Study On Radio Frequency Sterilization Of Black Pepper

As an emerging dielectric heating technique,radio-frequency heating is characterized by fast heating speed,strong penetrability and uniform heating,whose application has attracted increasing attention in the field of sterilization and insecticide of low-moisture agricultural products.Radio-frequency sterilization rapidly heats the material inside via radio-frequency heating,up to the thermal lethal temperature of target bacteria,thereby killing the bacteria.In the current research on the sterilization process of black pepper,the total amount of research is relatively small,and the result is difficult to apply in the industrial field.In addition,in the existing research on the sterilization of spices,most of them directly sterilize the materials,count the colonies,and then compare the total number of colonies before and after the treatment to confirm the sterilization effect of different sterilization methods.The experimental process is cumbersome.In industrial applications,the current main sterilization method is steam sterilization,which has a significant sterilization effect.However,subsequent drying is required to remove the increased water in the sterilization process.The process is cumbersome and affects the quality of black pepper.Therefore,in this study,polypropylene containers?24*15*6cm³?containing 1359.6 g of black pepper particles were selected as the research objects,Salmonella typhimurium and E.coli O157:H7 were the experimental strains.Foodborne pathogens.In the experiment,combined with numerical simulation,a finite element radio frequency sterilization model of black pepper particles was established,and the validity of the model was verified through experiments.Finally,the quality of black pepper before and after treatment was analyzed,and the results were compared to confirm the better sterilization process.In this paper,the operating mechanism of radio-frequency heating system and its application in food processing were first introduced.Meanwhile,the significance and current application status of finite element simulation were also introduced,combined with the advantages and disadvantages of radio-frequency,and some prospects about the application of radio-frequency heating were proposed.The research in this paper mainly includes four parts:1)The finite element numerical simulation was established for the radio-frequency heating of black pepper,and the validity of simulation was verified using experiments.2)The radio-frequency heating process of black pepper was explored.3)The thermal death and Bactericidal kinetic equation of Salmonella typhimurium and Escherichia coli O157:H7 in pepper was detected,and then subjected to simulation solution to determine the sterilization process.Finally,the radio-frequency sterilization assay was conducted.4)The quality of black pepper was analyzed under different treatment methods.The details are as follows:?1?The corresponding dielectric property of granular black pepper should be calculated using the mixture equations of dielectric properties?CRIME,LLLE and BE equation?.Then the electrostatic field-heat conduction two-way coupling model was established using COMSOL Multiphysics software to simulate the radio-frequency heating.At the same time,the experiment was performed using the plane-parallel radio-frequency system?27.12 MHz,12 k W?.The results showed that the dielectric properties obtained by the CRIME equation were the most similar to the temperature distribution of the experimental results in the simulation,indicating that the results from the CRIME equation were more consistent with the actual values,and the model established in the experiment was also valid and could be further applied in the exploration of sterilization process.?2?In the preliminary experiment,the formal container was determined as the polypropylene container?24*15*6 cm³?based on the matching voltage of machine and the heating rate of material.Then the distance between the upper and lower plates was altered?12-13 cm?,and a better heating process was selected.According to the temperature distribution uniformity index of the upper surface and intermediate layer,it was found that the index of the upper surface and intermediate layer was 0.097 and 0.074,respectively,slightly lower than that at 12 and 13 cm.The temperature rise curve of the intermediate point and corner point was compared.It was found that the initial temperature of material was 23°C,and the temperature rise curves were relatively flat under three conditions.The time spent on reaching the target end point?the temperature of the center point reached 70°C?was at an interval of about 30 s between two.Combined with the temperature distribution map and the temperature rise curve,it was confirmed that the distance of 12.5 cm was superior.To sum up,the size of the container for heating process was finally determined as 24*15*6 cm³,and the plate spacing was 12.5 cm.?3?To determine the radio-frequency heating sterilization process of black pepper,the lethality of the target bacteria?Escherichia coli O157:H7 and Salmonella typhimurium?was first determined.The results revealed that the corresponding D value of Escherichia coli O157:H7 at 65,70 and 75°C was 1.15,0.55 and 0.16 min,respectively.Then the log₁₀D-T relational graph was plotted and the curve was fitted,and the Z value was about 11.6°C.The corresponding D value of Salmonella typhimurium at 65,70 and75°C was 4.88,3.44,and 1.95 min,respectively.The log₁₀D-T relational graph was plotted and the curve was fitted,and the Z value was about 17.2°C.According to the regression analysis of the log₁₀D-T curve,the linear regression equations were obtained:y=5.68787-0.08695x?R²=0.94654?,and y=4.50238-0.05812x?R²=0.91081?.Based on the linear regression equation,the D-T relation was determined and taken into the computer model,so that the target colonies killed in the samples for different time of radio-frequency heating could be quickly detected.Considering the influence of factors,such as the ambient temperature and the initial temperature of the material,on the actual experiment,the heating time was prolonged to detect the target bacteria in the black pepper in the vacuum bag.It has been proved that the initial value of the target bacteria after inoculation of black pepper was 7?8 log,Escherichia coli O157:H7 and Salmonella typhimurium were completely killed after 9 min and 10 min of radio frequency heating respectively.Therefore,under this condition,radio frequency heating can kill target bacteria of 8 log or more.?4?After the sterilization experiment,the quality of black pepper was analyzed under different processing methods,in order to prove that the influence of radio-frequency sterilization on the quality of black pepper is slight.The water activity,ash content,piperine content,flavor components and color difference were selected as quality indexes.The results manifested that at 9 min after radio-frequency heating,the water activity of black pepper declined from 0.673±0.021 to 0.573±0.063,while it rose to 0.757±0.030after steam sterilization.Besides,the ash content of the original sample,steam sterilization at 121°C for 5 min and radio-frequency heating for 9 min was 3.28%,3.07%and 3.18%,respectively,without obvious differences.It can be seen that either steam treatment or radio-frequency treatment rarely affects the ash content of black pepper.Radio-frequency treatment had a smaller impact on ash of black pepper.The piperine content after steam sterilization at 121°C for 5 min was significantly different from that in the original sample?P<0.05?,in which case the piperine content was the lowest and the loss was the greatest.After 9 min of radio-frequency heating,the piperine content almost had no difference with the least loss.As can be seen from the GC-MS graph about the flavor components of black pepper,the flavor components of the three kinds of black pepper were consistent,and the four major substances were caryophyllene,3-pinene,D-limonene and?-pinene.The results of color difference analysis showed that compared with those of the original sample,the brightness,redness and yellowness of black pepper were increased after radio-frequency heating,while they were decreased after steam treatment.The brightness and yellowness of black pepper were greatly affected by steam treatment,and both treatment methods had a certain impact on the redness of black pepper.According to the comprehensive analysis,the color of black pepper was better after radio-frequency heating.This paper aims to provide more possibilities for the industrialized radio-frequency sterilization of black pepper and the combination of finite element simulation and radio-frequency sterilization.