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Applications of NMR imaging in processing of foods

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  1. Introduction.
  2. Theory of magnetic resonance imaging.
  3. Gradients.
  4. Crystallization and freezing.
    1. Crystallization.
    2. Freezing.
  5. Diffusion.
  6. Emulsions.
  7. Synersis.
  8. Flow.
    1. Time-of-flight techniques.
    2. Velocity-encoding techniques.
    3. Aseptic processing applications.
    4. Extrusion applications.
  9. Conclusion.

Magnetic resonance imaging (MRI), originally developed for medical applications, has recently been exploited for observation and characterization of foodstuffs and their manufacture. MRI and its related techniques have already proven useful in non-invasive observations of fruit and vegetable quality, ripening and fruit defects. The value of MRI has been demonstrated in several processes involved in cheese making: syneresis; formation of eyes during ripening of Swiss cheese; fat droplet size determination in the finished product; and diffusion of salt into cheese (brining). MRI observations of water and oil phase changes include drying, fat crystallization, and freezing.The magnetic resonance (MR) phenomenon, and its utilization in exploring the properties of food materials, depends on the inherent magnetic properties of certain atomic nuclei in a magnetic field. The hydrogen nucleus, a proton, behaves as a spinning charged particle; it possesses angular momentum and generates a polar field.

[...] Sodium imaging allows the non-invasive observation of the progress of sodium chloride diffusion into a milk protein gel, a model of cheese brining during ripening. Emulsions Food emulsions, including margarine, ice cream, mayonnaise and salad dressing, represent another economically important system observable by MRI. The potential for measurement by MRI of oil/water ratios in separated salad dressings emerged early on. Manufacturers of these products and of non-food emulsions must understand the affect of emulsifiers on phase separation of emulsions, and control particle size distribution. [...]

[...] Diffusion As in freezing, diffusion studies have exploited attenuation of the NMR signal; in contrast to freezing, this attenuation results from molecular mobility, and not its absence. Most diffusion imaging experiments make use of the spinecho pulse sequence: briefly, an rf pulse disturbs the magnetization of the sample nuclei; the nuclei dephase by T2 mechanisms, diffusion and magnetic field non uniformities; a refocusing pulse is applied; and the signal regains coherency spin echo). The refocusing pulse can only reverse dephasing due to a non-uniform magnetic field, not that due to T2 relaxation or diffusion. [...]

[...] Gradients The foundation of magnetic resonance imaging lies in the response of resonance frequency to changes in the magnetic field; difference in frequency or phase due to application of magnetic field gradients provides information on position, diffusion rates and flow velocity. Slice selection in a spin-echo experiment allows excitation of a relatively narrow band of frequencies, which, in the presence of the magnetic field gradient described above, corresponds to a limited volume of resonating nuclei. Excitation of nuclei from a designated volume necessarily restricts the origin of the acquired signal to nuclei from that volume, and facilitates non-invasive observation of the interior of objects. [...]

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