Experimental Study Of Static Magnetic Field Effect On Food Freezing Process

Cryogenic is a kind of widely used food preservation technology with perfect effect. The methods using athermal technology aided food freezing process have been a research hotspot in recent years at home and abroad. Within these athermal techniques, high voltage auxiliary refrigeration has been the stage of production application, while, magnetic field auxiliary quick freezing technology is currently in the stage of research and development. When various kinds of optimization on existing refrigeration technique conducted, we look forward to speeding up the chilling rate and reducing energy consumption, as well as, to make food retain the original nutrition, flavor and color maximally. This paper focuses on the influence and influence mechanism of weak strength static magnetic field (0～11G) on food freezing rate.Experiment was the main research method in this paper, the laboratory furniture of magnetic field auxiliary frozen was set up. Carp and potato were selected as the research objects.4times experiments were conducted for each kind research object, and are respectively the natural freezing without magnetic field; the freezing processes of samples were placed in3.6G,7.2G and10.8G static magnetic field. Comparing the experimental data, especially the freezing curves and the freezing time of three stages, and a series of conclusions were obtained. The results indicated that magnetic field could influence the freezing process; it promoted the phase change stage and restrained the frozen stage. In the carp freezing experiment, the consumed time undergone phase change stage with10.8G magnetic intensity is only52.9%of the natural freezing, the consumed time with10.8G magnetic intensity is175.3%of the natural freezing at the frozen stage. Meanwhile, in the potato freezing experiment, the consumed time undergone phase change stage with10.8G magnetic intensity is20%shorted than the natural freezing, the consumed time with7.2G magnetic intensity is46.4%longer than the natural freezing at the frozne stage. The time disparity was not obvious for these4kinds of magnetic field from the view of whole process, and the maximum deviation was within10%. The influence of magnetic field on these two kinds of food freezing point and the temperature range of maximum ice crystals generated belt was little. It can be seen that magnetic fields have different effects on different freezing stages, therefore, another potato freezing experiment were arranged, different magnetic field intensities were applied at different freezing stages, the results shown that the slowest refrigeration effect had not appear, it may be because that the influence of magnetic field on food materials exists hysteresis phenomenon.In order to explore the influence mechanism of magnetic field on food frozen stage, the influence of magnetic field on convective heat transfer was experimentally researched. A copper piece was selected as study object and it was freezing respectively under the circumstance of natural convection and forced convection. The lumped parameter method was employed to analysis these questions and a series of convective heat transfer coefficient were obtained, it shown that the convective heat transfer coefficients under the influence of magnetic fields were less than the case under natural freezing condition. The results indicated that the magnetic field inhibited the convective heat transfer and its impacts on natural convection and forced convection are different. This conclusion was also verified through both the carp and potato freezing experiments.The influence mechanisms of magnetic field on food freezing were explained. At the phase change stage, magnetic field affected the association phenomenon of water molecules, a large number of small water molecular groups were formed, the larger ice crystals were not easy to formed when the water crystallized and the crystal growth rate were also changed, therefore, the phase change process were accelerated. At frozen stage, magnetic field could produce magnetizing force on air which as a paramagnetic mixture medium, and then change the convective heat transfer characteristics.