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Industrial Crops and Products 52 (2014) 118– 124

Contents lists available at ScienceDirect

Industrial Crops and Products

journa l h om epa ge: www.elsev ier .com/ locate / indcrop

hanges of essential oil content and composition during convectiverying of lemon balm (Melissa officinalis L.)

imitrios Argyropoulos ∗, Joachim Müllerniversität Hohenheim, Institute of Agricultural Engineering, Tropics and Subtropics Group, Garbenstrasse 9, 70599 Stuttgart, Germany

r t i c l e i n f o

rticle history:eceived 29 July 2013eceived in revised form 1 October 2013ccepted 9 October 2013

eywords:edicinal plantsryingeranialeral

a b s t r a c t

The impact of hot-air drying on essential oil content and composition of lemon balm leaves (Melissa offi-cinalis L.) was investigated at different temperatures within the range of 30 and 90 ◦C, constant specifichumidity of 10 g kg−1 dry air and uniform air flow of 0.2 m s−1. Essential-oil reduction was determined byhydrodistillation of samples during drying and the experimental data was fitted to a first-order reactionkinetics model. The dependence of temperature on the rate constant was expressed by the Arrhenius-typerelationship. The volatile compounds of the essential oil were analyzed by GC/FID. The multifunctionalsurface structures of leaves were also viewed by a scanning electron microscope. In all cases, most of theoil loss was observed at the beginning of the drying process and it was proportional to drying tempera-ture. Convective drying of leaves at 30 and 45 ◦C resulted in 16% and 23% loss in essential oil respectively,

◦

EM images whereas drying at higher temperatures caused significant essential oil losses, for instance 65% at 60 C.Pronounced changes in the major essential oil components occurred at 60 ◦C: neral, geranial and citro-nellal were decreased, while citronellol indicated an increasing tendency. Micrographs showed that theoil glands are sparsely distributed on the lower epidermis of the leaf. Apart from the temperature sensi-tivity of the oil constituents, the loss of essential oil can also be attributed to the structural modificationscaused by drying.

. Introduction

Fresh and more often dried leaf preparations of lemon balmMelissa officinalis L.) are traditionally taken as a tea. Owing to theromatic and the therapeutic properties of the essential oil, therocessed plant is used in herbal medicine and natural cosmetics.he essential oil of M. officinalis is a prominent antimicrobial agentgainst food-borne pathogens and spoilage bacteria (Gutierrezt al., 2008). It has also shown a potential antiviral activity withnhibitory effect on herpes simplex virus (Schnitzler et al., 2008).mong the Lamiaceae, the essential oil is produced in specializedecretory organs, for instance a glandular trichome (Turner et al.,000). Such structure contains the monoterpenes and sesquiter-enes, characteristic of the essential oil of the herb. The totalssential oil content in M. officinalis is relatively low and variabilityan even occur between different harvest cut heights (Mrlianovát al., 2002). The major chemical compounds are citrals (geranialnd neral), citronellal accompanied by �-caryophyllene. These oil

omponents are also responsible for the antibacterial and antifun-al properties of the dried drug (Mimica-Dukic et al., 2004). Due tohe high cost and the sensitivity of essential oils, special attention

∗ Corresponding author. Tel.: +49 711459 23112; fax: +49 711459 23298.E-mail address: dimitrios.argyropoulos@uni-hohenheim.de (D. Argyropoulos).

926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.indcrop.2013.10.020

© 2013 Elsevier B.V. All rights reserved.

is given to the conservation of the heat-sensitive oil constituents inmedicinal and aromatic plants during processing.

Convective hot-air drying of herbs with essential oils as activeingredients usually involves low drying temperatures between30 ◦C and 50 ◦C in order to protect the volatile oil components. Thethermal sensitivity of the active ingredients is often a limiting fac-tor, which results in a comparably long drying process reducingthe capacity of dryers. In a review on convective drying of medici-nal, aromatic and spice plants, significant essential oil losses withincreasing temperature were reported, however, the extent of theoil loss varied between the species (Müller, 2007). More specifi-cally, in the case of Salvia officinalis L. the loss in essential oil wasmore severe during the final phase of drying and especially whenexceeding the recommended moisture content of the dried herb.On the contrary, a work published by Arabhosseini et al. (2006) onArtemisia dracunculus L. indicated that the oil losses may fluctu-ate with an increase of drying temperature. Nevertheless, the driedleaves subjected to 45 ◦C showed the least quality changes in termsof essential oil during a storage period of 120 days (Arabhosseiniet al., 2007).

The impact of the traditional convective drying techniques on

essential oil content and composition of some plant species of theLamiaceae such as Mentha longifolia L. subsp. Capensis (Asekunet al., 2007) and Satureja hortensis L. (Sefidkon et al., 2006) hasbeen examined. Furthermore, the influence of combined drying of

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ot-air and microwave-vacuum on essential oil constituents of Ori-anum vulgare L. (Figiel et al., 2010) and Rosmarinus officinalis L.Szumny et al., 2010) has been reported in the literature. Also, theffect of drying temperature on essential oil profiles of Thymus vul-aris L. (Sárosi et al., 2013) and Ocimum basilicum L. (Calín-Sánchezt al., 2012) has been documented. However, none of the aboveorks studied the course of the essential oil losses during hot-

ir drying of herbs belonging to Lamiaceae and especially for M.fficinalis.

Thus, the objectives of this study were (i) to investigate theinetics of essential oil changes of M. officinalis during convec-ive hot-air drying at different temperatures ranging between 30 ◦Cnd 90 ◦C, (ii) to assess the influence of drying temperature onssential oil content and composition and (iii) to examine the multi-unctional surface structures of the leaves under scanning electron

icroscopy.

. Materials and methods

.1. Plant material

Herbs of lemon balm (Melissa officinalis L.) cultivar Citronellaere harvested in the middle of June (2010) before flowering from

two year old crop of an organic farm in Magstadt, approximately0 km west of Stuttgart (Germany). The harvest was always carriedut in the morning after dew. The height of the collected plantsas 70 cm on average by cutting them to a height of 10 cm above

he ground. Since European Pharmacopoeia defines the leaf as theain source of the active ingredients, prior to drying experiments

he leaves of the top 40 cm were picked manually from the stems.erbal material was stored in the refrigerator at 12 ◦C for consecu-

ive drying experiments. The fresh leaves were controlled in termsf moisture content and essential oil content on a daily basis andere never stored longer than one week.

.2. Laboratory hot-air dryer

The hot-air drying experiments were conducted using the highrecision convection-type laboratory dryer (Argyropoulos et al.,011a,b) designed at the Institute of Agricultural Engineering, Uni-ersität Hohenheim in Stuttgart (Germany) at air temperatures of0, 45, 60, 75 and 90 ◦C and specific humidity of 10 g H2O per kgf dry air, resulting in a corresponding relative humidity of 36, 16,.7, 4 and 2.2%, respectively. An uniform air flow through the sam-le was maintained constant at 0.2 m s−1. Loading density of fresh

eaves was 2.2 kg m−2. The leaves were dried to the equilibriumoisture content of each drying condition (Argyropoulos et al.,

012). During the drying process, samples were collected at reg-lar intervals and analyzed in terms of essential oil and moistureontent. The experiments were repeated at least three times forach drying condition.

.3. Moisture content

The moisture content (MC) of the leaves was determined by theven method (103 ± 2 ◦C for 24 h) in triplicate and expressed in wetasis (% w.b.).

.4. Hydro-distillation

The essential oil of the leaves was isolated by hydro-distillationsing a Clevenger apparatus. About 40 g of fresh and 30 g of

emi-dried and dried leaves (mesh 3 mm) were added to a 1000 ml-ound-bottom flask filled with 500 ml of distilled water. The flaskas then heated for 4 h estimated from the time after condensa-

ion of the first drop of vapour in the calibrated tube. The amount

s and Products 52 (2014) 118– 124 119

of oil extracted was estimated from three consecutive analyses andexpressed in ml 100 g−1 dry matter (ml 100 g−1 D.M.).

2.5. Analysis of essential oil components by GC/FID

The chemical composition of the oil extracted from the fresh anddried samples at temperatures ranging between 30 ◦C and 60 ◦C wasanalyzed. The essential oil components were isolated by hydrodis-tillation and identified by a Hewlett Packard (HP 5890/FID 5972Series II) gas chromatograph (GC), equipped with a flame ioniza-tion detector (FID) and an electronic pressure control injector (EPC).A DB-Innowax capillary column (60 m × 0.25 mm, film thickness0.25 �m) was used. Analysis was carried out using N2 as carriergas at a flow rate of 1.0 ml min−1 in a split ratio of 1:60 and thefollowing temperature programme: 60–220 ◦C at 5 ◦C min−1, thenisothermal for 1 min. The injector and the FID detector temperaturewere maintained at 230 and 240 ◦C, respectively.

2.6. Multifunctional surface structures

The multifunctional surface structures of the randomly freshand dried leaves were examined using a digital microscope (Model:VHX-1000, Keyence Cooperation, Osaka, Japan) and a scanningelectron microscope SEM (Phenom-World BV, Eindhoven, TheNetherlands). Before image acquisition, a thin slice of about2 mm × 5 mm was cut from each leaf, glued on the metal stuband mounted onto the sample container. Then, the samples weresprayed with pure N2 and loaded instantly in the loading bay of theapparatus with low vacuum load-lock technology.

2.7. Kinetic considerations

A first order kinetic model was used to describe the changes ofessential oil during drying:

−dY

dt= k(Y − Ye) (1)

where, Y is the essential oil content (ml 100 g−1 D.M.) during dry-ing, Ye is the oil content at the equilibrium moisture content, k isthe rate constant (min−1) and t is the time of drying (min). At thebeginning of drying (t0), the essential oil content is equal to Y0, thus,by integration of Eq. (1) the following expression is derived:

Y − Ye

Y0 − Ye= exp(−k · t) (2)

The influence of temperature of drying air was embodied in the rateconstant by an Arrhenius type equation:

k = k1 · exp(

−k2

Ta

)(3)

k2 = Ea

R(4)

where, k1 is the Arrhenius factor (min−1), k2 is temperaturedependent constant (K), Ea is the activation energy (kJ mol−1), R isthe universal gas constant (8.314 J mol−1 K−1) and Ta is the absolutetemperature (K).

2.8. Statistical analysis

The values of k were determined by fitting the model to the

experimental data using the non-linear least squares procedure(Matlab v. R2010b, Mathworks, Inc.). The accuracy of fit was evalu-ated by the R-squared (R2), the sum of squared error (SSE) and theroot mean squared error (RMSE). The effect of drying temperature

120 D. Argyropoulos, J. Müller / Industrial Crops and Products 52 (2014) 118– 124

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Fig. 2. Changes in essential oil content of M. officinalis leaves during convective

ig. 1. Single spiny hairs on the upper epidermis (a) and sparsely distributed oillands on the lower epidermis (b) of M. officinalis leaf.

n the model constant was assessed by multiple linear regres-ion analysis. One-way analysis of variance (ANOVA) followed byukey’s test was performed to the data at p < 0.05 significance level.

. Results and discussion

.1. Essential oil content losses during drying

In almost all plant species of the Lamiaceae, essential oil is syn-hesized in two basic types of glandular hair such as peltate andapitate (Werker, 1993). In the case of M. officinalis, oil glands cane found on both sides of the leaf but they are mainly distributed onhe lower epidermis (Fig. 1b), while a high density of single spinyairs exists on the upper epidermis of the leaf (Fig. 1a). In Fig. 2he experimental data (points) of the essential oil content changesnd curves (solid lines) predicted by the model during convectiverying at temperatures of 30, 45, 60, 75 and 90 ◦C is shown. Dryingas terminated when a constant mass was achieved, regarded as

he equilibrium moisture content for the respective drying condi-ion. The oil content was decreased gradually as the drying processrogressed. In particular, most of the loss of oil was observed at theeginning of drying for all examined temperatures. For instance,

◦

eaves subjected to 30 C resulted in less than 16% oil reductionithin the first 540 min indicating a remarkable stability as drying

ontinued. A similar behaviour was also noticed at 45 ◦C, how-ver, greater essential oil losses were reported in a shorter time

drying at temperatures of (�) 30, (�) 45, (�) 60, (�) 75 and (♦) 90 ◦C fitted by afirst-order reaction kinetics model (lines).

(approx. 21% after 180 min). This phenomenon was more severewith a progressive increase of drying temperature up to 75 ◦C. Morespecifically, drying for 30 min at 60 ◦C and 75 ◦C caused a consider-able loss of 45% and 70% in oil content respectively. During drying,moisture moves by diffusion from the interior of the leaves to thesurface and may carry oil with it. Since drying rate is higher inthe initial phase of drying followed by a subsequent decrease, oilremoval can be greater at the beginning of the process. Moreover,the sensitivity of leaves of M. officinalis to higher temperatures canbe ascribed to the volatile constituents found in essential oils. Inaddition, high drying temperatures might have ruptured the oilglands causing rapid evaporation of oil.

According to the statistical analysis there is a significant influ-ence of the progressive increase in temperature on essential oillosses up to 75 ◦C (p < 0.05). A positive correlation between dry-ing temperature and loss of essential oil has been reported forvarious herbs of the Lamiaceae family such as O. basilicum (Calín-Sánchez et al., 2012), O. Vulgare (Di Cesare et al., 2003), S. officinalis(Venskutonis, 1997; Müller, 1992). In general due to the volatilityof essential oils, a reduction in total yield can be expected with anincrease of drying temperature however, the losses also depend onthe plant species. For instance, Sellami et al. (2011) found that nat-ural air drying at ambient temperature caused an increase in thetotal essential oil content of L. nobilis. Besides that, the oven dryingof the herb either at 45 or 65 ◦C resulted in the same oil recovery.Furthermore, moderate essential oil losses of C. recutita even at ahigh temperature of 80 ◦C have been documented in the literature(Müller et al., 1996). This indicates that essential oil losses can varyconsiderably from one species to another. It can be speculated thatdifferences in temperature sensitivity between the species can beascribed to secretory structures and their localization in the plantas well as to chemical composition of essential oil.

3.2. Influence of moisture content during drying

Essential-oil reduction as a function of moisture content (%w.b.) during drying at 30–90 ◦C is shown in Fig. 3. The majoramount of oil was lost at MC > 50% followed by minor changesas drying proceeded at 30 ◦C. Drying at 45 ◦C indicated a similarbehaviour, however, the effect was more evident during the entireprocess. Although a significant loss of oil was recorded at high

◦ ◦

moisture content (MC > 50%) between 60 C and 90 C, a substantialdecrease in essential oil was gradually continued until the equilib-rium moisture content of each condition had been reached. Studieson convective drying of R. officinalis (Szumny et al., 2010) and O.

D. Argyropoulos, J. Müller / Industrial Crops and Products 52 (2014) 118– 124 121

Fig. 3. Essential oil content versus moisture content of M. officinalis during convec-tive drying at different temperatures.

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Table 1Estimated rate constant k and accuracy of fit for the first order kinetic model todescribe the changes in essential oil of leaves of M. officinalis for convective dryingat different temperatures T.

T (◦C) k (min−1) R2 SSE RMSE

30 0.003941a 0.9015 0.0003043 0.00616745 0.01379b 0.9137 0.0004153 0.00679360 0.03394c 0.9881 0.0004721 0.00768275 0.06054d 0.9751 0.00118 0.0140290 0.06227d 0.984 0.0008297 0.01176

A closer view of a peltate gland along with some spine like hairs

ig. 4. Essential oil losses caused by convective drying at different temperatures athe target moisture content of 11%.

ulgare (Figiel et al., 2010) revealed a similar tendency. In con-rast to these herbs, Müller (1992) found that most of the essentialil loss of S. offcinalis, another important medicinal species of theamiaceae, occurred at MC ≤ 10% and especially at equilibriumoisture content in which the reduction was proportional to drying

emperature above 60 ◦C. However, at temperatures below 50 ◦Cven the over-drying of the plant did not cause any losses in essen-ial oil.

It is obvious that at lower drying temperatures between 30 ◦Cnd 45 ◦C the oil yield was not affected when the moisture contentas reduced below 11%, which is the maximum value for microbial

afe storage of leaves of M. officinalis (Argyropoulos et al., 2012).or drying at 75 ◦C a slight decrease in essential oil content wasetected when drying has been continued after reaching the targetoisture content of 11%. Hence, it should be pointed out that over-

rying is not a decisive factor diminishing the total oil content of. officinalis since significant essential losses already occurred in

he initial phase of drying. This indicates that besides drying tem-erature, a decrease of the volatile constituents of essential oils

s also associated with the amount of water removed during therying process. The oil content isolated from the leaves dried tohe recommended moisture content at either 75 or 90 ◦C was sta-istically identical. The effect of drying temperature on reduction

f total essential oil content at the target final moisture contentf 11% is summarized in Fig. 4. For drying at 30 ◦C, total essentialil losses of 16% occurred, which were increased to 23% at 45 ◦C.

Mean value of k with the different letters is significantly different for Tukey’s test atp < 0.05.

Significant losses of essential oil of 65% were recorded at a temper-ature of 60 ◦C and of 73% at 75 ◦C. An increase of temperature to90 ◦C did not further increase the oil losses significantly. Accord-ing to the above described losses in total essential oil content, atemperature of 60 ◦C is considered as being extremely high for theconvective hot-air drying of M. officinalis.

3.3. Modelling of essential oil changes

The values of the rate constant k at different drying tempera-tures along with the corresponding statistical criteria selected toassess the accuracy of fit are presented in Table 1. The model fittedthe experimental data adequately as observed by the relative highR2 and the low SSE and RMSE values. The value of the rate constantincreases with an increase in temperature from 30 to 90 ◦C, how-ever, the k values for drying temperatures of 75 and 90 ◦C were notsignificantly different (p > 0.05). More specifically, an increase intemperature of drying air from 30 to 45 ◦C increased the value of kfrom 0.003941 to 0.01379, which was further increased to 0.03394at 60 ◦C.

Based on the multiple linear regression analysis, the parametersk1 and k2 of the Arrhenius-type equation were obtained and theresulting rate constant k was computed as:

k = 7.374 · 106 · exp

(−6.436 · 103

Ta

)(R2 = 0.984) (5)

Thus, the oil losses during hot-air drying at 30–75 ◦C can becalculated by the following mathematical expression:

Y = (Y0−Ye) · exp

[−

(7.374 · 106 · exp

(−6.436 · 103

Ta

))· t

]+Ye

(6)

The activation energy as calculated by Eq. (4) is 53.51 kJ mol−1.This value implies that the loss of essential oil content is moredependent on physical properties such as transfer rates (heator mass transfer coefficients) rather than on chemical reactionscaused by drying. The magnitude of the activation energy indicatesthe temperature sensitivity of the process rate. Chemical reactionsin various food process operations are typically characterized bylarger values of activation energy. In addition, due to the complexityof the essential oil composition, k would denote a pseudo-first-order rate constant and not an intrinsic reaction rate constant of aspecific chemical process.

3.4. Structural alterations of leaves caused by drying

The plant multifunctional structures of a leaf of M. officinalis asviewed under a scanning electron microscope is shown in Fig. 5.

could be observed on the lower and upper leaf epidermis. The non-glandular trichomes serve as a multi-protective function for theplant, whereby the glandular hair secrete the organic compounds

122 D. Argyropoulos, J. Müller / Industrial Crops and Products 52 (2014) 118– 124

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ig. 5. SEM micrograph of a peltate gland on the lower epidermis of the leaf of M.fficinalis.

ssociated with the characteristic odour of the essential oil of theerb (Corsi and Bottega, 1999). In Fig. 6, SEM images show the effectf convective drying on leaves of M. officinalis. The cell structuref a fresh leaf is also shown (a). It is apparent that drying causedignificant changes on the leaves’ surface structure. After dryingt 30 ◦C, the samples exhibited some shrinkage of the cuticle (b),hich was more severe at 60 ◦C (c). Collapse of the cuticle layer and

xtensive cell damage of the epidermis was noticed on the materialubjected to higher temperatures. A similar observation was alsoeported for air dried leaves of O. basilicum (Yousif et al., 1999). Itan be hypothesized that apart from the temperature sensitivityf the oil constituents, the loss of essential oil might have beennduced by the structural changes occurring during hot-air drying.

.5. Effect of drying temperature on essential oil components

The relative amounts (%) of the major components foundn essential oil of the fresh leaves of M. officinalis cultivaritronella were mainly monoterpenes and monoterpenoids: citro-ellal (17.9 ± 1.8%), neral (12.4 ± 2.2%), geranial (16.1 ± 2.7%),itronellol (10.4 ± 3.6%), geraniol (9.7 ± 1.1%) and sesquiterpenes:-caryophyllene (3.9 ± 0.7%). These results are in agreement withrevious published data on fresh M. officinalis (Shalaby et al.,995). The boiling points of these compounds vary between 208 ◦Cnd 260 ◦C. Drying of leaves caused minimal quality deteriora-ion of the essential oil, but resulted in quantitative changesf the relative amount of the components. Fig. 7 shows theffect of temperature on the main constituents of the essentialil during drying within the range of 30 and 60 ◦C. The mainhemical compounds fluctuated with increasing drying temper-tures. Some of the compounds decreased with an increase ofemperature, whereas others exhibited an increasing tendencyt higher temperatures. It should be emphasized that the mostredominant changes occurred at a temperature of 60 ◦C andhey were dependent on the volatility and the chemical struc-ure of the constituent. In addition, components with lower

oiling points appeared earlier on the chromatogram. Althoughhe behaviour of the various compounds during drying at differ-nt temperatures is not fully understood, it can be hypothesizedhat compounds with a lower retention time will show a lower

Fig. 6. Structural modifications of leaves of M. officinalis caused by convective hot-air drying: fresh (a), dried at 30 ◦C (b) and 60 ◦C (c).share in the total oil at higher drying temperatures. Accordingly,

compounds with a high retention time are expected to show ahigher share by residual accumulation. In particular, the relativeamounts of citronellal, neral and geranial were decreased at highertemperatures, whereas the amount of citronellol and caryophyllene

D. Argyropoulos, J. Müller / Industrial Crop

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ig. 7. Effect of drying temperature on the main components of essential oil of M.fficinalis at the target moisture content of 11%.

xide were increased at a temperature of 60 ◦C. It is worth mention-ng that caryophyllene oxide was only detected in the oil of driedeaves but not in that of fresh samples. This compound was pre-umably formed by oxidation of �-caryophyllene during the dryingrocess and indicates essential oil of inferior quality. According torth et al. (1993) the formation of caryophyllene oxide dependedn the presence of both citral and molecular oxygen. Furthermore,nother study documented that �-caryophyllene was immedi-tely oxidized on exposure to air (Sköld et al., 2006). Although-caryophyllene and geraniol showed a tendency to increase withn increase of temperature only minor changes were documentedven at a temperature of 60 ◦C. Significant concentrations of �-aryophyllene have been detected in several herbs of the Lamiaceaeamily. A slight decrease of �-caryophyllene content was reportedor air-drying of S. officinalis at 60 ◦C in comparison to the samplesried at 30 ◦C (Venskutonis, 1997). In contrast to S. officinalis, theercentage of �-caryophyllene in the essential oil of O. basilicumas found to be unaffected by drying temperature (Calín-Sánchez

t al., 2012). The intensive lemon odour scent of the herb driedetween 30 ◦C and 50 ◦C can be associated with the relatively highontent of citrals (mostly geranial) and citronellal. Previous stud-es reported similar proportions regarding the relative amountsf neral, geranial and citronellal of air-dried M. officinalis leavesMimica-Dukic et al., 2004; Mrlianová et al., 2002). Basta et al.2005) identified �-pinene and sabinene as main components ofhe essential oil of M. officinalis from Greece, while citral or citro-ellal was not detected. The absence of citrals in the essential oil of. officinalis has also been documented in another work published

y Adinee et al. (2008). However, in the later study the oil compo-ents of the flower of M. officinalis were identified. This indicateshat a great variability of oil compounds is expected to occur evenetween the different parts of the plant.

. Conclusions

The changes in essential oil of M. officinalis during convectiverying were adequately described by a first-order reaction kinet-

cs model, in which the influence of drying temperature on theate constant was modelled as an Arrhenius-type equation. Mostf the oil losses were observed in the initial period of drying for allxamined temperatures. For drying of leaves at 30 ◦C, a loss of 16%as found that increased to 23% at 45 ◦C. Significant essential oil

osses of 65% already occurred at 60 ◦C and higher losses than 73%bove 75 ◦C. The content of the key essential oil components, neral,

eranial and citronellal decreased, whereas citronellol showed anncreasing tendency at elevated temperatures. Caryophyllene oxide

as only detected in the oil of the dried leaves. SEM images indi-ated that the oil glands are sparsely distributed on the lower

s and Products 52 (2014) 118– 124 123

epidermis of the leaves. Essential oil loss can also be ascribed toshrinkage and subsequent cell damage caused by hot-air drying.Further works should investigate the kinetics of selected com-pounds in essential oils of medicinal and aromatic plants.

Acknowledgements

This study was conducted within the framework of the project“Prompt and sustainable improvement of existing conveyor-,cabinet-, and flat-bed dryers for chamomile, lemon balm, and vale-rian. Subproject: Fundamental research and process optimizationfor the drying of chamomile, lemon balm and valerian” (sup-port code: 22012509). The authors are thankful for the financialsupport of the Fachagentur Nachwachsende Rohstoffe e.V. (FNR)as project executing organization of the Bundesministerium fürErnährung, Landwirtschaft und Verbraucherschutz (BMELV), Bonn,Berlin (Germany).

References

Adinee, J., Piri, K., Karami, O., 2008. Essential oil component in flower of lemon balm(Melissa officinalis L.). Am. J. Biochem. Biotechnol. 4, 277–278.

Arabhosseini, A., Huisman, W., van Boxtel, A., Müller, J., 2007. Long-term effects ofdrying conditions on the essential oil and color of tarragon leaves during storage.J. Food Eng. 79, 561–566.

Arabhosseini, A., Padhye, S., Van Beek, T.A., Van Boxtel, A.J.B., Huisman, W., Posthu-mus, M.A., Müller, J., 2006. Loss of essential oil of tarragon (Artemisia dracunculusL.) due to drying. J. Sci. Food Agric. 86, 2543–2550.

Argyropoulos, D., Alex, R., Kohler, R., Müller, J., 2012. Moisture sorption isothermsand isosteric heat of sorption of leaves and stems of lemon balm (Melissa offi-cinalis L.) established by dynamic vapor sorption. Lebensm. Wiss. Technol. 47,324–331.

Argyropoulos, D., Heindl, A., Müller, J., 2011a. Assessment of convection, hot-aircombined with microwave-vacuum and freeze-drying methods for mushroomswith regard to product quality. Int. J. Food Sci. Technol. 46, 333–342.

Argyropoulos, D., Khan, M.T., Müller, J., 2011b. Effect of air temperature and pre-treatment on color changes and texture of dried Boletus edulis Mushroom. DryingTechnol. 29, 1890–1900.

Asekun, O.T., Grierson, D.S., Afolayan, A.J., 2007. Effects of drying methods on thequality and quantity of the essential oil of Mentha longifolia L. subsp. Capensis.Food Chem. 101, 995–998.

Basta, A., Tzakou, O., Couladis, M., 2005. Composition of the leaves essential oil ofMelissa officinalis s. l. from Greece. Flavour Fragr. J. 20, 642–644.

Calín-Sánchez, A., Lech, K., Szumny, A., Figiel, A., Carbonell-Barrachina, A.A., 2012.Volatile composition of sweet basil essential oil (Ocimum basilicum L.) as affectedby drying method. Food Res. Int. 48, 217–225.

Corsi, G., Bottega, S., 1999. Glandular hairs of Salvia officinalis: new data on mor-phology, localization and histochemistry in relation to function. Ann. Bot. 84,657–664.

Di Cesare, L.F., Forni, E., Viscardi, D., Nani, R.C., 2003. Changes in the chemical com-position of basil caused by different drying procedures. J. Agric. Food Chem. 51,3575–3581.

Figiel, A., Szumny, A., Gutiérrez-Ortíz, A., Carbonell-Barrachina, Á.A., 2010. Compo-sition of oregano essential oil (Origanum vulgare) as affected by drying method.J. Food Eng. 98, 240–247.

Gutierrez, J., Barry-Ryan, C., Bourke, P., 2008. The antimicrobial efficacy of plantessential oil combinations and interactions with food ingredients. Int. J. FoodMicrobiol. 124, 91–97.

Mimica-Dukic, N., Bozin, B., Sokovic, M., Simin, N., 2004. Antimicrobial and antiox-idant activities of Melissa officinalis L. (Lamiaceae) essential oil. J. Agric. FoodChem. 52, 2485–2489.

Mrlianová, M., Tekel’ ová, D., Felklová, M., Reinöhl, V., Tóth, J., 2002. The influenceof the harvest cut height on the quality of the herbal drugs Melissae folium andMelissae herba. Planta Med. 68, 178–180.

Müller, J., 2007. Convective drying of medicinal, aromatic and spice plants: a review.Stewart Posthar. Rev. 3, 1–6.

Müller, J., Köll-Weber, M., Kraus, W., Mühlbauer, W., 1996. Trocknungsverhalten vonKamille (Chamomilla recutita (L.) Rauschert). Z. Arznei Gewürzpfla. 1, 104–110.

Müller, J., 1992. Trocknung von Arzneipflanzen mit Solarenergie. Verlag EugenUlmer, Stuttgart, Germany, pp. 1992.

Orth, M., Van Den Berg, T., Czygan, F.C., 1993. Aldehyde-dependent caryophylleneoxide formation in essential oils during storage. Planta Med. 59.

Sárosi, S., Sipos, L., Kókai, Z., Pluhár, Z., Szilvássy, B., Nová k, I., 2013. Effect of differentdrying techniques on the aroma profile of Thymus vulgaris analyzed by GC–MSand sensory profile methods. Ind. Crops Prod. 46, 210–216.

Schnitzler, P., Schuhmacher, A., Astani, A., Reichling, J., 2008. Melissa officinalis oilaffects infectivity of enveloped herpesviruses. Phytomedicine 15, 734–740.

Sefidkon, F., Abbasi, K., Khaniki, G.B., 2006. Influence of drying and extraction meth-ods on yield and chemical composition of the essential oil of Satureja hortensis.Food Chem. 99, 19–23.

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24 D. Argyropoulos, J. Müller / Industri

ellami, I.H., Wannes, W.A., Bettaieb, I., Berrima, S., Chahed, T., Marzouk, B., Limam,F., 2011. Qualitative and quantitative changes in the essential oil of Laurus nobilisL. leaves as affected by different drying methods. Food Chem. 126, 691–697.

halaby, A.S., El-Gengaihi, S., Khattab, M., 1995. Oil of Melissa officinalis L., as affectedby storage and herb drying. J. Essent. Oil Res. 7, 667–669.

köld, M., Karlberg, A.T., Matura, M., Börje, A., 2006. The fragrance chemical �-

caryophyllene – air oxidation and skin sensitization. Food Chem. Toxicol. 44,538–545.

zumny, A., Figiel, A., Gutiérrez-Ortíz, A., Carbonell-Barrachina, Á.A., 2010. Com-position of rosemary essential oil (Rosmarinus officinalis) as affected by dryingmethod. J. Food Eng. 97, 253–260.

s and Products 52 (2014) 118– 124

Turner, G.W., Gershenzon, J., Croteau, R.B., 2000. Distribution of peltate glan-dular trichomes on developing leaves of peppermint. Plant Physiol. 124,655–663.

Venskutonis, P.R., 1997. Effect of drying on the volatile constituents ofthyme (Thymus vulgaris L.) and sage (Salvia officinalis L.). Food Chem. 59,219–227.

Werker, E., 1993. Function of essential oil-secreting glandular hairs in aromaticplants of the Lamiaceae. A review. Flavour Frag. J. 8, 249–255.

Yousif, A.N., Scaman, C.H., Durance, T.D., Girard, B., 1999. Flavor volatiles and physicalproperties of vacuum-microwave- and air-dried sweet basil (Ocimum basilicumL.). J. Agric. Food Chem. 47, 4777–4781.