Dielectric properties of almond kernels associated with radio frequency and microwave pasteurization

To develop advanced pasteurization treatments based on radio frequency (RF) or microwave (MW) energy, dielectric properties of almond kernels were measured by using an open-ended coaxial-line probe and impedance analyzer at frequencies between 10 and 3000 MHz, moisture contents between 4.2% to 19.6% w.b. and temperatures between 20 and 90 °C. The results showed that both dielectric constant and loss factor of the almond kernels decreased sharply with increasing frequency over the RF range (10–300 MHz), but gradually over the measured MW range (300–3000 MHz). Both dielectric constant and loss factor of almond kernels increased with increasing temperature and moisture content, and largely enhanced at higher temperature and moisture levels. Quadratic polynomial equations were developed to best fit the relationship between dielectric constant or loss factor at 27, 40, 915 or 2450 MHz and sample temperature/moisture content with R2 greater than 0.967. Penetration depth of electromagnetic wave into samples decreased with increasing frequency (27–2450 MHz), moisture content (4.2–19.6% w.b.) and temperature (20–90 °C). The temperature profiles of RF heated almond kernels under three moisture levels were made using experiment and computer simulation based on measured dielectric properties. Based on the result of this study, RF treatment has potential to be practically used for pasteurization of almond kernels with acceptable heating uniformity.

Scientific RepoRts | 7:42452 | DOI: 10.1038/srep42452 kernel should be raised before dielectric heating. Therefore, the dielectric properties of almond kernels as influenced by moisture content need to be explored.
Dielectric properties of agricultural products influence the absorption and dissipation of electromagnetic energy when they are subjected to RF or MW heating and are generally expressed in terms of relative complex permittivity, ε = ε′ − jε″ . The real part, ε′ , known as the relative dielectric constant, represents a material's ability to store the applied electrical energy. The imaginary part, ε″ , known as the relative dielectric loss factor, measures the dissipation of applied electric energy in the form of heat 14,15 .
Dielectric properties of agricultural products have been studied over different frequency, temperature and moisture ranges for drying [16][17][18][19][20] , pest control 21,22 and pasteurization [23][24][25][26] . For example, Zhang et al. 17 reported that the dielectric properties of ground peanut kernels decreased with increasing frequency and increased with increasing moisture content and temperature. Gao et al. 23 studied the dielectric properties of almond shells over the frequency range of 10-1800 MHz and temperatures between 20 and 90 °C with moisture content from 6% to 36% w.b. Dielectric properties of almond and walnut kernels at the temperature between 20 and 60 °C with moisture content of 3% in the range of 1-1800 MHz were reported by Wang et al. 27 . Up to now, dielectric properties of almond kernel over different moisture contents at RF and MW ranges are still not available for dielectric pasteurization.
The objectives of this study were to (1) determine the dielectric properties of almond kernel over the frequency range from 10 to 3000 MHz, five moisture contents (4.2-19.6% w.b.), and the eight temperatures between 20 to 90 °C, (2) provide the empirical equations describing dielectric properties of almond kernels as a function of moisture content and temperatures at interested frequencies (27.12, 40.68, 915 and 2450 MHz), (3) evaluate the penetration depth of electromagnetic energy into the almond kernels at four industrial application frequencies, and (4) confirm the RF heating rate of almond kernels at three moisture content levels using experiment and computer simulation.

Results
Density, moisture content and water activity of almond kernels. Table 1 showed that as the moisture content of almond kernel increased, the true density was within 1.039 to 1.060 g/cm 3 , which are in agreement with the trend of peanut and pistachio kernels reported by Ling et al. 16 and Zhang et al. 17 . The true density increased slightly since the increased weight of almond kernels was higher than its volume expansion on moisture gain. Because the density changes in almond kernels were negligible, the density effect of the almond kernels was not considered in this study.
Moisture sorption isotherm of almond samples at 25 °C is shown in Fig. 1. The water activity of samples increased with increasing moisture content. For almond kernel, each moisture content level corresponded to a fixed water activity. Therefore, water activity dependent dielectric properties could be used to develop practical applications for quickly estimating the moisture content of almond kernel.
Frequency-dependent dielectric properties of almond kernels. Figures  and loss factor values were less than 4 at all measured temperatures due to low moisture and high fat contents in the almond kernels. The dielectric constant of almond kernels decreased with increasing frequency when the frequency was below 1600 MHz, and then increased above 1600 MHz, the peak value for dielectric constant was between 2000 and 2800 MHz. The loss factor peaked in the range between 500-1000 MHz. This phenomenon was completely disappeared when the moisture content was higher than 11.9% w.b. At the moisture content of 19.6% w.b., both dielectric constant and loss factor decreased with increasing frequency at all temperatures. Temperature and moisture content of samples had a significant effect on the frequency-dependent dielectric constant and loss factor, especially within the RF ranges. For example, when the frequency increased from 10 Tables 2 and 3. Regression models for dielectric constant were  shown in Eqs (1-4), and regression models for dielectric loss factor were shown in Eqs (5)(6)(7)(8). Quadratic polynomial relationships were best fit for dielectric constant and loss factor at 27, 40, 915 and 2450 MHz. ANOVA results for Eqs (1-4) and Eqs (5)(6)(7)(8) were shown in Tables 4 and 5. All the equations provided a good fit with a coefficient of determination (R 2 ) greater than 0.967 at the significance level of 0.0001 (p < 0.0001). The data suggested that the dielectric constant and loss factor for almond kernels at any given moisture content between 4.2% and 19.6% w.b. and temperature range from 20 to 90 °C at the four specific frequencies (27,40,915 and 2450 MHz) can be precisely predicted through these models. Also these models can be applied to estimate moisture and temperature-dependent dielectric properties used in computer simulation. Penetration depth. The penetration depths calculated from the obtained dielectric constant and loss factor of almond samples at four specific frequencies, five moisture levels, and eight temperatures are shown in RF heating rates of almond kernel at three moisture contents. The experimental and simulated temperature-time histories of almond kernels are shown in Fig. 6 with three moisture contents of 4.2%, 11.9% and 19.6% w.b. when subjected to RF heating for 5 min with electrode gap of 120 mm. It is shown that the computer simulation and experimental data agreed well with each other under three moisture levels. The sample temperatures increased almost linearly with the RF heating time but the heating rates of almond kernels with 11.9% was larger than those at 4.2% and 19.6% w.b., which were similar to those found by Huang et al. 28 since increasing loss factor caused an initial increase and then a decrease in RF heating rates.   of 1000-1600 and 500-1000 MHz 18,27 , respectively. These frequency-dependent trend differences may be caused by complex interaction among frequency, temperature, moisture and fat content 29 . However, when the moisture content was higher than 11.9% w.b., both dielectric constant and loss factor decreased with increasing frequency. Similar trends of frequency-dependent dielectric properties have been reported for nuts, wheat, beans, fruits and vegetables 18,30-33 . That's because with higher moisture content, dipolar rotations may play a dominant effect on the dielectric properties 18 . The dielectric constant and loss factor increased with increasing moisture content. Similar trends at a given frequency have also been found in red pepper powder, chestnut flour and macadamia nut kernels 18,34,35 . At the moisture content of 4.2% w.b., little increase in the dielectric constant and loss factor of samples was observed when the temperature ranged from 20 to 90 °C. That's because at low moisture content, most water molecules are bound to starch or proteins in the nut kernels, which caused little ionic conduction and less free water dispersion 36 . Moreover, the dielectric constant and loss factor of almond kernels increased with increasing temperature since higher temperature reduced viscosity of biomaterials, which may raise ionic conductivity 18 .
The dielectric constant of samples determines the electric field distribution when the loss factor is far smaller than dielectric constant. Therefore, optimum RF heating uniformity in almond kernels could be achieved using a container material with similar dielectric constant to the sample 28 . For example, Huang et al. 37 improved the RF heating uniformity of dry soybean samples by polystyrene container compared to polypropylene one through both experiment and computer simulation. The corn samples in the polyetherimide container had a better RF   Where ε′ container is the dielectric constant of the surrounding material and ε′ sample is the dielectric constant of the sample. Thus, to improve the RF heating uniformity, appropriate surrounding material should be chosen according to dielectric constant of almond kernels and the above equation 39 . Loss factor measures the energy dissipated in the materials from the applied electric field 29 . The higher the loss factor, the higher rate of temperature increases. The behaviour of decreasing moisture content caused low loss factor at RF and MW frequencies may lead to low sample temperature, which is potential drying advantages, referred to "moisture levelling effect" 40,41 . During RF or MW heating, almond kernels with high moisture content absorb more energy, and thus heated preferentially and more rapidly, causing more water vaporization. Since the vapour pressure gradient could be developed from the kernel centre to the surface, surface hot air heating in combination with internal RF and MW heating might be an appropriate way to improve drying efficiency. The higher sample temperature resulted in larger loss factor, this phenomenon is referred to "thermal run away" during pasteurization of almond kernels using RF or MW energy 42,43 . The temperature distributions need to be uniform before and during the RF or MW heating.
Due to larger penetration depths at RF ranges than at MW frequencies for almond kernel with the same moisture content and temperature, the uniformity and throughput under RF heating could be better than MW heating, providing practical large scale treatments for pasteurization of almond kernels.  Table 6. Penetration depth (cm) of electromagnetic wave into samples calculated from the measured dielectric properties of almond kernels at four specific frequencies over three temperatures and five moisture contents.

Materials.
The variety of almond kernel used in this study was "Nonpareil", which was purchased from Paramount Farming Company (Modesto, CA, USA). The almond kernel was sealed into polyethylene bags and stored at 4 ± 1 °C until testing. The initial moisture content (MC) and water activity (Aw) of almond kernels were 4.18 ± 0.02% w.b. and 0.47, respectively.

Sample preparation.
To prepare almond samples with five needed moisture content levels (4.2, 8.0, 11.9, 15.8, and 19.6% w.b.) for dielectric properties measurement, 200 g of almond kernels with the original moisture content (4.2% w.b.) were placed in 5 prepared plastic bottles. Predetermined amounts of distilled water were sprayed on the almond kernels. Then, the bottles were tightly capped and placed in a refrigerator at 4 °C for 7 d. During this period, they were thoroughly mixed three times a day to ensure the uniformity of moisture distribution.
Moisture content, water activity and density measurement. MC of almond kernel was determined according to the AOAC Official Method 925.40. Aw of almond kernel was determined by a water activity meter (Aqua Lab 4TE, Decagon Devices, Inc., Pullman, WA, USA). The true density of almond kernels at different moisture content was determined at room temperature using the liquid displacement method. Toluene (C 7 H 8 ) was used as the displacement liquid due to low surface tension 44 and little absorption by almond kernels. Detailed measurement procedures can be found by others 16,17,45 .

Dielectric properties measurement.
The open-ended coaxial probe method is widely used to measure dielectric properties of many materials over a broad frequency range due to its easy operation and high accuracy. It is usually used to measure liquid or semi-solid materials. However, for irregularly shaped nut kernel, ground samples of the nut were used for measuring dielectric properties since they can be closely contacted to flat tip of the probe [16][17][18]23 . Dielectric properties of ground almond kernels were measured in two replicates at 20, 30, 40, 50, 60, 70, 80, and 90 °C between 10 and 3000 MHz using an open-ended coaxial probe system (Fig. 7). This system consisted of an impedance analyzer (E4991B-300, Keysight Technologies Co. LTD., Palo Alto, California, USA) with a calibration kit (E4991B-010), a high-temperature coaxial cable, the coaxial probe with keysight dielectric probe kit (85070E-020), a custom-built sample test holder, an oil circulated bath (SST-20, Guanya Constant Temperature Cooling Technology Co. LTD., Wuxi, China) and a computer. Before the measurement started, the impedance analyzer and auxiliary computer were turned on and kept in a standby condition for at least 30 min. Then the E4991B calibration kit was used to calibrate the coaxial probe with open, short and 50 Ω resistance in order. The impedance analyzer system was calibrated by air, short and 25 °C distilled water in sequence. Once the system was calibrated, the samples were put into custom-built sample test holder (21 mm in diameter and 50 mm in height) and confined with a pressure spring to ensure a close contact between the tip of the coaxial probe and the sample during the measurements. The sample MC was maintained during the measurements because the sample holder was air tight. The sample temperature was controlled by circulating oil from an oil bath into the jacket of the test holder. The sample temperature during the measurement was monitored by a thermocouple (HH-25TC, Type-T, OMEGA Engineering Inc., Stamford, Connecticut, USA). After each measurement, the probe and the sample holder were cleaned with deionized water and wiped dry.