Radio Frequency Vacuum Drying Research Articles

Effect of Radio Frequency Vacuum Drying on Drying Characteristics and Physicochemical Quality of Codonopsis pilosula Slices

In this study, the radio frequency vacuum drying (RFVD) technique was used to dry Codonopsis pilosula slices. The effects of the drying temperature, slice thickness, plate spacing, and vacuum degree on the drying characteristics and physicochemical properties of the slices were investigated. The results showed that as the drying temperature and vacuum degree increased and the slice thickness and plate distance decreased, the drying rate and effective moisture diffusion coefficient of the Codonopsis pilosula slices improved, and the required drying time was shortened by 11.11% to 29.41% compared to that after hot air drying (HAD). Through comparison, it was found that the Midilli and Weibull models could better describe the moisture variation trend during the RFVD of Codonopsis pilosula. After RFVD, the retention of lobetyolin and syringin in Codonopsis pilosula significantly increased, with maximum contents of 135.74 mg/100 g and 19.16 mg/100 g respectively, which were 75.2% and 124.28% higher than those obtained by HAD. The contents of polysaccharides, total phenolics, and total flavonoids and antioxidant performance were also enhanced. The color, shrinkage rate, and internal tissue structure were significantly improved. In conclusion, RFVD not only increases the drying speed of Codonopsis pilosula slices but also ensures the good quality of the dried products.

Radio Frequency Vacuum Drying Study on the Drying Characteristics and Quality of Cistanche Slices and Analysis of Heating Uniformity.

To fully leverage the advantages of both hot air drying and radio frequency vacuum drying, a segmented combination drying technique was applied to post-harvest Cistanche. This new drying method involves using hot air drying in the initial stage to remove the majority of free water, followed by radio frequency vacuum drying in the later stage to remove the remaining small amount of free water and bound water. During the radio frequency vacuum drying (RFV) phase, the effects of temperature (45, 55, and 65 °C), vacuum pressure (0.020, 0.030, and 0.040 MPa), plate spacing (65, 75, and 85 mm), and slice thickness (4, 5, and 6 mm) on the drying characteristics, quality, and microstructure of Cistanche slices were investigated. Additionally, infrared thermal imaging technology was used to examine the surface temperature distribution of the material during the drying process. The results showed that compared to radio frequency vacuum drying alone, the hot air-radio frequency combined drying significantly shortened the drying time. Under conditions of lower vacuum pressure (0.020 MPa), plate spacing (65 mm), and higher temperature (65 °C), the drying time was reduced and the drying rate increased. Infrared thermal imaging revealed that in the early stages of hot air-radio frequency vacuum combined drying, the center temperature of Cistanche was higher than the edge temperature. As drying progressed, the internal moisture of the material diffused from the inside out, resulting in higher edge temperatures compared to the center and the formation of overheating zones. Compared to natural air drying, the hot air-radio frequency vacuum combined drying effectively preserved the content of active components such as polysaccharides (275.56 mg/g), total phenols (38.62 mg/g), total flavonoids (70.35 mg/g), phenylethanoid glycosides, and iridoids. Scanning electron microscopy observed that this combined drying method reduced surface collapse and cracking of the material. This study provides theoretical references for future drying processes of Cistanche.

Developing Effective Radio Frequency Vacuum Drying Processes for Moutan Cortex: Effect on Moisture Migration, Drying Kinetics, Physicochemical Quality, and Microstructure.

This study aims to maximize the post-harvest quality of Moutan Cortex and reduce energy consumption. Radio frequency vacuum (RFV) technology was used to dehydrate Moutan Cortex in this study to investigate the effects of different drying temperatures, plate spacing, and vacuum degree on the drying kinetics, physicochemical quality, and microstructure of Moutan Cortex. The results showed that RFV drying shortened the dehydration time of the Moutan Cortex by 10.71-28.57% and increased the drying rate by 15.79-54.39% compared to hot-air drying. The best color (∆E = 6.08 ± 0.28, BI = 26.97 ± 0.98) and relatively high retention of polysaccharides, total phenolics, total flavonoids, antioxidant properties, paeonol, gallic acid, paeoniflorin, and benzoylpaeoniflorin contents were observed in the dried products of Moutan Cortex at a drying temperature of 50 °C, spacing of 90 mm, and vacuum of 0.025 MPa. Analyzing the microstructure, it was found that RFV drying could effectively inhibit the shrinkage and collapse of the cellular structure, and a regular and loose honeycomb pore structure appeared inside the samples, which contributed to the rapid migration of the internal moisture. This study can provide a theoretical reference basis for the selection and application of industrialized processing methods of high-quality Moutan Cortex.

Effect of ultrasound combined with chemical pretreatment as an innovative non-thermal technology on the drying process, quality properties and texture of cherry subjected to radio frequency vacuum drying

To obtain high-quality cherry products, ultrasound (US) combined with five chemical pretreatment techniques were used on cherry prior to radio frequency vacuum drying (RFV), including carboxymethyl cellulose coating (CMC), cellulase (CE), ethanol (EA), isomaltooligosaccharide (IMO), and potassium carbonate + ethyl oleate (PC + AEEO). The effect of different pretreatments (US-CMC, US-CE, US-EA, US-IMO, US-(PC + AEEO)) on the drying characteristics, quality properties, texture, and sensory evaluation of cherries was evaluated. Results showed that the dehydration time and energy consumption were decreased by 4.17 − 20.83 % and 3.22 − 19.34 %, respectively, and the contents of individual sugars, soluble solid, total phenolics (TPC), natural active substances, total flavonoids (TFC), and antioxidant properties (DPPH, ABTS and FRAP) were significantly increased after US combined with five chemical treatments (P < 0.05). Moreover, the pretreatment played important role in improving texture properties and surface color retention in the dried cherries. According to the sensory evaluation analysis, the dehydrated cherries pretreated with US-CMC exhibited the highest overall acceptance, texture, crispness, color, and sweet taste showed lower off-odor, bitter taste and sour taste compared to control and other pretreatments. The findings indicate that US-CMC pretreatment is a promising technique for increasing physicochemical qualities and dehydration rate of samples, which provides a novel strategy to processing of dried cherry.

Developing effective radio frequency drying processes for tiger nuts: Dynamic analysis of moisture state, dielectric properties and quality

Radio frequency (RF) heating technology is widely applied to dry agricultural products due to the advantages of fast and volumetric heating, but it lacks systematic and in-depth studies on combination drying methods. This study aimed to investigate the drying mechanism of tiger nuts with hot-air drying (HD), radio frequency drying (RFD), radio frequency assisted hot-air drying (RFHD), and radio frequency-vacuum drying (RFVD), in terms of analyzing dynamic changes of dehydration kinetics, moisture migration, and dielectric properties during the drying processes, as well as quality attributes of dried tiger nuts. The results indicated that the RF drying time (including RFD, RFHD and RFVD) was significantly reduced by 38.60–54.39%, compared with HD. Although RF drying had a positive effect on physical qualities, it caused clear changes to the microstructure. Among all drying methods, the free water content (A23) in the tiger nuts decreased as drying processed, while the immobilized water content (A22) increased first and then decreased. The A23 exhibited strong positive correlations with dielectric constant (ε′) and loss factor (ε″). Correspondingly, ε′ and ε″ gradually decreased due to a reduction in the A23 value. The results may comprehensively understand the RF drying mechanism of tiger nuts to provide high-quality products and optimize the drying process.

Developing a two-stage strategy in the dehydration treatment of fresh apple slices: Combined freeze and radio frequency-vacuum drying

Freeze drying (FD) usually provides the best final quality but results in the high drying energy consumption, and the slow drying speed. To improve the FD efficiency, a two-stage dehydration strategy using a radio frequency-vacuum assisted freeze-drying process (FD + RFVD) was explored by taking apple slices as the representative samples. The drying rate changes during the FD treatment process of apple slices were experimentally determined. The best transition point of the two drying stages was also clearly obtained based on the changes of drying speed and product quality. The best processing parameters would be the FD drying time of 10.5 h followed by the RF-vacuum treatment at the electrode gap of 100 mm and vacuum pressure of 0.02 MPa. The optimized FD + RFVD process required only 81.25% of the FD drying time to achieve the same target moisture content, and similar or better product quality as compared to FD alone. This study may provide a technical guidance for developing effective and practical FD + RFVD technology to improve singe FD processes.

Effects of Different Drying Methods on the Drying Characteristics and Quality of Codonopsis pilosulae Slices.

The present study aimed to investigate the effect of rotary microwave vacuum drying (RMVD), radio frequency vacuum drying (RFVD), vacuum far infrared drying (VFID), vacuum drying (VD), hot air drying (HD) and natural drying (ND) on the drying characteristics, active ingredients and microstructure of Codonopsis pilosulae slices. Compared with the fitting results of the four models, the Weibull model is the most suitable drying model for Codonopsis. The RFVD and HD color difference values were smaller compared to ND. The effective moisture diffusivity (Deff) under different drying methods was between 0.06 × 10-8 m2/s and 3.95 × 10-8 m2/s. RMVD-dried products had the shortest drying time and retained more active ingredients. The microstructure analysis revealed that the porous structure of RMVD is more favorable for water migration. RMVD is a promising dehydration method for obtaining high-value-added dried Codonopsis products.

Effect of Ultrasonic Pretreatment on Radio Frequency Vacuum Drying Characteristics and Quality of Codonopsis pilosula Slices

In this paper, the effects of ultrasonic pretreatment on the drying kinetics, bioactive components (polysaccharides, total phenols, total flavonoids and antioxidant), qualitative characteristics (color index, lobetyolin and syringin) and microstructure of Codonopsis pilosula during radio frequency vacuum drying (RFVD) were studied. The average drying rate curve showed that the whole drying process could be divided into three stages: accelerating period, constant drying rate period and falling drying rate period. Deff values ranged from 6.61425 to 9.46745 × 10−8. Analysis of the drying rate constants revealed that different conditions of pretreatment were effective in increasing the drying rate. Ultrasonic pretreatment has a positive effect on the retention of polysaccharide content; low frequency favors retention of total phenols, flavonoids and syringin; and with the increase in ultrasonic time and ultrasonic power, the antioxidant capacity was higher than that without ultrasonic treatment. Ultrasonic pretreatment significantly improved color and microstructure. In summary, the pretreatment condition of ultrasonic frequency 20 kHz and power 60 W for 30 min is suitable, which provides a certain reference for the application of ultrasonic pretreatment technology in RFVD of Codonopsis pilosula slices.

Comparative analyses of five drying techniques on drying attributes, physicochemical aspects, and flavor components of Amomum villosum fruits

Amomum villosum fruit is an attractive spice owing to its pleasant aromatic properties, and drying is an efficacious way to maintain its therapeutic and trading values. In this study, Amomum villosum fruits were dried using five drying technologies namely, freeze drying (FD), hot air impingement drying (HAID), hot air drying with temperature and humidity process control (HADTHPC), HADTHPC combined with radio frequency-vacuum drying (HADTHPC + RFVD), as well as medium-and short-wave infrared drying (MSWID). Quality and drying characteristics were compared. Result showed that FD achieved the best color retention, the lowest shell burst ratio, and the best retention of flavor profiles due to the complete glandular trichomes structure; however, it recorded the longest drying time and the highest energy consumption. HADTHPC + RFVD, combining the advantages of HADTHPC and RFVD, had the shortest drying time, the highest conservation of total flavonoids content (TFC), the best antioxidant activity, and low energy consumption compared with the other thermal drying methods. After drying, the content of esters decreased, while alcohols content increased. GC-IMS indicated the fingerprints of the four thermal dehydrated samples exhibiting similar signal intensities. Therefore, in terms of maintaining quality and saving energy, HADTHPC + RFVD would be a promising drying technology for Amomum villosum fruits.

Radio frequency vacuum drying of Eucalyptus nitens juvenile wood

Wood drying is an important process for adding value and manufacturing innovative products. Eucalyptus nitens wood is inherently difficult to dry because of its natural propensity for checking as well as collapse and shrinkage. Lumber recovery after industrial drying of eucalypts is also very low. This study measured the wood quality of E. nitens juvenile wood (13 mm thickness) after radio-frequency vacuum (RFV) drying and wood dried in a conventional kiln dryer (KD). Drying cycles were performed using a radio frequency vacuum dryer with a 3 m3 of capacity and convective kiln-dryer equipment with a 3.5 m3 of capacity. The results showed that the drying time using the radio frequency vacuum method was reduced by 47% when compared to conventional kiln drying. The shrinkage was significantly lower in the RFV than in the conventional KD. The volumetric collapse decreased by approximately 60% in the RFV drying. RFV drying of E. nitens juvenile wood improves the wood quality for solid wood products because the intensity of surface checking and collapse are reduced.

Radio Frequency-Vacuum Drying of Kiwifruits: Kinetics, Uniformity, and Product Quality

To overcome long drying time, low energy efficiency and poor product quality associated with conventional drying, a radio frequency (RF) vacuum technology is proposed for drying kiwifruit slices using a 27.12 MHz, 3 kW RF-vacuum drying system. The results demonstrated that the process variables, electrode gap, vacuum pressure, and sample thickness, had major effects on the RF-vacuum drying. The RF-vacuum drying was associated with internal heating and rapid drying resulting in 65% reduction of hot air drying (60 °C) time. Moreover, kiwifruits dehydrated by RF-vacuum drying were associated with better color stability, higher vitamin C retention, and higher rehydration capacity (p < 0.05) as compared with hot-air-dried samples. Based on acceptable drying rate, stable temperature and avoiding arcing, a RF-vacuum drying protocol with the electrode gap of 60 mm, vacuum pressure of 0.02 MPa, and sample thickness of 8 mm was identified. Despite some differences observed in individual fruit slices, the RF-vacuum drying technique achieved better and more uniform drying patterns among the samples. Overall, the RF-vacuum drying process may provide a more effective and practical method for high-quality dehydration of kiwifruits.

Characteristics of Timbers Dried Using Kiln Drying and Radio Frequency-Vacuum Drying Systems

Heavy hardwoods are difficult-to-dry timbers as they are prone to checking and internal stresses when dried using a conventional kiln drying system. These timbers are usually dried naturally to reach 15% to 19% moisture content with an acceptable defects. Besides long drying time, timbers at these moisture contents are not suitable for indoor applications since they will further dry and causing, for example, jointing and lamination failures. Drying to a lower moisture content could only be achieved in artificial drying kilns such as conventional kiln, dehumidification kiln, solar kiln, radio frequency-vacuum, etc. The objective of this study was to evaluate the characteristics of 30 mm and 50 mm thick kekatong ( Cynometra spp.) timber dried using kiln drying (KD) and radio frequency-vacuum drying (RFV) system. The investigation involved drying time, moisture content (MC) variations between and within boards, drying defects, shrinkage, and drying stress. Drying defects include checks (surface, end, and internal checks) and warping (bowing, cuping, spring, and twisting). The results showed that RFV drying time was reduced to 50% compared to the KD. RFV dried boards demonstrated a more uniform MC between and within boards. Shrinkage in width and thickness, as well as tangential/radial and volumetric shrinkages were substantially less in RFV boards. The amount of cupping, bowing and spring were very low and negligible in all drying runs. There was no twisting observed in all drying methods. The number of stress-free RFV board was higher than KD. With proper procedure, the RFV technology could be used for drying heavy hardwoods which are difficult to dry in conventional kilns due to excessive drying times and degradation.

Radio Frequency Vacuum Drying of Timber: Mathematical Model and Numerical Analysis

The mass and heat transfer mechanisms during radio frequency/vacuum (RF/V) drying of square-edged timber were analyzed and discussed in detail. Based on the mass and heat transfer theory of porous materials, a one-dimensional mathematical model was developed from conservation equations. Compared with conventional models, it has the following advantages: (1) Each independent variable has a separate governing equation and is solved independently by converting the partial differential equation into a difference equation with the finite volume method; and (2) The calculated data from different parts of the wood specimen are displayed in the evolution curves because the specimen is divided into several units along fiber direction. Therefore, the change law of the parameters can be better described. The software Matlab, which has the advantages of fast calculation speed and high precision, was used for programming and calculations. In addition, the square-edged timber Sugi specimen was dried in a laboratory RF/V dryer, and the total average moisture content (MC) and local temperature were monitored as a function of time. By analyzing the calculated and experimental results, it can be concluded that most of the important phenomena observed during RF/V drying can be adequately described by this model.

Effect of different drying methods on liquid penetration of Chinese fir plantation wood

Chinese fir plantation sapwood and heartwood boards were treated by three drying methods: radio frequency-vacuum drying (RFVD), conventional kiln drying (KD) and high temperature drying (HTD). The maximum amount of dyeing solution uptake by the capillary rising method was used to evaluate the liquid penetration of the treated wood. The pit aspiration ratio was determined by a semithin section method. Changes in wood microstructure were investigated using scanning electron microscopy. The results show that liquid penetration of Chinese fir sapwood after RFVD is significantly higher than that after KD and HTD. Liquid penetration of Chinese fir heartwood after RFVD is higher than that after HTD. Liquid penetration of Chinese fir sapwood is significantly higher than that of heartwood after three drying treatments. Low pit aspiration ratio and cracks of some bordered pits are the main reasons for the increase in liquid penetration after RFVD treatment.

Predicting Thermal Efficiency in Timber Radio Frequency Vacuum Drying

In this work, the efficiency of transforming dielectric energy into evaporated water is analyzed for the case of timber radio frequency vacuum drying. Based on well-known heat and mass transfer equations, a simplified mathematical model is proposed that estimates the drying efficacy in regards to the thermo-physical properties of wood. Although not exact, the theoretical results are close to the experimental observations and elucidate some phenomena like the tendency of the timber to dry from inside to outside, and the drying rate increase with the rise of the timber gas permeability. The theoretical efficiency model also predicts a range of wood permeability values for which the drying efficiency changes from 100 to 0%, thus providing a quantitative scale for classifying the spectrum of “difficult-to-dry” all the way to “easy-to-dry” wood species when using radio frequency vacuum technology.

Radio Frequency Vacuum Drying of Wood. III. Two-Dimensional Model, Optimization, and Validation

A simple two-dimensional model is presented for the radio frequency vacuum drying of thick timbers. The developed code is linked to powerful optimization methods to account for the variability of the permeability and the internal pressure deviation from the saturated vapor pressure above the fiber saturation point. The model is compared with recently obtained experimental results where the radio frequency heating source is an amplifier. Despite the simplicity of the model, good agreement with experimental data was obtained for a variety of drying conditions, especially in predicting the average moisture evolution and the drying time as well as the development of the temperature profiles in two types of softwood timbers.

Non-Deterministic Description of Wood Radio Frequency Vacuum Drying

Summary A numerical method is proposed to predict the stochastic dispersion of data that unavoidably occurs in a real drying process. The method is based on the use of discrete frequency distribution curves to predict a random variation on some parameters of the process and is applied to the case of radio frequency vacuum (RFV) drying of wood. Experimental data of RFV drying of western hemlock timbers were used to obtain the numeric values of the stochastic parameters and their standard deviation. A mathematical model was designed to simulate the final moisture content dispersion on hypothetical examples. Results were quantified using three indices: average, range and standard deviation.

STOCHASTIC NUMERICAL MODEL FOR RADIO FREQUENCY VACUUM DRYING OF TIMBERS

ABSTRACT A new numeric method to simulate stochastic dispersion (a natural phenomenon that occurs when a magnitude cannot be associated to a specific value, but to the probability of being within a range of values) is proposed and applied to predict Radio Frequency Vacuum (RFV) drying of timber. A theoretical formulation of the method is described and complemented to take into account the frequency distribution of the timber initial moisture content, so that it can be applied to industrial runs. Experimental data obtained from mixed western hemlock and amabilis fir dried in a commercial RFV kiln are used to validate the stochastic model, and the results are compared through moisture content histograms and probability charts. A numerical example is shown in order to provide an idea of the movement of the moisture profiles during RFV drying.

Moisture flow characteristics during radio frequency vacuum drying of thick lumber

Moisture flow characteristics during radio frequency vacuum drying of thick lumber

Moisture flow characteristics during radio frequency vacuum drying of thick lumber

Thick specimens of two softwoods were dried in a laboratory radio frequency/vacuum (RF/V) kiln in order to investigate the internal moisture flow patterns. The spatial moisture distributions in the longitudinal arid transverse directions were monitored as a function of time in a number of runs. The results indicated that both longitudinal and transverse moisture transfer modes contribute on the overall moisture flow occurring within wood, but there was no distinction as to the percentage contribution of each to the overall flux. There were no abrupt drying front changes and no moisture discontinuities observed during drying. A second group of runs was also carried out using end-matched specimens to study its length effects on drying characteristics, such as drying rate, and internal vapor pressure and temperature profiles. The results demonstrated that short specimens dried faster than long ones at moisture contents above the fiber saturation point.