| Peanuts(Arachis hypogaea)are widespread leguminous crops and they are considered as naturally nutrient-rich foods due to their massive amount of fatty acids,proteins,minerals and vitamins.However,aflatoxins contamination has always been a preservation issue of peanuts.Aflatoxins are mainly produced by Aspergillus flavus,and Aspergillus parasiticus,which are frequently isolated from peanuts during storage period.Radio frequency(RF)heating is a new thermal processing technology which has been considered as an effective method for postharvest pasteurization due to its high heating rate,deep penetration depth and no pollution to environment.However,the non-uniform heating in food material caused by uneven distributed electric field is the major problem associated with RF heating.Computer simulation and mathematical modeling could help rapidly predict the temperature distribution among food materials after RF heating treatment or investigate the main factors affecting RF heating uniformity.The reliability and accuracy of computer modeling depends significantly on the physical properties of food samples input into the simulation model,such as dielectric properties and thermal properties.The main objectives of this study include:(1)measuring the dielectric properties of peanut kernels at different frequencies,moisture contents and temperatures with the open-ended coaxial-line probe technique,(2)studying the thermal inactivation kinetics of Aspergillus flavus in peanut samples to help determine the RF pasteurization processing parameters(such as temperature and heating time),(3)developing computer simulation model with the Joule module in commercial software,COMSOL Multiphysics,for better understanding the interaction between electromagnetic field and food components,(4)using the computer simulation model to study the effect of different methods on improving RF heating uniformity of peanut kernels,and(5)applying computer model to predict temperature distribution and microorganism survival within peanut samples after RF heating treatment by combining the thermal inactivation kinetics of Aspergillus flavus.The main results were described as follows:(1)The open-ended coaxial-line probe method was used to measure the dielectric properties of peanut samples over a frequency range of 10 to 4500 MHz.Regression equations of both dielectric constant and loss factor with respect to temperature and moisture content were established.The regression equations were input into the computer model to determine the RF heating temperature profiles of peanut samples at three moisture content levels.The results showed that both the dielectric constant and loss factor of peanut samples increased with increasing temperature and moisture content and decreased with increasing frequency.At RF frequencies(10-300 MHz),dielectric properties values of peanut samples decreased sharply with increasing frequency.The penetration depths at RF frequencies were much larger than those at microwave frequencies.Both the simulated and experimental results showed that the RF heating rate of peanut samples was higher at moisture content of 20.13%w.b.than that at 9.75%and 29.87%.The estimated highest heating rate can be achieved at moisture content of 20.40%w.b.at 45℃.(2)Thermal inactivation kinetics of Aspergillus flavus in peanut flour at different water activities and temperatures were determined.The survival curves of Aspergillus flavus were fitted with first-order kinetic and Weibull distribution models.The model parameters were further fitted with modified Bigelow model to predict D,δ,and p values at any given temperature and water activity.The influence of heating rate on thermal resistance of Aspergillus flavus was also studied at 59℃ and water activity of 0.921.The results showed that D values decreased with increasing temperature and water activity.The coefficient of determination showed that the Weibull model was more suitable than the first-order kinetic model to describe the inactivation of Aspergillus flavus.All the p values were smaller than 1,and upward concavities were found with the survival curves.When the heating rate was higher than 1℃/min,the D values were significantly lower than that at slower heating rate levels.The reduced thermal resistance of Aspergillus flavus at higher heating rate indicates that the rapid RF heating process is advantageous for peanut pasteurization in food industry.(3)A finite element model of peanuts heated in RF system was developed using software COMSOL Multiphysics to help exploring new methods to improve RF heating uniformity.Peanut samples packaged in three dimensions of mica containers were processed by RF system to validate the computer model.After validation,the model was used to analyze the effect of adding mica plates and polypropylene block in the container on RF heating uniformity.The results showed that adding mica plates on the cold spot area(center)can effectively optimize the RF heating uniformity of peanuts,especially when the size of mica plate was similar to the cold zone.Furthermore,increasing the thickness of mica plate to 9,15 and 17 mm for large,medium and small container,respectively,can obtain the optimal RF heating uniformity.The best heating uniformity was observed when adding mica plate at dimension of 300×200×15mm~3 in medium container.The average temperatures increased by adding polypropylene blocks among peanut samples,resulted in better RF heating uniformity.(4)A new strategy of improving RF heating uniformity was investigated based on the developed computer simulation mode.In the computer model,the whole peanut samples were divided into different layers to study the effect of sample layers on RF heating uniformity.The model was validated by placing 4.5 kg of peanut kernels in a 6 k W,27.12 MHz RF unit and heated to target temperatures.The heating uniformity of peanut samples was evaluated by the temperature uniformity index(TUI).The results showed that the heating uniformity could be improved by segregating peanut kernels into different layers or structured blocks with air gaps between them.The methods developed in this study could be further used to optimize RF processing protocols in food industry.(5)The microbial reduction of Aspergillus flavus in peanut samples after a thermal processing can be calculated according to the local temperature profile.The RF heating simulation model developed earlier could be combined with thermal inactivation kinetics to predict microbial reduction in peanut kernels.In terms of experimental validations,three 2-g inoculated peanut samples were placed at the center of three horizontal layers of peanut kernels and processed by the hot-air assisted RF heating unit.Both the simulated average temperatures and microbial reductions of Aspergillus flavus matched well with the experimental results.Nonuniform distributed temperature among peanut samples resulted in the least lethality of Aspergillus flavus at the center area.The experimental microbial reductions were slightly lower than the simulated ones due to the water activity change during RF treatments.The validated computer model can be used to design industrial-scale RF treatment protocols. |
PDF Full Text Request