Influence of Electrohydrodynamics on the Drying Characteristics and Volatile Components of Ginger

Abstract

This article studies the electrohydrodynamic drying of ginger. In this work, drying experiments were performed in an electrohydrodynamic drying (EHD) system at various AC voltages (0 kV (control), 15 kV, 20 kV, 25 kV, and 30 kV). The drying properties and volatile components of ginger were thoroughly examined and studied using IR spectroscopy and GC–MS. The findings revealed that electrohydrodynamics significantly increased the drying rate and reduced the drying time of ginger, with a sevenfold increase in drying rate and a one-third reduction in drying time at 30 kV. The peak of the infrared spectrum of ginger remains unchanged. We found 240 volatile chemicals under different drying voltages. The primary volatile chemicals were esters and aldehydes. The ginger products dried at 30 kV had the lowest aldehyde concentration. These findings give an experimental and theoretical foundation for applying electrohydrodynamics to the subject of ginger drying.

1. Introduction

Ginger is a perennial herbaceous rhizome of the zingiberaceae with a distinct pungent flavor that is grown extensively in China and comes in a range of types. Ginger is a dual-use herb that has been utilized in traditional Chinese medicine (TCM) since ancient times. It has various medical and food properties, including warming the belly, halting vomiting, expelling colds, and purifying and stopping bleeding and pain [1]. Modern research has discovered that ginger includes a few minerals and functional compounds, such as curcumin, ginger oleoresin, ginger polysaccharides, ginger protease, and others, which have anti-inflammatory, anticancer, antiplatelet, and antioxidant properties [2]. But fresh ginger has a high water content. It is susceptible to rot and deterioration, making it difficult to store and transport for an extended time. Drying can effectively reduce ginger’s moisture content while inhibiting microbe growth and reproduction, extending its shelf life. Furthermore, drying can improve ginger’s taste and flavor while increasing its processing adaptability.

Ginger drying techniques now encompass several processes, including natural drying, hot air drying, microwave drying, infrared radiation drying, and vacuum freeze-drying. Each approach has distinct advantages and applications, but there are certain drawbacks and challenges [3]. Although natural drying is less expensive, it is heavily influenced by weather conditions and has a low drying efficiency [4]. Hot air drying is simple to use, but it wastes a lot of energy and makes it difficult to keep ginger’s original color. Microwave drying and associated technologies are widely employed in the field of fruit and vegetable dry processing due to their advantages, such as rapid heating, ease of control, safety, and cleanliness [5]. Infrared radiation drying is more efficient, produces a higher quality product, and is better suited to the protection of heat-sensitive materials than conventional drying technologies. Furthermore, infrared radiation drying has no emissions, pollution, or environmental impact and is increasingly demonstrating green energy-saving benefits. However, the price is considerable and it is not ideal for large-scale drying [6]. Vacuum freeze-drying refers to the process wherein the material is frozen to below the eutectic point temperature in a vacuum state so that the water sublimates from the solid state to the gaseous state, resulting in a loose porous dry product. This method maximizes the preservation of the material’s color, aroma, taste, shape, and active ingredient activity but requires expensive equipment [7].

Therefore, developing innovative drying techniques with cheap equipment cost, high drying rate, and good nutritional value is crucial. The introduction of electrohydrodynamic drying technology solves this problem [8]. Electrohydrodynamic drying is a convective drying technology that effectively removes moisture from the product. Electrohydrodynamic drying has an electrode on the top and bottom; the upper electrode is connected to the power supply and is called the emitting electrode, and the lower plate is grounded and is called the grounding electrode. The potential difference between the electrodes directly affects the drying rate of the sample. When using electrohydrodynamic drying materials, a high voltage is passed through, the needle plate electrode discharge increases, and the surrounding air is ionized, which is known as “corona wind” or “ion wind” [9]. In conclusion, this method accelerates the charged ions by means of an upper and lower voltage difference, causing a forced convection of the ions. The primary factors influencing the effectiveness of the drying process are the dimensions of the electrodes, the intensity of the electric field, and the distance between the electrodes [10].

Elmizadeh et al. examined the electrohydrodynamic drying and hot air drying of quince slices and discovered that electrohydrodynamic drying consumed half the energy of hot air drying [11]. Polat et al. used electrohydrodynamic drying on apricot samples and obtained a higher rehydration rate than the hot air drying method [12]. Yang et al. used electrohydrodynamic drying to dry potato flours. The findings indicated that electrohydrodynamic drying could improve the functional qualities of potato flours while also increasing their diversity [13]. Athari et al. used an electrohydrodynamic device to dry cellulose extracted from sugarcane bagasse. Their findings revealed that the EHD-processed cellulose fibers exhibited superior textural qualities, making them suitable to produce a fat-reducing cream [14]. Han et al. employed an electrohydrodynamic (EHD) drying technique to dehydrate garlic. Their findings indicated that the EHD-dried garlic slices were of exceptional quality, featuring a rapid rehydration capability and minimal nutrient loss. Additionally, the EHD method significantly enhanced the drying rate, suggesting its suitability for garlic drying applications [15]. Several researchers have also coupled discharge plasma drying with traditional drying methods. Bai et al. studied the drying of sea cucumber and chose electric hydrodynamic (EHD) drying and vacuum freeze-drying (FD) as the drying methods. Their findings revealed that sea cucumbers dried using this combined method exhibited reduced shrinkage, a higher rehydration rate, increased protein content, and superior sensory qualities compared to those processed with other methods [16]. Murali et al. used a solar electric hybrid dryer (S-EHD) to dry Indian mackerel. Their research showed that the moisture content of salted mackerel after drying decreased to 31.8% and the drying efficiency was 23.81% [17].

Food odor is a mixture of many volatile components, and variations in the type and amount of these volatiles result in distinct odors [18]. During the drying process, a variety of chemical reactions and factors contributed to changes in food flavor. Current research suggests that flavor in meals is influenced by three major pathways: lipid oxidation, precursor degradation, and the Maillard reaction [19]. Johnson et al. investigated the three processing grades of ginger; ginger is known for its pungent flavor and health benefits, both of which are imparted by a variety of curcumin derivatives and other volatile constituents. They discovered a total of 100 volatile compounds, including 54 terpene derivatives, in the dried ginger samples [20]. Ding et al. utilized various drying methods, including air drying (AD), microwave drying (MD), vacuum drying (VD), and freeze-drying (FD) to dehydrate fresh ginger slices. An analysis of the primary volatiles through principal component analysis showed that these drying techniques led to an increase in the content of 1-(1,5-dimethyl-4-hexenyl)-4-methyl- and benzene. Among these methods, microwave drying (MD) emerged as the most favored approach [21].

The goal of this research is to investigate the drying properties and volatile components of ginger using electrohydrodynamics, as well as to compare the benefits and drawbacks related to drying rate, rehydration, and ginger color at various voltage levels. Through a thorough examination of the main parameters and influential elements in the ginger drying process, theoretical assistance and technical direction for the development of the ginger processing sector are provided. At the same time, this article addresses the retention of volatile components throughout the ginger drying process, nutritional retention, and quality enhancement issues, simultaneously providing consumers with healthier, safer, and more delicious ginger.

2. Materials and Methods

2.1. Raw Materials

Ginger for the study in this paper was purchased from fresh food supermarkets near the New Town Campus of the Inner Mongolia University of Technology. Excess ginger was stored in a refrigerator at 4 °C and humidity controlled at 25 ± 1%.

2.2. Instruments and Equipment

In this paper, electrohydrodynamic drying is taken, using the following equipment. The EHD drying system comprises a YD (JZ)-1.5/50 high-voltage power supply, a KZX-1.5 kVA controller (Wuhan, China), and a multi-needle-plate electrode system. The high-voltage power supply can output either DC or AC voltage. The controller has an adjustable range of AC voltage (0–50 kV). The upper plate of the dryer unit is a needle-plate electrode with dimensions of 400 mm × 240 mm. The needle spacing is 40 mm × 40 mm. The lower electrode is a grounded-plate electrode with dimensions of 1000 mm × 450 mm. The distance between the upper and lower electrodes is 100 mm.

2.3. Test Methods

As shown in Figure 1, fresh ginger has been cleaned thoroughly. The ginger is then cut into cylindrical slices with a thickness of 3 mm and a certain cross-sectional area. Ginger slices are placed under the needle plate electrode. Ensure that the external environmental conditions of the experiment are controlled under the same conditions. The temperature is 23 ± 3 °C, the relative humidity is 34 ± 2%, and the air speed is 0 m/s. The control group is dried under these conditions. The experimental group begins the drying experiment after applying AC voltages of 15, 20, 25, and 30 kV. Use an electronic balance (MTQ200, Shanghai, China) to accurately measure the mass of ginger slices every 0.5 h. Observe its dryness level. When the moisture content of the sample on a wet basis reaches 10%, the ginger drying experiment ends. Each experiment is independently repeated three times.

2.4. Determination of Water Content

Moisture content is an important indicator for identifying dried ginger. The water content and moisture content of ginger slices during drying were defined as follows [22]:

$$M_i={\displaystyle \frac{{m_i-m}_g}{m_g}},$$

(1)

$$MR={\displaystyle \frac{{M_i-M}_e}{M_0-M_e}},$$

(2)

where m_g is the mass of ginger before it starts drying, m_i is the mass of ginger slices dried at the moment t_i, M_i is the moisture content of ginger slices dried at the moment t_i, MR is the moisture content ratio of ginger, M_e is the water content of ginger at equilibrium, and M₀ is the water content of ginger at the initial moment. Because the equilibrium moisture content of ginger is very small, the moisture content equation can be simplified as [23]

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

(3)

2.5. Determination of Drying Rate

Drying rate is an important indicator for the process of drying ginger. The drying rate indicates the rate of evaporation of water from the material per unit time [23], calculated as

$$DR={\displaystyle \frac{{M_t-M}_{t+∆t}}{∆t}},$$

(4)

where M_t is the water content of ginger slices at the moment t and M_t+Δt is the water content of ginger slices at the moment t + Δt.

2.6. Rehydration Rate

The ginger dried by electrohydrodynamic drying is placed in a beaker containing 250 mL of pure water. Then, heat the beaker to 95 °C. Maintain 95 °C for 10 min. Take ginger slices and measure them. Use filter paper to absorb the moisture on the surface of the ginger. Measure the quality of ginger before and after rehydration using an electronic balance. The rehydration rate of ginger is calculated according to the following formula [24]:

$$RR={\displaystyle \frac{m_g}{m_0}},$$

(5)

where m_g is the mass after rehydration and m₀ is the mass before rehydration.

2.7. Shrinkage

Ginger tends to crumple easily after drying, and calculating the shrinkage rate helps to better judge the degree of drying. The masses of ginger slices after each drying treatment and before drying were measured by the mass method. Shrinkage was calculated based on the measurement of sample mass using the equation [25]

$$S={\displaystyle \frac{m_1-m_2}{m_1}}\times 100,$$

(6)

where S is the shrinkage of ginger slices, m₁ is the mass of ginger slices before drying, and m₂ is the final mass of ginger slices after drying.

2.8. Effective Diffusion Coefficient of Ginger Water

The water diffusion coefficient of ginger was calculated based on Fick’s second law. The exact calculation expression is as follows [26]:

$${\displaystyle \frac{dM}{dt}}=D_{eff}{\displaystyle \frac{d^{2}M}{d{r}^2}},$$

(7)

For a long drying process with MR < 0.6, the equation can be expressed as

$$MR={\displaystyle \frac{8}{{\pi }^2}}\mathit{exp}\left(-{\displaystyle \frac{{\pi }^2{D}_{eff}t}{4L^2}}\right),$$

(8)

where D_eff denotes the effective diffusion coefficient of moisture in ginger. The value of L is half the thickness of the ginger slices. Taking logarithmic values for both sides at the same time,

$$\mathit{ln}\left[MR\right]=-{\displaystyle \frac{{\pi }^2{D}_{eff}}{4L^2}}t+\mathit{ln}\left[{\displaystyle \frac{8}{{\pi }^2}}\right]$$

(9)

The value of D_eff can be found from the slope in the above equation:

$$\mathrm{k}=-{\displaystyle \frac{{\pi }^2{D}_{eff}}{4L^2}},$$

(10)

The value of D_eff can be obtained from the above equation.

2.9. Color Analysis

The change in color gives a good indication of the extent of the dryness of the ginger. The color of the ginger samples was measured using a colorimeter (3nh-NR60CP, Shenzhen, China). Three measurements were taken for each group, and, with the drying time, measurements were taken not more than half an hour after the end of drying. The color difference formula is as follows [27]:

$$∆E=\sqrt{{\left(L_i^{\ast }-{ L}_0^{\ast }\right)}^2+{\left(a_i^{\ast }-a_0^{\ast }\right)}^2+{\left(b_i^{\ast }-{ b}_0^{\ast }\right)}^2},$$

(11)

where $L_0^{\ast }$, $a_0^{\ast }$, and $b_0^{\ast }$ represent the color parameters of fresh ginger slices. $L_i^{\ast }$, $a_i^{\ast }$, and $b_i^{\ast }$ represent the color parameters of dried ginger.

2.10. Infrared Spectral Analysis

The sliced dried ginger items were pulverized and thoroughly mixed with potassium bromide powder in a 1:100 ratio, sieved, and pressed into tablets using a tablet press (HY-12; Shimazu, Shizuoka, Japan). The ginger slices’ infrared scanning spectra were acquired by placing them in a Fourier transform infrared spectrometer (S10 FT-IR spectrometer, Nicorette, Danbury, CT, USA) with a wave number range of 4000–400 cm⁻¹ and 32 scans at a resolution of 4 cm⁻¹. The spectra were compared to identify the differences in various components in ginger.

2.11. Volatile Components

The volatile constituents in electrohydrodynamically dried ginger were measured and analyzed by gas chromatography–mass spectrometry (GC–MS). The dried ginger sample was powdered and a sample of 3 g was weighed and placed in a headspace sample bottle. The samples were placed in 20 mL vials and subjected to solid-phase microextraction at 50 °C with an adsorption time of 40 min, followed by desorption at 250 °C for 3 min before testing. The chromatographic column was DB-5MS, and the injection port was 250 °C. The helium heating process was first maintained at 40 °C for 2 min, then raised to 200 °C at a rate of 6 °C/min, and finally raised to 300 °C at a rate of 15 °C/min and maintained for 3 min.

2.12. Statistical Analysis

All experiments were repeated three times, and the final data were produced by averaging the findings of each group of experiments and calculating the standard deviation. The analysis software used in this article includes Origin 2021, SPSS statistics 24.0 software, SIMCA-P 14.1, and Peak Fit 4.12. Origin 2021 mainly draws line charts, bar charts, and pie charts. Statistical software was used for variance analysis and significance testing, with p < 0.05 indicating significant differences.

3. Results and Discussion

3.1. Effect of Different Voltage Levels on the Water Content of Ginger Slices

As shown in Figure 2a, the control group showed the slowest decrease in moisture content, while the decrease in moisture content in the experimental group began to increase with increasing voltage. We also found that the water content versus time curves almost overlapped at 25 kV and 30 kV, indicating that a maximum voltage exists for ginger drying under the current conditions and that further increase in drying voltage will not accelerate the drying process. Iranshahi et al. [28] found that electrohydrodynamic (EHD) drying is mainly driven by ionized air on the tip of a needle, which creates an ionic wind on the fiber surface. The ionic wind diffuses the water molecules on the surface of ginger, causing these water molecules to accelerate away from the surface. However, with the prolongation of drying time, the water content in ginger decreases, the decrease of water content becomes slower, and the drying rate gradually slows down. Figure 2c shows the average drying time and rate under different drying methods and voltages. Compared with the control group, the drying time of the experimental group outside 15 kV was reduced by half, and the average drying time was also reduced with increasing voltage. Electrohydrodynamic (EHD) drying significantly reduced the drying time (p < 0.05) and increased the drying rate compared to the control. This implies that the electrohydrodynamic (EHD) drying process can also be used to dry ginger material.

3.2. Effect of Different Drying Voltage on Drying Rate of Ginger Slices

Figure 2b shows that the drying rate of the experimental group was higher than that of the control group, and the drying rate of the ginger slices followed a similar trend at different drying voltages. There was a rapid decrease in the first period and a slow decrease in the later period. The drying rate of electrohydrodynamic (EHD) drying depends on the rate of the evaporation of surface moisture and the rate of the melting of internal moisture. In the early stage, the action of electric field and ionic wind increased the movement of water molecules, which stimulated the evaporation of water from the surface of ginger slices and accelerated the drying rate. When the water content of the ginger slices was reduced to a specific level, the amount of water available for evaporation was reduced and the drying rate decreased; this result corresponds with the conclusion of Muhamaruesa et al. [29]. The average drying times at 15, 20, 25 s and 30 kV showed that the drying time of ginger slices decreased with increase in voltage as compared to the control.

3.3. Effective Diffusion Coefficient of Moisture

Figure 2d shows that the effective water diffusion coefficient of the control group is 0.82, which is the smallest. The water diffusion coefficients of all experimental groups were higher than those of the control group (p < 0.05). The variation law of the water diffusion coefficient of ginger increases with the increase in voltage, reaching 1.87, 2.22, 2.71, and 3.34 times that of the control group at 15, 20, 25, and 30 kV, respectively. This indicates that electric fluid dynamic treatment can effectively improve the effective water diffusion coefficient of ginger slices. Li et al. [30] found that the effective moisture diffusion coefficient is positively correlated with the drying voltage when drying apple slices under different electrohydrodynamic drying conditions, which is consistent with the results in this study.

3.4. Rehydration and Shrinkage

The higher the rehydration rate, the better the quality of ginger dried products, as this causes the least damage to the structure. Figure 3a shows the effect of different voltage levels on the hydration ability of ginger. Figure 3a shows that the rehydration rate varies at different voltage levels, with the control group having the lowest rehydration rate, while the rehydration ability is improved after electrohydrodynamic drying (EHD). The best rehydration ability is achieved at a voltage of 30 kV. Internal structural damage reduces rehydration capacity [31]. Thus, EHD drying does not destroy the internal structure of ginger. From Figure 3b, it can be observed that, as the voltage increases, the shrinkage rate of ginger remains basically unchanged. During the drying process of ginger, the shrinkage rate gradually increases due to volume loss. At the initial stage of drying, the volume of ginger samples was reduced by 15% due to the removal of free water. In the middle drying stage, the charged particles caused cell shrinkage and the deformation of the capillary structure, which further increased the volume shrinkage. At the end of drying, the volume of the sample was reduced by >80%. Therefore, there is a significant relationship between the shrinkage volume of the material and its moisture content. During the microwave infrared synergistic drying process of ginger, Zeng et al. [32] also found a similar relationship between material shrinkage and moisture content. The main reasons for volume shrinkage during food drying are structural damage and the removal of moisture. The high rehydration rate and essentially constant shrinkage rate suggest that electrohydrodynamics are well-suited to drying ginger.

3.5. Color Difference Analysis

The color of a product is a key parameter that reflects its overall characteristics and significantly influences consumer perception. Color is an important quality in evaluating the merit of a food product and influences consumer choice and the value of the product [33]. Figure 4 shows how electrohydrodynamic drying affects the surface color of ginger, with significant differences seen at different voltage levels (p < 0.05). After electrohydrodynamic drying, the brightness value of ginger increases with voltage, reaching a maximum of 11.85 ± 0.96 at 30 kV. Ginger has a maximum redness value of 6.23 ± 0.41 at 20 kV and a maximum yellowness value of 10.12 ± 1.37 at 30 kV. Electrohydrodynamic drying (EHD) has higher brightness and yellowness because electrohydrodynamic drying (EHD) is a low-temperature drying process. The color change is caused by enzymatic browning, which is the main influential factor [34]. In addition, the changes in the volume and density of ginger slices before and after drying can also cause differences in color difference measurement results. The lesser color difference in EHD compared to the control group could be attributed to the faster drying rate, which results in a rapid evaporation of water [35]. In conclusion, EHD provides distinct advantages in maintaining the color of ginger. According to Polat et al. [36], an increase in ΔE with increasing temperature was observed in the combined EHD–hot air and hot air methods. In the EHD-only method, ΔE value did not change significantly with a change in the kV value but, in our study, ΔE value decreased with an increase in the kV value. The relationship between color difference and voltage varies with different materials and is not fixed.

3.6. Infrared Spectral Analysis of Ginger

Fourier transform spectroscopy is considered a powerful tool for analyzing the chemical composition of foods [37]. Figure 5a depicts the infrared spectra of ginger slices at various drying voltage levels following electrohydrodynamic drying, as well as the attribution and comparison of the key distinctive absorption peaks in the spectra. It can be found that the broad and strong absorption peaks near 3414 cm⁻¹ are mainly caused by the N-H stretching of proteins and the O-H stretching vibration of polysaccharides; the absorption peaks near 2927 cm⁻¹ are the CH₃ symmetric stretching of esters. The absorption peaks near 1629 cm⁻¹ are the Amide I of proteins: 80% C=O stretching, 10% N-H bending, 10% C-N stretching, and the peak at 1514 cm⁻¹ is Amide II: 60% N-H bending, 40% C-N stretching. The COO- symmetric stretching of fatty acids is observed near 1384 cm⁻¹, the characteristic peak near 1236 cm⁻¹ is attributed to the stretching vibration of C-O in lipid esters, the strong absorption band near 1035 cm⁻¹ is mainly attributed to the stretching vibration of C-O in polysaccharides, and near 586 cm⁻¹ is the CH₂ rocking and C=H bending (cis-disubstituted olefin) [38]. These stretching and bending vibrations can combine to form a range of nutrients, including sugars, proteins, amino acids, starch, and carbs [39]. The infrared spectra of ginger slices under various drying voltages were essentially identical and the positions of the characteristic peaks were relatively close together. This confirms that the chemical bonds and functional groups of ginger slices did not change significantly after drying and the molecular structures remained essentially the same. The distinctive peak intensities of ginger slices under various drying voltages were notably different and the absorption peak intensities were in the following order of decreasing strength: 30 kV, 25 kV, 20 kV, 15 kV, and 0 kV. Electrohydrodynamic drying contributes to the retention of ginger nutrients.

3.7. Protein Secondary Structure of Ginger

The secondary structure of ginger protein was detected by Fourier infrared spectroscopy (FTIR). The secondary structure of the protein has different types, which can be specifically classified into α-helix, β-folding, β-turning, and β-antiparallel folding, and the spectral band region of its conformation is the amide I band at 1600~700 cm⁻¹ [40]. The amide I band shows bands for β-folded structure (1610–1642 cm⁻¹), α-helix structure (1650–1660 cm⁻¹), β-turned corner structure (1660–1680 cm⁻¹), and β-antiparallel folded structure (1680–1700 cm⁻¹). α-helix, β-folding, and β-antiparallel folding are ordered structures, while β-turning is a disordered structure. The transmission spectra in Figure 5a were analyzed through baseline adjustment and the application of second-order derivatives using the appropriate software (Peak Fit 4.12). The statistical analysis of the various structural components within the protein’s secondary structure yielded results presented in Figure 5b. Upon observing Figure 5b, it becomes evident that α-helix and β-sheet constitute the primary forms of the secondary structure in ginger sheet proteins. Figure 5b shows that the contents of ordered structures treated at 0, 15, 20, 25, and 30 kV drying voltage were 87%, 86%, 83%, 86%, and 84%, respectively, whereas the contents of disordered structures were 13%, 14%, 17%, 14%, and 16%. When the voltage is increased, electrohydrodynamic (EHD) drying discharge weakens the hydrogen bonding inside amino acids, causing ruptures in the α-helix and β-folding. That is, electrohydrodynamic (EHD) drying causes the protein secondary structure to change from ordered to disordered. Duppeti et al. [41] found that this unfolds or denatures the high-level structure of proteins and reorganizes the peptide chains, altering the structural and functional properties of material proteins. As such, the EHD drying process has a significant impact on the internal structure of materials, which in turn affects product quality.

3.8. Volatile Components

Ginger includes several chemical ingredients, including phenolic components, volatile compounds (VOCs), and polysaccharides. Volatile organic compounds (VOCs) are among the most beneficial components in ginger because of their functional qualities, which include anti-inflammatory, antioxidant, and analgesic activities [42]. To investigate the volatile components in dried ginger under different drying conditions, gas chromatography–mass spectrometry (HS-SPME-GC–MS) was used to determine the volatile components and compounds in dried ginger under different drying voltage conditions. The volatile components and their relative concentrations in dried ginger were examined and identified. A total of 248 volatile organic compounds (VOCs) were found in dried ginger products, but the amounts of volatile compounds varied depending on the drying method used. Among these, 151 volatile compounds were found in the 30 kV group, 163 in the 25 kV, 160 in the 20 kV, 152 in the 15 kV, and 168 in the control group. These volatile chemicals belonged to seven classes: 44 alkanes, 34 esters, 27 aldehydes, 72 alcohols, 32 terpenes, 22 ketones, and 17 others. Figure 6 shows the number of volatile component species in ginger after drying at various drying voltages.

The concentration of volatile components can be utilized as a key indicator to distinguish ginger that was dried using different processes. As shown in Figure 7, the volatile components in ginger are primarily terpenoids and hydrocarbons. After electrohydrodynamic drying, the terpenoids in ginger were reduced, with 30 kV having the lowest terpenoids content (36.68%) and the control group having the highest (47.18%). The second-most abundant material was hydrocarbons, which remained rather stable following electrohydrodynamic drying, varying around roughly 20%. Ester compounds were the next largest level, increasing when compared to the control group, with the 15 kV experimental group having the highest ester content of 20.94%. Alcohol compounds also increased, reaching a maximum of 14.32% at 15 kV. After electrohydrodynamic drying, the content of ketones fell and that of aldehydes increased, while the concentration of other chemicals remained relatively constant.

Yu et al. [43] found that terpenes were the main volatile components of ginger, representing the main odor information of ginger; the volatile content of fresh ginger was higher than that of dried ginger, and the odor of hot air-dried ginger was the strongest compared with that of vacuum-dried, sun-dried, and vacuum-freeze-dried. The results of the rapid GC–electronic olfactory sensory evaluation showed that the hot air drying could retain more of the unique pungent and irritating odors of dried ginger. This was consistent with our results.

Figure 8 depicts the fingerprint spectra of volatile compounds in ginger after electrohydrodynamic drying at different voltages. Fingerprint spectra were drawn based on the peak retention time and peak area parameters of these compounds. In Figure 8, at 1.2–2.2, the peak value of the control group is higher. At 30.5–32.0, the experimental group has a high front. The rest of the positions have the same position of the common peaks of each sample, indicating that the samples have good consistency. The established fingerprints can be used to evaluate the quality of ginger.

This paper presents the structural identification of the volatile chemical constituents of ginger in combination with retention indices after a search of the mass spectrometry standard spectral library (NIST). The retention index indicates the behavior of the retention value of a substance in a stationary solution and has the advantages of good reproducibility, homogeneity, and small temperature coefficient.

Table 1. Total volatile components and relative content.

NO.	Compounds	CAS	Structural Formula	Ret. Index
1	Tricyclo[2.2.1.0(2,6)]heptane, 1,7,7-trimethyl-	508-32-7		729
2	2-Heptanone	110-43-0		853
3	2-Heptanol	543-49-7		879
4	Bicyclo[3.1.0]hexane, 4-methylene-1-(1-methylethyl)-	3387-41-5		897
5	5-Hepten-2-one, 6-methyl-	110-93-0		938
6	.beta.-Pinene	127-91-3		943
7	.alpha.-Pinene	80-56-8		948
8	.beta.-Myrcene	123-35-3		958
9	trans-.beta.-Ocimene	3779-61-1		976
10	Octanal	124-13-0		1005
11	1,3-Cyclopentadiene, 1,3-bis(1-methylethyl)-	123278-27-3		1012
12	2-Nonanone	821-55-6		1052
13	Eucalyptol	470-82-6		1059
14	Linalool	78-70-6		1082
15	Camphene	79-92-5		1088
16	Nonanal	124-19-6		1104
17	Bicyclo[2.2.1]heptan-2-one, 1,7,7-trimethyl-	464-48-2		1121
18	Citronellal	106-23-0		1125
19	Terpinen-4-ol	562-74-3		1137
20	Fenchol	1632-73-1		1138
21	3-Cyclohexene-1-methanol, .alpha.,.alpha.4-trimethyl-	98-55-5		1143
22	Cyclohexanol, 1-methyl-4-(1-methylethenyl)-	7299-40-3		1158
23	Neral	106-26-3		1174
24	Citronellol	106-22-9		1179
25	Acetic acid, octyl ester	112-14-1		1183
26	Benzenemethanol, .alpha.,.alpha.,4-trimethyl-	1197-01-9		1197
27	Decanal	112-31-2		1204
28	2-Decenal, (Z)-	2497-25-8		1212
29	Oxiranecarboxaldehyde, 3-methyl-3-(4-methyl-3-pentenyl)-	16996-12-6		1215
30	2-Nonanol, acetate	14936-66-4		1218
31	Geraniol	106-24-1		1228
32	2-Undecanone	112-12-9		1251
33	Isoaromadendrene epoxide	0-00-0		1281
34	6-Octen-1-ol, 3,7-dimethyl-, acetate	150-84-5		1302
35	Myrtenyl acetate	1079-01-2		1314
36	2,6-Octadien-1-ol, 3,7-dimethyl-, acetate	141-12-8		1352
37	Cyclohexene, 4-ethenyl-4-methyl-3-(1-methylethenyl)-1-(1-methylethyl)-, (3R-trans)-	20307-84-0		1377
38	Aromandendrene	489-39-4		1386
39	(E)-2-Decenyl acetate	2497-23-6		1389
40	Cyclohexane, 1-ethenyl-1-methyl-2,4-bis(1-methylethenyl)-, [1S-(1.alpha.,2.beta.,4.beta.)]-	515-13-9		1398
41	trans-.alpha.-Bergamotene	13474-59-4		1430
42	.gamma.-Elemene	29873-99-2		1431
43	cis-.beta.-Farnesene	28973-97-9		1440
44	Cyclohexene, 3-(1,5-dimethyl-4-hexenyl)-6-methylene-	20307-83-9		1446
45	1,3-Cyclohexadiene, 5-(1,5-dimethyl-4-hexenyl)-2-methyl-	495-60-3		1451
46	.alpha.-Farnesene	502-61-4		1458
47	1-Isopropyl-4,7-dimethyl-1,2,3,5,6,8a-hexahydronaphthalene	16729-01-4		1469
48	(R)-1-Methyl-4-(6-methylhept-5-en-2-yl)cyclohexa-1,4-diene	28976-67-2		1480
49	.beta.-Bisabolene	495-61-4		1500
50	trans-Sesquisabinene hydrate	145512-84-1		1523
51	Benzene, 1-(1,5-dimethyl-4-hexenyl)-4-methyl-	644-30-4		1524
52	Farnesene epoxide, E-	83637-40-5		1540
53	Cyclohexanol, 3-ethenyl-3-methyl-2-(1-methylethenyl)-6-(1-methylethyl)-	35727-45-8		1555
54	6,10-Dodecadien-1-yn-3-ol, 3,7,11-trimethyl-	2387-68-0		1572
55	(1R,4R)-1-methyl-4-(6-Methylhept-5-en-2-yl)cyclohex-2-enol	58334-55-7		1591
56	2-Naphthalenemethanol, decahydro-.alpha.,.alpha.,4a-trimethyl-8-methylene-	473-15-4		1593
57	3(10)-Caren-4-ol, acetoacetic acid ester	0-00-0		1605
58	2,6,10-Dodecatrienal, 3,7,11-trimethyl-	19317-11-4		1656
59	Bergamotol, Z-.alpha.-trans-	88034-74-6		1673
60	2,6,10-dodecatrien-1-ol, 3,7,11-trimethyl-	3790-71-4		1710
61	Phenol, 5-(1,5-dimethyl-4-hexenyl)-2-methyl-	30199-26-9		1744
62	geranyl-.alpha.-terpinene	0-00-0		1962
63	(E)-1-(6,10-Dimethylundeca-5,9-dien-2-yl)-4-methylbenzene	55968-43-9		2006

shows the analysis of volatile components in ginger based on retention indices. There were 63 common substances, including 15 alcohols, 13 terpenoids, 5 ketones, 8 aldehydes, 7 lipids, 12 hydrocarbons, and 3 other substances. Table 1 includes their chemical names, CAS, molecular formulae, and retention indices.

According to Bartley et al. [44], pungent compounds are destroyed during drying and the pungency is reduced. The relative concentration of most of the monopods and sesquiterpene hydrocarbons increased. Terpenoids give ginger its distinct scent and are one of its most volatile components. They possess a variety of biological actions, including antioxidant and anti-inflammatory properties. After electrohydrodynamic drying, the percentage content of terpenoids (α-pinene, camphorene, β-pinene, β-laurene, and γ-pinene) decreased due to varying voltage levels. This can be linked to the production of short-chain olefins and the isomerization of related molecules [45]. It could also be related to the conversion of sesquiterpenes to monoterpenes after extended exposure to hot air. During drying, only the control group produced esters such as vinyl formate, linalyl acetate, citronellyl acetate, and (E)-2-decenyl acetate. This was due to prolonged exposure to oxygen, which facilitated the esterification of alcohols into their corresponding esters. Under varied drying settings, 27 aldehydes were found, with octanal, nonanal, citral, and geranial being the most volatile ingredients. The aldehydes were mostly formed by fat oxidation and amino acid decomposition and they had a refreshing aroma. The total aldehyde content was higher in the EHD group. This shows that aldehydes are more volatile in high-voltage environments [46].

In general, the change of volatile components in materials is related to microbial metabolism, bacterial proliferation, and some chemical reactions, and the drying characteristics of different drying methods differ, in which the active substances produced by the discharge plasma during the drying process react with the volatile components of the materials, thus changing the volatile components of ginger. Natural air-drying causes a substantial oxidation of ginger throughout the drying process due to the abnormally extended drying time, which damages ginger’s volatile components. As a result, the volatile contents of the various drying techniques differed significantly.

The research results of gas chromatography–mass spectrometry (GC–MS) were analyzed using chemometrics. The first stage of data analysis used principal component analysis (PCA) to provide an overall picture of all sample differences and identify the biomarkers that caused these differences. Principal component analysis (PCA) was conducted on the volatile components of ginger under different drying procedures, with 70 common volatile components as independent variables and several drying methods as independent factors [47]. PCA has been found to be effective at simplifying data and revealing interrelationships between samples. This approach, which describes unsupervised categorization trends between data, is increasingly commonly utilized [48]. Figure 9a shows the results, with PC1 (39.3%) and PC2 (24.6%) accounting for 63.9% of the total variation. Ginger dried using different ways may be clearly recognized based on volatile components, with electrohydrodynamic drying (EHD) concentrated in quadrants two, three, and four and natural air drying concentrated in quadrant one. As can be observed, the effect of different drying processes on volatile components varies greatly. The PCA helps us to identify different drying methods.

In addition, using the content of volatile compounds as the dependent variable and different drying voltages as independent variables, orthogonal partial least squares discriminant analysis (OPLS-DA) can effectively distinguish ginger under different drying conditions (Figure 9b). The results revealed that the experimental group was in the first, second, or fourth quadrants, but the control group was in the third quadrant. In the OPLS-DA model analysis, the model fitting index R²_X is 0.978, R²_Y is 0.999, and Q² is 0.998. Both R² and Q² are greater than 0.5, indicating a good fit of the model [49]. In addition, 200 repeated permutation tests were conducted to evaluate the robustness of the OPLS-DA model. As shown in Figure 9c, the intersection points between the Q² regression line and the vertical axis is less than zero, indicating that the model is not overfitting and is effective. The findings are valuable for identifying and analyzing volatile chemicals in ginger using various drying processes. The variable projected importance (VIP) was utilized to assess each variable’s influence and explanatory power in classification and discrimination. When the VIP surpassed 1, the variable was thought to play a significant effect.

3.9. Statistical Analysis

By normalizing the experimental data and plotting the relevant cyclic heatmap (Figure 10), we can better analyze the drying characteristics of ginger. These data include average drying rate, average drying time, effective water diffusion coefficient, rehydration rate, color difference, shrinkage rate, a*, b*, and L*. According to Figure 10, the correlation coefficients of average drying rate, shrinkage, b*, color difference, and effective water diffusion coefficient are the largest in the 30 kV treatment group; the a* correlation coefficient is the largest in the 20 kV group; and the correlation coefficient of average drying time is the largest in the control group. The results showed that the ginger dried at 30 kV had the best comprehensive quality.

Figure 11 shows a clustered heat map for screening differential volatiles in ginger under various drying settings. The relationships between various individuals in the clustered samples are shown. The vertical axis displays 36 distinct volatile chemicals (VIP > 1, p < 0.05), whereas the horizontal axis represents various drying voltages. The 36 differentially volatile substances (VIP > 1, p < 0.05) were screened. The following compounds have been identified: Rhamnol, α-Cadarenol, γ-Pinene, 2-Methoxy-1,7,7-trimethylbicyclo[2.2.1]heptane, Carbon, 9,10-Dehydrocyclic, Decahydro-2-naphthalenemethanol, 1-vinyl-1-methyl-2,4-dicyclohexane, 4-vinyl-alpha-cyclohexanemethanol, trans-Sedum, p-Toluic, β-Opulinone, 1-(6,6-Dimethylbicyclo[3.1.0]methylene)ethanone, Andrographolide, 2,7-Octadien-1-ol, 2-Methylene-3-(1-methylethenyl)cyclohexanol, Linalool, 3-Vinyl-3-methyl-2-(1-methylethyl)cyclohexanol, α-Hydrocotylethylene, 1,7,7-Trimethylbicyclo[2.2.1]heptan-2-ol, endoborn, α-Muurolene, 1H-cyclopropan[a]naphthalene, τ-Cardanol, Ultramarine, (E)-2-Hexen-1-ol, 3-Hydroxy-3-methylpentanoic, (1aR,4S,4aR,7R,7aS,7bS)-1,1,4,7-tetramethyl, 2-((3,3-dimethyloxapropan-2-yl)methyl)-3-methyl-2-butenol, 1,7,7-trimethylbicyclo[2.2.1]hept-2-acetic, decahydro-4a-methyl-1-methylnaphthalene, 2,6-dimethyl-6-methylenebicyclo[3.1.1]hept-2-ene, 2-carene, (1aR,3aS,7S,7aS,7bR)-1,1,3a,7-tetramethyl-something, and 2-Nonanone.

At 30 kV, 4-vinyl-alpha-cyclohexanemethanol, α-Muurolene, and decahydro-4a-methyl-1-methylnaphthalene all showed a positive correlation. The hierarchical chemical clustering demonstrates that the clustering of 36 different volatile chemicals reveals a link between ginger at different drying voltages. It is worth noting that the difference in volatile matter content can effectively distinguish dried ginger products.

4. Conclusions

This article uses electrohydrodynamic drying to control different voltages for drying ginger. It analyzes the drying characteristics, infrared spectra, and volatile components of dried ginger. The results indicate that increasing the voltage can shorten the drying time and improve the drying rate. Among them, the 30 kV experimental group shortened the drying time by 65.51%, increased the drying rate by 4.3 times, increased the rehydration rate by 39.07%, and increased the effective diffusion coefficient of moisture by 47.64%. Meanwhile, the ginger brightness value (L*) of the 30 kV treatment group was the highest. Eight volatile organic compounds (VOCs) were found in ginger. Terpene compounds were the most prevalent chemicals in ginger, and they were responsible for the distinctive odor. The results of gas chromatography–mass spectrometry (GC–MS) tests revealed that differences in voltage caused various changes in the volatile components of ginger during electrohydrodynamic drying. According to the literature [50,51,52], the energy consumption of electrohydrodynamic drying is relatively low compared to conventional hot air drying. The power (DC or AC) applied during electrohydrodynamic drying, the magnitude of the voltage, the geometry and material of the electrodes, and the distance between discharge and collection can influence the energy efficiency. Through the above analysis, it was determined that the electrohydrodynamic drying of ginger is a good drying method. Technology that moves from the laboratory to industrialization progresses through three stages: laboratory research, pilot testing, and industrialization. These experimental results are from the research stage of EHD drying technology in the laboratory. The main purpose is to explore the laws of EHD drying and provide experimental data and theoretical basis for further research. Our goal is to move EHD drying technology from the laboratory to industrialization, but a lot more work needs to be carried out.