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   

Moisture flow characteristics and variation patterns of dielectric constants in bitter gourd slices during microwave hot-airflow vibrating drying

Abstract

The uniformity of microwave-dried bitter gourd slices was poor, and a microwave hot-airflow vibrating drying (MHAVD) apparatus was used to carry out an in-depth elucidation of moisture flow characteristics underlying the variation in dielectric characteristics. Low-field nuclear magnetic resonance and imaging (LF-NMR/MRI) was used to measure the moisture state and distribution during different drying stages, and the relationship model between moisture semaphore and moisture ratio were analyzed. A vector network analyzer was used to measure the patterns in the variation of dielectric characteristics in bitter gourd slices at different drying stages. As a result, the temperature and moisture distribution of bitter gourd slices during MHAVD was uniform caused by hot air and vibration, and the water semaphore in NMR gradually decreased, but the bound water semaphore increased during certain drying stages. There is a significant linear relationship between the total semaphore A₂ and the moisture ratio M_R in bitter gourd slices per unit dry basis mass in the NMR spectrum (R² = 0.99). As moisture decreases, the ability of the material to store microwaves and convert electromagnetic energy to heat energy decreases, the ability of the material to absorb microwaves decreases, and the microwave penetration significantly increases. The moisture flow characteristics in bitter gourd slices is related to the variation of dielectric characteristics during MHAVD, and drying uniformity was improved.     

Spouted bed drying of agricultural grains

It is shown that the presence of a “slotted draft tube” results in reduced air requirements for spouting and improved drying performance. Experimental data are presented on batch as well as continuous spouted bed drying of wheat, paddy, maize and peas. The variables studied are feed moisture content (Q_o), inlet air temperature (T_o), bed mass hold-up (M_p), inlet superficial air velocity (u_o) and bed diameter (D_c) in batch drying, and the above variables and solids feed rate (F_s) in continuous drying. The data on average overall drying rate, ?_m, in kg moisture evaporated per unit time per kg bed solids, is found to be correlatable as ?_m, = k (50Q_o + 0.118T_o ? 12.5) 10^?5, and the single parameter k is presented for wheat, paddy, maize and peas for both batch and continuous modes of spouted bed drying. The correlation obtained should be useful in dryer design for the grains studied as well as for other similar materials.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

ABSTRACT

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

Measurement of water content and moisture distribution in sludge by 1H nuclear magnetic resonance spectroscopy

Moisture distribution in sludge is essential for the examination of dewatering problems; however, sufficiently rapid and accurate methods of sludge moisture measurement are currently lacking. Hence, this study investigated a low-field ¹H nuclear magnetic resonance (NMR) method for measuring water content and moisture distribution in sludge. Moisture content measured by NMR was closely correlated with the thermal drying method (R² = 0.999). The loss of mechanical bound water from sludge was the primary cause of the decrease in water content from 96 to 37% during thermal drying at 40°C. NMR is more accurate, rapid, and nondestructive than other water distribution measurement methods.     

Mass and Heat Transfer in a Continuous Plate Dryer

Abstract

A mathematical model for the mass-heat transfer in a continuous plate dryer has been developed. Along with a new mass transfer model, the formulas for several important parameters, such as height, volume of each granular heap and retention time, are provided. According to the penetration model of particle heat transfer, the average drying rate ([mdot]) is predicted together with the mean bulk temperature (T _out) and moisture content (X _out) by a straightforward stepwise calculation procedure. The only empirical parameter N _mix can be predicted by the method, provided that experimental data with various initial moisture contents (X _in) are given. According to the model, the optimization of a plate dryer should aim at the maximizing of the effective covering ratio (μ) and the total area-averaged heat transfer coefficient (α). The model and equations were applied in an experimental plant. In the result, the theoretical predictions are shown to be in satisfactory agreement with experimental data.     

Moisture conditioning of wood using a continuous microwave dryer

Lower moisture content in wood, preferably 5–6%, is desirable for many chemical modification reactions. Economically, it is not feasible to dry timber to such low moisture content by conventional drying without drying degrades. Microwave heating was evaluated and found to be effective in reducing the moisture content of radiata pine from 13% to 6% in a microwave cycle of two minutes. The energy consumption is about 55 kWh/m³. Moisture distribution profiles demonstrate very uniform drying across the sample thickness. The findings suggest that microwave heating can potentially be applied to condition wood in a very short period of time.     

Osmotic Dehydration of Apple Cylinders: III. Continuous Medium Flow Microwave Heating Conditions

Continuous flow osmotic drying permits a better exchange of moisture and solids between the food particle and osmotic solution than the batch process. Osmotic drying has been well studied by several researchers mostly in the batch mode. Microwave heating has been traditionally recognized to provide rapid heating conditions. Its role in the finish drying of food products has also been recognized. In this study, the effects of process temperature, solution concentration on moisture loss (ML), solids gain (SG), and mass transport coefficients (k _m and k _s ) were evaluated and compared under microwave, assisted osmotic dehydration (MWOD) versus continuous flow osmotic dehydration (CFOD). Apple cylinders (2 cm diameter, 2 cm height) were subjected to continuous flow osmotic solution at different concentrations (30, 40, 50, and 60°Brix sucrose) and temperatures (40, 50, and 60°C). Similar treatments were also given with samples subjected to microwave heating. Results obtained showed that solids gain by the samples was always lower when carried out under microwave heating, while the moisture loss was increased. The greater moisture loss strongly counteracted solids gain in MWOD and thus the overall ratio of ML/SG was higher in MWOD than in CFOD.     

Effect of carbonic maceration pre-treatment on the drying behavior and physicochemical compositions of sweet potato dried with intermittent or continuous microwave

Drying behavior and change of physicochemical compositions of sweet potato in intermittent microwave drying (IMD) and continuous microwave drying (CMD) combined with or without carbonic maceration (CM) pre-treatment were investigated. A quantity of 100?g of fresh or CM pre-treated samples were dried in IMD at 700?W with power on–off ratio of 1/6 (5?s/30?s, t_on/t_off) or dried in CMD with power density of 1?W/g. The results indicated that the process of CM?+?IMD as compared with IMD, CMD, and CM?+?CMD had the shortest drying time, the lowest activation energy (E_a), and the biggest effective moisture diffusivity (D_eff) and exhibited the highest total content of phenols, anthocyanins, flavonoids, β-carotene, ascorbic acid retention rate, as well as scavenging free radical capability 2,2-diphenyl-1-picrylhydracyl of dried product. Meanwhile, color measurements indicated that CM?+?IMD could get the most desirable color of dried sweet potatoes. Therefore, CM pre-treatment is promising for industrial application.     

Update for combustion properties of wood components

The combustion properties of various biomass and wood materials from various references and from our laboratory were reanalysed. The net heat of combustion for cellulosic materials was found to be 13.23 kJ/g times the ratio of stoichiometric oxygen mass to fuel mass, r_o, regardless of the material composition. Bomb calorimeter data for original, charred and volatilized material components provide gross heating values, while elemental analysis of the materials for carbon, hydrogen, oxygen and ash provide direct evaluation for r_o. We corrected these data as provided in various references by converting gross heating values to lower heating values and converting elemental compositions, char fractions and r_o to a moisture‐free and ash‐free basis. Some existing formulae were found to disagree with data from vegetation, charred wood with high ash content, and with volatiles from cellulose treated with the fire retardant NaOH. We also established various functional correlations of r_o with elemental compositions, or volatization fractions of untreated and treated materials, or material fractions for cellulose, lignin and extractives, or volatile fractions for tar, combustible gases and inert gases in pure nitrogen carrier gas. An interesting predictive result provides nearly constant heat of combustion while the volatile tar fraction is decreasing and combustible and inert gas fractions are increasing with time during the charring of Douglas‐fir wood. Published in 2002 John Wiley & Sons, Ltd.     

Microwave Drying of Porous Materials

Abstract

Experimental results on microwave drying of the porous particles exposed to air stream at 40°C are presented. The temperature and moisture distribution inside a particle were measured for gypsum spheres of 9, 18, 28, and 38 mm. The mass reduction was monitored during the drying process. The rate of drying and changes in temperature and moisture profiles for different drying conditions were analyzed and compared with the ones for convective drying.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

ABSTRACT

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

Moisture conversion and migration in single-wheat kernel during isothermal drying process by LF-NMR

Conversion among different moisture-binding types of single Hebei wheat kernels during isothermal drying processing at 60?°C was studied by low field nuclear magnetic resonance (LF-NMR), and moisture migration was studied by MRI. Water inside wheat kernels exists in four types: chemically combined water (T₂₁), strong bound water (T₂₂), loosely bound water (T₂₃), and free water (T₂₄). A new method to obtain the contents of different moisture-binding types is forwarded according to the transverse relaxation time distribution curve of dried wheat kernels. Moisture conversion inside wheat kernels during drying occurs mainly through the following mechanisms: high-temperature drives T₂₄ to diffuse to lower moisture areas and transform into other moisture-binding types, and moisture-gradient drives T₂₂ to transform into T₂₄. The drying process can be divided into two stages. Moisture migrates from endosperm to epidermis during drying. As the drying rate of the wheat kernel significantly decreases, the drying conditions and parameters need to be adjusted to improve the drying rate while ensuring the quality of wheat kernels.     

TEMPERATURE AND MOISTURE CHANGES DURING MICROWAVE DRYING OF SLICED FOOD

Microwave drying characteristics of sliced foods were investigated using potatoes (Solarium tuberosum) as a test model. Sliced samples were dried to 7-10% moisture content at microwave power levels between 2.2 W/g and 3.6 W/g (raw material). Moisture and temperature changes during drying were monitored. Semi-empirical models were developed that followed temperature and moisture changes during microwave drying. Sliced potatoes experienced three distinct periods: a warming-up period with little removal of moisture; a constant temperature period in which most of the drying took place; and a heating up period in which the drying rate decreased and sample temperature increased rapidly, often causing partial charring. Product temperature during the second period of microwave drying increased with sample thickness and microwave power. Drying rates were not affected by slice thickness, but increased with the microwave power/mass ratio. Product charring towards the end of drying may be avoided by reducing microwave power and increasing ambient air velocity.     

DRYING OF CARROTS IN A FLUIDIZED BED. I. EFFECTS OF DRYING CONDITIONS AND MODELLING

ABSTRACT

Drying curves were determined in a mechanically agitated fluidized bed dryer, at temperatures between 70°C and 160°C, air velocities between 1.1 m/s and 2.2 m/s and stirring rates between 30 rpm and 70 rpm for batch drying of 3 kg lots of carrot slices, measuring the moisture content and shrinking of the particles in time. This was complemented by a study of the rate and degree of swelling of dried carrot particles in water between 20 and 75°C. Drying kinetics were modeled by Fick's second law, for which an optimal agreement with the experimental data was obtained when the effective diffusivity (D _e ) was determined by a correlation based on the air velocity (v), the air temperature (T) and the dimensional moisture content of the carrot particles (X/X _o ). Loss of carotenes is minimized when dehydration is carried out at about 130°C with a drying time below 12 min.     

Drying kinetics and effective moisture diffusivity of purslane undergoing microwave heat treatment

The effects of microwave drying on moisture content, moisture ratio, drying time and effective moisture diffusivity of purslane leaves (Portulaca oleracea L.) were investigated. By increasing the microwave output power (180–900W) and the sample amounts (25–100 g), the drying time decreased from 43 to 12.5 minutes and increased from 27 to 54 minutes, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among the models proposed, the semi-empirical Midilli et al. model gave a better fit for all drying conditions applied. By increasing the microwave output power and decreasing the sample amount, the effective moisture diffusivity values ranged from 5.913×10⁻¹¹ to 1.872×10⁻¹⁰ m²/s and from 9.889×10⁻¹¹ to 3.292×10⁻¹¹ m²/s, respectively. The activation energy was calculated using an exponential expression based on the Arrhenius equation.     

LF-NMR online detection of water dynamics in apple cubes during microwave vacuum drying

ABSTRACT

In this study, changes in various states of water in apple cubes were assessed using low-field nuclear magnetic resonance (LF-NMR) during microwave vacuum drying. Apple cubes were dried at different microwave power levels (100, 150, and 200?W). Indicators including moisture content and water activity were measured. The Carr–Purcell–Meiboom–Gill (CPMG) sequence was used to measure transverse relaxation times (T₂). The results showed that three water fractions with different T₂ relaxation times (around 11, 126, and 1335 ms) were detected in fresh apples, which corresponded to different cell compartments. The transverse relaxation time of bulk water (T₂₃) and the signal per mass of the bulk water (A₂₃/g) decreased significantly with increasing drying time at different microwave power levels. The signal per mass of the total water (A_Total/g) had significant correlation with the total moisture content (R²?=?0.9919). Furthermore, good correlation (R²?=?0.9799) between water activity and NMR parameters based on partial least square regression model were observed. This research revealed that LF-NMR spectroscopy, a nondestructive technique, can detect the changes of water in different populations in the matrix.     

MEASUREMENT AND ANALYSIS OF MOISTURE CHANGES IN AGRICULTURAL PRODUCTS USING FFT NOISE IMPEDANCE SPECTROSCOPY

ABSTRACT

Macroscopically, cellular foods including agricultural products are regarded as the electrical systems in which cell membranes isolate intracellular electrolytes from extracellular electrolytes. Moisture movement in the materials during drying causes the changes in moisture of the intracellular and extracellular electrolytes. Therefore, the moisture movements can be detected by an electrical measurement

The authors measured the impedance of carrot roots during drying by a FFT (fast Fourier transform) noise impedance method at frequency of 40 Hz to 100 kHz. The equivalent circuit model agreed with the measured impedance. The parameters in the model were determined as the intracellular and extracellular resistances and membrane capacitance

As a result, it was found that the increase in the extracellular resistance showed a good correlation with a decrease in moisture content between 200 and 400% d.b (dry basis) and that the membrane capacitance had the maximum near the second critical moisture content. It was concluded that the FFT noise impedance method was applicable to the moislure measurement of agricultural products during drying.     

Analysis and Evaluation of Power Formulations for Wood and Hardboard Using Radio Frequency and Microwave Energy

This article analyzes the influence of frequency, temperature, moisture content, and structural orientation on the applicability of the Beer-Lambert law for various wood species using radio frequency and microwave radiation. To achieve this objective, the study compares the power dissipation computed from Maxwell's equation and Lambert's power law. The wood species considered are white oak (Quercus alba), Douglas fir (Pseudotsuga menziesii), trembling aspen (Populus tremuloides), white birch (Betula paperyfera), yellow birch (Betula alleghaniensis), sugar maple (Acer saccharum), and four commercial hardboards. The dielectric constant and dielectric loss factor are examined as a function of moisture conditions, temperature, frequencies, and the three principal structural orientations. The study involved 3,000 complex dielectric constants. It was found that the radial critical thickness is somewhat smaller than the tangential critical thickness (0.95 times smaller) and the longitudinal critical thickness is significantly smaller than the radial (0.52 times). It was demonstrated that the critical thickness L _crit above which the Beer-Lambert law is valid for all of the wood species studied under various conditions obeys the following conditions: log₁₀(L _crt) = 0.999 log₁₀(β^?1) + 0.4122, where β^?1 is the penetration depth (cm). In the case of microwave radiation, the critical thickness can be estimated from L _crt = 2.615 β^?1 ? 0.0626. Finally, a model is proposed to take into consideration the effect of moisture content with frequency (or with attenuation constant).     

Effect of particle size and ZnO addition on microwave dielectric properties of nanocrystalline (Zr0.8Sn0.2) TiO4 ceramics

Abstract

Polycrystalline samples of (Zr_0.8Sn_0.2)TiO₄ have been synthesised by solid state reaction method. The effects of ball milling and the addition of ZnO (1–2.5 wt-%) on the microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ ceramics are investigated. As the ball milling time increases, the density of the (Zr_0.8Sn_0.2)TiO₄ ceramics increases. The influence of ZnO as a sintering aid on the sintering temperature of (Zr_0.8Sn_0.2)TiO₄ is also studied. As the weight percentage of ZnO increases, the density increases up to 1.5 wt-% of ZnO and thereafter it starts decreasing. The dielectric constant εγ _r and the product of quality factor Q and resonant frequency f_o of TE_{01 °} mode (Qxf_o ) followed the same trend with increasing density, irrespective of whether the density increased with ball milling time or by addition of ZnO. The value of Qxf _o is found to increase with increasing grain size. The microwave dielectric constant and the Qxf _o values were respectively ranged from 32 to 39.3 and from 32 000 to 49 600 at 10.5 GHz for (Zr_0.8Sn_0.2)TiO₄ ceramics. Furthermore, the influence of the processing parameters on the microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ is discussed.