| Fruits and vegetables contain various nutrients necessary for the human body,and it is a necessity for people to live a good and healthy life.China is a big country in the production and consumption of fruits and vegetables,however,the postharvest processing technology of fruits and vegetables is backward,and the postharvest loss and economic loss are huge.Drying is the main form of post-harvest processing of fruits and vegetables.The phenomenon of "sugar exudation" in the drying of fruits and vegetables not only leads to crusting on the surface of materials,reducing drying efficiency and increasing energy consumption,but also leads to a serious decline in the quality of dried products.The traditional drying model didn’t fully consider the influence of microporous structural characteristics,solute migration,microscale effects and other factors on the loss of nutrients during the drying process.In view of this,this thesis used the principle of transfer process and pore network theory to analyze the drying process of porous media of fruits and vegetables.A bidirectional fully coupled pore network drying model of heat and moisture transfer and solute migration at the pore scale was established,and the drying process of porous media of fruits and vegetables was simulated.The main research work in this thesis includes:(1)Based on molecular dynamics method,the migration process of solutes in dried microporous nutrient solutions of fruit and vegetable porous media was analyzed.Using regular ensemble,the Lennard-Jones potential energy model was selected to simulate and observe the changes of sucrose molecules during water evaporation.By calculating the radial distribution function and mean square displacement of the sucrose solution system,the influence of microporous wall roughness and pore diameter on the sucrose diffusion process was obtained.The water diffusion coefficient and sucrose diffusion coefficient in sucrose solution both decrease with the increase of wall roughness and increase with the increase of phase area fraction.With the increase of the diameter of the throat,the diffusion coefficient of the water molecules in the solution gradually increases,and the expression of the relationship between the throat diameter and the diffusion coefficient of the solution was obtained.(2)According to the microstructure parameters of fruit and vegetable porous media,the physical model of the pore network was constructed,the influence mechanism of nutrient loss during the drying process was analyzed,the pore scale coupling model of drying heat and moisture transfer and solute migration of the porous medium of fruits and vegetables was established,and the simulation software was developed to simulate the drying process.A parameter characterization system to describe the microstructure characteristics of porous media such as fruits and vegetables was constructed.Established the "materialmodel" parameter mapping relationship to obtain the physical model of the drying pore network of porous media of fruits and vegetables.The "pore microelements"were selected as the center of any pore node,and the coupled mathematical model of heat and moisture transfer and solute migration of porous media of fruits and vegetables was constructed by equilibrium analysis method.The model solving software was developed and designed using MATLAB and Visual C++joint programming.The software can handle the physical structure information of the corresponding position of the material in a timely manner,and simulate the heat and moisture transfer and solute migration in the drying process of the material.(3)Hot air drying experiments of beetroots and apples were carried out.The experiment measured the moisture ratio,moisture status distribution,temperature and sugar content of different parts during the drying of the material.The experimental results showed that the drying rate of both materials first increases and then decreases,and the maximum drying rate occurs at about 40 min.During the drying process,the drying time after the moisture ratio was less than 0.1 occupied almost half of the total drying time.The content of free water in the initial state of beetroot was the largest,accounting for 81.8%of the total water,the content of bound water was 4.4%,and the content of water that was not easy to flow was 13.8%.With the increase of drying time,free water drops the most,semilow water and bound water were first increased and then decreased.The simulated values of the drying curve,moisture status distribution,average temperature curve,maximum temperature difference curve and sugar content curve were compared with the experimental values.The relative error of both was within ±10%,which proved that the established mathematical model and numerical solution had high reliability and accuracy.In the process of hot air drying,the sugar will migrate with the drying,but the total sugar content in the material was almost unchanged,and the reduction of the sugar content in a certain part will inevitably lead to an increase in the sugar content in its adjacent parts.(4)On the basis of the reliability verification of mathematical model and numerical method,the simulation analysis of different drying conditions,different mass transfer mechanisms and different model structures was further carried out.The results shown that the increase of hot air temperature,the decrease of the relative humidity of hot air and the increase of hot air speed will increase the drying rate and lead to an increase in the loss of nutrients in the material.The greater the relative humidity of the hot air,the faster the temperature rise rate of the material in the early drying stage.The existence of the Haines jump phenomenon made the solution inside the material quickly migrate to the surface of the material,and the water evaporated on the surface of the material,leaving sugar on the surface layer.At the same time,the drying process absorbed a large amount of energy,resulting in a slow temperature rise.The Haines jump phenomenon causes a large amount of sugar to migrate to the outer layer,which was the main reason for the formation of the "sugar exudation" phenomenon.The Raoul effect caused a decrease in the drying rate,which in turn reduced the loss of nutrients.(5)Since the solute on the surface of the material was mainly formed in the early stage of hot air drying with the migration of the Haines jump process of the solution in the throat,vacuum freeze drying can eliminate the influence of the Haines jump process.Therefore,the vacuum freezing and hot air combined drying experimental research and equipment design of beetroot and apple were carried out,and it was concluded that the vacuum freezing and hot air combined drying method can retain the nutrients in the material without reducing the drying rate.Fresh apples had 4.287 mg/100g of vitamin C and fresh beetroots had 4.9 mg/100 g.The vitamin C content of the inner layer of the material was higher than that of the outer layer,the different of vitamin C content of the inner layer and the outer layer of vacuum freeze-drying was 1.23%,and the different of content of vitamin C in the inner and outer layers of hot air drying was 28.39%.When the moisture content of the intermediate conversion point was less than 60%,the vitamin C content in the dried material was not much different.So,the moisture content of the greater intermediate conversion point for vacuum freezing-hot air combined drying was 60%.The model established in this thesis elucidated the relationship between dry nutrient loss in porous media of fruit and vegetable and microporous structure and microscale effects,and revealed the interaction mechanism of pore interface capillary force and liquid film flow on solute migration and heat and moisture transfer process in the drying process of fruits and vegetables,which can effectively describe and predict the phenomenon of "sugar exudation".The work in this thesis can establish a theoretical foundation and provide technical support for the analysis and optimization of the drying quality of porous media of fruit and vegetables,and the energy saving and efficiency improvement of the drying process. |
