Study on the Drying Characteristics and Physicochemical Properties of Alfalfa under High-Voltage Discharge Plasma

Abstract

High-voltage discharge plasma drying (HVDPD) is a non-thermal and efficient drying technique. In this study, we compared the drying characteristics and physicochemical properties of alfalfa dried via HVDPD, hot-air drying (HAD), and natural air-drying (AD) and analyzed the experimental results using infrared spectroscopy, scanning electron microscopy, colorimetry, and other detection techniques. The results showed that HVDPD had a higher drying rate than AD and saved more energy than HAD. Alfalfa dried via HVDPD had the most intact internal structure, the highest rehydration ratio (RR), the best color quality, and the best nutrient retention, resulting in the highest relative quality and feeding value. Therefore, HVDPD is an excellent drying technique for alfalfa, providing a new and effective way to improve the storage, transportation, and drying quality of alfalfa.

1. Introduction

Alfalfa has high nutritional value and strong adaptability and can improve soil quality, making it a perennial leguminous, herbaceous plant with multiple applications [1]. Alfalfa has been consumed as a vegetable for thousands of years, and it has antioxidant, cardiovascular, and cholesterol-lowering effects on human health. Alfalfa is a beneficial and functional edible plant [2,3,4]. Alfalfa can also replace some grains as feed, and it has an important strategic position and role in agriculture and animal husbandry; it is known as the “king of forage” [5]. The processes of storing and drying alfalfa restrict the development of the alfalfa industry. At present, the main methods of drying alfalfa are natural air-drying (AD) and hot-air drying (HAD). During natural drying, the humidity is relatively high and the temperature is low. The harvested alfalfa needs a long time to dry, and the quality of the alfalfa will be reduced due to leaf loss and nutrient consumption during the drying process [6]. Hot-air drying is fast, but high temperatures will damage the nutritional structure of the alfalfa, which is not conducive to retaining nutrients [7]. The drying method has a significant impact on the nutritional components, color, and rehydration ratio (RR) of forage [8,9,10]. When drying alfalfa, there is much room for improvement in terms of nutrient retention, drying speed, and human control over drying. We urgently need more scientific and environmentally friendly technology for drying alfalfa.

High-voltage discharge plasma technology has been widely studied and is highly valued in sterilization, food drying, and seed pretreatment for its non-thermal nature, low energy consumption, and other technical advantages [11,12,13,14]. High-voltage discharge plasma drying (HVDPD) technology can produce high-voltage electric fields (tens of thousands of volts) in a given space, releasing a plasma wind containing various active particles [15,16]. The joint action of the non-uniform high-voltage electric field and plasma wind can enhance the polarity of water molecules within the material, and the effects of physical and chemical etching on the material’s surface can inhibit the activity of bacterial and biological cell enzymes, reducing their metabolic capacity, which not only improves the drying rate of the material but also extends the shelf life of the material and improves nutrient retention [17,18,19,20]. HVDPD has been used for drying garlic, apple slices, tomato slices, etc. When HVDPD technology is applied to garlic, it can increase the rehydration ratio (RR) and the contents of pyruvic acid and other active ingredients after drying [12]. In the case of apple slices, HVDPD was found to significantly increase the drying rate with very low energy consumption [21]. The drying of tomato slices revealed that HVDPD-treated tomato slices had greater shrinkage and better color compared to untreated and hot-air-dried samples [22]. These studies showed that samples treated with HVDPD had improved color, rehydration capacity, and active ingredient contents, with very low energy consumption compared to samples dried using conventional drying methods. Studying the effects of HVDPD treatment on alfalfa could provide a low-energy, high-quality, non-thermal drying method, solving a series of problems in drying alfalfa, such as a slow drying speed and low nutrient retention. Currently, there is no relevant research on the application of HVDPD for drying alfalfa.

In this study, HAD, AD, and HVDPD were used to dry alfalfa; the drying characteristics of the alfalfa (such as the drying rate and rehydration ratio (RR)) were measured under different drying methods and different drying voltages, and the power consumption of the HVDPD and HAD methods was compared. We measured the chromaticity value of dried alfalfa with a colorimeter and analyzed changes in the surface microstructure and nutrient composition of alfalfa leaves dried using different methods and field voltages via scanning electron microscopy and infrared microscopy. The effects of different drying methods and discharge voltage conditions on the drying characteristics of alfalfa were comprehensively investigated with the goal of obtaining high-quality alfalfa hay and providing theoretical guidance for drying and processing alfalfa after harvest.

2. Materials and Methods

2.1. Experimental Materials

This study was conducted in the Experimental Building of the Inner Mongolia University of Technology, Hohhot, Inner Mongolia Autonomous Region, China (111°40′ E, 40°50′ N). The alfalfa used in the experiment (Zhongmu 3, provided by the Grassland Research Institute, Chinese Academy of Agricultural Sciences, Hohhot, China) was harvested in the first flowering stage, leaving a stubble height of ~5 cm after cutting. The harvested alfalfa was processed immediately by manually removing deteriorated and damaged samples and dividing plants with similar heights, leaf areas, and cluster densities into stem and leaf portions and cutting them to lengths of approximately 3 cm to maintain a uniform sample. The initial moisture content of the alfalfa was also measured to be 85 ± 3%.

2.2. Instruments and Equipment

As shown below, the experimental setup for the hot-air-drying group consisted of a hot-air-drying oven (DGX-9053, Shanghai Fangyuan Experimental Instrument Co., Ltd., Shanghai, China) with a temperature range continuously adjustable from 10 °C to 250 °C. Experiments in the AD group were conducted at room temperature. The HVDPD group utilized a needle array to plate the dielectric blocking discharge device. The upper plate, which served as the high-voltage electrode, consisted of an array of needles spaced horizontally and vertically at 4 cm, 2 cm in length and 1.56 ± 0.02 mm in diameter. The upper and lower plates were connected using polypropylene insulating rods, with a distance of 8.5 cm between the needle tips and the dielectric plate. The grounding terminal consisted of a flat aluminum plate and a 4 mm thick Plexiglas plate. The experimental power supply was alternating current (AC), with a discharge frequency of 50 Hz and an adjustable voltage range of 0 to 50 kV.

2.3. Empirical Method

As shown in Figure 1, the prepared fresh alfalfa samples were evenly spread in polyethylene containers with dimensions of 170 mm × 120 mm and a height of ~20 mm, with a mass of between 19.5 and 20.5 g per group. Then, the samples were subjected to different drying treatments: HAD experiments were performed at an air temperature of 50 °C and an air velocity of 2 m/s. The AD experiments were performed indoors (air velocity of 0 m/s, temperature of 27 ± 3 °C, and relative humidity of 29 ± 4%). Since excessive voltages can lead to air breakdown, which makes it impossible to proceed, whereas insufficient voltages do not have a significant drying effect on the samples, it is necessary to select a suitable voltage. In this study, 20 kV, 25 kV, and 30 kV were selected as the experimental voltages for HVDPD, so as to avoid the occurrence of air breakdown under certain conditions of pole distance and ensure a more significant drying effect on the samples. The HVDPD experiments were first performed by placing the prepared alfalfa samples on top of a dielectric plate and then drying the samples under high-voltage fields and plasma winds generated by different voltages (20 kV, 25 kV, and 30 kV), with the distance between the tip of the needle and the Perspex barrier plate being maintained at 8.5 cm and with the ambient temperature and relative humidity maintained at the same level as for the AD group. The mass of the alfalfa samples was measured using an electronic scale (BS124S, Shanghai Guanglu Electronic Technology Co., Ltd., Shanghai, China) every 2 h until the mass ceased to change and the drying process was finished.

2.4. Voltage, Current, and Power Parameter Diagnosis

The discharge morphology was captured with a single-lens reflex camera (Nikon D7000, Nikon Corporation, Tokyo, Japan). A hot-wire anemometer (405i, Ruice Electronics Technology Co., Ltd., Guangzhou, China) was used to measure the ion wind speed changes under different voltages. The power was measured using a high-voltage probe (TektronixP6015A, 75 MHz, Tektronix Technologies, Inc., Beaverton, OR, USA) and a current probe (PEARSON 6877, 120 MHz, Pearson Electronics Corporation, Palo Alto, CA, USA) to send the signal to an oscilloscope (RIGOL MSO5204, Suzhou Puyuan Precision Technology Co., Ltd., Suzhou, China). The discharge power of the HVDPD device could be intuitively reflected by the area of the Lissajous figure [23].

$$P=f\times C_M\times S$$

(1)

where C_M (unit: F) is the sampling capacitance with a capacitance size of 1.00 ± 0.05 μF, f (unit: Hz) is the discharge frequency of the experimental power supply at 50 Hz, and S (unit: V²) is the area of the Lissajous figure.

The energy consumption of HAD can be calculated using the product of the effective power and the drying time of the oven, while the energy consumption of HVDPD can be calculated using the product of the discharge power and the drying time under different voltages, which can be expressed using the following formula:

$$W=PT$$

(2)

where W (unit: kJ) is the power consumed by drying, P (unit: W) is the working power, and T (unit: h) is the average drying time. Specific energy consumption (SEC) is calculated as the amount of energy used to remove the unit mass of the moisture. The calculation formula is as follows:

$$SEC=\frac{W}{m_0-m_t}$$

(3)

where SEC (unit: kJ/kg water) is the specific energy consumption for drying, m₀ (unit: g) is the mass of the grass before drying, and m_t (unit: g) is the mass of alfalfa when drying is completed.

2.5. Analysis of Drying Characteristics

2.5.1. Determination of Moisture Content

The drying rate was evaluated based on the change curve of the dry-basis moisture ratio [24], and the dry-basis moisture content and the moisture content ratio were calculated using Equations (4) and (5), respectively:

$$M_i=\frac{m_i-m_g}{m_g}$$

(4)

$$MR=\frac{M_i-M_e}{M_0-M_e}$$

(5)

In Equation (4), M_i (unit: g water/g dry matter) is the real-time dry-basis moisture content of the alfalfa, m_i (unit: g) is the real-time mass of the alfalfa during the drying process, and m_g (unit: g) is the mass of the material after the alfalfa is completely dry. In Equation (5), MR is the moisture content ratio of the alfalfa, M_e (unit: g water/g dry matter) is the equilibrium moisture content, and M₀ (unit: g water/g dry matter) is the initial dry-basis moisture content of the alfalfa. Since the equilibrium moisture content of food materials is generally very low, the equilibrium moisture content was not evaluated and M_e did not actually affect the calculation results. For convenience of calculation, we treated M_e as 0 [25], so Equation (5) can be simplified as follows:

$$MR=\frac{M_i}{M_0}$$

(6)

2.5.2. Drying Rate

The alfalfa drying rate can be expressed as follows:

$$DR=\frac{M_t-M_{t+\Delta t}}{\Delta t}$$

(7)

where DR (unit: g water/g dry matter·h⁻¹) is the drying rate, M_t (unit: g water/g dry matter) is the dry-basis moisture content of the alfalfa at time t, M_t+Δt (unit: g water/g dry matter) is the dry-basis moisture content of the alfalfa at time t + Δt (unit: h), and Δt (unit: h) is the time interval. The average drying rate (VR), i.e., the amount of water evaporated per hour per unit mass of alfalfa, can be expressed as follows:

$$VR=\frac{m_0-m_z}{m_{0}T}$$

(8)

where VR (unit: g water/g solid·h⁻¹) is the average drying rate of the alfalfa, m₀ (unit: g) is the weighed mass of the alfalfa before drying, m_z (unit: g) is the mass of the alfalfa at the completion of drying, and T (unit: h) is the drying time.

2.6. Analysis of Rehydration Characteristics

The rehydration ratio (RR) was measured as described by Zhang et al. [26]. An electronic scale (BS124S, Shanghai Guanglu Electronic Technology Co., Ltd., Shanghai, China) was used to weigh 5.0 ± 0.05 g of dried alfalfa, which was then put into a beaker, mixed with 50 mL of deionized water, immersed in a constant-temperature 37 °C water bath for 8 h (until the mass of the sample no longer changed), and taken out of the bath; the sample was then weighed after its surface was completely dried using absorbent paper. The RR was calculated according to the following formula:

$$RR=\frac{m_b}{m_a}$$

(9)

where RR is the rehydration ratio, m_a (unit: g) is the mass of the alfalfa before rehydration, and m_b (unit: g) is the mass of the alfalfa after rehydration.

2.7. Scanning Electron Microscopy (SEM)

To observe the effects of the electromagnetic radiation and ion wind generated by the electric field on alfalfa, alfalfa leaves were selected for this study. The leaves were placed face up, facing the needle tip of the upper electrode, and fixed on the drying platform so that they would not flip due to the ion wind and electromagnetic effects. When the leaves achieved the required drying conditions, a thin metal film was sprayed on them, and then they were glued to the sample stage with conductive double-sided tape. Then, a scanning electron microscope (SU8020, Hitachi High-Tech, Tokyo, Japan) was used to observe the surface structure of the dried alfalfa leaves at the same position under the same magnifications of 500× and 2000× and under an accelerating voltage of 5 kV.

2.8. Color Analysis

The color assessment of the samples was based on CIE Lab color space coordinates, and the measuring instrument was a 3nh automated colorimeter (3nh-NR60CP, Shenzhen, China) calibrated with a blackboard and a whiteboard before use. The luminance value L*, redness value a*, and yellowness value b* of the leaves were measured with the colorimeter, and the experiment was repeated 10 times for each group of samples, from which the average value was taken. The color difference before and after drying can be expressed using the following formula [27]:

$$\Delta E=\sqrt{{(L_i^{*}-L_0^{*})}^2+{(a_i^{*}-a_0^{*})}^2+{(b_i^{*}-b_0^{*})}^2}$$

(10)

where ΔE is the overall color change degree of the sample relative to the fresh sample after drying; $L_0^{*}$, $a_0^{*}$, and $b_0^{*}$ are the brightness, redness, and yellowness values of the fresh alfalfa leaves, respectively; and $L_i^{*}$, $a_i^{*}$, and $b_i^{*}$ are the brightness, redness, and yellowness values of the dried alfalfa leaves, respectively.

2.9. Infrared Spectroscopic Analysis

The sample was mixed thoroughly with KBr at a mass ratio of 1:100 and ground to a powder with a particle size less than 2 μm. Then, the powder was pressed into a transparent thin film under high pressure. The sample was placed in the sample chamber for infrared scanning analysis. To reduce the error and eliminate the interference of water and carbon dioxide, a mid-infrared spectrometer (Nicolet lS10, Thermo Nicolet Corporation, Madison, WI, USA) was used for multiple scans. The scanning range was 400–4000 cm⁻¹, and the resolution was 4 cm⁻¹. The spectra were compared to identify the differences in various components in alfalfa.

2.10. Determination of Nutritional Components

The near-infrared reflectance spectroscopy technique has many operational advantages, including its ease of use, shortcuts, and accuracy, and it has been widely used to analyze the nutrient contents of various foodstuffs and forages [28]. The relative contents of nutrients in alfalfa were measured with reference to the test method of Yuan et al. [29]. The dried samples were ground into 1 mm particles and analyzed for crude protein (CP), non-fibrous carbohydrates (NFC), neutral detergent fiber (NDF), acid detergent fiber (ADF), and lignin via near-infrared reflectance spectroscopy (FOSS 2500, FOSS NIR Systems, Inc., Laurel, MD, USA). The relative feed value (RFV), relative forage quality (RFQ), and hay milk yield (HM) were also calculated for the samples. The database source was Dairy One (USA).

2.11. Statistical Analysis

All experiments were performed in triplicate, and the final data were averaged. The standard deviation was obtained by averaging the results of each group. The results are expressed as the mean ± standard deviation. Intergroup differences were determined by one-way analysis of variance (ANOVA). The differences between the means were determined at a significance level of 0.05. The obtained data were normalized, and heatmaps were generated using Origin software (Origin 2022) after processing. Pearson correlation analysis was performed on different indicators to analyze the correlations between different drying methods and each indicator, as well as the correlations between different indicators.

3. Results

3.1. HVDPD Parameters

3.1.1. Discharge Morphology and Plasma Wind

Figure 2a,b show that as the voltage increases, the blue-purple glow emitted by the needle tip becomes stronger, and the plasma wind speed also increases with the voltage.

3.1.2. Discharge Power

HVDPD can effectively utilize electrical energy while keeping the material temperature stable, which has advantages of low energy consumption, energy savings, and environmental protection [30]. Figure 3A–C show the HVDPD voltage and current parameters. Figure 3a–c show Lissajous figures, corresponding to 20 kV, 25 kV, and 30 kV areas, respectively, with sizes of 0.9814 × 10⁶ V², 4.3618 × 10⁶ V², and 4.4080 × 10⁶ V², respectively, which correspond to a power of 49 W, 218 W, and 221 W, respectively, and the drying power increases with the increase in voltage. The drying oven used for HAD has a rated power of 800 W. As shown in Figure 4, the energy consumption of different drying methods differed significantly. It is interesting to note that 25 kV was used at a lower voltage than 30 kV but had a higher SEC than 30 kV. This was due to the fact that the drying time taken at 25 kV is longer than that at 30 kV and, therefore, more energy is consumed. Alemrajabi A. A. et al. dried carrots using electrohydrodynamic drying, oven drying, and ambient air; they found that the energy consumption of electrohydrodynamic drying increased with the drying time [31], which is consistent with the results of this study. The SEC of the different voltages was lower than that of HAD, with 21~56% of the SEC of the HAD group, and the difference in SEC between the different voltage groups was significant (p < 0.05). Bai et al. compared the power consumption of electrohydrodynamic drying and HAD of sea cucumbers and found that the cost of electrohydrodynamic drying was only 21.31% of the electrical energy required for thermal drying [32], which is in general agreement with the results of this study. This proves that HVDPD is an effective method for achieving low-energy drying.

3.2. Analysis of Drying Characteristics

As shown in Figure 5a,b, the moisture content of all experimental groups showed a decreasing trend, and the drying rate decreased as the time increased. HAD had the fastest decrease in water content, while AD had the slowest decrease in water content and the least degree of change in the drying rate, with a relatively flat curve. The water content of HVDPD decreased with increasing voltage, which is consistent with the results of Han and Xiao et al. [12,14]. Within the first 3 h of drying treatment, the moisture content at 25 kV and 30 kV decreased faster than for HAD. This was due to the fact that the temperature of the sample cannot be raised to the experimental temperature (50 °C) as soon as the sample is placed into the oven for HAD, which is a relatively slow process. Additionally, at the beginning of the experiment, the water content of the alfalfa samples was very high, and a large number of water molecules migrated to the surfaces of the samples under the action of the high-intensity electric field and plasma wind before evaporating rapidly, which is a relatively fast process without the need to increase the temperature [33]. This resulted in a higher drying rate for HVDPD than for HAD in the early stages of drying under sufficiently high discharge voltage conditions. As shown in Figure 5c, there are significant differences in VR between different drying methods (p < 0.05), and the VR is ranked in order of magnitude as HAD > 30 kV > 25 kV > 20 kV > AD. HAD is characterized by high temperatures, high air circulation, and homogeneous drying; therefore, the water in the samples evaporates very quickly. The drying effect of the HVDPD is mainly driven by the non-uniform electric field and the plasma wind. Water molecules are polar molecules; under the action of the electric field force, water molecules are more likely to run to the surface of the sample than in their natural state, coupled with the physicochemical etching and blowing effect of the plasma wind, so the different voltages of the VR of HVDPD are higher than those of AD, and the higher the voltage used, the stronger the ionized wind and the electric field force and the faster the drying speed. The drying time also verified this result; when reaching the same drying level, with an increase in voltage, the HAD group used the shortest time, followed by the HVDPD group, while the AD group used the longest time. This is consistent with the findings of Wang and Zhang et al. [26,34]. This demonstrates the feasibility of HVDPD for increasing the alfalfa drying rate.

3.3. Analysis of Rehydration Characteristics

During the drying process, irreversible changes occur inside the material, causing the dried product to lose some of the properties and characteristics of the raw material. The more severe the structural damage, the lower the RR, so rehydration is an important indicator for evaluating the quality of dried products [35]. As shown in Figure 5d, there were no significant differences (p > 0.05) in the RRs of the HAD, AD, and 20 kV groups, while the RR of the 30 kV group was significantly different (p < 0.05) from that of the other treatment groups, and the RR of the 25 kV group was significantly different (p < 0.05) from that of the HAD and AD groups. Overall, alfalfa with the 30 kV treatment had the highest RR, HAD had the lowest, and alfalfa with different voltage treatments had higher RR than HAD and AD. This is because the temperature used by HAD was 50 °C, which was much higher than that of the other treatment groups, and HAD easily caused the irreversible collapse of the internal structure of the sample and caused the texture to harden [36]. Because the moisture content of the AD group decreased slowly, the alfalfa maintained a high moisture content for a long time, which increased the enzymatic activity of the internal cell structure, causing irreversible damage to the sample structure. Compared with AD, the RR of the 20 kV, 25 kV, and 30 kV groups increased by 3%, 8%, and 15%, respectively, and the RR of the 30 kV treatment group was the highest. This is due to the fact that HVDPD is a non-thermal drying method in which the depth of penetration of the discharge plasma is limited to the micron-to-nanometer scale. Not only does this not damage the internal structure of alfalfa, it also increases its hydrophilicity due to the physicochemical etching effect of the plasma wind on the surface structure of the sample, which, in turn, increases the RR of the alfalfa samples. The higher the voltage, the stronger the etching effect of the discharged plasma wind, which leads to a higher voltage being used to treat the alfalfa, along with a higher RR of the alfalfa. Luan and Wang et al. found that the inhomogeneous electric field and discharge plasma of HVDPD can cause damage to the coating of plant seeds, thereby increasing the hydrophilicity of the seed coat, which is consistent with the findings of this paper [20,37]. The results show that HVDPD has better rehydration performance, and the higher the voltage, the higher the rehydration rate. This is consistent with the findings of Esehaghbeygi A. et al. [38].

3.4. SEM

Figure 6 shows SEM images of alfalfa leaves after they were dried using different methods. There are scale-like protrusions, powder-like attachments, and stomata on the leaf surfaces. The scale-like structures of the AD group collapsed to some extent, while the samples treated with 20 kV, 25 kV, and 30 kV showed surface structural damage, ranging from light to severe. The collapse of the AD group was caused by the natural degradation process. During the HVDPD treatment, the ion wind contained large amounts of active nitrogen and oxygen particles, and the intensity of the ion wind increased with increasing voltage. The ion wind caused physical and chemical etching on the surface of the alfalfa, resulting in structural damage. The scale-like structures on the surface of alfalfa leaves are a special type of epidermal cell that can increase the reflectance of the leaves, reduce water evaporation, and improve photosynthetic efficiency and drought resistance. This indicates that HVDPD made the leaf surface of alfalfa more transparent, thereby increasing the drying rate. The HAD and AD groups had obvious stomatal opening phenomena [39], and the AD group had the largest stomatal opening, while the stomata of the leaves treated with HVDPD were not obvious. Thamkaew et al. found that a reversible pulsed electric field treatment resulted in the electrical evaporation of guard cells from Thai basil leaves, which led to the inactivation of guard cells and an inability to regulate stomatal size [40]. The HVDPD treatment had a similar effect on alfalfa leaves, and the stomata showed different degrees of closure. In the 2000× magnified electron microscope image, it can be seen that there were still many powder-like attachments on the leaf surface after the HAD and AD treatments, while the powder-like attachments on the leaf surface after the HVDPD treatments at 20 kV, 25 kV, and 30 kV became sparser with increasing voltage. In the 30 kV treatment group, due to the high-intensity ion wind, powder-like attachments were not observed. This also indicates that the ion wind of HVDPD had a damaging effect on the leaf surface. HAD accelerated the evaporation of water by increasing the temperature. HVDPD damaged the surface structure of the alfalfa leaves through the ion wind, and the electric field force caused water adsorption, which made the drying rate much higher than that of the AD group, and the higher the voltage, the more obvious the effect and the faster the drying rate. This conclusion is consistent with the results of the drying rate measurements.

3.5. Color Analysis

The color of alfalfa hay is an indicator of its quality. Generally, the greener the color of alfalfa hay, the better its quality, because this indicates a high content of alfalfa leaves, high nutritional value, and good palatability. In contrast, the yellower the color of alfalfa hay, the worse its quality and palatability. As shown in Figure 7 and

Table 1. Color changes in alfalfa before and after being dried using different methods.

Method	L*	a*	b*	ΔE
Fresh	23.04 ± 1.22	−8.98 ± 2.18	12.89 ± 1.30	/
HAD	41.03 ± 1.33 c	−4.42 ± 0.81 a	20.45 ± 1.71 a	20.10 ± 1.70 bc
AD	44.85 ± 2.25 a	−5.69 ± 0.89 c	21.24 ± 1.61 a	23.62 ± 2.53 a
20 kV	42.92 ± 1.77 b	−5.48 ± 0.91 bc	20.65 ± 2.40 a	21.76 ± 1.74 b
25 kV	40.61 ± 1.78 c	−4.89 ± 0.63 ab	19.61 ± 1.76 a	19.29 ± 2.19 cd
30 kV	39.47 ± 1.58 c	−4.66 ± 0.60 a	17.54 ± 0.87 b	17.64 ± 1.55 d

Note: Different letters indicate significant differences (p < 0.05) between sample means. L*: lightness; a*: redness; b*: yellowness; ΔE: chromatism.

, compared with HAD- and AD-dried alfalfa leaves, HVDPD-dried leaves were less yellow, and the yellowness of the leaves decreased with increasing voltage, with the lowest being observed in the 30 kV group. In terms of greenness, the HAD group had the lowest greenness value on the front of the leaves, the AD group had the highest, and the greenness of the HVDPD group decreased with increasing voltage. The ion wind effect produced by HVDPD caused damage to the chloroplasts in the leaf cells, and the chlorophyll was not well preserved, resulting in lower greenness [41]. HAD’s high temperature caused irreversible damage to the internal structure of the leaves. The lower the ΔE value of the dry and fresh samples, the lower the enzymatic browning reaction and color change in the sample, indicating that the quality of the dry sample is better [42]. As shown in Table 1, the brightness of the samples in the AD and 20 kV groups differed significantly from that of the other groups (p < 0.05), and the AD group had the highest brightness. The yellowness value of the 30 kV group was the lowest and was significantly different from that of the other groups (p < 0.05), while there were no significant differences between the other groups (p > 0.05). The magnitude of the ΔE of leaves was ranked as follows: AD > 20 kV > HAD > 25 kV > 30 kV. Among them, the ΔE of the AD group was the largest and was significantly different from that of the other groups in terms of color difference (p < 0.05), and the ΔE of the 30 kV group was the smallest, differing significantly from the HAD, AD, and 20 kV groups (p < 0.05). The results show that HVDPD does not cause severe damage to the internal structure of the cells due to high temperatures, can reduce the natural color degradation of alfalfa cells, and has a positive effect on slowing the color degradation of alfalfa. Consistent findings were also obtained for carrots dried by Alemrajabi A. A. et al. [31].

3.6. Infrared Spectroscopic Analysis

Figure 8 shows the effects of different drying methods on the infrared spectra of alfalfa. By referring to some common characteristic peak tables, the attribution of the main absorption peaks in the infrared spectra can be determined. For example, the peak near 3280 cm⁻¹ is the aromatic C-H stretching vibration; the peaks near 2918 cm⁻¹ and 2849 cm⁻¹ are the aliphatic C-H stretching vibrations; the peaks near 1735 cm⁻¹ are the C=O stretching vibrations of aldehydes, ketones, and esters; the peaks near 1632 cm⁻¹, 1400 cm⁻¹, and 1240 cm⁻¹ are the amide I band, amide III band, alkaloids, unsaturated esters of amino acids, proteins, etc.; and the broad and strong peak near 1024 cm⁻¹ is mainly attributed to the C-OH bending vibration of carbohydrates such as glycosides and polysaccharides [11,43]. The characteristic peaks of different drying methods have similar peak numbers, but the peak intensity varies significantly, indicating that different drying methods affect the retention degree of functional groups. Since the protein content is an important indicator for evaluating alfalfa, the bands corresponding to proteins were analyzed. The characteristic peaks were ranked from strong to weak, as follows: 30 kV > 25 kV > HAD > 20 kV > AD. According to the spectral analysis results, the protein content of alfalfa in the AD group was the lowest, while the protein content in the 30 kV group was the highest. The protein content of alfalfa dried at 30 kV and 25 kV was higher than that of the HAD group. The results show that the use of an appropriate electric field voltage for drying can effectively reduce the loss of alfalfa protein during the drying process. Compared with HAD and AD treatments, HVDPD at 30 kV and 25 kV is more conducive to the retention of alfalfa’s nutrients. The above results are consistent with the conclusions of Han et al. [12].

3.7. Nutritional Analysis

Quality traits such as CP, NFC, ADF, NDF, and lignin are key indicators of forage crop quality [44]. The contents of CP and NFC had significant effects on the growth of animals, with high contents favorable to growth [45,46]. As shown in Figure 9a, the highest CP content was found in the 25 kV group, and the CP content under different voltages was higher than that in the HAD and AD groups. The CP content of the 20 kV and 25 kV groups differed significantly from that of the other groups (p < 0.05), and the 25 kV group’s CP content was slightly higher than that of the 20 kV group, but the difference was not significant (p > 0.05). The highest NFC content was found in the 20 kV group, while the lowest was found in the AD group, and the differences between the groups with different voltage conditions and the HAD group were not significant (p > 0.05). A longer drying time would lead to the degradation of CP and NFC via cell and microorganism uptake, and a higher drying temperature would directly lead to the destruction of CP and NFC structures [47]. In contrast, HVDPD has both non-thermal and efficient drying characteristics, which are favorable to the retention of CP and NFC. This indicates that the CP and NFC contents after HVDPD are better than those after HAD and AD.

High relative contents of ADF, NDF, and lignin can reduce feed intake and the digestive efficiency of roughage in animals [45,46,48]. As shown in Figure 9b, the NDF and ADF contents after HVDPD at different drying voltages were significantly lower than those after HAD and AD (p < 0.05), where the NDF and ADF contents of the 25 kV group were the lowest, but the difference from the 20 kV group was not significant (p > 0.05). The lignin content of the HAD group was the highest, while that of the 30 kV group was the lowest, and with an increase in voltage, the lignin contents also decreased, with significant differences in lignin content between groups with different voltage conditions (p < 0.05). This indicated that HVDPD-treated alfalfa was superior to the HAD and AD groups in the three indices of NDF, ADF, and lignin content.

HM is an important index for evaluating the nutritional value of forage for ruminants. The RFQ and RFV are used for assessing the quality of forage based on the relative contents of material nutrients. Higher values of HM, the RFQ, and the RFV represent better nutritional quality in alfalfa [46,49]. As shown in Figure 9c,d, HM was the highest at 20 kV, followed by 25 kV, both of which were significantly different from the other groups (p < 0.05), and HM decreased with increasing voltage. The RFV was the lowest in the HAD group, the RFQ was the lowest in the AD group, and the differences in the RFQ and RFV values between HAD and AD were not significant (p > 0.05). The RFV and RFQ were the highest at 25 kV, and the difference in the RFQ value from the other groups was significant (p < 0.05). The RFV was not significantly different from that of the 20 kV group (p > 0.05) but was significantly different from that of the HAD, AD, and 30 kV groups (p < 0.05). These results indicated that HVDPD preserved the nutrients in alfalfa better than HAD and AD, while the drying voltage also affected the preservation of nutrients. As far as HM values are concerned, the best-quality alfalfa hay was obtained via the 20 kV treatment, while the worst was obtained via AD. Combining the RFV and RFQ values to assess the quality of alfalfa hay, the hay obtained from the 25 kV treatment was the best and had the highest feeding value, while the quality of the hay obtained through the HAD and AD treatments was the poorest.

3.8. Correlation Analysis

The experimental data (VR, RR, color values, nutrient content, and dry matter quality) were normalized and correlated. Figure 10a shows the correlation between different drying methods and alfalfa drying indicators. The results showed that the HAD rate was better than the HVDPD rate, while other indicators showed that HVDPD had more advantages. The color quality of the alfalfa dried at 30 kV was the best, with the least yellowness and the smallest color difference. At 30 kV, the correlation coefficient of the RR was the largest, indicating that the alfalfa rehydration ability was the best at 30 kV. In terms of nutrient composition, the alfalfa dried at 25 kV had the smallest correlation coefficients for ADF and NDF, the largest correlation coefficient for CP, and the largest correlation coefficients for the RFQ and RFV, indicating that the alfalfa dried at 25 kV had low levels of ADF and NDF, high levels of CP, and the highest values for RFV and RFQ; thus, the alfalfa treated at 25 kV had the best nutritional quality.

Figure 10b shows a Pearson correlation coefficient matrix depicting the correlation between different indicators. The RR was negatively correlated with lignin, NDF, ADF, b*, and ΔE, and positively correlated with CP and NFC, with the RR had a very high negative correlation with lignin (−0.99). This indicates that the RR reflects the lignin content; specifically, the higher the RR, the lower the lignin content. The color difference (ΔE) value was negatively correlated with the rehydration rate and average drying rate (VR) and positively correlated with lignin. It is worth noting that the RFV had a very high negative correlation (−1.00) with NDF and ADF, and the RFQ had a high negative correlation (−0.97) with NDF and ADF, indicating that the contents of NDF and ADF had a direct effect on the RFV and RFQ values, and the higher the NDF and ADF contents, the lower the RFV and RFQ values. This also verifies the claim that NDF and ADF would affect the nutritional quality of alfalfa. The results of statistical analysis showed that there was a certain correlation between the color quality, nutrient contents, and rehydration characteristics of the dried samples, providing a rapid and comprehensive method for evaluating the quality of alfalfa hay.

4. Conclusions

The findings of this study show that alfalfa subjected to HAD, AD, and HVDPD treatments had significant differences in drying characteristics, morphology, and nutrient contents. Among the treatments, HAD had the highest drying rate. HVDPD increased the alfalfa drying rate compared with AD, and its energy consumption was much lower than that of HAD. HVDPD significantly improved the quality of alfalfa hay compared with HAD and AD, and HVDPD-treated alfalfa had advantages in color quality, rehydration ability, and nutrient retention. In summary, HVDPD is a promising drying technology that has great future application potential for drying food and materials. In this study, we dried alfalfa using three drying methods (HVDPD, HAD, and AD) and found that compared with traditional drying technology, HVDPD can not only shorten the drying and processing time but can also maintain the integrity of the dried materials and enhance nutrient retention in food and feed.

In this study, we compared the effects of three drying methods (HAD, AD, and HVDPD) on the physicochemical properties of alfalfa, and satisfactory results were obtained. However, the research conducted on the micro-mechanisms of the action of HVDPD on alfalfa, the effects of biomolecular compounds, and the physiological state of the cells during the drying process was not sufficiently in-depth, and we will explore this in greater depth in future studies.