Influence of Radio Frequency Heating on the Pasteurization and Drying of Solid-State Fermented Wolfiporia cocos Products

Rice bran and soybean residue are high in nutrients and active ingredients. They are used as media in the solid-state fermentation of Wolfiporia cocos. They not only reduce raw material costs, but also raise the economic value and applications of soybean residues and rice bran. After 30 days of fermentation, the moisture content (w.b.) of the W. cocos product was approximately 40%, requiring it to be pasteurized and dried later. The objective of this research is to use radio frequency (RF) rapid heating technology to pasteurize and dry the solid-state fermented product. A 500 g bag of solid-state fermented W. cocos product took only 30 and 200 s at the RF electrode gap of 15 cm to pasteurize and reduce the moisture content (w.b.) below 15%, respectively; therefore, the methods can be used instead of the traditional 60 min autoclave sterilization and 100 min hot air drying at 45 °C. After RF treatment, the fermented W. cocos product was white, indicating that browning was prevented; the product contained 5.03% mycelium, 9.83% crude polysaccharide, 4.43% crude triterpene, 3.54 mg gallic acid equivalent/g dry weight (DW) of total polyphenols, and 0.38 mg quercetin equivalent/g DW of flavonoid contents and showed a good antioxidant capacity.


Introduction
Wolfiporia cocos is a medicinal and edible fungi that is mainly harvested in China. Polysaccharides, triterpenoids, and bioactive compounds are abundant in W. cocos. The current demand for fungal health foods is high, with their market growing each year. If grains can be used as solid-state fermented media to provide suitable carbon and nitrogen sources for W. cocos, the mycelial growth and production of biologically active metabolites can be promoted [1]. The biofunctions of W. cocos include tumor inhibition [2], immunity improvement [3], and anti-inflammatory, antiaging, hypoglycemic [4], hypolipidemic, antibacterial [5], and antioxidant capacities. Soybean residues from soybean milk production can be rapidly dried by radio frequency (RF) energy [6], and rice bran from milled brown rice can be stabilized by RF heating [7]. When soybean residues and rice bran are mixed in a 1:1 ratio to form a solid-state medium for W. cocos fermentation, the cost of the process is reduced and a mycelial product rich in polysaccharides and triterpenoids is produced.
RF treatment is a type of dielectric heating. When food is placed between parallel top and bottom electrodes, the polar water molecules and charged ions in food can absorb electromagnetic radiation to generate heat via dipolar polarization and ionic movement. As a result, RF heating can overcome the issues caused by heat conduction and the convection of hot air heated from the outside to the inside, and RF energy provides volumetric and more uniform overall heating, deep penetration, and a moisture self-balance effect [8][9][10]. Because RF heating is an emerging processing technology, RF equipment has been used to study the rapid sterilization and drying of agricultural products or food [11].
The dielectric properties of vegetable powders, such as onion, chill, broccoli, tapioca flour, and potato starch, decrease with frequency and compacted density and increase with

Sample Preparation 2.3.1. Maintenance and Pre-Activation of W. cocos
Wolfiporia cocos (BCRC 36022) was inoculated on a PDA plate medium and cultivated in a 25 • C incubator for 7 days. The strain was cut into 1 cm 2 pieces and the pieces were inoculated in a 500 mL flask with 150 mL of presterilized PDA medium at 25 • C and 150 rpm shaking for 7 d of pre-activation.

Solid-State Fermentation of W. cocos
The RF-dried soybean residue [6] and RF-stabilized rice bran [7] were mixed as a 1:1 ratio to form a solid-state medium (40% moisture content) in a plastic bag, and then sterilized in a 121 • C autoclave. After cooling, 10 mL of the pre-activated W. cocos solution was inoculated into 500 g of the solid-state medium and cultured for 30 d at 25 • C in an incubator. The polypropylene (PP) plastic bag (8 × 9 × 29 cm) with 500 g solid-state fermented W. cocos product was horizontally placed between two parallel electrode plates in the hotair-assisted RF equipment, and the RF output power was measured for different electrode plate gaps. Because the maximum current and output power of the 5 kW, 40.68 MHz RF equipment were 1.6 A and 5 kW, respectively, the output current (I, A) was measured three times and the output RF power was calculated using the following formula: power output (kW) = (I/1.6) × 5

RF Pasteurization
The solid-state fermented W. cocos products were heated by RF energy and samples were taken out every 10 s, and the surface temperature at three locations was measured with an infrared thermometer. The fermented products of various RF pasteurization times were inoculated on a PDA plate and cultured for 7 days in a 25 • C incubator to observe the lethal situation of W. cocos.  (20 s) to measure the weight change with an electronic balance and surface temperature with an infrared thermometer to determine the drying curve and temperature profile during RF drying. Moreover, the center and internal (2 cm from the edge) temperatures of the sample were determined by inserting a fiber optic thermometer. The PP plastic bag containing 500 g of solid-state fermented W. cocos product was heated in a 121 • C autoclave for 60 min, and then it was dried in a 45 • C hot-air-drying apparatus for 180 min. To determine the drying curve and temperature profile during hot air drying, the weight and temperature changes of the sample were measured at fixed time intervals.

Extraction of Solid-State Fermented W. cocos Products
The solid-state fermented W. cocos product was weighed to obtain a 2.5 g sample, and 50 mL of water or ethanol was added for 5 min microwave extraction, as described by Chen and Chen [25]. The hot water or ethanol extract was freeze-dried and dissolved to prepare the 20 mg/mL hot water or ethanol extract.
2.6. Analytical Methods 2.6.1. Moisture Content Weigh 5 g of ground almonds in an aluminum dish and dry them in an oven at 105 • C for 12 h; then, remove and weigh them after reaching a constant.
where W f is the weight (g) of the dried sample and W i is the initial weight (g) of the sample. The weight (W t ) of sample during drying was measured, and then the dry basis moisture content of the sample during drying was calculated with the following equation: Moisture content (dry basis) = (W t − W o )/W o (g water/g dry material) (4) where W t is the weight (g) of the sample at drying time t and W o is calculated by W i × solid content.

Mycelium Determination of Solid-State Fermented W. Cocos Products
The ergosterol content represents mycelium in the mycelial fermentation product, following the process described by Chang et al. [26], with some modifications. The methanol supernatant from extract was filtered with a 0.22 µm membrane after centrifugation; its ergosterol content was determined by HPLC using a C18 column (250 × 4.6 mm) using 100% methanol as the mobile phase, a flow rate of 1 mL/min, an injection volume of 20 µL, and a UV detector at 282 nm. With some modifications, the crude polysaccharide content was determined using the method described by Dubois et al. [27]. The hot water extract of the dried sample was mixed with four volumes of 95% ethanol and stirred the mixture vigorously, which was then collected by centrifugation at 5000× g for 20 min. The crude polysaccharide precipitate was washed twice with 95% (v/v) ethanol and then dried at 80 • C to remove residual ethanol. Then, the crude polysaccharides were dissolved in 1 mL of 1 N NaOH and determined the reducing sugar content of the supernatant using the phenol-sulfuric acid method.

Crude Triterpenoid Analysis
The crude triterpenoid content was measured according to Sun et al. [28] with some modifications. In an 80 • C dry bath incubator, the ethanol extract (0.1 mL) was evaporated to dryness. The dried extract was redissolved in 0.4 mL of 5% vanillin-acetic acid solution and 1 mL of perchloric acid solution at 60 • C. After a 15 min reaction, the extract was cooled to room temperature in an ice bath, and 5 mL of acetic acid was added. After 15 min of reaction, the absorbance at 548 nm was measured using a spectrophotometer to determine the triterpenoid content.

Total Phenol Analysis
The concentration of total phenolic compounds was determined using the method described by Antolovich et al. [29], with some modifications. A 0.2 mL sample of ethanol extract was mixed with 1 mL of Folin-Ciocalteu phenol reagent and 0.8 mL of 7.5% Na 2 CO 3 . Following the addition, the solution was incubated in the dark for 30 min at room temperature. The absorbance of the solution was measured at 765 nm and compared to a gallic acid calibration curve (0-500 ppm) using a linear regression equation of y = 0.0046x + 0.22 (R 2 = 0.993).

Total Flavonoid Analysis
Total flavonoids were analyzed using a modified version of the method described by Christel et al. [30]. A 1 mL extract sample was mixed with 1 mL of 2% methanolic AlCl 3 . The solution was incubated at room temperature for 10 min. The absorbance of the solution was measured at 430 nm and the result was compared to a quercetin calibration curve (0-100 ppm) using a linear regression equation of y = 0.0206x + 0.0241 (R 2 = 0.999).

DPPH Radical Scavenging Activity
The DPPH free-radical-scavenging capacity of the extracts of the mycelial products obtained by solid-state fermentation was determined by following the method of Xu and Chang [31], with minor modifications. Briefly, 2 mL of the ethanol extract sample was mixed with a 2 mL ethanol solution of DPPH radical (final concentration was 0.2 mM). The mixture was vortexed vigorously for 1 min and then left at room temperature in the dark for 30 min. The absorbance of sample was then measured using a spectrophotometer at 517 nm against an ethanol blank. Ascorbic acid and BHA were used as controls. DPPH scavenging activity (%) = [1 − (ABS sample /ABS control )] × 100%.

Ferric-Reducing Antioxidant Power (FRAP) Assay
The FRAP assay was carried out by following the method of Xu and Chang [31]. A 2 mL ethanol extract sample was added to a 1 mM FeCl 2 ·4H 2 O (0.1 mL). The reaction was started by adding 0.25 mM ferrozine (0.2 mL). The mixture was vigorously shaken and allowed to stand for 10 min at room temperature. The absorbance was taken at 562 nm using a visible spectrophotometer. Ascorbic acid and EDTA were used as controls. Ferric reducing antioxidant power (%) = [1 − (ABS sample /ABS control )] × 100%.

Reducing Power
A 2.5 mL ethanol extract sample was mixed with 0.2 mL of 0.2 M phosphate buffer and 2.5 mL of 1% potassium ferricyanide. The mixture was incubated at 50 • C for 20 min. Approximately 2.5 mL of 10% trichloroacetic acid was added to the mixture. The mixture was then centrifuged for 10 min at 3000 rpm and the supernatant (5 mL) was mixed with 5 mL of distilled water and 1 mL of 0.1% ferric chloride. The absorbance was monitored at 700 nm using a spectrophotometer. Ascorbic acid and BHA were used as controls [32].

Statistical Analysis
The experimental results were presented as mean ± standard deviation (SD). A oneway analysis of variance (ANOVA) was performed and subsequently subjected to Duncan's multiple range tests of treatment mean by using Statistical Analysis System (SAS 9.4, SAS Institute, Cary, NC, USA), and the significant level was set at 0.05. Figure 1 shows the RF output power of 500 g solid-state fermented W. cocos product placed between of two parallel electrode plates with different electrode gaps. The smaller the gap, the larger the RF output power; therefore, a gap for the electrode plate of 14, 15, and 16 cm was chosen for comparison of the temperature profile.

RF Pasteurization Conditions of Solid-State Fermented W. cocos Products
Foods 2022, 11, x FOR PEER REVIEW 6 of 13 A 2.5 mL ethanol extract sample was mixed with 0.2 mL of 0.2 M phosphate buffer and 2.5 mL of 1% potassium ferricyanide. The mixture was incubated at 50 °C for 20 min. Approximately 2.5 mL of 10% trichloroacetic acid was added to the mixture. The mixture was then centrifuged for 10 min at 3000 rpm and the supernatant (5 mL) was mixed with 5 mL of distilled water and 1 mL of 0.1% ferric chloride. The absorbance was monitored at 700 nm using a spectrophotometer. Ascorbic acid and BHA were used as controls [32].

Statistical Analysis
The experimental results were presented as mean ± standard deviation (SD). A oneway analysis of variance (ANOVA) was performed and subsequently subjected to Duncan's multiple range tests of treatment mean by using Statistical Analysis System (SAS 9.4, SAS Institute, Cary, NC, USA), and the significant level was set at 0.05. Figure 1 shows the RF output power of 500 g solid-state fermented W. cocos product placed between of two parallel electrode plates with different electrode gaps. The smaller the gap, the larger the RF output power; therefore, a gap for the electrode plate of 14, 15, and 16 cm was chosen for comparison of the temperature profile. The average temperature profile of RF pasteurization ( Figure 2) shows that the fermented product reached temperatures above 80 °C in less than 30 s with electrode plate gaps of 14, 15, and 16 cm. The temperature began to stabilize above 95 °C after 40 s of RF pasteurization. Because of the current output stability, we chose an electrode plate gap of 15 cm as the RF pasteurization condition for the solid-state fermented W. cocos products.  The average temperature profile of RF pasteurization ( Figure 2) shows that the fermented product reached temperatures above 80 • C in less than 30 s with electrode plate gaps of 14, 15, and 16 cm. The temperature began to stabilize above 95 • C after 40 s of RF pasteurization. Because of the current output stability, we chose an electrode plate gap of 15 cm as the RF pasteurization condition for the solid-state fermented W. cocos products.

RF Pasteurization Conditions of Solid-State Fermented W. cocos Products
Foods 2022, 11, x FOR PEER REVIEW 6 of 13 A 2.5 mL ethanol extract sample was mixed with 0.2 mL of 0.2 M phosphate buffer and 2.5 mL of 1% potassium ferricyanide. The mixture was incubated at 50 °C for 20 min. Approximately 2.5 mL of 10% trichloroacetic acid was added to the mixture. The mixture was then centrifuged for 10 min at 3000 rpm and the supernatant (5 mL) was mixed with 5 mL of distilled water and 1 mL of 0.1% ferric chloride. The absorbance was monitored at 700 nm using a spectrophotometer. Ascorbic acid and BHA were used as controls [32].

Statistical Analysis
The experimental results were presented as mean ± standard deviation (SD). A oneway analysis of variance (ANOVA) was performed and subsequently subjected to Duncan's multiple range tests of treatment mean by using Statistical Analysis System (SAS 9.4, SAS Institute, Cary, NC, USA), and the significant level was set at 0.05. Figure 1 shows the RF output power of 500 g solid-state fermented W. cocos product placed between of two parallel electrode plates with different electrode gaps. The smaller the gap, the larger the RF output power; therefore, a gap for the electrode plate of 14, 15, and 16 cm was chosen for comparison of the temperature profile. The average temperature profile of RF pasteurization ( Figure 2) shows that the fermented product reached temperatures above 80 °C in less than 30 s with electrode plate gaps of 14, 15, and 16 cm. The temperature began to stabilize above 95 °C after 40 s of RF pasteurization. Because of the current output stability, we chose an electrode plate gap of 15 cm as the RF pasteurization condition for the solid-state fermented W. cocos products.   The central and internal temperatures of the samples were substantially higher than the surface temperature. After 1 min of RF pasteurization and 10 min of room-temperature cooling, the central and internal temperatures of the fermented product remained above 80 • C (Figure 3), which exceeded the conditions for inactivating W. cocos (W. cocos died above 80 • C in previous experiments; data not shown in this paper). The central and internal temperatures of the samples were substantially higher than the surface temperature. After 1 min of RF pasteurization and 10 min of room-temperature cooling, the central and internal temperatures of the fermented product remained above 80 °C (Figure 3), which exceeded the conditions for inactivating W. cocos (W. cocos died above 80 °C in previous experiments; data not shown in this paper).  As a test of the pasteurization effect, solid-state fermented W. cocos products were RF pasteurized for 0 to 60 s and then incubated in PDA for 7 d (Figure 4). We observed that the fermented products only required 30 s of RF heating to achieve the pasteurization requirements. Because RF generates heat by making the polar water molecules in the sample rapidly rotate and oscillate, the heating rate of the samples during RF heating depended on the moisture content, loading, dielectric constant, and RF electrode gap. In previous studies, white bread (37.1% moisture content) was treated for less than 5 min with HARF to reach 58 ℃ or higher, which reduced the fungi spores and thereby extended the storage time [15]. Walnut shells (9% moisture content) were heated to over 60 °C by RF pasteurization for 10 min to kill Staphylococcus aureus. RF heating inhibited nucleic acid metabolism, translation, cell membrane transport, and cell wall biosynthesis, which eventually led to the cell death of Staphylococcus aureus [18]. Therefore, RF pasteurization is suitable for low-moisture-content foods due to the large difference in the dielectric loss between the target microorganisms and host foods. The method selectively heated and killed the microorganisms without damaging the food product [19]. Figure 5 presents the temperature profile of the solid-state fermented W. cocos products produced by an autoclave at 121 °C for 60 min and a cooling process. The figure shows that the temperature rose very slowly from 0 to 30 min. At 100 min, the highest central temperature of 114.1 °C was reached, which then gradually decreased. This indicated that the heat transfer of the solid-state fermented W. cocos products was poor when using an autoclave.  As a test of the pasteurization effect, solid-state fermented W. cocos products were RF pasteurized for 0 to 60 s and then incubated in PDA for 7 d (Figure 4). We observed that the fermented products only required 30 s of RF heating to achieve the pasteurization requirements. Because RF generates heat by making the polar water molecules in the sample rapidly rotate and oscillate, the heating rate of the samples during RF heating depended on the moisture content, loading, dielectric constant, and RF electrode gap. The central and internal temperatures of the samples were substantially higher than the surface temperature. After 1 min of RF pasteurization and 10 min of room-temperature cooling, the central and internal temperatures of the fermented product remained above 80 °C (Figure 3), which exceeded the conditions for inactivating W. cocos (W. cocos died above 80 °C in previous experiments; data not shown in this paper).  As a test of the pasteurization effect, solid-state fermented W. cocos products were RF pasteurized for 0 to 60 s and then incubated in PDA for 7 d (Figure 4). We observed that the fermented products only required 30 s of RF heating to achieve the pasteurization requirements. Because RF generates heat by making the polar water molecules in the sample rapidly rotate and oscillate, the heating rate of the samples during RF heating depended on the moisture content, loading, dielectric constant, and RF electrode gap. In previous studies, white bread (37.1% moisture content) was treated for less than 5 min with HARF to reach 58 ℃ or higher, which reduced the fungi spores and thereby extended the storage time [15]. Walnut shells (9% moisture content) were heated to over 60 °C by RF pasteurization for 10 min to kill Staphylococcus aureus. RF heating inhibited nucleic acid metabolism, translation, cell membrane transport, and cell wall biosynthesis, which eventually led to the cell death of Staphylococcus aureus [18]. Therefore, RF pasteurization is suitable for low-moisture-content foods due to the large difference in the dielectric loss between the target microorganisms and host foods. The method selectively heated and killed the microorganisms without damaging the food product [19]. Figure 5 presents the temperature profile of the solid-state fermented W. cocos products produced by an autoclave at 121 °C for 60 min and a cooling process. The figure shows that the temperature rose very slowly from 0 to 30 min. At 100 min, the highest central temperature of 114.1 °C was reached, which then gradually decreased. This indicated that the heat transfer of the solid-state fermented W. cocos products was poor when using an autoclave.  In previous studies, white bread (37.1% moisture content) was treated for less than 5 min with HARF to reach 58°C or higher, which reduced the fungi spores and thereby extended the storage time [15]. Walnut shells (9% moisture content) were heated to over 60 • C by RF pasteurization for 10 min to kill Staphylococcus aureus. RF heating inhibited nucleic acid metabolism, translation, cell membrane transport, and cell wall biosynthesis, which eventually led to the cell death of Staphylococcus aureus [18]. Therefore, RF pasteurization is suitable for low-moisture-content foods due to the large difference in the dielectric loss between the target microorganisms and host foods. The method selectively heated and killed the microorganisms without damaging the food product [19]. Figure 5 presents the temperature profile of the solid-state fermented W. cocos products produced by an autoclave at 121 • C for 60 min and a cooling process. The figure shows that the temperature rose very slowly from 0 to 30 min. At 100 min, the highest central temperature of 114.1 • C was reached, which then gradually decreased. This indicated that the heat transfer of the solid-state fermented W. cocos products was poor when using an autoclave. After 60 min of pasteurization in the traditional 121 °C autoclave, the solid-state fermented W. cocos products blackened due to the Maillard reaction ( Figure 6). We observed no notable color change between the control group and the RF pasteurized solid-state fermented W. cocos product. This finding indicated that RF pasteurization can replace traditional autoclave treatment, reduce operation time, and improve product color preservation.

RF Drying Conditions of Solid-State Fermented W. cocos Products
After pasteurization, the solid-state fermented W. cocos products contained about 40% moisture content and required drying steps for storage. A 500 g packet of solid-state fermented W. cocos product was laid flat on a plate and dried under an electrode gap of 15 cm under the same conditions as the RF pasteurization treatment. The internal temperature of the sample increased from 45 °C to more than 100 °C after 50 s of RF drying. Due to water evaporation, the internal temperature remained around 100 °C, while the surface temperature increased from 25 °C to more than 60 °C. The surface temperature increased to more than 70 °C after 80 s of RF drying, and it also showed a flat phenomenon, indicating that it entered the latent heat stage of moisture evaporation (Figure 7). After 60 min of pasteurization in the traditional 121 • C autoclave, the solid-state fermented W. cocos products blackened due to the Maillard reaction ( Figure 6). We observed no notable color change between the control group and the RF pasteurized solid-state fermented W. cocos product. This finding indicated that RF pasteurization can replace traditional autoclave treatment, reduce operation time, and improve product color preservation. After 60 min of pasteurization in the traditional 121 °C autoclave, the solid-state fermented W. cocos products blackened due to the Maillard reaction ( Figure 6). We observed no notable color change between the control group and the RF pasteurized solid-state fermented W. cocos product. This finding indicated that RF pasteurization can replace traditional autoclave treatment, reduce operation time, and improve product color preservation.

RF Drying Conditions of Solid-State Fermented W. cocos Products
After pasteurization, the solid-state fermented W. cocos products contained about 40% moisture content and required drying steps for storage. A 500 g packet of solid-state fermented W. cocos product was laid flat on a plate and dried under an electrode gap of 15 cm under the same conditions as the RF pasteurization treatment. The internal temperature of the sample increased from 45 °C to more than 100 °C after 50 s of RF drying. Due to water evaporation, the internal temperature remained around 100 °C, while the surface temperature increased from 25 °C to more than 60 °C. The surface temperature increased to more than 70 °C after 80 s of RF drying, and it also showed a flat phenomenon, indicating that it entered the latent heat stage of moisture evaporation (Figure 7).

RF Drying Conditions of Solid-State Fermented W. cocos Products
After pasteurization, the solid-state fermented W. cocos products contained about 40% moisture content and required drying steps for storage. A 500 g packet of solid-state fermented W. cocos product was laid flat on a plate and dried under an electrode gap of 15 cm under the same conditions as the RF pasteurization treatment. The internal temperature of the sample increased from 45 • C to more than 100 • C after 50 s of RF drying. Due to water evaporation, the internal temperature remained around 100 • C, while the surface temperature increased from 25 • C to more than 60 • C. The surface temperature increased to more than 70 • C after 80 s of RF drying, and it also showed a flat phenomenon, indicating that it entered the latent heat stage of moisture evaporation (Figure 7). Figure 8 shows the average temperature profile and drying curve of the solid-state fermented W. cocos product during RF drying. The temperature of the fermented product gradually increased and the moisture content slowly decreased during the first 50 s of RF drying. As a result, the sensible heat condition appeared in this first stage. However, after 50 s of RF drying, the average temperature of the fermented product gradually leveled off at approximately 85 • C and entered the latent heat moisture evaporation condition. The dry base moisture content of the fermented product linearly decreased with the drying time. The linear regression equation was y = − 0.0027x + 0.6806, R 2 = 0.9982, indicating that the drying rate was constant in this period, with the drying rate at this time being 0.8419 g water/min, and only 200 s of RF drying time reduced the moisture content of the solid-state fermented W. cocos product from 40% to 15% or less.   Figure 8 shows the average temperature profile and drying curve of the solid-state fermented W. cocos product during RF drying. The temperature of the fermented product gradually increased and the moisture content slowly decreased during the first 50 s of RF drying. As a result, the sensible heat condition appeared in this first stage. However, after 50 s of RF drying, the average temperature of the fermented product gradually leveled off at approximately 85 °C and entered the latent heat moisture evaporation condition. The dry base moisture content of the fermented product linearly decreased with the drying time. The linear regression equation was y = − 0.0027x + 0.6806, R 2 = 0.9982, indicating that the drying rate was constant in this period, with the drying rate at this time being 0.8419 g water/min, and only 200 s of RF drying time reduced the moisture content of the solidstate fermented W. cocos product from 40% to 15% or less. If the solid-state fermented W. cocos product was used for continuous RF pasteurization and drying treatment, the temperature rising time during the drying process would be reduced by 50 s, thereby reducing the time required for the pasteurization and drying treatment of the fermented product to 180 s, which was extremely fast (Figure 9).  If the solid-state fermented W. cocos product was used for continuous RF pasteurization and drying treatment, the temperature rising time during the drying process would be reduced by 50 s, thereby reducing the time required for the pasteurization and drying treatment of the fermented product to 180 s, which was extremely fast (Figure 9). Compared with a 45 °C hot-air-drying treatment, increasing the surface temperature of the solid-state fermented W. cocos products to more than 40 °C took 60 min. The constant drying rate period lasted from 0 to 60 min, the dry base moisture content gradually decreased, and then the drying rate decreased. A total of 100 min was required to reduce the moisture content of the fermented products to less than 15% in 45 °C with the hot-airdrying treatment ( Figure 10). Therefore, the rapid overall effect of RF heating was able to quickly dry the solid-state fermented W. cocos products, demonstrating a highly efficient drying technology.  Compared with a 45 • C hot-air-drying treatment, increasing the surface temperature of the solid-state fermented W. cocos products to more than 40 • C took 60 min. The constant drying rate period lasted from 0 to 60 min, the dry base moisture content gradually decreased, and then the drying rate decreased. A total of 100 min was required to reduce the moisture content of the fermented products to less than 15% in 45 • C with the hot-airdrying treatment ( Figure 10). Therefore, the rapid overall effect of RF heating was able to quickly dry the solid-state fermented W. cocos products, demonstrating a highly efficient drying technology. Compared with a 45 °C hot-air-drying treatment, increasing the surface temperature of the solid-state fermented W. cocos products to more than 40 °C took 60 min. The constant drying rate period lasted from 0 to 60 min, the dry base moisture content gradually decreased, and then the drying rate decreased. A total of 100 min was required to reduce the moisture content of the fermented products to less than 15% in 45 °C with the hot-airdrying treatment ( Figure 10). Therefore, the rapid overall effect of RF heating was able to quickly dry the solid-state fermented W. cocos products, demonstrating a highly efficient drying technology. Comparing the total drying time required for in-shell walnuts using RF drying, vacuum drying, and hot-air drying, the former was the fastest (138 min), followed by vacuum drying (185 min) and hot-air drying (300 min) [9]. To improve drying efficiency, a combination of RF and hot-air drying may overcome the resistance of both heat transfer and mass transfer.

Active Component and Antioxidant Activity of the Solid-State Fermented W. cocos Mycelia Products
The solid-state fermented W. cocos product was subjected to RF pasteurization and drying, 121 °C autoclave and 45 °C hot-air-drying treatment; there was no significant difference in the mycelium, crude polysaccharides, crude triterpenes, total polyphenols, flavonoids, or antioxidant activity of the microwave extract. The solid-state fermented W. cocos mycelia product after RF pasteurization and drying contained 5.03% mycelium, 9.83% crude polysaccharides, 4.43% crude triterpenoids, 3.54 mg gallic acid equivalent/g DW of total polyphenols, and 0.38 mg quercetin equivalent/g DW of flavonoids. The Comparing the total drying time required for in-shell walnuts using RF drying, vacuum drying, and hot-air drying, the former was the fastest (138 min), followed by vacuum drying (185 min) and hot-air drying (300 min) [9]. To improve drying efficiency, a combination of RF and hot-air drying may overcome the resistance of both heat transfer and mass transfer.

Active Component and Antioxidant Activity of the Solid-State Fermented W. cocos Mycelia Products
The solid-state fermented W. cocos product was subjected to RF pasteurization and drying, 121 • C autoclave and 45 • C hot-air-drying treatment; there was no significant difference in the mycelium, crude polysaccharides, crude triterpenes, total polyphenols, flavonoids, or antioxidant activity of the microwave extract. The solid-state fermented W. cocos mycelia product after RF pasteurization and drying contained 5.03% mycelium, 9.83% crude polysaccharides, 4.43% crude triterpenoids, 3.54 mg gallic acid equivalent/g DW of total polyphenols, and 0.38 mg quercetin equivalent/g DW of flavonoids. The supernatant of 50 mg/mL ethanol extract was assayed for antioxidant activity. The results show that the scavenging ability of DPPH free radicals and the chelating ability of ferrous iron were as high as 93.15% and 91.35%, respectively and had similar antioxidant activities to 5 mg/mL of ascorbic acid (93.44% and 91.40%, respectively), and the reducing power was as high as 0.71 (Table 1). However, in the color analysis, the L* and b* values and whiteness of the solid-state fermented W. cocos product by RF pasteurization and drying treatment were significantly higher than those produced by 121 • C autoclaving and the 45 • C hot-air-drying treatment. It showed that the color of the solid-state fermented W. cocos product was better preserved through the RF pasteurization and drying process compared with the autoclaving and hot-air-drying process ( Table 2).

Conclusions
In this study, RF heating required only 30 s to stop fermentation and 200 s to dry 500 g of solid-state fermented W. cocos product, so it is superior to the traditional sterilization treatment requiring 60 min of autoclaving at 120 • C and followed by the 180 min of 45 • C hot-air-drying method. There was no significant difference in the active components or antioxidant activity between RF and traditional process. RF heating offers a novel method for pasteurizing and drying solid-state fermented fungi products.

Data Availability Statement:
The data presented in this study are available in this article.