Radio Frequency Dielectric Properties of Bulk Canola Seeds under Different Temperatures,Moisture Contents,and Frequencies for Feasibility of Radio Frequency Disinfestation

The parallel capacitances and resistances of the bulk canola seeds were measured to determine the dielectric properties of canola seeds using a radio frequency dielectric fixture at different levels of temperature (30–80°C) and moisture content (5–11% w.b.), over the frequency range of 5 to 30 MHz. The dielectric constant (ε′) increased from 3.82 to 7.85 with increasing temperature and decreasing frequency regardless of the seed moisture contents. The dielectric loss factors (ε”) of the bulk seeds increased with increasing temperature and moisture content and decreased with frequency, ranging between 0.11 and 13.0. The penetration depth of the electromagnetic power in the bulk seeds varied from 1.30 to 48.0 m depending upon temperature, frequency, and moisture content. The distinct correlation of sensitivity of dielectric properties with moisture content was not observed except for that of penetration depth. The dielectric properties of the bulk canola seeds were higher than those of other oil seeds at various moisture contents; it might be affected by relatively higher linolenic acid content among other fatty acids. The large difference of the dielectric loss factors of insect pests cf. those of canola seeds showed potential for radio frequency disinfestation based on selective heating of insect pests in the canola seeds. The dielectric properties determined can be used for simulating temperature distribution within the bulk canola seeds during the radio frequency process.     

Dielectric properties of ground almond shells in the development of radio frequency and microwave pasteurization

To develop pasteurization treatments based on radio frequency (RF) or microwave energy, dielectric properties of almond shells were determined using an open-ended coaxial-probe with an impedance analyzer over a frequency range of 10–1800 MHz. Both the dielectric constant and loss factor of almond shells decreased with increasing frequency, but increased with increasing temperature and moisture content. The absolute value of the slopes of log–log plots between loss factor and frequency increased with increasing temperature at low frequencies, especially at high temperatures and moisture contents. The effective electrical conductivity of shell samples was close to zero at the lowest moisture content (6% w.b.) and 3–9 times larger at 90 °C than 20 °C for the highest moisture content (36% w.b.). A good linear relationship was observed between permittivity and density at 1800 MHz. The power penetration depths at RF range (27 and 40 MHz) were about 6–24 times as deep as those for microwave frequencies (915 and 1800 MHz) at each corresponding temperature and moisture content. It is likely that RF energy may provide uniform heating and high throughput treatments for controlling Salmonella in in-shell almonds after washing.     

Dielectric properties of microwave-baked cake and its constituents over a frequency range of 0.915–2.450 GHz

There is a large market for microwave foods, with one of the most prevalent growth areas being low-density bakery products. The dielectric constant and loss factor of Madeira cake batter and its constituents (sugar, cake concentrate, margarine, flour, egg) were measured at a range of moisture contents (0.429–1.000 kg kg⁻¹, dry basis), temperatures (20–80 °C), and over a frequency range of 915–2450 MHz. In general, for a given material, the dielectric properties of samples increased with increasing frequency. Irrespective of frequency and sample moisture content, the dielectric constant of batter and flour was relatively independent of sample temperature, whereas that of the sugar samples increased with increasing temperature, particularly within the lower moisture range. At 2450 MHz the loss factor of batter and flour samples decreased by a limited amount as temperature was increased, whereas at 915 MHz they were relatively unaffected. The sugar samples showed a significant decrease in loss factor with increasing temperature, irrespective of frequency. During microwave baking, the dielectric properties of Madeira cake batter initially increased sharply and then decreased steadily until the end of the baking process.     

Dielectric properties of chestnut flour relevant to drying with radio-frequency and microwave energy

Dielectric properties of compressed chestnut flour samples with 11.6–48.0% w.b. moisture content were determined with a network analyzer and an open-ended coaxial-line probe over a frequency range from 10 to 4500 MHz and a temperature range from 20 to 60 °C. The results showed that the permittivities of chestnut flour were a function of frequency, moisture content and temperature. Both dielectric constant and loss factor decreased with increasing frequency but increased with increasing moisture content and temperature. The relationship between permittivities and moisture content and temperature at 27, 40, 915 and 2450 MHz could be described by second- or third-degree polynomial models, with the coefficients of determination higher than 0.993. The analysis of variance showed that moisture content and temperature had strong significant effects on permittivity values. The penetration depth decreased with increasing frequency, moisture content and temperature. Large penetration depth at radio frequencies below 100 MHz may provide practical large-scale dielectric drying of chestnut.     

Temperature and Moisture Dependent Dielectric Properties of Chinese Steamed Bread Using Mixture Equations Related to Microwave Heating

In-package pasteurization treatments based on radio frequency or microwave energy can greatly reduce the danger of microbial spoilage for Chinese steamed bread. The mixture equations methods were used to obtain dielectric properties at the frequency from 1 to 2450 MHz for the temperature range of 25–100ºC and the moisture content range of 40.1–48.5%. The effects of frequency, moisture content, and temperature on Chinese steamed bread dielectric properties were analyzed. The Bottcher Equation was adapted for calculating dielectric constant, the Landau and Lifshitz, Looyenga Equation for dielectric loss factor. Both dielectric constants and dielectric loss factors increased with increasing moisture content and temperature. Radio frequency energies had several times deeper in Chinese steamed bread than microwave energies.     

Dielectric properties of guava,mamey sapote,prickly pears,and Nopal in the microwave range

Dielectric properties (DPs) of selected fruits: guava (Psidium guajava), mamey sapote (Pouteria sapota), red prickly pear (fruit from Opuntia streptacantha), and white prickly pear (fruit from Opuntia ficus-indica), as well as nopal (young cactus pads or cladodes from Opuntia ficus-indica) were studied. DPs were determined using the open-ended coaxial probe method at microwave frequencies (500 MHz–2 GHz) and temperatures of 20, 40, and 60ºC. At 20ºC and 915 MHz for unripe fruits, dielectric constant values ranged from 65.7 to 70.9, while the loss factor had values between 8.4 and 20.7. Both dielectric constant and dielectric loss factor were affected by temperature and frequency (p < 0.05). For example, loss factor for red prickly pear at 2450 MHz decreased from 13.88 at 20ºC to 12.8 at 40ºC and 11.7 at 60ºC. DPs of fruits were also affected by their ripening (quantified through the maturity index). In addition, penetration depth decreased with increasing frequency, ranging from 1.98 to 4.80 cm at 915 MHz and from 0.25 to 0.41 cm at 5800 MHz. Results are valuable to develop further applications with microwave technology for these foods, such as microwave-assisted disinfestation treatments or microwave drying.     

Dielectric Properties of Sweet Potato Purees at 915 MHz as Affected by Temperature and Chemical Composition∗

A process for rapid sterilization and aseptic packaging of sweet potato puree using a continuous flow microwave system operating at 915 MHz has been successfully developed. In microwave processing, dielectric properties have a major role in determining the interaction between purees and the electromagnetic energy. The objective of this research was to determine how dielectric properties are affected by temperature and chemical composition of purees derived from thirteen sweet potato cultivars with varying flesh colors. Results indicated that temperature, moisture, sugar and starch content had a pronounced effect (p < 0.001) on dielectric properties measured from 15°C to 145°C at 915 MHz. Dielectric constant decreased with increasing temperature, while dielectric loss factor increased quadratically. Power penetration depth of all cultivars decreased with increasing temperature. Predictive equations were developed for dielectric constant (R²?=?0.82) and dielectric loss factor (R²?=?0.90) as a function of temperature, moisture, sugar, and starch. The predictive equations would be useful in determining the dielectric properties of sweet potato purees for the microwave processing technology.     

DIELECTRIC PROPERTIES of RICE BRAN

The dielectric properties of rice bran have been of interest because of their correlation with moisture content and because to a large extent they determine the absorption of energy in dielectric heating applications. the dielectric properties, namely, dielectric constant, loss tangent, dielectric loss factor and a.c. conductivity of rice bran were measured experimentally at a frequency of 13.56 MHz by a resonance method using a Q-meter along with a coaxial, cylindrical sample holder. In order to study the effects of various factors, namely, the moisture content, temperature and bulk density of rice bran, the experiments were designed and performed based on Response Surface Methodology. the dielectric constant, dielectric loss factor and a.c. conductivity were found to have positive nonlinear relationships, while the loss tangent had a positive linear relationship within the ranges of factors studied, namely 3.95 to 14.05% (w.b.) moisture content, 26.5 to 53.5°C temperature, and 299.5 to 400.5 kg/m³ bulk density.     

Effect of expansion by instantaneous controlled pressure drop on dielectric properties of fruits and vegetables

The instantaneous controlled pressure drop (DIC) treatment is used for creation of a porous structure during drying of fruits and vegetables. DIC is based on high temperature, short time heating followed by an abrupt pressure drop into a vacuum. This abrupt pressure drop provokes auto-vaporization of the superheated liquid, expansion and breaking of the cell walls and instantaneous cooling. This process step is inserted between two drying stages at a moisture content of about 20% wet basis. The use of microwave radiation would provide more rapid and homogeneous heating than using steam in the DIC treatment and hot air during the final stage of drying. For that purpose the dielectric properties of the raw and DIC treated products were measured. The measurements were carried out with an open-ended coaxial probe at a frequency of 915 MHz in the range of temperatures 20-90 °C and moisture content 5-80% w.b. Three regions were revealed for the dependences of the dielectric constant and loss factor on moisture content. At low moisture content, these properties increased linearly with moisture content. At the middle moisture content, the increase was also linear but much steeper. At high moisture content, the dielectric properties were constant. The limits of these regions were different for ε′ and ε″ as well as for the products. The dielectric properties were slightly temperature dependent. The penetration depth increased with decreasing moisture content. The DIC treated products exhibited slightly lower dielectric properties than the raw products.     

Dielectric properties of salmon (Oncorhynchus keta) and sturgeon (Acipenser transmontanus) caviar at radio frequency (RF) and microwave (MW) pasteurization frequencies

Radio frequency (RF) and microwave (MW) heating provide an important advantage of more rapid heat penetration in pasteurization processes for heat labile high value foods, which to date, have only been pasteurized by conductive heating. The objectives of this work were to determine the dielectric constant, loss factor and power penetration depth for salmon (0.8% and 2.3% total salt) and sturgeon (0.20 and 3.3% salt) caviars at RF frequency of 27 MHz and MW frequency of 915 MHz (20–80 °C). The dielectric constant (ε′) and dielectric loss factor (ε″) for salmon and sturgeon caviar increased markedly with increasing temperature at 27 MHz but not at 915 MHz. Power penetration depth was higher at 27 MHz compared to 915 MHz, and in unsalted compared to salted roe. Power penetration depth tended to decrease as temperature increased.     

Dielectric spectroscopy measurements for moisture prediction in Vidalia onions

Microwave sensing offers an opportunity to determine nondestructively the amount of moisture in materials by sensing the dielectric properties of the material. Dielectric properties of Vidalia onions grown in southeastern Georgia were measured with an open-ended coaxial-line probe and network analyzer in the range from 200 MHz to 20 GHz. Frequency dependence and moisture dependence of dielectric properties were analyzed for moisture contents between 8% and 91%. Moisture content was linearly correlated with the dielectric constant at higher frequencies for the entire moisture range. A density-independent function that incorporates both the dielectric constant and loss factor was tested across multiple frequencies and moisture ranges. Use of this function enabled prediction of moisture content with high accuracy (R² = 0.99) up to 40% moisture content.     

Dielectric Properties and Thermal Conductivity of Marinated Shrimp and Channel Catfish

Peeled shrimp (Peneaus spp.) and channel catfish (Ictalutus punctatus) fillets were either mixed with commercial lemon pepper marinade and vacuum tumbled at 4°C for 30 min or soaked in 2% tripolyphosphate solution overnight. Dielectric constant and loss factor of marinated seafood and the penetration depth of microwaves were functions of temperature. When cooking temperature increased, the dielectric constant increased, while the loss factor and depth of penetration decreased. Because of the large variation in thermal conductivity measured for individual shrimp, no correlation between thermal conductivity and temperature was found. At constant temperature, thermal conductivity of 2% sodium tripolyphosphate-treated shrimp was higher than that of both marinated and nonmarinated shrimp as a result of higher moisture content. However, no difference in thermal conductivity was found between marinated and nonmarinated shrimp or catfish.     

Dielectric properties of honey adulterated with sucrose syrup

Sucrose syrup is a common additive in honey adulteration. To provide information for developing a cheap, simple, convenient and rapid sucrose-adulterated honey detector or sucrose content sensor, the permittivities of pure jujube, yellow-locust and milk-vetch flower honey, pure sucrose syrup and honey-sucrose syrup mixtures with sucrose content from 0% (pure honey) to 80% (pure sucrose syrup) were studied from 10 to 4500 MHz with open-ended coaxial-line technology and a network analyzer at room temperature. The correlations between permittivities and sucrose contents were regressed. The results showed that the dielectric constants of all samples decreased with increasing frequency, while the pure honey had higher dielectric constant than pure sucrose syrup. Dielectric relaxation existed in all samples. The maximum loss factor decreased with increasing sucrose content. The relaxation frequency changed very little with sucrose content. Strong negative linear correlation, R² > 0.98, was found between loss factor around the relaxation frequency and sucrose content.     

Dielectric Properties of Red Pepper Powder Related to Radiofrequency and Microwave Drying

To provide useful information for drying red pepper with radiofrequency or microwave energy and designing dielectric drying equipment, dielectric properties (dielectric constant ε′ and loss factor ε?″) of red pepper powder with moisture contents ranging from 10.4 to 30.8 % wet basis were measured at different temperatures (from 25 to 85 °C) over a frequency range of 20–4,500 MHz with a vector network analyzer and an open-ended coaxial-line probe. The results showed that both ε′ and ε?″ of red pepper powder were dependent on electromagnetic wave frequency, moisture content, and temperature. The value of ε′ decreased with increases in frequency over the tested range. The ε?″ decreased with increasing frequency when the frequency was below 4,000 MHz, and increased slightly when it was above 4,000 MHz. Both ε′ and ε?″ increased as either moisture content or temperature increased, and the increases were a little greater at high moisture and temperature than those at low moisture and temperature. The moisture- and temperature-dependent permittivities of red pepper powder at several frequencies of interest could be described by quadratic models. The results of variance analysis exhibited that both moisture content and temperature had significant influence (p<0.01) on permittivities of red pepper powder. The power penetration decreased as frequency, moisture content, and temperature increased. Large penetration depth at radiofrequencies below 100 MHz could be used to dry red peppers in a large scale, while microwave energy could be used for drying red peppers in a small scale.     

Temperature and moisture dependent dielectric properties of legume flour associated with dielectric heating

Dielectric properties data are important in developing thermal treatments using radio frequency (RF) and microwave (MW) energy and are essential in estimating heating uniformity in electromagnetic fields. Dielectric properties of flour samples from four legumes (chickpea, green pea, lentil, and soybean) at four different moisture contents were measured with an open-ended coaxial probe and impedance analyzer at frequencies of 10-1800 MHz and temperatures of 20-90 °C. The dielectric constant and loss factor of the legume samples decreased with increasing frequency but increased with increasing temperature and moisture content. At low frequencies and high temperatures and moisture contents, negative linear correlations were observed between the loss factor and frequency on a log-log plot, which was mainly caused by the ionic conductance. At 1800 MHz, the dielectric properties data could be used to estimate the legume sample density judging from high linear correlations. Loss factors for the four legume samples were similar at 27 MHz, 20 °C and low moisture contents (e.g. <15 g/100 g). At the highest moisture content (e.g. 20 g/100 g) soybean had the highest loss factor at 27 MHz and 20 °C, followed by lentil, green pea, and chickpea. The difference in loss factor among the four legumes did not show clear patterns at 915 MHz. Deep penetration depths at 27 MHz could help in developing large-scale industrial RF treatments for postharvest insect control or other applications that require bulk heating in legumes with acceptable heating uniformity and throughputs.     

基于介质损耗因数的红小豆含水率测量方法研究

以红小豆为对象,研究测量信号频率(1kHz～1MHz)、含水率(13.8%～21.8%)、容积密度(747～851kg/m3)和温度(5～40℃)对介质损耗因数的影响规律,从理论上分析介质损耗因数的变化原因,建立500kHz条件下红小豆的介质损耗因数、容积密度和温度与含水率的数学模型,并验证模型的正确性。结果表明：在1kHz～1MHz频段内,红小豆的介质损耗因数随测量信号频率的增大而减小,却随含水率、容积密度和温度的增大而增大;可用三元二次方程表征含水率与介质损耗因数、容积密度和温度之间的关系,模型的决定系数为0.9882。     

DIELECTRIC PROPERTIES OF SHRIMP RELATED TO MICROWAVE FREQUENCIES: FROM FROZEN TO COOKED STAGES

Dielectric properties of shrimp (Penaeus sp.) were measured at frequencies between 0.3 and 3 GHz from -25 to 70C by using a network analyzer. The relationship between dielectric properties and temperature was determined at 915 MHz and 2450 MHz. Below the freezing point, the dielectric constant (?′) and dielectric loss factor (?″) increased rapidly with increasing temperature as ratio of frozen water in shrimp was decreasing. Above the freezing point, the dielectric constant (?′) decreased as frequency and temperature increased at both frequencies. The dielectric loss factor (?″) did not change significantly with temperature. The relationships between penetration depth and temperature at both frequencies were evaluated from the experimental data.     

Temperature-Dependent Dielectric Properties of Raw Cow’s and Goat’s Milk from 10 to 4,500 MHz Relevant to Radio-frequency and Microwave Pasteurization Process

To offer useful information for milk pasteurization with radio-frequency or microwave heating, the dielectric properties (dielectric constant and dielectric loss factor) of raw cow’s milk and goat’s milk were determined over the frequency range of 10–4,500 MHz and temperature range of 25–75 °C by a vector network analyzer and an open-ended coaxial-line probe. The mathematical models describing the relationship between permittivities and temperature were built, and the power penetration depth was investigated. The results showed that both for cow’s milk and goat’s milk, the dielectric constants decreased with an increase of frequency, and the dielectric loss factor decreased with increasing frequency below 1,000 MHz and increased after that. The dielectric constants decreased with increasing temperature at a given frequency. For raw cow’s milk, the dielectric loss factor almost increased with temperature below about 800 MHz, and decreased with temperature above that. For raw goat’s milk, the loss factor decreased with temperature in whole investigated frequency range. Quadratic equations could be used to describe the relationship between permittivities and temperature at interested frequencies with coefficient of determination higher than 0.96. The penetration depth decreased with increasing frequency. Contrasted to frequency, temperature had less effect on penetration depth. Microwave heating at 915 MHz has great potential for raw cow’s milk pasteurization, while radio-frequency heating at 27.12 and 40.68 MHz and microwave heating at 915 MHz can be used for goat’s milk pasteurization.     

Dielectric Properties of Potato Puree in Microwave Frequency Range as Influenced by Concentration and Temperature

Dielectric properties of potato flour-water dispersions (slurry) were measured in the frequency range of 500–2500 MHz by the open-ended coaxial probe method using a network analyzer as a function of concentration (10–25% w/w) and temperature (20–75^oC). Both commercial and laboratory prepared samples of potato flours were used. Results indicated that the dielectric constant (ε′) decreased with temperature and frequency while increased with concentration. The loss factor (ε″) increased with frequency and concentration; however, temperature showed mixed effect. Both ε′ and ε″ data in conventionally used microwave frequencies (915 and 2450 MHz) were studied as function of concentration and temperature for two sample types using a response surface methodology and found to follow 2^nd order polynomial models. Temperature and concentration contributed significantly on dielectric spectra of potato slurry and the sample source had some effect. A change in ε′ and ε″ above 70^oC could be attributed by starch gelatinization. Penetration depth (D_p) decreased with an increase in frequency and non-systematic with temperature. Addition of salt substantially reduced D_p of potato slurry.     

Dielectric properties of mirin in the microwave frequency range

Dielectric properties of mirin (sweet rice wine for seasoning) at the microwave frequency range of 300–3000 MHz and temperature range of 5–70 °C were measured. The effects of temperature, frequency and liquid concentration on dielectric properties were investigated. Dielectric constant and loss factor were related to frequency by using the modified Cole–Cole equation. The important parameters such as relaxation wavelength, static dielectric constant and ionic conductivity were determined numerically under experimental conditions. It was concluded that the index of spread in the relaxation times depended on mirin concentration and it spread with increasing concentration. Penetration depth was calculated based on the collected dielectric properties data and it had a trough at mixture ratio (v/v) of 1:1 (original:water). The relationship between dielectric properties and temperature was determined at 915 MHz and 2450 MHz for commercial significance.