An Investigation of the Use of Microwaves and Airborne Ultrasound in the Convective Drying of Kale: Process Efficiency and Product Characteristics

Abstract

This study evaluated different hybrid drying modes, combining traditional convective drying with microwave radiation and airborne ultrasound for the dehydration of green leafy vegetables. The central composite design method was used to analyze the impact of microwave and ultrasonic waves on kinetics, energy consumption, and various quality parameters, like color, ascorbic acid, polyphenol, carotenoid, and chlorophyll content in Brassica oleracea, var. acephala. The results of the applied experimental design, i.e., the surface response methodology, showed that the application of microwaves and ultrasound decreased the drying time considerably and enhanced the moisture evaporation from the kale leaves, significantly improving the drying rate and energy efficiency. The drying rate increase demonstrated varying results with changes in air temperature. Specifically, ultrasound resulted in a 69–100% increase, microwaves in a 430–698% increase, and a combination of ultrasound and microwaves in a 463–950% increase at 70 and 50 °C, respectively. Specific energy consumption decreased by 42–51% for ultrasound, 80–87% for microwaves, and 82–90% for ultrasound and microwaves at 70 and 50 °C, respectively. The drying parameters were also found to be better at a higher temperature, but the increase in the drying rate caused by microwaves and ultrasound was notably lower. Moreover, the analysis of the key kinetic parameters and material qualities led to the conclusion that the synergistic action of microwave- and ultrasound-assisted convection contributes to better drying effectiveness and product quality, demonstrating greater retention of vitamin C, phenolics, and natural dyes of up to 90%.

1. Introduction

Fresh plant materials, such as fruits, vegetables, and herbs, represent a significant source of valuable compounds, including vitamins, microelements, and minerals. The main constituent of these materials is water, which can make up to 96% of the total weight. Other components include saccharides, proteins, lipids, dyes, and more. However, the exact composition varies, depending on the species, variety, and level of ripeness. Leafy vegetables, in particular, are an important subset of vegetables, distinguished by their distinctive taste and color. The primary advantages of leafy products are their low calorie and fat content, as well as their abundance of bioactive compounds. Additionally, these vegetables offer an array of health benefits, including fiber, amino acids, folic acid, vitamins, calcium, manganese, iron, phosphorus, zinc, copper, magnesium, and potassium. These benefits make leafy products a cost-effective and easily prepared addition to any diet. Representatives of this vegetable group include savoy cabbage, lettuce, spinach, and kale, among others. Kale, a type of leafy vegetable that belongs to the Brassica oleracea cultivars, is a staple in many traditional dishes due to its beautiful green or purple leaves that are rich in dietary fiber, minerals, quercetin, kaempferol, glucosinolates, indoles, isothiocyanates, chlorophyll, β-carotene, lutein, and vitamin C [1]. In recent years, kale chips have been gaining in popularity because of their delicious flavor and positive impact on human health compared to common potato chips and snacks [2].

The health benefits of kale are mainly due to its high concentration of bioactive compounds, including ascorbic acid, polyphenols, chlorophylls, and carotenoids [3]. These compounds have powerful antioxidant properties, and their consumption in sufficient amounts can reduce the risk of several chronic diseases, including scurvy, arthritis, cardiovascular and neurodegenerative diseases, diabetes, and various cancers [4]. Unfortunately, kale and other green leafy vegetables are highly perishable due to their low microbial stability caused by their high moisture content and water activity. In turn, this leads to spoilage processes that cause the loss of valuable bioactive compounds. Therefore, it is crucial to monitor the retention of biologically active compounds, e.g., vitamins and antioxidants during storage and processing. [5].

One common and easy-to-implement method for preventing food spoilage, enhancing safety, easing handling, and extending shelf life is the drying process. Dehydration not only reduces moisture content and water activity but also significantly minimizes the decay processes caused by the growth of bacteria, yeast, and mold. Dry food products have a broad range of applications, including instant products, spices added to soups and sauces, muesli, and healthy snacks like fruit–vegetable bars or chips, which have gained popularity in recent times [6]. Convective drying is the most commonly used drying process due to several advantages, such as a simple apparatus and a well-known drying mechanism, which have led to its widespread use in the industry. On the contrary, hot airdrying is considered to be one of the most time- and energy-consuming techniques, as well as being destructive and causing a deterioration in the quality of the product [7].

An increasing demand for high-quality food products and limited energy consumption has led to the development of alternative and combined methods, as well as advancements in drying technology such as hybrid drying. To achieve green food processing, various techniques, such as microwaves and ultrasound, can be combined with hot air drying [8]. The absorption of microwave radiation generates heat throughout the material, resulting in overall heating. Ultrasound results in multiple mechanical phenomena, including vibration effects, increased water vapor pressure due to acoustic wave absorption, acoustic cavitation, stirring, microstreaming, the reduction of the boundary layer, and minor changes in the material structure that resemble a sponge effect. Such dynamic interactions and phenomena accelerate the rate of drying by enhancing the exchange of heat and mass (by increasing the effective coefficients of both moisture diffusion and heat transfer) in dried food products. The joint effect of ultrasound and microwave action enhances drying parameters, such as reducing total drying time and energy consumption, while also maintaining the quality of processed food at a high level [9,10].

Sustainable drying technologies are essential for reducing food waste and energy consumption in developing countries [11,12]. Traditional drying methods, such as open sun drying, are energy-intensive and unhygienic [13]. To address these issues, integrating thermal energy storage with heat pumps and using advanced integrated drying systems, such as solar with microwaves or heat pumps with microwaves, can improve drying efficiency and food quality [14]. Additionally, sustainable drying technologies should be energy-efficient and eco-friendly [15]. The proper selection of drying methods, based on the route of drug administration and protein properties, can enhance the stability and bioavailability of therapeutic proteins. By optimizing the drying process, stable protein formulations can be prepared for various delivery systems. Overall, sustainable drying technologies play a crucial role in reducing food waste and energy consumption and improving the stability and bioavailability of dried products.

Central composite design is a methodology used to optimize the operating conditions for drying technologies. It involves developing models for responses such as moisture content, drying rate, energy efficiency, and exergy efficiency, based on independent variables like air temperature, air velocity, and drying time [15]. This approach allows for the generation of response surfaces and contour plots, which help identify the optimum operating zone for drying processes [16]. By applying desirability functions, the optimal conditions can be determined, taking into account factors like temperature, drying time, and air velocity [17]. The use of a central composite design in drying technologies contributes to sustainability by improving energy efficiency and reducing greenhouse gas emissions [18]. It offers a systematic approach to optimize drying processes, leading to more sustainable and environmentally friendly practices [15].

Although there are many research reports on hybrid drying of cabbage [19,20,21], lettuce [22,23,24,25,26], and spinach [27,28,29,30,31] available in the literature, the lack of works on kale is truly visible. In our previous work [32], we provided findings from a study on microwave drying utilizing continuous and periodic electromagnetic waves. The outcomes drawn from the study indicate that the pulsed impact of particularly high-powered microwave radiation has a favorable effect on kinetics and nutrient content. Nonetheless, it has the potential to result in undesirable color changes. Therefore, this study took a pragmatic approach and used the central composite design method to analyze the impact of microwaves on drying kinetics under forced convection conditions. The effects of air temperature and mechanical-waves–ultrasound assistance on kinetics, energy consumption, and various quality parameters like color, ascorbic acid, polyphenol, carotenoid, and chlorophyll content were also examined.

2. Materials and Methods

2.1. Materials

Fresh kale (Brassica oleracea, var. acephala) was purchased from a local market and refrigerated at 4 °C for no more than 24 h. The leaves were then cut into smaller pieces, measuring approximately 30 × 20 mm in size and no thicker than 5 mm, using a ceramic knife. A digital ultrasonic thickness gauge, model 27MG (manufactured by Olympus in Tokyo, Japan), was used to measure the thickness. Only the leaves without major veins were selected for use. The initial moisture content, MC₀, was determined using an XM120 moisture analyzer (Precisa, Dietikon, Switzerland) with an accuracy of 0.01%. On average, it measured 5.23 ± 0.25 DB. Before drying, the samples were spread in a single layer on a rotating pan’s Teflon-coated glass fiber grid, which had a mesh diameter of 4 mm and a thickness of 1 mm. They were then placed into the laboratory hybrid dryer. The mass of one batch was approximately 15 g. Following drying, the samples were placed in polypropylene barrier bags and stored in a dark area (maintained at room temperature) for additional quality analysis.

2.2. Drying Procedure

Drying processes were carried out in a laboratory hybrid dryer constructed by PROMIS-TECH (Wrocław, Poland). The scheme of the hybrid dryer is presented in Figure 1.

The dryer enables drying through hot air (C) and microwaves (M) operating at a frequency of 2.45 GHz. Additionally, the process may be supported by high-power airborne ultrasound (U) at a frequency of 25 kHz and power up to 200 W, generated by the Airborne Ultrasound System produced by Pusonics (Madrid, Spain). Kowalski et al. [33] have provided a detailed description of the apparatus.

The air temperature, velocity, and relative humidity process parameters were measured using a Delta OHM (Caselle di Selvazzano, Italy) hot-wire anemometer HD29371TC1.5 with 0.1 °C and 0.01 m/s precision and a humidity sensor HD4817ETC1.5 with 0.01% precision. The air parameters, microwave power, airborne ultrasound power, sample mass, and surface temperature were continuously monitored during the drying process at 5-min intervals for convective schedules and 30-s intervals for hybrid schedules. The CT LT15 pyrometer from Optris (Berlin, Germany) ensured precision within 0.1 °C. Each drying experiment was conducted twice.

2.3. Drying Schemes—Central Composite Design (CCD)

In order to study the effects of ultrasound and microwaves on the kinetics of convective drying, a central composite design (CCD) was developed for the experiment using Design Expert ver. 13 (Stat-Ease, Minneapolis, MN, USA). Two independent procedural numerical variables (microwave and ultrasonic power) and a categorical variable (air temperature) were tested. A total of eighteen different drying programs were conducted. Two of the experiments were conducted using convection, while the other experiments used a hybrid approach which combined convection with microwave and ultrasound technologies. Additionally, separate convective experiments with microwaves or ultrasound were conducted to assess the effect of each technology individually. Two repetitions were conducted at the center point, with alpha set to one. The airflow velocity was maintained at a constant rate of 2 m per second throughout the drying experiments. A comprehensive overview of the drying procedures can be found in

Table 1. Drying programs.

Process	Order		Space Type	Power		Air Temperature (°C)
				Microwaves (W)	Ultrasound (W)
	Standard	Run
C_5	1	9	Factorial	0	0	50
CM_52	2	14	Factorial	200	0	50
CU_52	3	16	Factorial	0	200	50
CMU_522	4	5	Factorial	200	200	50
CU_51	5	11	Axial	0	100	50
CMU_521	6	12	Axial	200	100	50
CM_51	7	4	Axial	100	0	50
CMU_512	8	20	Axial	100	200	50
CMU_511	9	13	Center	100	100	50
CMU_511	10	6	Center	100	100	50
C_7	11	7	Factorial	0	0	70
CM_72	12	17	Factorial	200	0	70
CU_72	13	19	Factorial	0	200	70
CMU_722	14	8	Factorial	200	200	70
CU_71	15	18	Axial	0	100	70
CMU_721	16	2	Axial	200	100	70
CM_71	17	1	Axial	100	0	70
CMU_712	18	10	Axial	100	200	70
CMU_711	19	15	Center	100	100	70
CMU_711	20	3	Center	100	100	70

, which outlines the standard order of drying schemes.

Table 2. Independent variables and their levels.

Independent Variable	Coded Variable	Type	Level
			−1	0	+1
Microwave power (W)	X1	numerical	0	100	200
Ultrasound power (W)	X2	numerical	0	100	200
Temperature (°C)	X3	categorical	50	-	70

presents the type and name of independent variables utilized in the experiments.

Based on the experimental results, the dry basis (DB) moisture content MC (g/g) as a function of time was calculated using the following equation:

$$MC(t)=\frac{m(t)-m_s}{m_s}\hspace{1em}\mathrm{dry} \mathrm{basis},$$

(1)

where m_s is the dry matter content (g) and m(t) is the sample mass (g) at a given time t of drying.

The moisture ratio MR (–) at a given time of process t is:

$$MR(t)=\frac{MC(t)-M{C}_0}{M{C}_{eq}-M{C}_0},$$

(2)

where MC₀ and MC_eq are the initial and equilibrium moisture contents (g/g), respectively. As the value of the equilibrium moisture content MC_eq, is relatively small compared to MC(t) and MC₀; Equation (2) is usually simplified to the form of Equation (3) without a significant change in the value of MR [34]:

$$MR(t)=\frac{MC(t)}{M{C}_0},$$

(3)

The drying rate at a given time of process DR(t) (g/s) was calculated using the following formula:

$$DR(t)=\frac{m(t_{i-1})-m(t_i)}{t_i-t_{i-1}},$$

(4)

The average drying rate DRA (g/s) was calculated as follows:

$$DRA=\frac{\Delta m}{DT}=\frac{m_0-m_f}{DT},$$

(5)

where m_f is the sample mass at the end of drying (g), and DT is the total drying time (s). The specific energy consumption SEC (MJ/kg) was calculated with the following equation:

$$SEC=3.6\cdot \frac{EC}{\Delta m},$$

(6)

where EC is the total energy consumption (kWh) recorded in each drying experiment, and the denominator of 3.6 was applied to recalculate the kWh to MJ (1 kWh = 3.6 MJ).

2.4. Determination of the Diffusion Coefficient

The effective moisture diffusion coefficient D_eff (m²/s) was estimated from the slope of the normalized plot of the moisture ratio MR using the following equation [35]:

$$\ln MR=\ln \frac{8}{{\pi }^2}-\frac{{\pi }^2{D}_{eff}t}{4L^2},$$

(7)

where L is the half-thickness of the material (m).

The ln(MR) = f(t) curves were approximated using the linear function, and D_eff was calculated from the slope:

$$slope=\frac{{\pi }^2{D}_{eff}}{4L^2},$$

(8)

$$D_{eff}=\frac{4L^2}{{\pi }^2}\cdot slope,$$

(9)

2.5. Quality Assessment

The quality assessment of dehydrated products was based on objective measures such as the overall color difference (dE), ascorbic acid, and the percentage retention of phenolic, carotenoid, and chlorophyll (A and B).

The color measurements were obtained using the Konica Minolta CR400 colorimeter (Tokyo, Japan), which was equipped with a D65 light source and a 2° declination of the sensor to the light source (precision: 0.01). Prior to the measurements, the kale leaves, whether fresh or dry, were ground in an IKA A11 Basic laboratory mill (Staufen, Germany). The paste or powder was placed in a sample holder, and CIELAB color coordinates (L*, a*, and b*) were measured five times. This process was conducted five times, resulting in a total of 25 measurements for each sample. The average color parameter values were then used to determine the total color difference [36]:

$$dE=\sqrt{{\left(L_0^{*}-L_s^{*}\right)}^2+{\left(a_0^{*}-a_s^{*}\right)}^2+{\left(b_0^{*}-b_s^{*}\right)}^2},$$

(10)

where L* is lightness, a* is the color parameter from red to green, and b* denotes the color parameter from yellow to blue. The index 0 refers to fresh kale leaves, whereas index s refers to dry leaves.

The ascorbic acid content was determined using the modified spectrophotometric method proposed by Rutkowski and Grzegorczyk [37], using a phosphotungstate reagent (PR), which is reduced by the L-ascorbic acid and produces tungsten blue, the absorbance of which was then measured at 734 nm in a spectrophotometer, model U-5100 (Hitachi, Tokyo, Japan). The total polyphenol content was determined by means of the Folin–Ciocalteu spectrophotometric assay with some modifications [38]. The absorbance was determined at 750 nm, and the results were expressed as gallic acid equivalents (mg GAE/100 g). The chlorophyll and carotenoid content determinations were carried out using the modified spectrophotometric method based on the extraction in acetone proposed by Lichtenthaler et al. [39]. The absorbance for the chlorophyll A and B and total carotenoids was measured at 663, 647, and 470 nm, respectively. The retention of ascorbic acid, phenolics, carotenoids, and chlorophylls in percentage was calculated as follows:

$$retention=\frac{c_d\cdot 100\%}{c_0}$$

(11)

where c_d and c₀ are the ascorbic acid or dye concentrations in fresh and dry kale leaves, respectively. The analysis was performed in three replications.

2.6. Statistical Analysis

All presented data are means ± standard deviations. Additionally, the Pearson correlation coefficients (r) were calculated for DT, DRA, and SEC. The goodness of the fit during the determination of the diffusion coefficient was assessed based on the adjusted R-squared and the reduced chi-squared [32]. Spearman’s rank order correlation was calculated for the chosen parameters. All calculations and graphs were made in Origin (Pro), Version Number 2023 (OriginLab Corporation, Northampton, MA, USA).

3. Results and Discussion

3.1. Drying Parameters and Specific Energy Consumption

Central composite design was successfully applied in the drying process of various leafy vegetables. In the study by Yildiz and Sarimeseli, a CCD was used to determine the optimal processing conditions for drying celery using a microwave system [40]. The variables considered in the design were vegetable load, microwave power, and drying time. The moisture ratio and drying rate of the celery were analyzed using response surface methodology (RSM), and a second-order polynomial equation was used to optimize the process conditions. In the study by Gbaguidi et al., a CCD was used to optimize the oven-drying temperature and duration for the preservation of baobab leaf powder [41]. The study aimed to improve the availability and shelf life of the product. The variables considered in the design were oven-drying temperature and duration. The dry matter, hue, chroma, and lightness of the baobab leaf powder were significantly influenced by these variables. The optimal oven-drying conditions for baobab leaves were determined to be 45 °C for 23.5 h. Overall, the application of central composite design in these studies allowed for the determination of optimal drying conditions for leafy vegetables, leading to improved product quality and preservation.

Figure 2 shows the contribution of ultrasound and microwaves on the drying rate (DR) at two different air temperatures of 50 and 70 °C.

The drying rate values (DR) rose as the power of ultrasound and microwave increased from 0 to 200 W, along with an increase in air temperature. However, the influence of microwaves on the drying kinetics of kale leaves was comparatively more significant than that of ultrasonic waves. The convective drying processes (C_5 and C_7) exhibited the lowest drying rate. The inefficient performance of the convective drying method was largely attributed to the fact that plant-based materials are predominantly dried during the second drying phase, also known as the falling drying rate period. During this stage, the diffusion of moisture within the material primarily dictates the drying rate, which continually decreases throughout the drying process. This explains the low efficiency of convection drying and corroborates the findings of previous researchers [42,43,44]. The increase in the drying rate of the hybrid programs was highly dependent on the type of reinforcement used and the air temperature. At a lower temperature, the quantitative effects were greater, with a maximum of 100% for ultrasound (CU), 700% for microwaves (CM), and 950% for microwaves and ultrasound (CMU) with respect to the reference process (C). For drying at 70 °C, the drying rate increased by a maximum of 70% for ultrasound (CU), 430% for microwaves (CM), and 463% for microwaves and ultrasound (CMU).

Figure 3 illustrates the response surface plots of microwave and ultrasound power on the specific energy consumption. When the SEC is considered, it can be concluded that hybrid processes such as convective drying with microwaves (CM) and convective drying with microwaves and ultrasound (CMU) are more energy-efficient than the convective method (C) and ultrasound-assisted convection (CU).

Unfortunately, the incorporation of ultrasonic waves in convective drying (CU) resulted in elevated energy consumption due to the requirement of powering the ultrasound generation system. The benefits to drying rate and time were insufficient to offset this added energy, resulting in an increased SEC. Conversely, in CM processes, the SEC was reduced by 55–69%. In the CMU processes, the SEC decreased as microwave and ultrasound power increased. However, the difference was more pronounced at 50 °C compared to 70 °C. The range of SEC reduction in this scenario was between 56% and 81%. Furthermore, the rise in ultrasound power from 100 to 200 W did not result in a significant difference in SEC compared to the increase in microwave power from 100 to 200 W. These findings align with prior studies on the microwave-assisted convective drying of kale [39] and the hybrid microwave–hot air drying of onion slices [44].

Figure 4 illustrates the impact of ultrasound and microwaves along with air temperature on the effective diffusion coefficient.

The effective diffusion coefficients fell within the typical range for fruits and vegetables, ranging from 10⁻⁷ to 10⁻¹³ m²/s [45], consistent with our prior research on kale leaves [39]. Among the different processes, the convective method displayed the smallest Deff, while the fastest hybrid processes involving microwaves and ultrasound (CMU) at 70 °C demonstrated the highest Deff. The data in Figure 4 confirms that Deff is affected by temperature and radiation energy. The increase in diffusion coefficient was greater at a lower temperature (50 °C), ranging from 64% (for CU) to 640% (for CMU). At a higher temperature (70 °C), the relative change ranged from 36% (for CU) to 371% (for CMU). This aligns with earlier findings by Wiset et al. [46] on cherry tomatoes, Md Salim et al. [47] on broccoli stalks, and Zambra et al. [43] on Kageneckia oblonga leaves.

Table 3. Statistics for kinetic parameters and color change.

Parameter	p-Value		Fit Statistics
	Model	Lack of Fit	R2	Adjusted R2	Predicted R2
DR	<0.0001	0.5014	0.9873	0.9839	0.9760
SEC	<0.0001	0.7527	0.9463	0.9319	0.9093
Deff	<0.0001	0.8906	0.9908	0.9875	0.9819
dE	<0.0001	0.3059	0.9249	0.8703	0.7910

shows the basic statistical parameters for the determined models. It is evident that all the models’ results were significant and possess a high coefficient of determination.

The effect of ultrasound (as a drying agent, not a pretreatment) on the drying kinetics of leafy vegetables had not yet been studied. The microwave-assisted convective drying of leafy vegetables such as kale, spinach, and cabbage can be influenced by air temperature and microwave power, affecting the drying rate and diffusion coefficient. The drying rate and diffusion coefficient are affected by the air temperature and microwave power. Higher microwave power and air temperature can result in a shorter drying time and higher drying rate [48,49]. The effective moisture diffusivity and mass transfer coefficient increase with higher microwave power and a higher loading amount [50]. The use of microwave pre-drying can significantly increase the drying rate and shorten the drying time [51]. Additionally, microwave pre-drying can improve the quality of the dried product, resulting in less nutrient loss [52]. Overall, microwave-assisted convective drying with higher microwave power and air temperature can lead to faster drying rates and improved quality of leafy vegetables.

3.2. Influence on Color

After the drying processes, the kale leaves were subjected to quality analysis. First, the samples were assessed in terms of color change. Figure 5 shows the results of the quality parameter related to color, i.e., the relative color changes dE after drying at 50 and 70 °C.

Regardless of the analyzed schedules, the drying operation resulted in a significant color change in kale leaves. In nearly all cases, the delta E (dE) value exceeded 30, indicating a noticeable color change. It was observed that all samples treated with hybrid methods exhibited high dE values. When drying at a lower temperature (as depicted in Figure 5a), a reduction in dE with an augmentation in ultrasound and microwave power was detected. Drying at a higher temperature (Figure 5b) resulted in an increase in color change as the power of the microwaves increased. Contrastingly, ultrasound did not exhibit any noteworthy trend in dE. Drying in combined processes (CMU) caused the total color change to increase only by a few units, up to a value of 37 on average, in comparison to the CU and CM processes. Similar findings were reported by Alibas [53] for collard leaves (Brassica oleracea, var. acephala).

According to the analysis, the utilization of a non-linear function leads to the highest correlation between dE and ultrasound or microwaves. The fit statistic and p-value for dE were satisfactory (refer to Table 3).

Microwave-assisted convective drying has been found to have an effect on the color of dried leafy vegetables. In the study by Khodifad et al., the drying of coriander leaves using continuous microwave drying resulted in the best quality dried coriander leaves with the maximum retention of sensory attributes, including color [54]. Similarly, Izli et al. reported that the color of pepinos decreased with increasing drying temperature during convective drying and microwave drying [55]. Lüle and Koyuncu found that convective drying at 70 °C and microwave drying at higher power levels yielded better color retention in nettle leaves [56]. These findings suggest that microwave-assisted convective drying can have an impact on the color of dried leafy vegetables, with lower temperatures and higher power levels generally resulting in better color retention.

3.3. Influence of Drying on Bioactive Compounds

The next quality parameters assessed in these studies were the content of ascorbic acid, polyphenols, and natural dyes, such as carotenoids and chlorophylls (A and B).

Table 4. Relative change and synergy effect for particular quality parameters.

	Relative Change (%)
Process	AAR	TPR	TCR	ChAR	ChBR
C_5 (Absolute values in mg/100g FW)	35.41 ± 1.37	10.99 ± 0.56	7.57 ± 0.90	57.37 ± 4.11	28.20 ± 3.26
CM_51	6	−17	55	25	−1
CM_52	14	−9	42	7	−18
CU_51	14	1	20	1	−26
CU_52	19	−6	15	12	−17
CMU_511	3	−11 *	18	23	6 *
CMU_512	13	−9 *	20	24	9 *
CMU_521	14	−9	75 *	8	−21 *
CMU_522	13	−5 *	88 *	24 *	−6 *
C_7 (Absolute values in mg/100g FW)	37.60 ± 0.71	10.37 ± 0.20	10.68 ± 1.16	66.26 ± 2.00	30.76 ± 0.29
CM_71	3	3	61	25	2
CM_72	7	3	21	3	−14
CU_71	−10	−4	52	24	−1
CU_72	−10	−18	46	19	−10
CMU_711	12 *	14 *	106	45	14 *
CMU_712	6 *	4 *	100	45 *	20 *
CMU_721	10 *	10 *	107 *	47 *	21 *
CMU_722	21 *	13 *	77 *	33 *	22 *

C—convection, M—microwaves, U—ultrasound, AAR—ascorbic acid retention, TPR—total polyphenol retention, TCR—total carotenoid retention, ChAR—chlorophyll A retention, ChBR—chlorophyll B retention, and *—synergy effect.

shows the results of relative change in the ascorbic acid, total polyphenol, total carotenoid, and chlorophyll A and B retention, as well as the synergy effects for certain processes, meaning higher values in total compared to processes with only ultrasound or microwave assistance.

According to Table 4, increasing the air temperature leads to the greater retention of ascorbic acid, polyphenols, carotenoids, and chlorophylls in dehydrated kale leaves. Convective drying processes (C_5 and C_7) have the lowest vitamin C retention due to prolonged exposure to an oxygen-rich atmosphere compared to hybrid processes. Korus [5] also reported a loss of approximately 30% in ascorbic acid when kale was dried using hot air/oven methods. Thus, prolonged dehydration and a slower drying rate resulted in the highest losses. The amount of retained bioactive components was also influenced by the use of ultrasound and microwave power during convective drying. However, overall, there was an increasing trend in the retention of vitamin C and natural colors. A combination of hot air, microwaves, and ultrasound resulted in a significantly higher content of vitamin C in green pepper after hybrid drying [57]. Similarly, the microwave–convective drying of parsley and lovage leaves resulted in high stability of green pigments (up to 94%) [58].

Worth noting is the synergistic effect observed in all CMU processes at 70 °C and most of them at 50 °C. The phenomenon is evidenced by the fact that the overall reduction in the quantity of a particular bioactive component during the separate execution of the hybrid processes (CM + CU) surpasses that during their simultaneous application (CMU). The synergistic effect is not easily explained and is a result of the mutual interaction of microwaves and ultrasound. Further investigation is underway to establish the mechanism of these interactions.

Figure 6 shows Spearman’s rank correlation between the drying time (DT), drying rate (DR), maximum sample temperature (T_max), rate of temperature increase (T_max/t), content of ascorbic acid (AA), total polyphenols (TP), total carotenoids (TC), and chlorophylls A and B (Ch_A and Ch_B). As recommended by Evans [59], a strong correlation occurs when the coefficient value is above 0.6 (positive) or below −0.6 (negative).

It is evident that certain process parameters had an impact on the quality parameters. The most significant negative correlation (−0.78) was found between drying time and carotenoid content, while a strong positive correlation was observed between this quality parameter and the drying rate. Likewise, weaker yet significant correlations were noted for ascorbic acid content, which increased with the rise in the drying rate (0.61) or the reduction in drying time (−0.63). Long-lasting drying may result in the loss of carotenoids and ascorbic acid due to thermal degradation and oxidation.

The microwave- and/or ultrasound-assisted convective drying of kale had not been reported in the literature. Studies conducted on other leafy products have indicated a contradictory relationship between the content of bioactive compounds and drying conditions. Feng et al. [22] determined that an escalation in the rate of microwave drying (radiation power and/or air temperature) leads to an amplified loss of chlorophyll in lettuce. Dadali and Ozbek [31] discovered that increasing the microwave power–drying rate produces a rise in the loss of ascorbic acid in both okra and spinach; the degree of the quantitative impact also relied on the weight of the load.

Similar findings were presented by Tao et al. [20] regarding the ultrasound-assisted convective drying of white cabbage. They reported that an increase in the drying rate (acoustic power) resulted in a shorter drying time but also caused a reduction in ascorbic acid content. However, some researchers have indicated a beneficial impact of the drying rate on the concentration of bioactive compounds. Yanyang and co-workers [21] reported that the content of ascorbic acid and chlorophyll in wild cabbage increased following drying through forced convection and the microwave–vacuum technique. Similarly, Ozkan et al. (2015) [30] found that an increase in the drying rate during the microwave drying of spinach led to an increase in ascorbic acid content.

The negative correlation between the drying time and bioactive compound content, along with the positive correlation between the drying rate and compound content, indicates that quickly drying at high temperatures is advantageous. This is likely due to the temperature degradation and oxidation processes previously mentioned, but it depends on various energy sources. The use of microwaves and/or ultrasound accelerates the drying process, reducing drying time without a significant increase in the material temperature. This hypothesis was supported by a moderate or weak correlation between T_max and the retention of analyzed compounds. The high drying rate, resulting in shorter drying times and a minimal temperature increase, allows for the greater preservation of bioactive compounds in hybrid drying schedules. Figure 6 displays two robust correlations between the levels of ascorbic acid (AA) and chlorophyll A (Ch_A) and the increase in temperature (T_max/t). These results suggest that these factors are influenced more by the rate of temperature increases, rather than the maximum temperature.

Microwave power has a significant effect on the content of ascorbic acid, polyphenols, carotenoids, and chlorophyll during the microwave-assisted convective drying of leafy vegetables. Higher microwave power levels generally result in the better retention of these bioactive compounds. For example, in the drying of fenugreek leaves, antioxidant activity was better preserved at higher microwave power levels compared to conventional drying methods [60]. In the drying of bitter gourds, the application of microwave power resulted in a lower reduction of vitamin A and better retention of the surface color [61]. In the drying of carrot pomace, microwave drying enhanced the preservation of carotenoids and ascorbic acid compared to convective air drying [62]. In the drying of parsley leaves, the drying parameters, including microwave power, influenced the total phenolic content, antioxidant activity, chlorophyll, and lutein contents [63]. In the cooking of chicory leaves, microwave cooking increased the number of phenolic compounds, carotenoids, and lipid peroxidation, while chlorophyll and pigment contents decreased [64].

4. Conclusions

The utilization of a central composite design (CCD) in leafy vegetable drying has proven successful, determining optimal conditions for enhanced product quality and preservation. Exploring the combined effects of ultrasound and microwaves on kale leaf drying kinetics revealed the dominance of microwaves over ultrasound. Hybrid processes like convective drying with microwaves (CM) and convective drying with microwaves and ultrasound (CMU) demonstrated higher drying rates and greater energy efficiency compared to conventional methods.

The impact of ultrasound and microwaves on the effective diffusion coefficient identified temperature and radiation energy as influential factors. The hybrid microwave–ultrasound (CMU) process at 70 °C exhibited the highest effective diffusion coefficient, indicating improved mass transfer during drying.

Analyzing color changes in dried kale leaves demonstrated that microwave-assisted convective drying significantly affected color retention, with lower temperatures and higher microwave power enhancing color preservation. Ultrasound’s incorporation into convective drying (CU) increased energy consumption, while microwave-assisted convective drying (CM) and convection assisted with microwaves and ultrasound (CMU) proved more energy-efficient.

The evaluation of bioactive compounds in dried kale leaves revealed that higher air temperatures, coupled with hybrid processes, enhanced the retention of ascorbic acid, polyphenols, carotenoids, and chlorophylls. The synergistic effects observed in CMU processes emphasized the benefits of simultaneous microwaves and ultrasound application.

Spearman’s rank correlation analysis established significant relationships between process and quality parameters. The drying time, rate, and temperature notably influenced the bioactive compound retention. Microwave power emerged as a critical factor impacting the content of ascorbic acid, polyphenols, carotenoids, and chlorophylls during the microwave-assisted convective drying of green leafy vegetables. In conclusion, this comprehensive investigation provides valuable insights for optimizing drying processes, contributing to the production of high-quality, nutritionally enhanced dried leafy vegetables.