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。     

水分和盐分对Cheddar干酪介电特性的影响

利用同轴探头技术,研究室温(22℃)下10~3000 MHz频段内不同水分质量分数(25.76%~36.94%)和不同盐的添加量(0~3%)对干酪(成熟30 d)介电特性的影响。结果表明,干酪的相对介电常数和介电损耗因子随水分质量分数的增加而增大。相对介电常数因加盐明显减小,而介电损耗因子随盐分质量分数的增加线性增加。在特定频率200、915、2450 MHz下水分和相对介电常数相关性极显著,与介电损耗因子相关性显著。在添加盐的干酪中,盐的添加量和介电损耗因子相关性极显著,盐分质量分数对介电损耗因子影响极显著。     

不同等级兰州百合的电学特性

以不同等级兰州百合为研究对象,在50Hz～1MHz频率范围内,利用平行极板法研究电特性参数与百合等级的关系。结果表明:在同一加载频率下,随百合等级的提高,其相对介电常数和电导增大,复阻抗减小;随频率的增大,复阻抗和相对介电常数均减小,电导增加。因此,复阻抗、相对介电常数和电导可作为判断百合等级的指示参数。并联等效电感、相位角、品质因数和损耗因子则与百合等级的关联性规律不强。     

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。     

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。     

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.     

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。     

静压力对苹果介电特性和胞内水分的影响

对苹果施加不同大小静压力,同时利用平行板电极法监测其介电参数,用针状电极监测其电阻抗图谱,根据Hayden等效电路模型计算分析了胞内电阻、胞外电阻和细胞内、外液体积分布的变化。结果表明:静压力小于120 N,相对介电常数和损耗因子随静压力的增大而非线性增大,大于120 N后线性增大;同一静压力下,相对介电常数随加载频率增大而变小,损耗因子则在加载频率为20 k Hz时有极小值;随静压力增大,苹果组织胞内电阻增大,胞外电阻在小于120 N阶段基本保持水平,大于120 N后开始增大。在静压力增大至250 N的过程中,细胞外液体积百分比由5.69%增加至9.22%,苹果相对介电常数、损耗因子和细胞外液体积百分比之间呈极显著正相关,相关系数大于0.94,说明细胞外液的体积变化是引起果品介电参数变化的主要原因。本研究有望为果品受压过程中内部生理状态的同步监测研究提供新思路,也将为采用介电特性法快速评价其损伤提供理论指导。     

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。     

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。     

真空醋渍黄瓜品质的介电检测

通过测量醋渍黄瓜的介电特性来衡量其品质并作为一种潜在的非损伤性评价手段。研究采用平行板电极测量了经过不同时间真空醋渍处理的黄瓜于100 kHz~12 MHz的介电特性,同时测量了反映醋渍黄瓜理化品质的参数,如酸度、盐度、亚硝酸盐含量、质构参数。研究结果表明,在整个频率区间,真空醋渍黄瓜的相对介电常数和介电损耗因子都随频率的增加而单调递减,真空醋渍时间与介电参数存在负相关关系。真空醋渍黄瓜的介电参数与盐分和亚硝酸盐含量存在着较好的相关性,然而介电参数与酸度、质构指标之间的相关性不明显。在2.26 MHz和12 MHz频率下,相对介电常数与盐分呈现出良好的线性关系,线性拟合方程的决定系数R2高达0.9813和0.9858;该研究为真空醋渍蔬菜快速检测仪器的开发提供了理论参考。     

全脂乳粉和脱脂乳粉在1kHz～10MHz下的介电特征研究

分别研究了各4种不同品种的全脂乳粉和脱脂乳粉在1kHz～10MHz波段的介电特性。结果发现:随着频率的增加,全脂乳粉和脱脂乳粉的ξ’和ξ″值呈单调递减趋势;全脂乳粉的ξ’和ξ″值都高于脱脂乳粉,相同类型乳粉各品种间的ξ’和ξ″差异不大;全脂乳粉的ξ″值在103～105Hz频率段减小趋势明显,且ξ″值的对数与频率的对数成线性反比关系,而脱脂乳粉无此规律;全脂乳粉和脱脂乳粉的穿透深度Dp随频率的提高而减小,两类乳粉的介电特性差异可能与它们各自的脂肪和乳糖含量有关。     

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.     

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-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 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.     

发育后期苹果的介电特性与理化特性的关系

为了解发育后期苹果的介电特性、生理特性和内部品质的变化规律以及三者之间的关系,采用同轴探头技术测量发育后期3 个月内‘富士’苹果在20～4 500 MHz间的相对介电常数ε’和介质损耗因子ε”,同时测量苹果果实的生理特性（乙烯释放量和呼吸强度）和内部品质（可溶性固形物含量（soluble solids content,SSC）、硬度、pH值和含水率）。结果表明：ε’随频率的增加而减小,ε”在2 000 MHz附近存在极小值;随着果实逐渐发育成熟,ε’和ε”逐渐减小。整个发育后期,果实的SSC和果汁的pH值呈上升趋势,硬度呈下降趋势;乙烯释放高峰出现在2 个呼吸高峰之间。ε’和ε”与SSC、硬度和pH值在特定的频率范围内存在较好的线性相关性。研究说明,苹果的介电特性是生理特性与内部品质的综合反映,根据介电参数值判断苹果的成熟情况以及用单一频率下的介电参数值评价苹果的内部品质均可行。     

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 Properties of Pea Puree at 915 MHz and 2450 MHz as a Function of Temperature

Dielectric properties [dielectric constant (e′) and loss factor (e″)] of 20% (w/w) pea puree with and without fortification of 3.5 mg thiamin/ 100g pea puree at 915 MHz and 2450 MHz were measured from 25 to 125°C by the cavity perturbation techniaue. Thiamin did not affect dielectric properties at ihe concentration studied. The dielectric constant decreased as frequency and temperature increased. The loss factor increased as temperature increased at 915 MHz. However, the relationship between loss factor and temperature was not direct at 2450 MHz due to the contributions of dipolar and ionic losses.