Radio Frequency Combined With Pulsed Electric Field Treatment For Endogenous Enzyme And Microbes Inactivation In NFC Apple Juice

The polyphenol oxidase(PPO)has two existing forms in apple tissue,most of which is membrane-bound PPO(mPPO)in chloroplast membrane and has low catalytic activity.The other PPO is soluble PPO(sPPO),which is in cytoplasm and has catalytic activity.When the cells are broken,mPPO will be released to become sPPO and contact with the phenolic substrate,resulting in browning.During the apple juice processing,enzymatic browning is one of the main reasons for the decline of product quality.Therefore,it is essential to inactivate the mPPO and sPPO to inhibit browning.However,conventional heating needs large quantities of energy and adding chemicals will cause adverse influence on human body.It is necessary to find a method to inactivate mPPO and sPPO quickly.Radio frequency(RF)and pulsed electric field(PEF)are the two non-thermal processing technology.RF treatment can inactivate the PPO(sPPO and mPPO)before the PPO is released.While long time treatment will decrease the quality of apple.PEF can inactivate PPO but not thoroughly.Therefore,it is necessary to find a combined mehod to achieve a good inactivation effect for PPO and improve the juice qualities.In this paper,the enzymatic properties of sPPO and m PPO were studied.The results showed that sPPO was easier to combine with the phenols than mPPO.The sPPO and mPPO had similar optimal p H,both were 4.8.The activity of mPPO was lower than that of sPPO when the pH was less than 4.8.The optimal temperature of sPPO and mPPO was 45?and35?,respectively.When the temperature was higher than 45?,mPPO was more likely inactivated than sPPO.Therefore,RF may make it easier to inactivate PPO in apple tissue.The inactivation kinetics of PPO and the influence on the quality of apple juice by RF treatment were studied.The results illustrated that the PPO in apple tissue was more easily inactivated than that extracted.Compared with the conventional processing methods,RF treatment could inactivate most of PPO in apple tissue at relative low temperature and reduced the loss of total phenols in apple juice.The color of juice was increased.The pro-processed apple tissue by RF was squeezed and the apple juice was processed by PEF(15 kV/cm for 400?s,25 kV/cm for 400?s and 35 kV/cm for 400?s).The results showed that there was no significant change(p>0.05)on the color,pH,electric conductivity and ~oBrix of apple juice.After PEF treatment,the PPO in apple juice could be inactivated to less than 5%.The flavor of apple juice processed by RF combined with PEF(RF for 10 min,PEF 35 kV/cm for 400?s)was better than the conventional processing samples(60?for 10min and 70?for 10 min).Finally,the storage experiment was carried out at 4?for 30 days.The microbes,color,pH,electric conductivity,~oBrix,total phenol content and total antioxidant capacity were monitored each 5 days.The results showed that the number of microbes was in a safe range(<100 CFU/mL)and the pH,electric conductivity and ~oBrix were not changed significantly(p>0.05).The apple juice processed by RF combined with PEF had higher total phenol content and total antioxidant capacity than conventional processing methods(75?for 15min,90?for 30s)during the storage time.Meanwhile,the color was better than conventional processing samples.