ABSTRACT
Food products are dielectric materials. The ability to store and dissipate electromagnetic energy of food in dielectric heating is determined by its dielectric properties. To develop pasteurisation protocols based on radio frequency (RF) and microwave (MW) heating of chili powder, dielectric properties of chili powder were measured using an open-ended coaxial-line probe and an impedance analyzer over a frequency range from 8 to 3000 MHz. Penetration depth of chili powder was calculated at five selected frequencies. Results showed that the dielectric properties of chili powder decreased with increase in frequency from 8 to 3000 MHz, but increased with increase in moisture content from 4 to 15% (w. b.) and with increase in temperature from 25 to 85°C. Dielectric constant and loss factor of different density samples showed little difference at low temperature, but large difference at high temperature. RF waves have larger penetration depth in chili powder than MW, and therefore could result in better heating uniformity especially for thicker samples during pasteurisation process. Geometry of sample and heating runaway phenomena may also affect the application of RF pasteurisation for foods. The relationship between moisture content, temperature, and dielectric properties of chili powder at 13.56, 27.12, 40.68, 915 and 2450 MHz can be described by third-order polynomial models with high coefficient of determination (R2>0.89), which can be used to predict temperature- and moisture-dependent dielectric constant and loss factor of chili powder with high accuracy.
Introduction
Recent salmonellosis outbreaks related to low-moisture foods such as chili powder, peanut butter, whey powder, non-fat dried milk, etc. have raised concerns about their safety.[Citation1,Citation2] Although foodborne pathogens do not grow in these foods, they can survive for long periods of time. Spices, traditionally available as low-moisture food, have been reported to show Salmonella contamination, and sometimes international food trade between countries is impacted due to Salmonella spp contamination.[Citation3]
Spices and herbs are natural products and may carry a large microbial load, among them several foodborne pathogens. Furthermore, the spices are cultivated in various areas of the world, mainly in developing countries where food sanitation is often poor compared to the situation in western countries. These cause severe problems, which lead to an increased number of foodborne infections and intoxications.[Citation4] Owing to their low-moisture contents, spices are non-perishable commodities, but once they get in contact with water-rich food products, microbial populations may develop quickly.
Fire-drying and sun-drying are commonly used in spice production which may cause severe problems, since the spices are exposed to flue gas and contaminants like insects, birds, and rodents. Furthermore, high microbial loads of up to 8 log CFU/g may be found in the raw materials depending on the plant parts used, their origin and climatic, harvesting, processing, storage and transport conditions.[Citation5]
Between April and September 1993, a nationwide outbreak of salmonellosis traced to contaminated paprika and paprika-powdered potato chips occurred in Germany. An interesting observation from this outbreak was that even low levels of Salmonella contamination (≥0.04 salmonellae per gram) caused illness in 1000 cases.[Citation6] Salmonella is known to have a high tolerance to stress such as desiccation and has actually been reported to have been isolated from low-moisture food.[Citation7,Citation8] In this case, the spices were added to ready-to-eat (RTE) foods indicating the need for effective measures to be taken prior to spices being marketed. Pathogens do not grow in dry spices. However, when they are added to RTE foods, the opportunistic pathogens can grow at high water activity and cause foodborne illness.
Dry heat has traditionally been used to inactivate microorganisms in food powders such as egg white powder. Conventional hot air/steam heating takes long time to achieve desired sterilisation of the powder because heat resistance of dehydrated microorganisms is many times higher than the resistance of same organism in aqueous medium.[Citation9] The low thermal conductivity of powered product also causes longer time to heat the sample to the desired sterilisation temperature. Lengthy heat treatments adversely affect heat-labile nutrients, such as key vitamins. Traditional thermal processing takes such a long time so that the low-moisture products are rendered with poor quality products.
Radio frequency (RF - 13.56, 27.12, 40.68 MHz) and microwave (MW - 915 and 2450 MHz) pasteurisation are new electromagnetic heating pasteurisation technologies, which are used for industrial, scientific and medical applications.[Citation10] When the dielectric materials are exposed to the electromagnetic waves of RF and MW, the electromagnetic energy is converted into thermal energy to heat the products in a volumetric manner. Electromagnetic heating pasteurisation requires less processing time and can result in improved final product quality and reduced treatment cost.[Citation11] RF pasteurisation can be applied as an alternative method for low-moisture foods, such as chili powder.[Citation12,Citation13]
To understand, develop, and improve the RF and MW heating of chili powder for pasteurisation, it is important to measure the material properties of chili powder. Food products are dielectric materials that can store energy like a capacitor and dissipate energy like a resistor in the electromagnetic field.[Citation14] The ability to store and dissipate electromagnetic energy is described as dielectric constant (ε’) and dielectric loss factor (ε”), respectively. The dielectric properties (ε) are combinations of dielectric constant and loss factor, which can be described as a complex number as shown in Eqn. 1.
where . There are many factors that can influence dielectric properties (relative permittivity), such as frequency, temperature, density, moisture content, and chemical composition of food products.[Citation15–Citation17] The dielectric properties of chili powder have been determined by measuring the parallel capacitance (Cp) and resistance (Rp) with and Inductance Capacitance and Resistance (LCR) meter. The measured values of Cp, Rp by the LCR meter were used to calculate the values of dielectric constant and loss factor.[Citation18] However, the dielectric properties of chili powder were only studied in RF spectrum (1 to 30 MHz) at 22°C. To develop pasteurisation protocols based on RF and MW heating of chili powder, the density-, temperature-, and moisture-dependent dielectric properties of chili powder need to be studied. Therefore, the objective of this study was to measure the temperature-, moisture-, and density-dependent dielectric properties of chili powder at RF and MW frequency range of 8 to 3000 MHz.
Materials and methods
Chili powder
Chili sample was obtained from a local farm supermarket (Yangling, China). The sample was dried in a oven and grounded into ten mesh flour in a coffee grinder. Then, the grounded sample was sealed in a valve bag and packed up in corrugated carton to maintain the moisture content during storage. The initial moisture content of sample was 10.0% (wet basis, w. b.). The ash content and moisture content were measured in the laboratory following the standard procedures of AOAC 923.03 and AOAC 986.21, respectively, in triplicates. Other compositions were determined from the sample label provided by the manufacturer. The compositions of the chili sample were showed in .
Table 1. Chili powder compositions
To increase the moisture content to 15%, deionised water was added into the sample with moisture content of 10%, before sealing in a sterile homogeneous valve bag. The bag was shaken three times a day for 4 days in the refrigerator at 4°C to make the water distributed evenly. To decrease the moisture content to 4% and 7%, the original chili powder sample was put into a rectangular container made of polypropylene and was dried in an oven at 40°C until the average moisture content reached the desired ones. After drying, samples were blended in a polypropylene bag and placed in the refrigerator at 4°C for 4 days. The bag was flapped and shook three times a day to make the moisture content distributed uniformly.
The dielectric properties of chili powder were measured at four densities. The samples with different weight ranging from 6.5 to 9.3 g were added into a cylindrical stainless steel sample test holder for dielectric properties measurement at densities of 430, 490, 550, and 610 kg/m3 (). The test holder had an inner diameter of 21.0 mm and depth of 57.0 mm. Sample thickness in the test holder was 44.0 mm. The test holder was tapped 10 times to spread the sample uniformly. A coaxial probe used for dielectric properties measurement was fixed at the top and compressed the sample to make a good contact with the sample.
Dielectric properties measurement
The dielectric properties of the ground chili powder samples were measured using an open-ended coaxial method. The schematic diagram of the dielectric properties measurement system was showed in . The system consisted of an impedance analyzer (E4991B-300, Keysight technologies co., LTD.), a custom-built stainless test holder with a jacket which circulated with silicone oil, a SST-20 oil circulated bath (Wuxi Guanya constant temperature cooling technology co., LTD.), a high-temperature coaxial cable, and a coaxial dielectric probe (85070E-020). The silicone oil was circulated in the test holder jacket to heat or cool the sample in the holder and maintain the temperature. The sample temperature was controlled from 25 to 85°C at an interval of 15°C. The dielectric probe was installed through a solid sanitary end cap and was sealed with an O-ring. The probe and end cap were inserted at the top end of the test holder and sealed with a gasket held in place with a sanitary clamp. A stainless steel spring and stainless steel piston were used to keep the sample in good contact with the dielectric probe. To determine the sample temperature, a thin diameter of 1.02 mm rigid stainless steel thermocouple probe was inserted through the center of a spring and piston into the center of the sample. A pre-calibrated type-T thermocouple temperature sensor was used to monitor the central sample temperature.
The impedance analyzer and auxiliary computer were standby for at least 30 min before the measurement. An E4991B calibration kit was used to calibrate the impedance analyzer, with an open, a short, and a 50 Ω resistance in order. The coaxial probe was calibrated by air, short and 25°C deionised water in order. The high-temperature coaxial cable, probe and test holder were fixed on a custom-designed frame during the process of calibration and measurement to reduce test error. After the calibration procedure was completed, the dielectric properties of chili samples were measured at four densities (430, 490, 550 and 610 kg/m3), four moisture contents (4%, 7%, 10% and 15%), five temperatures (25, 40, 55, 70 and 85°C) at frequency range of 8 to 3000 MHz. This method was similar to studies about caviar, chickpea flour and almonds and walnuts.[Citation19–Citation21]
Calculation of penetration depth
The penetration depth (dp in m) was defined as the depth where the power is reduced to 1/e (e = 2.718) of the power at the surface when a planar wave penetrates normally to the product. It was calculated according to Von Hippel as Eqn. 2.[Citation22]
where c is the speed of light in free space (3 × 108 m/s) and f is the frequency (Hz). After obtaining the dielectric properties, the penetration depths of chili powder samples were calculated at the measured four moisture contents, five temperatures, and selected five frequencies (13.56, 27.12, 40.68, 915 and 2450 MHz).
Results and discussion
Effect of frequency on dielectric properties
The dielectric properties of chili powder at five selected RF and MW frequencies, four moisture contents and five temperatures at the natural density of 490 kg/m3 were showed in . Dielectric constant decreased considerably with increasing frequency, especially for samples with higher temperature and moisture content. The same trend was appeared to loss factor. The values of dielectric constant and loss factor were 2.41 and 0.21 respectively at room 25°C and 27.12 MHz, which is similar in extent to that of chili powder (6.85 and 0.39) studied by Ozturk et al.[Citation17] Loss factors were very low especially for lower moisture content chilies. It might be due to the irreducible water relaxation, less ionic conduction and free water loss in low-moisture content samples.[Citation23] This result was similar to that reported for almonds and walnuts.[Citation21,Citation24] Moisture content and temperature had a significant impact on dielectric constant and loss factor. At high moisture content and temperature, dielectric constant and loss factor decreased notably with the increase of frequency, especially in RF spectrum. The frequency-dependent dielectric constant and loss factor of chili powder with moisture content of 10%, density of 490 kg/m3, at five temperatures were showed in . When moisture content was 10%, with frequency increased from 8 to 3000 MHz, dielectric constant decreased from 37.09 to 5.20, dielectric loss factor changed from 112.86 to 2.15 at 85°C; While dielectric constant deceased from 2.61 to 1.83, dielectric loss factor from 0.29 to 0.12 at 25°C. Similar results have been reported in rape seeds, grains, fruits, and egg products.[Citation25–Citation30]
Table 2. Dielectric properties of chili powder at five frequencies, four moisture contents and five temperatures at density of 490 kg/m3
Figure 1. Schematic diagram of dielectric properties measurement system.
Moisture and temperature dependent dielectric properties
The moisture-dependent dielectric constant and loss factor of chili powder at five selected temperatures, density of 490 kg/m3 and frequency of 27.12 MHz were showed in . The dielectric properties of sample with 4% moisture content had the smallest value in all conditions. The dielectric constant was 2.06 and dielectric loss factor was 0.02 at 25°C. The dielectric constant of chili powder was 5.74 and dielectric loss factor was 2.40 at 85°C. The sample with 15% moisture content had the biggest dielectric property values at all conditions. The dielectric constant of chili powder was 3.90 and dielectric loss factor was 0.91 at 25°C; the dielectric constant of chili powder was 54.15 and dielectric loss factor was 336.44 at 85°C. The increase of dielectric properties with increase in moisture content could be attributed to less dissolved ions and free water in low-moisture content food. Most water was bounded by protein and fat.[Citation31] With the increase of moisture content, single bound water became into muti-layer combined water or free water, and more free water could be used to dissolve ions. Similar trend of moisture-dependent dielectric properties of chili powder were also observed at other temperatures, densities, and frequencies.
Figure 2. The frequency-dependent dielectric constant and loss factor of chili powder with moisture content of 10%, density of 490 kg/m3 at five temperatures.
The temperature-dependent dielectric constant and loss factor of chili powder at two selected frequencies with 27.12 and 2450 MHz and two moisture contents with 4% and 10% were showed in (a, b). The moisture-dependent dielectric properties of chili powder at two selected frequencies with 27.12 and 2450 MHz and two temperatures with 25 and 85°C were showed in (c, d). The dielectric constant and loss factor increased with the increase of moisture content and temperature, especially at 27.12 MHz. This is due to ionic conduction was the main loss mechanism in low frequency. With the increase of frequency, the contribution of ionic conduction decreased.[Citation32] A rise in temperature led to sharp decline of food material viscosity, so the characteristics of ionic conduction can be enhanced.[Citation33] The results were similar to studies about macadamia nuts and safflower seed.[Citation29,Citation34]
Figure 3. Moisture-dependent dielectric properties of chili powder at five temperatures, density of 490 kg/m3 and 27.12 MHz.
Effect of density on dielectric properties
The density-dependent dielectric constant and loss factor of chili powder at five selected temperatures with moisture content of 10% and frequency of 27.12 MHz were showed in . Dielectric constant and loss factor of different density samples showed little difference at low temperature, but large difference at high temperature, and without a trend. At 25°C, the sample with 610 kg/m3 density had the biggest dielectric constant value of 3.25, and the sample with 430 kg/m3 density had the smallest value of 2.22. Ozturk et al. reported that the dielectric properties first increased and then decreased with the density of several powders, including broccoli, chili and onion powder, tapioca flour, and potato starch, at room temperature and 27.12 MHz.[Citation18] This increasing and decreasing dielectric properties with density was attributed to the caking of powders at high pressure. Food powders are inclined to cake as they are exposed to external compression, and the level of stickiness and caking of food powder is based on the humidity, particle size and composition of the food.[Citation18,Citation35] In this study, the dielectric properties of chili powder increased with temperature, but at different density, this increase didn’t present consistent increase rate at different temperature. The trends of dielectric properties changed with density were not as clear as Ozturk et al.’s research, which may be due to the different range of density. Overall, the dielectric constant and loss factor of low density sample with 430 kg/m3 increased faster than that of higher densities. The caking effect at different density and temperature may contribute to the changing trend of dielectric properties of chili powder.
Figure 4. Temperature-dependent dielectric properties of chili powder at two selected frequencies and moisture contents (a, b) and moisture-dependent dielectric properties of chili powder at two selected frequencies and temperatures (c, d).
Prediction equations of dielectric properties
The dielectric properties of the chili powder at the density of 490 kg/m3, frequencies of 13.56, 27.12, 40.68, 915 and 2450 MHz, moisture content ranging from 4% to 15% and temperature ranging from 25 to 85°C were used for nonlinear regression analysis using SPSS 20.0 for windows (IBM inc., USA). The significant level was P < 0.05. The relationship between moisture, temperature and dielectric properties can be described by third-order polynomial models as shown in . The regression equations all reached the significant level with high coefficient of determination. The regression equations can be used for predicting dielectric constant and loss factor of chili powder at moisture content from 4% to 15% and temperature from 25 to 85°C.
Table 3. Regression equations relating dielectric properties to Moisture content and temperature at five frequencies
Penetration depth
The penetration depth of chili powder at five selected frequencies, four moistures and five temperatures at density of 490 kg/m3 were showed in . The penetration depths of chili powder with moisture content of 10% (w. b.), temperature of 25°C, and density of 490 kg/m3, were 1298.8 and 22.1 cm, respectively, for two frequencies of 27.12 and 2450 MHz. The influence of moisture content and temperature on the penetration depth was showed in . As the moisture content and temperature increased, the penetration depth decreased significantly, especially at 27.12 MHz. At 70°C, with the moisture content increased from 4% to 15%, the penetration depth at 27.12 MHz decreased from 796.41 to 25.64 cm, while penetration depth at 2450 MHz decreased from 5.00 to 2.58 cm. With the temperature increased from 25 to 85°C, the penetration depth at 27.12 MHz decreased from 1298.81 to 24.33 cm, while penetration depth at 2450 MHz decreased from 22.06 to 1.96 cm. The chili powder showed much higher penetration depth at RF than MW at the same moisture content and temperature. The similar results were found in different studies of other materials.[Citation14,Citation27,Citation36,Citation37] Lower penetration depth will lead to more surface heating which can seriously affect the uniformity of RF or MW pasteurisation. Higher penetration depth means more uniform electric field distribution and better heating uniformity. Dielectric heating uniformity was also affected by the geometry of samples. At radio frequencies, the loss factor increases with temperature, which lead to the thermal runaway phenomena and to non-uniform heating. The large penetration depth of RF spectrum makes it more capable of pasteurisation than MW. The obstacle have to be overcome of RF heating before it was used for food pasteurisation.
Table 4. Penetration depths of chili powder at four moisture contents, five frequencies and five temperatures at density of 490 kg/m3
Figure 5. The density-dependent dielectric constant and loss factor of chili powder at five selected temperatures with moisture content of 10% and frequency of 27.12 MHz.
Figure 6. Effect of moisture content and temperature on penetration depth of chili powder at the density of 490 kg/m3, two frequencies of 27.12 and 2450 MHz with temperature of 70°C (a) and moisture content of 10% (b).
Conclusion
The dielectric properties of chili powder at four densities, four moisture contents and five temperatures at frequencies ranging from 8 to 3000 MHz were fully investigated. Generally, both dielectric constant and loss factors decreased with increasing frequency, but increased as moisture content and temperature increased. The influence of density on dielectric property values increased with increasing temperature of samples. Compared with MW, chili powder had higher dielectric constant and loss factor resulting in larger penetration depth at RF spectrum. The regression equations were determined for temperature- and moisture-dependent dielectric properties, which can be used for predicting dielectric constant and loss factor of chili powder.
Additional information
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References
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