Study On Sea Cucumber Microwave Vacuum Drying And Influence Mechanism On Its Quality

Sea cucumber is a precious aquatic product,but because of its autolysis characteristics,it needs to be processed in time to avoid quality deterioration.Microwave vacuum drying speed is very fast with relatively good quality,which avoids long processing cycle by traditional means.However,the influence mechanism of microwave vacuum on sea cucumber drying and quality has not been understood,which leads to the instability of the dry sea cucumber quality,such as irregular bulge of the body wall and other quality deterioration,restricting the application of microwave vacuum drying technology in the field of sea cucumber processing.In view of this,sea cucumber microwave vacuum drying and influence mechanism on its quality were studied in this paper.By analyzing the influence law of microwave power density and vacuum degree on miosture migration and quality characteristics of sea cucumber during microwave vacuum drying,a mathematical model of drying kinetics and a multi-physical field coupling simulation model were established.The complex heat and mass transfer process of coexistence of gas,liquid and solid in the body wall of sea cucumber during microwave vacuum drying was investigated,as well as the internal influence mechanism on the quality of sea cucumber.At the same time,the influence of pretreatment on the processing quality of sea cucumber was studied,in order to provide theoretical basis and technical reference for the microwave vacuum drying of irregular materials such as sea cucumber.The main research results are as follows:1.The effects of blanching,boiling and steaming pretreatment on autolysis enzyme inactivation,protein degradation and processing quality of sea cucumber were studied,and a mechanical model of heat-induced autolysin inactivation was established.Different heating treatments significantly affected the autolysis enzyme activity and protein properties of sea cucumber,and then affected the yield,water holding capacity,texture and other processing qualities.Because of its high thermostablility,the autolysis enzyme cannot be completely inactivated by blanching at 40～80℃,but was inactivated at 100℃by boiling or steaming.Z value and E_a were 28.39℃and 91.38 k J·mol^-1,ΔG was 98.73～102.36 k J·mol^-1.D value of boiling and steaming groups was shortened to 32.84 min and 19.88 min respectively.The denaturation of autolysin may be caused by the change of molecular binding site.The non-isothermal model can accurately describe the thermal-induced sea cucumber autolysis enzyme inactivation process.Long-term processing at 100℃led to the decrease of protein content and moisture retention,and in turn affects the yield and texture.According to the principal component analysis,the physicochemical properties and enzyme inactivation level were taken into consideration.Boiling at 100℃for 45 min was more suitable as a pretreatment for drying.The yield was 70.3%and the protein content was 78.5%.For long-term preservation of ready-to-eat food,should be cooked at 100℃for at least 30 min,the relative activity is only 3.2%with good palatability.2.Microwave vacuum drying characteristics and kinetics of sea cucumber were studied.The drying characteristics of sea cucumber under different microwave power density,vacuum degree and pretreatment conditions were determined through the modeling of drying kinetics and characterization of thermodynamic characteristic parameters.The mismatch of drying energy and internal heat and mass transfer mechanism were mapped.Increasing the microwave power density,vacuum degree and the boiling pretreatment can increase the drying rate.The drying process was mainly composed of up-speed and down-speed drying stages,which changed from surface vaporization control to internal diffusion control.By comparison,the Page model can describe the water migration rule of microwave vacuum drying sea cucumber correctly.The D_effof sea cucumber was 9.69×10^-13～2.84×10^-11 m²/s,the E_a was 248.13 k J/mol（boiled groups）,and 107.72 k J/mol（unboiled groups）,and△H was13.56～13.80 k J/mol.The mismatch between start-up energy required in the early stage and the microwave energy consumed in the middle and late stage led to the excessive vaporization of water molecules and the bulging of the body wall.3.The multi-physical field of microwave vacuum drying of sea cucumber considering heat transfer and moisture migration process was simulated.The electromagnetic field,temperature field and moisture field coupling simulation model of microwave vacuum drying of sea cucumber was constructed in COMSOL Multiphysics software,and the simulation strategy and governing equation were determined.Based on the energy conservation,mass conservation and momentum conservation theories,the multi-physical field coupling simulation model was solved.The temperature and moisture distribution of sea cucumber under different microwave power density and vacuum degree were obtained,as well as the temperature and electric field distribution in drying chamber.In the drying process,because of the clumpy distribution of the electric field,temperature in the drying chamber and the sea cucumber body wall was uneven,and the focus of energy created hot spots.Gradient of temperature,moisture concentration and pressure was formed inside the sea cucumber,which promoted the rapid drying.The increase of microwave power significantly increased the temperature of the sea cucumber and electric field intensity of drying chamber,and changed the moisture migration form.The temperature difference between cold and hot spot of sea cucumber surface reached 4.53℃,18.08℃,22.1℃and42.79℃,respectively after drying for 10min with the microwave power density of 1 W/g,4W/g,7 W/g and 10 W/g.The influence of vacuum degree on the distribution of temperature and moisture content of sea cucumber was not obvious.4.The changing principles of sea cucumbers drying quality were studied,and the relationships between macroscopic quality and moisture migration,protein denaturation and pore formation were explored,and the quality formation mechanism and cause of body wall bulge were revealed.In the boiled group,the body wall was compact and well conserved,the shrinkage rate increased with drying time and tended to be stable in the middle and late drying period,the rehydration rate and water retention were high,and the sensory quality was excellent under low power density.The increase of power density was accompanied by bulging,but did not affect the sensory and texture characteristics after rehydration.The body wall of the sea cucumber in unboiled group is relaxed,and the deformation was serious after drying.After rehydration,the body wall was loose and sticky,which cannot provide the desired sensory and texture characteristics.The shrinkage rate increases with the increase of drying time and microwave power density,and the rehydration rate was low.Increasing microwave power density or prolonging drying time may lead to local hot spots,and the secondary and tertiary structure changes of protein molecules as well as the concentrated evaporation of water molecules are the main causes of bulging phenomenon.Different pretreatment,microwave power density and vacuum degree had a dominant effect on drying quality by affecting pore formation and protein characteristics in the body wall of sea cucumber.According to the principal component analysis,the microwave power density of 1 W/g and vacuum degree of 90 k Pa were the appropriate microwave vacuum drying conditions for sea cucumber after boiling pretreatment.After rehydration,the body wall was thick,elastic and chewy.The shrinkage rate was 23.58%,the color difference 9.40,the porosity 12.42%,the rehydration rate 597.60%,the water holding capacity 86.02%,crude protein and collagen content was 61.34%and 35.81%.