Experimental Study On Influencing Factors And Heat Transfer Characteristics Of Grape Vacuum Precooling

Grape has the reputation of "the treasure of fruit".As one of the most common seasonal fruits in our life,grape is colorful,delicious and juicy,rich in vitamins and minerals,and is popular among the public.But because of the high moisture content,tender pulp,high temperature after picking,vigorous breathing,easy to deteriorate and rot,resulting in huge resource waste and economic losses.In addition,with the improvement of people’s living standard,the requirements for fresh grape quality are also higher and higher.Therefore,how to keep the quality of grapes in the process of storage and transportation after picking has become an urgent problem.Vacuum precooling can reduce grape temperature rapidly,inhibit respiration,and ensure the quality of grape during transportation and storage.The vacuum precooling process determines the effect of vacuum precooling,and its parameters affect the efficiency of vacuum precooling in different aspects and to varying degrees.Firstly,through the analysis of the results of vacuum precooling experiment of eight grape species,we get the best scheme of vacuum precooling theoretically.The results show that three optimal schemes are produced in 8 grape varieties.The optimum schemes of vacuum precooling of red earth,rose fragrance,ruby and seedless dew varieties are vacuum degree of 600Pa,precooling time of 30min,water supplement rate of 8%and treatment capacity of 100g.The optimal schemes of Jufeng and Zuojin are vacuum degree of 600Pa,precooling time of 30min,water supplement rate of 8%and treatment capacity of 400g.The best schemes of autumn black and golden fingers are vacuum degree of 600Pa,water filling rate of 40 minutes and treatment capacity of 100g.Then,the process of grape vacuum precooling is analyzed,and it is found that the process can be divided into two stages.The cooling process mainly occurs in the second stage.When the pressure in the vacuum chamber is lower than the evaporation pressure,the moisture in the gauze on the grape surface evaporates rapidly and takes away a lot of heat,which makes the temperature of grape drop rapidly.In addition,the temperature of eight grapes in the process of vacuum precooling is the same with time.The curve fitting the temperature with time can converge to logistic function.Finally,the heat transfer process and vacuum environment of grape vacuum precooling are analyzed theoretically.The change of temperature field in grape vacuum precooling is simulated by fluent.The results are in agreement with the simulation results.Therefore,the model can be used to understand and predict the cooling process of grape vacuum precooling in production practice or experimental research.