Evaluating radio frequency heating uniformity using polyurethane foams

Radio Frequency Treatment of Food: A Review on Pasteurization and Disinfestation

Radio frequency (RF) is a novel technology with several food processing and preservation applications. It is based on the volumetric heating generated from the product's dielectric properties. The dielectric properties of each material are unique and a function of several factors (i.e., temperature, moisture content). This review presents a list of dielectric properties of several foods and describes the use of RF as an innovative technology for the food industry. This paper includes several examples of pasteurization, fungi inactivation, and disinfestation in selected food products. The aim of this review is to present the potential applications of RF in pasteurization and disinfestation and research needs that should be addressed. RF has been successfully applied in the inactivation of pathogens such as Salmonella spp., Listeria monocytogenes, and Escherichia coli in low- and high-moisture food. The disinfestation of crops is possible using RF because of selective heating. This process inactivates the insects first because of the different dielectric properties between the pests and the food. The products' final quality can be considerably better than conventional thermal processes. The processing time is reduced compared to traditional heating, and thermal damage to the food is minimized. The main drawback of the technology is the lack of uniform heating, mainly when the product is surrounded by a packaging material with different dielectric properties from the food.

Radio frequency pasteurization and heating uniformity of canned pineapple

This research investigated heating uniformity and pasteurization of canned pineapple using radio frequency (RF) energy. Experiments were conducted in a 6 kW, 27.12 MHz pilot-scale RF system. Results showed that the temperature difference was more than 16°C, and the standard deviation was 4.38°C at the end of heating when using RF heating alone. Water bath-assisted RF (WRF) heating effectively improved the heating uniformity, the temperature difference was less than 7°C and the standard deviation was 2.52°C at the end of heating in the condition of electrode gap (210 mm), chord length of the fruit block (26 mm), and the initial temperature of sugar solution (80°C). When the total number of colonies reached 4-log reduction, water bath (WB) heating alone needed 660 s, and WRF heating needed 180 s. Vitamin C, hardness, and color of fruit blocks were well preserved using WRF heating compared with WB alone. PRACTICAL APPLICATION: This study shows that the pasteurization of canned food by radio frequency heating can achieve better food quality than the traditional pasteurization methods. Therefore, this research can promote the application of radio frequency heating technology in canned food pasteurization.

Radio frequency heating of green peas (Pisum sativum L.): The improvement of heating uniformity and its dry blanching effect

A radio frequency (RF) heating system was used to process green peas and a rotating system was introduced to improve heating rate and heating uniformity. Results revealed that rotation treatment accelerated heating rate effectively and improved heating uniformity index significantly (p < 0.05) from 0.175 (0 rpm, 55°C) to 0.029 (60 rpm, 55°C). After being treated with RF and traditional hot water blanching at 85°C, the residual lipoxygenase activities of green peas were 1.90 ± 0.71% and 35.51 ± 5.25%, respectively, confirming RF possessed better blanching efficiency. Meanwhile, weight loss, electrolyte leakage rate, color, and texture of green peas all had significant changes (p < 0.05) through RF heating. PRACTICAL APPLICATION: A rotation device can effectively improve heating uniformity of RF electromagnetic heating. The rotation device could be expanded or further developed into continuous feeding conveying device for industrial production.

Analysis on the effect of polyetherimide on energy distribution of radio frequency heating of viscous sauce

Radio frequency (RF) sterilization of low-moisture, high-oil, high-protein, and viscous sauces for instant food (LHHVS) demonstrates many advantages, but uneven heating is a main problem that must be addressed. Main factors that affect heating uniformity are generally considered dielectric properties, shape and size of the sample and its position relative to the electrode plate, in addition the structure and voltage of RF electrode. A method based on texture characteristics of the solid–gel–liquid mixing system of LHHVS for adjustment and control of energy distribution in the RF field is proposed in this study to improve the heating uniformity. First, energy conversion principles and control equations of RF heating were analyzed on the basis of dielectric theory. Second, the influence of RF electromagnetic field-medium polyetherimide (PEI) on the RF heating of peanut butter (RHPB) was investigated on the basis of the numerical model of RHPB that was verified through experiments. Finally, the influence mechanism and its regulation and control effect were analyzed and discussed. The following conclusions can be drawn from this study: the increase of electrode gaps exerts minimal effect although it reduces the unevenness of the energy distribution. However, RF heating protocols must use the smallest possible electrode gap to heat agrifoods and increase the heating rate significantly. The energy distribution on the part of the sample close to PEI varies with the change of geometry and size of PEI when its placement is bias or symmetric. The area of energy enhancement continues to expand where the sample is in contact with PEI as PEI gradually increases. The area where the temperature increases under the influence of PEI will expand along the direction of the sample radius when the thickness of PEI remains unchanged and the radius gradually enlarges; otherwise, it will expand along the direction of the sample thickness. The influence of PEI on the energy distribution of RHPB demonstrates local characteristics. PEI significantly influences the energy distribution and heating mode of RHPB, which is easy to adjust and control, but does not reduce the processing speed and does not increases energy consumption. Hence, PEI is an effective means to interfere with energy distribution of RHPB. Uniform energy distribution can be obtained by selecting the appropriate PEI shape and size. Results of this study can help determine the experimental protocol for RHPB with the optimal uniform distribution and promote the fast commercial application of this technology.

A comprehensive review on recent developments of radio frequency treatment for pasteurizing agricultural products

Recent pathogen incidents have forced food industry to seek for alternative processes in postharvest pasteurization of agricultural commodities. Radio frequency (RF) heating has been used as one alternative treatment to replace chemical fumigation and other conventional thermal methods since it is relatively easy to apply and leaves no chemical residues. RF technology transfers electromagnetic energy into large bulk volume of the products to provide a fast and volumetric heating. There are two types of RF technology commonly applied in lab and industry to generate the heat energy: free running oscillator and 50-Ω systems. Several reviews have been published to introduce the application of RF heating in food processing. However, few reviews have a comprehensive summary of RF treatment for pasteurizing agricultural products. The objective of this review was to introduce the developments in the RF pasteurization of agricultural commodities and to present future directions of the RF heating applications. While the recent developments in the RF pasteurization were presented, thermal death kinetics of targeted pathogens as influenced by water activity, pathogen species and heating rates, non-thermal effects of RF heating, combining RF heating with other technologies for pasteurization, RF heating uniformity improvements using computer simulation and development of practical RF pasteurization processes were also focused. This review is expected to provide a comprehensive understanding of RF pasteurization for agricultural products and promote the industrial-scale applications of RF technology with possible process protocol optimization purposes.

Radiofrequency Heating for Enhancing Microbial Safety of Shell Eggs Immersed in Deionized Water

Radiofrequency (RF) heating was simulated for improving the microbial safety of a shell egg immersed in deionized water using a finite element model. A regression equation that relates the top electrode voltage to the gap between the electrodes and vertical position of the egg was also developed. The root mean squared errors between the simulation and validation results ranged from 0.76 to 2.86 °C. Concentrated heating occurred in the yolk in all the investigated configurations, with some configurations reaching close to 60 °C in the yolk after 20 min of RF heating. The cooling effect of the water along with lower electric field intensity in the egg caused the focused heating in the yolk. Extrapolation of the model revealed that a scaled-up RF heating process (10.5 kV at top electrode) followed by a hot water immersion process can achieve a minimum of 3 log reductions of Salmonella in the yolk within 37 min.

PRACTICAL APPLICATION

Radiofrequency (RF) heating with the assistance of deionized water was shown to have the potential of improving the microbial safety of shell eggs. The validated model results revealed that this effect was due to a combination of focused heating and surface cooling.

Pilot-scale radio frequency pasteurisation of chili powder: heating uniformity and heating model

BACKGROUND

Microbial contamination is a vital obstacle needed to overcome for food safety of condiments. Radio frequency (RF) pasteurisation is a new technology to solve this obstacle. Temperature distribution and heating uniformity of sample, which are influenced by different factors, are the most important things affecting the nutritional ingredients and microbial safety of sample in the process of RF pasteurisation. This study demonstrated the location of cold spot in chili powder by analysing temperature distribution in horizontal and vertical direction. The related models were established and the accuracy was verified.

RESULTS

Cold spot located on the centre of sample surface in the process of RF pasteurisation. The averaged temperature of sample increased linearly. The uniformity index decreased as the averaged temperature increased. Both the correlation coefficient of two equations were greater than 0.91. The error value of heating rate and heating uniformity index was 0.54% and 0.75% between the measured value and predicted value.

CONCLUSION

Electric field was not uniformly distributed between RF parallel-plate electrodes in the RF pasteurisation of chili powder. The heating models were reliable to predict experiment results with high precision and accuracy. © 2015 Society of Chemical Industry.