Water probe

Because of the vast amount of information that has been published on the use of NMR to investigate food systems, it is not feasible to present a comprehensive survey of the usefulness of NMR for foods in this review. Rather, the approach taken here is to illustrate the usefulness of NMR specifically for probing water and solids mobility in foods by presenting selected papers from recent literature. Review papers and recent developments in each of the following areas are also included. 

Sun, X. and Schmidt, S.J. 1995. Probing water relations in foods using magnetic resonance techniques. In Annual Reports on NMR Spectroscopy (G.A. Webb, P.S. Belton, and M J. McCarthy, eds), Vol. 31, pp. 239-273. Academic Press, New York. 

WISE) solid-state NMR technique has been used to probe water-starch interactions on the molecular distance scale (Kulik et al., 1994). 

Figure 1. Optical arrangement to probe water distribution inside membranes. The magnification is 7 to 20 times.

The static measurement is based on the addition of a water-swollen cellulose to a solution of the molecular probe. Water in pores accessible to the solute dilutes the solution. In the chromatographic techniques, either glass or standard liquid chromatography columns were packed with cellulose in various forms. The elution volumes of the molecular probes used were determined. Data is generally plotted as internal volume accessible to individual solutes against their molecular sizes. This is illustrated in Figure 5.43. 

Incoherent quasielastic neutron scattering measured as a function of hydration for powders of deuterated phycocyanin has been used to probe water motions (Middendorf et al., 1984). The simplest model accounting for the data was jump diffusion of water molecules between localized-sorption sites and the development of clusters of surface water at higher hydration (half-coverage of the surface, 0.15 h). This model is consistent with the picture developed from sorption thermodynamics. 

Zawodzinski et al. [64] have reported self-diffusion coefficients of water in Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) equilibrated with water vapor at 303 K, and obtained results summarized in Fig. 36. The self-diffusion coefficients were deterinined by pulsed field gradient NMR methods. These studies probe water motion over a distance scale on the order of microns. The general conclusion was the PFSA membranes with similar water contents. A, had similar water self-diffusion coefficients. The measured self-diffusion coefficients in Nafion 117 equilibrated with water vapor decreased by more than an order of magnitude, from roughly 8 x 10 cm /s down to 5 x 10 cm /s as water content in the membrane decreased from A = 14 to A = 2. For a Nafion membrane equilibrated with water vapor at unit activity, the water self-diffusion coefficient drops to a level roughly four times lower than that in bulk liquid water whereas a difference of only a factor of two in local mobility is deduced from NMR relaxation measurements. This is reasonably ascribed to the additional effect of tortuosity of the diffusion path on the value of the macrodiffusion coefficient. For immersed Nafion membranes, NMR diffusion imaging studies showed that water diffusion coefficients similar to those measured in liquid water (2.2 x 10 cm /s) could be attained in a highly hydrated membrane (1.7 x 10 cm /s) [69]. 

The value of the NMR spin-lattice relaxation time in each of the pixels of an image may be converted to a pore size by the adoption of a relaxation model. For a liquid imbibed in a pore space the relaxation rate is enhanced. This is believed to be due to interactions between a thin layer of liquid and the solid matrix at the solid/liquid interface increasing the relaxation rate. There is then also difllisional exchange between this surface-affected layer and the rest (bulk) of the liquid in the rest of the pore. In the case here where the pore sizes are several orders of magnitude smaller than the rms displacement of the probe water molecules employed, the "two-fraction fast-exchange" model of Brownstein and Tarr [7] will be used, where the overall measured value of T, is given by ... 

The best strategy to be followed in order to get accurate sets of A values has not been defined, so at present more or less complex statistical elaborations of some data are used. Among the numerical data that have been used we mention solvation and solvent transfer energies, intrinsic solute properties (electron isodensity surfaces, isopotential electronic surfaces, multipole expansions of local charge distribution), isoenergy surfaces for the interaction with selected probes (water, helium atoms), Monte Carlo simulations with solutes of various nature. All these sets of data deserve comments, that are here severely limited not to unduly extend this Section. 

Renaud made a similar observation and reported a mild and efficient radical-mediated reduction of organoboranes (Scheme 6.10).20 An in situ-generated fi-methoxycatecholborane methanol complex acts as a reducing agent. The radical nature of the process was demonstrated by using (+)-2-carene as a radical probe. Water, ethanol and trifluoroethanol can be used instead of MeOH with very similar efficiency. 

Probing Water-Solid Interactions in Crystalline and Amorphous Systems Using Vibrational Spectroscopy... 

In this presentation, two examples of the use of vibrational spectroscopy to probe water-solid interactions in materials of interest to the food and pharmaceutical sciences are described. First, the interaction of water vapor with hydrophilic amorphous polymers has been investigated. Second, water accessibility in hydrated crystalline versus amorphous sugars has been probed using deuterium exchange. In both of these studies, Raman spectroscopy was used as the method of choice. Raman spectroscopy is especially useful of these types of studies as it is possible to control the environment of the sample more easily than with infrared spectroscopy. 

This study focuses on the effect of water on the variations of surface characteristics of the drug substance and how these surfaces manifest themselves in terms of measurable physico-chemical quantities. Water plays a central role in many pharmaceutical and food situations, not only because of its unique interactive properties and behavior, but also because its importance, ubiquitous nature, and common use in pharmaceutical and food processing. In this study, the working hypothesis is that the surface energetics of the two polymorphs can be distinguished through the study of their interactions with a common probe, water, and that such differences can, in turn, be related to differences in the corresponding powder properties. 

Mei, Y., Kumar, A., and Gross, R.A. (2002) Probing water-temperature relationships for lipase-catalyzed lactone ring-opening polymerizations. Macromolecules, 35.14, 5444-5448. 

As a base for the development of these probes, water vapor partial pressure was firstly determined exactly with the help of a galvanic solid-electrolyte cell in the 1, g-equilibrium system H2O-NH3 in the temperature range from 0 to 30 °C [91]. The results were used in the investigation of the decomposition of metastable NH3 at the platinum electrode of a solid-electrolyte cell. It was proved that the NH3 decomposition in the temperature range from 500 to... 

NMR relaxation times have also proved useful in probing water mobility in foodstuffs. In some cases, exchange between three water sites, corresponding to bulk, surface and bound water, is used to explain observed relaxation behaviour. Another interesting area is using NMR relaxation times to study the water that does not freeze at the temperatures normally used in cryopreservation. 

See also. Amperometry. Conductimetry and Oscillometry. Coulometry. Electrogravimetry. Ion-Selective Electrodes Oven/iew Glass Solid-State Liquid Membrane Gas Sensing Probes Water Applications. pH. Polarography Overview. Process Analysis Sensors. Sensors Overview Amperometric Oxygen Sensors. Sulfur. Voltammetry Overview Anodic Stripping. Water Analysis Industrial Effluents. 

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