Drying Performance and Optimization of Ginger Slices Using Microwave Vacuum Drying

1  Introduction

Ginger (Zingiber officinale) is an important crop in China, valued for its culinary and medicinal applications [1,2]. To extend its shelf life and enhance its economic value, ginger is often processed into dried ginger and ginger powder, preserving its nutritional and sensory qualities [3,4]. Drying plays a critical role in this processing chain, significantly influencing storage stability, final product quality, and its suitability for further processing [5,6]. Current drying methods for ginger primarily include natural drying (ND), hot air drying (HAD), microwave drying (MD), and solar drying (SD), each of which presents notable limitations [7]. ND is inexpensive but slow, weather-dependent, and prone to inconsistent quality [8]. While HAD offers better control, it is still time-consuming [9,10]. MD is faster but often leads to uneven heating and quality degradation [11]. SD, although attractive for its renewable energy use and low operational costs, suffers from low energy density, climatic dependence, and poor process control. These drawbacks result in prolonged drying times, unstable drying rates, and risks of quality degradation, microbial growth, and contamination [12,13]. Furthermore, the extended exposure to solar radiation accelerates the degradation of sensitive bioactive compounds such as gingerols [14].

To address these limitations, Microwave Vacuum Drying (MVD) has emerged as a promising hybrid drying technology. MVD integrates the volumetric heating of microwaves with the low-temperature conditions of a vacuum, offering rapid drying while maintaining high product quality. Previous studies have demonstrated MVD’s potential for ginger processing. For example, Guo et al. [15] reported that MVD-produced ginger powder retained higher levels of total phenolics, and Lin et al. [16] found that slicing orientation affected shrinkage and microstructural development during MVD. Other studies on medicinal and oil-rich materials have shown that MVD efficiency is influenced by microwave power, vacuum degree, and material geometry [17,18]. Additionally, emerging studies have explored advanced methods to extract, stabilize, and utilize ginger’s active compounds, such as gingerols, in functional foods and packaging [19,20].

Despite these advances, the application of MVD to ginger drying remains underexplored. Specifically, a comprehensive investigation into the interactive effects of MVD parameters (microwave power, vacuum degree, and slice thickness) on drying efficiency, mass transfer mechanisms, and microstructural changes is lacking. Furthermore, there is no direct comparative evaluation of MVD, conventional HAD, and standard MD under controlled conditions.

Therefore, this study aims to provide a detailed evaluation of ginger drying using MVD, with the following key contributions and innovations:

1.   Mechanistic Elucidation of Unique Drying Kinetics: This study identifies and explains the absence of a constant-rate period in MVD, a key distinction from MD. This phenomenon is attributed to the vacuum-enhanced internal vapor pressure gradient, which disrupts the dynamic equilibrium between internal moisture migration and surface evaporation.

2.   Superior Drying Efficiency via Synergistic Mechanisms: MVD demonstrates significantly higher drying efficiency compared to both HAD and MD, reducing drying time to approximately one-sixth and three-fourths, respectively. This improvement results from the synergistic coupling of volumetric microwave heating and vacuum-induced boiling point depression, which accelerates internal moisture vaporization and mass transfer while mitigating thermal resistance at the material surface.

3.   Quantitative Hierarchy of Process Parameters: The influence of key MVD parameters on mass transfer was first interpreted mechanistically through effective moisture diffusivity (Deff) analysis, which demonstrated that microwave power, vacuum degree, and slice thickness all exert significant effects on internal moisture migration. While the quantitative ranking of their relative importance was established through response surface methodology (RSM). The RSM results consistently identified microwave power as the dominant factor, followed by vacuum degree and slice thickness, ensuring coherence between the mechanistic insights provided by Deff and the statistical optimization framework.

4.   Elucidation of Microstructural Evolution Mechanisms: The study clarifies how ginger tissue undergoes structural changes during MVD, linking distinct pathways—ranging from uniform shrinkage to puffing and rupture—to thermal and hygroscopic stresses. These findings provide a mechanistic understanding of how drying kinetics affect product quality and establish a microstructure-guided framework for process optimization.

Overall, this research offers an energy-efficient drying strategy, presenting key parameters and mechanistic insights to advance the industrial processing and quality control of ginger products.

2  Materials and Methods

2.1 Materials and Equipment

Fresh ginger (Zingiber officinale Roscoe) used in this study was purchased from a commercial supplier at Hongqi Commercial City, Xiangtan, Hunan Province, China, to ensure uniformity of the raw material. All samples were visually inspected and selected to be free from mechanical damage, decay, and surface defects, with firm texture and uniform size. Prior to experiments, the ginger samples were stored at 4°C and equilibrated to ambient temperature (25 ± 2°C) for at least 2 h before processing to minimize the influence of temperature gradients on drying behavior.

The drying experiments and quality measurements were conducted using the following instruments, with their main technical specifications summarized below to ensure reproducibility: Electric hot air oven (DHG series, Shanghai Yiheng Scientific Instrument Co., Ltd., China): operating temperature range of RT + 10 to 250°C, chamber volume of 136 L, and temperature fluctuation within ±1.0°C; Laboratory microwave dryer (NJL07-3, Nanjing Jiequan Microwave Equipment Co., Ltd., China): operating frequency of 2450 ± 15 MHz, adjustable microwave power from 0 to 700 W, chamber volume of 30 L, equipped with an internal mode stirrer to improve field uniformity; Microwave vacuum drying oven (RWBZ-08S, Nanjing Sunrui Drying Equipment Co., Ltd., China): microwave power range of 0–800 W, maximum achievable vacuum degree of −0.098 MPa, and a nominal dehydration capacity of 0.8 kg/h. The vacuum level was continuously monitored using a built-in calibrated vacuum gauge; Precision electronic balance (JY1003, Ningbo Yinzhou Huafeng Electronic Instrument Factory, China): weighing range of 0–1000 g with a resolution of 0.001 g; Power recorder (PO8S-16A, Yuyao Pinyi Electric Appliance Co., Ltd., China): accuracy class 1, maximum power tracking capacity of 4000 W, employed to continuously record and integrate the total electrical energy consumption of each drying process; Colorimeter (WR-10, Shenzhen Wave Optoelectronics Technology Co., Ltd., China): equipped with a D65 standard light source and CIELAB color system, with a repeatability of ΔE < 0.08, used for color evaluation of dried ginger slices; Scanning electron microscope (SEM) (JSM-IT810, JEOL, Japan): thermal field emission electron gun, secondary electron resolution of 0.6 nm at 15 kV, used to observe surface and cross-sectional microstructures of ginger samples before and after drying. A schematic diagram illustrating the experimental drying system and measurement configuration is shown in Fig. 1.

Figure 1: Drying process and experimental apparatus.

2.2 Experimental Procedure

2.2.1 Sample Preprocessing

Fresh ginger roots were washed thoroughly with distilled water to remove surface impurities, manually peeled, and sliced using a custom-made stainless-steel slicing mold to ensure uniform thicknesses of 2, 3, and 4 mm. These thicknesses were selected to represent the typical range used in commercial dried ginger products and to systematically evaluate the influence of diffusion path length on heat and mass transfer during drying. After slicing, the samples were sealed in polyethylene bags and stored at 4°C. Prior to drying experiments, all samples were equilibrated to ambient conditions (25 ± 2°C) for at least 2 h to minimize the effect of initial temperature differences on drying kinetics. The initial moisture content of fresh ginger was determined using the standard oven-drying method at 105°C until constant mass was achieved and was found to be 94.17% (wet basis). The drying endpoint for all experiments was defined as a moisture content of ≤10% on a dry basis, which is widely accepted as a safe level for the storage and handling of dried ginger products.

2.2.2 Hot Air Drying (HAD)

For each run, 30 ± 0.5 g of ginger slices were evenly spread in a single layer on an aluminum tray and dried in an electric hot air oven. Drying temperatures of 50, 60, and 70°C were selected based on preliminary experiments. Temperatures below 50°C resulted in excessively long drying times, while temperatures above 70°C led to noticeable quality deterioration, particularly excessive surface browning and color degradation. During drying, sample mass was recorded at 30 min intervals until the target moisture content (≤10% d.b.) was reached. These conditions were chosen to represent conventional industrial hot air drying practices and to provide a baseline for comparison with microwave-based drying methods.

2.2.3 Microwave Drying (MD)

For microwave drying experiments, 30 ± 0.5 g of ginger slices were uniformly arranged on a glass plate and placed at the center of a laboratory microwave dryer. Preliminary trials indicated that microwave powers below 200 W resulted in slow moisture removal, whereas powers above 500 W caused localized overheating, uneven moisture distribution, and structural damage due to excessive internal vapor pressure buildup. Accordingly, microwave power levels of 200, 350, and 500 W were selected for systematic evaluation. The mass of the samples was recorded at 5 min intervals until the moisture content reached ≤10% (d.b.).

2.2.4 Microwave Vacuum Drying (MVD)

For MVD experiments, 30.0 ± 0.5 g of ginger slices were evenly spread on a glass plate and dried in a microwave vacuum dryer. Sample mass was measured at 5 min intervals throughout the drying process until the target moisture content was achieved. Preliminary tests demonstrated that vacuum levels lower than 0.02 MPa provided limited enhancement compared to atmospheric microwave drying, whereas a vacuum level of 0.08 MPa significantly promoted moisture removal by reducing the boiling point of water and enhancing internal vapor-driven mass transfer, while remaining within the safe operating range of the equipment. Therefore, vacuum degrees of 0.02, 0.05, and 0.08 MPa were selected.

To investigate the individual effects of key MVD parameters, initial single-factor experiments were conducted under the following conditions:

(1)   Microwave power: 200, 350, and 500 W at a fixed vacuum of 0.05 MPa and slice thickness of 3 mm;

(2)   Vacuum degree: 0.02, 0.05, and 0.08 MPa at 350 W and 3 mm thickness;

(3)   Slice thickness: 2, 3, and 4 mm at 350 W and 0.05 MPa vacuum.

Given the strong coupling between microwave power, vacuum degree, and material geometry in MVD which leads to nonlinear heat and mass transfer behavior, response surface methodology (RSM) was employed to model and optimize the drying process. The experimental design and factor levels used in the RSM are presented in Table 1. All experiments were conducted in triplicate, and the average values were reported. The measurement uncertainties associated with key experimental parameters are summarized in Table 2.

2.3 Theoretical Basis

2.3.1 Moisture Content

Mt=mt−mcmc×100%(1)

where mt is the mass at time t, mc is the mass of the dry sample.

2.3.2 Moisture Ratio

The moisture ratio MR was calculated as:

MR=M−MeM0−Me(2)

Since Me is very small compared to Mt and M0 and can be neglected, the above formula can be simplified to:

MR=MtM0×100%(3)

where Mt is the moisture content of the material at any time t, M0 is the initial moisture content of the material, Me is the equilibrium moisture content of the material.

2.3.3 Drying Rate

DR=ΔMΔt=Mt−Mt+ΔtΔt(4)

where ΔM is the difference in moisture content between adjacent samples, Δt represents the time gap between successive measurements of sample weight.

2.3.4 Energy Consumption Calculation

To quantitatively assess the energy utilization efficiency of different drying technologies, Specific Energy Consumption (SEC) was introduced as a key evaluation metric. SEC is defined as the electrical energy consumed to evaporate a unit mass of water, calculated based on the total energy consumption of the drying process and the mass of water removed:

SEC=Emω(5)

where E is the total electrical energy consumed during the entire drying process (kWh), obtained by real-time integration via a power recorder; and mω is the mass of water removed (kg), calculated from the mass difference of the sample before and after drying.

2.3.5 Mathematical Modeling

To model the drying kinetics of ginger slices under MVD, for which no established model currently exists, five classic empirical and semi-empirical mathematical models [21,22] were selected to fit the experimental drying curves. The coefficient of determination (R2) and root mean square error (RMSE) were employed to assess the goodness of fit for each model. The forms of the evaluated models are presented in Table 3.

2.3.6 Correlation Coefficients and Error Analyses

In this experiment, several commonly used thin-layer drying models (Table 3) were employed to fit the experimental data. The goodness of fit for each model was evaluated using the coefficient of determination (R2), chi-square (χ2), and root mean square error (RMSE). A better fit is indicated by higher R² values coupled with lower χ2 and RMSE values. These statistical parameters were calculated using the following equations:

R2=1−∑i=1N(MRexp,i−MRpre,i)2∑i=1N(MRexp,i¯−MRpre,i)2(6)

χ2=∑i=1N(MRexp,i−MRpre,i)2N−n(7)

RMSE=1N∑i=1N(MRexp,i−MRpre,i)2(8)

where MRexp,i and MRpre,i represent the experimental and predicted MR values calculated from the i-th experimental data point, respectively; N denotes the number of experimental data points; and n represents the number of model parameters.

2.3.7 Calculation of Effective Diffusivities

It is well-established that the drying characteristics of biological materials during the falling-rate period can be effectively modeled using Fick’s second law of diffusion. For materials with slab geometry, assuming a uniform initial moisture distribution, the analytical solution derived by Crank can be applied. The general form of this solution is given in Eq. (9), which is applicable to particles with slab geometry.

MR=8π2∑n=0∞1(2n+1)2exp⁡(−(2n+1)2π2Defft4L02)(9)

where Deff is the effective diffusivity (m2/s); L0 is the half thickness of slab (m). For long drying period, Eq. (9) can be further simplified to only the first term of the series. Then, Eq. (9) is written in a logarithmic form as follows:

ln⁡MR=ln⁡8π2−π2Defft4L02(10)

The diffusion coefficient is usually determined by plotting the experimental drying data as lnMR against drying time t, as this plot will show a straight line, and its slope K is (π2Deff)/(4L02).

2.4 Methods for Quality Attributes Measurement

To evaluate the effects of MVD on the quality attributes of ginger slices, color characteristics, rehydration behavior, and microstructural features of the dried samples were analyzed, as these parameters are closely associated with consumer acceptance, structural integrity, and mass transfer behavior during drying.

2.4.1 Color Measurement

The surface color of dried ginger slices was measured using a precision colorimeter operating under a D65 standard illuminant and the CIELAB color space. The color parameters L* (lightness), a* (red–green coordinate), and b* (yellow–blue coordinate) were recorded. Fresh ginger slices were used as the reference (control). For each sample, measurements were taken at three randomly selected positions on the surface, and the average values were reported to minimize local heterogeneity. The total color difference (ΔE) between dried and fresh ginger slices was calculated according to Eq. (11):

ΔE=(L∗−L0∗)2+(a∗−a0∗)2+(b∗−b0∗)2(11)

where L0*, a0* and b0* are the color parameters of the fresh ginger slices.

2.4.2 Rehydration Ratio

The rehydration capacity of dried ginger slices was evaluated as an indicator of structural integrity and porosity development during drying. A known mass of dried sample (Wd) was immersed in an excess of distilled water at 25°C for 30 min under static conditions to ensure sufficient water availability. After rehydration, the samples were removed, gently blotted with filter paper to eliminate surface water, and weighed to obtain the rehydrated mass (Wr). The rehydration ratio (RR) was calculated using Eq. (12):

RR=(Wr−WdWd)×100%(12)

2.4.3 Microstructure Observation

To examine the microstructural evolution induced by MVD under different drying intensities, two representative MVD-treated samples—corresponding to the shortest drying time and the longest drying time—were selected for comparison, together with fresh ginger as a reference. The samples were fractured to expose the cross-sectional surface, freeze-dried to preserve internal structure, and sputter-coated with a thin layer of gold to enhance electrical conductivity. The cross-sectional morphology was observed using a scanning electron microscope (SEM, JEOL JSM-IT810, Japan) at an accelerating voltage of 5.0 kV. The observed microstructural features were used to qualitatively interpret differences in moisture migration behavior, structural collapse, and puffing phenomena under different MVD conditions.

3  Results and Discussion

3.1 Drying Characteristics of MVD

3.1.1 Effect of Microwave Power on Ginger Drying

Fig. 2 illustrates the evolution of dry-basis moisture content of ginger slices dried under different microwave power levels (200, 350, and 500 W). In all cases, moisture content decreased monotonically with drying time. Increasing microwave power significantly shortened the total drying duration, with drying times of approximately 60, 50, and 30 min at 200, 350, and 500 W, respectively. The acceleration of drying at higher microwave power can be attributed to the increased volumetric energy absorption within the material. Under microwave heating, electromagnetic energy is directly converted into thermal energy within the moisture-containing matrix, leading to rapid temperature rise and enhanced internal vapor generation. The resulting increase in internal vapor pressure intensifies the moisture migration driving force, thereby reducing both evaporation time and internal mass transfer resistance. This behavior is consistent with the fundamental principle that microwave drying kinetics are governed primarily by the absorbed power density rather than by external heat transfer limitations. Similar trends have been reported for other high-moisture biological materials, such as Moringa leaves extract, where increased microwave power density markedly reduced drying time [23].

Figure 2: The moisture ratio changes of ginger slices under different microwave powers.

The corresponding drying rate curves under different microwave power levels are presented in Fig. 3. Although higher microwave power resulted in more pronounced fluctuations in drying rate, particularly at 500 W, all curves exhibited a similar overall pattern, characterized by a rapid initial increase followed by a continuous deceleration. The absence of a distinct constant-rate period indicates that moisture removal during MVD is predominantly controlled by internal vapor-driven transport rather than steady surface evaporation. The initial rapid increase in drying rate can be associated with the abundant availability of free and weakly bound water and the strong internal heating effect of microwaves, which promotes rapid vaporization throughout the sample volume. As drying proceeds, the depletion of easily removable moisture and the progressive strengthening of moisture binding within the solid matrix lead to a sharp decline in drying rate, marking the transition to a diffusion-dominated regime. At higher microwave power levels, this transition occurs earlier because free water is removed more rapidly, exposing the diffusion-controlled stage sooner. The observed drying rate fluctuations, particularly under high microwave power (500 W), are likely associated with intermittent puffing or localized structural disruption caused by rapid vapor generation and release. Such phenomena are commonly observed during microwave drying of moist biological materials and reflect the dynamic interaction between internal vapor pressure buildup, structural resistance, and momentary vapor escape. These transient events further highlight the inherently unsteady nature of heat and mass transfer during MVD.

Figure 3: The drying rate changes of ginger slices under different microwave powers.

3.1.2 Effect of Vacuum Degree on Ginger Drying

Fig. 4 shows the evolution of dry-basis moisture content of ginger slices dried under different vacuum degrees (0.02, 0.05, and 0.08 MPa). The total drying times were approximately 70, 50, and 40 min, respectively, indicating that increasing the vacuum degree significantly accelerates the MVD process. The enhancement of drying performance at higher vacuum levels can be primarily attributed to pressure-induced thermodynamic and mass transfer effects. First, reducing the ambient pressure lowers the boiling point of water within the ginger matrix, allowing phase change to occur at lower temperatures. This facilitates internal moisture vaporization under milder thermal conditions, thereby reducing the thermal resistance required for evaporation and potentially mitigating heat-induced quality degradation. Second, an increased vacuum degree enlarges the water vapor partial-pressure difference between the product interior and the surrounding chamber environment. This amplified pressure gradient enhances vapor-driven moisture transport from the interior toward the surface and subsequently into the vacuum chamber. In contrast to atmospheric conditions, where surface evaporation may limit mass transfer, the low-pressure environment in MVD suppresses external resistance, enabling more efficient removal of internally generated vapor. Similar effects of vacuum-enhanced drying kinetics have been reported for tomato slices by Alvi et al. [24].

Figure 4: The moisture ratio changes of ginger slices under different vacuum degrees.

The corresponding drying rate curves under different vacuum degrees are presented in Fig. 5. At a vacuum degree of 0.08 MPa, the drying rate increased rapidly during the early stage, reached its maximum value earlier, and then declined sharply. Conversely, at 0.02 MPa, the drying rate exhibited a delayed peak with a lower maximum value and a more gradual decrease. These differences reflect the role of ambient pressure in governing the onset and intensity of internal vaporization. At higher vacuum levels, the reduced equilibrium vaporization temperature decreases the energetic barrier for phase change, enabling rapid moisture evaporation even at moderate internal temperatures. As drying progresses, the rapid depletion of free water leads to an earlier transition to a diffusion-dominated regime, resulting in a pronounced deceleration in drying rate. At lower vacuum degrees, vaporization is less intense, and moisture removal proceeds more gradually, delaying the peak drying rate and extending the overall drying duration.

Figure 5: The drying rate changes of ginger slices under different vacuum degrees.

3.1.3 Effect of Thickness on Ginger Drying

Fig. 6 illustrates the variation in dry-basis moisture content of ginger slices with different thicknesses (2, 3, and 4 mm). The total drying times were approximately 40, 50, and 60 min, respectively. The nearly linear increase in drying time with slice thickness indicates that geometric factors, particularly the internal moisture diffusion path length, play a dominant role in governing the overall drying duration. Thinner slices provide a shorter migration distance for moisture transport from the interior to the surface, thereby reducing internal mass transfer resistance. In addition, the larger surface-area-to-volume ratio of thinner slices facilitates more efficient vapor release into the surrounding low-pressure environment, which is especially beneficial under microwave vacuum conditions. These combined effects result in faster moisture removal and shorter drying times. Similar thickness-dependent drying behavior has been reported for Persimmon slices under microwave-assisted drying conditions [25].

Figure 6: The moisture ratio changes of ginger slices under different ginger thicknesses.

The corresponding drying rate curves for different slice thicknesses are shown in Fig. 7. Thicker slices exhibited a lower maximum drying rate and a more prolonged deceleration stage compared with thinner samples. This behavior can be attributed to the increased internal diffusion resistance associated with longer moisture transport pathways and a reduced surface-area-to-volume ratio. Moreover, in thicker slices, the central regions tend to experience slower temperature rise during the early stage of drying, resulting in lower internal vapor generation rates and a weaker vapor-driven mass transfer driving force. These results highlight that slice thickness primarily influences MVD performance through geometric constraints on moisture transport, rather than by enhancing intrinsic mass transfer properties. Therefore, optimizing slice thickness is essential to suppress diffusion-limited behavior and to maximize drying efficiency in microwave vacuum drying of ginger.

Figure 7: The drying rate changes of ginger slices under different ginger thicknesses.

3.2 Comparison of Three Drying Methods

3.2.1 Comparison between MVD and HAD

Figs. 8 and 9 compare the drying behavior of ginger slices under hot air drying (HAD) at 50, 60, and 70°C. As shown in Fig. 8, increasing the drying temperature significantly reduced the total drying time, which decreased from approximately 7 h at 50°C to 5 h at 60°C and 3 h at 70°C. This trend reflects the enhanced convective heat transfer and increased vapor pressure gradient at elevated air temperatures. Despite this temperature-dependent acceleration, HAD remained substantially slower than MVD. Under comparable moisture endpoints, the total drying time required for MVD was approximately one-sixth of that required for HAD at 70°C. This pronounced difference arises from the fundamentally different heat and mass transfer mechanisms governing the two processes. In HAD, heat is supplied externally and must be transferred from the hot air to the product surface and subsequently conducted into the interior. As drying progresses, the formation of a dried outer layer and the depletion of surface free water increase internal mass transfer resistance, causing a progressive decline in drying rate. A significant portion of the supplied thermal energy is therefore consumed in heating the solid matrix rather than directly driving moisture evaporation, particularly during the later stages of drying. By contrast, MVD combines volumetric microwave heating with a low-pressure environment, allowing energy to be deposited directly within the moisture-containing regions of the material. This internal energy generation promotes rapid vapor formation throughout the sample volume, generating a strong internal vapor pressure gradient that accelerates moisture transport toward the surface. Consequently, MVD effectively mitigates thermal lag and surface hardening phenomena commonly observed in HAD.

Figure 8: The moisture ratio changes of ginger slices under different hot air temperatures.

Figure 9: The drying rate changes of ginger slices under different hot air temperatures.

The drying rate curves shown in Fig. 9 further highlight these differences. HAD exhibited an accelerating stage followed by a prolonged deceleration stage, with no distinct constant-rate period. This behavior has been widely reported for agricultural products dried by hot air, including Daqu [26] and Red Onion Slices [27], and reflects the dominance of internal diffusion resistance once surface moisture is rapidly depleted. Overall, the comparison demonstrates that the superior performance of MVD over HAD is primarily attributable to the shift from externally limited heat transfer to internally driven vapor transport.

3.2.2 Comparison between MVD and MD

Fig. 10 presents the moisture ratio curves of ginger slices dried by conventional microwave drying (MD) at power levels of 200, 350, and 500 W. The total drying times required to reach the target moisture content were approximately 70, 65, and 55 min, respectively. Under the same microwave power settings, MVD consistently achieved shorter drying times, with an average reduction of approximately 25% compared to MD. This improvement highlights the synergistic effect of combining microwave heating with vacuum conditions. While both MD and MVD rely on volumetric microwave energy absorption, the presence of vacuum in MVD lowers the boiling point of water and reduces the external vapor pressure. As a result, moisture vaporization can occur at lower internal temperatures, enhancing vapor-driven mass transfer and reducing thermal stress on the product.

Figure 10: The moisture ratio changes of ginger slices under different microwave powers of MD.

The drying rate profiles of MD and MVD differ markedly, as shown in Fig. 11. Under atmospheric conditions, MD exhibited a characteristic multi-stage drying behavior, consisting of an initial rapid increase in drying rate, a pronounced constant-rate period, and a final falling-rate stage. The existence of a constant-rate period is associated with the high initial free-water content of ginger slices, during which absorbed microwave energy sustains a relatively steady evaporation rate at the surface. Similar drying kinetics have been reported for other high-moisture agricultural materials, such as lychee [28]and Castanea henryi [29]. In contrast, MVD drying rates displayed a sharp initial peak followed by continuous deceleration, without a prolonged constant-rate stage. The reduced ambient pressure and volumetric microwave heating jointly intensify internal vapor generation from the onset of drying, leading to rapid moisture expulsion and an earlier transition to a diffusion-controlled regime. This direct transition reflects the dominance of internal vapor pressure–driven transport over surface evaporation control under vacuum conditions.

Figure 11: The drying rate changes of ginger slices under different microwave powers of MD.

A direct comparison of MD and MVD at 350 W (Fig. 12) further emphasizes three key distinctions: (i) a steeper decline in moisture ratio under MVD, indicating faster moisture removal; (ii) the absence of the extended constant-rate plateau observed in MD; and (iii) a higher initial peak drying rate for MVD, reflecting enhanced mass transfer under reduced pressure. Collectively, these differences explain the shorter drying time achieved by MVD and suggest its potential advantages for reducing overall thermal exposure during ginger drying.

Figure 12: Comparison of MD and MVD at 350 W: (a) Moisture ratio; (b) Drying rate.

3.2.3 Energy Consumption and Efficiency Analysis

Energy consumption is a critical factor in evaluating the economic feasibility and environmental sustainability of drying technologies. In this study, the specific energy consumption (SEC) of HAD, MD, and MVD was calculated based on Eq. (5) for identical ginger slice samples dried to the same target moisture content. The detailed results are summarized in Table 4. From a drying kinetics perspective, MVD exhibited a clear advantage in terms of processing time. For example, under comparable drying endpoints, MVD (350 W, 0.08 MPa, 3 mm) required only 22.2% of the drying time needed for HAD at 70°C and approximately 61.5% of that required for MD at 350 W. This substantial reduction reflects the intensified internal heat and mass transfer achieved by combining volumetric microwave heating with vacuum conditions. However, the energy consumption analysis reveals a different trend. As shown in Table 4, MVD generally exhibited higher SEC values than MD under most investigated conditions, primarily due to the system-level energy demand of MVD, which includes not only the microwave power input but also the continuous operation of the vacuum pump and associated auxiliary components. HAD, although requiring the longest drying time, exhibited moderate SEC values, particularly at higher temperatures (e.g., 17.68 kWh/kg at 70°C), which are comparable to or slightly higher than MD but still lower than high-power MVD. This can be attributed to its simple system configuration and the absence of energy-intensive auxiliary equipment.

These results highlight an inherent trade-off between drying speed and energy efficiency among the investigated drying methods. MVD is particularly suitable for applications where rapid processing is critical and where the higher energy cost can be justified by product quality requirements or high added value. Conversely, when minimizing overall energy consumption is the primary objective, atmospheric MD or high-temperature HAD may represent more economical alternatives. The quantitative SEC data provided in this study offer a practical basis for selecting appropriate drying technologies for ginger slices under different production priorities.

3.3 Model Fitting

To quantitatively describe the drying kinetics of ginger slices under MVD, five commonly used thin-layer drying models—Lewis/Newton, Henderson and Pabis, Page, Logarithmic, and Two-term models—were fitted to the experimental moisture ratio data. Model fitting was performed using MATLAB 2023a based on the nonlinear least-squares method employing the Gauss–Newton algorithm. The goodness of fit of each model was evaluated using multiple statistical criteria, including the coefficient of determination (R2), sum of squared errors (SSE), root mean square error (RMSE), and chi-square value (χ2). A superior model was identified by a higher R² combined with lower SSE, RMSE, and χ2 values, indicating closer agreement between experimental and predicted moisture ratios. The use of multiple complementary criteria helps avoid bias associated with relying on a single statistical indicator and provides a more robust basis for model comparison.

The fitting results under different microwave power levels, vacuum degrees, and slice thicknesses are summarized in Tables 5–7, respectively. Across all investigated operating conditions, the Page model consistently exhibited the highest R2 values (generally exceeding 0.997), along with the lowest SSE and RMSE values and acceptably small χ2 values. These results indicate that the Page model provides the most accurate overall description of the MVD drying kinetics of ginger slices among the tested models. From a modeling perspective, the superior performance of the Page model can be attributed to its two-parameter semi-empirical structure, which offers sufficient flexibility to capture the strongly nonlinear moisture removal behavior observed during MVD, particularly the rapid initial moisture loss followed by a pronounced falling-rate period. At the same time, the Page model avoids excessive complexity, which is advantageous for parameter stability and interpretability. In contrast, more complex models, such as the Two-term model, occasionally exhibited parameter instability or physically unrealistic parameter values under certain drying conditions, suggesting potential overparameterization without a corresponding improvement in predictive accuracy.

To further evaluate the predictive capability and generalizability of the selected models, independent validation experiments were conducted under two drying conditions that were not included in the model-fitting dataset: (i) a microwave power of 400 W, vacuum degree of 0.05 MPa, and slice thickness of 3 mm; and (ii) a microwave power of 350 W, vacuum degree of 0.07 MPa, and slice thickness of 3 mm. The predicted moisture ratios obtained from the fitted models were compared with the corresponding experimental data. As illustrated in Fig. 13, the Page model exhibited the closest agreement between predicted and experimental moisture ratios under both validation conditions, with minimal systematic deviation across the entire drying period. This strong correspondence demonstrates that the Page model not only provides an excellent fit to the experimental data used for parameter estimation but also maintains robust predictive performance when applied to previously untested operating conditions within the investigated parameter space. In addition, the residual analysis presented in Fig. 14 further confirms the adequacy of the Page model. The small-magnitude, randomly distributed residuals indicate the absence of systematic bias. Together, the close agreement between predicted and experimental moisture ratios and the random residual distribution verify the strong capability of the Page model to accurately describe the moisture change kinetics of ginger slices during MVD across a wide range of microwave powers and vacuum degrees.

Figure 13: Comparison of experimental and predicted values of five models: (a) a microwave power of 400 W, vacuum degree of 0.05 MPa, and slice thickness of 3 mm; (b) a microwave power of 350 W, vacuum degree of 0.07 MPa, and slice thickness of 3 mm.

Figure 14: Residual plots of the page model under different microwave powers and vacuum degrees.

3.4 Effective Diffusivities of Ginger Slices

The effective moisture diffusivity (Deff) is a widely used parameter for characterizing internal moisture transport during drying and for comparing drying intensification under different operating conditions. In this study, Deff was estimated based on Fick’s second law using the analytical solution for an infinite flat plate geometry (Eq. (10)). The values of Deff were determined from the slope of the linear relationship between ln(MR) and drying time, and the results are summarized in Table 8. Under all investigated MVD conditions, the Deff ranged from 6.12 × 10−10 to 1.68 × 10−9 m2/s, which lies within the typical range reported for agricultural products during drying processes. This agreement with literature values supports the applicability and reliability of the adopted estimation method. The results indicate that microwave power, vacuum degree, and slice thickness significantly influence the effective moisture diffusivity, reflecting changes in the overall internal mass transfer behavior during MVD of ginger slices.

With increasing microwave power from 200 to 500 W, the Deff value increased from 6.12 × 10−10 to 1.68 × 10−9 m2/s. This increase reflects the intensification of volumetric microwave heating, which enhances internal energy absorption by polar water molecules, accelerates vapor generation, and promotes internal moisture transport. Consequently, the higher Deff quantitatively corresponds to the substantial reduction in drying time observed in Figs. 2 and 3, indicating strengthened internal mass transfer under higher microwave power levels.

Similarly, increasing the vacuum degree from 0.02 to 0.08 MPa resulted in a notable rise in the Deff value, from 5.86 × 10−10 to 1.42 × 10−9 m2/s. Under reduced pressure, the boiling point of water decreases, facilitating phase change at lower temperatures and enabling internal vapor pressure to exceed the surrounding pressure more readily. The resulting enhancement in vapor pressure gradients and reduction in external mass transfer resistance jointly promote moisture migration. The increase in Deff therefore reflects the combined contribution of vacuum-assisted evaporation and diffusion enhancement, consistent with the accelerated drying behavior shown in Figs. 4 and 5.

In contrast, slice thickness exhibited an inverse relationship with the Deff value. When thickness increased from 2 to 4 mm, the Deff decreased from 1.32 × 10−9 to 8.74 × 10−10 m2/s. This trend does not indicate a change in intrinsic diffusivity, but rather reflects the effect of increased diffusion path length embedded in the Fickian formulation used for Deff estimation. Thicker slices impose longer internal moisture migration distances and higher internal resistance, which are manifested as lower diffusivity values. This behavior provides a mechanistic explanation for the prolonged drying times observed for thicker ginger slices in Figs. 6 and 7.

Overall, the Deff values obtained in this study effectively capture the influence of key process parameters on internal moisture transport during MVD and provide a quantitative basis for comparing drying intensification under different operating conditions.

3.5 Optimization of Response Surface Methodology

3.5.1 Experimental Design

MVD is a multivariable and highly nonlinear process influenced by microwave power, vacuum degree, and slice thickness. Response Surface Methodology (RSM) was therefore employed to quantitatively model the combined effects of these factors and to optimize drying performance with a limited number of experiments. It should be noted that the second-order polynomial model adopted in RSM represents a local approximation of the response surface within the investigated experimental domain, and its applicability is therefore confined to the defined factor ranges. Based on single-factor results, a three-factor, three-level Box–Behnken design was constructed using Design-Expert 13, with drying time, color difference, rehydration rate and specific energy consumption as the responses. The experimental matrix and corresponding responses are listed in Table 9.

3.5.2 Regression Equation

Quadratic polynomial regression models were developed for four response variables—drying time (Y1), total color difference (Y2), rehydration ratio (Y3), and specific energy consumption (Y4)—as functions of microwave power (A), vacuum degree (B), and slice thickness (C). The corresponding ANOVA results are summarized in Table 10, and the regression equations are presented below.

Y1=50.8−24.25A−19.5B+9.25C+2.75AB−3.25AC−3.02A2(13)

Y2=9.08+2.34A−1.4B+1.99C+1.05A2+0.4225B2(14)

Y3=260.14−18.74A−8.8B−4.26C+2.82AB−3.6A2(15)

Y4=97.86−9.82A−22.2B+11.53C+7.53AB−4.28AC−9.04A2−9.74B2−6.74C2(16)

All four models were statistically significant (p < 0.0001), indicating that the selected model terms adequately describe the primary effects and interactions within the experimental design space. Importantly, the lack-of-fit tests for all responses were non-significant (p > 0.05), demonstrating that the models provide an acceptable empirical approximation of the experimental data within the investigated parameter ranges. The coefficients of determination (R2) ranged from 0.9914 to 0.9968, reflecting strong agreement between predicted and experimental values. In addition, Adequate Precision values for all models exceeded the recommended threshold of 4, supporting their ability to navigate the design space and to identify meaningful response trends.

Analysis of variance further clarified the relative influence of the process parameters. Microwave power (A) was identified as the most influential factor, exerting a highly significant effect (p < 0.01) on all responses, particularly drying time, color difference, and rehydration ratio. Vacuum degree (B) also exhibited a significant impact, especially on specific energy consumption. Slice thickness (C) significantly affected drying time and energy consumption, mainly by governing the effective moisture diffusion path length. Significant interaction effects were observed, most notably between microwave power and vacuum degree (AB) and between microwave power and slice thickness (AC), highlighting the coupled and nonlinear nature of heat and mass transfer during MVD.

Overall, the regression analysis established statistically adequate second-order polynomial models for all four responses. Within the investigated experimental domain, these models provide a sound empirical basis for response surface analysis and multi-objective optimization, while acknowledging the inherent limitations of polynomial approximations for highly nonlinear drying processes.

3.5.3 Response Surface Analysis and Optimization

Based on the developed quadratic regression models, three-dimensional response surface and contour plots were generated to visualize the interactive effects of process parameters on drying time (Figs. 15–17). These graphical representations provide an intuitive means to interpret the combined influence of microwave power, vacuum degree, and slice thickness within the investigated experimental domain.

Figure 15: Contour map (a) and response surface plot (b) of the interaction between microwave power and vacuum degree.

Figure 16: Contour plot (a) and response surface plot (b) of the interaction between microwave power and ginger slice thickness.

Figure 17: Contour plot (a) and response surface plot (b) of the interaction between vacuum degree and ginger slice thickness.

Fig. 15 illustrates the interaction between microwave power (A) and vacuum degree (B) at a fixed slice thickness of 3 mm. Drying time decreases markedly with increasing microwave power and vacuum degree, indicating that both parameters contribute to drying intensification. The curvature of the response surface and the elliptical contour lines are consistent with a synergistic interaction between these two factors, reflecting the combined enhancement of internal heat generation and vapor pressure driving force under higher power and stronger vacuum conditions.

Fig. 16 presents the interaction between microwave power (A) and slice thickness (C) at a vacuum degree of 0.05 MPa. Increasing microwave power significantly reduces drying time, whereas increasing slice thickness prolongs the process. The pronounced surface curvature suggests that slice thickness modulates the effectiveness of microwave heating by altering the internal moisture diffusion path length, which is characteristic of diffusion-controlled drying behavior.

Fig. 17 shows the interaction between vacuum degree (B) and slice thickness (C) at a microwave power of 350 W. Drying time decreases with increasing vacuum degree, whereas thicker slices require longer drying times. Compared with the other interaction plots, the response surface is relatively smooth, indicating a weaker interaction between these two factors within the studied range.

Based on the established regression models, a multi-objective numerical optimization was conducted using the desirability function approach, with the aims of minimizing drying time, total color difference, and specific energy consumption, while maximizing the rehydration rate. The predicted optimal conditions are summarized in Table 11. For practical implementation, the predicted values were adjusted to the nearest feasible operating levels: a microwave power of 200 W, a vacuum degree of 0.08 MPa, and a slice thickness of 2 mm. Validation experiments performed under these conditions yielded a drying time of (40 ± 5) min, a color difference of (5.0 ± 0.4), a rehydration rate of (265.8 ± 5.5)%, and a specific energy consumption of (38.2 ± 1.5) kWh/kg. The close agreement between predicted and experimental values demonstrates that the developed RSM models provide a reliable basis for identifying favorable operating conditions within the studied parameter space.

3.6 Microstructure Analysis

Scanning electron microscopy (SEM) was employed to investigate the microstructural evolution of ginger tissue under different MVD conditions and to establish links between tissue morphology, drying kinetics, and quality attributes. The observed changes in cellular structure are primarily associated with the development of thermal and hygroscopic stresses during dehydration, as reported for other plant-based materials [30,31]. Representative samples—including fresh ginger, as well as samples dried under the slowest (200 W, 0.02 MPa, 3 mm) and fastest (500 W, 0.08 MPa, 3 mm) MVD conditions—were selected for comparison (Figs. 18–20).

Figure 18: SEM micrographs of fresh ginger slices (200× and 300×).

Figure 19: SEM micrographs of ginger slices under the slowest drying conditions (200× and 300×).

Figure 20: SEM micrographs of ginger slices under the fastest drying conditions (200× and 300×).

Fig. 18 shows the native microstructure of fresh ginger tissue. The parenchyma cells are intact, turgid, and closely packed, with smooth cell walls and minimal intercellular voids. This dense and coherent cellular arrangement corresponds to the high initial moisture content of fresh ginger and reflects its strong water-holding capacity. The microstructure of ginger slices dried under the slowest MVD conditions is presented in Fig. 19. The tissue exhibits pronounced and relatively uniform shrinkage, with cells collapsing into a compact and folded structure accompanied by extensively wrinkled cell walls. This morphology is characteristic of a drying regime dominated by gradual moisture removal through capillary flow and diffusion, where the cellular framework progressively contracts as water is removed. Despite the severe shrinkage, the overall cellular connectivity is largely preserved, which provides a plausible structural explanation for the relatively high rehydration ratio observed under these conditions. However, the prolonged exposure to moderate thermal input during slow drying is consistent with the more pronounced color degradation measured in the quality analysis. In contrast, Fig. 20 illustrates the microstructure formed under the most intensive MVD conditions. The tissue displays a highly porous and expanded morphology, with extensive cell wall rupture, irregular cavities, and a fragmented internal network. This structure indicates rapid internal vapor generation, where moisture vaporization occurs faster than outward diffusion, leading to the buildup of internal pressure and subsequent mechanical disruption of cell walls. The resulting porous network substantially reduces internal mass transfer resistance, accounting for the significantly shortened drying time. However, the loss of structural integrity and reduction in effective water-binding sites are detrimental to rehydration performance and are accompanied by more severe color changes, likely due to localized thermal effects.

Overall, the SEM observations demonstrate that drying intensity strongly governs the resulting tissue architecture: slower drying promotes uniform shrinkage with preserved cellular connectivity, whereas faster drying induces pore formation and structural rupture. These microstructural differences provide a physical basis for the observed trade-offs among drying rate, rehydration capacity, and color quality, thereby linking MVD process parameters to final product attributes.

4  Conclusions

This study systematically investigated the microwave vacuum drying (MVD) of ginger slices, with particular emphasis on drying kinetics, mass transfer behavior, product quality, and process optimization. Compared with conventional hot air drying (HAD) and atmospheric microwave drying (MD), MVD significantly reduced the drying time, achieving the target moisture content in approximately one-sixth and one-quarter of the time required by HAD and MD, respectively. This improvement is attributed to the synergistic combination of volumetric microwave heating and vacuum-induced boiling point reduction, which enhances internal moisture vaporization and accelerates mass transfer.

Among the investigated parameters, microwave power was identified as the dominant factor influencing drying performance, followed by vacuum degree, while slice thickness mainly affected the internal diffusion path length and drying duration. The drying kinetics of ginger slices under MVD were accurately described by the Page model (R2 > 0.997), demonstrating its suitability for predicting moisture removal behavior within the studied operating range.

Based on response surface methodology, a multi-objective optimization strategy was established, yielding favorable operating conditions of 200 W microwave power, 0.08 MPa vacuum degree, and 2 mm slice thickness. Experimental validation showed good agreement with model predictions, resulting in a drying time of approximately 40 min while maintaining acceptable color retention (ΔE = 5.0) and a high rehydration rate (265.8%).

Microstructural observations revealed that drying intensity governs tissue architecture, with slower drying producing uniformly shrunken but connected cellular structures, and faster drying inducing porous, ruptured networks. These structural differences provide a physical explanation for the observed trade-offs between drying rate, rehydration capacity, and color quality.

Overall, the results indicate that MVD is an efficient and controllable drying approach for ginger slices, supported by reliable kinetic modeling and statistically validated response surface–based optimization within the investigated parameter space. While further studies on nutrient retention, aroma preservation, energy efficiency, and pilot-scale performance are required, the present work provides a solid experimental and modeling basis for the rational design and optimization of MVD processes for ginger and other high-value agricultural products.

Acknowledgement: The authors are grateful to the financial support for the work by the Regional Joint Funds of Natural Science Foundation of Hunan Province.

Funding Statement: This research was funded by the Regional Joint Funds of the Natural Science Foundation of Hunan Province, grant number 2022JJ50041 (received by Guohai Jia), URL: https://kjt.hunan.gov.cn/kjt/zxgz/zkjj/xmgljcx/202206/t20220609_25442185.html.

Author Contributions: The author confirms the following contributions to the paper: Study conception and design: Guohai Jia; Data collection: Yongjia Ma, Yuanyuan Li; Result analysis and interpretation: Yongjia Ma, Yuling Cheng, Dan Huang; Manuscript preparation: Guohai Jia, Yongjia Ma, Dan Huang. All authors reviewed and approved the final version of the manuscript.

Availability of Data and Materials: The data supporting the results of this study can be obtained from the corresponding author Dan Huang upon reasonable request.

Ethics Approval: Not applicable.

Conflicts of Interest: The authors declare no conflicts of interest.

Nomenclature

References

1. Spence C. Ginger: the pungent spice. Int J Gastron Food Sci. 2023;33(1):100793. doi:10.1016/j.ijgfs.2023.100793. [Google Scholar] [CrossRef]

2. Zhang M, Zhao R, Wang D, Wang L, Zhang Q, Wei S, et al. Ginger (Zingiber officinale Rosc.) and its bioactive components are potential resources for health beneficial agents. Phytother Res. 2021;35(2):711–42. doi:10.1002/ptr.6858. [Google Scholar] [PubMed] [CrossRef]

3. Dalsasso RR, Valencia GA, Monteiro AR. Impact of drying and extractions processes on the recovery of gingerols and shogaols, the main bioactive compounds of ginger. Food Res Int. 2022;154(11):111043. doi:10.1016/j.foodres.2022.111043. [Google Scholar] [PubMed] [CrossRef]

4. Yang X, Wei S, Lu X, Qiao X, Simal-Gandara J, Capanoglu E, et al. A neutral polysaccharide with a triple helix structure from ginger: characterization and immunomodulatory activity. Food Chem. 2021;350(10):129261. doi:10.1016/j.foodchem.2021.129261. [Google Scholar] [PubMed] [CrossRef]

5. Liu Y, Liu J, Zhang Y. Research progress on chemical constituents of Zingiber officinale roscoe. Biomed Res Int. 2019;2019(6):5370823. doi:10.1155/2019/5370823. [Google Scholar] [PubMed] [CrossRef]

6. Chen GT, Yuan B, Wang HX, Qi GH, Cheng SJ. Characterization and antioxidant activity of polysaccharides obtained from ginger pomace using two different extraction processes. Int J Biol Macromol. 2019;139(2):801–9. doi:10.1016/j.ijbiomac.2019.08.048. [Google Scholar] [PubMed] [CrossRef]

7. Khan MKI, Maan AA, Aadil RM, Nazir A, Butt MS, Rashid MI, et al. Modelling and kinetic study of microwave assisted drying of ginger and onion with simultaneous extraction of bioactive compounds. Food Sci Biotechnol. 2020;29(4):513–9. doi:10.1007/s10068-019-00695-5. [Google Scholar] [PubMed] [CrossRef]

8. Kanyal B, Pande C, Tewari G, Aabha, Rana L, Padalia RC, et al. Influence of post-harvest drying processes on the composition and biological activities of essential oils from leaves of camphor tree from Uttarakhand Himalaya. India J Essent Oil Bear Plants. 2023;26(1):161–75. doi:10.1080/0972060X.2023.2179426. [Google Scholar] [CrossRef]

9. Pantoja Espinosa DC, Rodríguez Cortina J, Hernández Carrión M, Osorio Mora O. Drying and cooking effects on the final quality of pea grains (Pisum sativum L.) varieties. Food Sci Technol. 2022;42(6):e32120. doi:10.1590/fst.32120. [Google Scholar] [CrossRef]

10. Zhang Q, Wang B, Zhang H, Lin R, Yang L, Lv W, et al. Effects of microwave-hot air combined drying on the moisture content, physical properties, flavor, and volatile components of prunes (Prunus domestica L.). Dry Technol. 2025;43(6):993–1005. doi:10.1080/07373937.2025.2479871. [Google Scholar] [CrossRef]

11. Bourdoux S, Li D, Rajkovic A, Devlieghere F, Uyttendaele M. Performance of drying technologies to ensure microbial safety of dried fruits and vegetables. Comp Rev Food Sci Food Safe. 2016;15(6):1056–66. doi:10.1111/1541-4337.12224. [Google Scholar] [PubMed] [CrossRef]

12. Hao WG, Wang XY, Wei LX, Wang L, Wang H. Research progress and development direction of theoretical simulation model of solar drying technology. J Chin Agric Mech. 2025;46(12):274–81. (In Chinese). doi:10.13733/j.jcam.issn.2095-5553.2025.12.03. [Google Scholar] [CrossRef]

13. Zhang YT, Wang WL, Yang ZY, Gong ZQ, Zhang J, Cui WJ, et al. Application of new energy-saving drying technology in fruit and vegetable processing and analysis of research hotspots. Food Ferment Ind. 2025;51(24):387–96. [Google Scholar]

14. Gonzalez-Gonzalez M, Yerena-Prieto BJ, Carrera C, Vázquez-Espinosa M, González-de-Peredo AV, García-Alvarado MÁ, et al. Determination of gingerols and shogaols content from ginger (Zingiber officinale Rosc.) through microwave-assisted extraction. Agronomy. 2023;13(9):2288. doi:10.3390/agronomy13092288. [Google Scholar] [CrossRef]

15. Guo F, Yang C, Zang C, Shang Y, Zhang B, Yu H, et al. Comparison of the quality of Chinese ginger juice powders prepared by different drying methods. J Food Process Eng. 2019;42(7):e13252. doi:10.1111/jfpe.13252. [Google Scholar] [CrossRef]

16. Lin X, Xu JL, Sun DW. Evaluating drying feature differences between ginger slices and splits during microwave-vacuum drying by hyperspectral imaging technique. Food Chem. 2020;332(1):127407. doi:10.1016/j.foodchem.2020.127407. [Google Scholar] [PubMed] [CrossRef]

17. Gomide AI, Monteiro RL, Laurindo JB. Impact of the power density on the physical properties, starch structure, and acceptability of oil-free potato chips dehydrated by microwave vacuum drying. LWT. 2022;155(7):112917. doi:10.1016/j.lwt.2021.112917. [Google Scholar] [CrossRef]

18. Carvalho GR, Monteiro RL, Laurindo JB, Augusto PED. Microwave and microwave-vacuum drying as alternatives to convective drying in barley malt processing. Innov Food Sci Emerg Technol. 2021;73(2):102770. doi:10.1016/j.ifset.2021.102770. [Google Scholar] [CrossRef]

19. Akamine LA, Vargas Medina DA, Lanças FM. Magnetic solid-phase extraction of gingerols in ginger containing products. Talanta. 2021;222:121683. doi:10.1016/j.talanta.2020.121683. [Google Scholar] [PubMed] [CrossRef]

20. Zhong L, Wang R, Wen QH, Li J, Lin JW, Zeng X, et al. The interaction between bovine serum albumin and [4]-, [6]- and [8]-gingerol: an effective strategy to improve the solubility and stability of gingerol. Food Chem. 2022;372:131280. [Google Scholar] [PubMed]

21. Li M, Chen Y, Wang X, Cheng S, Liu F, Huang L. Determination of drying kinetics and quality changes of Panax quinquefolium L. dried in hot-blast air. LWT. 2019;116:108563. doi:10.1016/j.lwt.2019.108563. [Google Scholar] [CrossRef]

22. Sun Y, Zhang M, Bhandari B, Yang P. Intelligent detection of flavor changes in ginger during microwave vacuum drying based on LF-NMR. Food Res Int. 2019;119(1):417–25. doi:10.1016/j.foodres.2019.02.019. [Google Scholar] [PubMed] [CrossRef]

23. Puttalingappa YJ, Natarajan V, Varghese T, Naik M. Effect of microwave-assisted vacuum drying on the drying kinetics and quality parameters of Moringa oleifera leaves. J Food Process Eng. 2022;45(8):e14054. doi:10.1111/jfpe.14054. [Google Scholar] [CrossRef]

24. Alvi T, Khan MKI, Maan AA, Rizwan M, Aamir M, Saeed F, et al. Microwave-vacuum extraction cum drying of tomato slices: optimization and functional characterization. Food Sci Nutr. 2023;11(7):4263–74. doi:10.1002/fsn3.3352. [Google Scholar] [PubMed] [CrossRef]

25. Wei Z, Duan Z, Tang X, Qin Y, Zhou S, Duan W, et al. Effects of microwave drying on nutrient component and antioxidant activity of persimmon slices. J Food Meas Charact. 2022;16(2):1744–53. doi:10.1007/s11694-021-01273-2. [Google Scholar] [CrossRef]

26. Xia Y, Luo HB, Zhou P. Study on hot-air drying characteristics and dynamic model of Daqu. Modern Food Sci Technol. 2018;34(4):206–14. doi:10.13982/j.mfst.1673-9078.2018.04.031. [Google Scholar] [CrossRef]

27. Hussein JB, My S, Abiona OO, Mo O. Drying characteristics of osmotically pretreated red onion slices via hot air oven. J Food Process Technol. 2018;9(5):1–8. doi:10.4172/2157-7110.1000733. [Google Scholar] [CrossRef]

28. Kriaa K, Nassar AF. Comparative study of pretreatment on microwave drying of Gala apples (Malus pumilaeffect of blanching, electric field and freezing. LWT. 2022;165(2):113693. doi:10.1016/j.lwt.2022.113693. [Google Scholar] [CrossRef]

29. Sunanda, Kumar S, Ramya HG, Alam MS, Gautam RB. Convective-cum-microwave drying characteristics of ginger (Zingiber officinale). Int J Bio Resour Stress Manag. 2017;8(1):153–9. doi:10.23910/ijbsm/2017.8.1.1704c. [Google Scholar] [CrossRef]

30. Jiang D, Xiao H, Zheng Z. Effects of different drying methods on drying characteristics, microstructure, quality, and energy consumption of Panax Notoginseng roots (Araliaceae). Dry Technol. 2022;40(6):1247–61. doi:10.1080/07373937.2020.1863978. [Google Scholar] [CrossRef]

31. Zeng S, Wang B, Zhao D, Lv W. Microwave infrared vibrating bed drying of ginger: DRYING qualities, microstructure and browning mechanism. Food Chem. 2023;424(6):136340. doi:10.1016/j.foodchem.2023.136340. [Google Scholar] [PubMed] [CrossRef]