Numerical Simulation And Experimental Study On The Freeze-Drying Process Of Pitaya Fruit Based On Fractal Theory

As a country with a large population,in order to meet people’s living needs,China’s agricultural production also ranks first in the world.In people’s daily life,people should eat a certain amount of fruits and vegetables every day to supplement the nutrition needed by the body.Fruits and vegetables contain a large amount of vitamins and dietary fiber,which was of great benefit to people’s health.With the continuous improvement of China’s economic level,people’s living standards were also improving,and the demand for healthy food was also rising.Fruit and vegetable products played an important role in the consumer market,but fresh fruits and vegetables contain a lot of water,has poor storage resistance,were easy to decay after harvest,and were prone to large losses during storage and transportation.Therefore,in order to prolong the storage time of fruits and vegetables,the fruits and vegetables should be processed to prolong the storage time of fruits and vegetables,and at the same time,the nutrients in fruits and vegetables should be preserved as much as possible.By drying fruits and vegetables,the moisture content in fruits and vegetables could be reduced,and the storage time of fruits and vegetables products could be greatly increased.The loss of dried fruits and vegetables in storage and transportation would be greatly reduced.Vacuum freeze-drying,referred to as freeze-drying,refers to the complete freezing of materials and sublimation of ice crystals under certain vacuum conditions to achieve the purpose of low-temperature dehydration.The vacuum freeze drying technology could solve the problem that fruits and vegetables are not easy to store.Compared with the traditional drying method,it has the advantages of less loss of nutrients,good color retention,and good rehydration performance.However,the vacuum freeze drying process took a long time and consumed a lot of energy.It was necessary to explore the best drying parameters of different foods to save energy and cost.Fractal theory was used to quantitatively express complex and regular geometric phenomena that cannot be described by traditional European geometry in nature.In1982,fractal geometry characterized by fractal dimension,self-similarity and infinite divisibility was formed and described by iterative algorithm.Fractal dimension can quantitatively describe the regularity and uniformity of pore distribution.In the process of food drying,due to the influenced of moisture evaporation and heat and mass transfer gradient,food volume shrinkage occurs during the drying process,which would affect the diffusion coefficient and drying rate of materials.The diffusion coefficient of water was the most important parameter to control the drying kinetics of food.Most foods belonged to capillary porous media.With the help of a large number of experiments and grid simulations,the fractal characteristics of foods has been confirmed.Pitaya was rich in vitamins,anthocyanins and water-soluble dietary fiber.Under natural conditions,pitaya fruit was easy to decay and damage during picking,transportation and storage.Using vacuum freeze-drying technology could not only better store pitaya fruit,but also retained the nutritional components of pitaya fruit.In this paper,the freeze-drying process of pitaya was studied to reduce the freeze-drying time and energy consumption.The contents of this study include the following aspects:(1)Carry out slice research on pitaya,measure various thermophysical parameters of pitaya,such as thermal conductivity,specific heat capacity,porosity and other parameters,analyze the heat and mass transfer process of pitaya slice in the sublimation drying stage with fractal theory,establish the corresponding numerical simulation model,and verify the accuracy of the simulation results through experiments to better analyze and understand the heat and mass transfer change process of the sublimation drying process.Based on the fractal theory,the principle of heat and mass transfer,and the basic theory of vacuum freeze-drying,the thermal-mass coupling simulation model of pitaya sublimation drying process was established on the COMSOL Multiphysics platform,and the numerical solution was carried out through the boundary conditions and control equations.The displacement of water vapor sublimation interface,the temperature distribution of pitaya slices,and the change of water content of pitaya slices were simulated during the sublimation drying process.According to the numerical simulation results,the actual freeze-drying cycle was predicted.The vacuum freeze-drying experiment of pitaya slices was carried out to obtain the energy consumption in the sublimation drying stage and the analytical drying stage.The numerical simulation model was verified.The numerical simulation result of the sublimation drying period of 10 mm thick pitaya slices was 19.5 h,the experimental result was 20 h,and the relative error was 2.5%.(2)According to the simulation results and experimental data,the effects of the thickness of pitaya slices,the pre-freezing speed,and the temperature of the partition in the sublimation drying stage on the freeze-drying cycle and energy consumption were investigated respectively through the factors affecting the freeze-drying cycle.At the same time,the numerical simulation results were compared with the experimental results.Compared with the experimental results,the relative errors of the sublimation drying period of pitaya slices with the thickness of 8mm,10 mm,12mm and 14 mm were2.7%,2.5%,0.9% and 1.3% respectively.Validate the accuracy and validity of the numerical simulation model.Under the condition of ensuring the validity and accuracy of the numerical simulation model,integrate the simulation results and experimental results,further explore the factors and laws that have a greater impact on the freeze-drying cycle during the freeze-drying process of pitaya fruit,and obtain the best freeze-drying parameters and operation process of pitaya fruit.(3)During the vacuum freeze-drying process of red pitaya,the freezing speed of the pre-freezing process has an impact on the sublimation drying cycle.With the increase of the freezing rate,the smaller the pore of the frozen material,the greater the resistance in the sublimation drying process,and the longer the sublimation drying cycle.The thicker the slice of pitaya,the longer the period of sublimation drying.During sublimation drying,the higher the temperature of the separator,the shorter the period of sublimation drying.The effects of slice thickness and partition temperature on the period of sublimation drying are nonlinear.The sublimation drying periods of pitaya slices with thickness of 8mm,10 mm,12mm and 14 mm were 18 h,20h,23 h and 24 h respectively,and the energy consumption in the sublimation drying stage was 41.2k W·h,45.3 k W·h,51.1 k W·h and 53.3 k W·h respectively.For 12 mm thick pitaya slices,the partition temperature is 0 ℃,-5 ℃ and-10 ℃ respectively,the experimental sublimation drying cycle is 12 h,14h and 23 h respectively,and the energy consumption in the sublimation drying stage is 26.5 k W·h,31.3 k W·h and 51.1 k W·h respectively.Considering the sublimation drying cycle and energy consumption,this study considers that the freeze-drying parameters with a thickness of 10 mm and a partition temperature of-5 ℃ are the best design parameters for red heart pitaya slices.