Effects of Drying Methods on the Physicochemical Aspects and Volatile Compounds of Lyophyllum decastes

In this study, fresh Lyophyllum decastes was dried using hot air drying (HAD), hot air combined with vacuum drying (HAVD), and vacuum freeze drying (VFD). Additionally, the quality and volatile compounds were analyzed. VFD achieved the best color retention, the highest rehydration capacity, and the slightest damaged tissue structure; however, it recorded the longest drying time and the highest energy consumption. HAD was the most energy-efficient of the three methods. Furthermore, the products with more hardness and elasticity were obtained by HAD and HAVD—this finding was convenient for transportation. In addition, GC-IMS demonstrated that the flavor components had significantly changed after drying. A total of 57 volatile flavor compounds was identified, and the aldehyde, alcohol, and ketone compounds were the primary ingredient of the L. decastes flavor component, whereby the relative content of the HAD sample was apparently higher than HAVD and VFD. Taken together, VFD was better at preserving the color and shape of fresh L. decastes, but HAD was more appropriate for drying L. decastes because of the lower energy consumption, and was more economical. Meanwhile, HAD could be used to produce a more intense aroma.


Introduction
Lyophyllum decastes is accepted by general consumers, owing to its delicious taste, special flavor, and abundant nutrients. Furthermore, its fruiting body has been reported to contain a wide array of phytochemicals which are claimed to exert many health benefits, including antioxidants [1], antidiabetics [2], hypolipidemics [3,4], immunoregulation [5], and antitumor properties [6]. With the development of industrialized cultivation techniques, the production of L. decastes has increased substantially in recent years. According to the data released by the Chinese Edible Mushroom Association, the production of L. decastes in 2018 increased by 257% year-on-year compared with 2017. However, the fresh L. decastes is highly perishable due to its high moisture content (around 89% wet basis), which has easily led to the quality deterioration and great economic losses for the L. decastes industry.
Dehydration, as a convenient and effective process in the food industry, can reduce moisture to a safe level to prolong the shelf life [7]. However, the different drying methods have significantly affected the characteristics of dried products, such as the physicochemical properties, flavor characteristics, drying characteristics, and texture properties [8,9]. Hot air drying (HAD), vacuum drying (VD), and vacuum freeze drying (VFD) are the three common drying process for mushrooms. Additionally, different methods have various advantages during the drying process of different types of edible fungi. HAD could be used as an appropriate drying method, rather than others, for shiitake mushrooms to produce a typical aroma [10]. VD is usually combined with other drying methods, such as hot air combined with vacuum drying (HAVD) and microwave-vacuum drying (MWVD), which can effectively reduce the loss of active constituents and nutrients, improve color retention,

Drying Treatment
The drying parameters were obtained from the previous experiments in our laboratory. The fresh L. decastes samples were dried by hot air drying (HAD), hot air combined with vacuum drying (HAVD), and vacuum freeze drying (VFD), until the final moisture content was ≤10% wet basis (g/g, w.b.) according to national standards; then, the drying was finished.

Hot Air Drying (HAD)
The fresh L. decastes sample (1 kg) was uniformly spread onto a tray, and then put into the drying room of an electro-thermostatic blast oven (DHG-9240A, Jinghong Experimental Equipment Co., Ltd., Shanghai, China). The drying parameters were: primary temperature at 42 • C, treated for 3 h, before rising to 55 • C until the moisture content of the sample was ≤10% (g/g, w.b.).

Hot Air Combined with Vacuum Drying (HAVD)
Firstly, the fresh L. decastes sample (1 kg) was evenly spread onto a tray, and then placed into the drying chamber of the electro-thermostatic blast oven (DHG-9240A, Jinghong Experimental Equipment Co., Ltd., Shanghai, China) at 45 • C and treated for 4 h, before rising to 55 • C until the moisture content of sample was below 75% (g/g, w.b.); then, it was transferred to the vacuum drying oven (Weifang North Pharmaceutical Equipment Co., Ltd., Weifang, China.) at 60 • C, and the pressure of the drying chamber was set at −0.08 MPa, until the moisture content of the sample was ≤10% (g/g, w.b.); then, the drying was finished.

Vacuum Freeze Drying (VFD)
The fresh L. decastes sample (1 kg) was pre-frozen in a −80 • C refrigerator (DW-86L626, Qingdao Haier Special Electrical Appliance Co., Ltd., Qingdao, China.) for 10 h, and then moved to a vacuum freeze dryer (LJG-12, Songyuan Huaxing Technology Development Co., Ltd., Beijing, China). The drying parameters were: degree of vacuum of < 5 Pa, cold-trap temperature of −45 • C until the moisture content of the sample was ≤10% (g/g, w.b.); then, the drying was finished. The moisture content of samples treated with hot air drying, hot air combined with vacuum drying, and vacuum freeze drying was tested every 0.5 h by a moisture analyzer (XY-110MW, Chongzhou Lucky Electronic Equipment Co., Ltd., Chongzhou, China.) after 12 h, 10 h, and 16 h, respectively. When the moisture content of the samples was below 10% (g/g, w.b.), the drying was finished, and the drying time was recorded.

Unit Energy Consumption (UEC)
The UEC in the drying process was calculated by Equation (1) according to the output power of equipment and drying time.
where P represented the output power of equipment (kW/h), T was the total of drying time (h), and m represented the amount of fresh sample (kg).

Rehydration Measurement
Rehydration experiments were carried out in a water bath at 40 • C. The weighted 5 g of dried production was put into a 250 mL beaker with 200 mL of distilled water. Samples were weighted after 0.5 h. The rehydration ratio (RR) of the samples after the drying treatment was calculated by Equation (2). All samples were carried in triplicates.
where m a was the weight of rehydration samples and m 0 represented the weight of dried products.

Color Measurement
The color of L. decastes samples with different drying method treatments was measured by a colorimeter (NR110, Shenzhen 3nh Technology Co., Ltd., Shenzhen, China). L, a, and b represented the three chromatic components of lightness, redness or greenness, and yellowness or blueness, respectively [18]. Then, the total color difference (∆E) was calculated by Equation (3). All samples were performed in triplicates.
where L * 0 , a * 0 , b * 0 represented the color parameters of fresh L. decastes, and L * , a * , b * were the color parameters of dried products. In order to examine the texture profiles of dried L. decastes, hardness (g), elasticity, and chewiness (g) were analyzed by a texture analyzer (TA-XT plus, Ultratech Instruments Shanghai Branch, Shanghai, China). The test speed and deformation value were 2 mm/s and 40%, respectively. The trigger force was 10 g. All samples were performed in triplicates.

Microstructure Observation
The microstructure of L. decastes with different drying technologies treatment was observed by a scanning electron microscope (Apreo 2, Thermo Fisher Technology (China) Co., Ltd., Shanghai, China) with the acceleration voltage set at 5.0 kV and observed under 5000× magnification.

Volatile Flavor Compound Analysis
The HS-GC-IMS (FlavourSpec ®® , Gesellschaft für Analytische Sensor systeme mbH, Dortmund, Germany) was employed to detect the volatile flavor compounds of L. decastes treated by different drying methods, as previously described in the literature [16]. The dried L. decastes powder (0.5 g, passing through a 100-mesh sieve) and fresh product (5 g) were precisely weighed, then put it into a 20 mL headspace vial, respectively. Thereafter, the samples were incubated at 60 • C, 500 rpm for 15 min. Whereafter, 200 µL of gas from the headspace was automatically injected into the warmed injector by a syringe in a splitting mode at 85 • C. At that point, the samples were driven into a RESTEK capillary column (30 m × 0.53 mm ID) by nitrogen at a programmed flow as follows: 2 mL/min for 10 min, 10 mL/min for 10 min, 100 mL/min for 10 min, before rising to 150 mL/min for the ending. The analytes were separated at 60 • C into a column, then subsequently were ionized in the IMS ionization chamber at 45 • C. The drift gas flow was set at a constant flow of 150 mL/min. The ketones (C4-C9) were used as the external standard to calculate the retention time (RI). Volatile compounds were identified by comparing RI and the drift time of the standard in the GC-IMS library. All samples were performed in triplicates. The relative content of the volatile flavor compounds was calculated by Equation (4) [19] The relative content(%) = The peak volume Total peak volume × 100 (4)

Statistic Analysis
All of the experimental data were presented as mean ± standard deviation (SD). One-way ANOVA was performed by Duncan's multiple range test with a significant level (p < 0.05). The software of IBM SPSS statistics (version 26.0, SPSS Inc., Chicago, IL, USA) was applied for significance analysis. PCA was performed by the HS-GC-IMS-assisted analysis software VOCal.   Figure 1a,b, VFD consumed more time (around 22 h) and the highest UEC among all drying technologies. Although the drying time of HAD was the second longest (around 14 h), it was the lowest in energy consumption. Compared with VFD and HAD, HAVD significantly (p < 0.05) reduced the drying time. However, it is worth noting that HAVD consumed more energy than HAD by 135% (p < 0.05), which was likely due to the high-energy consumption of the vacuum pump [20]. The most conventional vacuum dryers rely on conduction heat transfer from hot plates, which is slow, difficult to control, and requires a large surface area [21]. This resulted in a high-operating cost and lower drying efficiency of HAVD. Therefore, HAD exhibited not only the lowest in energy consumption, but also the lowest drying time among the three drying methods. These results agreed with the conclusions from the literature for Agaricus bisporus and shiitake mushrooms [22,23]. However, it has been reported that the energy consumption of the VFD process could be reduced by adjusting the working pressure and shelf temperature [24]. In the future, the drying efficiency could be further improved by promoting the drying process.

Results and Discussion
time. However, it is worth noting that HAVD consumed more energy than HAD by 135% (p < 0.05), which was likely due to the high-energy consumption of the vacuum pump [20]. The most conventional vacuum dryers rely on conduction heat transfer from hot plates, which is slow, difficult to control, and requires a large surface area [21]. This resulted in a high-operating cost and lower drying efficiency of HAVD. Therefore, HAD exhibited not only the lowest in energy consumption, but also the lowest drying time among the three drying methods. These results agreed with the conclusions from the literature for Agaricus bisporus and shiitake mushrooms [22,23]. However, it has been reported that the energy consumption of the VFD process could be reduced by adjusting the working pressure and shelf temperature [24]. In the future, the drying efficiency could be further improved by promoting the drying process.

Rehydration Behaviors
The rehydration ratio (RR) is an important evaluation index of structural quality for the dried agricultural products, which indicates the degree of cellular and structural disruption in the process of drying [25]. As found in Table 1, the RR of VFD, HAVD, and HAD was 5.66, 3.12, and 2.94, respectively. The RR of the L. decastes dried by VFD was the highest, while the HAD sample had the lowest RR value. The previous study showed that the variations in the pore size of the product's internal structure had a major influence on rehydration [23]. The material was exposed in the high-temperature environment for a long time during the HAD, which easily caused significant shrinkage and formed a dense structure with a reduced ability for moisture retention in the rehydration process [22]. As a result, the rehydration of HAD L. decastes was difficult. By contrast, the VFD sample was considered to have the best rehydration performance due to its protective effect on cells and tissue structure at a low temperature, and a highly porous structure could facilitate rehydration [26].

Rehydration Behaviors
The rehydration ratio (RR) is an important evaluation index of structural quality for the dried agricultural products, which indicates the degree of cellular and structural disruption in the process of drying [25]. As found in Table 1, the RR of VFD, HAVD, and HAD was 5.66, 3.12, and 2.94, respectively. The RR of the L. decastes dried by VFD was the highest, while the HAD sample had the lowest RR value. The previous study showed that the variations in the pore size of the product's internal structure had a major influence on rehydration [23]. The material was exposed in the high-temperature environment for a long time during the HAD, which easily caused significant shrinkage and formed a dense structure with a reduced ability for moisture retention in the rehydration process [22]. As a result, the rehydration of HAD L. decastes was difficult. By contrast, the VFD sample was considered to have the best rehydration performance due to its protective effect on cells and tissue structure at a low temperature, and a highly porous structure could facilitate rehydration [26].

Color
The color of dried mushrooms is an important parameter for food quality and consumer acceptability. It usually changes during the drying process, owing to the enzymatic browning reaction and pigment degradation [27]. As shown in Table 1, the drying technologies significantly affected the color of L. decastes. The VFD product had the highest value of L*, and the lowest values of a* and b*, which were significant differences from HAD and HAVD (p < 0.05). The similar tendency was found in ∆E-the VFD product exhibited the lowest ∆E value of 1.54, followed by HAVD, and the HAD product had the highest ∆E value. The result confirmed that VFD samples underwent less color degradation due to the vacuum and low-temperature environment that inhibited the enzymatic browning reaction and pigment degradation [28].

Texture and Microstructure
The texture property has often been used to assess the quality of dried agriculture products [29]. The drying method generally has significant effects on the texture of dried products [30]. As shown in Table 1, the hardness of the HAD sample had reached a maximum value of 1560 g, and that of VFD products had reduced to the lowest value of 405.83 g. In addition, the elasticity and chewiness indicated the same trend. The HAD sample significantly had the highest values of elasticity and chewiness compared to the others (p < 0.05). This result was expected, providing that the sample with thermal treatment (HAD and HAVD) exhibited more dense structures and a severely-damaged cell structure, as observed by SEM (Figure 2a,b). However, the skeleton structure of L. decastes with VFD treatment was well retained due to the removal of water, which occurred by sublimation from frozen substances with the simultaneous effect of the vacuum (Figure 2c). Therefore, the VFD sample showed an excellent rehydration capacity and the lowest hardness, elasticity, and chewiness. Similar results have also been reported by other studies [11,12,31]. It is noteworthy that Liu et al. reported that maintaining the hardness of potatoes could reduce the loss of products and save costs during the transportation [32]. Furthermore, the dried L. decastes samples with higher hardness and elasticity were obtained by HAD, which was economical for packaging and convenient for transportation.

Volatile Flavor Compounds of L. decastes under Different Drying Technologies
The flavor components of fresh and dried L. decastes treated by three drying methods are shown in Figure 3a, with the drift time as abscissa and the retention time as ordinate. Each point at the right of the reactive ion peak (RIP) represents one volatile compound split from the samples, respectively. The color of the point represents the substance intensity of the volatile components. The red represents a higher concentration, while the blue indicates a lower concentration level. Compared with the fresh sample, the picture offers a clear vision of the color intensity-representing a high concentration level in the three dried samples-meaning that the drying process significantly influenced the composition of the flavor compounds of L. decastes. This is consistent with the report that Lentinula edodes via the drying process detects more volatile compounds [12].

Volatile Flavor Compounds of L. decastes under Different Drying Technologies
The flavor components of fresh and dried L. decastes treated by three drying methods are shown in Figure 3a, with the drift time as abscissa and the retention time as ordinate. Each point at the right of the reactive ion peak (RIP) represents one volatile compound split from the samples, respectively. The color of the point represents the substance intensity of the volatile components. The red represents a higher concentration, while the blue indicates a lower concentration level. Compared with the fresh sample, the picture offers a clear vision of the color intensity-representing a high concentration level in the three dried samples-meaning that the drying process significantly influenced the composition of the flavor compounds of L. decastes. This is consistent with the report that Lentinula edodes via the drying process detects more volatile compounds [12]. sity of the volatile components. The red represents a higher concentration, while the blue indicates a lower concentration level. Compared with the fresh sample, the picture offers a clear vision of the color intensity-representing a high concentration level in the three dried samples-meaning that the drying process significantly influenced the composition of the flavor compounds of L. decastes. This is consistent with the report that Lentinula edodes via the drying process detects more volatile compounds [12].  In order to better recognize the differences in flavor components in the four samples, the visual gallery plot was built according to the signal peak intensity of each compound (Figure 3b). Each row in the gallery plot represented all of the signal peaks selected in a sample, and each column represented the signal peaks of the same volatile substance in different samples [15]. From Figure 3b, it can be seen that a total of 57 volatile compounds were successfully identified. Most of these compounds in region A of Figure 3b, such as limonene dimer, limonene, p-cymene, (Z)-6-nonenal dimer, acrolein, and benzaldehyde, were clearly abundant in fresh L. decastes samples, and they were extremely damaged during the drying stage because of volatilization and thermal degradation [33]. In contrast, acetone, 2-butanone, 2-butanone dimer, butanal, butanal dimer, cyclopentanone, butan-1-ol, pentanal, and so on, labelled as B, were considerably increased though the drying treatments; this can be attributed to the precursors of fresh samples which degraded or reacted with each other during the drying process [10,34]. In addition, the drying methods exerted profound effects on the volatile fingerprints of L. decastes. In the HAD sample, 3-hydroxy-2-butanone, (E)-2-hexenal, 1-pentanol, 1-hydroxypropan-2-one, (E)-2-heptenal, pentanal, and l-menthol exhibited obviously higher concentrations than other samples (in the yellow frame of Figure 3b). The amounts of acetic acid, pentyl acetate, 2-hydroxybenzaldehyde, and ethyl acetate were markedly higher in the HAVD sample than other samples (in the green frame of Figure 3b). Among these, octanal and heptanal were the highest in the VFD sample. Notably, compared with VFD, L. decastes samples dried via HAD and HAVD were detected with similar types of volatile compounds. This may be because when HAD and HAVD were both under thermal treatment, the increasing temperature of the samples promoted the Maillard reaction, which resulted in the similar flavor composition [10]. An earlier study by Zhang et al. on Allium mongolicum Regel also found that the flavors of VD and HAD AMR samples were relatively similar but different from the FD sample [35].
The volatile compounds in L. decastes subjected to different drying methods were listed in Table 2. These compounds were classified into five chemical types, including 24 aldehydes, 12 alcohols, 10 ketones, 5 alkenes, esters, and 6 other compounds. It has been reported that aldehydes, alcohols, esters, and ketones contributed significantly to the formation of the flavor profiles of foods [10,34]. To further compare the changes in flavor substances of L. decastes between various drying methods, the relative content of flavor components in different samples were calculated in Figure 4 according to the gallery plot. The aldehydes were the major chemical family of L. decastes after the drying process, and they contributed greatly to the flavor of mushrooms due to the low odor thresholds and intense flavor properties [36]. From Figure 4, except the other components, the relative content of aldehydes in the HAD sample was the highest with 32.87%, followed by VFD, and the HAVD sample exhibited the lowest content with 22.79%. Combined with Figure 3b, the signal intensities of nonanal, butanal, and pentanal were significantly increased by the three drying methods-this finding was consistent with the findings of others mushrooms (Luo et al., 2021). (E)-2-hexenal, (E)-2-heptenal, and pentanal appeared with the largest intensity in HAD sample. HAD and HAVD resulted in a higher level of 3-methylbutanal Foods 2022, 11, 3249 9 of 12 than VFD, which was mainly a result of the Strecker degradation [36]. Octanal (citrus orange with a green peel nuance) and heptanal (citrus odor), which were related to a desirable fruity aroma, were greatly increased via VFD.
position [10]. An earlier study by Zhang et al. on Allium mongolicum Regel also found that the flavors of VD and HAD AMR samples were relatively similar but different from the FD sample [35].
The volatile compounds in L. decastes subjected to different drying methods were listed in Table 2. These compounds were classified into five chemical types, including 24 aldehydes, 12 alcohols, 10 ketones, 5 alkenes, esters, and 6 other compounds. It has been reported that aldehydes, alcohols, esters, and ketones contributed significantly to the formation of the flavor profiles of foods [10,34]. To further compare the changes in flavor substances of L. decastes between various drying methods, the relative content of flavor components in different samples were calculated in Figure 4 according to the gallery plot.  Alcohols were mainly formed by the oxidation of fatty acids [37]. A total of 12 alcohols were detected in fresh and dried L. decastes samples. Among these, butan-1-ol had a sweet balsam whiskey aroma and 3-methyl-1-butanol provided the fruity banana flavor. In agreement with the published literature, they were substantially enhanced after the drying process in this study [38]. The increase in these alcohols could be due to the high decarboxylase activity in the reaction of keto acids [39]. Ketones also resulted in a rich aroma of dried mushrooms [10]. In this study, the relative contents of ketones were significantly increased after the drying process. Similar results were mentioned in other mushrooms [40,41]. As shown in Figure 4, ketones were the highest in the HAD sample-this might have been possible because ketones were formed by the oxidation of fatty acids and the occurrence of the Maillard reaction. In addition, the thermal treatment could promote the reaction, whereas the vacuum and low-temperature treatment had reverse effects for its formation [12,42,43].
In summary, regarding the three drying methods, dried L. decastes via HAD was detected with a higher relative content of aldehydes, alcohols, and ketones than via HAVD and VFD. This was consistent with the research on juice [44,45]. Other than the thermaltreatment effect, this finding could be due to the destruction of the cell wall and cytoplasm via HAD, more various intracellular components released, or the involvement with the reaction during drying [16]-if considered, more flavor compounds could be produced. This agreed with the microstructure result.

Principal Component Analysis (PCA) of L. decastes Flavor Volatile Components
PCA is a multivariate statistical analysis technique. The complex and difficult-to-find variables in the original samples were represented by a few principal component factors, therefore, the differences from the samples could be evaluated according to the contribution rate of principal component factors of the different treatments [44,45]. The PCA of volatile substances in both fresh and different treated L. decastes samples was clarified in Figure 5. The accumulative contribution of the first and second principal components was 87% (PC1 was 64% and PC2 was 23%, respectively). These components indicated that the differences in dried L. decastes samples under different drying methods could be separated. The value of PC1 increased in the following order: FP < VFD < HAVD < HAD. The regions of each group of L. decastes showed that the HAVD and HAD samples were close to each other, meaning that they had similar flavors. In addition, the VFD group was located closely to FP, indicating that the flavor of the VFD samples was more similar to the FP sample than HAD and HAVD. The result agreed with a recent study in which samples of Ganoderma lucidum, which were dried via different treatments, had different spatial distributions of PCA [46]. Figure 5. The accumulative contribution of the first and second principal components was 87% (PC1 was 64% and PC2 was 23%, respectively). These components indicated that the differences in dried L. decastes samples under different drying methods could be separated. The value of PC1 increased in the following order: FP < VFD < HAVD < HAD. The regions of each group of L. decastes showed that the HAVD and HAD samples were close to each other, meaning that they had similar flavors. In addition, the VFD group was located closely to FP, indicating that the flavor of the VFD samples was more similar to the FP sample than HAD and HAVD. The result agreed with a recent study in which samples of Ganoderma lucidum, which were dried via different treatments, had different spatial distributions of PCA [46].

Conclusions
Based on the result of the present investigation, we concluded that the drying methods had significantly affected the energy consumption, physicochemical properties, and volatile compounds of the L. decastes-dehydrated products. Among all the drying methods (HAD, HAVD, and VFD), VFD had a better rehydration capacity, and a lower color difference. Furthermore, VFD could also better retain the tissue structure of fresh L. decastes, which might be attributed to a lower operating temperature. However, it had a low efficiency and a high-power consumption. In contrast, HAD was preferred-it had a relatively lower drying time and was the most energy efficient. In addition, the higher hardness of the HAD sample was more economical for packaging and more convenient for transportation. For flavor components, HAD could effectively promote the formation of new volatile flavor compounds and provide dried L. decastes with a more intense aroma, while the VFD was more similar to the FP sample. Therefore, VFD would be suitable for attaining dried L. decastes samples with high requirements for color and appearance, while the HAD drying method was more energy-efficient, economical, and obtained a richer aroma. The above results will provide reference for selecting a drying method that maximizes the retention and the qualities of L. decastes in industrial productions.