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.     

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.     

Temperature- and Moisture-Dependent Dielectric Properties of Macadamia Nut Kernels

Conventional hot air drying for macadamia nuts takes several weeks, generating great interest in developing advanced drying technologies. Radio frequency (RF) and microwave (MW) heating hold potential for providing fast and uniform drying with acceptable product quality. To clearly understand the interaction between electromagnetic energy and macadamia nuts, information on dielectric properties of the kernels is needed for designing a practical and effective drying process. In this study, dielectric properties of the macadamia nut kernels were measured between 10 and 1,800 MHz using an open-ended coaxial-line probe technique at temperatures between 25 and 100 °C and moisture contents between 3 % and 24 % on a wet basis (w.b.). The results showed that both dielectric constant and loss factor of the kernels decreased sharply with increasing frequency over the RF range (10 to 300 MHz), but gradually over the measured MW range (300–1,800 MHz), which were largely enhanced by increasing moisture content and temperature. Penetration depth decreased with increasing frequency, moisture content, and temperature. Based on this study, uniform drying of macadamia nut kernels in thick layers could be effectively developed using RF energy.     

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 Pretreatment of Rapeseed 1: Influence on the Drying Characteristics of the Seeds and Physico-chemical Properties of Cold-Pressed Oil

The aim of this study was to explore the effect of dielectric pretreatments on the drying characteristics of rapeseeds and physico-chemical attributes of cold-pressed rapeseed oil. Rapeseeds were adjusted to moisture contents of 15% and underwent dielectric pretreatments prior to oil extraction by cold pressing at frequencies of 27, 915, and 2450 MHz for 45, 30, and 22 min, respectively. Results showed that 2450 MHz dielectric heating had the highest temperature rising rate but the most nonuniform temperature distribution. Compared to the control samples, oil extraction yields were increased by 12.28, 17.25, and 22.08% for 27, 915, and 2450 MHz dielectric pretreatments, respectively. The SEM analysis indicated that the cell structures of pretreated rapeseed samples were severely damaged, thereby improving the extraction efficiency. Additionally, dielectric pretreatments significantly increased the total tocopherol content and improved the oxidative stability of cold-pressed oil (p <?0.05). The oil extracted from the 27-MHz pretreated rapeseeds exhibited lower acid and peroxide values and less color change, indicating better quality is achieved. Regarding fatty acid (FA) composition of the oils, oleic acid slightly increased and docosenoic acid decreased, but the total FA composition was not altered significantly by dielectric pretreatments (p >?0.05). These results suggested that dielectric pretreatment, a promising and environment-friendly technique, could be useful in the food industry for oil extraction.     

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.     

Optimization of Short- and Medium-Wave Infrared Drying and Quality Evaluation of Jujube Powder

In this study, response surface methodology was used to optimize parameters of short- and medium-wave infrared drying processing for producing jujube powder. Optimization factors were power input (675–1,575 W), drying temperature (70–90 °C) and drying time (2.5–3.5 h), and responses were moisture content, L value, b value and vitamin C content of the jujube powder. Box–Benhnken center design was applied to the optimized the processing conditions. The optimal conditions for the production of jujube powder by the short- and medium-wave infrared drying method were power input of 1,125 W, drying temperature at 85.4 °C and drying time for 3.5 h. Under the optimal conditions, the values of moisture content, L, b and vitamin C content of jujube powder were 4.59 %, 64.46, 19.87 and 441.56 mg/100 g, respectively. Quality evaluation of the jujube powder was also investigated. The results were: hygroscopicity of 28.83 %, rehydration of 236.44 %, solubility of 53.59 %, bulk density of 0.70 g/ml, particle size of 154.2 μm, soluble solids content of 82.29 %, total sugar content of 46.87 %, reducing sugar content of 32.09 %, total acid content of 2.03 %, total flavonoids content of 1.45 % and cyclic adenosine monophosphate content of 75.32 mg/kg. The microstructure of jujube powder was irregular and 38 kinds of aroma components were generated.     

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.     

Effect of Amino Acids on Microwave Dielectric Properties of Minced Antarctic Krill (<Emphasis Type="Italic">Euphausia superba</Emphasis>)

The dielectric properties of ten solid amino acids, namely, glycine, proline, cysteine, glutamine, lysine, methionine, histidine, arginine, tyrosine, and tryptophan, and the effects of amino acid concentrations (0, 1, 3, and 5%) on the dielectric properties of minced Antarctic krill (Euphausia superba) were studied within a frequency range of 300–3000 MHz at 20.0 ± 0.5 °C. The dielectric constants and dielectric loss factor of amino acids were 2.0 ± 0.4 and 0.2 ± 0.3, respectively (moisture contents, 0.05–0.33%), except those of lysine (5.5 ± 0.5 and 1.5 ± 1.0, respectively) (moisture content, 4.5%). The dielectric properties of non-polar, neutral-polar, alkaline amino acids added to Antarctic krill were affected by the frequency and moisture contents, molecular weight, and isoelectric points (pI) of amino acids. As the molecular weight and pI values increased, the critical frequencies of aliphatic amino acid added to minced Antarctic krill decreased, whereas heterocyclic amino acid added to minced Antarctic krill increased. Quantitative relationships between parameters that define both the real and the imaginary parts of the Debye relations and the weight fraction of amino acid contents in Antarctic krill were determined. Moreover, glycine (at 5%) or lysine (at 5%) significantly influenced the dielectric constants and dielectric loss factor of minced Antarctic krill (p < 0.05) but did not have significant effects on temperature history and distribution (p < 0.05) during microwave heating at 2450 MHz.     

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.     

Contribution of Proteins to the Dielectric Properties of Dielectrically Heated Biomaterials

In this article, we report results on the behavior of selected proteins in solution and on how they could affect the overall absorption and distribution of electromagnetic energy within dielectrically heated biomaterials. Ovalbumin, bovine serum albumin (BSA), β-lactoglobulin (BLG), and lysozyme (Lys) proteins varying in molecular weight, structure, and isoelectric point were systematically screened for this study. Measurements were performed using an open-ended coaxial probe over a frequency range encompassing the industrial, scientific, and medical (ISM) bands (1–1800 MHz) at six concentration levels and 25 °C. Primary findings include discovery of an additional δ-dispersion region in addition to the previously observed two regions between the well-established β- and γ-dispersion regions for protein solutions, which we propose are a sub-set of multiple δ-dispersions in the said region. We hypothesize that the β-dispersion is a summation of these multiple δ-dispersions and their cumulative effect is manifested in the amount of heat generated within a dielectrically treated biomaterial. An individual protein’s contribution to the overall dielectric absorption was quantitatively determined to account for up to 10% of the energy absorbed by free water molecules. We derived a mixture formula that accurately predicted the dielectric constant (ε′) at the critical industrial 915 MHz frequency, which we propose to use in computational simulations for purposes of design and development of dielectric heating devices. Theoretical calculation of the local ε′ of individual proteins from their amino acid makeup resulted in an average of 2.7, closely matching previously reported results. An increase in the physical size of a protein resulted in a decrease in the overall electromagnetic absorption of the mixture. A similar effect was observed as protein concentration increased with more pronounced effects on the dielectric increments (?ε′) and absorption decrements (?ε″); the implication of such effect on energy absorption and heating is discussed.     

Electrical Properties of Fish Mince During Multi-frequency Ohmic Heating

A multi-frequency ohmic heating system with 30 Hz～1 MHz range which could deliver 250 watts was developed for measuring electrical conductivity and absolute dielectric loss of food samples. Pacific whiting surimi paste and stabilized mince in the 20～70°C range were tested at frequencies from 55 Hz to 200 kHz. Sample impedance decreased slightly with frequency. The DC electrical conductivity (σ_dc) and absolute dielectric loss (ε″) of Pacific whiting surimi paste increased with temperature and salt concentration; adc and ε″ of the stabilized mince increased with temperature. Empirical models of electrical properties for surimi paste (moisture content 79% and salt at 1, 2 or 3%) and stabilized mince (77% moisture and 0.74% salt) were derived. Electrolytic corrosion diminished with frequency.     

Quantitatively determining the total bacterial count of raw goat milk using dielectric spectra

The objective of this study was to evaluate dielectric spectra as a means of quantitatively determining total bacterial count (TBC) of raw goat milk. The dielectric spectra, including dielectric constant (ε′) spectra and dielectric loss factor (ε″) spectra, and TBC of 154 raw goat milk samples were measured using network analyzer and plate count methods, respectively. Owing to the poor linear relationship between TBC in logarithm and permittivities at a single frequency, chemometrics was used to reduce noise, identify outliers, select effective variables, and divide sample sets. Several linear models, such as multiple linear regression, ridge regression, and least absolute shrinkage and selection operator, were established to determine TBC based on the effective spectra of ε′, ε″, and their combination (ε′+ε″). The results indicated that the models built using the spectra of ε′+ε″ and ε′ had excellent TBC prediction performance. The best model was multiple linear regression based on ε′+ε″ spectra with the residual predictive deviation of 3.26. This study shows that the dielectric spectra had great potential to quantitatively and rapidly determine TBC of raw milk.     

Determination of microwave drying and rehydration kinetics of green peppers with the bioactive and textural properties

The aim of this study was to determine the effect of microwaves on drying and rehydration kinetics of green pepper varieties (sweet, green, and bell) and the bioactive and textural properties of dried and rehydrated peppers. Drying was performed at 90, 180, and 90 + 180 W and dried peppers were rehydrated at 25, 50, and 70°C. The best drying fits were obtained using the equations of both Sigmoid and Hii et al. for drying, and two term exponential decay was the most appropriate model for rehydration. The lowest and highest final rehydration ratio values were obtained for the peppers dried at 180 and 90 W, respectively at each temperature and pepper variety. Among peppers, bell pepper showed the lowest color change (ΔE) and it was the most potent to recover initial color in rehydration. Peppers had softer texture in both drying and rehydration treatment. Total phenolic content of peppers was reduced around 56–65% compared to the fresh pepper by drying and this decrement was reached up to 87% in rehydrated peppers. Results showed that drying at 180 W and rehydration at 70°C provided faster drying and rehydration as well maintained the quality characteristics of green peppers.     

Effect of salt and sucrose content on dielectric properties and microwave freeze drying behavior of re-structured potato slices

Potato puree was used to prepare re-structured potato slices. The effect of salt and sucrose content on microwave freeze drying (MFD) and the variation of the dielectric properties of the potato slices were studied. Results showed that the dielectric constant (ε′) and loss factor (ε″) of the potato purees decreased significantly after freezing. Both salt and sucrose addition increased ε′ and ε″ of the samples at low temperatures (−25 to −5 °C). The salt content had greater influence relative to sucrose content on dielectric properties of the samples. Sucrose addition shows insignificant influence on dielectric properties of the thawing samples; while for the salted samples, ε′ decreased gradually with increasing salt content and ε″ increased significantly with increasing salt content and temperature. MFD drying rate was significant higher after adding salt and sugar.     

Combination of digital images and laser light to predict moisture content and color of bell pepper simultaneously during drying

Optical devices are being increasingly used for more accurate and faster ways to investigate quality parameters of agricultural products. The main task of this research is to evaluate the use of a CCD camera in combination with laser diodes emitting at 532 nm and 635 nm to monitor changes in moisture content of red, yellow and green bell pepper during drying. Moreover, the feasibility of using the CCD camera alone to analyze color changes during drying, in alternative to the colorimeter, is assessed. Results showed that scattering area and light intensity were able to predict moisture content changes of bell pepper during drying. While excellent correlations were obtained for yellow colored samples (R = 0.93 and RMSEP = 7.28), the behavior of green and red wavelengths in peppers with red and green color, respectively, may raise some doubt on the ability to predict moisture content (R = 0.33 and RMSEP > 22). Additionally, light scattering varied according to the different depths on the surface where the images were taken. Furthermore, it was concluded that a change in tissue structure during drying may have caused dispersion of photons through the surface resulting in varying degrees of scattering. The presented technique based on a digital camera and laser diodes shows the potential to be applied in the future as a fast and modern method for in-line measurements of monitoring moisture content during drying. Digital images in terms of RGB values have been demonstrated to be a promising alternative to monitor changes of L^∗ (R = 0.87 and R = 0.90), and a^∗ values (R = 0.93 and R = 0.97), during drying of yellow and green pepper. Extensive studies are still required to determine the suitability of digital images for analysis of b^∗ values at different colors of bell pepper.     

Effect of water activity and temperature on the color change of red pepper (Capsicum annuum L.) powder

Isotherm characteristics of red pepper powder and the effect of temperature and water activity (Aw) on its color change were investigated. Monolayer moisture contents of red pepper powder decreased from 0.1218 to 0.0912 g water/g solid with increasing temperature from 25 to 50°C. The color change of red pepper powder was greatly dependent on temperature and Aw. As temperature and Aw increased, red color of pepper powder increasingly faded out to become brown and tarnish black, which is mainly attributed to the degradation of carotenoid pigments and development of browning compounds. Color parameters such as Hunter-L, a, b values and other color functions as well as browning index and ASTA color values represent color changes of red pepper powder as influenced by temperature and Aw.     

Effect of Pulsed-Spouted Bed Microwave Freeze Drying on Quality of Apple Cuboids

The effects of novel Pulse-Spouted Bed Microwave Freeze Drying (PSMFD) technology on the quality on natural food products have been investigated. The objective of this research was to study effects of this novel technology on dielectric properties and quality characteristics (moisture content, porosity, microstructure, texture, color, and flavor) of apple cuboids as compared with the conventional drying technologies (air drying and freeze drying). During the first 45 min of drying, the dielectric properties increased due to partial conversion of water from ice to liquid, and then gradually decreased due to the moisture removal. Microwave energy increased sample temperature from minus 20 °C to +?67 °C, which resulted in fast drying to 0.09 g/g within 270 min. Porosity increased almost linearly, reaching 0.87 at equilibrium moisture content. Hardness of apple cuboids increased to 350–450 kPa due to the glass transition in the final period of drying. Better preservation of apple color and volatile compounds demonstrated the benefits of the hybrid PSMFD technology for the production of premium quality dried fruits compared to air drying and freeze drying.     

Physical Property Effects on Drying of Chile Peppers

Understanding effects of pepper fruit physical properties on pepper drying allows for improvement in postharvest handling and processing plus development of better cultivars. Drying tests with jalapeño peppers demonstrated that while dimensions of pepper fruits were decreasing with progressive harvests there was essentially no difference in the drying trends of peppers from the first harvest to the last. Slicing produced a similar drying trend for cultivars distinctly different in physical properties. Speed in drying peppers averaging 89.4% moisture to the 10–11% (w.b.) safe storage level was promoted by slicing as well as drying temperatures as high as 115^oC.     

Investigation of drying characteristics in superheated steam drying of UF-impregnated Chinese fir

Urea formaldehyde (UF) resin-impregnated Chinese fir (Cunninghamia lanceolata (Lamb.)Hook.) was dried at different temperatures in an atmospheric pressure superheated steam dryer. Drying characteristics, moisture content, drying rate, temperature profile, drying defects, and color change were investigated. The moisture content was reduced from 66.21 to 11.79% within 30 h without causing severe drying defects; in contrast, the conventional hot air process required 7–8 days. After 25 h of drying, the temperatures at both the center and the surface of wood remained stable. After 34.5 h, the surface temperature gradually approached the steam temperature. The color of the superheated steam dried Chinese fir appeared slightly more intense yellow and red than the control. Investigation of the UF-impregnated Chinese fir wood by scanning electron microscopy (SEM) revealed that the majority of the lumens and voids, including the microvoids in wood structure, was filled with urea formaldehyde resin.