Study On Radiofrequency Pasteurization Process And Quality Of Black Pepper

Black pepper is loved for its special smell and is one of the most important economic spice crops in the world.Therefore,in recent years,the processing methods and food safety of black pepper and pepper products have been widely concerned by consumers,and the new food processing technology has a wide range of research and application prospects in the food processing industry.Radio frequency（RF）heating technology,as a new heating method,has the characteristics of fast heating speed,overall heating,good pasteurization effect,and other characteristics,while gaining the attention of researchers.The research content of this paper mainly includes four parts:1.Study of finite element simulation and temperature distribution of RF heating process of black pepper;2.Study of feasibility of Enterococcus faecalis as a substitute for Salmonella in RF heating of black pepper;3.Study of RF and combined pasteurization process of black pepper;4.Study of quality of black pepper after RF combined pasteurization.In this study,finite element numerical simulations of RF heating of black pepper were performed using both individual particle geometry models and equivalent property geometry models.RF heating experiments were performed on a single layer of black pepper kernels placed in a container（14×14×2 cm³）using an RF heating device at40.68 MHz.By grading the black pepper particle size,the sample particle size was determined to be about 0.5 cm（70%of the total）.The physical properties such as density,dielectric properties and thermal conductivity of black pepper kernels were measured in the temperature range of 20-100℃,and the dielectric properties of equivalent properties were calculated using the CRIME equation,and the thermal-electromagnetic coupling model was established using COMSOL Multiphysics software.The applicability of the two modeling approaches was compared by plotting the surface temperature distribution,center point and edge point temperature rise curves of RF heating experiments and simulation experiments on single-layer black pepper pellets,using the coefficient of determination R² as the judgment criterion.Comparison and analysis of the temperature distribution maps of the RF heating experiments and computer simulations revealed that the experimental results were consistent with the simulated results,and the surface of the monolayer sample was overheated at the edges and corners,and the cold point was located at the geometric center of the sample.The temperature profiles of the center and corner points of the experimental and simulated samples were compared,and the coefficients of determination R² were 0.99 and 0.95 for the center and corner points of the particle geometry modeling alone,and 0.98 and 0.92 for the center and corner points of the equivalent property geometry modeling,respectively.The simulation results of both geometric modeling methods fit well with the experiments,but the results of comparing the modeling and computational difficulties showed that the equivalent attribute modeling method was simpler and faster.Freshly harvested black pepper will be sun-dried,long-distance transportation and other processing,in the process of circulation will be infected with a variety of fungi,bacteria.Fungus in the mold will make black pepper spoilage and unusable,E.coli,Staphylococcus aureus,Salmonella will make people sick,which is the most harmful to humans is Salmonella.This study was conducted to explore the feasibility of using Enterococcus faecalis NRRL B-2354（E.faecalis）as a surrogate for Salmonella typhimurium ATCC 14028（S.typhimurium）in black pepper during thermal processing.Firstly,the thermal resistance parameters（D-and z-values）of E.faecalis and S.typhimurium in black pepper kernels（a_w=0.65）were measured in isothermal experiments.RF pasteurization experiment was then conducted to validate the effectiveness of surrogate bacteria by placing 2 g of inoculated black pepper kernels with both bacteria strains in the center of 45 g of original black pepper kernels and subjected to RF heating for 15 min at 40.68 MHz.Results showed that the D values of E.faecalis(D_65℃=8.14±0.75 min,D_70℃=4.79±0.48 min,D_75℃=3.42±0.08 min)were higher than those of S.typhimurium(D_65℃=5.05±0.54 min,D_70℃=4.33±0.29 min,D_75℃=2.19±0.29 min)in black pepper kernels.The thermal resistance of E.faecalis was higher than that of S.typhimurium,and the population reduction of S.typhimurium in black pepper kernels was greater compared to E.faecalis under the same RF heating treatment,which indicated E.faecalis could be used a suitable surrogate for S.typhimurium in RF treated black pepper kernels.RF pasteurization is a thermal pasteurization,with a rapid temperature rise,penetration depth,suitable for large volume sample heating,the quality of the food has a small impact on the characteristics.However,RF pasteurization due to the existence of the edge effect,will produce uneven temperature distribution phenomenon,resulting in the sample edge parts of the temperature is too high,the quality of deterioration.Ozone pasteurization,as a new cold pasteurization method,is suitable for pasteurization of porous samples such as black pepper because of its good pasteurization effect,low impact on quality,no pollution,and large pasteurization range.Therefore,we chose to combine ozone pasteurization with RF pasteurization to achieve the purpose of lowering pasteurization temperature,effective pasteurization and reducing quality degradation.In this study,we used Salmonella alternative,E.faecalis,and black pepper kernels with a_w=0.65 as the sample to study the pasteurization effect of RF combined with ozone pasteurization and the effect of different treatments on the quality of black pepper.Ozone pasteurization was performed for 40 min on the inoculated samples using a 60 W ozone pasteurization device.Ozone pasteurization alone showed that the pasteurization effect of ozone on black pepper was limited and only about 1 lg CFU/g of the target bacteria could be removed.Heating of black pepper to 85°C using a 40.68 MHz RF device was found to completely inactivate E.faecalis in black pepper.The results showed that the combined pasteurization method of 40 min ozone and 10 min RF heating and 10 min RF heating and 40 min ozone were both able to completely inactivate the target bacteria on black pepper at 70°C.This study demonstrates that the combined ozone and RF pasteurization treatment can effectively reduce the pasteurization temperature.After the pasteurization experiment was completed,the samples treated with effective pasteurization methods were tested for quality.The samples of black pepper treated with RF heating alone,ozone followed by RF and RF followed by ozone were tested for color difference,water activity and volatile flavor substances,respectively.The results showed that:the L*values of all the treated groups were significantly different from the control samples except for the samples treated with RF followed by ozone;the effect of the three treatments on the a*values was small,but the b*values of the treated samples were significantly lower,indicating that the color of black pepper became bluer.The black pepper treated with RF followed by ozone had the smallest total color differenceΔE value（1.09±0.14）.Comparing the moisture activity of the samples after different pasteurization treatments,the moisture activity of the samples decreased after all three treatments,and the ozone followed by RF treatment had the least effect on the moisture activity of the samples.The volatile flavor compounds of black pepper after different treatments were analyzed by SPME-GC-MS,and it was found that the main components of pungency,gastrodene and 3-carene,andβ-pinene,which represents its aroma,were changed,and the relative contents of gastrodene,3-carene andβ-pinene were26.77±3.75%,10.83±4.08%and 3.78±0.00%;32.57±1.53%,14.52±5.51%and 3.69±0.02%for gastrolene,3-carene andβ-pinene,respectively,in RF followed by ozone treated samples;30.46±13.77%,6.09±0.00%and 2.52±0.00%for gastrolene,3-carene andβ-pinene,respectively,in ozone followed by RF treated samples 2.52±0.00%,respectively.It was shown that all three fungicidal treatments adversely affected the flavor quality of black pepper,resulting in a decrease in both pungency and flavor.The results of the quality tests for color difference,water activity and volatile flavor substances showed that RF followed by ozone had the least effect on the quality of black pepper kernels.