1. Introduction
American ginseng (
Panax quinquefolium L.) is a perennial herb in the Ginseng genus of the Araliaceae family, which is renowned for its health-promoting functions, such as blood nourishment, fluid production, physique strengthening, mind calming, and cognitive enhancement [
1]. Being a very popular medicinal and edible herb, it has an important economic value [
2], resulting in its use in health products in 35 countries globally. Ginsenoside is its primary active component, which is known to enhance immunity [
3] and exhibit anticancer, anti-aging, and anti-fatigue activities [
4], key points in its popularity [
4]. Fresh American ginseng is high in moisture content (more than 70%, wet basis (w.b.)), which favors the growth of microorganisms. Furthermore, the high moisture content can easily enhance enzymatic and non-enzymatic reactions, resulting in its rapid deterioration and thus reducing its medicinal and commercial value [
5,
6,
7]. Drying is a common preservation technology for maintaining the quality of agri-food products. Drying American ginseng can extend its shelf-life by reducing the moisture content to a low level (usually 10% w.b.), which prevents the growth of microorganisms and minimizes the costs of packaging, transportation, and storage [
6,
8]. Additionally, drying also affects the quality of American ginseng products, including color change, degradation of active ingredients, and rehydration ratio, which ultimately reduces their medicinal and commercial value [
5,
9].
Lu et al. [
10] investigated the impact of the drying durations of the drying stages and the slice thickness of American ginseng during a vacuum freeze-drying process using a univariate experimental approach. They used ginsenoside Rb1 content as an indicator to optimize the process parameters. Vacuum freeze-drying was found to produce high-quality products, but it is characterized by a long drying time process, low efficiency, and high operational costs [
8].
Hot-air drying equipment has a wide range of adaptations, and a simple and easy operation, making it widely used to dry American ginseng [
7]. However, its high drying temperature in the presence of oxygen has a great influence on the quality of the medicinal components. In the case of American ginseng, hot-air drying causes ginsenoside degradation, color deterioration, shrinkage, and case hardening (which impede internal moisture migration) [
6,
7]. Du et al. [
11] found that the color of American ginseng roots dried at 70 °C was dark, while the total ginsenoside content decreased with the increase in drying temperature.
Wang and **ement drying (a type of hot-air drying) under different drying temperatures (35, 40, 45, 50, 55, 60, and 65 °C), air velocities (3, 6, 9, and 12 m/s), and sample thicknesses (1, 2, 3, and 4 mm) to analyze color, ginsenoside content, rehydration ratio, and microstructure. Their results indicated that the drying time was substantially shortened by increasing drying temperature and that both drying temperature and sample thickness had significant effects on the change in color, whereas air velocity did not have any significant effect. It was also observed that ginsenosides decreased with increasing drying temperature. Taking into account the drying time and dried sample quality, it was proposed that a drying temperature at 45 °C with a sample thickness of 2 mm should be used for thin-layer air im**ement drying of American ginseng slices.
Although the above study effectively reduced the processing temperature, it was still in an aerobic heating environment throughout the process, which would still affect active ingredients like ginsenosides. Zhou et al. [
13] found that the content of Rg1 (the main component of ginsenoside) in vacuum-dried samples was 17.1% higher than that in the hot-air-dried samples and suggested that this was due to the fact that vacuum drying avoids biochemical reactions, such as the oxidation of ginsenoside Rg1 during the drying process.
Kim et al. [
14] compared the balanced low-pressure vacuum drying and hot-air drying of ginseng and found that a longer process time and a higher drying temperature of hot-air drying significantly reduced volatile bioactive phenolic compounds, which resulted in lowering its antioxidant activities. In contrast, balanced low-pressure vacuum drying improved the retention rate of bioactive components (such as acidic polysaccharides) to a level similar to that of freeze-drying. Furthermore, the liquid extracts from ginseng dried by balanced low-pressure vacuum drying increased the growth of human B and T cells as well as the secretion of both IL-6 and TNF-α. However, balanced low-pressure vacuum drying required a longer process time than that of hot-air drying. So, a single drying technology has disadvantages that are difficult to overcome.
Hot-air and vacuum combined drying (HAVCD) is an emerging technology developed in recent years [
15]. It first uses hot-air drying to quickly remove a large amount of water in the material; then, it uses vacuum drying to create an anoxic environment to inhibit the occurrence of undesirable biochemical reactions, such as oxidative deterioration and browning reactions. Such a combination ultimately reduces the loss of active ingredients and improves the quality of dried products [
16]. HAVCD can achieve the complementary advantages of different drying technologies, such as good product quality, high drying efficiency, and low energy consumption [
15]. HAVCD has been successfully applied to dry a range of vegetables, such as lettuce [
15]; celery [
17]; as well as a number of Chinese herbal medicines, such as
Astragalus membranaceus [
18] and
Scutellaria baicalensis [
19]. However, to the best of our knowledge, there is no detailed report on the effects of HAVCD on American ginseng slices. Therefore, this study compared the effects of various drying methods, including HAVCD, on the properties of American ginseng slices.
Additionally, different drying technologies led to different microstructures of Chinese herbal medicines [
8], which in turn, might affect the dissolution of active ingredients. In many countries, American ginseng is often used as a precious herbal medicine [
2], which is soaked in water to dissolve its active ingredients for drinking. However, improper drying methods may affect the dissolution of ginsenosides like active ingredient, reducing its health or medicinal effects.
In order to select a high-quality and efficient drying method with optimized drying parameters, this study compared four drying methods, namely hot-air drying (HAD), vacuum drying (VD), HAVCD, and vacuum freeze-drying (VFD), based on various process parameters, including drying time, color, total ginsenoside content, total ginsenoside residual rate (which reflects the dissolution of ginsenosides), rehydration ratio, hardness, and microstructure. An improved multi-indicator test formula method was used to calculate and analyze comprehensive scores to select the optimal drying method. Then, the optimal drying method was analyzed through the response surface methodology (RSM), regression equations were established, the influence of factors on the experimental indicators was analyzed, and the processing parameters were optimized and verified. This study will provide a guidance for the application of new drying technologies to design a new processing equipment for drying American ginsengs with high quality, high efficiency, and low energy consumption.
2. Materials and Methods
2.1. Devices and Equipment
The main instruments used in this study included a BSA123S-CW one ten thousandth balance (Sartorius Scientific Instruments Co., Ltd., Bei**g, China); KQ-300VDE ultrasonic cleaner (Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China); DR6000 UV-visible spectrophotometer (HACH Corporation, Loveland, CO, USA); TA.XT Plus texture analyzer (Stable Micro Systems Ltd., Godalming, Surrey GU7 1YL, UK); multifunctional grinder (Bei**g Liren Technology Co., Ltd., Bei**g, China); YS3060-type grating spectrophotometer (Guangdong SUNSHI Technology Co., Guangdong, China); FA-C electronic balance (accuracy 0.01 g) (Shanghai Youke Instrumentation Co., Shanghai, China); and TF-HFD-1A vacuum freeze-dryer (Shanghai Tianfeng Industrial Co., Ltd., Shanghai, China); Vega3-SBH Scanning Electron Microscope (TESCAN, Brno, Czech Republic).
The self-developed laboratory-scale HAVCD equipment was from the laboratory of Agricultural Products Drying Processing Technology and Equipment of Shaanxi University of Science and Technology. The same equipment was used to carry out HAD, VD, and HAVCD, which ensured consistency of the experimental conditions among the proposed three drying methods.
A scheme of the HAVCD equipment is shown in
Figure 1. The equipment primarily consisted of a HAD system, a VD system, and a control system. Among them, the control system was designed based on the touch panel MT8071iE (WEINVIEW Co., Ltd., Shenzhen, China) and S7-200 PLC (Siemens (China) Co., Ltd., Bei**g, China).
During hot-air drying, the electric vacuum isolation butterfly valve was open. Hot air was accelerated by the main fan and heated by a hot-air electric heating tube. Subsequently, hot air was evenly distributed via airflow to a uniform distribution chamber before being jetted into the combined drying chamber for material drying. Finally, the hot air was recycled back through the return air chamber to the main fan return air area for continuous circulation. The automatic control system utilized temperature sensors (model HC2A-IC 102 with ±0.1 °C temperature resolution, Michell Instruments (Shanghai) Co., Ltd, Shanghai, China) installed in the return air chamber to detect current temperature values according to the current temperature based on the proportional–integral–derivative (PID) function module in PLC to accurately control the power of the hot-air electric heating tube and the temperature in the combined drying chamber (temperature control accuracy of ±1 °C).
The VD stage mainly relied on hot water from the hot water tank to heat the material. Before VD, the hot water temperature was detected by the temperature sensor (model Pt100,with temperature resolution ± 0.1 °C, Meicon Automation Technology Co., Ltd., Hangzhou, China) and fed back to the control system, which accurately controlled the power of the electric heating tube and the hot water temperature based on the set hot water temperature with the PID function module in PLC (the temperature control accuracy was ±1° C). The VD system and the HAD system shared a combined drying chamber. During VD, the atmospheric air was isolated by closing the electric vacuum isolation butterfly valve and the pressure in the combined drying chamber was reduced to a preset value (vacuum sensor model PT500-503H with accuracy ±0.5 kPa, Meicon Automation Technology Co., Ltd., Hangzhou, China) by a vacuum pump. Subsequently, hot water was circulated from the hot water tank to the hollow material rack by means of a circulating water pump, which heated the metal trays placed on the hollow material rack and further heated and dried the material.
In the process of HAVCD, the equipment first entered the hot-air drying stage to quickly remove a large amount of free water in the material, enhancing the drying efficiency and minimizing the energy consumption. After the material moisture content reached the conversion point, the material did not need to be translocated, while the system was switched to the VD stage, isolating oxygen to avoid oxidation reaction, which effectively ensured the product quality. HAVCD proceeded until the moisture content of the material was reduced to a safe moisture content (less than 10%).
The above-mentioned fans, vacuum pumps, heating tubes, sensors, and valves were all connected to the control system, which were controlled and powered by the control system.
2.2. Materials
American ginseng (aged four years) was used for the experiment, which was sourced from an American ginseng plantation area in Hanzhong, Shaanxi Province (China). To mitigate the impact of material variability on the experimental outcomes, specimens of uniform size were selected for analysis (approximately 130–150 mm in length and 20–25 mm in diameter). The average initial moisture content was 72.0% (wet basis), as measured by VD at 70 °C for 24 h (AOAC, 1990) [
6]. Prior to the experiment, the American ginseng was cleaned and drained, and the root head and lateral roots were excised and then cut into 2 mm segments [
6], which were then placed in a sealed plastic bag (
Figure 2). Mixing was performed to minimize the effect of individual differences in the ginseng roots on the experimental results. The samples were stored overnight (12 h) at 4 °C to equalize their moisture contents.
2.3. Experimental Procedure
2.3.1. Comparative Experiment
Approximately 200 g of American ginseng slices was obtained from each group. According to the results of **ao’s study [
6] on the optimal drying temperature of American ginseng slices, the drying temperature was chosen to be 45 °C; the air velocity for HAD was 3 m/s; and the vacuum level for VD was 10 kPa. HAVCD had the same HAD stage parameters and VD stage parameters as the aforementioned only-HAD or -VD parameters, respectively, with the moisture content at a conversion point of 30%. For the three drying methods mentioned above, the dehydrated samples were removed from the dryer to measure the moisture content; the drying was concluded once the moisture content fell below 10% [
6,
7,
12,
20].
VFD served as the control, which was acknowledged as the best drying method for dried product quality. For the VFD process, the material was frozen at −30 °C for 3 h, which was subjected to a primary drying process at −15 °C for 8 h and then to a secondary drying process at 30 °C for 5 h; a cold trap temperature of −45 °C and a vacuum of 1 Pa were used throughout the process [
10]. Since VFD requires a continuous high-vacuum environment, no samples were taken during drying to detect moisture content in this study.
2.3.2. Response Surface Experimental Design
Optimization methods are often used to obtain high-quality dried products. Response surface methodology (RSM) is a collection of mathematical and statistical technologies for empirical modeling, which is a powerful method of process optimization and product improvement [
15].
Following the Central Composite Design (CCD) guidelines, the variables examined were hot-air temperature (A), vacuum temperature (B), and moisture content at the conversion point (C). The evaluation indices included drying time (T), rehydration ratio (R), total ginsenoside content (G), color brightness (
L*), and comprehensive score (Y). According to the results of **ao’s study [
6] on the optimal drying temperature of American ginseng slices, the drying temperature was chosen to be 45 °C. Referring to the study of HAVCD by Yuan and Yang [
18,
21], the moisture content at the conversion point was chosen to be 40%. And, the results of the one-factor test were the same as those in the above study. On this basis, a three-factor and five-level RSM experiment was designed with hot-air and vacuum temperatures of 45 °C, and the moisture content at the conversion point of 40% as the central point to analyze the influence of each factor on the indexes and to optimize the process parameters of the HAVCD of American ginseng slices by using its comprehensive score (Y). Subsequently, a validation test was conducted; the design of the test factors and levels is detailed in
Table 1.
2.3.3. Data Processing and Statistical Methods
Excel (Version 2019, Microsoft Redmond, WA, USA), Origin (Version 2022, OriginLab, Northampton, MA, USA), SPSS (Version 23.0, SPSS Inc., Chicago, IL, USA), and Design Expert (Version 8.0.6, Stat-Ease Minneapolis, MN, USA) software were used to statistically analyze the data, to make graphs and tables, to build models, and to test them. The indicators were normalized to a comprehensive score according to reference [
22].
2.4. Drying Characteristic Analysis
2.4.1. Measurement of Moisture Content
Samples were weighted using an electronic balance (with an accuracy of 0.01 g) during drying. Drying was continued until the samples reached the desired final moisture content of 10.0% (w.b.). The product was cooled and heat-sealed in polyethylene (LDPE) bags. The experiments were performed in triplicate, and the means of the triplicate were used for drawing drying curves [
6,
7].
2.4.2. Dry Basis Moisture Content (Mt)
The dry basis moisture content of the samples was calculated from the wet basis moisture content (
Ms) using the following formula:
2.4.3. Drying Rate
The drying rate (
DR) was calculated according to Equation (2) [
23].
where
Mt+Δt is the dry basis moisture content at moment
t + Δ
t and Δ
t is the drying time of two adjacent moments.
2.4.4. Rehydration Ratio
Rehydration was carried out by immersing an approximately 5 g dried sample (random sampling) in distilled water at a constant temperature of 90 °C for 60 min. Then, the samples were removed from water, drained, and weighed [
24]. Calculations were performed according to Equation (3) with three replications for each set of tests, and the results were averaged.
where
RR is rehydration ratio;
m1 is the mass (g) after rehydration; and
m2 is the mass (g) before rehydration.
2.5. Determination of Ginsenoside Content
A sample (random sampling, about 1 g) was taken in a 100 mL volumetric flask and sonicated for 30 min with a little bit of water. Water was then added until the total volume reached 100 mL. After centrifugation at 6000 rpm for 10 min, a 1 mL aliquot of the supernatant was used for column chromatography. For column chromatography, a 10 mL syringe served as the chromatographic column, containing 3 cm of Amberlite-XAD-2 macroporous resin with 1 cm of neutral alumina. The column was first washed with 25 mL of 70% ethanol and then with 25 mL of water; both eluates were discarded. After that, 1.0 mL of the sample solution was eluted first with 25 mL of water and the elute was discarded. Then, 25 mL of 70% ethanol was used to collect the elute in an evaporating dish, which was evaporated to dryness in a water bath at 60 °C; such a dried sample was subsequently used for the ginsenoside analysis. Vanillin in glacial acetic acid (0.2 mL, 5%) was added to the evaporating disc to dissolve the dried residues, and 0.8 mL of perchloric acid was mixed thoroughly, which was then transferred to a 5.0 mL stoppered centrifuge tube. After heating for 10 min at 60 °C and cooling, 5.0 mL of glacial acetic acid was added and vigorously shaken. A colorimetric analysis was conducted alongside the standard tube using a 1 cm quartz cuvette at a wavelength of 560 nm. A ginsenoside Re standard solution (2.0 mg/mL) was dispensed in 100 μL aliquots into an evaporating dish and dried using a water bath (below 60 °C) [
25]. The total ginsenoside content was calculated using Equation (4).
where
x is the total ginsenoside content of the sample (g/100 g);
A1 is the absorbance of the measured solution;
A2 is the absorbance of the standard solution;
C is the standard tube ginsenoside
Re amount (μg);
V is the volume of sample dilution (mL); and m is the sample mass (g).
2.6. Calculation of Total Ginsenoside Residual Ratio
The total ginsenoside residual rate (
L) was defined as follows: Rp represented the total ginsenoside detection value (g/100 g), which was extracted and determined under the optimal dissolution condition (American ginseng powder) and referred to the total ginsenoside content in the sample;
Rs was the total ginsenoside content (g/100 g) extracted and detected in the sample under different drying methods, which reflected the extractable content of total ginsenosides in the sliced samples.
Rp values were always greater than Rs values, and the difference between
Rp and
Rs largely reflected the loss of total ginsenoside dissolution due to differences in the pore structure. This loss, when divided by
Rp, gave the percentage of total ginsenoside loss, as demonstrated in Formula (5):
The ginsenoside residual rate (L) represented the effect of different drying methods on the degree dissolution of ginsenosides in American ginseng slices.
2.7. Measurement of Hardness
The hardness was determined using a TA.XT Plus texture analyzer, employing the TPA (texture profile analysis) mode with an A/CKB type probe. The probe descended to the sample surface at 2 mm/s and continued downward at 1 mm/s. Upon reaching 70% compression, the probe returned to the starting position. After a 3 s pause, the second phase commenced, replicating the same compression before returning to the zero point at 2 mm/s [
20]. This procedure was replicated eight times for each sample (random sampling), and the final results were averaged.
2.8. Measurement of Color
A YS3060 grating spectrophotometer was employed to quantify the surface brightness
L* (0 <
L* < 100). For American ginseng slices, the higher the
L* value, the whiter the color, indicating more favorable consumer perception [
7]. Sampling for color measurement was performed at the point when the chrysanthemum pattern was in the center of the American ginseng slices. Three sample slices (random sampling) were taken for each drying method, and the results were averaged.
2.9. Microstructure Analysis
Scanning electron micrographs showed the microstructural features of the samples. The planar microstructures of the samples were observed at an accelerating voltage of 10 kV. The sample slices were gold-sprayed for 40 s before testing [
26]. The micrographs were obtained at 200× magnification, and representative micrographs were selected to represent each sample.
2.10. Synthesized Assessment
A multi-indicator test formula method can objectively assign weights to the indicators without changing the order of the indicators and the optimal program, which evaluate the test effect in the form of a comprehensive score [
27]. However, this method results in the indicator nature of large differences in the calculated weight coefficients being huge. Small extremum difference indicators’ weight coefficients are several times or even dozens of times higher than those of the large extremum difference indicators, which leads to the final score being too sensitive to the small extreme indicators. The coefficient of variation method can give appropriate weighting coefficients according to the size of the numerical extremes of different indicators. Therefore, this study applied the coefficient of variation method to improve this step.
The improved multi-indicator test formula method [
22] was as follows: The optimal value, denoted as (
xb), of each index was assigned a full score of 100 points. Subsequent values were then converted to scores, represented as (
Xij), with no score exceeding 100. Indices were classified as either positive (higher values indicated better quality) or negative (lower values indicated better quality).
Positive indicator score conversion formula:
Negative indicator score conversion formula:
where
Xij is the score of the
jth indicator for the
ith set of trials;
xij is the measured value of the
jth indicator for the
ith set of tests; and
xb is the optimization of the measured value of each indicator.
The coefficient of variation method was used to objectively assign weights to each indicator:
where
Wi is the weighting coefficient for the ith indicator and
V(
xj) is the coefficient of variation for the
jth indicator.
The comprehensive score
Pi was calculated for each group of trials: