Study On The Change Of Physicochemical Properties Of Apple Juice Treated By Induced Electric Field

Apple juice is popular among consumers due to its sweet taste and high nutrition.Although current electric field processing technologies can improve food quality,metal electrode that contacts food is necessary.It will cause food contamination due to the electrochemical reactions on electrode surface during long-term treatment.To avoid the use of electrode,induced electric field(IEF)was developed by converting magnetic energy to electrical energy,in the configuration of a transformer.In this study,compared with control(0 V),the effect of induced electric field(IEF)on the quality of apple juice was studied via research of enzyme activity,impedance,electrical conductivity,and storage stability of apple juice.The conclusion was presented as follows:The juice was flowed through IEF reactors and treated by the electric field at conditions: temperature 30~80°C,voltage 540~900 V,flow rate 17.5~87.5 mL/min,and the number of reactors 1~5.The effect of IEF on browning of apple juice was investigated from the changes in the activity of polyphenol oxidase(PPO)and peroxidase(POD).The effect of IEF on the enzyme activity primarily depended on the temperature.At 70 °C,IEF treatment dramatically decreased the enzyme activity.Compared with control,POD residual activity(RA)was decreased by 17.66% and the RA of PPO was decreased by 23.80%.However,IEF increased enzyme activity at 50 °C.Compared with control,the RA of POD and PPO was increased by 15.12% and 26.01%,respectively.There was no significant difference(p>0.05)in the RA of PPO and POD treated with IEF at 30 and 40 °C.The excitation voltage and flow rate also caused the changes in enzyme activity.Excitation voltage contributed to the inhibition of juice browning,which the browning increased with flow rate.At a flow rate of 17.5 m L/min,PPO was completely inactivated by IEF treatment,while the RA of POD was reduced to 13.80%.The RA of juice enzyme also decreased with the number of reactors.When the number of reactors was 5,RA of enzyme was the lowest.Orthogonal experiments showed IEF had the best effect on enzyme inactivation of apple juice at 70 °C,2.5 mL/min,4 reactors and 900 V.Compared with the control groups,PPO decreased by 42.41% and POD decreased by 45.78%.These effects could be explained by the fluid impedance and the conductivity of the juice.The fluid impedance and conductivity of apple juice was also recorded to analyze the factors that inhibit juice browning during IEF treatments.The factors that affects the potential difference between two the feed liquid coils changed the impedance of the sample fluid,including temperature and flow rate.However,the conductivity was only related to the temperature: the conductivity increased with the temperature increased.In addition,the effect of IEF on the conformation of horseradish peroxidase(HRP)was investigated.The circular dichroism and fluorescence chromatograms of HRP showed that the application of IEF at both 50 and 70 °C reduced the ?-helix content of HRP compared to the control.IEF reduced fluorescence intensity of HRP at 50°C but increased the intensity at 70 °C.The effect of IEF on the quality of apple juice during storage was explored.The changes in microorganisms,pH,titratable acid,solids content,color value,total phenolics,total flavones,and volatile components in juice treated by IEF,control(0 V)and heat during storage were determined.The sensory evaluation of apple juice and the quality during storage showed that the IEF and the fresh group had higher sensory scores,while the control group and the heat treated fruit had lower scores.The pH,titratable acid and solid content of the juice remained at a stable level.There was no significant difference(p>0.05)among different treatment methods,and also no obvious change in storage period(p> 0.05).Compared to untreated and heat-treated juices,IEF and control juices kept the total number of microorganisms below 1 log in the shortterm shelf life.The processed apple juice has a brighter color during storage and a lower degree of browning.The untreated apple juice produced a large amount of n-hexanol during the storage due to the presence of a large amount of microorganisms,while the content of n-hexanol in the IEF and the control juice was extremely low and has no tendency to increase.The content of nhexanol in heat-treated sample slowly increased.Except for n-hexanol,other volatile components in untreated juice had a decreasing trend during the storage.The volatile components of heat-treated juice slowly decreased,while the volatile components in IEF and control juice were more stable.IEF,control and heat samples had some damage to phenols and flavonoids,but as the storage period prolonged,the total phenol content and total flavonoid content of the untreated apple juice rapidly decreased.When the storage period was 15 days,the total phenolic content and total flavonoid content of the IEF-treated juices were the highest,which were 18.41 mgGAE/100 g fruit juice and 6.01 mgRE/100 g fruit juice,respectively.The control was better than the heat treatment.The total phenolic content and total flavonoids of untreated juice were the lowest,which were 13.80 mgGAE/100 g juice and 3.84 mgRE/100 g juice,respectively.This showed that the IEF was conducive to maintaining the quality of apple juice during storage.Continuous flow treatment of juice liquid under IEF was high-throughput processing.Meanwhile,it avoided electrochemical reaction and metal ion contamination of the sample,and might provide theoretical basis for the development of non-thermal processing of food.