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

Abstract

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.

1. Introduction

Codonopsis pilosula is the dried root of Codonopsis pilosula (Franch.) Nannf, Codonopsis pilosula Nannf.var.modesta (Nannf.) L.T.Shen, or Codonopsis tangshen Oliv., belonging to the Campanulaceae family [1]. Codonopsis pilosula is a valuable medicinal herb that serves both as food and medicine. It is rich in polysaccharides, phenolics, flavonoids, codonopsis saponins, syringin, volatile oils, triterpenes, trace alkaloids, and essential amino acids. These components provide various benefits, such as neuroprotection, blood sugar regulation, immune enhancement, microcirculation improvement, and vasodilation, playing a significant role in disease prevention and treatment [2,3]. However, fresh Codonopsis pilosula contains a high amount of free water and natural active substances, which can lead to spoilage, the loss of components, and bacterial growth during storage, transportation, and drying. These adverse effects cause changes in the internal structure and rapid deterioration of the physicochemical quality, affecting the medicinal value and sensory properties of the material. Drying, as a solid–liquid separation technique, is one of the most commonly used methods for reducing the water activity of agricultural products, preserving nutrients, and extending shelf life [4].

Currently, research on the drying process of Codonopsis pilosula is still in its early stages. In its place of origin, traditional natural drying and sulfur fumigation are primarily used. Although these methods are low-cost and widely applicable, they have many drawbacks. Traditional natural drying methods are highly weather-dependent, have long drying cycles, and result in suboptimal quality. Moreover, there is a significant loss of medicinal components, and under cloudy and rainy conditions, the material easily molds and rots [5]. Sulfur fumigation results in the better apparent quality of Codonopsis pilosula, but it leaves harmful residues of sulfur dioxide and other substances, making it unsuitable for human consumption [6]. Additionally, both natural drying and sulfur fumigation have uncontrollable drying processes, fail to meet hygiene standards, and cannot ensure product quality. They also cannot achieve standardized and large-scale processing. Studies by Zang [7] and Xu et al. [8] have shown that traditional drying methods such as shade drying and sun drying negatively affect the appearance and medicinal value of herbs. Therefore, finding a reasonable drying method and process to maximize the retention of the material’s original sensory quality, flavor, and color is crucial for enhancing the edible and medicinal value of Codonopsis pilosula.

The working principle of RFVD technology is to use the friction between water molecules under the action of a high-frequency electric field to achieve volumetric heating [9]. This causes intense collisions and polar movements between molecules within the material, converting electromagnetic energy into thermal energy and thereby evaporating the moisture from the inside [10]. Radio frequency vacuum heating is a non-ionizing electromagnetic radiation technology that can better preserve the natural active substances and color quality of medicinal materials under vacuum conditions. Therefore, applying radio frequency vacuum technology to the drying of medicinal materials is feasible. Compared to traditional direct heating methods such as hot air [11] and vacuum far-infrared [12] drying, RFVD has advantages such as non-contact heating, high efficiency, uniform drying, good quality, and accurate control of the moisture regain rate. Additionally, the penetrative nature of the electromagnetic field gives dried products a certain puffing effect, greatly improving elasticity, toughness, and taste. The study by Zang et al. [13] indicated that compared to hot air drying, RFVD significantly shortened the dehydration time of peony root bark, increased the drying rate, and resulted in the best color of the dried product. Additionally, the retention rates of active ingredients were higher with RFVD. Zhou et al. [14] found through their research on the drying kinetics, uniformity, and product quality of kiwifruit subjected to RFVD that RFVD not only improved the drying rate but also resulted in kiwifruit with better color, higher vitamin C retention, and better rehydration capacity. Currently, radio frequency technology has been preliminarily applied to the dehydration of agricultural products such as soybeans [15], corn [16], carrots [17], and purple sweet potatoes [18].

This study applied RFVD technology to the drying of Codonopsis pilosula slices, investigating the effects of different drying temperatures, slice thicknesses, vacuum levels, and plate spacing on the drying characteristics of Codonopsis pilosula slices. It also evaluated changes in quality and microstructure, aiming to provide technical guidance for the post-harvest processing of Codonopsis pilosula.

2. Materials and Methods

2.1. Experimental Materials

Fresh Codonopsis pilosula was purchased from Weiyuan County, Dingxi City, Gansu Province, and immediately refrigerated at 2–4 °C. Uniformly sized and intact primary roots of Codonopsis pilosula were selected as test materials. The initial moisture content (M0) of the Codonopsis pilosula slices was determined to be 72.50 ± 0.50% using the Official Analysis Method (AOAC) [19].

2.2. Instrument and Equipment

This experiment used the GJS-3-27-JY model high-frequency vacuum dielectric heating test device, with a voltage of 380 V, a high-frequency rated power of 27.12 MHz, a high-frequency oscillation power of 3 KW, and an ultimate vacuum degree of −0.09 MPa. The distance between the upper and lower plate was adjustable from 20 mm to 300 mm. The device was developed by Hebei Huashi Jiyuan High-Frequency Equipment Co., Ltd. (Langfang, China), and the specific structure and parameters of the equipment are shown in Figure 1.

The control group used hot air drying with a YQ101-0A-4A model electric blast drying oven, with a voltage of 220 V/50 Hz, a power of 1.2 KW, and an airspeed of 1.5 m/s. The equipment was manufactured by Beijing Yuqin Tengda Pharmaceutical Equipment Co., Ltd. (Langfang, China).

2.3. Experimental Methods

Uniformly sized and intact Codonopsis pilosula roots were selected, cleaned, sliced, and weighed (300.0 ± 0.5 g). The slices were then evenly and singly layered in a porous polytetrafluoroethylene drying tray and placed in an RFVD device (preheated for 30 min prior to the experiment). When the moisture content on a wet basis decreased to 10%, the experiment was stopped. To ensure reliability, all experiments were performed in triplicate, and the average value was used as the experimental result. Based on the results of preliminary trials, the drying temperature (50 °C, 55 °C, 60 °C), slice thickness (3 mm, 4 mm, 5 mm), plate spacing (80 mm, 90 mm, 100 mm), and vacuum degree (0.015 MPa, 0.025 MPa, 0.035 MPa) were selected as factors for single-factor orthogonal experimental design. The experimental plan for RFVD of Codonopsis pilosula slices was developed, as shown in

Table 1. RFVD test program for Codonopsis pilosula Slices.

Experiments Number	Experiments Condition
	Drying Temperature (°C)	Slice Thickness (mm)	Plate Spacing (mm)	Vacuum Degree (Mpa)
1	50	4	90	0.025
2	55	4	90	0.025
3	60	4	90	0.025
4	55	3	90	0.025
5	55	4	90	0.025
6	55	5	90	0.025
7	55	4	80	0.025
8	55	4	90	0.025
9	55	4	100	0.025
10	55	4	90	0.015
11	55	4	90	0.025
12	55	4	90	0.035
13	55	4	HAD

.

Hot air drying (HAD) was used as the control group experiment: Selected Codonopsis pilosula slices were cleaned and dried and then sliced (4 mm). Weighed precisely to 300 ± 0.5 g, the slices were evenly and singly layered in a stainless-steel tray. The weight of the Codonopsis pilosula slices was recorded every 15 min until the moisture content on a wet basis decreased to 10%.

2.4. Drying Characteristics

2.4.1. Dry Basis Moisture Content

The moisture content of the dried base in the drying process of the Codonopsis pilosula was calculated as shown in Equation (1):

$$M_t={\displaystyle \frac{W_t-W_d}{W_d}}$$

(1)

where $M_t$ is the dry basis moisture content of Codonopsis pilosula slices at time $t$ (%), $W_t$ is the mass of Codonopsis pilosula slices at time t (g), and $W_d$ is the mass of dry matter in Codonopsis pilosula slices (g).

2.4.2. Moisture Ratio

The moisture ratio of the Codonopsis pilosula slices was calculated as shown in Equation (2):

$$MR={\displaystyle \frac{M_t{-M}_e}{M_0{-M}_e}}$$

(2)

where $MR$ is the moisture ratio of Codonopsis pilosula Slices, $M_0$ is the initial moisture content of Codonopsis pilosula slices (%), and $M_e$ is the equilibrium moisture content of Codonopsis pilosula slices (%).

2.4.3. Drying Rate

The Drying rate of the Codonopsis pilosula slices was calculated as shown in Equation (3):

$$DR={\displaystyle \frac{M_{t2}-M_{t1}}{t_2-t_1}}$$

(3)

where $DR$ is the drying rate of the Codonopsis pilosula slices, $M_{t2}$,$M_{t1}$ is the weight of the Codonopsis pilosula slices at times $t_2$, and $t_1$ (g), $t_2-t_1$ is the time interval between the two drying processes (min).

2.4.4. Effective Moisture Diffusion Coefficient

The effective moisture diffusion coefficient of the material is calculated as shown in Equation (4).

$$MR={\displaystyle \frac{8}{{\pi }^2}}\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\pi }^2{\displaystyle \frac{D_{eff}}{{4d}^2}}t\right)$$

(4)

where Deff is the effective moisture diffusion coefficient of Codonopsis pilosula slices (m²/s), $t$ is the drying time (s), and $d$ is half the thickness of the Codonopsis pilosula slices (mm)$.$

2.4.5. Drying Kinetic Model

As shown in

Table 2. Common thin-layer drying models.

Drying Model	Model Equation	Model Parameter
Newton	MR = exp(−kt)	k
Midilli	MR = aexp(−ktn) + b	a; k; n; b
Weibull	MR = exp[−(k/α)β]	α; β
Handerson and Pabis	MR = aexp(−kt)	a; k
Logarithmic	MR = aexp(−kt) + b	a; k; b
Two-term exponential	MR = aexp(−kt) + (1 − a)exp(−kat)	a; k

, six drying models were used to describe the drying kinetics of Codonopsis pilosula slices to predict and analyze the moisture content variation.

The fitting performance of the model was evaluated by the coefficient of determination (R²) and the root mean square error (RMSE). The larger the R² and the smaller the RMSE, the better the fit. The calculation formulas are as follows:

$$R^2={\displaystyle \frac{\sum _{i=1}^N{\left(M{R}_{pre,i}-M{R}_{\mathrm{e}\mathrm{x}\mathrm{p},i}\right)}^2}{\sum _{i=1}^N{\left(M{R}_{\mathrm{e}\mathrm{x}\mathrm{p},i}-M{R}_{\mathrm{e}\mathrm{x}\mathrm{p}}\right)}^2}}$$

(5)

$$RMSE=\sqrt{{\displaystyle \frac{\sum _{i=1}^N{\left(M{R}_{\mathrm{e}\mathrm{x}\mathrm{p},i}-M{R}_{\mathrm{e}\mathrm{x}\mathrm{p}}\right)}^2}{N}}}$$

(6)

where $M{R}_{\mathrm{e}\mathrm{x}\mathrm{p},i}$ is the moisture ratio obtained from the i-th experiment, $M{R}_{pre,i}$ is the predicted moisture ratio of the i-th experiment, and N is the number of experimental groups.

2.5. Quality Attributes

2.5.1. Color Values

A CR-210 (Konica Minolta, Zhengzhou, China) precision colorimeter was used to measure the color change of Codonopsis pilosula slices, and the total color difference ($∆E$) was calculated using Equation (7) [20].

$$∆E=\sqrt{{\left(L^{\ast }-L_0\right)}^2+{\left(a^{\ast }-a_0\right)}^2+{\left(b^{\ast }-b_0\right)}^2}$$

(7)

where $∆E$ is the total color difference of Codonopsis pilosula slices. $L^{\ast }$, $a^{\ast }$, and $b^{\ast }$ are the brightness, red–green, and yellow–blue values of fresh Codonopsis pilosula slices, respectively. $L_0$, $a_0$, and $b_0$ represent the brightness, red–green, and yellow–blue values of dried Codonopsis pilosula slices.

2.5.2. Rehydration Ratio (RR)

In total, 5 g of dried Codonopsis pilosula slices were weighed, placed in a beaker containing 100 mL of distilled water, and left in a water bath at 37 °C for 60 min. Then, the samples were drained using a filter and weighed with an electronic balance with an accuracy of ±0.0001 g. The rehydration ratio of the material was calculated using Equation (8) [21]:

$$RR={\displaystyle \frac{A}{A_0}}$$

(8)

where $A$ is the mass of the rehydrated Codonopsis pilosula slices (g), and $A_0$ is the mass of the dehydrated Codonopsis pilosula slices (g).

2.5.3. Shrinkage Rate (SR)

The SR of Codonopsis pilosula slices was calculated using Equation (9).

$$SR={\displaystyle \frac{V_0-V}{V_0}}$$

(9)

where $V_0$ is the volume of Codonopsis pilosula slices before drying (mL), and V is the volume after drying (mL).

2.5.4. Lobetyolin and Syringin

The contents of Lobetyolin and Syringin in Codonopsis pilosula were determined by HPLC. The chromatographic conditions were as follows: the chromatographic column was Agilent Eclipse XDB-C₁₈ (250 mm × 4.6 mm, 5 μm); the mobile phase consisted of acetonitrile (B) and 1% aqueous acetic acid (D); gradient elution was performed for 0–4 min (15–40% B), 4–8 min (40–65% B), 8–10 min (65–85% B), 10–12 min (85–15% B), and 12–16 min (15–15% B); the flow rate was 1.0 mL/min; the column temperature was 25 °C; the detection wavelength was 280 nm; and the injection volume was 10 μL.

2.5.5. Polysaccharide Content (PC)

The determination of PC in Codonopsis pilosula slices was performed using the phenol-sulfuric acid method, following the procedure described by Dubois [22]. Sucrose was used as the calibration standard. The PC in Codonopsis pilosula slices was calculated based on Equation (10).

$$PC={\displaystyle \frac{\left(C_s\times V_{s2}\right)}{\left(V_{s1}\times M_s\right)}}$$

(10)

where $C_s$ is the mass concentration of polysaccharides (mg/mL), $V_{s1}$ is the volume of the sample solution aspirated (mL), $V_{s2}$ is the volume of the extraction solution (mL), and $M_s$ is the mass of Codonopsis pilosula slices’ dry matter (g).

2.5.6. Total Phenolic Content (TPC)

The TPC in Codonopsis pilosula slices was determined using the Folin–Ciocalteu method, as referenced by Beato [23], with gallic acid used as the standard. The TPC in Codonopsis pilosula slices was calculated using Equation (11).

$$TPC={\displaystyle \frac{\left(C_p\times V_{p2}\right)}{\left(V_{p1}\times M_p\right)}}$$

(11)

where $C_p$ is the mass concentration of gallic acid (mg/mL), $V_{p1}$ is the volume of the sample solution aspirated (mL), $V_{p2}$ is the volume of the extraction solution (mL), and $M_p$ is the mass of Codonopsis pilosula slices’ dry matter (g).

2.5.7. Total Flavonoid Content (TFC)

The TFC in Codonopsis pilosula slices was determined using the sodium nitrite–aluminum nitrate–sodium hydroxide method, following the procedure described by Lay [24], with catechin used as the standard for calibration. The TFC in Codonopsis pilosula slices was calculated using Equation (12).

$$TFC={\displaystyle \frac{\left(C_f\times V_{f2}\right)}{\left(V_{f1}\times M_f\right)}}$$

(12)

where $C_f$ is the mass concentration of catechins (mg/mL), $V_{f1}$ is the volume of the sample solution aspirated (mL), $V_{f2}$ is the volume of the extraction solution (mL), and $M_f$ is the mass of Codonopsis pilosula slices’ dry matter (g).

2.5.8. Antioxidant Properties (AP)

The total antioxidant capacity of organic active substances was measured using the DPPH method, following the procedure described by Nencini [25]. The antioxidant capacity of the samples was calculated using Equation (13).

$$I={\displaystyle \frac{\left(A_0-A\right)}{A}}\times 100\%$$

(13)

where $I$ is the inhibition rate of the sample solution (%), $A$ is the absorbance of the sample solution, and $A_0$ is the absorbance of the solution without the sample.

2.5.9. Microstructure

The Codonopsis pilosula slices were cut into small pieces of 5 × 5 mm, quickly immersed in 2.5% glutaraldehyde solution for 12 h, and then rinsed three times with 0.2 mol/L phosphate buffer (pH 7.4) for 15 min each time. They were then sequentially dehydrated in a graded ethanol series of 50%, 70%, 80%, 90%, and 100% for 15 min each. The permeated Codonopsis pilosula slices were finally transferred to tert-butyl alcohol for storage. After gold coating, the samples were observed using a scanning electron microscope with an accelerating voltage of 5.0 kV. The repeated samples were examined at a magnification of 300×, and images of representative areas were saved for further analysis.

2.6. Statistical Analysis

To ensure the accuracy of the experimental data and minimize random errors caused by improper operation, each experiment was repeated three times, and the average value was taken as the final result. Excel 2016 was used to process the data, and Origin 2021 was used to plot the curves and images. Analysis of variance (ANOVA) was conducted using SPSS 24.0 (Warren Duncan Model).

3. Results and Discussion

3.1. Effect of Different Drying Conditions on the RFVD Characteristics of Codonopsis pilosula Slices

3.1.1. Effect of Different Drying Temperatures on Drying Characteristics

The effect of the drying temperature on the drying characteristics of Codonopsis pilosula slices with a slice thickness of 4 mm, plate spacing of 90 mm, and vacuum degree of 0.025 MPa is shown in Figure 2. The corresponding drying times at temperatures of 50 °C, 55 °C, and 60 °C were 290 min, 260 min, and 240 min, respectively. Compared to HAD (340 min), the drying times were reduced by 14.71%, 23.53%, and 29.41%, respectively. The average drying rates at these temperatures were 0.96 g/g∙min, 0.97 g/g∙min, and 1.11 g/g∙min, which were 17.07%, 18.29%, and 35.37% higher than that of HAD (0.82 g/g∙min). This indicates that RFVD effectively reduces the internal diffusion resistance of the material, increases the drying rate, and enhances the absorption of thermal energy by Codonopsis pilosula slices as the radiation temperature increases. Under the influence of temperature and humidity gradients, the vapor pressure difference between the sample and the drying medium increased, enhancing the internal energy of water molecules, which accelerated the diffusion and evaporation of internal moisture, thereby improving heat transfer efficiency. Although high temperatures significantly increase the drying rate, they can also cause severe shrinkage, dry shrinkage, and surface hardening of the material, which reduces the internal permeability. Similar conclusions were reached in the study by Shi et al. [26]. Additionally, it was observed that the drying rate curves of Codonopsis pilosula slices under different temperature conditions exhibited a brief accelerating phase, followed by a significant decelerating phase, without a clear constant rate phase. First, as the internal moisture evaporates and the moisture content decreases, the free water content significantly reduces, leading to an increased resistance to moisture diffusion within the samples. Second, due to the physical and chemical interactions within the material, water becomes tightly bound to the material structure, further reducing the drying rate [27].

3.1.2. Effects of Different Slice Thicknesses on Drying Characteristics

The effect of slice thickness on the drying characteristics of Codonopsis pilosula slices at 55 °C, 90 mm, and 0.025 MPa is shown in Figure 3. It can be observed that the drying rate rapidly increases within the first 20 min, showing a trend of first increasing and then decreasing, with a distinct constant rate period. As the slice thickness decreases, the drying time shortens and the drying rate increases. This is likely because thinner slices absorb more electromagnetic energy per unit time, enhancing water permeability and thereby improving drying efficiency [21]. For different slice thicknesses, the times required for the sample moisture content to decrease to a safe level were 240 min, 270 min, and 290 min, respectively. Compared to 3 mm, the drying times for 4 mm and 5 mm slices increased by 12.50% and 20.83%, respectively. This may be due to the loss of polarization motion and the ionic oscillation migration of polar molecules in the sample during RFVD, which generates a large amount of heat energy through molecular friction, increases the internal temperature and pressure gradients, weakens intermolecular cohesion, enhances internal water turbulence, and thus accelerates moisture diffusion and migration [28]. Additionally, as electromagnetic waves interact with the material surface, the penetration intensity decreases with increasing slice thickness. Thinner samples exhibit more pronounced surface heating effects but may lead to local overheating and uneven temperature distribution [29].

3.1.3. Effects of Different Plate Spacing on Drying Characteristics

The effect of plate spacing on the drying characteristics of Codonopsis pilosula slices at 55 °C, 4 mm, and 0.025 MPa is shown in Figure 4. As shown in the figure, the time required for the samples to reach a safe moisture content was 250 min, 280 min, and 300 min for plate spacings of 80 mm, 90 mm, and 100 mm. Compared to HAD (340 min), the dehydration times were reduced by 26.47%, 17.65%, and 11.76%. This indicates that a reduction in plate spacing positively affects the shortening of the drying time and the increase in the drying rate, which is consistent with the variation pattern of the radio frequency load current with plate spacing. With the initial moisture content unchanged, a smaller plate spacing results in a shorter time for the sample to reach the desired temperature. This is because the intensity of the radio frequency electric field increases as the plate spacing decreases, leading to greater energy absorption [16,30]. Additionally, reducing the plate spacing increases the inter-plate capacitance, resulting in a decrease in the heating circuit frequency, thus increasing the drying rate. However, if the plate spacing is too low, it may cause the heating circuit frequency to fall below the operating frequency, leading to reduced coupling and uncontrolled heating at the corners or edges of the sample, thereby weakening energy transfer and reducing drying quality.

Furthermore, the dielectric properties of the material affect the distribution of the electromagnetic field, which in turn determines the heating rate of Codonopsis pilosula slices during RFVD. The heating rate formula for the radio frequency vacuum electric field indicates that the heating rate is inversely proportional to the square of the plate spacing, meaning that a smaller plate spacing results in a faster heating rate. This is because the high-intensity electric field disrupts the internal chemical bonds and molecular structure of the Codonopsis pilosula slices, reducing their dielectric properties and prolonging the time required for the sample to reach a safe moisture content.

3.1.4. Effects of Different Vacuum Degrees on Drying Characteristics

The effect of the vacuum degree on the drying characteristics of Codonopsis pilosula slices at 55 °C, 4 mm, and 90 mm is shown in Figure 5. As the vacuum degree increases, the dehydration time of the material decreases and the drying rate increases. At a vacuum degree of 0.035 MPa, the total drying time was 240 min, which was reduced by 17.24% and 11.11% compared to the drying times at 0.015 MPa (290 min) and 0.025 MPa (270 min), respectively. The average drying rates at the corresponding vacuum degrees were 0.87 g/g∙min and 0.97 g/g∙min, which were 20.91% and 11.82% lower compared to the drying rate at 0.035 MPa (1.10 g/g∙min). This may be due to the increase in the vacuum degree, which leads to a larger total pressure difference between the inside and outside of the sample. Under the influence of the electromagnetic field, the interaction of bipolar particles within the material causes molecular movement, friction, and heat generation. This process rapidly accumulates energy and internal pressure, thereby accelerating the rate at which moisture migrates to the surface of the material. Furthermore, it was observed that the drying rate curve showed minimal fluctuation. This is likely because, as drying progresses, the increase in chamber humidity increases the likelihood of the material being in a hygroscopic state. Hygroscopicity and moisture diffusion occur simultaneously, causing radio frequency energy to concentrate in areas with a higher moisture content, thereby maintaining a dynamic balance of moisture content through the moisture content self-equilibrium effect. Therefore, compared to HAD, RFVD technology offers advantages such as better drying uniformity and higher drying efficiency.

3.1.5. Effect of Different Drying Conditions on the Effective Moisture Diffusivity (Deff)

Table 3. Deff of Codonopsis pilosula Slices under Different Drying Conditions.

Experiments Number	Experiments Condition				Deff/(×10−10 m2/s)
	Drying Temperature (°C)	Slice Thickness (mm)	Plate Spacing (mm)	Vacuum Degree (Mpa)
1	50	4	90	0.025	3.6233
2	55	4	90	0.025	4.2633
3	60	4	90	0.025	5.5638
4	55	3	90	0.025	4.5032
5	55	4	90	0.025	4.0028
6	55	5	90	0.025	3.6025
7	55	4	80	0.025	4.0028
8	55	4	90	0.025	4.5032
9	55	4	100	0.025	3.7268
10	55	4	90	0.015	3.6024
11	55	4	90	0.025	3.7599
12	55	4	90	0.035	4.1568
13	55	4	HAD		3.0021

shows that the Deff of Codonopsis pilosula slices dried by RFVD ranges from 3.6024 × 10⁻¹⁰ to 5.5638 × 10⁻¹⁰. As the drying temperature increases, the radio frequency energy absorbed by Codonopsis pilosula increases, resulting in the formation of numerous micropore channels within the sample. This accelerates the diffusion and evaporation of moisture and reduces internal diffusion resistance, leading to an upward trend in Deff. With an increase in slice thickness, Deff initially increases and then decreases, which is contrary to general expectations. This may be due to the fact that thinner slices lose moisture more rapidly, leading to surface hardening that slows moisture diffusion, resulting in a smaller effective moisture diffusivity. Additionally, as the thickness increases, the plate spacing decreases, which enhances the radio frequency electric field strength to some extent. This increases the drying rate of the material, thereby raising the Deff value. Moreover, at a vacuum level of 0.035 MPa, the maximum Deff of Codonopsis pilosula was 4.1568 × 10⁻¹⁰ m²/s under single-factor experiments. This indicates that an appropriate vacuum level can reduce the adsorption of moisture molecules in the Codonopsis pilosula matrix and enhance the movement of moisture molecules, creating and expanding micropore channels, which facilitates moisture migration.

3.1.6. Drying Kinetics Model and Validation

Table 4. Fitting results of six drying models under different drying conditions.

Experiments Condition	Newton			Midilli
	R2	RMSE	SSE	R2	RMSE	SSE
50 °C/4 mm/90 mm/0.025 MPa	0.9548	0.0029	2.16 × 10−4	0.9964	0.0026	3.78 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9468	0.0077	3.79 × 10−4	0.9991	0.0048	5.64 × 10−4
60 °C/4 mm/90 mm/0.025 MPa	0.9579	0.0033	4.28 × 10−4	0.9979	0.0037	4.33 × 10−4
55 °C/3 mm/90 mm/0.025 MPa	0.9507	0.0044	4.49 × 10−4	0.9968	0.0029	3.21 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9365	0.0058	4.32 × 10−4	0.9947	0.0060	5.49 × 10−4
55 °C/5 mm/90 mm/0.025 MPa	0.9476	0.0049	3.88 × 10−4	0.9957	0.0054	4.49 × 10−4
55 °C/4 mm/80 mm/0.025 MPa	0.9552	0.0069	5.54 × 10−4	0.9948	0.0058	5.83 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9446	0.0059	4.82 × 10−4	0.9949	0.0061	5.57 × 10−4
55 °C/4 mm/100 mm/0.025 MPa	0.9527	0.0052	3.92 × 10−4	0.9961	0.0049	4.08 × 10−4
55 °C/4 mm/90 mm/0.015 MPa	0.9597	0.0038	3.00 × 10−4	0.9991	0.0010	8.29 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9374	0.0064	4.67 × 10−4	0.9987	0.0034	6.21 × 10−4
55 °C/4 mm/90 mm/0.035 MPa	0.9466	0.0054	4.86 × 10−4	0.9969	0.0028	3.16 × 10−4
Experiments condition	Weibull			Handerson and Pabis
	R2	RMSE	SSE	R2	RMSE	SSE
50 °C/4 mm/90 mm/0.025 MPa	0.9959	0.0010	5.29 × 10−4	0.9678	0.0031	3.89 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9989	0.0005	3.66 × 10−4	0.9527	0.0029	6.44 × 10−4
60 °C/4 mm/90 mm/0.025 MPa	0.9943	0.0014	2.75 × 10−4	0.9606	0.0033	4.59 × 10−4
55 °C/3 mm/90 mm/0.025 MPa	0.9948	0.0006	5.11 × 10−4	0.9629	0.0037	5.67 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9946	0.0007	5.64 × 10−4	0.9564	0.0046	5.34 × 10−4
55 °C/5 mm/90 mm/0.025 MPa	0.9956	0.0006	4.56 × 10−4	0.9638	0.0038	4.71 × 10−4
55 °C/4 mm/80 mm/0.025 MPa	0.9936	0.0007	5.78 × 10−4	0.9576	0.0048	6.80 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9948	0.0006	6.97 × 10−4	0.9489	0.0056	6.01 × 10−4
55 °C/4 mm/100 mm/0.025 MPa	0.9958	0.0009	4.39 × 10−4	0.9619	0.0040	4.79 × 10−4
55 °C/4 mm/90 mm/0.015 MPa	0.9977	0.0003	2.14 × 10−4	0.9707	0.0028	3.61 × 10−4
55 °C/4 mm/90 mm/0.025 MPa	0.9932	0.0008	6.98 × 10−4	0.9493	0.0052	5.89 × 10−4
55 °C/4 mm/90 mm/0.035 MPa	0.9950	0.0006	5.04 × 10−4	0.9561	0.0045	6.12 × 10−4
Experiments condition	Logarithmic			Two-term exponential
	R2	RMSE	SSE	R2	RMSE	SSE
50 °C/4 mm/90 mm/0.025 MPa	0.9921	0.0021	4.27 × 10−4	0.9645	0.0017	0.0029
55 °C/4 mm/90 mm/0.025 MPa	0.9923	0.0037	3.95 × 10−4	0.9660	0.0039	0.0128
60 °C/4 mm/90 mm/0.025 MPa	0.9894	0.0019	5.49 × 10−4	0.9801	0.0033	0.0046
55 °C/3 mm/90 mm/0.025 MPa	0.9946	0.0005	5.39 × 10−4	0.9718	0.0028	0.0280
55 °C/4 mm/90 mm/0.025 MPa	0.9868	0.0014	3.70 × 10−4	0.9672	0.0034	0.0411
55 °C/5 mm/90 mm/0.025 MPa	0.9865	0.0014	4.21 × 10−4	0.9737	0.0027	0.0356
55 °C/4 mm/80 mm/0.025 MPa	0.9841	0.0018	1.79 × 10−4	0.9696	0.0034	0.0374
55 °C/4 mm/90 mm/0.025 MPa	0.9883	0.0013	11.2 × 10−4	0.9603	0.0043	0.0520
55 °C/4 mm/100 mm/0.025 MPa	0.9886	0.0012	1.19 × 10−4	0.9869	0.0014	0.0179
55 °C/4 mm/90 mm/0.015 MPa	0.9969	0.0003	2.92 × 10−4	0.9794	0.0020	0.0254
55 °C/4 mm/90 mm/0.025 MPa	0.9905	0.0010	9.79 × 10−4	0.9605	0.0041	0.0447
55 °C/4 mm/90 mm/0.035 MPa	0.9934	0.0007	6.66 × 10−4	0.9668	0.0034	0.0337

shows the fitting performance of six drying models in terms of the moisture ratio during HAD and RFVD. The results indicate that the R² values for all six drying models were greater than 0.93, and the RMSE values were less than 0.05, indicating that these drying models can accurately describe the moisture change pattern of Codonopsis pilosula slices during RFVD. Among the six drying kinetics models, the R² values of the Midilli and Weibull models ranged from 0.9936 to 0.9991, the RMSE values ranged from 0.0003 to 0.0061, and the SSE values ranged from 2.14 × 10⁻⁴ to 8.29 × 10⁻⁴. Therefore, compared to other drying models, the Midilli and Weibull models better describe the moisture change pattern of Codonopsis pilosula slices during RFVD.

3.2. Effect of Different Drying Conditions on Quality Characteristics of Codonopsis pilosula Slices under RFVD

3.2.1. Color

The color parameters of Codonopsis pilosula slices under different drying conditions are shown in

Table 5. Color parameters under different drying conditions.

Experiments Condition	L*	a*	b*	ΔE
HAD	64.92 ± 0.49	1.81 ± 0.25	22.01 ± 2.48	17.85 ± 0.59
50 °C/4 mm/90 mm/0.025 MPa	64.63 ± 2.26	1.76 ± 0.89	20.41 ± 2.65	10.33 ± 1.21
55 °C/4 mm/90 mm/0.025 MPa	71.48 ± 1.15	3.42 ± 0.65	21.76 ± 2.55	12.36 ± 1.33
60 °C/4 mm/90 mm/0.025 MPa	79.91 ± 1.29	2.49 ± 0.44	18.93 ± 2.14	8.17 ± 0.45
55 °C/3 mm/90 mm/0.025 MPa	74.13 ± 2.01	2.40 ± 0.89	21.92 ± 1.59	10.31 ± 0.62
55 °C/4 mm/90 mm/0.025 MPa	74.93 ± 2.55	2.62 ± 0.58	20.75 ± 0.94	8.53 ± 0.42
55 °C/5 mm/90 mm/0.025 MPa	69.33 ± 1.29	3.87 ± 0.14	22.73 ± 2.88	14.68 ± 1.22
55 °C/4 mm/80 mm/0.025 MPa	76.2 ± 3.89	3.04 ± 0.33	22.71 ± 3.15	9.86 ± 0.59
55 °C/4 mm/90 mm/0.025 MPa	77.97 ± 4.48	2.86 ± 0.41	20.04 ± 2.45	6.77 ± 2.08
55 °C/4 mm/100 mm/0.025 MPa	72.75 ± 3.27	1.28 ± 0.55	20.59 ± 3.14	10.35 ± 2.01
55 °C/4 mm/90 mm/0.015 MPa	69.69 ± 1.29	1.77 ± 0.26	19.01 ± 2.66	12.38 ± 0.89
55 °C/4 mm/90 mm/0.025 MPa	71.38 ± 4.89	1.51 ± 0.18	18.81 ± 0.89	6.12 ± 2.11
55 °C/4 mm/90 mm/0.035 MPa	76.24 ± 2.11	0.82 ± 0.31	79.4 ± 2.59	6.83 ± 0.94

. Compared with HAD, the L* value of the samples after RFVD significantly increased, and the $∆E$ decreased, indicating that RFVD can improve sample brightness and reduce color difference. An et al. [31] compared the macroscopic structure and $∆E$ of fresh and dried peas and similarly found that $∆E$ increased after drying, indicating significant color changes. The impact of different RFVD conditions on color also varied. As the temperature increased, color difference values first decreased and then increased. This might be because at lower drying temperatures, prolonged contact with moist air increases the probability of oxidation reactions, leading to a significant reduction in brightness. However, at higher temperatures, phenolic and glycosidic substances are more likely to degrade, accelerating Maillard reactions, thus reducing the L* value and causing more severe browning. The effects of the slice thickness and vacuum degree on color were similar to those of the drying temperature, suggesting that appropriately increasing the vacuum degree and slice thickness can help preserve the color quality and medicinal components of Codonopsis pilosula. Considering the effects of different drying conditions on the color quality of Codonopsis pilosula, it was found that at 55 °C, 4 mm, 90 mm, and 0.025 MPa, the sample had a lower $∆E$ and better color quality.

3.2.2. Rehydration Ratio (RR) and Shrinkage Ratio (SR)

The RR and SR are critical indicators for assessing the degree of structural damage during drying. A higher RR and lower SR indicate less internal damage and better structural integrity. The RR and SR of Codonopsis pilosula slices under different drying conditions are shown in Figure 6. Under constant conditions, the RR at 55 °C (3.89 ± 0.38) was the highest, increasing by 17.88% and 16.45% compared to 50 °C (3.30 ± 0.29) and 60 °C (3.25 ± 0.47), respectively. This suggests that both excessively low and high temperatures damage the internal structure, causing the rupture and collapse of micropore channels. Furthermore, high temperatures can cause surface hardening, potentially hindering water penetration during rehydration, thus leading to a poorer rehydration ability at high temperatures. Compared to HAD, the RR of the samples after RFVD increased by 1.03% to 29.06%, while the SR decreased by 11.76% to 23.73%, indicating that RFVD effectively preserves the integrity of Codonopsis pilosula’s internal structure. In a vacuum environment, the damage to the internal pore structure is minimal, and a regular honeycomb-like micropore structure forms, maintaining the internal structural integrity while expanding the pore size. Thus, the rehydration performance is better, and the SR is lower. At a vacuum level of 0.035 MPa, the SR was 0.40 ± 0.03. Compared to 0.015 MPa, the SR increased by 3.08% due to the higher vacuum level accelerating moisture removal while increasing internal structural stress, causing a disordered cell arrangement and the partial destruction of micropore channels and water-retaining components (e.g., proteins, starches), leading to structural shrinkage and reduced rehydration ability.

3.2.3. Effect of Different Drying Conditions on the Content of Lobetyolin and Syringin

Lobetyolin and Syringin are the main natural active components in Codonopsis pilosula, with antioxidant, lipid-lowering, anti-atherosclerotic, antibacterial, and immune-regulating effects. Figure 7 shows the impact of different drying conditions on the natural active components in the samples. Compared to HAD, the retention of Lobetyolin and Syringin in Codonopsis pilosula significantly increased after RFVD (p < 0.05). This indicates that RFVD helps prevent thermal and oxidative damage to bioactive compounds, effectively reducing the loss of heat-sensitive nutrients. At 55 °C, 4 mm, 90 mm, and 0.025 MPa, the content of Lobetyolin (130.26 ± 5.48 mg/100 g) and Syringin (17.92 ± 1.24 mg/100 g) in the samples was the highest, increasing by 75.20% and 124.28% compared to HAD. At a drying temperature of 60 °C, the content of Lobetyolin in Codonopsis pilosula slices was lower due to the fact that many active substances in the samples are heat-sensitive and prone to decomposition under high temperature and oxygen conditions, which is unfavorable for the retention of active substances.

3.2.4. Effects of Different Drying Conditions on Polysaccharide Content (PC)

The effects of different drying conditions on the PC in Codonopsis pilosula slices are shown in Figure 8A. It is evident that with the increase in slice thickness, the PC first increases and then decreases. When the slice thickness is 3 mm, the PC is 33.05 mg/g, which is 43.36% lower than that at 4 mm (47.38 mg/g). This may be due to the uneven surface temperature distribution during drying, causing localized overheating and intensifying the Maillard reaction, leading to a decrease in PC. Additionally, RFVD causes the polarization of glycosidic bonds such as C-O-C, along with intense molecular motion, resulting in the breaking of sugar chains [32]. Different plate spacings also significantly affect the PC; at 80 mm, 90 mm, and 100 mm, the PC values are 36.56 mg/g, 41.56 mg/g, and 24.87 mg/g, which represent increases of 109.15%, 137.76%, and 44.28% compared to HAD. This indicates that a plate spacing of 90 mm retains the most polysaccharides. With an increasing temperature and vacuum degree, the PC initially increased and then decreased. This was because excessively high drying temperatures accelerated polysaccharide degradation and sped up the Maillard reaction, resulting in a decrease in PC. Additionally, during dielectric heating, excessively low vacuum degrees could cause “arc discharge” and “glow discharge” phenomena, disrupting the metabolic balance of carbohydrates inside Codonopsis pilosula and leading to the formation of large amounts of cellulose, starch, and other high-molecular compounds, thereby reducing PC [33]. The PC is highest at a vacuum degree of 0.025 MPa, likely because Codonopsis pilosula contains a large amount of starch, and its amylopectin easily undergoes depolymerization under the influence of electromagnetic fields, forming water-soluble amylose. The higher the vacuum degree, the greater the internal pressure of the material, leading to less degradation of starch and thus a minimal loss of polysaccharide substances [34]. Overall, RFVD can effectively reduce the loss of PC in Codonopsis pilosula slices.

3.2.5. Effects of Different Drying Conditions on Total Phenolic Content (TPC)

Phenolic compounds in Codonopsis pilosula slices are highly active and thermosensitive components with antioxidant properties [35]. As shown in Figure 8B, the TPC in Codonopsis pilosula slices under different drying conditions ranged from 75.26 ± 2.48 to 121.15 ± 4.51 mg/100 g. After RFVD, the TPC was significantly higher than that of HAD (69.73 ± 2.79 mg/100 g), indicating that RFVD is beneficial for preserving the TFC during dehydration. At vacuum degrees of 0.015 MPa and 0.025 MPa, the TPC values were 98.90 ± 1.49 and 114.33 ± 3.10 mg/100 g, respectively, which were 14.93% and 1.66% lower compared to the content at 0.035 MPa (116.26 ± 2.48 mg/100 g). This might be because increased vacuum degrees result in a denser and stronger electric field distribution in the radio frequency vacuum field. The radio frequency can cleave covalently bound polyphenolic compounds from plant cell tissues and break down cell walls to release phenolic substances [13]. By analyzing the changes in TPC at different plate spacings, it was found that the content was lowest at 80 mm (107.02 ± 1.08 mg/100 g) and highest at 90 mm (121.15 mg/100 g). The trend is similar to the effect of the slice thickness on TPC. This suggests that the smaller the plate spacing, the faster the temperature rises under the “ion migration” and “dipole rotation” loss effects. Phenolic compounds, formed by ester bonds with xylan in the cell walls, absorb more heat during the temperature rise, breaking their covalent bonds and promoting the dissolution and extraction of phenolic substances [30]. It is also noteworthy that when the temperature increased from 55 °C to 60 °C, the TPC decreased by 37.7%. This decline was likely due to the accelerated degradation rate of phenolic compounds at higher temperatures, which led to a reduction in TPC [36].

3.2.6. Effects of Different Drying Conditions on Total Flavonoid Content (TFC)

Flavonoids in Codonopsis pilosula have antioxidant properties, effectively preventing cell degeneration and aging, inhibiting the growth of cancer cells, and thus possessing anti-cancer effects. Figure 8C shows the variation in TFC in Codonopsis pilosula slices under different drying conditions. As shown in the Figure, the TFC after drying ranged from 99.48 ± 1.05 to 128.44 ± 2.11 mg/100 g. The TFC after RFVD was significantly higher than that of the HAD samples (81.07 ± 1.10 mg/100 g) (p < 0.05). At a drying temperature of 55 °C, the TFC in the dried product was 128.44 ± 2.11 mg/100 g, higher than that of products dried under other temperature conditions. This may be because the longer drying time under low temperature conditions leads to prolonged contact between the sample and moist hot air, making flavonoid compounds prone to oxidative degradation catalyzed by enzymes, resulting in reduced content [37,38]. When the temperature was raised to 60 °C, the TFC decreased. Rózek et al. [39] also found that flavonoids and polyphenols began to thermally decompose at drying temperatures above 60 °C. As the slice thickness increased, the TFC first increased and then decreased, reaching the highest content (126.34 ± 4.37 mg/100 g) at a thickness of 4 mm. This indicates that a slice thickness of 4 mm causes less damage to the internal tissue structure and increases polar molecules, thus better preserving the TFC in Codonopsis pilosula slices. Furthermore, the vacuum degree and plate spacing showed similar trends.

3.2.7. Effects of Different Drying Conditions on Antioxidant Properties (AP)

Phenols and flavonoids, as basic bioactive compounds, are directly related to the AP of the material. The AP is characterized by the inhibition rate (IR); the higher the IR, the stronger the AP. As observed in Figure 8D, there are significant differences (p < 0.05) in the IR of Codonopsis pilosula products under different drying conditions, and the trend of AP changes under different drying conditions is similar to that of TPC and TFC, indicating that the AP of the samples may be influenced by both TPC and TFC. Compared with HAD, the AP of the samples obtained by RFVD increased by 2.37% to 24.67% due to the high retention of phenolic and flavonoid bioactive substances by RFVD. Furthermore, in RFVD, the IR of the samples was 39.79%, 42.79%, and 34.22% at 50 °C, 55 °C, and 60 °C, respectively, indicating that a moderate increase in temperature is conducive to improving the AP of Codonopsis pilosula slices. The increase in temperature alters the structure and content of some monophenolic and flavonoid compounds, enhancing the AP of Codonopsis pilosula [40], but high temperatures can cause the oxidative degradation of phenolic and natural bioactive substances, thereby reducing their AP to some extent. When the plate spacing was 80 mm, the IR was 37.42%, slightly lower than that at 90 mm, which may be due to cell damage caused by high-intensity electromagnetic fields, leading to the oxidative degradation of glycosides, phenols, and flavonoids in Codonopsis pilosula, resulting in weaker AP.

3.2.8. Microstructure

Figure 7 shows the microstructure of Codonopsis pilosula slices under different drying conditions. It was observed that the internal cells of fresh Codonopsis pilosula slices were closely arranged, while the internal tissue structure of dried samples changed significantly. After HAD (Figure 9a), the material, affected by the temperature and humidity gradient stress exhibited a dense surface tissue structure with fewer microporous channels and significant shrinkage deformation, which is not conducive to moisture diffusion and migration. Under RFVD conditions (Figure 9b–d), it can be seen that in a relatively low-temperature environment, the internal tissue cells of Codonopsis pilosula, affected by the temperature and humidity gradient stress, showed a denser internal structure with severe deformation and shrinkage, fewer microporous channels, and higher blockage. As the radiation temperature increased, the internal pore structure became relatively loose, the number of closed or semi-closed microporous channels decreased, and adjacent cell walls were orderly arranged, which is conducive to reducing moisture diffusion resistance [41]. Additionally, it was observed that at a radiation temperature of 60 °C, the microporous channels inside the samples showed shrinkage and rupture, which might be due to the rapid loss of internal moisture and the rapid decrease in expansion pressure caused by high temperatures, leading to surface hardening, generating strong local stress, and destroying the tissue structure. This is consistent with the drying characteristics analysis, and similar conclusions were drawn by Jiang et al. [12]. Under a slice thickness of 3 mm (Figure 9e) and a plate spacing of 80 mm (Figure 9g), cell shrinkage and deformation were observed. This shrinkage and deformation were even more pronounced under a plate spacing of 100 mm (Figure 9h). Therefore, an appropriate slice thickness, plate spacing, and vacuum degree can inhibit cell shrinkage and structural collapse, positively contributing to the efficiency of moisture migration within Codonopsis pilosula and reducing the resistance to heat and mass transfer.

4. Conclusions

Addressing the issues of low efficiency, high energy consumption, and poor quality associated with traditional drying methods, this study used Radio Frequency Vacuum Drying (RFVD) technology to dehydrate Codonopsis pilosula slices. The results showed that as the drying temperature and vacuum degree increased and the slice thickness and plate spacing decreased, the drying time shortened and the drying rate increased. The effective diffusion coefficient (Deff) of Codonopsis pilosula slices under different drying conditions ranged from 3.6024 × 10⁻¹⁰ to 5.5638 × 10⁻¹⁰, displaying a trend similar to the drying characteristics. Among the six drying models tested, the Midilli and Weibull models better described the moisture change trends during the RFVD process. 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, representing increases of 75.2% and 124.28% compared to hot air drying (HAD). The contents of polysaccharides, total phenols, and total flavonoids, as well as the antioxidant activity, were all improved compared to HAD, showing a trend of initially increasing and then decreasing with higher drying temperatures, slice thicknesses, and electrode spacing. Compared to HAD, RFVD markedly enhanced the color and rehydration performance of the samples, reduced shrinkage, and resulted in a regular, uniform honeycomb-like pore structure with better boundary integrity. In summary, RFVD not only improved the drying speed of Codonopsis pilosula slices but also yielded higher-quality dried products, offering a more efficient and superior drying method for Codonopsis pilosula.