Influence Of Different Drying Parameters On The Composition
Of Volatile Compounds Of Thyme and Rosemary Cultivated In
Piga A.1*, Usai M.2, Marchetti M.3, Foddai M.2, Del Caro A.1, Meier H.P.4, Onorati V.4,
1Dipartimento di Scienze Ambientali Agrarie e Biotecnologie Agro-Alimentari, Università degli Studi di
Sassari, Viale Italia 39/A, 07100 Sassari – Phone and fax n. 079 229273, e-mail: email@example.com
2Dipartimento di Scienze del Farmaco, Università degli Studi di Sassari, Via Muroni 23, 07100
3CNR - Istituto di Chimica Biomolecolare sede di Sassari, Traversa la Crucca 3, 07040 Sassari
4Consorzio di Produttori Sardi di Piante Officinali e loro derivati, Viale Trieste 124, 09123 Cagliari
Written for presentation at the
2007 CIGR Section VI International Symposium on
FOOD AND AGRICULTURAL PRODUCTS: PROCESSING AND INNOVATIONS
24-26 September 2007
Abstract. The shelf life of spices is traditionally extended by drying. Fresh herbs, due to their high
water content, undergo microorganism growth and adverse biochemical reactions. On the other hand
drying may result in a lot of physical and chemical alterations. Air and oven-dehydration are the main
methods used to stabilize spices. During oven drying, in general, losses of volatile compounds are
directly dependent on the temperature and time used.
This paper deals with the effect of different drying temperatures and air fluxes on the volatiles in
rosemary (Rosmarinus officinalis L) and thyme (Thymus officinalis L.) cultivated in Sardinia. Fresh
leaves were collected and soon divided in two batches, which were subjected to hydro distillation and
GC-MS analysis, the first batch as fresh, the second one after drying in a laboratory pilot dryer.
Three drying temperatures were used, 30, 38 and 45°C, and for each one two airflow rates were set.
The fresh and dried plant material were hydro distilled for 4 hours using a Clevenger-type apparatus
(Italian Official Pharmacopeias X). The oils (liquid and light yellow) were recovered directly from
above the distillate without adding any solvent and stored at –20°C before analyses, which were
carried out on two replicates of each sample by gas chromatography, using a flame ionization
detector. The diluted samples were injected using a split/splitless automatic injector (using 2,6-
dimethylphenol as internal standard). Qualitative analysis was done by GC/Mass and mass units
were monitored from 10 to 450 at 70 eV.
Results of the influence of the different drying conditions on volatile compounds of the two herbs will
Keywords. Rosmarinus officinalis L., Thymus officinalis L., essential oils, drying, storage.
Proceedings of the 3rd CIGR Section VI International Symposium on
FOOD AND AGRICULTURAL PRODUCTS: PROCESSING AND INNOVATIONS
Naples, Italy, 24-26 September 2007
Aromatic herbs and spices are becoming increasingly requested from the consumers. Apart
from the flavoring use, in fact, people are interested also in medicinal and anti-inflammatory
properties (Rish, 1997). The use of plants is as old as the mankind. Natural products are cheap
and claimed to be safe. They are also suitable raw material for production of new synthetic
Most of herbs and spices are marketed dried, because, due to the high water content in the
fresh state, they undergo severe deterioration after microbial growth and biochemical reactions.
Water removal by dehydration, in fact, stabilizes microbiologically herbs and spices by lowering
the water activity (aw) values below the threshold for microbial growth, that is 0.6.
In general, hot air drying is the most used method, anyway, it can led to thermal damage and
can severely alters the volatile composition of herbs as well as the color. In fact, some
compounds can evaporate during air drying, while other are in part retained (Jerkovic, Matelic &
Milos, 2001). Some oxidation products can also appear during drying (Luning, Ebbenhorstseller,
Derijk & Rozen, 1995). In general, losses are correlated to temperature and time of drying
(Raghavan, Abraham, Shankaranarayana & Koller, 1994; Venskutonis, Poll & Larsen, 1996). In
fact, ambient temperatures and temperatures below 50°C are the best to retain volatile
compounds (Park, Vohnikova & Brod, 2002; Ulseth, 1996; Soysal & Oztekin, 2001). Changes,
anyway, are not only process dependent, but can be attributed to the specific compound and
The scientific research has shown that the thyme (Thymus officinalis L.) has a so strong
antiseptic effect to be able to kill the bacteria in 40 second. The ancient Egyptians knew it and
they used it to embalm their dead persons. The ancient Greek burned it as incense aromatic,
from which derives the name of the Greek word to burn thymon'.
The Romans associated it to the strength and the courage. The soldiers took a bath of thyme
before entering war. This superstition has had long life and in the Middle Ages the noblewomen
embroidered the thyme on the emblems of their knights. The thyme is also considered exciting
and invigorating substance, recommended in case of respiratory problems, bad circulation of
the blood or bad digestion. An infusion gives relief to the headache, nervousness, cough,
influence and it helps against the acne from the inside.
Rosemary (Rosmarinus officinalis Linn.) is a common household plant grown in many parts of
the world. It is used for flavouring food, a beverage drink, as well as in cosmetics; in folk
medicine it is used as an antispasmodic in renal colic and dysmenorrhoea, in relieving
respiratory disorders and to stimulate growth of hair. Extract of rosemary relaxes smooth
muscles of trachea and intestine, and has choleretic, hepatoprotective and antitumorigenic
activity. The most important constituents of rosemary are caffeic acid and its derivatives, such
as rosmarinic acid. These compounds have antioxidant effect. The phenolic compound,
rosmarinic acid, gains one of its phenolic rings from phenylalanine via caffeic acid and the other
from tyrosine via dihydroxyphenyl-lactic acid. Relatively large-scale production of rosmarinic
acid can be obtained from the cell culture of Coleus blumei Benth when supplied exogenously
with phenylalanine and tyrosine. Rosmarinic acid is well absorbed from gastrointestinal tract and
from the skin. It increases the production of prostaglandin E2 and reduces the production of
leukotriene B4 in human polymorphonuclear leucocytes, and inhibits the complement system.
Thus, that rosemary and its constituents especially caffeic acid derivatives such as rosmarinic
acid have a therapeutic potential in treatment or prevention of bronchial asthma, spasmogenic
disorders, peptic ulcer, inflammatory diseases, hepatotoxicity, atherosclerosis, ischaemic heart
disease, cataract, cancer and poor sperm motility.
Thyme and rosemary grow both wild in the Mediterranean basin and, as told before, they are
very much appreciated for their aromatic, antimicrobial and antioxidant properties (Dorman &
Deans, 2000; Nguyen, Takascova, Jakubik & Dang, 2000; Manou, Bouillard, Devleeschouwer &
Barel, 1998; Schwarz & Ternes, 1992). The effect of mechanical air drying on the volatiles of
T.officinalis and R. officinalis have been extensively reported (Blanco, Ming, Marques & Bovi,
2002; Deans & Svoboda, 1992; Di Cesare, Viscardi, Fusari, & Nani, 2001; Fadel & El-Massry,
2000; Jaganmohan-Rao, Meenakshi-Singh, Raghavan & Abraham, 1998; Koller & Raghavan,
1995; Raghavan, Abraham & Koller, 1995; Venskutonis, 1995, 1997; Venskutonis, Poll &
Larsen, 1996). However, we did not find any reference on the effect of air drying on volatile
composition of these two species cultivated in Sardinia.
Consequently, the aim of this work was to assess the best drying conditions, using
temperatures up to 45°C, to minimize volatile loss or degradation of essential oil extracted from
T. officinalis and R. officinalis cultivated in Sardinia, in order to decrease the seasonality
dependence of the essential oil market and toward increasing the value of the products.
Sun drying is also used for drying herbs, requiring low capital, simple equipment and low energy
input. Nonetheless, mechanical air dehydration has gained importance because it has many
advantages over sun drying, such as the following:
a) the process is carried out under better sanitary conditions as a result of reduced
contamination by dust and other foreign matter;
b) drying parameters can be accurately set, controlled and changed throughout the process,
thus a more uniform product can be achieved with less quality degradation;
c) dehydration is not conditioned by rain or weather changes;
d) labor costs are lower. We also have to remember that Aspergillus spp. fungi growth may be
dramatically increased when the whole process is too slow, due to the reasons specified above.
Aflatoxin contamination has been reported, in fact, in spices (Selim, Popendorf, Ibtrahim, El
Sharkawi, & El Kashory, 1996).
Thus, mechanical air drying of herbs can surely be a safer technology for drying herbs.
Materials and Methods
Plant material was furnished by “Consorzio Produttori Sardi di Piante Officinali e loro Derivati”.
Thyme and rosemary samples were collected during January and February in the south-west
(Serdiana) and south-east (Muravera) of Sardinia, respectively, and transported within two
hours to our laboratory, were they were immediately processed. In particular, both leaves and
stems were used. The samples were divided in two batches, one was immediately used for the
extraction of the essential oil, the other one was subjected to drying. Dried samples were
immediately sent to essential oil extraction.
Drying equipment and process parameters
Herbs were dried in a laboratory pilot dryer. The air drier was a tangential air-flow cabinet (a
modified model of “Scirocco”, Società Italiana Essiccatoi, Milan, Italy), equipped with automatic
temperature and air moisture control devices. Air flows tangentially to the shelves carrying the
herbs, while a particular air recycling system allows mixing exhaust air with fresh air and then
reheating and redirecting to the product, in order to achieve the desired air moisture. The
particular construction of the drier allows a continuous airflow on the herbs, avoiding turbulence,
and consequently it is particularly suited to calculate drying kinetics (Figures 1 and 2). Herbs
were placed on steel shelves (product load from 0.6 to 0.7 kg/m2) using three shelves per
treatment (the drier holds ten shelves). Herbs were removed when an estimated 10% water
content (based on weight loss calculations) was obtained. Processing parameters were as
- Air temperature at ambient conditions = 20 °C
- Drying air temperature = 30, 38 and 45 °C
- Relative humidity of air at entrance <40%
- Volumetric flow rate = 300 (low) -1250 (high) m3/h
- Air recycling to keep relative humidity below 40%.
Isolation and analysis of the essential oils
Oil distillation and yield
Fresh and dried plant materials were separately steam distilled for 4 h in a Clevenger-type
apparatus according with Italian Official Pharmacopoeias X (1999); the reached yield is reported
in Figure 3.
All the obtained oils were liquid and light yellow. Three replicate samples were distilled
simultaneously. The essential oil was recovered directly from above the distillate without
adding any solvent. The oils were stored at –20°C (under nitrogen atmosphere) until they
Oil analyses: Gas-cromatography analysis
Two replicates of each sample (three for every station) were analyzed by using a Hewlett-
Packard Model 5890A gas cromatograph (GC) equipped with a flame ionization detector (F.I.D.)
and fitted with a 60 m x 0.25 mm, thickness 0.25µm AT-5 fused silica capillary column (Alltech).
Injection port and detector temperature were 280°C. The column temperature was programmed
from 50°C to 135°C at 5°C/min (1 min), 5°C/min up 225°C (5 min), 5°C/min up 260°C and held
for 10 min. The samples, diluted 1/10, (using 2,6-dimethylphenol as internal standard) were
injected using a split/splitless automatic injector HP 7673 and using helium as carrier gas.
Measurements of peak areas were performed with a HP workstation; the threshold was set at 0,
peak width at 0.02. The quantization of each compound was expressed as absolute weight
percentage using internal standard and response factors. The detector response factors (RFs)
were determined for key components relative to 2,6-dimethylphenol and assigned to other
components on the basis of functional group and/or structural similarity, since oxygenated
compounds have lower detectability by F.I.D. than hydrocarbons (Dugo, Licandro, Cotroneo, &
Dugo, 1983) The standards were > 95% also, and actual purity was checked by GC. Several
response factor solutions were prepared and consisted of only four or five components (plus
2,6-dimethylphenol) to prevent interference from trace impurities.
Gas-Cromatography/Mass-Spectrum (GC/MS) analyses were carried out with a Hewlett
Packard G1800B-GCD System using the same conditions and column described above. The
column was connected with the ion source of the mass spectrometer. Mass units were
monitored from 10 to 450 at 70 eV. The identification of constituents was based on comparison
of the retention times (Rt) values and mass spectra with those obtained from authentic samples
and/or the NIST and Wiley library spectra (NIST98; Adams, 2001) or on the interpretation of the
EI-fragmentation of the molecules.
Fresh and dried samples were inspected for water and dry matter content (%), water activity
and colour. Drying kinetics were calculated by plotting the water content measured at regular
intervals during processing versus process times, while drying rates were computed from water
loss and process times. In particular, water content was determined in a vacuum oven for 12 h
at 70°C until constant weight. Water activity was assessed by an electronic hygrometer (model
Aw-Win, Rotronic, equipped with a Karl-Fast probe), calibrated in the range 0.1-0.95 with
solutions of LiCl of known activity.
Results and Discussion
Figures 1-2 show dehydration kinetics of the two herbs. The process was stopped when
samples reached an average water content of 0.11 kg of water per kg of dry matter, which is
considered a safe value from the microbiological standpoint and which is suggested by the
European Spice Association (2004). Drying times and rates were affected mainly by process
parameters. In our experimental conditions, the time to reach the estimated water content
ranged from 8 h for the combination 45°C and high fair flux to 25 hours for the combination 30°C
and low air flux. Drying times did not differ significantly between species or same processing
M ( 0,4
Drying time (hours)
Figure 1 - Drying kinetics of Thymus officinalis samples as M (g H2O g dry matter-
1) versus drying time (hours). Data are the mean of three determinations.
0 g2 0,8
M ( 0,4
Drying time (hours)
Figure 2 - Drying kinetics of Rosmarinus officinalis samples as M (g H2O g dry
matter-1) versus drying time (hours). Data are the mean of three determinations.
It is to highlight that only a falling drying period was observed. Times for drying agree with those
reported by other authors (Venskutonis et al., 1996; Jaganmohan-Rao et al., 1998). The aw
values ranged from 0.56 to 0.59, thus sample were surely microbiologically stable.
All the obtained samples were submitted to quantitative and qualitative essential oils analyses.
The first parameter evaluated was the essential oil yields in different drying processes. In all
cases, after data normalization, no significant differences were recorded, and this is in
accordance with data reported by Deans, Svoboda & Bartlett (1991). In Figure 3 are reported all
raw experimental data referred to the different drying procedure.
Figure 3 – Raw data (expressed in percentage) deriving from essential oil
extraction of fresh and dried herbs.
At first we consider the thyme (Figure 4): from the observation of the plot reported in figure, it is
possible to note the ?-thujene strongly decreased when drying at 45°C and low air flux were
used; this behavior is common to all volatile components of the essential oil, in fact under this
experimental conditions all non-volatile constituents of the essential oil increased significantly
(e.g. thymol and carvacrol), whereas all volatile hydrocarbons were lost in significant amount.
On the contrary, drying at 45°C and high air flow allowed a relatively small drying time, and this
resulted in a keeping of the volatile compounds in the oil glandular hair, therefore, the
composition of the essential oil deriving from dry plant is similar to that distilled from fresh herbs.
This behavior has been noted in all drying trials, while, when lower temperatures were used
(38°C and 30°C), the loss of volatile compounds was not high. It is noteworthy to observe some
phenomena of isomerization and/or oxidation that take place during long time drying operations.
In particular, using low air flux resulted, for example, in an increase of carvacrol and in some
cases thymol, and this changed the peculiar characteristics of essential oil.
The above observation induced us to consider 45°C and high air flux as the best drying
conditions from the essential oil production point of view. In fact, no significant changes were
recorded when comparing fresh herbs and the corresponding dried sample at the above cited
process parameters (Figure 5). The results obtained are in part in accordance with that reported
by Raghavan et al., (1995), Venskutonis et al. (1996) and Venskutonis (1995), even if a right
comparison can not be made because we do not know exactly all the characteristics of the air-
water mixture used by the authors. However, contrary to what reported by the above cited
authors, the lowest temperature gave the worst results.
Figure 4 – Variation of main components (as RFs) of essential oil of thyme extracted from
fresh and dry plants obtained under different drying conditions.
3 8 , 7 4
3 9 , 5 6
19 , 9
2 1, 8 1
9 , 9 8
1, 6 1
0 , 7
1, 67 9 1, 9 810 , 2 6
3 , 9 1
0 , 3
1, 4 8
2 , 4 7
0 , 2
0 , 7
1, 23 8
1, 9 5
0 , 8 400 , 2 0 , 2 31, 160 0 00, 95
0 , 3 50 , 2 21, 17
2 , 0 2
0 0 0 0 0 , 180 , 103,2 5
0 , 1
0 8,3 3
0 , 110 , 17
0 , 2
0 3,7 400 , 3 3
0 , 120 0 0
1, 9 5
1, 0 3,7 2
2 , 0 9
2 , 9
4 , 13
0 , 3 6
0 , 150 , 20 5,20 , 15 0 , 7 3
0 , 8 3 0 , 8 4
1, 6 4
00 , 2 90 , 2 80 , 9 900 , 1800 , 80 3,6
1, 7 6
00 , 12 0
0 0 0 0 0 , 1100 , 00 9,1500 , 190
00 , 15
0 , 2 00 , 7 7
0 , 1
0 , 3 8
0 , 1 0 0 0
Figure 5 – Comparison of whole essential oil (as RFs) of fresh and dry thyme
using the best parameters of drying.
The same criteria above described have been also used to establish the best drying parameters
for rosemary plants. In this case, the percentage of main and more significant constituents of
the essential oil (verbenone and ?-pinene) remain practically the same when 38°C and low air
flux is used (Figures 6 and 7). It is very interesting to note that if low temperature and high air
flux during the drying are employed, some oxidation phenomena occur dramatically, for
example limonene is almost completely converted into 1,8-cineol, when the herb is dried at
30°C using a high air flux. Our results partly agree with those reported by some other authors
(Jaganmohan et al., 1998; Fadel & El-Massry, 2000; Blanco et al., 2002).
Rosmarinus officinalis L.
30 low, dry
38 high, dry
38 low, dry
45 high, dry
45 low, dry
Figure 6 - Variation of main components (as RFs) of essential oil of rosemary
deriving from fresh and dry plants obtained under different drying conditions.
2 1 , 3 5
2 2 , 9 8
2 1 , 1 3
2 2 , 9 5
9 , 6 9
4 , 1
9 , 5 2
3 , 4 5
1 9, 6
1 , 5
0 1, 629
1 , 4
5 13 , 1 8 3 , 4 7
6 , 2 9
4 , 1 2
0 , 2 6
, 5 3
0 ,01, 64 6
3 , 6 6
1 6, 2
5 2, 0 7
1 , 4 9
00 , 3
1 1,3 1 2 , 2 9
000 , 9 6
8, 21 1 , 8
2 , 5 43 , 0 7
0 , 206
1 , 4 2
3 , 7 6
7 , 0 1 00
0 , 3 3
0 , 2
0 , 1 4
0 0 ,
0 , 5 3 1 , 6 1
0 , 3 8
1 , 8 3
0 , 6 4 1 , 6 1
00 , 4
1 3,1 3
2 , 6 6
0 00 0 0 000 0000 0,4600,25
Figure 7 - Comparison of whole essential oil (as RFs) of fresh and dry rosemary
using the best parameters of drying.