Study On Microstructure Damage Of Apple Tissues Treated By Ohmic Heating Treatment

Apple not only has ornamental value,but also has a very high nutritional composition,and Shaanxi is a large province of apple production in China.Apple deep-processing products play an important role in increasing their added value.Pretreatment is required before deep processing.Ohmic heating has unique advantages as an excellent pretreatment method,but the effect of changes in heating conditions on the microstructure damage is not clear.This study explored the different heating temperatures(55,60,63,65,70°C),electric field intensity(40,55,70,90,110V/cm),frequency(50,250,500,800 Hz)during heating pretreatment,the effect of three parameters on the impedance and phase angle changes of the apple tissues and the Cole-Cole plot.And experimental analysis and comparison of three different biological tissue equivalent circuit model to select the most suitable for apple tissues.In addition,the selected model was used to fit the parameters of the equivalent circuit model to analyze the damage of the cell membrane to the heating conditions.In addition,the compression experiment and creep-recovery experiment of the treated sample were performed,and the influence of the heating condition on the compression characteristics of the sample was analyzed by calculating the correlation index of the curve,and the heating condition was analyzed by fitting the parameters of the compliance curve equation.The effect of the change on the creep characteristics of the sample.Finally,the experimental results were verified by scanning electron microscope and transmission microscope.The conclusions are as follows:.(1)The analysis of the electrical impedance characteristics shows that the impedance and peaks of phase angle decrease with increasing heating temperature and electric field intensity,increase with increasing heating frequency.Up to 90 V/cm and lower than 250 Hz,the sample impedance was almost the same,which indicating that the higher heating temperature and electric field intensity,the lower heating frequency,the greater damage on cell structure.The heating field intensity reached 90V/ cm,and the heating frequency was reduced to 250 Hz,the cell structure was almost destroyed.With the increase of heating temperature,electric field strength and decrease of heating frequency,the intracellular resistance value increases,the extracellular resistance value and the capacitance of the cell membrane decrease,and the heating electrical field intensity increases up to 110 V/cm and the heating frequency decreaseto 50 Hz,the cell membrane capacitance slightly increased,indicating that the cell destruction was due to the cell membrane electroporation.Thus,50 Hz and 90V/cm heating to 63 ?considered to be the best heating condition in this study.(2)After comparing and analyzing the goodness of fit among the three models,it is concluded that the standard deviation of the modified model using the constant-phase element is relatively minimal,and the corresponding coefficients of the real and imaginary components of the corresponding complex impedance are 0.998 and 0.979.Thus,the modified model is the most suitable equivalent circuit model of apple tissue.(3)In the experimental study of compression characteristics,the compressive characteristic of untreated sample shows a typical solid hard material,and with the increase of ohmic heating pretreatment temperature and electric field intensity,the decrease of heating frequency,the stress and strain value of fracture point decreased,the lower elastic modulus and the calculated toughness value are.The experimental results also show that the higher heating temperature and electric field intensity,the lower heating frequency,the greater the damage on the apple tissue cells and the elasticity of the sample.(4)In the creep-recovery experiment,it was found that as the heating temperature and the electric field intensity increase,the heating frequency decrease,the higher corresponding strain in the creep recovery curve.From the fitting results of the flexibility equation of the pretreated sample,it can be seen that the higher the heating temperature and electric field intensity,the lower heaing frequency,the corresponding initial compliance of creep compliance decreases,and the delay flexibility decreases,reflecting that as the heating temperature and electric field strength increase,the heating frequency decreases,the cell wall structure is destroyed,and the overall structure of the tissue becomes weaker.(5)It can be seen by scanning electron microscopy and transmission microscopy that,when heated to different temperatures,the entire cell structure collapses due to thermal stress and the overall structure is destroyed.When the heating is performed with different electric field intensity,the sample will be mainly subjected to thermal stress and the separation of the plasmic wall occurs when the field strength is lower than 90 V/cm,and the membrane shrinks.After the field intensity higher than 90 V/cm,The cell membrane will be broken and destroyed by the electroporation effect before the contraction under high field strength.