Study The Influential Mechanism Of HPEF On Freezing And Vacuum Freeze-drying Process Of Fruits And Vegetables

In order to prolong the preservation time,fruits and vegetables are preserved and stored in low temperature environment,but the quality of thawed fruits and vegetables is greatly reduced.Vacuum freeze-technology could keep the original flavor,taste and nutritional ingredients of fruits and vegetables,and had good application in fruit and vegetable processing.Unfortunately,the technology has not generalized because of the high energy consumption and big cost,so it is an important practical significance to explore a low power consumption vacuum freeze-drying processing.Aiming at freezing optimization in freeze-drying process,this paper studied the influential mechanism of high pulsed electric field(HPEF)on freezing and vacuum freeze-drying process of fruits and vegetables,which would provide foundation support for further understanding the freezing mechanism and exploring freeze-dried process parameters optimization of fruit and vegetables with low energy-consumption.The research contents of this subject include:(1)The study of HPEF influence on pretreatment of freezing process of water and solution.Water and sodium chloride solution is the main composition of fruits and vegetables.The paper,taking water and sodium chloride solution as the test material,measured their freezing curve after HPEF pretreatment and found the relationship between HPEF and the freezing characteristics.The results showed that appropriate high-voltage pulsed electric field could reduce the lowest crystallization temperature of water and sodium chloride solution,increase supercooling degree,extend the time in the supercooling state,delay the nucleation time,shorten crystallization time of phase change and increase the crystallization rate.The higher supercooling degree,the higher crystallization rate.(2)The effect of HPEF pretreatment on solution crystallization process.For better results,the paper took potassium permanganate solution with color instead of sodium chloride solution as test material,and observed crystallization behavior of potassium permanganate solution after HPEF pretreatment.The color of frozen of potassium permanganate became shallow and relatively uniform after treatment.The results showed that ions in solution was no longer precipitated directly in crystallization process,but changed into salty ice with water molecules.After the pretreatment of HPEF,the way of ion precipitation in the solution was changed,the content of water ice decreased,the content of salty ice increased,and the size of ice crystal reduced.It was good for keeping at low temperature.(3)The study about the effect of HPEF pretreatment on physical properties of solution.The paper,taking water,sodium chloride solution,glucose and sucrose solution as the researched objects,measured the change of contact angle on solid surface and the optical rotation of glucose and sucrose solution after HPEF pretreatment.The results showed that the intermolecular bonding,state reunion and polarity in solution were changed,so the surface tension of solution was changed and more isomer formed in the solution.(4)The influence of HPEF pretreatment on freezing process of fruits and vegetables.With sweet potatoes,potatoes,apples and pears for research objects,the paper observed their frozen curve after treatment and gained frozen characteristics of fruit and vegetable under HPEF influence.The results showed that suitable dose HPEF can induce the nucleation of fruits and vegetables ahead of time,and the time of the largest ice crystals formation stage was shortened,so the ice damage to the fruit and vegetable was slowed down in freezing process.HPEF technology,as an important way,could be used to improve the quality of frozen fruits and vegetables.(5)The influence of HPEF pretreatment on texture properties of thawed fruits and vegetables.For evaluating the influence of HPEF on fruit and vegetable quality,the paper measured the related physical properties of thawed fruits and vegetables.Analysis of the data was made:1000v/cm,60?s,15 of HPEF treatment could keep sweet potato with better adhesiveness,springiness,cohesiveness and chewiness;1250v/cm,90?s,30 and 1500v/cm,120?s,45 of HPEF treatment significantly reduced the sweet potato hardness,adhesiveness and resilience,but the effect on springiness,cohesiveness and chewiness was not significant;1000v/cm,60?ps,15 treatment could significantly improve pear hardness and chewiness;1250v/cm,90?s,30 treatment significantly increased the hardness of the thawed apple.The lower the degree of supercooling and the shorter time through the largest ice crystals formation stage,the better fruit and vegetable quality.(6)The influence of HPEF on tissue structure of fruits and vegetables.With sweet potato as the research object,the paper made frozen sections of sweet potato and observed tissue structure in low temperature.The cellular structure was all changed with different dose of HPEF pretreatment.The stronger the HPEF,the greater the cell deformation degree.Some cell walls were broken and cellular structure collapsed.The results showed that the lower the degree of supercooling the shorter time through the largest ice crystals formation stage,the better microstructure of fruit and vegetable.After HPEF pretreatment,the ice crystal size was decreased in phase change process,so the damage to cell microstructure was reduced.(7)The mechanism of HPEF on improving vacuum freeze-drying rate of fruits and vegetables.During the vacuum freeze-drying process,ice crystals' sublimation begins at the material surface,and then goes into the inside gradually;the drying layer gets thicker and thicker,and the frozen layer thinner and thinner.Based on the characteristics of vacuum freeze-drying process of fruits and vegetables,the paper constructed a porous media model that the porosity increased linearly from the center to the edges during the drying procedure.By using the LBM combined with the conservation laws of mass,momentum and energy,the vacuum freeze-drying velocity distribution in the model was simulated.The results showed that the bigger the change of porosity,the higher the vacuum freeze-drying velocity,which suggested that the change of porosity in porous media could express the drying rate in vacuum freeze-drying process.Then the experiment took apples as samples to validate the results in the simulation.The HPEF technology could improve vacuum freeze-drying rate,because the porosity in fruits and vegetables treated by HPEF was bigger than untreated.The basic cause was that cell membrane permeability was enhanced by HPEF treatment and porosity increased,so the velocity of moisture migration was accelerated.