Research Article | Open Access
The Influence of Drying Method on Volatile Composition and Sensory Profile of Boletus edulis
The objective of this study was to evaluate the influence of different drying methods on aroma and sensory profile of Boletus edulis (cepe). The drying methods tested were convective drying (CD), freeze-drying (FD), vacuum microwave drying (VMD), and a combination of convective predrying and vacuum microwave finish-drying (CPD-VMFD). Fresh and dried cepe volatiles, analyzed by SPME and GC-MS, showed the presence of 53 volatile compounds, most of them present in all dried samples but with quantitative variation. The major volatile compounds in fresh and dried cepe were 1-octen-3-ol (3405 µg 100 g−1·db), 3-octanone (429 µg 100 g−1·db), and hexanal (355 µg 100 g−1·db). The results showed that drying of cepe mushrooms caused major losses of aroma compounds; however, the highest content of volatile compounds and the highest intensity of most of the key positive sensory attributes were found in samples after (i) CD at 80°C (3763 µg 100 g−1·db), (ii) CD at 70°C (3478 µg 100 g−1·db), and (iii) CPD at 60°C and VMFD at 480/240 W (2897 µg 100 g−1·db).
Over the last decades, the consumption of edible wild-grown mushrooms has significantly increased because they are traditionally recognized as valuable sources of nutrients . Mushrooms are rich in digestible proteins, carbohydrates, dietary fibre, certain vitamins, and minerals but low in calories and fat . Moreover, mushrooms contain a huge variety of bioactive compounds and proved to be effective as antioxidants, anticancer, and antimicrobial agents . Among the bioactive molecules, phenolic compounds and tocopherols are the most responsible for their antioxidant activity. Apart from medicinal and nutritional properties, mushrooms are strongly appreciated for subtle flavor, aroma, texture, and unique taste . Fruiting bodies are consumed as food and food-flavoring material in almost every national cuisine of many countries all over the world. For some regions of Europe, the rate of mushroom production and consumption is relatively high. In terms of value, European apparent consumption increased from €158 million in 2010 to €177 million in 2014 . For instance in Poland, according to FAO statistics, mushroom production was estimated at almost 200.000 tons per year . Furthermore, the annual consumption in Poland in the last two years was about 1.5 kg of fresh product by capita .
Boletus edulis is commonly known as cepe and king bolete in English-speaking countries. The fruiting body consists of 30 cm diameter brown cap and is 3–23 cm long and 3–11 cm thick white and/or grayish brown stripes and finely covered with delicate reticulated light brown pattern over the upper half . Besides, its exceptional taste and aroma make B. edulis one of the most highly prized mushrooms in Poland. It was previously reported that this species is rich in nutrients and bioactive molecules, such as phenolic acids and tocopherols, that are related with their antioxidant activity.
Fresh mushrooms are very perishable products with a limited shelf life of 1 to 3 days at room temperature; thus, postharvest treatments are very important to extend their shelf life. One of the most frequently used methods of preserving mushrooms is canning, drying, or marinade. Dehydration is one of the simplest and oldest methods of preserving mushrooms, allowing them to be available throughout longer periods of time . However, different dehydration methods may affect texture, nutritional value, volatile components, and sensory quality of final product . Therefore, choosing the best drying method could make a significant contribution to the mushroom processing industry. Several techniques for mushroom drying have been reported; the most popular are convective and sun drying. Freeze-drying is considered as one of the best dehydration methods, providing the best structural integrity and flavor retention . Due to the lack of liquid water presence, most of microbiological reactions are stopped, and the low temperatures required for the process gives a final product of excellent quality. Convective drying is still the most widely technique used to reduce the moisture content of vegetables, fruits, and herbs. However, relatively long time of the process and high temperature may negatively affect color, texture, flavor, and nutritional value of the final product . An alternative way to avoid these inconveniences is vacuum microwave drying. The low temperature used during process combined with fast mass transfer conferred by vacuum and fast energy transfer by microwave heating induces very rapid and low temperature drying. Furthermore, the absence of air during drying may inhibit oxidation, and therefore, the color and nutrients content can be largely retained . The combination of an initial convective predrying step (CPD) with a second and final step of vacuum microwave finish-drying (VMFD) reduces the mass to be loaded in the VMD system, resulting in a reduced total cost of dehydration process and improving the product quality .
In the present work, the influence of different drying methods on volatile profile of fresh cepe (Boletus edulis), originated from Polish forests, was examined. Furthermore, the aroma composition found in each sample was related with their descriptive sensory evaluation.
2. Materials and Methods
Boletus edulis (cepe) samples were collected from the forest in the region of Wrocław (Poland, 51°10′25.2″N 16°51′38.3″E) in September 2015. Moisture content of samples was determined using the vacuum dryer (SPT-200, ZEAMIL Horyzont, Krakow, Poland). The initial moisture content of all samples was 11.58 kg kg−1·db. Whole mushrooms of similar size were selected and cut into cubes of 1 cm of side and dehydrated using 4 different methods: (i) freeze-drying (FD), (ii) convective drying (CD), (iii) vacuum microwave drying (VMD), and (iv) combined drying consisting of convective predrying followed by vacuum microwave finish-drying (CPD-VMFD).
2.2. Drying Methods
Freeze drying (FD) was performed in a freeze dryer OE-950 (Labor, MIM, Hungary) for 24 h at reduced pressure 65 Pa. Frozen mushrooms were placed on a heating plate, arranged in a one single layer. The temperature within the drying chamber was −60°C, while the heating plate reached 30°C.
Convective drying (CD) was carried out in a dryer designed and built at the Institute of Agricultural Engineering (Wrocław University of Environmental and Life Sciences, Poland). Mushroom samples (100 g) were placed in a tray of 100 mm diameter, arranged in one single layer, and were dried at four different temperatures: 50, 60, 70, and 80°C; the air velocity was 0.8 m s−1. An increased mode of temperature 50/70°C and 50/80°C, consisting of convective finish drying at 70 and 80°C preceded by convective predrying at 50°C, was applied in order to reduce the drying time by increasing the drying rate at the final stage of CD.
Vacuum microwave drying (VMD) was done using a Plazmatronika SM 200 dryer (Wrocław, Poland). A cylindrical drum made of organic glass (18 cm of diameter × 27 cm of length) was rotated with 6 rev·min−1. The drum was connected to a vacuum system consisting of a vacuum pump BL 30P (“Tepro”, Koszalin, Poland), vacuum gauge MP 211 (“Elvac”, Bobolice, Poland), and compensation reservoir of 0.15 m3 capacity. In this study, 2 power levels were assayed, 240 and 480 W; a reduced mode of microwave power 480/240, consisting of VMD at 480 W followed by drying at 240 W, was also applied to assure a possible fast drying at relatively low temperature of the dried material. The maximum temperature reached by the mushroom samples was measured with an infrared camera i50 (Flir Systems AB, Stockholm, Sweden), immediately after removing samples from the VM dryer.
Combined drying (CPD-VMFD) consisted of convective predrying (CPD) at temperatures 50, 60, 70, and 80°C until a moisture content around 4.0 kg·kg−1 db was reached, followed by VM finishing-drying at 480/240 W. Samples of ∼100 g of fresh mushrooms were used for all abovementioned drying methods.
2.3. Modeling of Drying Kinetics
The drying kinetics of CD, VMD, and CPD-VMD was performed based on the mass losses of cepe samples. During CD, weight losses were monitored every 5 min for the initial 20 min, and then, the measurement time intervals were extended to 10, 15, and 30 min after 20, 60, and 120 min of drying, respectively. On the contrary, VMD samples were monitored every 2 and 4 min for 480 and 240 W, respectively, to get a similar energy input regardless of the microwave power level. The moisture ratio (MR) was calculated using the following equation:where is the moisture content at time τ, is the initial moisture content, and is the equilibrium moisture content.
Table Curve 2D Windows v2.03 was used to fit the basic drying models to the measured MR determined accordingly to Equation (2). There are several drying models which can be used for describing drying kinetics of plant materials including mushrooms. The good fitting of a specific model to the experimental data was evaluated using two parameters: (i) coefficient of determination (R2) and (ii) root-mean squared error (RMSE). The model fit is better if the value of R2 is closer to 1, and the RMSE value is closer to 0. Five drying models, such as modified Page, Henderson–Pabis, logarithmic, Midilli-Kucuk, and Weibull were considered for describing the drying kinetics. However, preliminary tests conducted in this study proved that the best fitting was obtained for the modified Page model (as given by equation (3)); consequently, only this model was used in this study:where , , and are constants.
2.4. Isolation of Volatile Compounds
The isolation of the aroma compounds was performed using a headspace solid phase microextraction (HS-SPME) procedure, previously reported by Politowicz et al. . A manual SPME holder (Supelco, Bellefonte, PA, USA) with a divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS, 50/30 µm, coating 1 cm) fiber (Supelco) was used to extract headspace volatiles from Boletus edulis.
2.5. GC-MS and GC-FID Analyses
The GC-MS analysis was performed on GC-MS Clarus SQ8 (PerkinElmer, MA, USA) equipped with an Elite-5 MS (PerkinElmer, MA, USA) capillary column (30 m × 0.25 µm film × 0.25 mm i.d.). Samples were injected with a splitless ratio and helium, at 1 mL·min−1, was used as carrier gas. Oven temperature was held at 60°C for 3 min, raised to 120°C at 3°C min−1, then to 300°C at 15°C·min−1, and held for 2 min. Mass spectra were recorded in electron impact (EI) ionization mode at 70 eV, scanning the 35–550 m/z range. The injector was held at 250°C, while the detector temperature was 300°C.
The quantification was performed by gas chromatograph (GC) coupled to a flame ionization detector (FID) (PerkinElmer 580, MA, USA) with an Elite-5 MS (PerkinElmer 580, MA, USA) column (30 m × 0.25 µm film × 0.25 mm i.d.). 2-Undecanone was used as an internal standard (0.5 μg 2-undecanone in cyclohexane per each sample of cepe mushrooms (∼5 g)), and comparing its concentration with that of the other compounds, by supposing a general response factor of 1 for all volatile compounds, no standard curves were conducted for each of the compounds found in the samples. The GC conditions were the same as those for GC-MS. The analyses were carried out using helium as a carrier gas.
2.6. Descriptive Sensory Analysis
Nine trained panelists belonging to the “Food Quality and Safety” research group, UMH (Universidad Miguel Hernández de Elche), use descriptive sensory evaluation to test dried samples of B. edulis. The panel consisted of 5 males and 4 females, with ages ranging between 26 and 55 years. The panel was selected and trained following the ISO standard 8586–1 , and it is specialized in descriptive sensory evaluation of fruits and vegetables and has a wide expertise in studying the effects of drying on different matrixes, such as herbs and mushrooms [16, 18].
One orientation session was conducted, and the panel evaluated different coded samples of Spanish and Polish dried and fresh mushrooms similar to B. edulis. During this session, the panel decided to use 10 attributes (i) appearance: inner color and piece size; (ii) flavor: mushroom ID (flavor resembling that of freshly picked mushrooms), fresh, fried, nutty, earthy, and burnt, and (iii) texture: hardness and sponginess. Reference products of these attributes, with intensity similar to those of the samples under evaluation, were prepared and provided to the panel.
Samples of B. edulis (coded with 3-digit numbers) were randomly presented, in 100 mL plastic cups with lids, to panelists in normalized individual booths with controlled illumination and temperature. Freeze-dried B. edulis sample was used as control. The intensity of the sensory attributes was scored using a scale from 0 to 10, where 0 = none or not perceptible intensity and 10 = extremely high intensity.
2.7. Statistical Analysis
To compare the experimental data, two consecutive tests were performed: (i) one-way analysis of variance (ANOVA) and (ii) Tukey’s multiple range test. Homogenous groups and the least significant difference (LSD) were determined at significance level of . Statgraphics Plus 5.0 software (Manugistics, Inc., Rockville, MD, U.S.A.) was the program used for the statistical analyses.
3. Results and Discussion
3.1. Drying Kinetics
Figure 1 shows changes with time of the moisture ratio (MR) of cepe samples dehydrated by CD at temperatures in the range 50 to 80°C (Figure 1(a)), VMD at three magnetron powers 240, 480, and 480/240 W (Figure 1(b)), and combined (CPD-VMFD) drying consisting of CPD at 60 and 70°C and VMFD under the reduced mode 480/240 W (Figure 1(c)). The drying times, together with the final moisture content, the maximum temperatures, and the constants of the modified Page model (Equation (3)) are shown in Table 1. Because of the high values of the coefficient of determination (R2>0.99) and low values of RMSE (<0.05), it was demonstrated that this model (modified Page) can be successfully used to describe drying kinetics of cepe dehydrated by CD, VMD, and CPD-VMFD methods. Similar to earlier research, the good fitting of modified Page model was found in the case of chanterelle and oyster mushrooms [16, 19].
†Treatment means of the ANOVA test (values are the mean value of three replications). ‡Values followed by the same letter, within the same column, were not significant different (), Tukey’s multiple range test.
In the case of CD, increasing the air temperature from 50 to 80°C shortened the drying time from 390 to 210 min, respectively. With regard to VMD, where the conventional water diffusion occurring according to Fick’s law is supported by the pressure diffusion mechanism of the Darcy type, a radical reduction of total drying time was observed  as compared to CD. Under VMD at 480 and 240 W, the drying time was 40 and 72 min, respectively. Calín-Sanchez et al. also observed extended drying time while decreasing the microwave power . Combined CPD and VMD at a reduced mode 480/240 W reduced process duration ∼4 times when compared to simple CD. It was also found that increasing hot air temperature during CPD resulted in higher temperature of VMFD samples, from 55 to 57°C (Table 1). This experimental finding may be related to the water molecules distribution constituted by CPD inside the sample as the location of water molecules effects the generation of heat energy under microwave radiation during VMFD .
The results of previous studies [23, 24] revealed that energy consumption for VMD of plant materials is significantly lower than that for convective drying particularly for higher microwave powers. However, from the practical point of view, combined drying consisting of convective predrying followed by vacuum microwave finish-drying (CPD-VMFD) is recommended for industrial conditions . This is due to the fact that CPD is very effective at the beginning of drying process when the most of water is removed from the dried material and requires more and more time at the final stage of drying when the residual water strongly binds with the cellular structure of this material . That kind of water is effectively evacuated during VMFD when the process of diffusion is assisted by the pressure transport mechanism as it was stated above. Moreover, the mass of CPD material is largely reduced, and VMFD can be performed in much smaller installations which leads to the lowering of the investment costs. Therefore, from the technical point of view, the combined drying CD 70°C -VMD 480/240 seems to be the better option than CD 60°C -VMD 480/240 taking into account shorter drying time and lower operating costs at the comparable temperature of the dried material. However, the final recommendations regarding drying conditions of cepe for industrial applications should be formed considering also the quality aspects of the dried material, such as volatile composition and sensory attributes.
3.2. Volatile Composition of Fresh and Dried Cepe
The volatile compounds of fresh and dried cepe mushrooms identified by HS-SPME are listed in Tables 2 and 3. In this context, it is also worth mentioning that the aroma profile of Polish B. edulis has not previously been well examined. Fifty-three volatile compounds, most of them short-chain organic compounds amounting on average 95.6% of total volatile fraction, were identified by means of GC-MS (Tables 2 and 3). Alcohols, aldehydes, and ketones, amounting on average to 69.2, 11.0, and 6.5% of total volatiles, respectively, were the dominating chemical families and represented more than 80% of individual components in fresh and dried material. It was previously found that these C8 odorants usually represent between 44 and 98% of the total volatile fraction in mushrooms . Aprea et al. reported the presence of 66 aroma components identified by HS-SPME in dried cepe, while only 25 compounds were identified in previous works where solvent extraction techniques were used [30, 31].
†Treatment means of the ANOVA test (values are the mean value of three replications). ‡Values followed by the same letter, within the same row, are not significant different (), Tukey’s multiple range test.
The total concentration of volatile compounds in the fresh B. edulis was 5089 µg 100 g−1·db (Table 2). The main components found in cepe were (i) 1-octen-3-ol (66.9%), (ii) 3-octanone (8.43%), and (iii) hexanal (6.99%). It was previously found that 1-octen-3-ol and 3-octanone are secondary metabolites of most mushrooms and give them their typical “mushroom-like” odor . According to Csóka et al., the percentage of the 8-carbon volatiles in cepe was notably high (65.41%), and 1-octen-3-ol was the predominant compound . The high ratio of this “mushroom alcohol” in B. edulis was reported by other authors as well [1, 34].
In the present study, heterocyclic compounds were also identified, including 9 pyrazines and 1 furan. In a previous work, Thomas identified in dried cepe 12 pyrazines, 6 furans, and 9 pyrroles . These compounds are products of the Maillard reaction occurring during the drying process and are responsible for typical odor notes of “dried” mushrooms . Of the 13 aldehydes found in dried cepe, 11 were previously reported by Aprea et al. and only 4 (hexanal, octanal, nonanal, and benzaldehyde) by Thomas .
According to the literature survey, only few studies have described the influence of drying on the volatile composition of cepe mushrooms. Furthermore, there is no information about the impact of CPD-VMFD and/or VMD on their volatile composition. Table 3 shows the total volatile compounds content and the content of each individual compound, expressed as µg volatiles per 100 g db, in fresh and dried cepe mushrooms. The preliminary working hypothesis was that FD (control treatment) and CD, contrary to CPD-VMFD, will provide the best quality product, with high content of volatiles and the highest intensities of key sensory attributes . With regard to the assayed drying methods, CD at 70 and 80°C can be considered as suitable dehydration methods, because these methods allowed to maintain high content of volatiles. Moreover, the increase in hot air temperature decreased the drying time which seems to more harmful to volatiles that the drying temperature.
Remarkable differences were noticed among tested drying methods as regards the total concentration of volatile compounds (Table 3). With regard to FD, it was found that, under this method, the losses of volatiles were intermediate as compared to the fresh product. For example, fresh cepe samples dried using FD lost only 56% of their characteristic aroma, contrary to adjusted modes 50/80°C and 50/70°C, where the loss of volatiles accounted for almost 80% of the initial total content of volatiles; the sublimation of water during FD is linked with the loss of water soluble volatile compounds. In the study by Baranauskiene et al., the loss of volatile components depended on the nature of aroma components; for example, the vapor pressure of a nonpolar compound is higher than that of a polar compound . Accordingly, Calin-Sanchez et al. proved that reduction in pressure in the drying chamber of the freeze dryer may result in losses of volatiles into the environment .
In the case of CD, it can be stated that increasing the air temperature led to higher retention of the total volatile compounds from fresh cepe; however hot-air-dried mushrooms showed characteristic color deterioration and structure deformation . The total concentration of aroma constituents increased from 1538 to 3763 µg 100 g−1, when the air temperature increased from 50 to 80°C; a similar trend was observed for the “mushroom alcohol” 1-octen-3-ol with concentrations being significantly higher when the drying temperature was 80°C (1521 µg 100 g−1·db) as compared to 50°C (330 µg 100 g−1·db).
Increasing the microwave wattage from 240 to 480 W resulted in decrease in total amount of volatile components from 1492 to 1335 µg 100 g−1, although differences were not statistically significant. The adjusted mode of microwave power 480/240 W provided a final product with a total volatiles concentration of 1225 µg 100 g−1. It has been previously stated that the VMD method is suitable for the drying of heat-sensitive materials, such as mushrooms and herbs; however, in that study, the changes in the content of volatiles was not investigated .
To improve the quality of dried mushrooms, vacuum drying has been combined with microwave drying . Under CPD-VMFD, the losses of volatiles were intermediate as compared to the fresh product. For instance, sample predried at 60 and 70°C and finish dried at reduced mode 480/240 W presented total volatile concentration of 2897 and 2835 µg 100 g−1, respectively This indicates that using a lower temperature during CPD is a more advantageous option for combined drying. In the previous section, combined drying was recommended for the preservation of cepe from the technical point of view. In the study by Argyropoulos et al., it was reported that the combined method CPD-VMFD was proposed as an alternative method to improve the quality of dried mushrooms, especially when considering their structural and textural properties .
3.3. Descriptive Sensory Analysis with Trained Panel
It is clearly proven in Table 4 that the drying method significantly affected the descriptive sensory profile of dried cepe mushrooms. FD proved to be the best drying treatment, as expected, and that was why it was selected as the control treatment. But besides FD, CD gave the best results and had the highest (after FD) intensities of key sensory attributes in dried mushrooms, such as inner color, piece size, mushroom ID, and fresh mushroom (Figure 2). CPD-VMFD method showed intermediate intensities, while VMD was the worst drying method (Figure 2). These trends agreed well with previous results reported on chanterelle mushrooms .
†NS = not significant F ratio (); ∗,∗∗, and ∗∗∗ = significant at , respectively. ‡Treatment means of the ANOVA test (values are the mean value of 9 panelists). Values followed by the same letter, within the same row, are not significantly different (), Tukey’s multiple range test.
Data on Table 4 are useful in selecting the best one within each type of the drying method (CD, VMD, or CPD-VMFD). In this way, CD at the highest temperatures (70 and 80°C) led to the best results in this specific drying method. There were no significant differences between the two conditions assayed in CPD-VMFD according to the sensory profile, and similar conclusions can be reached for VMD. Thus, the conclusion of this section could be that CD at high temperatures was successful in keeping high intensities of the key positive attributes in B. edulis, and this experiment finding must be due to the short drying time needed to reach the final moisture content.
To make a final conclusion on the grouping of all sensory attributes and volatile compounds linked to specific drying treatments, a principal component analysis (PCA) was conducted and explained 62.89% of the data variability (PCA 1 explained 46.56% and PCA 2 16.34 %). The drying treatments could be visually grouped into 3 clusters of samples (Figure 3):(1)The first one included only the FD and CD samples at 50, 60, 70, or 80°C and was characterized by high intensities of inner color, piece size, mushroom ID, fresh and earthy notes, and sponginess and also by high contents of hexanal, 4-(Z)-hexenol, 2,5-dimethypyrazine, 2,3-dimethylpyrazine, 1-octen-3-one, and 2,3,5-trimethylpyrazine. This group consisted of the best drying treatments.(2)The second one included samples CD at 50/70 and 50/80 and CPD-VMFD samples and was linked with 2-tridecanol. This second cluster consisted of drying treatments leading to dried products of intermediate quality.(3)The third and last group included all VMD treatments and was characterized by the highest intensities of negative sensory attributes, such as hardness, burnt, and fried. This experimental finding proved that VMD cannot be recommended to dry Boletus edulis.
Fresh cepe was dried using the following methods: convective (CD), vacuum microwave drying (VMD), freeze-drying (FD), and CPD-VMFD (convective predrying and vacuum microwave finish-drying). The drying kinetics were successfully described with the modified Page model. Convection drying of Boletus edulis in 70 and 80°C gave the highest retention of key volatile compounds, which was 74 and 69%, respectively. In the case of CD in 50 and 50/80°C provides product, with loss around 70% of aromatic substances. In all conducted variants of drying, several pyrazine derivatives were found, having impact on aroma description. Combined methods of drying lead to products whose aroma descriptor was far from fresh one. Contrary to this, freeze-drying and CD in 70 and 80°C gives product with highest mushroom aroma. Eventually, CD at 80°C could be most recommended approach for Boletus preservation taking into account both the simplicity of drying procedure and the quality of dried product. On the contrary, combined drying of mushrooms consisted in CPD at 60°C followed by VMFD at reduced mode of microwave power 480/240 W can be considered for industrial applications due to high performance and energy efficiency with intermediate quality of the dried product.
|A, n:||function parameters|
|k:||drying constant (min-1)|
|M:||moisture content (kg kg-1·db)|
|M0:||initial moisture content (kg kg-1·db)|
|Me:||equilibrium moisture content (kg kg-1·db)|
|Mf:||final moisture content (%)|
|R2:||coefficient of determination|
|ANOVA:||analysis of variance|
|DSA:||descriptive sensory analysis|
|GC-MS:||gas chromatography-mass spectrometry|
|SPME:||headspace solid-phase microextraction|
|LSD:||least significant difference|
|RMSE:||root-mean squared error|
|VMD:||vacuum microwave drying|
|VMFD:||vacuum microwave finish-drying.|
The authors declare that all results can be found in Department of Chemistry, Wrocław University of Environmental and Life Sciences.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The study was supported by Wroclaw Centre of Biotechnology Programme “The Leading National Research Centre (KNOW)” for years 2014–2018.
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