Determination of Various Drying Methods’ Impact on Odour Quality of True Lavender (Lavandula angustifolia Mill.) Flowers

True lavender flowers (Lavandula angustifolia Mill.) is a critical source of essential oils and a flavouring agent used in numerous industries like foods, cosmetics and pharmaceuticals. Its main volatile constituents are linalool and linalyl acetate, which are commonly considered as main odour-active constituents (OACs). Nevertheless, the quality of true lavender flowers is highly dependent on its post-harvest treatment, mainly the preservation method. Recognising that drying is the most frequently used preservation method, the influence of various drying methods, including convective drying (CD) at 50, 60 and 70 °C, vacuum-microwave drying (VMD) with powers 240, 360 and 480 W and combined convective pre-drying at 60 °C followed by vacuum-microwave finish-drying with power 480 W (CPD-VMFD), on the quality of true lavender flowers was verified. The evaluation of influence was carried out by HS-SPME(HS, solid-phase microextraction), GC-MS, GC-MS-O (gas chromatography–mass spectrometry–olfactometry) techniques. Moreover, the sensory panel has assessed the sample odour quality. As a result, the optimal drying methods regarding the requirements for products were established. Overall, for total essential oil recovery, CD at 50 °C is the optimal drying method, while for odour quality concerning the sensory panel evaluation, VMD with power 360 W combined CPD-VMFD and CD at 50 °C is the optimal drying method.


Introduction
Lavandula genus is a large group of tremendously useful plants and, along with such herbs like rosemary, basil or sage, belongs to the Lamiacae family. The most well-known representative of Lavandula sp. is Lavandula angustifolia Mill.-true lavender. Its visibility is caused by numerous applications, both of the plant and its derivatives in pharmacy, aromatherapy, perfumes and cosmetics, food flavouring and preservation, household products or just as a decorative plant [1,2]. True lavender plants, with its characteristic violet flowers and narrow leaves, are native to Europe, mainly the Mediterranean area, North America and Australia. Nowadays, the largest cultivations are located in France, Bulgaria and Turkey. In Bulgaria its cultivation area reaches more than 6000 ha, and the Turkish production of lavender flowers oscillates around 50-75 tons per ha [3][4][5][6].  Table 1. flower dehydrated by the CD, VMD and CPD-VMD methods, characterized by high values of the determination coefficient (R 2 > 0.99) and low root-mean squared error (RMSE) values (<0.05). A good adaptation of the applied Page model to the description of the drying kinetics was found in many earlier publications of dill leaves, true lavender leaves, chanterelle and oyster mushrooms [25][26][27][28].    The Page model can be successfully used to describe the drying kinetics of the true lavender flower dehydrated by the CD, VMD and CPD-VMD methods, characterized by high values of the determination coefficient (R 2 > 0.99) and low root-mean squared error (RMSE) values (<0.05). A good adaptation of the applied Page model to the description of the drying kinetics was found in many earlier publications of dill leaves, true lavender leaves, chanterelle and oyster mushrooms [25][26][27][28].

Drying Kinetics
In the case of CD, increasing the drying air temperature from 50 to 70 • C decreased the time of drying from 245 to 150 minutes, respectively. With respect to VMD, radical reductions in the total drying time have been observed: the time was shortened from 44 to 20 minutes with a power change from 240 to 480 W. This radical reduction in the total drying time of VMD compared to CD is a result of the conventional water diffusion occurring according to Fick's law, which is supported by a pressure diffusion mechanism of the Darcy type [29]. Combined CPD and VMFD using 480 W shortened the drying time of flowers almost four-fold compared to CD at 50 • C. The use of CD and 480 W power caused a drop in the material temperature during VMD by 4 • C for flowers in reference to VMD 480 W. This condition is caused by the molecular distribution of water particles inside the dried material, and the distribution of water particles has an effect on the generation of heat energy under microwave radiation during VMD [30,31]. Energy consumption during the CD of plant materials is much lower than in VMD [32,33]. In industrial conditions, the best solution is combined drying consisting of CPD and VMFD. The CD is very effective at the beginning of the drying process (the largest loss of water occurs during the phase) and VMD at the final stage of drying (removal of water strongly bound to the cellular structure of the material being dried) [23,30,31]. The final choice of recommendations drying process should be related to the aspects of the dried material (volatile composition and sensory attributes) [30].

Volatile Profile of True Lavender Flowers Cultivated in Poland
The volatile profile of true lavender flowers consisted of seventy-four constituents, among which seventy-two were identified (the mass spectra of unidentified compounds are available in supplementary data). This high number of volatile compounds agreed quite well with results obtained byŚmigielski et al. (2018) [34], which had identified sixty-seven compounds in the true lavender flower EO. However, they achieved higher sensitivity in the case of dried flowers, which may be caused by applying different analytical methods. It should be underlined that they had used true lavender flowers harvest in 2013, delivered by the same supplier that was used in this study (harvest in 2018).
Other major constituents revealed in this study were cis-β-ocimene (4.97% ± 0.74%), trans-β-ocimene (3.99% ± 0.11%), 1-octen-3-ol acetate (1.57% ± 0.20%), terpinen-4-ol (3.36% ± 0.29%), lavandulyl acetate (5.17% ± 0.48%), caryophyllene (7.57% ± 1.45%) and alloaromadendrene (3.80% ± 0.76%). All of these constituents are reported in ISO and pharmacopeial standards for true lavender EO [1]. All results are listed in Table 2.  [37] obtained results from some true lavender varieties (French blue, H-701 and C-179(2)) collected in China, similarly to this study. Moreover, some other constituents in less amounts like cis-β-ocimene, trans-β-ocimene, lavandulyl acetate [37] and caryophyllene [36] also overlap in this and mentioned studies. Nevertheless, Da Porto and Decorti (2008) [36] also report high amounts of eucalyptol (7.95%-10.78%) and camphor (8.60%-12.59%) found in the volatile profile of true lavender flowers, which is in opposition for this and to our recent study regarding the volatile profile of true lavender leaves [25]. It appears that, for the chemotype of true lavender used in these studies, the characteristic is high ratio camphor/eucalyptol and low amounts of linalool and linalyl acetate in the volatile profile of true lavender leaves, and the opposite situation in the volatile profile of true lavender  [40], who obtained on average of 6.52% of EO cultivated in the Czech Republic. Moreover, also according to Aprotosoaie et al.'s (2017) [1] overview, this result is within the boundaries reported by numerous studies. It should be underlined that steam distillation, rather than hydrodistillation, is applied in industry practice. Despite that, for analytical purposes in laboratory scale, it is common to apply hydrodistillation with Clevenger or Deryng apparatus, which is supported by methods used in numerous works [41][42][43][44][45][46][47]. However, there may be an observed difference in the distribution of major constituents. In the current study, similar amounts of linalyl acetate (18.98% ± 0.33%) and linalool (18.37% ± 0.12%) were found, which is in opposition toŚmigielski et al.'s (2018) [34] result (respectively 19.7%-23.4% and 26.5%-34.7%). This result may again be caused by different years of cultivation of true lavender flowers. Nevertheless, applied drying methods had significantly affected the EO composition and major volatile constituents' retention, which is presented in Table 3 (list of all identified constituents of true lavender flowers EO is available in supplementary data). The most efficient drying methods, which are not significantly different, were CD at 50 • C and 70 • C or CPD-VMFD, although those methods still decreased the EO yield from 3.8 to 4.5 times. This result is higher than the one reported by Prusinowska andŚmigielski (2015) [48], however, they considered a different drying method-shade drying. Also, Sadowska (2012) [49] had pointed out that naturally dried lavender was characterized by a higher amount of EO than that dried mechanically. On the contrary, Figiel et al. (2010) [50] and Nöfer et al. (2018) used the same drying methods respectively on oregano (Origanum vulgare) and king bolete (Boletus edulis) and obtained results similar to those found in this study. In the case of major volatile constituents of EO, the most efficient drying method (not for all compounds) overlapped with the most efficient method for total EO retention, although for most CD at 50 • C it was still the most attractive one. This result is in contrast to our previous work, focused on true lavender leaves [25], where the CD at 50 • C was the less effective for total EO. Nevertheless, it is worth underlining that, in the case of true lavender flowers, twelve out of fourteen compounds demonstrate the highest retention for this drying method. These different results may be caused by differences in the morphological structure of true lavender leaves and flowers. Also Chua et al. (2019a) [42] and Chua et al. (2019b) [51] had observed more efficient results for CD than for VMD drying methods. The drying process had also affected the volatile constituents present in the volatile profile of true lavender flowers determined by HS-SPME technique, which is presented in Table 4. Obtained results differ from results for EO constituents. The efficient drying method is more diversified for specific constituents. In the case of two major odour-shaping constituents of true lavender flowers, linalool and linalyl acetate, respectively CD at 50 • C and CD at 60 and 70 • C, improved the highest compounds share in volatile profile, which partially coincides with EO analysis results. Overall, these results are similar to those obtained for true lavender leaves [25].

Odour-Active Compound Determination and Sensory Value of True Lavender Flowers Affected by Various Drying Methods
GC-MS-Olfactometry analysis of true lavender flower EO had revealed that only ten constituents from seventy-four identified in its volatile profile may be classified as OACs and are presented in Table 5 within their aroma description and variability. The majority of them were also identified by Xiao et al. (2017) [52] although they identified thirty constituents as OACs in total. The difference may be caused by the origin of the true lavender plants, among other factors. Moreover, in this study OACs were identified for dried true lavender flowers. The sensory panel results, given in Figure 2, had shown that true lavender flowers dried by the VMD 360 W method were recognized as the material with the most intense odour, while the CD 70 • C method was the less suitable since it had the lowest intensity (.pdf file with all ranks is available in supplementary data). Also, with lower ranks but not scientifically different was the assessment of combined CPD-VMFD and CD at 50 • C products. These results were surprising because they do not agree with the total volatile contents discussed previously here, where CD methods were more effective than VMD methods in retaining a higher content of EO. Such findings demonstrated that the highest amounts of constituents or their share in a volatile profile do not always guarantee the best sensory quality. It seemed that, in the case of major true lavender flower OACs, the most favourable linalool:linalyl acetate ratio is approximately 1:2, while linalyl acetate share is around 43%-47%. Similar findings were reported by Beale et al. (2017), who showed that lower amounts of linalool over linalyl acetate improved the quality of EO [55]. Moreover, higher amounts of linalyl acetate over 50% strongly decreased the sensory panel score. This fact agreed with the literature, which indicates that the linalyl acetate share in EO should not become excessive by exceeding 46% [20]. The sensory panel results, given in Figure 2, had shown that true lavender flowers dried by the VMD 360 W method were recognized as the material with the most intense odour, while the CD 70 °C method was the less suitable since it had the lowest intensity (.pdf file with all ranks is available in supplementary data). Also, with lower ranks but not scientifically different was the assessment of combined CPD-VMFD and CD at 50 °C products. These results were surprising because they do not agree with the total volatile contents discussed previously here, where CD methods were more effective than VMD methods in retaining a higher content of EO. Such findings demonstrated that the highest amounts of constituents or their share in a volatile profile do not always guarantee the best sensory quality. It seemed that, in the case of major true lavender flower OACs, the most favourable linalool:linalyl acetate ratio is approximately 1:2, while linalyl acetate share is around 43%-47%. Similar findings were reported by Beale et al. (2017), who showed that lower amounts of linalool over linalyl acetate improved the quality of EO [55]. Moreover, higher amounts of linalyl acetate over 50% strongly decreased the sensory panel score. This fact agreed with the literature, which indicates that the linalyl acetate share in EO should not become excessive by exceeding 46% [20].

Plant Material
The study was performed on true lavender flowers cultivated in 2018 in Poland (Kawon-Hurt Nowak Sp.j. Company, Gostyń, Poland). The initial moisture content of the material has been determined for 2.2 kg·kg −1 . The drying processes were stopped after no further change in weights was observed. The moisture content of samples was determined using a vacuum dryer (SPT-200. ZEAMIL Horyzont, Krakow, Poland). The flowering aerial parts of true lavender were separated from leaves and straws and dried using three methods CD, VMD and CPD-VMFD.

Drying Methods
All drying performances were carried out at the Institute of Agricultural Engineering (Wrocław University of Environmental and Life Sciences, Wrocław, Poland). CD was performed using equipment engineered and constructed therein. True lavender flowers were placed in the round container (d = 100 mm) and dehydrated at 50 °C, 60 °C and 70 °C, all temperatures with an air velocity of 0.5 ms -1 .
VMD technique was carried on SM 200 dryer (Plazmatronika, Wrocław, Poland). Cylindrical drum (made of glass, 18 cm of diameter × 27 cm of length) was rotated with 6 rev·min −1 . The vacuum conditions were obtained by applying BL 30P vacuum pump (Tepro, Koszalin, Poland), an MP 211 vacuum gauge (Elvac, Bobolice, Poland), a compensation reservoir of 0.15 m 3 capacity and a cylindrical tank. Samples were dried with applying three power levels-240, 360 and 480 W, and pressure ranged from 4 up to 6 kPa. The maximum temperature of dried true lavender flowers was

Plant Material
The study was performed on true lavender flowers cultivated in 2018 in Poland (Kawon-Hurt Nowak Sp.j. Company, Gostyń, Poland). The initial moisture content of the material has been determined for 2.2 kg·kg −1 . The drying processes were stopped after no further change in weights was observed. The moisture content of samples was determined using a vacuum dryer (SPT-200. ZEAMIL Horyzont, Krakow, Poland). The flowering aerial parts of true lavender were separated from leaves and straws and dried using three methods CD, VMD and CPD-VMFD.

Drying Methods
All drying performances were carried out at the Institute of Agricultural Engineering (Wrocław University of Environmental and Life Sciences, Wrocław, Poland). CD was performed using equipment engineered and constructed therein. True lavender flowers were placed in the round container (d = 100 mm) and dehydrated at 50 • C, 60 • C and 70 • C, all temperatures with an air velocity of 0.5 ms −1 .
VMD technique was carried on SM 200 dryer (Plazmatronika, Wrocław, Poland). Cylindrical drum (made of glass, 18 cm of diameter × 27 cm of length) was rotated with 6 rev·min −1 . The vacuum conditions were obtained by applying BL 30P vacuum pump (Tepro, Koszalin, Poland), an MP 211 vacuum gauge (Elvac, Bobolice, Poland), a compensation reservoir of 0.15 m 3 capacity and a cylindrical tank. Samples were dried with applying three power levels-240, 360 and 480 W, and pressure ranged from 4 up to 6 kPa. The maximum temperature of dried true lavender flowers was measured just after removal from the dryer using an i50 infrared camera (Flir Systems AB, Stockholm, Sweden).
The combinate CPD-VMFD consisted of two steps. First, the material was pre-dried applying CD at 60 • C until the moisture content of flowers approximately 0.44 kg·kg −1 was reached. Then the drying was finished by using VMD at 480 W.

Modelling of Drying Kinetics
The drying kinetics of convective drying (CD), vacuum microwave drying (VMD), and combined drying consisting of convective pre-drying followed by vacuum microwave finish-drying (CPD-VMFD) were fitted based on the mass losses of the true lavender samples. The drying kinetics of convective drying (CD) and weight losses were monitored every 2 min for the initial 20 min, and then every 5 min until the end of the drying process.
Vacuum microwave drying (VMD) samples were monitored every 2, 3 and 4 min for 480, 360 and 240 W. Different drying time intervals were applied in order to a similar energy input between subsequent measurements regardless of the microwave power level.
The moisture ratio (MR) of lavender flowers during drying experiments was calculated using the following equation: where M(t) is the moisture content at time τ, Mo is the initial moisture content, and M e is the equilibrium moisture content (kg water kg -1 dw). The values of Me are relatively little compared to those of M (t) or M o , the error involved in the simplification is negligible [56][57][58]; thus moisture ratio was calculated as: Table Curve 2D Windows v2.03 was used to fit the basic drying models to the measured MR determined according to Equation 2. Several drying models in the literature can be used to describe the kinetics of drying plant materials. For drying model selection, drying curves were fitted to five well-known thin drying models: the modified Page, Henderson-Pabis, logarithmic, Midilli-Kucuk, and Weibull models. The best of fit was determined using two parameters: the values for the coefficient of determination (R2) and root-mean squared error (RMSE). A model fits better if the value of R2 is closer to 1 and the RMSE value is closer to 0 using the following equations: where MR is moisture ratio, (MR) is the mean value of moisture ratio, "pre" and "exp" indicate predicted and experimental values, respectively, while "i" indicates subsequent experimental data, and N is the number of observations. Tests conducted in this study proved that the best fitting was obtained for the modified Page model: where A, n and k are constants.

Hydrodistillation of EO
To obtain EOs from true lavender flower samples, Deryng apparatus was applied according to Szumny et al.'s [59] method. Briefly, fresh or dried flowers were placed in 250 mL round bottom flask; then, 150 mL of distilled water was added. The flask was heated up to boiling point and, then, it was kept for 1 h at this temperature. Immediately after reaching the boiling point, 1 mL of cyclohexane with 2 mg of 2-undecanone as internal standard (Sigma-Aldrich, Saint Louis, MO, USA) was added to collect EO. After extraction, a solvent with EO was collected and kept in −18 • C until GC-MS analysis was performed. Hydrodistillations for all samples were run in triplicates.

EO GC-MS Analysis
GC-MS analysis was carried out on Shimadzu GCMS-QP2020 (Shimadzu Company, Kyoto, Japan) equipped with Zebron ZB-5 MSI (30 m × 0.25 mm × 0.25 µm) column (Phenomenex, Torrance, CA, USA). The GC oven temperature was programmed from 50 • C kept for 2 min to 130 • C at a rate of 4.0 • C, then to 270 • C at a rate of 10.0 • C and kept for 5 min. Scanning was performed from 50 to 400 m/z in electronic impact (EI) mode at 70 eV. Samples were injected at a 1:100 split ratio, and helium gas was used as the carrier gas at a flow rate of 1.1 mL·min −1 . Analyses were run in triplicate.

Headspace Solid-Phase Microextraction (HS-SPME)
HS-SPME analysis was applied with 2 cm DVB/CAR/PDMS fibre (Supeclo, Bellefonte, PA, USA). About 0.100 g of fresh flowers or 0.150 g of dried flowers were put into a headspace glass vial and 0.5 mg of 2-undecanone (Sigma-Aldrich, Saint Louis, MO, USA) as an internal standard was added. Then, the vial was placed in a laboratory water bath at 70 • C, and the fibre was exposed above the sample (headspace) for 30 min and followed by analyte desorption in GC injector for 3 min at 220 • C. A GC-MS analysis was performed on Varian CP-3800/Saturn 2000 apparatus (Varian, Walnut Creek, CA, USA) equipped with Zebron ZB-5 MSI (30 m × 0.25 mm × 0.25 µm) column (Phenomenex, Torrance, CA, USA). The GC oven temperature was programmed from 50 • C to 130 • C at a rate of 4.0 • C, then to 180 • C at a rate of 10.0 • C, then to 280 • C at a rate of 20.0 • C. Scanning was performed from 35 to 550 m/z in electronic impact (EI) mode at 70 eV. Samples were injected at a 1:10 split ratio, and helium gas was used as the carrier gas at a flow rate of 1.0 mL·min −1 . Analyses were run in triplicate.

Identification and Quantification of Volatile Compounds and EO Constituents
Identification of both volatile compounds and EOs constituents was based upon a comparison of experimentally obtained mass spectra and Kovats retention indices (RI) with those available in NIST WebBook, NIST14 database and literature data [35]. The data was processed using Shimadzu software GCMS Postrun Analysis (Shimadzu Company, Kyoto, Japan) and ACD/Spectrus Processor (Advanced Chemistry Development, Inc., Toronto, ON, Canada). The quantification of identified constituents was performed by calculation based on the amount of added internal standard (2.0 mg of 2-undecanone) and the percentages of particular peaks area.

Determination of Odour-Active Compounds of True Lavender Flowers
The identification of OACs was performed on Shimadzu GCMS-QP2020 (Shimadzu Company, Kyoto, Japan) with sniffing port, equipped with Restek Rxi-5MS (30 m × 0.25 mm × 0.25 µm) column (Bellefonte, PA, USA). The GC oven temperature was programmed from 50 • C and kept for 2 min, to 130 • C at a rate of 4.0 • C, then to 180 • C at a rate of 10.0 • C, then to 280 • C at a rate of 20.0 • C and kept for 1 min. Scanning was performed from 35 to 550 m/z in electronic impact (EI) mode at 70 eV. Samples were injected at a 1:10 split ratio and helium gas was used as the carrier gas at a flow rate of 3.8 mL·min −1 .

Sensory Evaluation
A group of thirty-nine trained judges was organized at the Orihuela Campus (Escuela Politécnica Superior de Orihuela) of Universidad Miguel Hernández de Elche to evaluate the sensory quality of dried true lavender flowers. The panel was selected and trained following ISO standard 8586-1 [60,61].
Samples were presented coded and in random order in one orientation session. Panellists' assessments were made in individual booths with controlled illumination (70-90 footcandles) and temperature (23 ± 2 • C). Panellists were asked to rank the samples according to the intensity of sensory attributes (fresh lavender aroma).

Statistical Analysis
The data from drying kinetics were subjected to the analysis of variance using Tukey's test (p < 0.05) and the data from quantitative EO and volatile constituents were subjected to the analysis of variance using Duncan's test (p < 0.05), all using the STATISTICA 13.3 software for Windows (StatSoft, Krakow, Poland). The data obtained during the sensory panel were analysed using Friedman's rank-sum analysis (α = 0.05). The means were compared with Tukey's Honest Significance Difference (HSD) with significance at p < 0.05.

Conclusions
This study on the influence of various drying methods on true lavender (Lavandula angustifolia Mill.) flowers cultivated in Poland had revealed that the optimal drying method is highly dependent on the purpose of the dried product. In respect of drying kinetics, it seems that combined CPD-VMFD is the most favourable drying method, nevertheless, in the case of volatile constituents and preferable odour, other methods should be chosen. For highest total EO recovery, the optimal drying method is CD at 50 • C (1.35% of EO in dried true lavender flowers, while in fresh ones it is 5.18%). Also, CD at 50 • C is the optimal drying method for almost all specific major EO constituents. Concerning odour quality, the sensory panel indicated that VMD with power 360 W, as well as combined CPD-VMFD and CD at 50 • C products, possess the strongest aroma compared to fresh true lavender flowers. This result may be caused by specific changes of OACs ratio in the volatile profile of true lavender flowers. HS-SPME analysis had revealed that in those cases of linalool to linalyl acetate, considered as a min OACs of true lavender flowers, the ratio is close to 1:2, which appears to be most favourable distribution.