| Acidovorax citrulli,one of the phytosanitary pathogens,was detected inside and outside of the watermelon seeds in commercial circulation,which raised the risk of spread of bacterial fruit blotch during seed transportation.Therefore,seeds need to be pasteurized to meet circulation and quarantine requirements.As a novel heating technology,radio frequency(RF)heating has a deep penetration depth and rapidly volumetric heating,and the potential to efficiently pasteurize pathogens inside and outside of watermelon seeds.To achieve the target of maintaining seed vigor and effective pasteurization,before developing a reliable RF pasteurization process for watermelon seeds,it is necessary to systematically measure the dielectric and thermal properties of watermelon seeds,clarify the changing trends of seed germination rate,reveal the thermal death kinetics(TDK)of A.citrulli,establish a calculation model for parameters of thermal pasteurization,determine the RF heating uniformity and electric field distribution of samples,validate the thermal pasteurization parameters and measure the seed vigor before and after RF pasteurization.Based on this,the main contents and results were summarized as follows:(1)The dielectric and thermal properties of watermelon seed coat and embryo in different moisture contents,temperatures,and RF frequencies were systematically studied by using the open-ended coaxial probe method.The polynomial model could calculate the dielectric(R~2=0.935～0.999)and thermal properties(R~2=0.974～0.994)of seed coats and embryos at27.12 MHz within the given range of moisture content and temperature.Values for dielectric and thermal properties of watermelon seeds with a moisture content of 14.70%±0.11%wet basis(w.b.)calculated by the mixture equation gradually increased as the temperature increased from 20 to 80°C.(2)The variation trend for germination rate of watermelon seeds under different moisture contents,temperatures,and heating rates was studied by using the heating block system(HC-HBS).To maintain the germination rate of watermelon seeds greater than or equal to 90%,temperature of heat treatment does not exceed 75°C.The heat-resistance of watermelon seeds increased with the decrease of moisture content.The increasing holding time could be achieved by appropriately reducing the moisture content and heating rate of the seeds.In addition,the Weibull model(R~2=0.800～0.999 and RMSE=0.024～0.450)was a feasible and effective mathematical model for predicting the germination rate curve of watermelon seeds.(3)The variation for heat-resistance of A.citrulli inoculated into seed embryos with different water activities and temperatures was clarified by using the heating block system(WSW-HBS),and the potential parameters of thermal pasteurization were determined by TDK of pathogen combined with the heating uniformity index and curve of germination rate.The heat-resistance of A.citrulli ACCC 05732 inoculated into watermelon seed embryos was significantly(P<0.05)higher than that in seed coats.The heat-resistance of A.citrulli inoculated into samples increased as water activity decreased.Compared with the first-order kinetic model(R~2=0.927～0.988 and RMSE=0.164～0.467),the Weibull model(R~2=0.965～0.999 and RMSE=0.023～0.304)fitted better with the data for TDK of A.citrulli inoculated into seed embryos.According to the prediction model,when the heating uniformity index was 0.04,the reference values for average heating temperature and treatment time at potentially high or low germination rates were 63.6°C and 136.2 min or 65.3°C and 98.1 min,respectively.(4)The method of finite element software(COMSOL)combined with practical experiments were applied to systematically analyze the effect of adding dielectric or metallic materials on the uniformity of RF heating in watermelon seeds.The electric field distribution in the second layer of sample with inserting polypropylene(PP)or aluminum(AL)plates was different.The volumetric RF heating uniformity index of the second layer in seeds with inserting the AL plate was smaller than that of the sample with inserting the PP plate.As the thickness of the AL plate increased from 0.5 to 16.0 mm,the volumetric heating uniformity index of the whole sample increased from 0.166 to 0.281.As the air gap increased,the electric field lines gradually concentrated in corners and edges of the second layer in seeds.The average value of volumetric electric field intensity of the AL plate varied with the change of electrical conductivity.A composite plate(210×150×2 mm~3)composed of central part with AL plate and other part with PP plate was proposed and validated to improve the uniformity of RF heating based on simulation results.RF heating uniformity could be effectively improved by inserting 1 horizontal composite plate and 3 AL plates into watermelon seeds at an interval of 1/4(10 mm)of the sample height.(5)After further improving the heating uniformity and validating the potential parameters of thermal pasteurization,the RF-hot air combined pasteurization processes of watermelon seeds were developed and optimized,and the influence of different pasteurization processes on the vigor parameters of watermelon seeds was also studied.The temperature difference between the different layers in sample would be further reduced by adding a PP frame(134×94×17 mm~3)with the surface area of 40%of the top surface in sample to center part for the first layer in watermelon seeds.After validating the potential parameters of thermal pasteurization with the high and low germination rates,the optimized process was to treat watermelon seeds by RF heating for 6.5 min,and the samples with stacked form were put into a hot air oven at 64.5°C for 50 min,and then the sample was divided into a single layer for hot air drying of 30 min,and finally sample was cooled for 6 min with forced room air.In addition,there were no significant differences(P>0.05)in germination energy,germination rate,germination index,vigor index and leachate electrical conductivity of watermelon seeds after optimized pasteurization process and in control group.Therefore,the RF-hot air combined pasteurization process identified in this study would provide a reference for the subsequent industrial application for seeds. |
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