Study And Optimization Of Vacuum Freeze Drying Technology Of Pineapple Slices

As one of the famous fruit varieties in tropical and subtropical regions of China,Pineapple contains a lot of fructose,glucose,vitamin C,protein and other trace elements.However,pineapple is listed in large quantities in summer,microorganisms can breed,and it will be easy to rot and deteriorate in the process of transportation.Therefore,it is necessary to further process pineapple.At present,the drying methods of pineapple mainly include microwave,heat pump,hot air and variable temperature and differential pressure expansion.Vacuum freeze-drying technology is the best way to preserve the quality of fruits and vegetables at present.It can retain nutrients to a great extent and its structure is not easy to deform,However,vacuum freeze-drying process is complex and energy consumption is large.Different freeze-drying conditions will have a great impact on the quality of dried products.Therefore,it is of great significance to explore the quality changes of pineapple in the process of vacuum freeze-drying and optimize the best freezedrying process.From the perspective of macro quality research,this topic explores the effects of different pretreatment and freeze-drying parameters on the quality,evaluates the quality after freeze-drying with different detection indexes,and optimizes the vacuum freezedrying process of pineapple based on single factor experiment.the cooling and freezing process of pineapple slices was numerically simulated.The main research contents of this paper are as follows:(1)Based on experimental research and empirical formula calculation,the thermal physical parameters of pineapple were determined,which provided basic data support for the simulation of pineapple slice cooling and pre-freezing.The density of pineapple slices before and after freezing was measured by drainage method at room temperature.The thermal conductivity of pineapple slices before and after freezing was measured by hot disk thermal constant analyzer.The eutectic point,eutectic point,freezing point,latent heat of phase transformation and specific heat capacity were measured by Q1000 differential scanning calorimeter(DSC).The surface heat transfer coefficient of pineapple during prefreezing was determined by empirical formula.The moisture content of pineapple was obtained by direct drying method.(2)The pretreatment experiment was carried out with ultrasonic and 0.9%Na Cl solution.The control group was set up for comparative analysis with dry basis moisture content,rehydration rate,hardness,color difference,vitamin C and soluble solid content and sensory evaluation as quality evaluation indexes,The results showed that ultrasonic and0.9%Na Cl solution pretreatment could improve the quality of freeze-dried pineapple slices to a certain extent.(3)Single factor experiment was carried out to explore the influence of parameters on the quality of pineapple slices during vacuum cold drying,and the best value range of four factors was determined.The parameters are set as follows: slice thickness(3mm,4mm,5mm,6mm,7mm,8mm),shelf cooling rate(0.5?/min,1?/min,1.5?/min,2?/min,2.5?/min,3?/min),partition sublimation temperature(5?,10?,15?,20?,25?),shelf resolution temperature(30?,35?,40?,45?,50?);Quality inspection indexes:moisture content,rehydration rate,hardness,color difference,vitamin C and soluble solid content on dry basis,sensory evaluation.(4)Based on the single factor experiment,the response surface experiment was designed by using the box benhnken central combination design principle.Taking the slice thickness,cooling rate,sublimation temperature and analysis temperature as factors,sensory evaluation,rehydration rate and vitamin C content as response values,the optimal process optimization of pineapple slice vacuum freeze-drying was explored,and the significance of the four factors on the quality was obtained.Through the analysis,the best process conditions are as follows: slice thickness of 6mm,cooling rate of 1?/min,sublimation temperature of 15?,12 h,resolution temperature of 36?,2h.The final results are as follows: the moisture content is 3.02%,the rehydration rate is 336.5%,the hardness is 2.47 n,the color difference value is 17.89,the content of vitamin C is 144mg/100 g,the content of soluble solids is 7.6g,and the sensory evaluation is good.(5)Taking pineapple as porous medium,the three-dimensional physical model and mathematical model of pineapple slice pre freezing process were established by COMSOL Multiphysics multi physical field simulation software,the variation law of shelf and ambient temperature with time in the cooling process of Lyophilizer is applied to the model,The model can well predict the distribution of temperature field in the freezing process of pineapple slices.The simulation results of pineapple slices with temperature change trend are basically the same as the experimental results,which verifies the accuracy of the model.