Pulsed Field Gradient Spin-echo Technique Research Articles

Determining Effects of Freezing on Pasta Filata and Non-Pasta Filata Mozzarella Cheeses by Nuclear Magnetic Resonance Imaging

The formation of ice during freezing of pasta filata and non-pasta filata Mozzarella cheeses, and the spatial redistribution of water T2 relaxation time and the changes of water self-diffusion coefficient (D) within the unfrozen and frozen-stored cheese samples were observed by nuclear magnetic resonance imaging. Images of water spin number density and water T2 relaxation time were obtained using spin-echo imaging pulse sequence. The water self-diffusion coefficient was measured by pulsed-field gradient spin-echo technique. The ice formation was accompanied by loss of signal intensity in the affected areas of the cheese sample. There was a significant change in T2 and D values of water following freezing-thawing, which can be used to characterize the effect of freezing on cheeses. The D values of the frozen-stored pasta filata Mozzarella cheese samples were higher than those for the unfrozen samples. Such a difference was not observed for the non-pasta filata Mozzarella cheese samples. The T2 distributions of frozen-stored pasta filata Mozzarella cheese samples were narrower, and those for the non-pasta filata Mozzarella cheese samples were broader T2. This may be attributed to the microstructure differences between the two cheeses.

Anomalous Surfactant Diffusion in a Gel of Chemically Cross-Linked Ethyl(hydroxyethyl) Cellulose

The interactions of sodium dodecyl sulfate (SDS) with chemically cross-linked gels of ethyl(hydroxyethyl) cellulose (EHEC) were studied. Above the so-called critical association concentration (cac), binding of SDS gives rise to an increased swelling of the EHEC gels. The binding of SDS to the gels was measured with flame emission analysis of the sodium ion. The self-diffusion of the surfactant ion (DS) in the gels was studied by the NMR pulsed field gradient spin-echo technique. Both experiments were performed on gels swollen to equilibrium in SDS solutions of varying concentrations. Comparisons with the DS diffusion in a solution of non-crosslinked EHEC were also made. In the EHEC solutions the observed spin-echo decays for DS were always describable in terms of a single surfactant diffusion coefficient (Gaussian diffusion). In contrast, the DS diffusion in the gels above the cac (SDS) was clearly non-Gaussian, or anomalous. The echo decays in the gels could be fitte to a log-normal distribution of diffusion coefficients. When the time during which the diffusion was measured was increased, the width of the distribution increased, while the average diffusion coefficient remained constant. An increase in the width of distribution was also seen when the SDS concentration was increased. The anomalous diffusion is ascribed to inhomogeneities in the gel. (Less)

Diffusion of ethane in silicalite-1 by frequency response, sorption uptake and nuclear magnetic resonance techniques

The diffusion of ethane in ZSM-5 has been studied using four different experimental methods: a frequency-response technique, a single-step frequency-response method (an improved sorption uptake–desorption technique), an n.m.r. pulsed field gradient spin–echo technique and, finally, an n.m.r. tracer desorption technique. Large and uniform silicalite-1 crystals were prepared and used in all four techniques. Different concentrations of tetrapropylammonium ions in the synthesis batch alter systematically both the crystal habit and the relative amounts of internal silanol groups in the silicalite-1 framework, indicating different concentrations of defect Si—O—Si bonds. For the genesis of the defective siloxane bonds, a model has been presented. Comparing the translational mobility of ethane on a selected sample, the diffusion coefficients measured by the two n.m.r. methods were found to be ca. 150 times larger than the corresponding coefficients derived from the frequency response and sorption uptake–desorption methods. This experimental finding is explained by crystal bed depth influences on the molecular uptake in the latter sorption experiments.