Theoretical Analysis And Experimental Research On Vacuum Pre-cooling Performence Of Leaf Vegetables

With the rapid development of national economy and the improvement of people’s living standard,the demand on the quality of fresh vegetables is higher and higher.Picking vegetables more easily get degenerate at high temperatures with breathing,evaporation and other metabolic activities.It is of significance to keep the vegetables freshness in the pre-cooling process of leafy vegetables cold storage.In this study,root vegetables and leafy vegetables as the research object,comparing the freshness effect of vacuum pre-cooling between two kinds vegetables.We can conclude that leafy vegetables are suitable for vacuum pre-cooling.By the test of vacuum cooling of leafy vegetables,we can analyze the relationship between the power consumption of the compressor and water acquisition of water trap.Based on the physical properties of leafy vegetables and the heat and mass transfer mechanism in the process of vacuum pre-cooling,the non-steady-state calculation model of heat and mass transfer was established.We can analyze the effects of pre-cooling temperature,sprinkling water and pre-cooling pressure,in addition to test a small of leaf vegetables.The results can effectively optimize the pre-cooling process of leafy vegetables,which can provide reference for the farmers to build small-scale cold storage and further study the suitable pre-cooling technology and storage scheme of the vegetables in the cold chain.The main contents are as follows:(1)A case study of leafy vegetables(with Chrysanthemum as example)and rhizome vegetables(with ginger as examples)for vacuum pre-cooling test.The results showed that under the pressure of 650 Pa vacuum pre-cooling,the ginger was cooled from 20 degrees to 10 degrees for 60 minutes,the Chrysanthemum was cooled from 20 degrees to 3degrees for 15 min that much less than the time of pre-cooling ginger.So the leafy vegetables including Chrysanthemum are suitable for vacuum pre-cooling.(2)The performance of the water catcher has a great influence on the vacuum pre-cooling process.By the test of vacuum cooling of leafy vegetables,we can analyze the relationship between the power consumption of the compressor and water acquisition of water catcher.The results show that comprehensive comparison of catch water and compressor power consumption,when the evaporator temperature is-5 degrees,the unit energy consumption of water capture reached the highest,reaching 14.6g / W.(3)The mathematical model of heat and mass transfer in the process of vacuum pre-cooling was established,and the temperature changes of leaf vegetables were simulated.Compared with the experimental results,we can conclude that the pressure drop rate in the vacuum chamber is very fast in the early cooling stage.After the flash point is reached,the rate of decrease is slowed down and tends to be stable.The cooling rate at the outside surface of cylindrical vegetables is greater than that in the central part.The vacuum pressure and the temperature of the vegetables are consistent,and the change trend is consistent.(4)By testing different pre-cooling conditions of chrysanthemum,the optimum pre-cooling conditions can be list as follows: pre-cooling pressure is 900 Pa,water spray is 5%,pre-cooling temperature is 3-5degrees.