Effect of moderate electric field pretreatment on damage degree and microstructure of apple tissue

Abstract: Apple deep processing in China is developing rapidly with the increasing demand of apple slices, fresh apple juice and apple canned products in recent years. All necessary processing technologies such as dehydration, diffusion, pressing and drying essentially require acceleration of two-way mass transfer into or out of the apple tissue. Apple tissue consists of thousands of cells and cell structure is the key factor that influences the mass transfer from inside to outside and vice versa. Therefore, the pretreatment method for damaging the apple tissue cell is needed before further deep processing. In recent years, moderate electric field (MEF) is receiving more and more significant attention due to its characteristics with the advantages of thermal and electrical treatment. To explore the effect of MEF pretreatment parameters such as electric field intensity, frequency and temperature on the damage degree and damage process of apple tissue, the paper analyzed the structural change of apple tissue under different MEF conditions, and evaluated the influence of pretreatment temperature on the damage process at macro and micro scale by detection of disintegration index of apple tissue structure and scanning electron microscope (SEM) observation respectively. The results indicated that apple tissue structure disintegration index from MEF was greater than that resulted from thermal treatment of waterbath. For waterbath pretreatment, the disintegration index of apple tissue structure firstly increased slowly with increasing temperature and then raised sharply when the temperature was above 60 ℃. The same tendency was observed for low electrical field intensity pretreatment (<30 V/cm), but the disintegration index increased more rapidly. The disintegration index changed slowly under hot water bath and low electrical field intensity treatment (< 30 V/cm), but when the temperature was above 60 ℃, the disintegration index increased sharply. The disintegration index increased linearly with increasing temperature when electrical field intensity was higher than 45 V/cm, and a greater disintegration index could be obtained under lower temperature and higher electric field intensity (90 V/cm). The reason was that the thermal and electrical synergistic effect of MEF pretreatment applied to the apple tissue simultaneously brought up the electroporation of cell membrane and the degradation of cell wall. In addition, the disintegration index of apple tissue structure increased with the increase of electric field intensity and decreased with the increase of frequency when the temperature was lower than 70 ℃. All the curves of disintegration index with holding time could be approximated using the first-order reaction kinetics model which could describe the characteristics of the damage process. The rate constant of the first-order model increased with the increasing treatment temperature and the characteristic damage time showed contrary tendency. The rate constants at different temperatures could be formulated by the Arrhenius equation, the same as the characteristic damage time. The activation energy calculated from the rate constant was 122.88 kJ/mol, and that from the characteristic damage time was 157.01 kJ/mol. The activation energy obtained from the hot water bath pretreatment was 143.82 kJ/mol.The micro-structure observation of apple tissue indicated that MEF pretreatment can effectively destroy the apple tissue at lower temperature. Apple tissue is destructed more easily by moderate electric field treatment because of the synergistic effect from electrical field and temperature field. The findings are useful to analyze the electro-thermal synergistic regime from MEF pretreatment and improve the deep processing technology of apple products.