| Liquid nitrogen is one of the few ideal refrigerants which is colorless,tasteless,chemically inert,and can contact with food without causing any adverse effect.Liquid nitrogen has outstanding advantages both in heat efficiency and refrigeration temperature after atomization.It can also produce the low enough temperature for deep-freezing of food.Therefore,liquid nitrogen had been widely used in food freezing field for nearly 40 years.Wolfberry and cordyceps are high value-added products.How to ensure their freshness while prolonging the storage period is an urgent problem to be solved.For the defects in liquid nitrogen freezer such as uneven temperature field distribution,insufficient utilization of liquid nitrogen and high thermal stresses during the process of deep-freezing,temperature field distribution in liquid nitrogen freezer is improved using the method of numerical simulation based on the theories of liquid atomization.Then,the accuracy of the model is verified by experiments.After that,based on the first two chapters,taking wolfberry and Cordyceps as the research object.The effects of different freezing temperature?-40±1?with the fan speed of 2500r/min in an air-blast freezer,-60±2?,-80±2?and-100?±2?in a liquid nitrogen freezer?on the freezing characteristics of wolfberry and Cordyceps were studied.The main conclusions are as follows:1.Numerical simulation:The temperature distribution in the cavity during the liquid nitrogen spray cooling process is calculated by the phase-to-phase coupling between the continuous phase and the discrete phase.The larger the flow of nozzle,the faster cooling of the cavity,and the temperature field is more uniform.The cooling rate is not correlation with wind pressure.However,within a certain wind pressure range of 10 pa to 30 pa,the larger the wind pressure,the better the uniformity of temperature field.After the wind pressure exceeds 30 pa,the change in temperature field uniformity is not obvious.Under the disturbance of the fan,the uniformity of the temperature field of the double-sided jet is better than that of the single-sided jet.2.Experimental verification:It is verified by experiments that the temperature value of the measuring points in cavity is quite different from the temperature value of the simulation results which one side injection is 23.83%and double-sided injection is 20.94%.However,the experimental results are consistent with the simulation results in terms of temperature field uniformity which one side injection is 7.87%and double-sided injection is 8.45%.3.Liquid nitrogen freezing:The glass transition temperature is determined to be-39.2°C by the glass transition curve of wolfberry.For the freezing characteristics of the wolfberry,the condition at NF-100°C performed well in the freezing rate,cell membrane permeability and polyphenol oxidase activity,but the color,soluble sugar content,ascorbic acid content,peroxidase activity,water distribution and epidermal microstructure were inferior to NF-80?group.Comprehensive consideration,NF-80°C80°C is the most suitable freezing conditions for wolfberry.4.Liquid nitrogen freezing:The freezing temperature and the storage temperature of the cordyceps were determined to be-34.86?by the glass transition curve of cordyceps.For the freezing characteristics of cordyceps,all of the indicators including freezing rate,energy absorption rate,electrical conductivity,drip loss,color difference and soluble sugar were superior under the condition of NF-100?.For the storage characteristics of cordyceps,all above the indicators become worse with the extension of storage time.But the NF-100?group can minimize the deterioration of the indicators during storage.Therefore,NF-100?is the most suitable freezing condition for cordyceps. |
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