Effect of Radio-Frequency Drying on the Physicochemical Properties and Isoflavone Contents of Fermented Black Bean Dregs

Abstract

We treated dry black bean dreg fermentation products with radio-frequency (RF) waves. With RF treatment (RF output power of 3 kW and electrode gap of 12 cm), a sample was dried within 1 h, which would take 10 h to dry via hot-air drying at 60 °C (sample weight reduction from 100 to 28.2 g). RF treatment thus accelerated fermented black bean dreg drying approximately 10-fold. The fermented black bean dregs were dried for 60 min at a fixed RF output power of 3 kW with different electrode gaps (12, 16, and 20 cm). Samples treated with electrode gaps of 12 and 16 cm were completely dried. When the fermented black bean dregs were dried for 60 min with an RF output power of 3 kW with a 16 cm electrode gap, their water activity decreased from 0.98 to 0.65. Colorimetric analysis showed that the sample color darkened significantly (ΔE = 5.73 ± 2.71), whereas its solubility (28.8 ± 0.1%) and antioxidant capacity (EC₅₀ = 51.7 ± 0.7 mg/mL) increased. In addition, deglycosylated isoflavone daidzein content significantly increased, whereas that of the glycosylated isoflavone genistin decreased. These results reveal that RF application can efficiently dry fermented black bean dregs.

1. Introduction

In recent years, processing technologies such as infrared heating, microwave heating, and radio-frequency (RF) heating have been applied to the food industry. RF heating has the advantages of rapid heating and favorable penetration, and it is a dielectric drying method that can be used for food [1,2]. Traditional food drying methods rely on conduction or convection to transfer heat energy from the heat source to the food surface, whereas RF processing is a volumetric heating method in which electromagnetic waves are directly coupled with the food to generate thermal energy. When a dielectric material with polar molecules and charged ions is subjected to an alternating electric field, the positive ions in the material are displaced to the negative region of the electromagnetic field, and the negative ions are displaced to the positive region of the electromagnetic field, a process referred to as ionic migration. In addition to the migration of charged ions, dipolar molecules, such as water molecules, tend to align according to the alternating polarity as the electric field changes, which is known as dipole rotation. In both ion migration and dipole rotation, heat energy is produced owing to friction, which promotes the loss of moisture in the food to achieve drying or sterilization effects [3]. In RF, microwave, and infrared heating, electromagnetic waves are used to heat the target object for drying purposes. The main difference lies in the frequency. The frequency of RF heating is 10⁵ to 10⁷ (Hz), the frequency of microwave heating is 10⁷ to 10¹⁰ (Hz), and the frequency of infrared heating is 10¹⁰ to 10¹² (Hz). Among the three, the penetrating power of RF is better than that of microwaves and infrared rays [4]. The results of Gou et al. [5] indicated that after drying purple potatoes with infrared, microwave, and RF heating, the samples treated with RF heating dried faster than those treated with infrared heating and retained more phytochemicals, such as total phenols and isoflavones, among others, resulting in improvements in the antioxidant capacity and DPPH and ABTS free radical-scavenging activities.

Jiang et al. [6] treated strawberries with RF drying, hot-air drying at 70 °C, freeze drying, and microwave drying. Here, the drying rate was higher for strawberries treated with RF drying and microwave drying compared to that with hot-air drying and freeze drying. Moreover, strawberries subjected to RF drying had better color and higher carotenoid and total phenolic contents. The results of Zhang et al. [7] indicated that when mango slices were subjected to hot-air-assisted RF drying, the moisture content was reduced from 40% to 18% after drying for 45 min with an electrode gap of 7.5 cm, a sample thickness of 4.5 cm, and hot air at 60 °C. The drying efficiency of this method was also better than that with hot-air drying or vacuum drying alone, with advantages such as uniform moisture distribution and a decrease in the loss of vitamin C contents. Chen et al. [8] suggested that when drying bean dregs, the rate of drying with a combination of RF treatment and cold air drying at 45 °C is increased by 25 times compared to that with traditional cold air drying at 45 °C. Moreover, the rate of drying increased as the sample amount decreased. In addition, the RF-dried samples had higher total phenolic contents and better DPPH free radical-scavenging activity.

Soybean [Glycine max (L.) Merr.] is an excellent source of protein (such as glycinin and β-conglycinin) and isoflavones (such as genistin, daidzin, and glycitin) for humans. Roughly 10% of the world’s soybean supply is used in the processing of other products, such as soymilk and tofu. Black soybean [Glycine max (L.) Merr.] is a soybean varietal with a black seed coat, which is widely used to produce soybean products. The term black bean dregs refers to a by-product of processing black soybeans to make commercial products, with a moisture content of approximately 81.7–84.5%. These dregs are rich in nutrition, with dietary fiber accounting for 42.4–58.1%, protein accounting for 15.2–33.4%, fat accounting for 8.3–10.9%, carbohydrates accounting for 3.9–5.3%, and ash accounting for 3.0–4.5% [9,10]. In recent years, guided by the concept of sustainable development, black bean dregs have also been used for microbial fermentation. Black bean dregs can be used to produce a fermented soy product referred to as Tempeh, which has been a staple in Indonesia for more than 300 years. Tempeh is usually made from soybeans fermented using Rhizopus spp. (Rhizopus oligosporus) and/or Yarrowia lipolytica [11,12]. Note that during fermentation, Rhizopus oligosporus produces β-glucosidase, while Yarrowia lipolytica produces extracellular and cell wall-bound lipases. During this process, the black soybeans are coated with a thick white mycelium. The hydrolyzation of biomacromolecules results in small-molecule nutrients with high bioavailability by fungal enzymes, thereby enhancing the nutritive value and functional properties of soybeans [13]. In addition, the Chinese traditional fermented okara (Meitauza) was produced by fermenting soybean dregs with Bacillus subtilis B2 in China [14]. In Indonesia, red oncom is generally made from soybean dregs through a fermentation process involving Neurospora sp. while black oncom is made from peanut dregs fermented by Rhizopus oligosporus and Mucor sp. [15,16]. The dietary fiber in bean dregs is mostly insoluble, and fermented black bean dregs are biologically active. The acidic environment during fermentation with lactic acid bacteria results in disruption of the glycosidic bonds in polysaccharides and the conversion of insoluble dietary fibers into soluble dietary fiber, which can be used to reduce blood cholesterol levels [17]. In addition, although the solubility of the protein in bean dregs is lower than that of soy milk protein, the physical and chemical characteristics can be changed through modifications in an acidic environment, such as the hydrolysis of protein peptide bonds and deamination, to improve its emulsifying properties, surface hydrophobicity, and foaming behaviors, among others, thus increasing the utilization rate of bean dreg proteins as a food ingredient [18]. Linoleic acid accounts for the majority of the black bean dreg oil contents, and it is an ω-3 essential fatty acid, which provides essential nutrients for the human body [19].

Drying facilitates the preservation of fermented black bean dregs, and traditional drying methods include hot-air drying, freeze drying, and roasting. Among them, hot-air drying and freeze drying take 12 h or even up to 56 h, whereas roasting at 150–200 °C significantly shortens the drying time to 50–75 min. However, during roasting, the temperature increase is uneven, and the black bean dregs can easily stick to the pan, agglomerate, and scorch, among other issues, which can easily change their appearance and flavor. RF heating has the advantages of rapid heating and strong penetration. Since research on the physical properties of fermented black bean dregs during RF drying is lacking, hot-air and RF drying were performed on fermented black bean dregs in this study to explore the effect of different RF treatment conditions, specifically the RF output power, electrode gap, and drying time, on the drying of this product. Moreover, changes in the moisture content, color, solubility, water activity, isoflavone levels, and antioxidant capacity during drying were analyzed to form a basis for the future application of RF treatment to drying fermented black bean dregs.

2. Materials and Methods

2.1. Preparation of Black Bean Dregs and Fermentation of Their Products

Black soybeans [Glycine max (L.) Merr.] were used as raw materials (purchased from Hongyu Food Enterprise Co., Ltd., Taipei City, Taiwan). Black soybeans (5 kg) were washed and soaked in water at a bean-to-water ratio of 1:5 in the refrigerator for 7 h. After soaking, a grinder (CH-102, Cheng Huei Machinery Co., Ltd., Taichung City, Taiwan) was used to grind the black soybeans. Then, an automatic filter (Chang Sheng Machinery Industry Co., Ltd., Taoyuan City, Taiwan) was used to separate the soybean milk and black bean dregs. A water spray retort sterilizer (CY-3000H-RD-770-1P, Chang Yu Machinery, Changhua, Taiwan) was used to heat the black bean dregs at 121 °C for 15 min, and then, the fermented black bean dregs were prepared according to the method of Zhang et al. [20]. The black bean dregs were mixed with a Rhizopus oligosporus spore solution (10⁵ spores/mL) at a ratio of 15:1 and homogenized for 3 min. The homogenized black bean dregs (150 g) were placed into a sterilized glass dish with a diameter of 15 cm and incubated at 30 °C for 24 h. After the culturing was completed, the black bean dregs were mixed with the bacterial liquid of Yarrowia lipolytica (10⁶ CFU/mL) at a ratio of 15:1 and homogenized for 3 min. The homogenized black bean dregs (150 g) were then placed in a sterilized glass dish with a diameter of 15 cm and incubated at 40 °C for 72 h to obtain the fermented black bean dregs (with a moisture content of 71.8 ± 0.3%).

2.2. Effect of Hot-Air and RF Drying on the Weight of Fermented Black Bean Dregs

Fermented black bean dreg samples (100 g) were placed in a laboratory oven (model RHD-452, Chuanhua Precision Co., Ltd., Taipei City, Taiwan) to dry at 60 °C for 10 h. The weight of the fermented black bean dregs was recorded every hour to assess the effect of hot-air drying on the sample. In addition, the fermented black bean dregs (100 g) were dried with an RF dryer (model EDB-5D, frequency 40.68 MHz, YH-DA Biotech Co., Ltd., Yilan, Taiwan). RF drying was performed for 1 h at an output power of 3 kW with an electrode gap of 12 cm. The weight of the fermented black bean dregs was recorded to assess the effect of RF treatment on the drying of the fermented black bean dreg sample. All experiments were repeated in triplicate.

2.3. Effect of RF Output Power and Electrode Gap on the Weight of Dried Fermented Black Bean Dregs

To explore the effect of different RF output powers and electrode gaps on the weight of dried fermented black bean dregs, these samples (100 g) were subjected to RF drying for 1 h, during which the weights were recorded. This experiment was performed in three stages. First, we assessed the effects of RF output power (1, 3, or 5 kW) on the weight of dried fermented black bean dregs using a fixed electrode gap (12 cm) and fixed processing time (60 min). Second, we assessed the effects of electrode gap (12, 16, or 20 cm) on the weight of dried fermented black bean dregs using a fixed RF output power (3 kW) and fixed processing time (60 min). Third, we assessed the effects of RF drying time (60 min) on the water activity, appearance, color, solubility, isoflavone content, and antioxidant activity of fermented black bean dregs using a fixed electrode gap (16 cm) and fixed RF output power (3 kW). All experiments were repeated in triplicate.

2.4. Determination of Water Activity and Color of Fermented Black Bean Dregs

Fermented black bean dregs (100 g) were dried for 1 h with an RF output power of 3 kW and an electrode gap of 16 cm. The samples were then collected and freeze-dried. The method of Wang et al. [21] was used to measure the water activity of fermented black bean dregs. For this, the sample was placed in a round plastic container (diameter, 4.5 cm × height, 1.5 cm), which was placed into a water activity meter (AQUA LAB, model CX-2, Decagon Devices Inc., Pullman, WA, USA) to measure the relative humidity of the sample. The water activity was calculated as follows: water activity = relative humidity/100. The color change in the fermented black bean dregs was measured with reference to the method of Dias et al. [22]. The color parameters (L*, a*, and b*) were measured with a colorimeter (Ci60, X-Rite Color Technology Co., Ltd., Grand Rapids, MI, USA). The L* value ranged from 0 to 100, where 0 indicated black and 100 indicated white. A negative a* indicated that the color was greenish, whereas a positive a* indicated that the color was reddish. A negative b* indicated that the color was bluish, whereas a positive b* indicated that the color was yellowish. The ΔE indicated the difference between the two colors, and its calculation formula was as follows: ΔE = (ΔL*² + Δa*² + Δb*²)^1/2.

2.5. Determination of Fermented Black Bean Dreg Solubility

The fermented black bean dregs (100 g) were dried at an RF output power of 3 kW with an electrode gap of 16 cm for 1 h. Samples were collected and freeze-dried. The solubility of the fermented black bean dregs was subsequently measured. The method of Chang et al. [23] was used, with modifications, to measure solubility. The sample (0.15 g) was mixed with 10 g of water with shaking in a centrifuge tube for 30 min, left to stand at 4 °C for 30 min, and centrifuged at 12,000× g and 4 °C for 20 min (CT15R, Hitachi, Tokyo, Japan). The supernatant was dried at 120 °C to a constant weight to calculate the solubility as follows: solubility (%) = (weight of supernatant/dry weight of sample) × 100.

2.6. Determination of Isoflavone Contents in Fermented Black Bean Dregs

Fermented black bean dregs (100 g) were dried at an RF output power of 3 kW and an electrode gap of 16 cm for 1 h. Samples were then collected and freeze-dried. The isoflavone content of the fermented black bean dregs was subsequently measured. Genistin and daidzein account for a high proportion of the isoflavones in fermented black bean dregs. Therefore, the contents of these molecules were analyzed according to the method of Hsiao and Hsieh [24]. For this, 100 mg of the sample was added to 1.0 mL of mixed solvent (acetone/water/acetic acid = 70:29.5:0.5, v/v) for extraction with oscillation at 30 °C for 3 h. Then, the mixture was centrifuged at 12,000× g and 4 °C for 20 min. The supernatant was used for high-performance liquid chromatography (HPLC) analysis. The column used for the analysis was a C18 column (Vydac 218TP54, 4.6 × 250 mm, 5 μm, Grace Vydac, Hesperia, CA, USA). The mobile phase was 100% acetonitrile and pure water. Gradient analysis was performed at a flow rate of 2 mL/min. The gradient condition was an increase in the acetonitrile content from 8% to 50% from 0 to 16 min, and the absorbance value at 262 nm was measured.

2.7. Determination of the Antioxidant Activity of Fermented Black Bean Dregs

Fermented black bean dregs (100 g) were dried at an RF output power of 3 kW and an electrode gap of 16 cm for 1 h. Samples were then collected and freeze-dried. The antioxidant activity of the fermented black bean dregs was measured. The ABTS radical-scavenging activity was measured as per the experimental method of Lee et al. [25], and Trolox was used as the standard. ABTS reagent (500 μL, 7 mM) was mixed with 8.8 μL of 140 mM potassium persulfate. The mixture was then diluted with 5 mM phosphate buffer solution, pH 7.4, until the absorbance value at a wavelength of 734 nm fell within 0.7 ± 0.02. Next, 25 μL of the sample or standard (Trolox) was added to 125 μL of the diluted ABTS reagent. The mixture was reacted in the dark at room temperature for 5 min, and the absorbance value at 734 nm was measured. ABTS radical-scavenging activity result was expressed as the half maximal effective concentration (EC₅₀). First, the following formula was used to calculate the ABTS radical-scavenging activity. Then, 50% of the scavenging rate was substituted into the linear regression equation of the sample scavenging rate to obtain the EC₅₀. The blank was a phosphate buffer solution, which was used to replace the sample.

$$\mathrm{ABTS}\mathrm{scavenging}\mathrm{activity}(\%)=(1-\frac{\mathrm{O}\mathrm{D}734\mathrm{of}\mathrm{sample}}{\mathrm{O}\mathrm{D}734\mathrm{of}\mathrm{blank}})\times 100\%$$

2.8. Statistical Analysis

Each set of data in this study comprised triplicate measurements, and the data were expressed as the mean ± standard deviation. The experimental data were subjected to a one-way analysis of variance using SAS (Statistical Analysis System, Version 9.4, SAS Institute Inc., Cary, NC, USA). Duncan’s multiple range test was performed to compare the differences between each group. A p-value < 0.05 indicated significant differences between each group of data.

3. Results and Discussion

3.1. Effects of Hot-Air and RF Drying on the Weight of Fermented Black Bean Dregs

The fermented black bean dregs (moisture content, 71.8 ± 0.3%) were dried with hot air at 60 °C. The sample weight decreased as the drying time increased. After heating for 10 h, the weight of the fermented black bean dregs was 28.2 g, and the sample was completely dried (Figure 1). In hot-air drying, hot air is blown onto the sample, leading to water evaporation from the sample surface due to heat, and the moisture is removed through the wind blowing, such that the sample is gradually dried from the outside to the inside. However, with RF drying (output power, 3 kW; electrode gap, 12 cm), it only took 1 h to completely dry the sample. The weight of the fermented black bean dregs was 28.2 g. In RF drying, between the parallel electrode plates, the rotation of polar molecules in the sample and the rapid movement of ions are converted into heat energy. Compared with traditional heating, RF drying can heat food more quickly and more evenly. Moreover, hot air can be combined to bring out the moisture and accelerate the removal of moisture. The aforementioned findings showed that RF drying is more efficient than hot-air drying and that the drying time was shortened by 90%. RF drying is an alternative to heat treatment that can be used to heat various foods more uniformly with a high heating rate. Moreover, it can be used to dry various foods [26,27]. Zhou and Wang [3] pointed out that RF drying is characterized by volumetric heating, deep penetration, and moisture self-balance effects and is, therefore, an effective and practical dielectric drying method for food. Shewale et al. [28] also used RF processing combined with low-humidity air to dry apples. Such methods can be applied to the drying of foods with poor thermal stability.

3.2. Effects of the RF Output Power and Electrode Gap on Fermented Black Bean Dreg Drying

To explore the drying conditions for RF treatment, the effect of different RF output powers on fermented black bean dreg drying was investigated (Figure 2). As shown in Figure 2a, the fermented black bean dregs were dried for 60 min with a fixed electrode gap (12 cm) and an RF output power of 1, 3, and 5 kW. After treatment with an RF output power of 1, 3, and 5 kW, the sample weight decreased as the drying time increased. After heating for 60 min, the weight of the fermented black bean dregs was 40.2 ± 3.4, 30.3 ± 0.2, and 28.5 ± 0.0 g, respectively. An RF output power of 3 and 5 kW completely dried the samples. Zhang et al. [29] pointed out that during RF drying, energy consumption is affected by the drying time. Therefore, when drying for 60 min, the energy consumption with an RF output power of 3 kW is lower than that with 5 kW. In addition, after the fermented black bean dregs were treated with an RF output power of 1, 3, and 5 kW for 60 min, the sample temperatures were 55.5 ± 0.5, 59.0 ± 1.0, and 63.5 ± 0.5 °C, respectively (Figure 2b), with the temperatures of the fermented black bean dreg treatments as follows: 5 kW > 3 kW > 1 kW. This indicated that the temperature of the fermented black bean dregs also increased as the RF output power was increased during drying. Huang et al. [30] showed that RF treatment is a dielectric heating method in which RF energy is converted into thermal energy within the material to heat the material through ionic conduction and dipole rotation.

In this study, the effect of different electrode gaps on the drying of fermented black bean dregs was also explored (Figure 3). As shown in Figure 3a, the fermented black bean dregs were dried for 60 min at a fixed RF output power of 3 kW with electrode gaps of 12, 16, and 20 cm. The sample weight decreased as the drying time increased. After heating for 60 min, the weight of the fermented black bean dregs was 29.1 ± 0.2, 30.0 ± 0.4, and 33.7 ± 0.4 g, respectively. The results showed that a lower electrode gap facilitated the drying of the fermented black bean dregs. Furthermore, an electrode gap of both 12 and 16 cm completely dried the samples. Zhang et al. [29] demonstrated that during RF drying, different electrode gaps would affect the dehydration rate of the sample and that a lower electrode gap would lead to a higher dehydration rate. In addition, after the fermented black bean dregs were dried for 60 min with electrode gaps of 12, 16, and 20 cm, the temperature of the fermented black bean dregs also increased as the electrode gap was decreased. After heating for 60 min, the temperature of the samples was 57.0 ± 1.0, 55.0 ± 0.0, and 50.5 ± 0.5 °C, respectively (Figure 3b). The temperatures of the fermented black bean dregs with different electrode gaps were ranked as follows: 12 cm > 16 cm > 20 cm. The results showed that a closer electrode gap was associated with a higher temperature of the sample.

3.3. Effect of RF Drying Time on the Water Activity, Appearance, Color, and Solubility of Fermented Black Bean Dregs

The effect of RF drying time on the physical and chemical properties of fermented black bean dregs, including water activity, appearance, color, and solubility, was explored. As shown in Figure 4, the fermented black bean dregs were dried for 60 min at an RF output power of 3 kW and with an electrode gap of 16 cm. The water activity of the sample decreased with an increase in the drying time and decreased to 0.87 ± 0.01 after 50 min of RF treatment, further declining to 0.65 ± 0.02 after 60 min of RF drying. Wang et al. [31] showed that RF drying can be used to dehydrate Hawaiian beans, reducing the water activity and extending the shelf life. Peleg [32] suggested that a water activity below 0.9 is not suitable for the growth of most organisms, including bacteria, yeasts, and molds. Therefore, when the water activity of the fermented black bean dregs is reduced to 0.65, the growth of most microorganisms is fully inhibited, which could extend the storage time of the fermented black bean dregs. RF technology is considered a form of rapid heating. Chen et al. [33] reported that an electrode gap of 6 cm was sufficient for drying a small sample of rice bran (1 kg) by inducing a temperature increase from 25 °C to 100 °C in 2 min. Note that the RF process also proved effective in stabilizing the rice bran. In the current study dealing with small samples (100 g) of fermented black bean dregs, we adopted an electrode gap of 16 cm with an RF output power of 3 kW and a processing time of 1 h. Under these drying conditions, RF processing proved highly effective in drying the fermented black bean dregs, as indicated by a reduction in water activity to 0.65. This process significantly reduced moisture content, sample weight, and the water activity of the fermented black bean dregs. Note that RF drying is superior to freezing or commercial sterilization in terms of product preservation and transportability.

In this study, the effect of the RF drying time on the appearance and color of fermented black bean dregs was also explored.

Table 1. Effect of radio-frequency (RF) drying time on color differences in fermented black bean dregs.

RF Drying Time	Colorimetric Analysis
(min)	L*	a*	b*	ΔE*
0	21.13 ± 0.15 a	4.24 ± 0.04 a	7.96 ± 0.06 a	0 ± 0 a
10	21.84 ± 0.24 a	4.26 ± 0.02 a	7.94 ± 0.04 a	0.71 ± 0.29 a
20	22.48 ± 0.40 a	4.32 ± 0.02 a	8.06 ± 0.11 ab	1.35 ± 0.71 a
30	25.91 ± 0.93 b	4.47 ± 0.04 ab	8.72 ± 0.09 bc	4.83 ± 1.30 b
40	26.81 ± 0.88 b	4.67 ± 0.08 bc	9.15 ± 0.14 cd	5.81 ± 1.21 b
50	26.58 ± 0.38 b	4.79 ± 0.10 bc	9.31 ± 0.21 cd	5.63 ± 0.51 b
60	26.62 ± 1.08 b	4.85 ± 0.13 c	9.51 ± 0.27 d	5.73 ± 2.71 b

In the same column, values with different lowercase letters (a–d) are significantly different (p < 0.05).

shows the effect of the RF drying time on the color of fermented black bean dregs. As the RF drying time increased, the L*, a*, and b* values all increased. After drying for 60 min, the L*, a*, and b* values of the sample increased from 21.13 ± 0.15, 4.24 ± 0.04, and 7.96 ± 0.06 to 26.62 ± 1.08, 4.85 ± 0.13, and 9.51 ± 0.27, respectively. Moreover, the L*, a*, and b* values of the dried samples all increased significantly (p < 0.05). Bhavya and Prakash [34] suggested that the L* value represents the lightness of the color, with a larger L indicating a brighter color. The a* value represents the color intensity on the axis of blue to red, whereas the increase in the b* value represents an increase in the yellow intensity. Our results showed that after drying, the color of fermented black bean dregs became reddish and yellowish, with increased lightness. We also found that the ΔE of the fermented black bean dregs also increased as the RF time increased (i.e., the ΔE of the fermented black bean dregs increased to 5.73 ± 2.71 after drying for 60 min; Table 1). Mokrzycki and Tatol [35] showed that when 1 > ΔE > 0, the observer would not notice the color difference between the samples. Meanwhile, when ΔE > 5, the observer would notice that the sample colors are different. Thus, the ΔE might be used as a reference for color changes. Furthermore, a ΔE > 5 indicates a color difference distinguishable by the unaided eye. As shown in Table 1, when the RF time exceeded 40 min, the ΔE (5.81 ± 1.21) was greater than 5, indicating that the color change of the fermented black bean dregs could be observed. Figure 5 shows the effect of the RF drying time on the appearance and color of fermented black bean dregs. From the photos taken during the sample RF drying process, after 40 and 60 min, the color had changed, with significant differences from that in the undried group (Figure 5c,d). In addition, the effect of the RF drying time on the solubility of fermented black bean dregs was also explored in this study. As shown in

Table 2. Effect of radio-frequency (RF) drying time on the solubility and antioxidant capacity of fermented black bean dregs.

RF Drying Time (min)	Mean Solubility (%)	EC50 of Trolox Equivalent Antioxidant Capacity (mg/mL)
0	24.5 ± 0.3 a	43.6 ± 0.9 a
10	27.6 ± 0.4 b	52.1 ± 0.6 c
20	28.1 ± 1.2 b	52.5 ± 1.7 c
30	28.2 ± 0.6 b	55.0 ± 1.0 d
40	28.6 ± 0.6 b	47.6 ± 1.0 b
50	28.9 ± 0.1 b	50.7 ± 1.5 c
60	28.84 ± 0.1 b	51.7 ± 0.7 c

In the same column, values with different lowercase letters (a–d) are significantly different (p < 0.05).

, the solubility of the sample after RF drying treatment for 10, 20, 30, 40, 50, and 60 min was higher than that in the group without drying (p < 0.05). Guo et al. [36] reported that the RF drying induced notable changes in free sulfhydryl groups. It was also shown to increase the surface hydrophobicity (H₀) of secondary and tertiary structures in soy protein isolate. Voutsinas et al. [37] reported that surface hydrophobicity was positively correlated with protein emulsifying capacity. This means that the rapid heating and surface group modification induced by RF drying could theoretically be used to enhance the solubility and emulsifying capacity of proteins. Hassan et al. [38] suggested that the solubility of corn protein increases after RF drying. This could be because RF drying increases the activity of proteolytic enzymes, leading to protein hydrolysis and increased solubility.

3.4. Effect of RF Drying Time on the Isoflavone Content and Antioxidant Activity of Fermented Black Bean Dregs

Glycosylated isoflavones (genistin) and deglycosylated isoflavones (daidzein) account for high proportions of the isoflavones in fermented black bean dregs. Therefore, the effect of the RF drying time on genistin and daidzein contents in the fermented black bean dregs was explored. Figure 6 shows that when the fermented black bean dregs were dried for 60 min at an RF output power of 3 kW and with an electrode gap of 16 cm, the genistin content decreased from 557.0 ± 6.0 μg/g to 391.5 ± 0.5 μg/g as the RF drying time was increased (Figure 6a), whereas the daidzein content increased from 162.0 ± 7.4 μg/g to 266.9 ± 4.9 μg/g with increases in the RF drying time (Figure 6b). Yen and Chen [39] reported on the pasteurization and drying of fermented soybean residue mixed with rice bran using hot-air-drying or RF treatment. After hot-air-drying treatment at 45 °C, they obtained the following results: total polyphenols (3.55 mg gallic acid equivalent/g DW), flavonoids (0.33 mg quercetin equivalent/g DW), DPPH free radical-scavenging (92.11%), and ferrous iron chelation (90.98%). After RF drying, they obtained the following results: total polyphenols (3.54 mg gallic acid equivalent/g DW), flavonoids (0.38 mg quercetin equivalent/g DW), DPPH free radical scavenging (93.15%), and ferrous iron chelation (91.35%). Hsiao and Hsieh [24] showed that the genistin content of black soybean milk decreases during soaking and heating, but the daidzein content increases. Our findings showed that during the RF drying of fermented black bean dregs, the daidzein content increased significantly after 60 min, but the genistin content decreased significantly.

In addition, the effect of the drying time on the antioxidant activity of fermented black bean dregs was also explored. Trolox was used as the standard for analyzing the antioxidant activity (Table 2). After the fermented black bean dregs were dried via RF treatment for 0, 10, 20, 30, 40, 50, and 60 min, the EC₅₀ values were 43.6 ± 0.9, 52.1 ± 0.6, 52.5 ± 1.7, 55.0 ± 1.0, 47.6 ± 1.0, 50.7 ± 1.5, and 51.7 ± 0.7 mg/mL, respectively. Moreover, the antioxidant activity of the fermented black bean dregs after RF drying for 10, 20, 30, 40, 50, and 60 min was lower than that in the undried group (p < 0.05). Nicoli et al. [40] suggested that degradation during thermal processing might decrease the antioxidant activity of thermally unstable antioxidants. Studies by Liu et al. [41] and Shen et al. [42] showed that the antioxidant capacity of dragon fruit and Sichuan pepper samples is decreased after RF drying, which is similar to the results of this study. It was speculated that this could be due to the degradation of phenolic compounds mediated by the heat generated during RF drying. In the current study, RF drying was also shown to induce a slight increase in the antioxidant activity of fermented black bean dregs. Note that this can perhaps be attributed to changes in the content of isoflavones, anthocyanins, and/or total phenolic compounds. Nonetheless, further research will be required to elucidate the mechanisms underlying these effects.

4. Conclusions

In this study, the samples were completely dried with RF drying at an RF output power of 3 kW and with an electrode gap of 16 cm for 1 h. The water activity of the sample was reduced to 0.65. Compared with traditional hot-air drying, RF drying had better drying efficiency for fermented black bean dregs. After the RF drying of fermented black bean dregs for 1 h, the L*, a*, and b* values of the sample all increased and the sample color was significantly different from that of the undried group. Moreover, the solubility of the sample was higher than that of the undried group. In addition, daidzein contents in the dried samples were higher than those in the untreated samples, whereas the genistin content and antioxidant capacity were lower than those in the untreated samples. In summary, RF drying can be used to dry fermented black bean dregs more rapidly.