Experimental study on drying characteristics of wheat by low-field nuclear magnetic resonance

The characteristics of transverse relaxation time (T₂) of water in wheat were studied by measuring the relaxation time of low-field nuclear magnetic resonance. Analysis of the exponential distribution of T₂ revealed that wheat contains five water components. The T₂ relaxation time and distribution significantly changed during drying. The dynamic characteristics of five water components during wheat drying were determined using the signal quantity of their characteristic peaks, which showed different features. Weakly chemically bound water (T₂₂) and water ascribed to cell wall (T₂₃) were the main source of water loss. Moreover, most T₂₃ and extracellular water (T₂₄) were removed during drying. Water migration between strongly chemically bound water (T₂₁) and the other water components was bidirectional. This process was not only affected by temperature but also by wheat moisture content and proportion of the five water components. The start time of water migration advanced and growth rate of T₂₁ at the end of drying to that before drying increased at 60, 70, and 80°C. Drying at varied temperatures should be applied according to the characteristics of five water components during the drying process. In addition, high initial temperature was found to be necessary to achieve high drying rate of T₂₃, T₂₄, and free water (T₂₅). The use of drying temperature of 80°C at the early stage and then changing to 70°C reduced the heat consumption by 4.81% and increased the drying time by 9.61%.     

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 transfer and stress development during high-temperature drying of Chinese fir

Abstract

The knowledge of moisture content (MC) and drying stress are crucial parameters to control the drying process and maintain the quality of dried wood. Herein, we investigated the pattern of moisture transfer and stress development in Chinese fir during the high-temperature drying process. The MC in each layer of lumber was separated into bound water and free water via nuclear magnetic resonance (NMR), and the drying and residual stress were measured using prong test method. There was different MC in each layer along the thickness, resulting in an MC gradient that initially increased and then decreased, which is consistent with the trend of drying stress. The T₂₁ peak indicating bound water shifted to the left especially when MC was below the fiber saturation point, signifying that the discharge of moisture became difficult with prolonged drying time. The ratio of bound water to free water was different in each layer, indicating that the moisture transfer was different along the thickness. Furthermore, the residual stress was greater than the corresponding drying stress though the disparity reduced gradually, which suggests that the MC gradient was the largest affecting factor for drying stress at high MC stages but decreased to some extent as the drying process continued.     

Use of low-field-NMR and MRI to characterize water mobility and distribution in pacific oyster (Crassostrea gigas) during drying process

Low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI) were used to monitor the water mobility and distribution of Pacific oyster during drying process. The results show the mobilities of bulk, immobilized, and free water were reduced, and the immobilized water was removed dramatically. T₂-weighted images displayed the water decrease from the external surface to inner center of oyster during drying. In addition, excellent correlations between the total moisture content and T₂₂ and A₂₂ were observed with coefficients 0.9777 and 0.9832, respectively. Principal component analysis showed the drying degree of oyster could be monitored based on raw relaxation data. Thus, the result revealed that LF-NMR and MRI have great potential in assessing water mobility and distribution in oyster during drying process.     

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.     

Development of cantaloupe (Cucumis melo) pulp powder using foam-mat drying method: Effects of drying conditions on microstructural of mat and physicochemical properties of powder

In this study, the effects of drying conditions on moisture content, water activity (a_w), dissolution time, solubility, hygroscopicity, β-carotene, color, glass transition temperature (T_g), and sticky point temperature (T_s) of foam-mat-dried cantaloupe pulp powders and microstructure of dried cantaloupe pulp foams were investigated. Drying was performed in three temperatures (40, 55, and 70°C) on 3- and 5-mm thicknesses. The analysis of scanning electron microscopy micrographs with grey-level co-occurrence matrix showed that there is wide porous structure of dried foams at higher speeds drying. The temperature increase reduced moisture content and a_w, and increased hygroscopicity, and thickness rise increased moisture content and a_w and consequently decreased powders’ hygroscopicity under the same thickness and drying temperature, respectively. Increase in drying temperature would increase the reconstitution speed of powders into water and therefore the dissolution time decreased. In addition, results showed that the powder produced at 40°C have higher β-carotene content than those of produced at 55 and 70°C. With increasing drying temperature from 40 to 70°C, Lightness parameter (L) was increased while redness parameter (a) was decreased. The T_g and T_s were compared by plotting them in a graph against moisture content. For all drying processes the T_s was higher than the T_g. The drying conditions at 70°C (higher drying temperature) and 3?mm (lower thickness) led to a shorter drying time and consequent lower energy demand to produce a powdered cantaloupe pulp with high stability (low moisture content, a_w, and high T_g and T_s) and reconstitution speed of powder into water.     

Determination of moisture content and moisture content profiles in wood during drying by low-field nuclear magnetic resonance

In this article, the low-field nuclear magnetic resonance was used to measure the moisture content (MC) and MC profiles in poplar wood during real-time drying. The T₂ distribution curve at each drying stage measured using the Carr–Purcell–Meiboom–Gill pulse sequence provided detailed information in the dynamic change of free water as well as bound water of the whole wood sample. In addition, a new developed SE-SPI pulse sequence was first used to evaluate the spatially resolved T₂ distribution of the successive nondestructive sliced layer of wood. Combined with the area integration method, the moisture content in each layer was calculated, and the change of MC profiles within wood at the MC above and below fiber saturation point was well reflected.     

LF-NMR to explore water migration and water–protein interaction of lamb meat being air-dried at 35°C

To decrease the drying time and energy consuming, water migration and water–protein interactions of lamb meat being air-dried at 35°C were investigated. Low-field nuclear magnetic resonance T_2b indicated the water–protein interactions changed during air-drying. Area of T₂₁ (intramyofibrillar water) decreased, meanwhile the area of T₂₂ (extra-myofibrillar water) increased when the moisture content decreased from 55 to 45%, indicating the water migrated from myofibril to extramyofibril. Drying rate (the rate of water migration from meat to air) could be predicted by the area of T₂ populations, and the correlation coefficient was 0.990. Change of water binding and hydration in myofibrils was evident by the increase in hydrophobicity and decrease in solubility of myofibrillar protein. Differential scanning calorimetry showed denaturation of myosin in dried meat which might result in water migration from myofibril to extramyofibril space. In conclusion, water–protein interactions changed, and then influenced the drying rate during drying.     

Differences between constant and intermittent drying in surf clam: Dynamics of water mobility and distribution study

Low-field nuclear magnetic resonance (LF-NMR) has been increasingly popular as analytical tools for evaluating the dynamics of water mobility and distribution. In this study, dynamics of moisture mobility and constitution of surf clam during constant drying process and intermittent drying process were evaluated by LF-NMR, while the differences of physical and chemical indexes were measured. Intermittent drying improved the product quality of clam, such as moisture content, shear force, color indices, sugar content, peroxide value, thiobarbituric acid value, and the bulk water ratio, which were closely related with moisture distribution and microstructure. The moisture constitution of constant drying process and intermittent drying process were distinctly different. Tempering process reduced drying time and resulted in lower moisture content in dried surf clam. In the meanwhile, the boundary between A₂₁ and A₂₂ was acquired by LF-NMR, revealed that bound water and immobilized water transformed from each other. During tempering process, the myofibril stretched out, verifying that moisture approached a relatively homogeneous. In addition, R² value reached 0.9897 and 0.9926 for calibration and validation, respectively, displaying good linear correlations between the T₂₁ parameters and moisture content. This study interpreted the dynamics of water mobility and distribution on the proton level to explain the reason that tempering processes to improve physicochemical indexes of surf clam.     

Effects of gluten and moisture content on water mobility during the drying process for Chinese dried noodles

To investigate the water mobility during a drying process, noodles were prepared with different gluten contents (10.0%, 12.5%, 15.0%, 17.5%, 20.0%, 22.5%, or 25.0%) and moisture contents (30%, 32%, or 34%), and dried on a food moisture analysis technology platform. Three types of water were deduced from relaxation signals measured by low-field nuclear magnetic resonance (LF-NMR) spectroscopy. Peak time T₂₂ increased with gluten and moisture content but decreased with drying time from 4–6 to 1.0–1.7?ms after 1.5?h. Gluten content mainly affected the drying rate (DR) in the middle drying period, whereas initial moisture content had an influence in the middle drying period and final drying period.     

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.     

MOISTURE DISTRIBUTION IN SPHERICAL FOODS IN MICROWAVE DRYING

Abstract

Moisture distribution in spheres of potato (Solanum tuberosum) and yam (Dioscorea japonica) tubers during microwave drying was investigated. Moisture variation at various locations within the samples was experimentally determined for different drying times. A numerical simulation model was developed based on a modified form of Lambert's law for microwave penetration in spheres and on moisture transfer in both liquid and vapor phases. This model was used to study various factors influencing moisture distribution. Moisture profile during microwave drying was strongly influenced by non-uniform microwave energy distribution within the foods. The core moisture contents were lower than other parts of potato and yam spheres of the various sizes tested in this study (2.6 cm to 6.0 cm diameter). The sphere radius to microwave penetration ratio (2αr_o) was an important parameter affecting the moisture profile in spherical foods. For 2αr_o of less than 2.0, the moisture content in the center of potato and yam spheres was markedly lower than the rest of the food, but for 2αr_o between 2.5 and 4, moisture contents at the center and close to the surface were lower than the average moisture content.     

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS*

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g ). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.

     

Effect of hot-air oven dehydration process on water dynamics and microstructure of apple (Fuji) cultivar slices assessed by LF-NMR and MRI

Abstract

Non-destructive analysis of water dynamics during drying is of importance for quality control of food products. In this study, different water dynamics and migration in Fuji apple slices dried at various hot-air oven temperatures, i.e. 50, 60, 70, and 80?°C and air velocity at 0.2 m/s were monitored using low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI). Multi-exponential fitting of the transverse relaxation T₂ parameter demonstrated four distinct water peaks in all samples corresponding to strongly bound, lightly bound, entrapped/immobilized, and free water as follows: T₂₁ in the range of 0.01–1?ms, T₂₂ in the range of 1–10?ms, T₂₃ in the range of 10–100?ms, and T₂₄ in the range of 100–1000?ms, respectively. The water content was measured and analyzed by the traditional technique using the oven drying method. The overall results were highly significant, depicting that the transverse relaxation times T₂₄, signal per mass of the free water A₂₄, and water content significantly decreased (p< 0.05), while the color (L*, a*, b*) and shear force (SF) curves increased with extended drying. Furthermore, good correlations were observed between the LF-NMR parameters and color, water content, and SF in differently processed samples during the dehydration process. Scanning electron microscopy (SEM) and MRI provided the structural changes and spatial water distributions during the drying process. LF-NMR exhibited great potential in evaluating the various water dynamics and quality of Fuji cultivar apples during the drying process.

Abbreviations: AA: ascorbic acid; ANOVA: analysis of variance; CPMG: Carr-Purcell-Meiboom-Gill; DW: distilled water; F: Fuji; LF-NMR: low-field nuclear magnetic resonance; MRI: magnetic resonance imaging; SE: spin-echo; SEM: scanning electron microscopy; SF: shear force; SIRT: simultaneous iterative reconstruction technique; TE: echo time; TR: repetition time.     

Effects of ultrasonic pretreatments on thermodynamic properties,water state,color kinetics,and free amino acid composition in microwave vacuum dried lotus seeds

Abstract

Ultrasonic pretreatments were applied to lotus seeds at acoustic energy densities of 0.29, 0.40, and 0.51?W mL^?1 for 10?min. After pretreatments, lotus seeds were subjected to microwave vacuum drying (MVD). Parameters of glass transition temperature (T_g), gelatinization temperature (T_p), water state, color kinetics, and free amino acid content of microwave vacuum dried lotus seeds were determined. With increasing acoustic energy density, MVD elevated the T_g values appreciably by decreasing the content of cytoplasmic bulk water in lotus seeds tissues. The T_p had a positive relationship with the relaxation times of cytoplasmic bulk water (T₂₂), while T_g had a negative relationship with T₂₂. Color kinetics were analyzed by the divisional method during MVD due to different browning reactions, which failed to appear with ultrasonic pretreatment. Free amino acid content ranged from 517.65 to 666.13?mg/100?g dry weight at 0.51?W mL^?1.     

MATHEMATICAL SIMULATION OF TEMPERATURE AND MOISTURE FIELDS WITHIN A GRAIN KERNEL DURING DRYING

ABSTRACT

Non-linear partial differential equations are presented for two dimensional heat and mass transfer within a single grain kernel during drying. In this model, the moisture evaporation inside the kernel is considered. The moisture is assumed to diffuse to the outer boundary of the kernel in liquid form and evaporate on the surface of the kernel. The influence of temperature and moisture content on grain properties is also considered in the simulation. The Non-linear partial differential equations are solved using the finite element method and simulation data is verified on a thin layer dryer for wheat kernels. The comparison shows that the simulated results have a high accuracy with average relative error of about 5%. The results of the finite element analysis can be used for grain quality evaluation, drying simulation studies and stress analysis of grain kernel.     

High power short time microwave finish drying of paprika (Capsicum annuum L.): Development of models for moisture diffusion and color degradation

The commercially available paprika at 16.25% (db) moisture was quickly finish-dried using microwaves at higher power density (5–25?W?g^?1). The moisture diffusivity was estimated using Fick’s second law of diffusion and the generalized kinetic model was used to estimate the color degradation rates. The moisture diffusivity and color degradation showed a close correlation with the difference between the average product temperature (T) achieved due to microwave heating, and average glass transition temperature (T_g) of paprika. Acceleration in moisture diffusion and color degradation was observed with the rise in the difference between the T and T_g. Further, the color degradation rate showed correlation with monolayer moisture content, average moisture content, T, and T_g of paprika during finish drying. The constants of the Gordon and Taylor model showed the less plasticization effect of water. Also, T_g showed a good correlation between water activity and moisture content. The activation energies for moisture diffusion and color degradation were found to be 92.53 and 11.03?kJ mol^?1_, respectively_. The microstructural analysis of finish-dried paprika showed the expanded and newly formed intercellular spaces. The developed correlations can be used to simulate heat and mass transfer operations such as drying and sterilization.     

An NMR determination of the physiological water distribution in wood during drying

Proton magnetic resonance has been used to study water in Douglas fir and western red cedar. The free induction decay, when combined with a knowledge of chemical composition of the wood, gives an accurate measure of the absolute moisture content. Spin-lattice relaxation was found to be significantly different for the two species. In sapwood, three distinct spin—spin relaxation times, T₂, were measured and assigned, with the help of anatomical data, to water in and on the cell wall, water in the ray and latewood tracheid lumens, and water in the earlywood tracheid lumens. This T₂ behavior was explained by a model in which free water in a void exchanges with a small fraction of bound water on the lumen surface. The three T₂'s were almost independent of moisture content, suggesting physically separate compartments. The behavior of the three water components during drying was studied. The fiber saturation point could be determined from a single T₂ measurement on a green sapwood sample. Magnetic resonance imaging of logs was investigated.     

Effects of Wood Drying Schedules on Fluid Flow in Paulownia Wood

Effects of three drying schedules on fluid flow were studied in the sapwood and heartwood of Paulownia wood (Paulownia fortunei). Boards with a commercial thickness of 5 cm were randomly dried to a final moisture content of 8 ± 2% using a mild (T₆E₃), a moderate (T₆E₄), and a severe (T₇E₄) drying schedule. Permeability measurement was carried out when specimens reached the final moisture content. Results showed a significant difference in the specific gas permeability as well as liquid permeability of the boards dried under the three drying schedules. Furthermore, a significant difference was observed in the gas permeability of sapwood and heartwood, dried under the different schedules, but not much significant difference was seen in the liquid permeability between sapwood and heartwood. T₆E₃ had the highest liquid permeability; furthermore, it was reported to have resulted in the lowest warping and most homogeneous moisture profile. This mild schedule is therefore recommended for commercial drying of Paulownia wood when further preservation and impregnation processes are planned for the dried boards. In the meantime, it is concluded that the age and drying schedule have significant effects on the formation of tyloses, significantly affecting gas and liquid permeability in Paulownia wood.     

Drying characteristics and water dynamics during microwave hot-air flow rolling drying of Pleurotus eryngii

Abstract

The aim of this study was to improve the drying uniformity and quality of Pleurotus eryngii by using the combination of microwave drying and hot-air flow rolling drying. The moisture content, drying uniformity, and water migration of P. eryngii during microwave hot-air flow rolling drying (MARD) were analyzed in detail. The temperature distribution images were obtained via infrared thermal imaging techniques and the relationship between relaxation time and signal amplitude were obtained via low-field nuclear magnetic resonance analysis and imaging (LF-NMR/MRI). The curves of signal amplitude with moisture content changes were fitted by a linear model with good linearity correlation. It was found that the hot-air and rolling bed could improve the drying uniformity of microwave drying. And the free water was found to transform into immobilized water and bound water during the drying process. Only a small amount of water was left in the dried P. eryngii in the final stage. The results could provide supportive information for improving the uniformity and quality of the drying processes of the edible fungi.