation on capillary suction pressure. The effects of changing particle size and surface tension on the profiles is investigated quantitatively, and it is shown that high capillary pressures create anomalies in the drying profiles.

Quantitative radial imaging of porous particles beds with varying water contents.

Radial imaging protocols suitable for monitoring water transport in biopolymer and food materials during processes such as drying and rehydration are developed and tested on a well-characterized model sample. This model consisted of a randomly packed bed of Sephadex beads with varying water content. The results are interpreted with theoretical models for the dependence of the initial water magnetization, transverse relaxation, and diffusive attenuation on water content for two slice-selective radial imaging pulse sequences. It is shown that volume shrinkage and changes in packing density complicate the dependence of the initial magnetization on water content, so that the transverse relaxation rate provides the most reliable monitor of water content. Radial imaging is shown to offer many advantages over conventional two-dimensional imaging whenever the sample can be made with cylindrical symmetry.

NMR microimaging studies of water diffusivity in saturated, microporous systems.

A novel microimaging method is described for measuring the effective diffusion coefficient (D infinity) of water in saturated, microporous materials. A simple multicompartment computer simulation was used to show how the value of D infinity, when combined with measurements of water proton relaxation times and pulsed field gradient spin-echo amplitudes, allows the pore size distribution and pore connectivity to be characterized.