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 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.     

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 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.     

Characterization of 50-ohm radio frequency heating of bulk canola seeds (Brassica napus. L) in a tubular applicator with parallel electrodes and post-treatment quality

A pilot-scale 50-ohm (Ω) radio frequency (RF) heating system was used to determine the temperature distribution of bulk canola seeds (Brassica napus L.), 9% moisture content (MC) wet-basis (w.b.) in a tubular applicator with parallel electrodes. Non-uniformity of the temperature distribution of bulk canola was observed during the RF heating process of the canola seeds. The hottest spot was observed at the front side of the tubular cavity of the applicator adjacent to the hot electrode. T_avg of the canola seeds was 38 °C, 42.2 °C, and 40 °C at 3 kW, 5 kW, and 7 kW respectively. Temperature distribution was relatively uniform in the back zone (0.287, 0.433, and 0.278 for 3 kW, 5 kW, and 7 kW respectively). The physicochemical properties of canola seeds changed significantly after 50-ohm RF heating at various end temperatures and power levels.Industrial relevance textRadio frequency heating is an emerging technology and has immense potential in food grains, pulses, and oilseeds processing. The radio frequency heating can be used for disinfestation of insects, drying, and removal of anti-nutritional compounds in stored food grains, pulses, and oilseeds. Radio frequency heating is based on dielectric heating principle thus allowing selective and volumetric heating. The radio frequency waves have wavelengths ranging from 1 mm to about 100 km with a frequency of 3 kHz to about 300 GHz, the penetration depth is comparatively higher than microwaves, which has wavelengths ranging from 1 mm to about 1 m with frequencies between 300 MHz and 300 GHz. Thus, radio waves penetrate larger volume objects compared to microwaves. This research investigated a 50-ohm radio frequency heating system with a through field parallel plate type applicator to observe the temperature distribution throughout the bulk canola at different power levels. Understanding the characteristics of 50-ohm radio frequency heating of bulk canola seeds at different power levels can help design effective heating processes involved in canola seeds storage and handling.     

Frequency- and temperature-dependent dielectric properties of fruit juices associated with pasteurization by dielectric heating

Dielectric properties of apple, pear, orange, grape and pineapple juices were studied from 20 to 4500 MHz at 15-95 °C. The dielectric constants ε^′ of fruit juices decreased as frequency increased. The loss factors ε^″ decreased with increasing frequency to a minimum between about 1000 and 3000 MHz, depending on temperature, and then increased as frequency increased. The frequency of that turning point increased, whereas the minimum ε^″ decreased with increasing temperature. The ε^″ values increased with increasing temperature below about 1000 MHz, and decreased with increasing temperature above 3000 MHz. Dielectric constants decreased consistently and linearly with increasing temperature. Linear or polynomial correlations for the dependence of dielectric properties on temperature were developed. The power penetration depth of all fruit juices decreased with increasing frequency. The depth decreased as temperature increased at lower frequencies and increased with temperature above 3000 MHz. The dielectric properties obtained in the study provide important parameters for juice pasteurization by dielectric heating.     

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.     

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 Mashed Potatoes Relevant to Microwave and Radio-frequency Pasteurization and Sterilization Processes

ABSTRACT: Dielectric properties of mashed potatoes relevant to microwave and radio-frequency (RF) pasteurization and sterilization processes were measured over 1 to 1800 MHz and 20 °C to 120 °C. Effects of moisture content (81.6% to 87.8%, wb) and salt content (0.8% to 2.8%, wb) were investigated. Dielectric loss factors and constants decreased with frequency. Dielectric loss factors increased with temperature and salt content, but dielectric constants were not significantly affected. Ionic conductivity played a dominant role at low frequencies. Power penetration depth increased with moisture content and decreased with temperature, frequency, and salt content. Regression equations were developed to relate the dielectric properties to temperature, moisture, and salt contents.     

Dielectric Properties of Beans at Different Temperatures and Moisture Content in the Microwave Range

Dielectric properties of common Mexican beans (Phaseolus vulgaris L.) were determined and analyzed at microwave frequencies (800–2500 MHz). The free-space transmission technique was employed for the measurements of three varieties (“Flor de mayo,” “Bayo,” and “Negro”) with different moisture content (8.8–12.3%, w.b.) at 20, 30, 40, 50, and 60°C. The dielectric constant and loss factor of beans decreased with increasing frequency for a fixed temperature, and increased with increasing temperature at a fixed frequency. The dielectric constant increased with increasing moisture content, while the loss factor remained nearly constant. With these results, disinfestation or quality control measurements can be proposed for beans using microwaves.     

Sugar and water contents of honey with dielectric property sensing

The dielectric properties of pure yellow locust, jujube and rape flower honey and their water-adulterated products with water content from 18% to 42.6% were measured with open-ended coaxial-line probe technology and a network analyzer from 10 to 4500 MHz at 25 °C. Dielectric constants of pure honeys and water-added honey samples decreased monotonically with increasing frequency, and increased with increasing water content. Dielectric relaxation was evident in the dielectric loss factors. The critical frequency and the maximum loss factor increased with increasing water content. There were strong linear correlations between the dielectric constant and the total soluble solids and water contents. The linear coefficients of determination were higher than 0.995 from 650 to 960 MHz. The good linear correlations and the sufficient penetration depth >20 mm below 960 MHz, suggest that microwave dielectric properties could be used in developing sensors to determine sugar and water contents.     

An investigation of the dielectric and thermal properties of frozen foods over a temperature from –18 to 80°C

The dielectric properties (dielectric constant [ε′] and dielectric loss factor [ε″]) for three frozen meals (basil fried chicken, green curry with chicken, and congee with minced pork) were measured at frequency 2.45 GHz from –18 to 80°C. Thermal properties (thermal conductivity [k] and specific heat capacity [c]) of each food item were characterized using their composition for improving the modeling of microwave thawing process of frozen products. For all frozen meals, similar trends in the dielectric and thermal properties values were observed as a function of temperature. In all samples of frozen foods, the dielectric properties (ε′, ε″) rapidly increased with temperature for the range from –10 to 0°C. Thereafter, dielectric properties linearly increased with temperature for basil fried chicken and congee with minced pork but linearly decreased with temperature for green curry with chicken from 0 to 80°C. The dielectric properties data bases were used to calculate power reflected (P_r), power transmitted (P_t), dielectric loss tangent (tanδ), and penetration depth (d_p). These parameters were related to the dielectric properties. The thermal conductivity values of all samples decreased with increasing temperature in the frozen stage and little changed after thawing. While the specific heat capacity values increased first and then do not change considerably. The temperature-dependent material properties can give insight into how the food product interacts with the incident electromagnetic radiation.     

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 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.     

Radio frequency pasteurization and disinfestation techniques applied on low-moisture foods

Abstract

The shelf life of foods is usually limited due to the frequent contamination by pests and microorganisms. Although low risk of pathogen contamination and no growth potential compared to those in high water activity animal- or vegetal-derived products, the low-moisture food has still significantly contributed to the total number of foodborne infections and outbreaks. Radio frequency (RF) treatments can be classified as a dielectric heating, which is a promising technology for achieving effective food pasteurization and disinfestations because of the associated rapid and volumetric heating with large penetration depth. The RF technique could be applied at low-moisture food as both the dipole dispersion and ionic conductivity may play effective roles. It can selectively heat and kill the microorganisms/pests without damaging the agricultural product because of the large difference of dielectric loss factors between target microorganisms/pests and host foods. In this article, the low-moisture foods sterilized and disinfested by RF energy are reviewed through basic theories, dielectric properties, heating effect, and uniformity. The potential research directions for further RF heating applications are finally recommended in low-moisture foods.     

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.     

Effect of Temperature (−5 to 130 °C) and Fiber Direction on the Dielectric Properties of Beef Semitendinosus at Radio Frequency and Microwave Frequencies

ABSTRACT:  The dielectric properties must be defined to design efficient radio frequency (RF) and microwave (MW) processes by the food manufacturers. The objective of this study was to understand how frequency, temperature, and muscle fiber orientation influence the dielectric properties. The eye of round ( Semitendinosus ) muscle was selected because it contains large, relatively uniform muscle cells with similar muscle fiber orientation and relatively uniform chemical composition throughout the tissue. Dielectric properties were measured using an open-ended coaxial probe technique at 27, 915, and 1800 MHz and temperatures between −5 and 130 °C. Power penetration depth was calculated. Since many commercially prepared, thermally processed, ready-to-eat entrees are made with frozen meat, dielectric property measurements were started from −5 °C. The dielectric constant and dielectric loss factors were often higher for muscle with the muscle fiber measured in a parallel orientation to the probe compared to samples of the same treatment (for example, fresh or frozen) in a perpendicular tissue orientation at the same frequency and temperature. Dielectric constant and loss values for frozen beef tended to be higher than fresh beef at the same temperature and frequency. Tissue orientation appeared to have a greater effect on dielectric loss values at lower frequencies. Penetration depth tended to be greater when the direction of propagation was perpendicular to the muscle fiber.     

DIELECTRIC PROPERTIES of STARCH SOLUTIONS AS INFLUENCED BY TEMPERATURE, CONCENTRATION, FREQUENCY and SALT

Dielectric properties of starch solutions (1 to 4% w/w) were evaluated at temperatures ranging from 20 to 80C at 10, 20 and 30 MHz. the effect of added salt (0.2 and 0.5% w/w) was investigated in relation to changes in trends exhibited by the relative permittivity, loss factor and penetration depth. the relative permittivity ranged from 46 to 308 and 65 to 92 for solutions with and without salt, respectively. the corresponding loss factor ranged 266 to 4133 and 9 to 266, respectively. Temperature, frequency, concentration and their interactions had different levels of significance on the dielectric properties of starch solutions. Salt enhanced the relative permittivity, and its effect conformed to the anomalous dispersion phenomenon. the loss factor increased with increasing temperature and salt content, and penetration depths associated with salt‐enriched samples were low compared to samples without salt. Generally, the effects of temperature, frequency, concentration and salt on the dielectric properties of starch solutions were attributed to the complex interaction between conductivity, density, moisture content, loss angle and starch rheological properties. Excellent correlations were developed that could be used for estimating the dielectric properties of starch solutions with and without salt.     

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,heating rate,and heating uniformity of wheat flour with added bran associated with radio frequency treatments

The purpose of the study was to determine influences of dielectric properties (DPs) on radio frequency (RF) heating rate and uniformity and provide essential information for developing effective pasteurization processes of wheat flour. DPs of the wheat flour as a function of frequency, moisture content (MC), wheat bran content (WBC) and temperature were determined using an impedance analyzer. A 27.12 MHz, 6 kW pilot-scale RF system at an electrode gap of 100 mm was used to evaluate the heating rate and temperature distribution in each wheat flour-bran mixture. The results showed that DPs of wheat flour increased with increasing temperature and MC but with decreasing WBC due to high fat content in wheat bran, and their relationship can be represented by cubic or quadratic models. The heating rate of wheat flour decreased first and then increased with increase in MC while decreased with decreasing WBC due to the increasing difference between dielectric constant (ε′) and loss factor (ε″). Decreased WBC also resulted in decreasing uniformity index (λ) value. Reducing MC and WBC in wheat flour-bran mixtures could help to improve RF heating uniformity.