基于微波干燥的江南年糕介电特性的研究

本文以江南年糕为对象,利用网络分析仪测量并研究了频率(915~2450 MHz)、年糕含水率(16~48%)和温度(25~60℃)对相对介电常数ε′和介质损耗因数ε″的影响;计算了微波穿透深度并分析频率、含水率和温度对其影响;建立了部分频率下年糕介电参数与主要影响因素的关系模型,检验了基于介电参数预测含水率的精度和可行性。结果表明,在915~2450 MHz频率段内,年糕的ε′随频率的增大而减小,ε″随频率的增大而增加,ε′和ε″均随含水率的增大而增加,但随温度的增大而减小;能量穿透深度随含水率、频率的升高而降低,随温度的升高而增加。可用二元三次方程描述介电参数和主要影响因素的关系,且各模型的决定系数皆大于0.99。本研究对了解年糕的介电特性以及含水率检测仪的开发有重要的指导意义。     

芋头的介电特性

利用矢量网络分析仪和末端开路的同轴探头,测量了30~3 000 MHz频段内含水(以质量分数表示)分别为25%、40%、55%、70%、85%的芋头片在20、40、60℃和80℃下的介电特性。研究表明:介电常数与介质损耗因子随频率的增加而降低,介电常数随着含水率的增大而增大;介电常数与介质损耗因子都随着温度的升高而减小;915 MHz下介电常数与介质损耗因子的数学模型的显著水平均小于0.000 1,可预测芋头的含水率;同一含水率与温度的芋头片穿透深度随频率的增加而降低,一定频率下的芋头片的穿透深度随温度的增加而增加。     

冬枣片热风干燥过程介电参数变化规律研究

为确定冬枣片热风与射频联合干燥射频介入的适宜条件,以冬枣为试验材料,应用终端开路同轴电缆技术测量不同频率(1～300 MHz)、温度(25℃、40℃、55℃、70℃和85℃)以及含水率(20％、35％、45％、60％和80％)条件下冬枣片在热风干燥过程中介电参数的变化规律.试验结果表明:1～300 MHz频段内,各温度条件下,冬枣片的介电常数与介电损耗因子均随频率的增大而逐渐减小;对比射频加热常用频段,27.12 MHz时的介电常数与介电损耗因子均明显高于40.68 MHz.射频频率为27.12 MHz时,各含水率条件下,冬枣片介电常数和介电损耗因子均随温度的升高而增大;各温度条件下,冬枣片介电常数随含水率先增大后趋于稳定,介电损耗因子随含水率的增大而增大.通过对各条件下介电参数变化规律的分析与比较,确定冬枣片热风与射频联合干燥中射频介入的适宜条件:频率27.12 MHz、温度55～75℃、含水率(湿基)45％～80％,为后续冬枣片热风与射频联合干燥技术与工艺研究提供理论依据.     

不同含水率稻谷电学物理量变化规律

为探究高水分稻谷含水率与其电学物理量之间的关系,探索电学物理量在高水分稻谷含水率快速检测方面的可行性,以高水分稻谷(含水率23.14%)为研究对象,利用LCR阻抗测试仪测定信号频率1～8 MHz下不同含水率稻谷的13个电学物理量。结果表明,阻抗的相位角、损耗系数、介电常数和等效并联电容与稻谷含水率具有良好相关性。在1～8 MHz频率范围内,稻谷的阻抗的相位角和损耗系数均随着频率的增加呈先增大后减小趋势,介电常数和等效并联电容均随着频率的增加而增大。1 MHz频率下稻谷的等效并联电容与其含水率具有更好的相关性(R2=0.998)。通过对不同含水率稻谷的电学物理量进行研究,为开发一种利用电学物理量预测稻谷含水率的快速检测技术提供理论基础。     

水和盐对猪里脊肉糜介电特性的影响

应用网络分析仪和同轴探头技术,研究室温(25℃)下10～4500MHz 频段内0%～20% 的加水率和0%～4%的加盐率对新鲜猪里脊肉糜射频和微波介电特性的影响。研究结果表明,肉的相对介电常数随着频率的增大而减小;介质损耗因子在约2500MHz 时出现最小值。相同条件下肉的相对介电常数和介质损耗因子随加水率的增加而增大。整个频段内,介质损耗因子随加盐率的增加线性增加,当频率大于约200MHz 时,相对介电常数随含盐量的增加而减小。加水率和加盐率分别对相对介电常数和介质损耗因子有明显的影响,且其间的线性决定系数大于0.98。     

基于电特性的大豆含水率测量研究

为了对大豆含水率进行快速无损测量,提出了利用大豆电特性进行含水率测量的原理及装置。由大豆介质套筒式电容器、信号发生器、信号测量仪组成了检测装置,研究不同测试频率时大豆的介电特性(等效电容、损耗角正切、等效电阻)与含水率的关系。研究表明:合适的测试频率为1kHz;经单纯形法拟合,等效电容与含水率呈幂函数关系。为大豆含水率的快速测试提供了一种新方法。     

基于微波加热的馒头介电特性的分析与研究

基于一种同轴探头网络分析仪技术和混合方程的途径研究微波加热馒头的介电特性。测量馒头在不同密度下的介电特性值,运用回归分析方法得出介电特性与频率的关系,通过比较馒头介电特性混合方程计算值和测量值,得到Bottcher方程适用于计算馒头的介电常数,Landau and Lifshitz,Looyenga方程用于计算馒头的损耗因子。研究频率范围915~2 450 MHz、含水率范围40.12%~48.50%(w.b.)和温度范围25~100℃下馒头介电特性的变化规律。研究表明:馒头的介电常数随着频率的增加而减小,损耗因子随着频率的增加呈现出先减小后很缓慢增大的趋势;介电常数和损耗因子均随含水率和温度的增加而增加。馒头微波加热的穿透深度随着各影响因素的增加而减小,且在25~100℃温度范围内,915 MHz和2 450 MHz微波加热馒头最大厚度分别不超过12.82 cm和10.62 cm。     

频率、温度和含水率对白果粉介电特性的影响研究

以白果粉为研究对象,利用网络分析仪研究频率(1~1000)MHz、温度(30~80)℃和含水率11.6%~31.6%(w.b.)范围内,被压缩的白果粉的相对介电常数ε′和介电损耗因子ε′′随频率、温度和含水率变化的影响;计算了射频穿透深度并分析频率、含水率和温度对其影响,运用回归分析方法得出介电特性与温度及含水率的理论方程。研究表明,白果粉的介电特性与其含水率和温度,以及频率密切相关。白果粉的介电特性随着其含水率和温度的增大而增加,随频率的增加而减小。处于较低的射频频率时介电常数和损耗因子比处于较高频率下的变化更加明显;穿透深度随着频率,样品含水率和温度的增加而减小。研究结果对了解白果粉的介电特性以及白果粉的干燥及保存有重要的指导意义。     

基于电容法的藜麦水分快速检测仪设计

利用LCR仪研究了5~40 ℃下,不同含水率(10.14%~29.55%)藜麦的电容随激励频率(1~100 kHz)的变化规律。基于多元回归分析法,确定了藜麦含水率检测的最佳激励频率(21 kHz)及含水率检测模型。以MSP430F149单片机最小系统板为检测系统核心,结合AD5933阻抗测量芯片、自制同心圆式电容传感器和DS18B20数字温度传感器,设计了一种藜麦水分快速检测仪,并对其进行了精度检验,结果显示:藜麦含水率检测结果的平均相对误差为4.17%,测量时间小于2 s,检测仪取得了较为满意的检测结果。     

915MHz和2450MHz下扇贝柱介电特性的研究

利用介电网络分析仪测量温度20~100℃,频率0.3~3GHz下扇贝柱介电常数和介电损耗因数,从而得出扇贝柱在微波加热过程中温度和频率对介电特性的影响。结果表明,温度一定时,介电常数和介电损失因数都是先随着频率增加而降低,随后变化不显著;频率915MHz和2450MHz时,介电常数与温度成负相关,介电损耗因数与温度成正相关。且915MHz和2450MHz下,穿透深度随温度的升高而减小。扇贝柱介电特性的研究为微波杀菌技术的研究提供了理论基础。     

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.     

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.     

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.     

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.     

Temperature-dependent dielectric properties of honey associated with dielectric heating

Dielectric properties of pure yellow-locust and jujube honey and their water solutions at seven final water content levels from 17.4% to 42.6% were measured with an open-ended coaxial-line probe from 10 to 4500 MHz at 20-80 °C. The results showed that the dielectric constants of pure honey increased with increasing temperature when frequency was above 40 MHz. For honey solutions, the dielectric constant decreased with increasing temperature at the lower frequencies, but increased at the higher frequencies. At any given water content, the relaxation frequency shifted to a higher frequency with increasing temperature, while the loss factor peaks showed little change. There was a strong negative linear correlation between penetration depth and frequency in log-log plot for pure honey at 20-80 °C. The research will be helpful in developing effective honey pasteurization processes with radio-frequency and microwave dielectric heating.     

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 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.     

频率和温度对鲍鱼介电特性的影响

采用同轴探针方法分别测定了20～120℃,频率300～3000MHz内鲍鱼的介电常数和介电损失。温度一定时,鲍鱼的介电常数和介电损失均随频率的升高而减小。频率一定时,介电常数随温度升高,先增大后减小,介电损失随温度的升高而增大。本研究提出了在915、2450MHz下,介电常数和介电损失随温度变化的预测方程,预测值与实验值有较好的一致性。在915、2450MHz下,鲍鱼的穿透深度均随着温度的升高而减小。     

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.