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. [...]
[...] Aseptic Processing Applications Use of velocity profiles provided by MRI could prevent either under- or overcooking of foods during aseptic processing. Aseptic processing requires the heating of food at a prescribed temperature, for time sufficient to sterilize it, as it moves through a pipe. Packaging and sealing while still hot protect the product from spoilage. Undercooking, leading to spoilage, and overcooking, resulting in loss of desired sensory attributes (taste, texture) present a problem addressable by MRI flow techniques. Obviously the solution involves ensuring that all of the food material remains in the heating pipe for a time sufficient to sterilize, but no longer, to preserve quality. [...]
[...] Mobility of nuclei determines the behavior of the NMR signal, and analysis of the NMR signal can in turn define the mobility of the nuclei, ranging from slow to fast, from the slow molecular movements of crystallization to flow rates of meters per second. Crystallization and Freezing Decrease in mobility during freezing or crystallization greatly attenuates MR signal amplitude, providing a means of following these related processes. Intensity of the MR signal depends on the relaxation rate of the interrogated nuclei; the very fast relaxation rates found in solids prevent observation by use of liquid MR techniques. [...]
[...] Extrusion Applications Residence time as well as shear rate govern product quality in food extrusion; consequently velocity profiles provided by MRI flow techniques could assist in the construction and adjustment of extruders. As in aseptic processing, residence time determines cooking time, and often some portions of a batch cook less than others. This results in undesired quality variations. As shear rate influences texture, the data supplied by velocity profiles allow adjustment in screw speed and design to optimize shear rate and therefore quality and reproducibility. [...]
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