| In recent years,the application of moderate electric field(MEF)technology in the field of fruit and vegetable pretreatment has attracted much attention.Compared with the traditional heating pretreatment method,when MEF acts on fruit and vegetable tissues,it can damage the tissue cells faster due to the significant electrothermal synergy.Although MEF technology retains significant advantages in fruit and vegetable pretreatment,the contribution rate of MEF electrical and thermal effects on fruit and vegetable tissues are still unclear.Therefore,a moderate electric field pretreatment apple test system was designed in this paper.The effects of electric field intensity,heating temperature and frequency on the macroscopic mechanical properties and microstructure of apple tissue were studied.The synergistic mechanism of MEF electric and thermal effects on apple tissue was clarified.Moreover,the contribution rate of MEF electrical and thermal effects to apple tissue damage was revealed by the equivalent circuit model parameters of apple tissue.The main results are as follows:(1)The design of MEF pretreatment apple test system was completed.The MEF pretreatment apple test system is composed of power supply,temperature collector,thermocouple,PC and sample processing chamber.The sample processing chamber is chiefly composed of ratchet force measuring device,push rod,upper and lower electrode plate,upper and lower electrode insulation plate and bearing.(2)The effects of treatment conditions on the rheological and compressive properties of apple tissues were studied.The results showed that the stress relaxation curve of apple tissue treated with MEF was significantly different from that treated with water bath.The rheological parameters of MEF treated apple tissue were significantly different from those of water bath treatment.MEF treatment could significantly reduce the elasticity and viscosity of apple tissue,and the proportion of elastic properties decreased.Compared with the water bath treatment,the hardness,fracturability and elasticity of apple tissue treated with MEF decreased,the biological yield point of apple tissue treated with MEF disappeared,and the second compression peak time was advanced.(3)The effects of treatment conditions on cell wall components of apple tissues were studied.The results show the presence of electric field promotes the decomposition of WSP and the formation of CSP and NSP at the same temperature.With the increase of field strength,WSP content increased,CSP,NSP and hemicellulose content decreased.When the electric field intensity was higher than 60 V/cm,the heating temperature was higher than 60℃and the frequency was lower than 250 Hz,the impedance of apple tissue changed little in the whole measurement frequency range.Through R_e/R_i and C_m,it was discovered that MEF treatment significantly reduced the integrity of apple tissue cell membrane and reduced extracellular resistance.(4)When the electric field strength changes,the thermal contribution rate is only about4%,and the electrical contribution rate is about 95%.When the temperature changes(below70°C),the heating temperature increases,and the thermal contribution rate increases from0.76%to 55.37%.When MEF exists,the electrical contribution rate decreases from 99.24%to 44.63%.The impedance of MEF-treated apple tissue showed a strong dependence on electric field intensity and temperature.(5)The effects of treatment conditions on the microstructure of apple tissue were studied.Through the microstructure,it was found that the effect of temperature on tissue cells mainly caused the plasmolysis,and the plasmolysis was significant with the increase of temperature until the cell membrane was damaged at 70℃.When MEF treatment,low temperature and low electric field treatment of cell membrane plasmolysis and electroporation occur at the same time.And high temperature and high field treatment,cell membrane without plasmolysis is completely destroyed,under the synergistic effect of electro-thermal effect on cell membrane and cell wall caused irreversible damage.(6)The mechanism of MEF treatment of apple tissue was discussed.The electric field causes the cell membrane to produce electroporation and cell turgor loss,which is a significant reason for the decrease of mechanical properties of apple tissue.Secondly,the change of cell wall component content leads to the change of network structure of apple tissue cell wall,which is another important reason for the decrease of mechanical properties of apple tissue.In addition,temperature is the main factor affecting the plasmolysis of apple tissue cells.Temperature can decompose cell wall and dissolve cell membrane.Electric field can cause electroporation of cell membrane and promote cell wall decomposition.However,the cell wall is more sensitive to temperature,and the electric field has a greater impact on the cell membrane. |
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