Theoretical And Experimental Study On Optimization Of Heat Pump Drying Process Parameters Of Pitaya

Pitaya is a high-quality fruit that is rich in various vitamins and natural pigments such as anthocyanins,which are rarely found in common fruits and vegetables.Due to the soft tissue structure of pitaya,it can easily lead to a large amount of decay if it is not sold quickly after harvesting.Drying is an important technique to extend the shelf life of fruits,and heat pump drying has the advantages of energy saving and pollution-free.The lower drying temperature of heat pump drying can better preserve the flavor and nutritional content of dried products.Currently,there is limited literature on the drying of pitaya using heat pump drying method,and heat pump drying can significantly improve the quality of dried pitaya products,providing theoretical guidance for industrial production.Therefore,this study focuses on pitaya as the research object,studying the moisture migration law and optimization of dried product quality during heat pump drying.Firstly,the optimal drying process parameters are determined;secondly,the optimal drying and shrinkage kinetic models are determined;then,the heat and mass transfer process during pitaya heat pump drying is simulated;finally,the optimal pretreatment method is determined.The main research content and conclusions of this study are as follows:（1）The influence of drying temperature（50-70）,relative humidity（10-30%）,and slice thickness（8-10 mm）on the browning degree and surface microstructure of pitaya during heat pump drying was analyzed through experiments.The results showed that the browning degree increased with the increase of drying temperature and relative humidity,and the decrease of slice thickness.Based on fuzzy comprehensive sensory evaluation results and response surface design experiments,the optimal drying process parameters predicted by the quadratic regression model were:drying temperature of 55.9,relative humidity of 15.7%,and slice thickness of 8.6 mm,with a sensory score of 73.87.The error between the predicted and experimental values was less than 10%.（2）The effects of different heat pump drying process parameters on the moisture ratio and volume shrinkage ratio of pitaya during drying were analyzed.The results showed that increasing the drying temperature from 50 to 70 reduced the drying time by 28.57%;reducing the slice thickness from 10 mm to 8 mm shortened the drying time by 12%;and the volume shrinkage ratio decreased with the increase of drying temperature and the decrease of slice thickness and relative humidity.The Avhad and Marchetti models could better describe the moisture migration during pitaya heat pump drying,and the Quadratic model could accurately describe the volume shrinkage behavior.The average error between the models and experimental values were 5.56%and 5.01%,respectively.Under the heat pump drying conditions described in this study,the effective moisture diffusion coefficient of pitaya ranged from 6.4167×10^-10 m²/s to 9.8156×10^-10 m²/s,and increased with the increase of drying temperature,and the decrease of slice thickness and relative humidity.The drying activation energy and shrinkage activation energy of pitaya were 19.628 k J/mol and 27.185 k J/mol,respectively.（3）A mathematical model coupling heat and mass transfer with solid mechanics was established to study the temperature and moisture content changes of pitaya during heat pump drying.It was found that the model considering volume shrinkage had higher calculation accuracy,with a maximum relative error of 9.2%compared to experimental values.The temperature distribution in pitaya slices during the initial stage of drying decreased gradually from the surface to the center,reaching equilibrium in the later stage,while the moisture distribution showed the opposite trend.The temperature and humidity gradient forces during heat pump drying were the main factors causing shrinkage deformation of pitaya slices.The calculated values of moisture content were validated to have errors of less than 10%compared to experimental values,demonstrating the accuracy of the model.（4）Five pretreatment methods including blanching,steam blanching,ultrasonication,osmotic dehydration,and freeze-thaw,were evaluated for their effects on the quality of pitaya during heat pump drying based on drying rate,volume shrinkage,color difference,browning index,and sensory evaluation.The results showed that pitaya slices subjected to freeze-thaw pretreatment had the highest drying rate,with a reduction in drying time of 76.6%relative to the control group,and the smallest volume shrinkage of 6.7%.Pitaya slices subjected to osmotic dehydration pretreatment with zinc acetate（mass fraction of 0.2%）had the smallest color difference of 24.893 and browning index of 17.225.The sensory evaluation score of pitaya slices subjected to osmotic dehydration pretreatment with zinc acetate（mass fraction of 0.2%）was the highest at 71.55.Therefore,osmotic dehydration pretreatment with zinc acetate（mass fraction of 0.2%）was identified as the optimal pretreatment method.