Dielectric properties of thermosonically treated Agaricus bisporus slices during microwave vacuum drying and correlation with the water state

Abstract

The dielectric properties of thermosonically treated Agaricus bisporus slices during microwave vacuum drying were determined, by using a network analyzer and an open-ended coaxial-line probe at microwave frequency (915 and 2450?MHz). The permittivity at 25?°C was also correlated to the water state determined by low field nuclear magnetic resonance (LF-NMR) analysis. Results showed that the dielectric constant ε' and loss factor ε″ of A. bisporus slices were affected significantly by the composition of water state in the sample. Thermosonic pretreatment enhanced the influence of semi-bound water (M₂₂) and bound water (M₂₁) on the permittivities in sample slices during microwave vacuum drying (MVD). The dependency of permittivities on the moisture content and temperature at 915 and 2450?MHz for thermosonically treated A. bisporus samples could be described by third-degree polynomial models, with R² higher than 0.998. Infrared thermal imaging analysis confirmed that thermosonic pretreatment is a potential technology to promote the uniformity of A. bisporus slices during the MVD process.     

Effect of three drying methods on the drying characteristics and quality of okra

The objective of this study was to compare the drying characteristics and quality of dried okra obtained by vacuum freeze drying (VFD), microwave vacuum drying (MVD), and pulse-spouted microwave vacuum drying (PSMVD). The quality parameters include color (L*, a*, and b*), texture (hardness and fracturability), shrinkage ratio (SR), nutrient retention (chlorophyll, flavonoids, and vitamin C), and sensory score was investigated. Vacuum freeze drying maximally preserved the original properties of samples. And the samples dried by PSMVD showed higher quality and more uniformity compared to MVD. Considering of all aspects comprehensively, such as cost-effectiveness, practicability, and scale-up, PSMVD was a promising technique which would match results obtained by vacuum freeze drying.     

Effect of microwave energy dissipation on drying process of berry puree under microwave foam drying conditions

ABSTRACT

Microwave foam drying (MFD) is a novel drying technology particularly for the drying of agricultural products with high viscosity and thermal-sensitive components. The effect of dissipation of microwave energy on the drying process of berry puree was investigated in MFD conditions. The results show that the dissipation of microwave energy induces the vapor pressure inside the berry puree to improve the formation of bubbles. The changes of pressure inside bubbles of berry puree follow Sine equations. The favorable conditions of bubbles formation inside berry puree are the temperature of 70–80°C with the corresponding viscosity of 70–80?pa?s and moisture content of 80–85% (wb). The dissipation of microwave energy in berry puree tends to decaying trend due to the changes of dielectric properties. Moisture content has the most significant positive effect on both dielectric constant and dielectric loss factor of berry puree, followed by the density, and the drying temperature has the least but significant effect on both indicators. The focusing phenomenon of microwave irradiation depended on the dielectric properties determines the distribution of temperature of berry puree. The findings in current work contribute to a better understanding the MFD behavior of berry puree and will help to optimize the drying conditions of MFD for enhancing energy efficiency and drying quality.     

Ultrasound-assisted osmotic process on quality of microwave vacuum drying sweet potato

The effects of pretreatment before microwave vacuum drying (MVD) on texture, color, expansion, rehydration, drying rate, microstructure, sensory evaluation, and other properties of sweet potato were investigated in this study. The pretreatment consisted in five processing conditions, using blanching; osmotic dehydration at 35°Brix of sucrose (OD); ultrasound in distilled water (US); ultrasound in distilled water before osmotic dehydration (US?+?OD), and ultrasound-assisted osmotic dehydration (USOD). Pretreatments of sweet potato before MVD have shown success in reducing drying time with US treatment relatively more effective regarding drying time than other treatments. Compared with other treatments, US showed the highest rehydration ratio values. The osmotic group pretreatment exhibited a pronounced effect on water loss and solid gain, improved the color, aroma, and taste of dried sweet potato, whereas sucrose impregnation resulted in a hard texture observed with OD sample. USOD samples had a higher expansion ratio, lower hardness and color difference values, appeared less cell damaged, and recorded better overall quality than the other samples. There was a slight difference between USOD and US?+?OD samples. Combining osmotic dehydration with ultrasound as a pretreatment can significantly accelerate the heat transfer rate, reducing the dried time accordingly and increasing energy efficiency.     

Microwave vacuum drying of lotus seeds: Effect of a single-stage tempering treatment on drying characteristics,moisture distribution,and product quality

This study evaluated the effects of a single-stage tempering treatment during microwave vacuum drying (MVD) on drying characteristics and quality of lotus (Nelumbo nucifera Gaertn.) seeds using two parameters: intermediate moisture content (IMC) and tempering temperature (4 and 25°C). Magnetic resonance imaging (MRI) was used to examine moisture migration and distribution in individual lotus seed during tempering. Results from MRI showed tempering could reduce the moisture gradient in lotus seeds during MVD. The tempering treatments led to increased moisture diffusivity (3.96–43.56%) and a shortened drying time (6.25–31.25%) when compared with continuous MVD. Furthermore, tempered samples exhibited a greater rehydration capacity, a limited overall color change, and increased amounts of taste-active amino acids when compared with nontempered lotus seeds. High IMC improved rehydration ratios of dried samples. Low tempering temperature provided favorable free amino acid content and desirable product color.     

微波干燥和热风干燥对金萱茶叶品质影响

将微波干燥和热风干燥应用于新鲜金萱茶叶的干燥,在3个不同的微波功率（350 W,470 W,800 W）和4个不同的空气温度（70 ℃,80 ℃,90 ℃,100 ℃）下,研究了干燥参数即微波功率或温度对干燥时间、失水量的影响,同时还考察了茶叶中茶多酚总量和儿茶素含量的变化、干茶叶的复水性。实验得出,与热风干燥相比,微波干燥的时间缩短了76%～95%,而且干燥后的茶叶有很好的复水性,但干燥后茶多酚总量少于热风干燥;同时随着功率增加,微波干燥时间由11 min降为6 min,随着温度升高,热风干燥时间由127 min缩短为45 min。这两种方法的茶叶干燥动力学模型均满足Page’s半经验公式。     

热风和微波干燥对煤泥品质的影响

采用热风和微波两种方式对煤泥进行干燥处理,从主要组分、燃烧特性和水分复吸特性等方面对煤泥干燥产品进行了分析。通过工业分析对比了干燥前后煤泥主要组分的变化,发现两种煤泥干燥过程中煤泥的挥发分、灰分和固定碳含量没有明显变化。采用热重分析研究了原煤泥和两种煤泥干燥产品的燃烧特性,发现除了失水阶段,3种煤泥样品的热重曲线没有较大差异,3种样品的燃烧特性指数十分接近。最后探究了两种干燥方式下煤泥干燥产品的水分复吸特性,发现环境湿度和煤泥含水率对水分复吸特性有显著影响,而干燥方式对水分复吸特性影响并不明显。     

Mathematical modeling and influence of ultrasonic pretreatment on microwave vacuum drying kinetics of lotus (Nelumbo nucifera Gaertn.) seeds

The people of Southeast Asia often use lotus as a highly sought-after food source. Here, the effects of ultrasonic pretreatment on the drying kinetics of lotus (Nelumbo nucifera Gaertn.) seeds under microwave vacuum drying were investigated. The best fit model to predict the drying kinetics was also proposed. Lotus seeds were subjected to ultrasonic pretreatment at frequencies of 20, 35, and 80?kHz and power intensity of 0.75 and 1.50?W/g for 10?min using an ultrasonic bath and then to microwave vacuum drying. Five different mathematical models were fitted to the experimental data and a newly proposed model was selected based on model with highest regression coefficient (R²), lowest root mean square error (RMSE), sum square error (SSE), and chi-square (χ²), respectively. Time-domain nuclear magnetic resonance and field scanning electron microscope were used to describe the water state of ultrasonic samples and examine microstructure, respectively. The results showed that ultrasonic pretreatment performed at a relatively low frequency and relatively high power intensity had a positive effect on reducing the drying time (6.25–31.25%) during microwave vacuum drying because of the redistribution of water and the formation of microchannels. In parallel studies, the new model showed the best fit to the drying curve.     

Effect of combined infrared freeze drying and microwave vacuum drying on quality of kale yoghurt melts

Abstract

Kale yoghurt melts are proposed as a novel snack that can melt in the mouth. However, such melts with thin fresh pulp were difficult to shape when drying at elevated temperature. In order to obtain a desirable shape and high-quality snack, freeze drying (FD), infrared freeze drying (IRFD), and combination of infrared freeze drying and microwave vacuum drying (IRFD-MVD) were attempted. The effects of these drying techniques on the drying time, energy consumption, bulk density, rehydration time, texture, color, chlorophylls, carotenoids and total phenolics contents as well as antioxidant activity and sensory characteristics of the differently dried products were compared. Nuclear magnetic resonance and magnetic resonance image analyses were also conducted to illustrate the state and proportion of water during the IRFD process. Drying time of IRFD was noted to be significantly shorter than that of FD. IRFD-MVD could further save the time as well as energy consumption of drying and, when properly conducted, could maintain the quality of snacks.     

Online measurement of moisture content,moisture distribution,and state of water in corn kernels during microwave vacuum drying using novel smart NMR/MRI detection system

Moisture content is unevenly distributed and hard to measure when agricultural products are dried using microwave drying. A low-field nuclear magnetic resonance/imaging (NMR/MRI) and microwave vacuum drying (MVD) combination equipment was developed. The residual moisture content, distribution, and state of water (free, immobilized, and bound) in fresh corn kernels during MVD were quickly measured in real time. NMR results indicated that the amplitude of free and immobilized water decreased very rapidly at the early stage of MVD, while the amplitude of bound water experienced a similar rapid decrease at the last stage. MRI results indicated that the moisture content was always distributed unevenly during MVD, especially at the early stage. The moisture distribution tended to become uniform when drying progressed and the bound water became dominant. The residual moisture content of corn kernels and integral (total) amplitude of NMR were found to fit well with a linear model (R²?>?0.991, P?相似文献   

The effect of freezing on the hot air and microwave vacuum drying kinetics and texture of whole cranberries

Abstract

The aim of this study was to evaluate the effect of convective and cryogenic freezing, hot air convective drying (HACD) at 60, 70, 80, and 90?°C and microwave vacuum drying (MWVD) at 100, 150, 200, 300, 450, and 500?W on the drying kinetics and texture of whole cranberries. Effective moisture diffusivities and drying rates were higher, whereas drying times were shorter for the samples dried by MWVD compared with the samples processed by HACD. The drying kinetics of cranberries during MWVD was discussed based on the hypothesis postulating that changes in the drying rate of cranberries during MWVD can be explained by and correlated with changes in the pressure gradient on material surface. Cranberries processed by MWVD were characterized by significantly greater hardness, gumminess, and chewiness in comparison with HACD samples. MWVD was found to be an effective method for producing dried snacks characterized by hard and crispy texture and considerable resistance to stress associated with manufacturing, packaging, storage, and delivery. HACD produced brittle fruit that were difficult to store and transport and were not fully suitable for direct consumption. Convective freezing before MWVD improved the overall appearance of cranberries, whereas cryogenic freezing combined with high temperature HACD adversely influenced the drying rate and produced dried cranberries with suboptimal overall appearance.     

Improvement of microwave dielectric properties of the LiNb0.6Ti0.5O3 M-phase by doping with (Co1/3Nb2/3)4+

Structural evolution and microwave dielectric properties of LiNb_0.6(Ti_1-x[Co_1/3Nb_2/3]_x)_0.5O₃ (.05≤x≤.2) ceramics have been studied in this paper. Although the doped compositions maintain the M-phase solid solutions, compositional fluctuation due to nonuniform dispersion of minor dopants could be observed as x < .05, and trace amount of Li₂TiO₃-based solid solution (Li₂TiO₃ss) secondary phase presents in the x > .05 compositions. The microwave dielectric properties could be remarkably improved by the doping of (Co_1/2Nb_1/2)⁴⁺ in comparison to the undoped counterpart. Optimized microwave dielectric properties with Q × f = ∼6500 GHz, ε_r = ∼74 and τ_f = +8.2 ppm/°C could be obtained at x = .10 after sintering at 1050°C/2 h. The sintering temperature could be further reduced to 900°C/2 h by adding .2 wt% B₂O₃ without affecting significantly its microwave dielectric properties: ε_r = 73, Q × f = 6000 GHz, τ_f = +8.5 ppm/°C. The LiNb_0.6(Ti_1-x[Co_1/3Nb_2/3]_x)_0.5O₃ ceramics obtained in this case exhibit large dielectric permittivity coupled with much improved Q × f values, near zero τ_f, and low sintering temperature simultaneously, which makes it a promising high-k microwave dielectric material for low temperature cofired ceramic applications.     

Effect of different forms of poly o‐toluidine/poly vinyl chloride composites on dielectric properties in the microwave field

In this paper we report the preparation and dielectric properties of poly o‐toluidine:poly vinyl chloride composites in pellet and film forms. The composites were prepared using ammonium persulfate initiator and HCl dopant. The characterization is done by TGA and DSC. The dielectric properties including dielectric loss, conductivity, dielectric constant, dielectric heating coefficient, absorption coefficient, and penetration depth were studied in the microwave field. An HP8510 vector network analyzer with rectangular cavity resonator was used for the study. S bands (2–4 GHz), C band (5–8 GHz), and X band (8–12 GHz) frequencies were used in the microwave field. Comparisons between the pellet and film forms of composites were also included. The result shows that the dielectric properties in the microwave field are dependent on the frequency and on the method of preparation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2253–2260, 2007     

Comparison of four pretreatments on the drying behavior and quality of taro (Colocasia esculenta L. Schott) slices during intermittent microwave vacuum-assisted drying

ABSTRACT

Effect of different pretreatments on drying behavior and quality of taro slices during intermittent microwave vacuum-assisted drying were investigated. Taro slices were subjected to the following pretreatments: blanching (B), blanching and freezing (B?+?F), blanching and immersion in maltodextrin solution (B?+?M), and blanching and immersion in maltodextrin solution and freezing (B?+?M?+?F). The reduction in amylose content, the increase in amylose to amylopectin ratio, the change of mechanical properties, the increase in sample bulk density and reduction in sample porosity and structure collapse during pretreatment processes facilitated heat and moisture transfer and quality changes during drying of taro slices. During drying process, all the pretreatment reduced the drying time of samples. The drying rate included a warming-up period and a falling rate period. The values of effective moisture diffusivity increased gradually as the moisture content decreased. The B?+?M?+?F pretreatment had the shortest drying time, the highest drying rate and the biggest effective moisture diffusivity. Moreover, for B?+?M?+?F pretreatment, the parameters including hardness, crispness, color, and microstructure had remarkable changes as compared with the other pretreatments.     

Effects of adding B2O3 on microwave dielectric properties of 0.9625MgTiO3-0.0375(Ca0.5Sr0.5)TiO3 composite ceramics

A series of 0.9625MgTiO₃-0.0375(Ca_0.5Sr_0.5)TiO₃ composite ceramics added with different amounts of B₂O₃ (1-5 wt%) were prepared via the solid state sintering method using the pre-synthesized raw MgTiO₃ and (Ca_0.5Sr_0.5)TiO₃ powders by molten-salt reaction. The sintering temperature of 0.9625MgTiO₃-0.0375(Ca_0.5Sr_0.5)TiO₃ composite ceramics can be reduced from 1275°C to 1175°C due to the liquid phase sintering effect of B₂O₃. When the adding amount of B₂O₃ was more than 2 wt%, a new phase MgTi₂O₅ could be detected by X-ray diffraction, which would substantially degrade the dielectric properties of the obtained ceramics. Resultantly, the quality factor (Q·f) and dielectric constant (ε_r) of the samples increase first and decrease later with increasing addition amount of B₂O₃. In addition, the temperature coefficient of resonant frequency (τ_f) progressively increases with increasing content of B₂O₃. By sintering at 1175°C for 4 hours, the obtained 0.9625MgTiO₃-0.0375Ca_0.5Sr_0.5TiO₃ composite ceramics with 2 wt% B₂O₃ possess the optimal microwave dielectric properties of ε_r = 18.9, Q·f = 57 000 GHz and τ_f = −1.2 ppm/°C.     

Ba缺位和烧结工艺对Ba1-xZn1/3Nb2/3O3陶瓷离子有序度和微波介电性能的影响

利用常规固相法制备了Ba1-xZn1/3Nb2/303(x=O～0.02)陶瓷,研究了Ba缺位对Ba1-xZn1/3Nb2/3O3陶瓷的相成分、B位离子长程有序度(LRO)和微波介电性能的影响规律.X射线衍射(XRD)结果显示,适量的Ba缺位可以提高材料的阳离子有序度,x=0.01时陶瓷具有最大的阳离子有序度;Ba缺位...     

Effect of Li nonstoichiometry and TiO2 addition on the microwave dielectric properties of Li3PO4 ceramics

Li₃PO₄ ceramic is a promising microwave ceramic material with low dielectric constant. The effect of Li nonstoichiometry on phase compositions, microstructures, and microwave dielectric characteristics of Li₃PO₄ ceramics, on the other hand, has been examined infrequently. Therefore, in the first part of this study, the stoichiometry and Li nonstoichiometry compositions based on Li_3+xPO₄(x = 0, 0.03, 0.06, 0.09, 0.12 and 0.15) were prepared by conventional solid-phase method. The results show that a few nonstoichiometric lithium ions enter the lattice of Li_3+xPO₄. Compared with the chemical content of Li₃PO₄, the sintering characteristics, relative dielectric constants and quality factors of Li_3+xPO₄ ceramics can be improved by slightly excessive Li ions, while the properties of Li₃PO₄ ceramics can be deteriorated by excessive Li ions. Li_3.12PO₄ ceramics sintered at 975 °C for 2 h have good dielectric properties (ε_r = 5.89, Q×f = 44,000 GHz, τ_f = ?206 ppm/^°C). In order to improve its large negative temperature coefficient of resonant frequency, in the following study, rutile nano TiO₂ particles were added as τ_f compensator. Adding TiO₂ powders not only effectively improve the temperature stabilities of the multiphase ceramics, but also make the grain growth more uniform. With the increase of TiO₂ content from 0.40 to 0.60, τ_f increases from ?73.5 ppm/^°C to +42.3 ppm/^°C. The best dielectric property of 0.45Li_3.12PO₄-0.55TiO₂ composite ceramic is ε_r = 13.29, Q×f = 40,700 GHz, τ_f = +8.8 ppm/^°C.     

CaTiO3添加剂对氧化铝陶瓷烧结、显微结构及微波介电性能的影响

CaTiO3添加剂通过湿法球磨与Al2O3粉料混合,并通过无压烧结制备了氧化铝陶瓷,研究了CaTiO3添加剂对氧化铝陶瓷烧结性能、相组成、显微结构和微波介电性能的影响.CaTiO3可以使Al2O3的烧结温度降低至1450℃,但在该温度下烧结的样品由于CaTiO3的加入会产生CaAl12O19第二相.样品中存在大、小两种晶粒,根据EDS能谱分析,大晶粒主要是CaAl12O19,而小晶粒为Al2O3和CaAl12O19的混合相.添加CaTiO3有利于Al2O3陶瓷介电常数的提高,1450℃下掺杂2.5wt％ CaTiO3的氧化铝陶瓷具有较好的烧结性能和微波介电性能,相对密度可达到97.74％,εr～10.86,Q×f～ 8061 GHz.     

Effect of ZnO doping on the microstructure and microwave dielectric properties of 0.2CaTiO3-0.8(Li0.5Sm0.5)TiO3 ceramics

0.2CaTiO₃-0.8(Li_0.5Sm_0.5)TiO₃-xZnO(x = 0, 0.3, 0.6, 0.9, 1.2 wt%, 0.2CT-0.8LST-xZnO) with orthogonal perovskite structure were fabricated by the solid state method. The effects of ZnO additives on the microwave dielectric properties of 0.2CT-0.8LST ceramics were systematically investigated. With increasing the dopant (x) concentration, the dielectric constant (ε_r) and the temperature co-efficient of resonance frequency (τ_f) decreased, however, the Q × f values increased. The relationship between vibration mode and microwave dielectric properties was studied using Raman spectroscopy. The Q × f value of ceramics was related to the half-height width of Raman scattering. Narrower Raman scattering peaks corresponded to longer microwave energy propagation decay times and higher Q × f value. Based on X-ray photoelectron spectroscopy (XPS), the addition of Zn²⁺ ions limited the reduction of Ti⁴⁺ cations. The excellent dielectric properties were obtained when x = 1.2 wt% with ε_r = 100.25, Q × f = 6525 GHz, and τ_f = ?12.12 ppm/°C.