Comparative Study of Raw and Dehydrated Boletus edulis Mushrooms by Hot Air and Centrifugal Vacuum Processes: Functional Properties and Fatty Acid and Aroma Profiles

Abstract

A study on Boletus edulis mushrooms subjected to either hot air drying (HAD) or centrifugal vacuum drying (CVD) was performed to evaluate and compare their functional properties, fatty acids, and aroma compounds. Better flowability and a higher rehydration ratio were observed for HAD powders, while enhanced indices of water solubility, emulsifying activity, and stability were noticed for CVD ones. The composition of 21 identified fatty acids varied between raw and dried samples, the most relevant being the decrease of oleic acid and the increase of linoleic acid during drying. The PUFAs/SFAs ratio was >3.3 in all samples, thus meeting the requirements for healthy lipids. Of the 15 aroma compounds identified in raw mushrooms, only hexanal, 1-octen-3-ol, and (Z)-2-octen-1-ol were also found in dried samples, to different extents; 1-octen-3-ol was the major volatile constituent in all samples. Low amounts of new alcohols, aldehydes and ketones, D-limonene, and caryophyllene were detected in HAD powders, while dimethyl disulfide and 2-n-pentyl-furan were detected in CVD ones. The drying of mushrooms resulted in a total loss of 2-methyl-2-butenal, (E)-2-octenal, and 1-octen-3-one. These findings become important milestones for food manufacturers and researchers in selecting the desired drying technique of B. edulis based on the powder/emulsifying properties and preservation of fatty acids and aroma molecules.

1. Introduction

Edible wild-grown Boletus edulis mushrooms (porcini) have been largely appreciated for their flavor, nutritional value, and the extra health benefits provided by micronutrients and bioactive compounds (non-nutrient constituents) with biological significance, in particular antioxidant activity. Due to a high moisture content of >80%, fresh mushrooms are highly perishable [1] and are usually dehydrated into dried slices or powders, not only with the purpose of being consumed throughout the year, but also to be used as functional ingredients.

Drying is an ancient traditional procedure embraced for its ease and for providing a year-round supply of mushrooms. In more recent years, researchers have focused on the compositional changes linked to the applied conditions of drying. In order to prolong the shelf life of mushrooms, different drying techniques have been employed, such as solar (open sun, solar dryers), hot air, lyophilization, microwave, or combined treatments, the specific applied drying conditions being responsible for the quality of the final product [2,3,4]. In relation to the conditions of operation, the complex process of mushroom dehydration may negatively impact the composition of some nutrients and bioactive compounds as well, but, on the other hand, may enhance the mushrooms’ flavor [5]. The main mechanism for production of flavor/aroma compounds is based on the metabolism of fatty acids as a key pathway for C8 volatile components, and of sulfur and non-sulfur amino acids that act as precursors of aldehydes, alcohols, acids, or sulfur-containing aroma compounds [6]. Generally, drying at high temperatures changes the profile of phenolic and organic acids, decreases the content of polysaccharides and proteins, and increases the level of saturated fatty acids, due to different chemical reactions (oxidation, hydrolysis, Maillard reaction). Despite the fact that the total lipid content is low, fresh mushrooms constitute an advantageous source which can contribute to the uptake of essential fatty acids (in particular linoleic acid), but investigations on the recovery of some unsaturated fatty acids is required for their dried counterparts. The most frequently identified fatty acids in Basidiomycota are palmitic, oleic, and linoleic acids, with higher levels of the unsaturated ones [7]. New insights on the role of unsaturated fatty acids confirm a beneficial anti-inflammatory and antioxidant activity of omega 3-6-9 acids encapsulated in chitosan [8], while other studies on omega-9 fatty acids, such as oleic acid, have shown their neuroactive potential as analgesic agents [9]. The antioxidant activity and the content of total polyphenols, flavonoids, and volatile compounds may increase after drying. However, completing the conclusions of Marçal et al. given in their published review paper [5], it is no easy task to recommend the most suitable type of drying, due to the great variability of chemical composition even of the same species of investigated mushrooms, the different analytical methods involved for quantifying compounds (in particular aroma), and other factors that should also be considered, such as functional properties required for the final powder, biological activity, and cost of the drying technology.

It has been considered that dried slices of B. edulis mushrooms exhibit a strong attractive aroma [10], so that studying the changes in the profile of volatile compounds after drying is of high scientific and practical interest. Most studies on volatile compounds of B. edulis, using techniques such as SDE (steam distillation extraction)-GC–MS, SPME (solid-phase microextraction)-GC–MS, and LE (liquid extraction)-GC–MS, indicated a great loss of 1-octen-3-ol, the major contributor (86%) to the aroma of fresh B. edulis, and a total loss of 1-octen-3-one and (E)-2-octen-1-ol [11,12]. Zhang et al. investigated the aroma compounds in raw and dried porcini from China, using SAFE (solvent-assisted flavor evaporation)-GC–MS, a technique which employs a mild treatment for extracting volatile fractions [10]. Of the total quantified aroma compounds, the authors found that raw porcini contained high amounts of 1-octen-3-ol, (E)-2-octen-1-ol, (E)-2-octenal, and 1-octen-3-one, while dried porcini contained 3-(methylthio)propanal and pyrazines as major aroma components, but the applied drying conditions have not been described. There are not many studies reporting the health benefits of biogenic volatile compounds, despite the fact that the so-called mushroom alcohol 1-octen-3-ol is broadly used in perfumes or as a food and flavoring agent; papers also describe some negative health effects [13].

In addition to conventional drying methods, a different method of drying B. edulis based on a centrifugal vacuum process at 60 °C has been reported by our group [14]. Because our previous results showed that drying a B. edulis paste is efficient in preserving the phenolic content and antioxidant activity, the aim of the present work was to explore the effects of two different types of drying on the following: (1) the physical and powder properties of mushrooms (loose and tapped density, Hausner ratio, solubility, rehydration, and emulsifying parameters); (2) the profile of fatty acids using the GC–MS approach, and (3) the volatile aroma compounds using headspace in-tube extraction coupled with gas chromatography–mass spectrometry (HS-ITEX/GC–MS), properties which additionally affect the industrial technologies of mushroom powders processing.

2. Materials and Methods

2.1. Mushroom: Samples and Preparation

Mushrooms (B. edulis L.), cap and stipe, were wild-collected from the Avrig region of Romania, located at 45.661123 N, 24.445704 E at an altitude of 500 m, during the mushroom season in 2021. All samples were collected from the same natural forest (deciduous forest), with small distances between mushrooms, on the same day. A total of 20 samples of the same species, B. edulis, were cleaned, sliced, and blended (Blendforce BL 438831, Germany) in order to make a product in the form of a paste. One part of the paste (200 g) was distributed in Petri glass dishes of Φ 7 cm in a layer of 1 cm thickness and subjected to hot air drying (HAD) using a forced-air oven preheated to 60 °C (UFE 400 with forced air circulation, Memmert, Germany) at a maximum fan speed (100%), for 275 min, while another part (200 g) was distributed in conical glass vials and dried by centrifugal vacuum drying (CVD) using a speed-dry vacuum concentrator (RVC 2-18 CD plus, Christ, Germany), at 60 °C and a speed of 1200 rpm for 749 min, as previously described by our group [14]. The dried samples were further ground to a size of 700 μm. Raw mushroom paste stored at -70 °C was used as a control sample. The physical chemical features of the raw sample, as determined using a refractometer (DR 301-95 A, Krüss Optronic, Germany), were as follows: total soluble solids (TSS) value = 13.35° Brix, refractive index = 1.3530, and salinity 11.45%. The moisture content of raw and dried samples was determined at 105 °C using a moisture analyzer (Mac 210/NP Radwag, Poland).

2.2. Loose and Tapped Bulk Density, Hausner Ratio

Loose bulk density (ρL) and tapped bulk density (ρT) of the mushroom powders were determined using the method described by Atalar et al. [15]. The Hausner ratio (HR) was calculated according to Equation (1):

$$HR=\frac{\rho T}{\rho L}$$

(1)

2.3. Solubility, Water Solubility Index (WSI), and Rehydration Ratio

The water solubility of mushroom powders was determined as described in [15] by dispersing 0.1 g powder in 24.9 g distilled water. The mixture was stirred in a water bath at 25 °C and centrifuged for 20 min at 4500 rpm. Then, 10 g of accurately weighed supernatant was dried to constant weight at 105 °C. The solubility was expressed as a percentage of the mass of dry residue in the supernatant per mass of water. The water solubility index (WSI) was determined as a percentage of the mass of dry solids in the supernatant per the original mass of the sample [16].

The rehydration ratio (RR) was determined as described in [17] by immersing 1 g of mushroom powder into boiling water for 10 min. The mixture was centrifuged at 7000 rpm and weighed. The RR of the dried mushroom was calculated according to Equation (2):

$$RR=\frac{rehydratedsample\left(g\right)}{driedsample\left(g\right)}$$

(2)

2.4. Determination of Emulsifying Properties

The emulsifying activity index (EAI) and emulsion stability index (ESI) were determined spectrophotometrically as described in [15,18] by mixing 0.3 g of mushroom powder with 30 mL of distilled water and 10 mL of sunflower oil, followed by homogenization using an Ultra Turrax (T18 digital model, Ika, Germany) at 20,000 rpm for 1 min. Then, 2 aliquots of emulsion were diluted, after 0 and 10 min of homogenization, with 0.1% (w/v) sodium dodecyl sulfate (SDS) solution. The absorbance of the diluted solution was measured at 500 nm using a spectrophotometer (Specord 200 Plus UV-Vis, Analytik Jena, Germany).

The EAI and ESI were calculated according to Equations (3) and (4), respectively:

$$EAI\left(m^2{g}^{-1}\right)=\frac{2\times 2.303\times A_0\times DF}{c\times \theta \times L\times 10000}$$

(3)

$$ESI\left(min\right)=\frac{A_0\times ∆t}{A_0-A_{10}}$$

(4)

where:

A₀ and A₁₀ = absorbance measured at initial time and after Δt = 10 min;

DF = dilution factor;

c = weight of powder per unit volume (g/cm³);

L = width of the optical path (1 cm); and

θ = oil volumetric fraction of the emulsion.

2.5. Determination of Fatty Acids by GC–MS Analysis

The lipids of mushroom samples were extracted in chloroform/methanol (2/1). The final mixture was filtered under vacuum using a vacuum pump (N 810 FT.18, Laboport, KNF, Germany), and the residue was washed with 0.88% KCl. The organic phase was separated and evaporated to dryness using a rotary evaporator (Hei-VAP Precision, Heidolph, Germany). The dried lipids were resuspended in methanol and stored at −18 °C until analysis. Fatty acid methyl esters (FAMEs) of the total lipids (0.2 g) were produced by acid-catalyzed transesterification using 1% sulfuric acid in methanol [19,20]. The methylated fatty acids were determined using a gas chromatograph (GC) coupled to a mass spectrometer (MS) (PerkinElmer Clarus 600 T GC–MS; PerkinElmer, Inc., Shelton, CT, USA) as follows: 0.5 μL sample was injected into a 60 m × 0.25 mm i.d., 0.25 μm film thickness SUPELCOWAX 10 capillary column (Supelco Inc., USA), at injector temperature 210 °C; helium carrier gas flow rate 0.8 mL/min; split ratio 1:24; oven temperature 140 °C (hold 2 min) to 220 °C at 7 °C/min (hold 19 min); electron impact ionization voltage 70 eV; trap current 100 μA; ion source temperature 150 °C; mass range 22–395 m/z (0.14 scans/s with an intermediate time of 0.02 s between the scans). The FAMEs were identified based on their mass spectra by comparison with those of reference compounds from the NIST mass spectral library (National Institute of Standards and Technology NIST MS Search 2.0). The amount of each fatty acid was expressed as an area percentage calculated from the total area of identified FAMEs.

2.6. Extraction and Analysis of Volatile Compounds by Headspace In-Tube Extraction Coupled with Gas Chromatography–Mass Spectrometry (HS-ITEX/GC–MS)

The analysis was performed by extraction of volatile compounds by the in-tube extraction (ITEX) technique, followed by their separation and identification using GC–MS [21]. Briefly, the extraction of volatile compounds was carried out by incubating a headspace vial containing an aliquot of sample (1000 μL) at 60 °C for 20 min under continuous agitation. After the incubation period, a microtrap fiber (ITEX-2TRAPTXTA, Tenax TA 80/100 mesh) was inserted into the headspace of the vial, and the volatile compounds were adsorbed continuously (30 strokes) with a headspace syringe (CombiPAL AOC-5000 autosampler, CTC Analytics, Switzerland). The volatile compounds were thermally desorbed directly into the GC–MS injector. The separation and identification of volatile compounds was carried out on a GC–MS system (QP-2010 model, Shimadzu Scientific Instruments, Kyoto, Japan). The separation was performed using a Zebron ZB-5 ms capillary column of 30 m × 0.25 mm i.d and 0.25 mm film thickness. The carrier gas was helium, 1 mL/min, split ratio 1:20, injector temperature 250 °C. The temperature program of the column oven was 40 °C (held for 5 min) to 50 °C at 4 °C/min, then to 250 °C at 10 °C/min (held for 5 min). The MS detection was performed on a quadrupole mass spectrometer operated in full scan (35–350 m/z) electron impact (EI) at ionization energy of 70 eV. The volatile constituents of the samples were identified based on their mass spectra, by comparison with those of reference compounds from NIST27 and NIST147 mass spectra libraries (considering a minimum similarity of 85%). The relative percentage of each compound was estimated as a fraction of its integrated ion area from the total ion chromatograms (TIC) area (100%).

2.7. Statistical Analysis

The obtained data of experiments performed in triplicate were reported as mean ± standard deviation (SD) for each sample. Statistically significant differences between samples were determined using ANOVA in Microsoft Excel 2016, the significance being defined at p < 0.05. Pearson’s correlation test was conducted to analyze inter-relationships between the level of fatty acids and aroma compounds of the investigated samples.

3. Results and Discussion

As previously shown by our group, hot air drying and centrifugal vacuum drying of B. edulis mushrooms in the form of a paste proved their efficacy particularly in terms of phenolic content and antioxidant activity; CVD mainly emerged as a new efficient mushroom drying technology [14].

Considering that the hereby described powder formulation may find further useful food applications as a flavoring or thickening agent, and as a functional ingredient with pharmacological properties, other key attributes have been investigated, such as physical characteristics, emulsifying properties, and fatty acid and aroma profiles.

3.1. Physical and Emulsifying Properties of B. edulis POWDERS

The physical properties of B. edulis mushroom powders (moisture content, bulk densities, Hausner ratio, solubility, water solubility index, rehydration ratio) are shown in

Table 1. Physical properties of mushroom powders according to the type of drying.

Sample	Moisture (%)	Loose Density (g/cm3)	Tapped Density (g/cm3)	HR	Solubility (g/100 g Water)	WSI (%)	RR (g/g DM)
Dried by HAD	6.52	0.885 ± 0.007	1.050 ± 0.037	1.186	0.266 ± 0.025	24.688 ± 2.670	4.020 ± 0.216
Dried by CVD	5.89	0.765 ± 0.057	1.047 ± 0.074	1.368	0.316 ± 0.015	28.525 ± 1.475	3.012 ± 0.441

.

According to the type of drying conducted on the mushroom paste, the moisture content decreased from the initial value of 84.58% to 6.52% after conventional HAD (60 °C, 100% fan speed, 254 min), and to 5.89% after CVD (0.1 mbar process vacuum, 1200 rpm, 60 °C, 749 min), values identified as suitable for their storage or further processing.

Loose and tapped densities are essential physical features of powders, indicating their ability to flow. The CVD process determined slightly lower values of loose and tapped bulk density of the mushroom powder than those for HAD samples. Based on the calculated HR, the flow ability of mushroom powders obtained by HAD was “good” compared to that of samples dried by CVD method, which may be considered “poor” according to the classification given in the literature [22]. In relation to the cited paper, the flow character of a powder is considered “good” for HR values of 1.12–1.18, and “poor” for values of 1.35–1.45. The lower the HR index, the better flowability and less cohesiveness of the powder. Such properties offer advantages of handling, storage, and transportation of powder products. Similar HR values produced by HAD at 60 °C have been reported in the literature for green mango powders (1.20) [23]. Other studies showed that cabinet drying of Agaricus bisporus mushroom slices at 50 °C generated a lower flowability of the final powder due to a higher HR value (1.298) [24] than that of the hereby investigated B. edulis paste dried by HAD at a higher temperature, 60 °C (1.186), because of different operational conditions which produced higher intermolecular forces. On the other hand, increasing the drying temperature from 60 °C to 70 °C has been shown to significantly decrease the HR value in red pepper powders obtained using either a convection or a vacuum oven (20 mbar), while a further increase to 80 °C determined a significant HR increase [25]. However, the authors showed that in order to improve the flow properties of red pepper powder, several food-grade additives (maltodextrin, gum arabic) can be added or an agglomeration process can be applied to the obtained powders. The study of Atalar et al. [15] confirmed that an agglomeration process applied to A. bisporus mushrooms significantly decreased the HR value of powders, from 1.44 in the control sample dried at 45 °C to 1.05 ÷ 1.23 depending on the employed operational conditions (inlet air temperature, atomization pressure, pure water amount).

The poor flow characteristics and high cohesiveness based on the obtained HR value of the B. edulis sample dried by CVD technique may be due to a more compact/sticky product that resulted after this type of drying (concentration to dryness), which may increase the forces of attraction between the particles. On the other hand, studies have shown that the presence of liquid/water at a certain content on the particle surface may decrease the van der Waals interaction and improve flowability due to capillary forces, which will bind particles together [22]. This may further explain the better flow characteristics of the hereby investigated mushroom dried by HAD, which had a slight greater water content (by ~11%) than that of the sample dried by CVD.

Solubility and WSI are key physical characteristics of food powders, measuring the ability of powders to dissolve in water and form a homogeneous solution [23,26]. The WSI value of HAD samples was lower (24.688 ± 2.670%) than that of CVD samples (28.525 ± 1.475%). Apart from some factors affecting the powder solubility, e.g., type of drying, powder particle size, and hydrophobic groups, the lower solubility observed in HAD samples might be due to the higher moisture content and different distribution of water molecules into the powder, which may lead to physical chemical changes, such as the formation of hydrates and aggregates. It has been demonstrated that solubility and bulk density are inversely correlated [24], a similar trend being observed for both investigated mushroom powders as indicated in Table 1.

Regarding the RR, samples dried by HAD showed higher values (4.020) compared to mushrooms dried by CVD (3.012). The rehydration of powder is a multi-step process consisting of water absorption, water transfer to the porous structure, water distribution within the solid matrix, solid matrix swelling, and solids’ dissolution into the aqueous medium [27].

The results on the functional properties of B. edulis powders are presented in Figure 1, which comparatively depicts the emulsifying activity and stability indices, EAI and ESI, respectively, for the mushroom powders obtained by the two investigated drying procedures, HAD and CVD. Samples dried by CVD procedure showed slightly larger values of EAI (4.963 ± 0.828 m²/g) and ESI (34.406 ± 7.701 min) than values of EAI (4.687 ± 0.923 m²/g) and ESI (33.741 ± 7.510 min) for HAD samples, indicating a better emulsion stability of CVD samples.

The emulsifying properties, described in the literature by EAI and ESI indices, of different mushroom powders (A. bisporus, Pleurotus geesteranus, Coprinus attrimentarius) or their extracted individual compounds, indicate the ability of their biomolecules (mainly proteins, but also polysaccharides/β-glucans) to form an emulsion [15,28,29] or improve the emulsion characteristics of further developed mushroom-powder-based products, such as meat [15]. In our investigation, the observed differences of EAI and ESI might be explained by the presence of both hydrophobic and hydrophilic residues, probably exposed to a different extent due to the distinct types of drying, which interact with the oil and aqueous phases, respectively. Similar ESI values but much higher EAI values were obtained for a protein solution at pH 7, the investigated proteins being isolated through different precipitation procedures from Pleurotus geesteranus dried mushrooms [28]. The study of Atalar et al. [15] reported higher EAI and ESI values of A. bisporus mushroom powders, 61.47 ± 0.26 m²/g and 42.87 ± 0.90 min, respectively, the obtained values being significantly increased due to the applied agglomeration process using top-spray fluidized bed. To our knowledge, no published study has been found on B. edulis emulsifying properties determined using the spectrophotometric method and expressed by EAI and ESI, so that the hereby obtained results could not be compared to ones reported in the literature. In addition to the most common spectrophotometric method employed for measuring the emulsifying activity and emulsion stability of the powders, other methods have also been described and applied, such as visual inspection of the emulsion layer [29].

3.2. Fatty Acid Content of Raw and Dried B. edulis Mushrooms by GC–MS Analysis

The content of 21 individual fatty acids identified in the raw mushroom paste and in dried samples (hot air and centrifugal vacuum-dried powders) is shown in

Table 2. Fatty acid profile of raw and dried B. edulis mushrooms (% of total fatty acid methyl esters FAMEs); data values are mean ± SD.

No.	Common Name of Fatty Acids (Shorthand and Omega Type)	Content (%)
		Raw Sample	Dried by HAD	Dried by CVD
1	Caproic acid (C6:0)	0.06 ± 0.01	0.09 ± 0.02	0.02 ± 0.01
2	Caprylic acid (C8:0)	0.04 ± 0.01	0.14 ± 0.04	0.02 ± 0.01
3	Capric acid (C10:0)	0.04 ± 0.01	0.09 ± 0.01	0.03 ± 0.01
4	Myristic acid (C14:0)	0.24 ± 0.02	0.39 ± 0.04	0.22 ± 0.02
5	Pentadecanoic acid (C15:0)	0.39 ± 0.04	0.39 ± 0.04	0.30 ± 0.03
6	Palmitic acid (C16:0)	11.31 ± 0.62	11.68 ± 0.64	12.49 ± 0.69
7	Hypogeic acid (C16:1 n-9)	0.21 ± 0.03	0.24 ± 0.04	0.22 ± 0.03
8	Palmitoleic acid (C16:1 n-7)	0.63 ± 0.03	0.69 ± 0.03	0.74 ± 0.03
9	Margaric acid (C17:0)	0.13 ± 0.01	0.05 ± 0.01	0.11 ± 0.01
10	Stearic acid (C18:0)	2.39 ± 0.11	1.64 ± 0.07	1.75 ± 0.08
11	Oleic acid (C18:1 n-9)	28.75 ± 1.28	22.31 ± 0.99	23.94 ± 1.07
12	Vaccenic acid (C18:1 n-7)	1.95 ± 0.09	1.66 ± 0.07	2.07 ± 0.09
13	Linoleic acid (C18:2 n-6)	51.91 ± 2.08	58.52 ± 2.34	56.45 ± 2.26
14	α-linolenic acid (C18:3 n-3)	0.08 ± 0.02	0.03 ± 0.01	0.03 ± 0.01
15	Arachidic acid (C20:0)	0.34 ± 0.02	0.35 ± 0.04	0.19 ± 0.02
16	Gondoic acid (C20:1 n-9)	0.30 ± 0.05	0.00 ± 0.00	0.25 ± 0.04
17	Eicosadienoic acid (C20:2 n-6)	0.22 ± 0.02	0.38 ± 0.04	0.25 ± 0.03
18	Behenic acid (C22:0)	0.30 ± 0.06	0.42 ± 0.08	0.25 ± 0.05
19	Erucic acid (C22:1 n-9)	0.16 ± 0.03	0.22 ± 0.04	0.17 ± 0.03
20	Lignoceric acid (C24:0)	0.31 ± 0.03	0.34 ± 0.03	0.25 ± 0.03
21	Nervonic acid (C24:1 n-9)	0.23 ± 0.03	0.36 ± 0.05	0.24 ± 0.04

.

The fatty acid composition of raw B. edulis showed a reasonably low content of saturated fatty acids SFAs (15.56 ± 0.70%), which is similar to most edible vegetal oils ranging from 6.3% (rapeseed oil) to 22.5% (rice bran oil) [30]. The major contributors of SFAs were palmitic and stearic acids, and these results are in agreement with other reported ones on B. edulis originating from the Sălaj region of Romania [21] or different regions of Poland [31]. Regarding mono- and polyunsaturated fatty acids (MUFAs and PUFAs), they represent the major part of the total fatty acids (84.45 ± 3.46%), the PUFAs being the predominant type in all hereby investigated samples. The PUFAs/SFAs ratio, which denotes the nutritional quality of food lipids, should be >0.4, according to the recommendation of health guidelines [32]. In our study, the PUFAs/SFAs ratio for raw mushrooms was 3.36. Among MUFAs, the greater part is represented by omega-9 fatty acids, also called ω-9 or n-9 (C16:1, C18:1, C20:1, C22:1, and C24:1), which belong to the non-essential group of fatty acids [33]. In particular, oleic acid was found in a great amount (28.75 ± 1.28%), as previously reported by other authors [21], with some variations in wild B. edulis samples from Poland, which ranged from 19.56 ± 3.11% to 44.75 ± 0.70% according to the region [31]. Other MUFAs identified in raw samples, in low amounts, were the omega-7, palmitoleic, and vaccenic acids. Similar to the particular PUFAs of raw mushrooms reported in the above-mentioned papers, the only essential omega-3 fatty acid was α-linolenic acid, hereby found in very low amounts, while the predominant one was omega-6 linoleic acid (51.91 ± 2.08%). Comparable content of linoleic acid has been reported for B. edulis originating from Israel, Poland, and Romania, while lower content (~33%) was reported for samples from Turkey and India, and higher content (75.6%) for some samples from Turkey [7,21,31]. Despite the fact that B. edulis mushrooms are deficient in essential omega-3 fatty acids, they remain a good source of the following: (i) omega-6 linoleic acid, an essential fatty acid which may be consumed moderately for its role as precursor of other essential omega-6 fatty acids in the human body, such as arachidonic acid; and (ii) omega-9 fatty acids, which have been shown to possess anti-inflammatory and anticancer properties [34].

As shown in Figure 2, drying mushroom samples at 60 °C did not affect the level of SFAs compared to the one in the raw sample and did not negatively impact the total content of PUFAs. On the contrary, a slight increase of the PUFAs level was noticed. Most studies have shown that drying the starting raw biomaterials of different origins, such as seeds, at temperatures below 80 °C did not generate significant changes in the fatty acids profile, while drying at temperatures above 100 °C caused a significant decrease of the content of PUFAs [35]. The results presented in Table 2 indicate an increase of the linoleic acid content in mushrooms dried either by HAD or CVD, accompanied by a decrease of some MUFAs (oleic and gondoic acids). One explanation might be the changes produced in the lipid structures during the processing of raw materials (homogenization, drying, powdering), which may favor the action of some enzymes, e.g., desaturase and elongase, involved in the conversion of oleic acid to linoleic acid, and elongation of gondoic acid to erucic and nervonic acids, respectively [36,37]. A slight increase of linoleic acid was reported by other authors in Kinnow mandarin citrus seed oil after seed drying at 60 °C or 70 °C [38].

As shown in Figure 2, the SFAs content of mushroom samples did not change with drying, while the MUFAs content decreased by 14–21% in dried samples, and the PUFAs level increased by 7–13% in dried samples compared to those of the raw sample. Despite the fact that no statistically significant differences at p < 0.05 between groups of samples and total fatty acids were found (f-ratio 0.00379, p-value 0.996215), it seems that the HAD method produces more changes in the unsaturated fatty acids profile than the CVD technique. Regarding the detected trend of PUFAs, some studies in the literature also reported a higher content of PUFAs in chicken fat heated at temperatures of 60–80 °C compared to that of the sample not subjected to thermal treatment, while temperatures >140 °C conducted to a lower PUFAs content [39].

3.3. Profile of Volatile Aroma Compounds in Raw and Dried B. edulis Mushrooms by HS-ITEX/GC–MS Analysis

The aroma of mushrooms is given by a combination of volatile and non-volatile compounds of different chemical structures, such as alcohols, ketones, esters, aldehydes, hydrocarbons, acids, and heterocyclic and aromatic molecules [6].

As indicated in

Table 3. Mean relative peak areas (expressed as % from total peak areas) of aroma compounds from raw and dried B. edulis mushrooms as determined by HS-ITEX/GC–MS; data values are mean ± SD.

No.	Compound	Content (%)
		Raw Sample	Dried by HAD	Dried by CVD
Alcohols
1	3-Methyl-1-butanol	nd	0.28 ± 0.04	0.37 ± 0.03
2	1,7-Octadien-3-ol	0.03 ± 0.001	nd	nd
3	1-Octanol	nd	0.16 ± 0.01	nd
4	1-Octen-3-ol	70.5 ± 1.13	91.71 ± 1.32	91.25 ± 1.41
5	€-2-Octen-1-ol	0.04 ± 0.001	0.20 ± 0.01	nd
6	(Z)-2-Octen-1-ol	1.12 ± 0.03	2.48 ± 0.09	2.99 ± 0.03
7	Octen-1-ol, acetate	nd	0.22 ± 0.02	nd
8	(Z)-3-Octen-1-ol, acetate	nd	0.07 ± 0.001	nd
Aldehydes
9	Benzaldehyde	0.20 ± 0.008	0.10 ± 0.001	nd
10	Benzene acetaldehyde	nd	0.30 ± 0.01	0.20 ± 0.02
11	Dodecanal	0.03 ± 0.001	nd	nd
12	2-Ethyl-2-hexenal	nd	0.28 ± 0.03	nd
13	2-Ethyl-trans-2-butenal	0.29 ± 0.01	nd	nd
14	Heptanal	nd	0.39 ± 0.39	0.41 ± 0.02
15	Hexanal	0.41 ± 0.01	0.98 ± 0.04	1.44 ± 0.02
16	2-Methyl-2-butenal	11.88 ± 0.15	nd	nd
17	2-Methyl-2-hexenal	0.12 ± 0.02	nd	nd
18	Nonanal	0.07 ± 0.001	nd	nd
19	(E)-2-Octenal	5.74 ± 0.06	nd	nd
20	(E)-2-Pentenal	0.16 ± 0.02	1.61 ± 0.02	nd
Ketones
21	2-Heptanone	nd	0.61 ± 0.02	0.59 ± 0.04
22	3-Octanone	0.34 ± 0.05	nd	nd
23	1-Octen-3-one	9.06 ± 0.11	nd	nd
Others (hydrocarbons, sesquiterpenes, sulfur compounds, etc.)
24	Caryophyllene	nd	0.07 ± 0.001	nd
25	Dimethyl disulfide	nd	nd	1.59 ± 0.02
26	D-Limonene	nd	0.52 ± 0.04	0.35 ± 0.04
27	2-n-Pentyl-furan	nd	nd	0.82 ± 0.09

nd = not detected.

, 15 aroma compounds were detected by HS-ITEX/GC–MS in raw B. edulis samples, 16 aroma compounds in samples dried by the HAD technique, and 10 aroma compounds in samples dried by the CVD method. A more diverse composition of aroma compounds was identified in HAD powders.

The most abundant aroma compounds identified in the raw sample were 1-octen-3-ol (70.52 ± 1.13%), 2-methyl-2-butenal (11.88 ± 0.15%), 1-octen-3-one (9.06 ± 0.11%), and (E)-2-octenal (5.74 ± 0.06%). From the odor point of view, among C8 aroma compounds, 1-octen-3-ol and 1-octen-3-one have been described as being “earthy/mushroom-like”, while different attributes have been given to the others, as following: (E)-2-octenal—“green, fatty”; 3-octanone—“sweet, fruity, musty”; 1-octanol—“detergent, soap, orange-like”; (E)-2-octen-1-ol—“grass-like”; and 1-octen-3-one also “metallic” [6,40]. The aroma profile of fresh edible mushrooms may vary according to their maturity [6], storage time and temperature, and analysis techniques [41].

An increase of the diversity of aroma compounds have been recorded with the drying process of raw wild B. edulis. The only aroma compounds commonly identified both in raw and dried samples, but at different extents, were hexanal, 1-octen-3-ol, and (Z)-2-octen-1-ol. The statistical analysis indicates a strong positive correlation of these volatile compounds with C18 unsaturated fatty acids, oleic, linoleic, and α-linolenic acids, R = 0.8837, p < 0.05. Low amounts of new volatile compounds, such as caryophyllene and D-limonene were detected in HAD-dried samples, and D-limonene, dimethyl disulfide, and 2-n-pentyl-furan in CVD-dried samples. The new volatile generated compound, dimethyl disulfide, might be a degradation product of the amino acid methionine [42]. Dimethyl disulfide has been previously identified at different extents in wild edible mushrooms, air-dried at 25 °C for 24 h, e.g., B. edulis from the Sălaj region of Romania (11.56 ± 0.48%) and Calvatia gigantea from Louisville Kentucky, USA (trace amounts) [21]. New volatile compounds such as hydrocarbons, furans, carbonyls (benzene acetaldehyde, heptanal, 2-heptanone) or sulfur compounds may be the result of fatty acid decomposition or amino acid degradation due to the thermal treatment applied to mushrooms. Further, 1-octen-3-ol was the major contributor to the volatile composition of dried mushrooms, whose content increased to mean relative peak area values of 91.25–91.71%, as shown in Table 3. These results correlate with the increase of linoleic acid in B. edulis samples during the drying process (Table 2), this PUFA being considered the oxidation precursor for the enzymatic formation of 1-octen-3-ol [43]. Such synthesis has been commercialized for the production of 1-octen-3-ol, used as a food additive approved by the US FDA due to its low odor threshold [11]. However, the high concentration of 1-octen-3-ol may cause significant off-flavors in some food products [11]. Several important aldehyde aroma contributors in raw B. edulis [2-methyl-2-butenal, (E)-2-Octenal] and other minor aldehydes (dodecanal, 2-ethyl-trans-2-butenal, 2-methyl-2-hexenal, nonanal) were not identified in the dried samples, probably due to their thermal-oxidative degradation during the drying process. Other authors reported lower levels of a rarely detected aroma compound (3-methylbutanal) in dried porcini [10]. We have noticed a total loss with drying of some aroma C8 ketones identified in the raw sample, 1-octen-3-one and 3-octanone, but a new 2-heptanone is generated in dried samples. A decrease of aldehydes and ketones with drying has been frequently reported for different edible mushrooms, e.g., B. edulis, Flammulina velutipes [6,10].

4. Conclusions

The present study described the effects of different drying methods of B. edulis mushrooms on their physical and powder properties, and on their fatty acid and aroma compounds profiles.

Better flowability based on a low Hausner ratio and higher rehydration ratio were observed for HAD samples, while an enhanced water solubility index and better emulsifying activity and emulsion stability indices were noticed for CVD samples, despite the fact that results were not statistically significant.

The composition of 21 fatty acids identified in B. edulis varied between raw and dried samples. Drying at 60 °C, either by a convective or centrifugal vacuum method, did not affect the total content of saturated fatty acids but led to a decrease of some unsaturated fatty acids (mainly oleic acid) and an increase of linoleic acid. The PUFAs/SFAs ratio was >3.3 in raw and dried samples as well, meeting the requirements for healthy lipids according to health guidelines.

Drying determined important changes in types and contents of volatile aroma compounds of B. edulis. Among more than 10 aroma compounds, only 3 compounds (hexanal, 1-octen-3-ol, and (Z)-2-octen-1-ol) were commonly identified either in raw or dried samples, but to different extents. The compound 1-octen-3-ol was the major contributor to the volatile composition of dried mushrooms, whose content increased from a mean relative peak area value of 70.52 ± 1.13% in the raw sample to ~91% in dried ones. Statistical analysis indicates a strong positive correlation of this volatile compound with its precursor, linoleic acid. Low amounts of new volatile substances, alcohols, aldehydes and ketones, or others, such as D-limonene and caryophyllene, were detected in HAD powders, while D-limonene, dimethyl disulfide, and 2-n-pentyl-furan were detected in CVD ones. Drying led to a total loss of 2-methyl-2-butenal, (E)-2-octenal, and 1-octen-3-one.

The description of the major contributors to volatile aroma compounds and fatty acids, both saturated and unsaturated, helps scientists to understand the mechanism of action involved during thermal treatment and supports food manufacturers in selecting the desired drying technique of B. edulis based on the powder/emulsifying properties and on the recovered valuable biocompounds.