| The vacuum pre-cooling technology has been extensively used and studied from 1980s. It is widely applied in pre-cooling meat, cures food, aquatic product, and flowers and so on. Besides the rapid cooling speed, the vacuum pre-cooling technology is predominant at assuring the food quality, cooling uniformly, high COP, etc. These advantages make it a potential broad cooling means in the future.The theory research on pre-cooling process is usually focusing on heat and mass transfer. In this paper, vegetables are considered as porous media, theories of which are used to analyze the heat and mass transfer in the pre-cooling process. Besides, a nonlinear mathematical model is established on the base of non-equilibrium thermodynamic theory and phase equilibrium principle. The coupled heat and mass transfer phenomena is considered enough in the model, which will assist in the theory research on the heat and mass transfer in the pre-cooling process.The experiments on cabbages, Chinese cabbages, tomatoes, and radishes were carried out. The experimental data were analyzed and plotted into figures. Furthermore, the effects of the vacuum pressure, the beginning temperature and ending temperature and spraying water were studied. We find that the weight losing get quicker with the lower pressure. The pressure in the chamber is not only a decisive factor in the water evaporation but also a key factor in the migration of moisture from the internal towards the surface. The beginning and ending temperature has little effect on the pre-cooling process. The cooling and water losing rate vary with food sorts. Moreover, the sprinkling water on the surface affects the pre-cooling process undoubtedly. The more spraying water is, the quicker evaporating and cooling are. Therefore, we should connect the pre-cooling pressure and the amount of water to achieve the best cooling effect to meet with the quality requirements in actual vacuum cooling operation, according to the properties of materials. |
PDF Full Text Request