Effects of Drying Methods on Drying Characteristics and Physicochemical Quality of Turnip Slices (Brassica rapa L.)

Abstract

To identify the optimal drying methods for turnip slices, vacuum freeze-drying (VFD), air impingement drying (AID), infrared-assisted hot air drying (IR-HAD), and conventional hot air drying (HAD) were evaluated. The physicochemical properties of dried samples were comprehensively assessed under varied drying conditions. The results demonstrated that AID achieved the shortest drying time (240 min). At identical temperatures, AID samples exhibited significantly lower total color difference (ΔE) compared with IR-HAD and HAD, alongside superior retention of total phenols and enhanced antioxidant activity. VFD yielded the highest quality attributes, including optimal rehydration capacity, maximal phenol retention, and the strongest antioxidant activity (DPPH: 16.56 ± 0.26 μmol Trolox/g; FRAP: 13.99 ± 0.04 μmol Trolox/g). SEM analysis revealed that VFD produced a loose, porous microstructure, explaining its enhanced rehydration. Overall, both AID (efficiency) and VFD (quality) show promise for industrial turnip processing.

1. Introduction

Turnip (Brassica rapa L.), a biennial herbaceous plant of the Brassicaceae family, is characterized by its globular roots and is cultivated worldwide. According to Food and Agriculture Organization (FAO) data, global turnip production reached 42.14 million tons in 2022, with China ranking first (18.68 million tons), followed by Uzbekistan (3.92 million tons), the United States (1.38 million tons), and Russia (1.36 million tons). In China, cultivation is primarily concentrated in Xinjiang and Tibet [1]. As a nutritious cruciferous vegetable, turnip is recognized as a traditional medicinal and edible homologous food [2,3,4]. Chemically, it is rich in dietary fiber, vitamin C, minerals, phenols, glucosinolates, and other bioactive components [5,6,7]. Pharmacological studies confirm its antioxidant, anti-diabetic, and anti-cancer properties [8,9,10], supporting its historical use in Chinese folk medicine for treating asthma, fatigue, cough, hypoxia, and other diseases [11,12].

Fresh turnips typically exhibit a high moisture content of approximately 92% (wet basis), rendering them prone to rapid germination and quality deterioration at ambient temperatures post-harvest. This degradation significantly compromises their nutritional value, pharmacological potential, and commercial viability [13]. To address these preservation challenges, drying technologies have been widely employed to reduce the moisture content to approximately 13% (wet basis), effectively inhibiting microbial growth and retarding deterioration reactions. Among the conventional drying methods, hot air drying (HAD) remains prevalent in fruit and vegetable processing due to its cost-effectiveness, utilizing convective heat transfer mechanisms. However, this method presents notable limitations, including prolonged drying duration, inferior product quality, and substantial nutrient degradation [14,15]. The factors affecting HAD mainly include drying temperature, wind velocity, and medium relative humidity [16]. Infrared drying offers distinct advantages in processing time reduction through rapid thermal energy transfer. Nevertheless, its efficacy is constrained by the material’s inherent optical properties and structural characteristics [17,18]. To optimize drying performance, hybrid systems combining infrared radiation with convective hot air have demonstrated synergistic effects, enhancing both processing efficiency and product quality [19,20]. Air impingement drying (AID) represents another advanced technique characterized by enhanced drying kinetics. This method employs high-velocity hot air jets directed at the material’s surface, creating a boundary layer that accelerates moisture removal [21]. Successful applications have been documented for various agricultural products including strawberry slices [22], kiwifruit [23], and American ginseng [24]. For premium quality preservation, vacuum freeze-drying (VFD) has emerged as a superior alternative. By facilitating direct sublimation of ice crystals under reduced pressure, VFD optimally maintains structural integrity, chromatic characteristics, and nutritional profiles [25]. This technology has been effectively implemented in the dehydration of apple slices [26], strawberries [27], carrot slices [28], and peach slices [29].

Given the significant nutritional and medicinal value of turnip and the critical need for drying, the quality and drying efficiency of dried turnip can be improved through appropriate drying technology. To find a feasible drying technology for turnip, the effects of different drying methods such as IR-HAD, AID, HAD, and VFD on the drying kinetics, physicochemical properties, and microstructure of turnip slices were explored in this work. The findings aim to establish a scientific foundation for turnip drying and provide practical insights for selecting appropriate drying methods to enhance product quality.

2. Materials and Methods

2.1. Raw Materials

Fresh turnips (Brassica rapa L.) were sourced from Alar, Xinjiang, China, and stored in the refrigerator (4 ± 1 °C) for no more than one week. In order to ensure the reliability of the results, turnips from the same batch with no mechanical damage and uniform size were selected for the experiment. Before the experiments, the turnips were removed from refrigerator and equilibrated to ambient temperature. Surface impurities were rinsed with tap water, and residual moisture was removed using tissue paper. The turnips were then uniformly sliced to dimensions of 5.0 ± 0.2 mm thickness and 40.0 ± 1.0 mm diameter. The initial moisture content of turnips was found to be 92.00% ± 0.85 (wet basis), which was estimated by vacuum drying at 70 °C for 24 h following the AOAC standard method [30].

2.2. Drying Experiments

Four distinct drying methods were employed in this study: HAD, AID, IR-HAD, and VFD. Prior to each drying experiment, the respective drying equipment was preheated to achieve stable operational conditions. Approximately 300 g of prepared turnip slices was evenly distributed on stainless steel wire mesh trays and loaded into the drying chambers. Throughout the drying process, sample weights were recorded at 30 min intervals until the moisture content reached the target value of <13% (wet basis). To ensure experimental reliability, all drying trials were conducted in triplicate. Schematic diagrams of the four drying systems are presented in Figure 1.

2.2.1. Hot Air Drying (HAD)

HAD experiments were conducted in a laboratory-scale electrically heated blast drying oven (DHG-9140A, Shanghai Yiheng Technology Co., Ltd., Shanghai, China). The samples were spread evenly in a single layer on a drying tray. The drying temperature was 60 °C, and the air velocity during the drying process was set to 1 m/s [31]. The schematic diagram of the HAD setup is shown in Figure 1a.

2.2.2. Air Impingement Drying (AID)

The second set of turnip drying experiments was carried out in the air impingement dryer. The AID experimental setup and its working principle have been described in detail by Liu et al. [32]. On the basis of preliminary experiments, the AID temperature was set at 60 °C, and the air velocity was set at 3 m/s. The structural sketch of the equipment is shown in Figure 1b.

2.2.3. Infrared Heating-Assisted Hot Air-Drying (IR-HAD)

The experiments were conducted using an infrared heating-assisted hot air dryer (STC Taizhou Senttech Infrared Technology Co., Ltd., Taizhou, China). The dryer’s details have been described in detail by Zhang et al. [33]. The dryer features 3 parallel infrared tubes uniformly distributed at the top of the drying chamber, with 18 annular airflow nozzles arranged in an array configuration above the infrared heating elements. On the basis of preliminary optimization trials, the operational parameters were set to a drying air temperature of 60 °C and an air velocity of 3 m/s, with an infrared lamp distance of 13 cm from the material’s surface. The schematic diagram of the dryer is shown in Figure 1c.

2.2.4. Vacuum-Freeze Drying (VFD)

Turnip slices were first pre-frozen at −50 °C for 3 h in a vacuum freeze-dryer (Model LGJ-25C, Sihuan Scientific Instrument Co., Ltd., Beijing, China), followed by VFD under the following conditions: cold trap temperature maintained at −50 °C, heating plate temperature set to 30 °C, and chamber pressure held at 20 Pa. The experimental setup is illustrated in Figure 1d.

2.3. Drying Characteristics

The weight loss of turnip slices during drying was recorded, and the moisture ratio (MR) at different time points was calculated using the following equation [34]

$$MR={\displaystyle \frac{M_t-M_e}{M_0-M_e}}$$

(1)

where M₀ represents the initial moisture content on a dry basis (d.b.) of the material, in g/g; M_t is the moisture content (d.b.) at drying time t, in g/g; and M_e indicates the equilibrium moisture content.

The drying rate (DR), representing the speed of moisture removal, was calculated using the following equation [35]

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

(2)

where t₁ and t₂ are the drying times, in min; $M_{t_1}$ and $M_{t_2}$ are the dry basis moisture contents of the material at drying times t₁ and t₂, respectively, in g/g (d.b.).

The effective moisture diffusivity (D_eff), describing the water removal capacity through diffusion, was calculated using Fick’s second law following Equation (3) [36]

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

(3)

where D_eff stands for the effective moisture diffusivity, in m²/s; r is the volume equivalent radius of turnip slices, in m; and t is the drying time, in s. D_eff could be calculated by the natural logarithm from through the following Equation (4):

$$\ln \mathit{MR} = \ln {\displaystyle \frac{8}{{\pi }^2}}-{\displaystyle \frac{{\pi }^2{D}_{eff}}{r^2}}t$$

(4)

And the D_eff could be obtained as shown in Equation (5)

$$D_{\mathrm{eff}}=-{\displaystyle \frac{r^2}{{\pi }^2}}k$$

(5)

where k is the slope value of the regression line between ln MR versus drying time.

The Weibull and Page models were subsequently used for fitting the drying kinetics of turnip slices according to Equations (6) and (7), respectively

$$\mathrm{MR}=\exp (-k{t}^n)$$

(6)

$$\mathrm{MR}=\exp \left[-{\left({\displaystyle \frac{t}{\alpha }}\right)}^{\beta }\right]$$

(7)

where t is the drying time, in h; n and k are the model constants; and α and β are the scale parameter and shape parameter, respectively.

The fit of the model between the predicted data and the experimental values was conducted by coefficient of determination (R²), root mean square error (RMSE), and reduced chi-square (${\chi }^2$) tests, which were calculated by the following

$${\mathrm{R}}^2=1-{\displaystyle \frac{{\sum }_{i=1}^n{\left(M{R}_{pre,i}-M{R}_{\exp ,i}\right)}^2}{{\sum }_{i=1}^n{\left(\overline{M{R}_{pre,i}}-M{R}_{\exp ,i}\right)}^2}}$$

(8)

$${\chi }^2={\sum }_{i=1}^n{\displaystyle \frac{{\left(M{R}_{pre,i}-M{R}_{\exp ,i}\right)}^2}{n-z}}$$

(9)

$$RMSE=\sqrt{{\displaystyle \frac{1}{n}}{\sum }_{i=1}^n{\left(M{R}_{pre,i}-M{R}_{\exp ,i}\right)}^2}$$

(10)

where MR_pre,i is the experimental values, MR_exp,i is the predicted values, and n is the number of experiences.

2.4. Texture

The hardness of the turnips was determined by a texture profile analyzer (TA. XTPLUS/50, Stable Micro System, Surrey, UK). Each turnip slice was placed on a flat plate with a round hole in the center, and then a P/2 cylindrical probe was used to puncture the turnip at a test speed of 0.5 mm/s to a depth of 5 mm. Hardness was recorded from the maximum force fracturing of the sample [37].

2.5. Rehydration Ratio (RR)

Dried turnip slices were rehydrated following Zhou et al.’s method [38] with slight modifications. A 10 g sample was immersed in 200 mL of distilled water at 40 °C for 2 h, with the temperature maintained by a water bath. The rehydration ratio was calculated using Equation (11) [39]

$$RR={\displaystyle \frac{m_2}{m_1}}$$

(11)

where RR is the rehydration ratio, in g/g; m₁ and m₂ are the mass of turnip slices before and after rehydration, respectively, in g.

2.6. Microstructure Measurement

Dried turnip samples were sectioned into 2 mm × 2 mm × 1 mm pieces with a scalpel, mounted on aluminum stubs using conductive adhesive, and gold-coated for 50 s in a vacuum with an ion sputter coater (E-1010, Hitachi, Tokyo, Japan). Surface morphology was observed by scanning electron microscopy (SEM) (SU3500, Hitachi, Japan) at a 10 kV accelerating voltage at 100×, 200×, and 300× magnifications.

2.7. Color Measurement

The colorimetric values (L*, a*, b*) of turnip slices were measured using a LabScan XE spectrophotometer. The total color difference (ΔE*) between the dried and fresh turnip samples was calculated by Equation (12) [40]

$$\Delta E*=\sqrt{{(L_0*-L*)}^2+{(a_0*-a*)}^2+{(b_0*-b*)}^2}$$

(12)

where L* represents the lightness value, a* represents the redness/greenness value, and b* represents the yellowness/blueness value; L₀*, a₀*, and b₀* represent the color parameters of the fresh turnip slices, respectively.

2.8. Determination of Total Phenolic Content (TPC)

Extractions were prepared following Zhou et al. [38] with modifications. Briefly, 0.6 g (±0.01 g) of the pulverized sample was mixed with 10 mL of 70% (v/v) ethanol and ultrasonicated for 30 min (in triplicate). After centrifugation (10,000× g, 4 °C, 10 min), supernatants were collected as extracts. TPC was determined using the Folin–Ciocalteu method [41]. Specifically, 1 mL of the extract was mixed with 1 mL of Folin–Ciocalteu reagent and 4 mL of distilled water and incubated for 5 min (dark, room temperature), followed by the addition of 4 mL 10% Na₂CO₃ and 60 min incubation (dark). Absorbance was measured at 750 nm using a spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co., Ltd., Beijing, China). TPC was expressed as gallic acid equivalents per gram of dry weight (GAE, mg/g DW).

2.9. Antioxidant Capacity Determination

Antioxidant capacity was evaluated using two methods: (1) DPPH radical scavenging activity, and (2) ferric reducing antioxidant power (FRAP). The extraction followed the same protocol as phenolic compounds. The results were expressed as μmol Trolox equivalents (TE/g DW), with triplicate determinations.

DPPH scavenging activity was determined following Brand-Williams et al. [42] with modifications. Briefly, each sample extract was mixed with a 0.1 mM DPPH solution and incubated in the dark (30 min, room temperature), and the absorbance was measured at 517 nm using a spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co., Ltd.).

For the FRAP assay [36], the sample extract was mixed with 10 mM TPTZ, 0.1 M acetate buffer (pH 3.6), and 20 mM FeCl₃ (10:1:1, v/v/v), then incubated at 37 °C in the dark for 30 min. Absorbance was measured at 593 nm using a spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co., Ltd.).

2.10. Statistical Data Analysis

The experimental data were statistically analyzed and compared by ANOVA and Duncan’s multiple range test using SPSS statistical software (version 26.0, IBM Corp., Armonk, NY, USA). The results on drying and quality are presented as the mean ± standard deviation (SD) of three determinations. Statistical significance was set at the 5% probability level (p < 0.05).

3. Results and Discussion

3.1. Drying Characteristics Analysis

The effects of various drying techniques on the moisture content and drying rate curves of turnip slices dried at 60 °C are shown in Figure 2. As expected, the moisture content decreased with the extension of drying time, which was simulated by the Weibull and Page models. As shown in

Table 1. Fitting results of drying kinetics for turnip slices (Brassica rapa L.) under various methods.

Models	Drying Method	Parameters
		α	β	χ2/(×10−4)	R2	RMSE	Deff/(10−8, m2·s−1)
Weibull	HAD	124.03565 ± 0.660	1.26917 ± 0.0137	0.5954	0.9994	0.0013	2.467
	AID	42.61838 ± 0.53255	1.09985 ± 0.02323	0.1084	0.9988	0.0014	4.863
	IR-HAD	65.0599 ± 0.52267	1.23456 ± 0.01873	0.7852	0.9992	0.0011	4.068
		k	n	χ2/(×10−4)	R2	RMSE	Deff/(10−8, m2·s−1)
Page	HAD	0.00221 ± 0.00016	1.26819 ± 0.015	0.5956	0.9994	0.0013	2.467
	AID	0.01612 ± 0.00156	1.10004 ± 0.02455	1.0844	0.9988	0.0014	4.863
	IR-HAD	0.00579 ± 0.00050	1.23399 ± 0.02014	0.7852	0.9992	0.0011	4.068

Note: α and β are the scale parameter and shape parameter in the Weibull model, respectively; k and n are the empirical constants in the Page model.

, the predicted data fit well with the experimental data, with high R² (0.9988~0.9994) and low χ² (0.5954 × 10⁻⁴~1.08444 × 10⁻⁴) and RSEM (0.11 × 10⁻²~0.14 × 10⁻²) values. Figure 2A demonstrates that the different drying methods had a significant (p < 0.05) effect on the time required to reach the final moisture content. The drying times of HAD, AID, and IR-HAD were 510 min, 240 min, and 300 min, respectively. HAD required the longest drying time (510 min), while AID showed the shortest drying time (240 min). Compared with HAD and IR-HAD, the drying times of AID were shortened by 52.9% and 20%, respectively. This was mainly due to the high air velocity, short air flow, and thin boundary layer between the surface of the material and the air, which usually has a high convective heat transfer coefficient [43]. For the AID of turnips, the rapid airflow and high convective heat transfer coefficient enabled more efficient heat transfer in a shorter time, thereby promoting faster water evaporation. Generally, shorter drying times are associated with lower energy consumption. As an energy-intensive industrial process, drying accounts for approximately 7–15% of total industrial energy use [36]. Therefore, a comprehensive evaluation of drying technologies should explicitly incorporate energy efficiency considerations. Future research should prioritize the development of strategies to enhance energy efficiency in drying processes, thereby contributing significantly to energy conservation and carbon emission reduction in industrial operations.

D_eff, a function of the moisture content at the same temperature, reflects the dehydration ability of the material under drying conditions. D_eff values are assumed to be constant at a given temperature, even though they in fact vary with moisture content. As presented in Table 1, the effective moisture diffusivity values for AID, IR-HAD, and HAD were 4.863 × 10⁻⁸, 4.068 × 10⁻⁸, and 4.863 × 10⁻⁸ m²/s, respectively. These results align with the general principle of dehydration, where lower effective moisture diffusivity corresponds to longer drying times.

For the Weibull model, the scale parameter (α) of AID was significantly lower (p < 0.05) than that of HAD at a drying temperature of 60 °C. The α value decreased from 124.035 in HAD to 42.618 in AID, indicating a shorter drying time. The shape parameter (β) reflects the drying rate during the initial stage, with a lower β value corresponding to a higher initial drying rate. As shown in Table 1, the β values for AID, IR-HAD, and HAD were 1.099, 1.234, and 1.269, respectively, suggesting that AID had the highest initial drying rate. This result is consistent with the drying kinetic curve presented in Figure 2.

For the Page model, the n values exceeded 1 across all drying methods, suggesting the occurrence of super-diffusion behavior. The k value of AID was significantly higher (p < 0.05) than that of IR-HAD and HAD. Since the k value represents the initial drying rate, a larger k corresponds to a faster drying rate at the beginning stage.

The relationship between the drying rate and moisture content of turnip slices under different drying methods is shown in Figure 2B. At the identical drying temperature (60 °C), AID demonstrated a significantly higher drying rate than both IR-HAD and HAD (p < 0.05). Throughout the drying process, the drying rate progressively decreased with reducing moisture content. The entire drying process of turnip slices occurred in the falling rate period, indicating that internal water diffusion dominated the process [19]. In AID drying, the drying rate peaked during the initial stage owing to the rapid airflow impinging on the material surface, which promoted substantial moisture loss [44]. However, during the later drying stage, IR-HAD exhibited a higher drying rate than AID. This phenomenon primarily occurred because infrared radiation could penetrate the material’s surface during IR-HAD, providing additional driving force for internal moisture migration and thereby accelerating moisture transfer [45]. Consequently, while the total drying times for AID and IR-HAD were comparable, their drying rate profiles differed substantially across various drying stages.

3.2. Rehydration Ratio and Texture

Rehydration capacity, defined as the degree to which dried products regain their original fresh state upon rehydration, serves as a crucial indicator reflecting structural changes in the product [46]. A higher rehydration ratio corresponds to less structural damage in the dried product and consequently better product quality [47]. As shown in Figure 3A, the rehydration rate of turnip slices varied significantly among different drying methods. VFD-treated turnip slices exhibited the highest rehydration rate, while AID and IR-HAD showed comparable rehydration capacities. HAD samples demonstrated the lowest rehydration capacity. The vacuum and freeze-drying environment better preserved the microstructure of turnip slices, forming a stable solid matrix that maintained the original cellular structure. This structural integrity allowed for more efficient water reabsorption [48]. The freeze-drying process created a porous, well-organized structure that significantly enhanced rehydration ability. In contrast, the other three drying methods caused varying degrees of structural collapse in turnip slices, thereby impeding water absorption. Among these, HAD showed the most severe structural damage and lowest rehydration capacity, which can be attributed to its prolonged drying duration.

Texture is a physical property determined by the composition and microstructure of turnip, and the drying method has a great influence on the texture [32]. The hardness of the sample after drying by different drying methods is shown in Figure 3B. It can be seen that different drying methods have a significant effect on the hardness of turnip slices (p < 0.05). The hardness of HAD samples was the largest (9.06 ± 0.62 N). The analysis may be due to the long drying time, the serious collapse of tissue cells, and the surface being crusted by continuous heat. After VFD, turnips had the lowest hardness (2.39 ± 0.77 N) due to the moisture sublimating during the freeze-drying process, forming a loose and porous structure [49]. Those with strong rehydration capacity had the lowest hardness, and those with weak rehydration capacity had the highest hardness.

3.3. Microstructure by SEM

Microstructure can reflect the quality of dried products. The microstructure analysis can help us to understand the mechanism of mass change of the material at the cellular level, thereby improving the drying performance [50]. The SEM images of turnip slices were obtained to explore the effect of different drying methods on the microstructure, as shown in Figure 4. The changes in microstructure were mainly manifested in the size and uniformity of microcracks, holes, and cavities. There was crusting on the surface of hot air-drying materials—which may be due to the long drying time—the cell wall was seriously damaged, the pore structure in the drying process was seriously collapsed, and the surface pores disappeared. Li et al. [31]. discovered a similar phenomenon, observing that the tissue cells of Cistanche deserticola were damaged severely at a 60 °C drying temperature. There were apparent folds on the surface of the turnip after IR-HAD, which may be due to the fact that the energy of infrared radiation was absorbed by the surface. The moisture on the surface of a turnip slice evaporates rapidly. The internal moisture migration speed was much smaller than the speed of surface water evaporation, thus forming folds on the surface. After VFD, the structure of the turnip slices collapsed slightly, a relatively intact cell structure was retained, and the pore size was similar and evenly distributed. This was because the sublimation of water from a solid state to a gaseous state in freeze-drying did not destroy the complete structure of the cell, so the microstructure was loose and porous. This is also why turnip chips were less hard and had higher rehydration capacity after VFD.

3.4. Color

Color is an important index used to evaluate the quality of dry products, and it has an important impact on the market value and consumption choice of products [51,52]. The values of L*, a*, b*, and ΔE, along with the corresponding images for different drying methods, are presented in

Table 2. Images and color parameters of turnip (Brassica rapa L.) under various methods.

	Fresh	IR-HAD	AID	HAD	VFD
Image
$L^{\ast }$	93.53 ± 0.36 a	85.24 ± 1.18 d	91.58 ± 0.28 b	88.39 ± 0.99 c	94.67 ± 0.40 a
$a^{\ast }$	−3.82 ± 0.30 b	−3.72 ± 0.91 b	−3.55 ± 0.35 b	−5.21 ± 0.44 c	−1.34 ± 0.10 a
$b^{\ast }$	2.80 ± 0.08 d	15.22 ± 0.69 a	7.33 ± 0.55 c	11.54 ± 0.34 b	1.88 ± 0.18 d
$\Delta E$	—	14.98 ± 0.64 a	4.96 ± 0.43 c	10.24 ± 0.83 b	2.88 ± 0.26 c

Note: Different letters within a row denote significant differences (p < 0.05).

. After VFD drying, the L* value was the highest and the b* value was the lowest compared with the other samples. This may be due to the drying environment being at a low temperature under vacuum freezing, which can inhibit the occurrence of browning and the Maillard reaction, and VFD improving the brightness and whiteness of turnip slices. Similar results have been reported for garlic drying by Zheng et al. [53], who found that the lightness value of garlic slices increased from 79.36 ± 0.68 to 91.57 ± 0.41. IR-HAD samples showed the lowest L* (85.24 ± 1.18), highest b* (15.22 ± 0.69), and highest ΔE values (14.98 ± 0.64). This resulted from infrared radiation damaging the surface structure, as evidenced by microstructural analysis. The uneven moisture distribution between internal and external regions, combined with sustained high surface temperatures, accelerated browning reactions. Bondaruk et al. [54] found that when the ΔE value is greater than 5, the change in color is noticeable to the observer. The higher the ΔE value, the greater the color change during the drying process. The ΔE values of IR-HAD and HAD were both greater than 5, which were 14.98 ± 0.64 and 10.24 ± 0.83, respectively, indicating that the color after drying changed significantly (p < 0.05). The ΔE values of VFD and AID were less than 5, which were 2.88 ± 0.26 and 4.96 ± 0.43, respectively, indicating that the VFD and AID samples were close to the color of fresh turnip slices.

The images in the table demonstrate that VFD-treated turnips exhibited minimal shrinkage and deformation, whereas other thermal drying methods caused significant structural changes in the samples. This phenomenon occurs because thermal drying induces rapid surface water evaporation, while internal moisture migration cannot keep pace with surface water loss. Consequently, the resulting surface shrinkage generates stress that leads to substantial shape alteration [53].

3.5. Total Phenolic Content

Various preclinical in vivo animal and in vitro cell culture studies have shown that antioxidant substances and antioxidant capacity can prevent various peroxidation-related diseases [55]. Phenols are closely related to antioxidant activity [56]. Statistical analysis showed that different drying methods significantly affected the total phenol content of turnip. As shown in Figure 5, in IR-HAD, AID, and HAD, the retention of total phenols was 14.92 ± 0.07, 15.12 ± 0.02, and 14.01 ± 0.04 mg/g, respectively. The total phenol retention of VFD was 20.98 ± 0.09 mg/g. This was significantly higher than that of other drying methods, probably because the low-temperature vacuum environment helps to retain heat-sensitive and oxidizing components [57]. Although thermal drying accelerates phenolic degradation, its shorter processing time partially offsets this negative effect. Similar results were reported in a study of apple drying, where drying temperature and drying time affected the product’s nutrient retention [58].

3.6. Antioxidant Activity In Vitro

The healthy properties of fruits/vegetables in preventing diseases are based on the antioxidant capacity to reduce oxidative damage in the body through scavenging free radicals [59]. As shown in Figure 6, there were significant differences in the DPPH and FRAP radical scavenging abilities of turnip with different drying methods. The DPPH and FRAP radical scavenging capacities were the strongest for the VFD samples, which were 16.56 ± 0.26 μmol Trolox/g and 13.99 ± 0.04 μmol Trolox/g, respectively. This may be due to the formation of ice crystals during VFD that disrupted the cell structure’s integrity and increased the release of intracellular antioxidant active substances. Shireen et al. [60] found similar phenomena in the study of the drying process of mangoes. The antioxidant activity of hot air drying was low, possibly due to the degradation of active antioxidant substances promoted by prolonged thermal processing. The antioxidant activity of different drying methods showed the same trend as that of total phenol content, suggesting that phenolic compounds might play a dominant role in the antioxidant activity, consistent with the result in goji berry by Zhang et al. [61]. As shown in Figure 7, significant correlations (p < 0.05) were observed between DPPH and TPC, and between FRAP and TPC, consistent with the findings by Liu et al. [62], who reported that the antioxidant activity was significantly correlated with TPC.

4. Conclusions

This study investigated the effects of four drying methods, namely IR-HAD, AID, HAD, and VFD, on the drying kinetics, physicochemical properties, and microstructure of turnip slices. The results showed that different drying methods significantly affected the overall quality of dried turnip slices (p < 0.05). AID showed the shortest drying time, while VFD required the longest duration. The SEM analysis revealed that VFD produced a well-structured porous network, resulting in superior rehydration capacity. Furthermore, VFD-treated samples exhibited minimal non-enzymatic browning, the smallest color deviation, the highest phenolic compound retention, and the strongest antioxidant activity among all samples. Among the thermal processing technologies, the overall quality of AID was better than that of IR-HAD and HAD; the color difference was smaller, and the active ingredient retention was higher. The current research results showed that the turnip slices obtained by VFD had the best quality, but the drying time of VFD was long and the cost was high. The drying process needs to be further optimized to improve its potential for industrial application. On the other hand, AID has the potential for industrial applications due to its short drying time and low cost.