Biosystems Engineering (2007) 96 (4), 495–502
doi:
10.1016/j.biosystemseng.2006.12.011
Energy Consumption and Colour Characteristics of Nettle Leaves during
Microwave, Vacuum and Convective Drying
Ilknur Alibas
Faculty of Agriculture, Department of Agricultural Machinery, Uludag University, 16059 Bursa, Turkey; e-mail:
(Received 27 March 2005; accepted in revised form 15 December 2006; published online 5 February 2007)
Nettle leaves (Urtica dioica L.) were dried from an initial moisture content of 441 to 01 (dry basis) by
involving microwave, convective and vacuum drying, respectively. Energy consumption and colour
parameters for the nettle leaves were compared at these different drying conditions. In particular, the
experiments were carried out at four different microwave power levels (500, 650, 750 and 850 W) and air
temperatures (50, 75, 100 and 125 1C) to investigate the effect of these factors on the microwave and
convective drying, respectively. Instead, under vacuum drying conditions both the influence of vacuum (20
and 50 mm [Hg]) and drying temperature (50 and 75 1C) were considered. Drying periods ranged from 4 to 6,
30 to 120 and 35 to 65 min for microwave, convective and vacuum drying, respectively. The semi-empirical
Page’s equation was able to reproduce the experimental drying curves at all operating conditions under
microwave, convective and vacuum drying. The optimum method with respect to the drying period, colour
and energy consumption was the microwave drying at 850 W.
r
2007 IAgrE. All rights reserved
Published by Elsevier Ltd
1. Introduction
Stinging nettle Urtica dioica L. belongs to the family
of Urticaceae. In the past few years, nettle has been
noted as a healing plant because of its considerable
effects on human health both in Turkey and in the other
countries all over the world (
). It is
considered to be a nutritive food. Nettle leaf has a long
history as an herbal remedy and nutritious addition to
the diet. The seeds and leaves of nettle contain minerals
(especially iron), vitamin C, pro-vitamin A (
), amino acids (
ascorbic acid (
Martinez-Para & Torija-Isasa, 1980
), rare
carbohydrates (
), and several
mineral elements (
). It is also known that
nettle has anti-oxidant, antimicrobial, anti-ulcer and
analgesic properties (
). Shoots of
nettle cooked as a potherb are added to soups and can
also be dried for winter use (
). Although the plants are used principally in
pottage, a tea made from the leaves has traditionally
been drunk (
).
Nettle is a vegetable which rapidly perishes after
harvest and which is currently consumed only in season.
Drying is the one of the storage methods, which has the
capability of extending the consumption period of
nettles, yet maintaining its nutrition content. Drying is
the process of removing the moisture in the product up
to certain threshold value by evaporation. In this way,
the product can be stored for a long period, since the
activities of the micro-organisms, enzymes or ferments
in the material are suppressed (
Different drying methods are used in the drying of
fruits and vegetables. Such as worsening of the taste,
colour and nutritional content of the product, decline in
the density and water absorbance capacity, as well as
shifting of the solutes from the internal part of the
drying material to the surface (
Yongsawatdigul & Gunesekaran, 1996
;
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495
r
2007 IAgrE. All rights reserved
Published by Elsevier Ltd
;
).
The use of microwave rays in the drying of
agricultural products such as grains (
;
), vegetables (
;
and fruits (
;
;
) has become widespread
because it minimises the decline in food quality and
provides a rapid and an effective head distribution in the
material (
), which leads to energy
savings (
). Microwave drying creates an effect
for moisture transfer, leading to a water vapour pressure
gradient between the bulk and the surface of the
material, as in the convectional drying methods (
).
Microwave drying creates an effect for moisture
transfer, leading to a water vapour pressure gradient
between the bulk and the surface of the material, as in
the conventional drying methods (
Microwave energy applications in the drying of vege-
tables have several advantages including the shortening
of drying time, a homogenous energy distribution on the
material and, formation of suitable dry product
characteristics due to the increase in temperature in
the centre of the material. Among the other benefits of
using microwave drying are inhibition of high surface
temperatures, continuation of the product respiration,
lowered product temperatures when combined with
vacuum drying, reduction in the loss of water-soluble
components and energy savings (
).
Vacuum drying is a drying technique which is used for
drying of various products, retaining colour and vitamin
content (
). Vacuum enhances the
mass transfer because of an increased pressure gradient
between the inside and outside of the sample to dry and
maintains a low temperature level essential for thermo-
labile products (
). Better product
quality with respect to traits such as taste, flavour and
rehydration can be retained via high-degree vacuum
treatment (
). The key benefits
of vacuum drying include lower process temperatures,
less energy usage and hence greater energy efficiency,
improved drying rates, and in some cases, less shrinkage
of the product (
). Vacuum
drying has been successfully applied to many fruits and
vegetables and other heat-sensitive foods. Vacuum dried
materials are characterised by better quality retention of
nutrients and volatile aroma. However, the cost of the
process is high (
The objectives of this study were to: (i) evaluate the
efficacy of microwave, convective and vacuum drying
technique for nettle leaves; (ii) compare the measured
findings obtained during the drying of nettle with the
predicted values obtained with Page’s thin-layer drying
semi-empirical equation; (iii) determine the changes in
the colour values of the product after drying; and (iv)
determine the optimum drying method for the drying of
nettle, with respect to energy consumption, colour and
drying period.
2. Materials and methods
2.1. Drying experiments
The leaves used in the drying experiments were 25
(
7003) g in weight and were selected from healthy
plants of fresh nettle (Urtica dioica L.) provided from
Karacabey county of Bursa. All the samples were stored
at the temperature of 4
705 1C before being dried.
Fresh chard leaves were pre-treated in chamber of
steamy cooker (Raks Buharlim, Manisa, Turkey) before
drying to reduce enzymatic changes. In order to prevent
colour changes, the cooker was set to produce 100 1C
steam and the chard leaves were exposed to steam for
30 s.
Microwave and convective drying treatment was
performed in a domestic digital combine oven (Arcelik
MD 592, Turkey) with technical features of 230 V,
50 Hz and 2900 W. Microwave energy is capable of
polarising substances. The microwave oven has the
capability of operating at eight different microwave
stages, being 90, 160, 350, 500, 650, 750, 850 and
1000 W. The convective oven has the capability of
operating at nine different temperature stages, being 50,
75, 100, 125, 150, 175, 200, 225 and 250 1C at 1 m/s air
velocity. The area on which microwave and convective
drying is carried out was 327 mm by 370 mm by 207 mm
in size, and consisted of a rotating glass plate with
280 mm diameter at the base of the oven. Glass plate
rotates for 5 min
1
and the direction of 3601 rotation
can be changed by pressing the on/off button. Time
adjustment is done with the aid of a digital clock located
on the oven. The drying temperature of convective oven
can be reached after every weight measured within 5 s.
Vacuum drying treatment was performed in a
laboratory-type vacuum oven (Nuve EV 0180, Turkey)
with technical features of 220 V, 50 Hz, 35 A and
800 W. The temperature of vacuum oven has a
sensitivity of 1 1C, with a maximum temperature of
250 1C. The area on which vacuum drying is carried out
was 300 mm by 200 mm by 250 mm in size. An
analogous vacuum-meter which indicates the vacuum
value in terms of mm [Hg] exists on the vacuum oven.
Time adjustment is done with the aid of a program-
mable clock located on the oven.
ARTICLE IN PRESS
I. ALIBAS
496
Drying experiments were conducted using three
different drying methods, namely, microwave, convec-
tive and vacuum drying. Three different experimental
designs were performed for each method. Microwave
drying trial was carried out at four different microwave
generation powers being 850, 750, 650 and 500 W.
Convective drying trial was carried out at four different
temperatures being 50, 75, 100 and 125 1C. Four
different vacuum-temperature combinations were ob-
tained in vacuum trials by combining two different
vacuum levels i.e., 20 and 50 mm [Hg] and two different
temperature regimes at 50 and 75 1C, and the trials were
realised under the combinations of 50 1C–20 mm [Hg],
50 1C–50 mm [Hg], 75 1C–20 mm [Hg] and 75 1C–50 mm
[Hg]. A laboratory type greasy vacuum pump (Carpa-
nelli MMDE80B4, Italy) was used in the vacuum drying
with operating conditions were 220/240 V, 50/60 Hz
and 51/48 A. The vacuum pump is increased the least
vacuum value within 20 s.
All experiments were conducted at each drying
technique and the values obtained from these trials
were averaged and the drying parameters were deter-
mined. Dried nettle leaves which were being dried were
removed from the oven periodically (every 30 s for
microwave drying and every 5 min for vacuum and
convective drying) during the drying period, and the
moisture loss was determined by weighing the plate
using digital balance (Sartorious EX 2000A, Germany)
with 001 g precision (
). All
weighing processes were completed in 10 s during the
drying process. Energy consumption of microwave,
convective and vacuum oven with together vacuum
pump was determined using a digital electric counter
(Kaan, Type 101, Turkey) with 001 kW h precision.
Drying process continued until the moisture content of
nettle fell down to 01
70005 on dry basis.
2.2. Data analysis and empirical drying model
The following common semi-empirical Page’s equa-
tion [Eqn (1)] was used to describe the thin-layer drying
kinetics of nettle leaves (
M
R
¼
X X
e
X
0
X
e
¼
expððktÞ
n
Þ
,
(1)
where: M
R
is the moisture content ratio; X is the
moisture content at any drying time dry basis (db); X
e
is
the equilibrium moisture content % db; X
o
is the initial
moisture content in % db; t is the drying time in min; k
is the drying constant in min
1
; and n is the dimension-
less exponent.
2.3. Colour parameters
Leaf colour was determined by two readings on the
two different symmetrical faces of the leaf in each
replicate, using a Minolta CR 300 colorimeter (Konica-
Minolta, Osaka, Japan), calibrated with a white
standard
tile.
The
colour
brightness
coordinate
L measures the whiteness value of a colour and ranges
from black at 0 to white at 100. The chromaticity
coordinate a measures red when positive and green
when
negative,
and
the
chromaticity
coordinate
b measures yellow when positive and blue when
negative. Also, the chroma C [Eqn (2)] and hue angle
a [Eqn (3)] were calculated from the values for L, a, b
and used to describe the colour change during drying
(
):
C ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
a
2
þ
b
2
p
(2)
a ¼ tan
1
ð
b=aÞ
(3)
2.4. Data analysis
The research was conducted using randomised plots
factorial experimental design. Determination of the
investigated components was carried out in three
replicates. Mean differences were tested for signifi-
cance by using an least significant difference (LSD)
(MSTATC) test at 1% level of significance.
Non-linear regression analysis was performed using
NLREG (NLREG version 63) to estimate the para-
meters k and n of semi-empirical Page’s equation [Eqn
(1)]. Regression results include the coefficients for the
equation and coefficient of determination R
2
.
3. Results and discussion
3.1. Drying curves
Moisture–time diagram of nettle along the drying
period on dry basis is given in
for microwave,
convective and vacuum drying, respectively.
A reduction in drying time occurred with increasing
the microwave power level. The time required for the
lowering of moisture content of nettle leaves to 01 from
441 on dry basis varied between 4 and 6 min depending
on the microwave power level. A marked decline was
noted in the drying period of leaves with the increasing
microwave power level (
). The required time for microwave drying
ARTICLE IN PRESS
ENERGY CONSUMPTION AND COLOUR CHARACTERISTICS OF NETTLE LEAVES
497
at 500 W was 15 times longer than that at 850 W. The
drying time reduced by 30 and 15 times in the drying
treatment realised at 50 and 75 1C temperatures and at 1
m/s air velocity compared with the drying treatment
realised at 850 W microwave powers. The moisture
content of the material was very high during the initial
phase of the drying which resulted in a higher
absorption of microwave power and higher drying rates
due to the higher moisture diffusion. As the drying
progressed, the loss of moisture in the product caused a
decrease in the absorption of microwave power and
resulted in a fall in the drying rate. Higher drying rates
were obtained at higher microwave output powers.
Thus, the microwave output power had a crucial effect
on the drying rate (
).
The convective drying process which reduced the
nettle leaves moisture contents from 441 db to moisture
content of 010 db took 30–120 min, depending on the
applied temperature. As the temperature was increased,
the drying time of leaves was significantly reduced
probability (P
o001) (
;
Mwithiga &
). By working at
125 1C instead of 50 1C, the drying time up to the
moisture content of 010 db could be shortened by 75%.
A marked decline was observed in the drying period
of nettle leaves with the increasing temperature level and
decreasing vacuum level (
).
Drying time at 50 1C temperature was found as 55 and
65 min for 20 and 50 mm [Hg], respectively, and at 75 1C,
it was found as 35 and 45 min for 20 and 50 mm [Hg]
vacuum values, respectively. Increase in temperature
level in vacuum drying had an important effect on the
reduction of drying time. The extent of drying realised at
50 1C temperature and 50 mm [Hg] vacuum value with
the longest drying period was 186 times higher
compared with the drying process realised at 75 1C and
20 mm [Hg], with the shortest drying period. When the
drying process realised at 50 1C temperature and 1m/s
air velocity without vacuum effect was compared with
the drying processes at 50 1C temperature and with 20
and 50 mm [Hg] vacuum values, the drying period was
shortened by 218 and 185 times, respectively, com-
pared with the drying without vacuum effect. Similarly,
when the drying applications realised at 75 1C with 20
and 50 mm [Hg] vacuum values were compared with
drying process without vacuum at 75 1C and 1m/s air
velocity, the drying period was reduced by 171 and 133
times, respectively, under vacuum.
ARTICLE IN PRESS
0
1
2
3
4
5
0
10
20
30
40
50
60
70
80
90 100 110 120
Convective drying time, min
M
o
isture content,
db
Fig. 2. The convective drying curve of nettle leaves on dry basis;
comparing experimental curve with the predicted one (-) through
semi-empirical Page’s equation [Eqn (1)] for nettle leaves at
various temperatures; K, 125 1C; m, 100 1C; &, 75 1C;—,
50 1C
0
1
2
3
4
5
0
1
2
3
4
5
Microwave drying time, min
Moisture content, db
Fig. 1. The microwave drying curve of nettle leaves on dry basis;
comparing experimental curve with the predicted one (-) through
semi-empirical Page’s equation [Eqn (1)] for nettle leaves at
various microwave levels; K, 850 W; m, 750 W; &, 650 W;—,
500 W
0
1
2
3
4
5
0
10
20
30
40
50
60
70
Vacuum drying time, min
Moisture content, db
Fig. 3. The vacuum drying curve of nettle leaves on dry basis;
comparing experimental curve with the predicted one (-)
through semi-empirical Page’s equation [Eqn (1)] for nettle
leaves at various vacuum and temperature combinations; K,
75 1C and 20 mm [Hg]; m, 75 1C and 50 mm [Hg]; &, 50 1C
and 20 mm [Hg];—, 50 1C and 50 mm [Hg]
I. ALIBAS
498
During the drying of 50 g nettle leaves at three
different drying methods, a total of 3987 g of weight
loss occurred from each drying sample.
3.2. Energy consumption
The energy consumption values obtained during
microwave, convective and vacuum drying of nettle
leaves are given
. When the three drying methods
were compared with respect to energy consumption
values, it was noted that the lowest energy consumption
occurred in microwave drying method and this was
followed by convective- and vacuum- drying methods.
The best result with regard to energy consumption was
obtained from 850 W microwave levels among all drying
methods.
Energy
consumption
at
this
level
was
006 kW h. The highest value in all drying methods
regarding energy consumption was noted in vacuum
drying process consisting of 50 1C temperature and
50 mm [Hg] vacuuming rate, with 081 kW h. There was
a 135—fold difference between the highest (50 1C-
50 mm [Hg]) and the lowest (850 W) energy consump-
tion values.
3.3. Modelling drying data
The parameters k and n of a semi-empirical Page’s
thin layer drying equation [Eqn (1)] for a given
microwave, convective and vacuum drying condition
were estimated using non-linear regression technique
(
) and the fitness is illustrated in
respectively.
The model gave an excellent fit for all the experi-
mental data points with values for the coefficient of
determination of greater than 09982 at 850 W in
microwave drying, 09994 at 75 1C in convective drying
and 09976 (75 1C–20 mm [Hg]) in vacuum drying. It is
determined that the value of the drying constant k
increased with the increase in microwave power. This
data points out that following the increase in microwave
output power, drying curve becomes steeper, indicating
faster drying of the product. As a result, measured
moisture ratio values and predicted moisture ratio
values were found similar to each other (
). The drying constant k values
increased with the increasing temperature at all tem-
perature values but 75 1C (
The best coefficient of determination at 50 1C was 09999
at convective drying. The vacuum drying level estab-
lished by combining 50 1C temperature and 20 mm [Hg]
vacuuming rate gave the best result (09999) with respect
to coefficients of determination.
ARTICLE IN PRESS
0
0.8
0.6
0.4
0.2
1
50
°
C, 1 m/s
75
°
C, 1 m/s
100
°
C, 1 m/s
125
°
C, 1 m/s
20 mm[Hg], 75
°
C
20 mm[Hg], 50
°
C
50 mm[Hg], 75
°
C
50 mm[Hg], 50
°
C
500 W
650 W
750 W
850 W
Con
v
ecti
ve
V
acuum
Microw
ave
Energy consumption, kWh
Fig. 4. Energy consumption during the drying of nettle leaves at
three different drying methods
Table 1
Non-linear regression analysis results of semi-empirical Page’s
equation [Eqn (1)] for microwave, vacuum and convective drying
of nettle (
Urtica diocia L.) leaves; k, drying rate constant, min
1
;
n, exponent; SEE, standard error of estimate; R
2
, coefficient of
determination
Drying method
k
(NS)
n
SEE(
7)
(NS)
R
2(NS)
Microwave drying
850 W
01865
20276
001603
09982
750 W
01472
20911
001219
09990
650 W
01171
20335
001185
09990
500 W
00925
19988
001291
09988
Vacuum drying
75 1C–20 mm [Hg]
00373
12348
001721
09976
75 1C–50 mm [Hg]
00345
12155
001148
09989
50 1C–20 mm [Hg]
00368
11492
000396
09999
50 1C–50 mm [Hg]
00354
11302
000665
09996
Convective drying
50 1C
00697
11383
000841
09995
75 1C
00483
11520
000633
09997
100 1C
00251
12006
000826
09994
125 1C
00261
10446
000348
09999
Column mean values with different superscripts are significantly
different.
Probability Po001,
NS
not significant.
ENERGY CONSUMPTION AND COLOUR CHARACTERISTICS OF NETTLE LEAVES
499
3.4. Colour parameters
The colour parameters (L, a, b) formed in microwave,
convective and vacuum drying of nettle leaves are
compared in
The colour criteria obtained as a result of microwave
(500, 650, 750 and 850 W), convective (50, 75, 100 and
125 1C) and vacuum (50 1C–50 mm [Hg], 50 1C–20 mm
[Hg], 75 1C–50 mm [Hg] and 75 1C–20 mm [Hg]) drying
the nettle leaves, and the colour criteria obtained from
the fresh nettle leaves. An important loss was noted in
the values for L, a, and b of dried nettle leaves when
compared with fresh leaves.
The highest loss in brightness was determined in
500 W microwave power levels in microwave drying. A
higher value was obtained at 850 W in comparison with
the other power levels. Values similar to the results
obtained for L values were obtained with a, b values.
The colour values nearest to those of fresh nettle leaves
were reached in the drying applications made by using
850 W microwave powers. Similar results were obtained
by
. They examined the colour
change in carrot as influenced by 100, 300 and 500 W
microwave powers and determined the slightest colour
change in 500 W microwave output powers. According
to
, high moisture bio-products
undergoing microwave drying have the advantage.
Microwave drying removes the moisture on the surface
by converting it to water vapour; it results in drying
without causing surface overheating phenomena. There-
fore, in terms of surface colour degradation, preserva-
tion of the product colour was good. It is estimated that
the products are subjected to high temperature with the
increasing power levels during microwave drying (
). For this reason, the product colour is
also adversely affected, since the microwave drying
period is longer in the drying process realized at very
low microwave powers, and thus the product is
subjected to heat for a longer time (
). Also, in this study, the low values measured in L,
a, and b colour criteria measured at 500 W microwave
power are supported by the results in the previous
research given above.
The greatest loss in brightness was determined at 125 1C
temperature value in convective drying. Higher values
were obtained at 50 1C temperature compared with the
other treatments. Similar values were obtained also for a,
and b values. Colour values nearest to those of fresh nettle
leaves were reached with drying at 50 1C temperature.
examined the colour change in
red pepper at 50 and 60 1C; and determined the slightest
colour change at 50 1C temperature.
dried bay leaves at 40, 50 and 60 1C and obtained the best
colour value at 50 1C. The products are subjected to high
temperatures with the increasing temperature during
convective drying. Therefore, the product colour is
adversely affected by convective drying applications
realised at high temperatures.
ARTICLE IN PRESS
Table 2
Comparison between microwave, vacuum and convective drying methods for colour parameters during nettle leaves drying
(
L, brightness (+100)/ darkness (+0); a, redness (+50)/greenness (50) coordinate; b, yellowness (+50)/blueness (50)
coordinate;
C, chroma; a1, hue angle)
Drying method
Colour parameters
L
a
b
C
a1
Fresh
2764
a
(
70330)
686
a
(
70176)
967
a
(
70146)
1185
a
(
70216)
12535
ab
(
7037)
Microwave drying
850 W
2693
ab
(
70209)
665
a
(
70116)
919
ab
(
70180)
1134
ab
(
70210)
12588
a
(
7023)
750 W
2650
bcd
(
70227)
641
ab
(
70169)
898
bc
(
70068)
1103
bc
(
70153)
12553
a
(
7051)
650 W
2612
bcde
(
70087)
613
bc
(
70069)
874
bcd
(
70099)
1068
bcd
(
70120)
12507
ab
(
7006)
500 W
2571
de
(
70207)
586
cd
(
70144)
833
de
(
70197)
1019
de
(
70244)
12514
ab
(
7003)
Vacuum drying
75 1C–20 mm [Hg]
2659
bc
(
70217)
646
ab
(
70154)
899
bc
(
70076)
1107
bc
(
70152)
12570
a
(
7042)
75 1C–50 mm [Hg]
2606
cde
(
70030)
610
bc
(
70029)
866
cd
(
70032)
1059
cd
(
70043)
12515
ab
(
7003)
50 1C–20 mm [Hg]
2569
de
(
70155)
584
cd
(
70092)
832
de
(
70142)
1017
de
(
70170)
12506
ab
(
7003)
50 1C–50 mm [Hg]
2485
fg
(
70147)
546
de
(
70115)
783
ef
(
70104)
954
ef
(
70151)
12489
ab
(
7022)
Convective drying
50 1C
2544
ef
(
70121)
566
cd
(
70093)
808
e
(
70127)
986
e
(
70158)
12504
ab
(
7004)
75 1C
2418
g
(
70072)
502
e
(
70047)
745
f
(
70049)
899
f
(
70067)
12398
b
(
7008)
100 1C
2219
h
(
70199)
323
f
(
70177)
610
g
(
70139)
691
g
(
70205)
11788
c
(
7077)
125 1C
1963
i
(
70383)
200
g
(
70139)
508
h
(
70194)
556
h
(
70230)
11139
d
(
7067)
Probability Po001 Column mean values with different superscripts are significantly different.
I. ALIBAS
500
In vacuum drying, the greatest loss in brightness was
determined at 50 1C temperature and 50 mm [Hg]
vacuum combinations. Values nearest to the colour
values (L, a, and b) of fresh nettle leaves were obtained
with the combinations of 75 1C–20 mm [Hg] and
75 1C–50 mm [Hg].
In all drying methods, colour values closest to those
of fresh nettle leaves were obtained in microwave drying.
The brightness L value most approximate to those of
fresh nettle leaves was determined in 850 W microwave
power levels, while the most distant brightness L value
was found at 125 1C temperature.
4. Conclusion
The effects of three different drying methods on the
drying of nettle leaves were evaluated based on the
drying parameters such as the drying time, the moisture
content on dry basis, the drying rate, energy consump-
tion and colour criteria. The microwave drying period
was completed between 45 and 6 min at the microwave
powers between 750 and 500 W, while the energy
consumption was constant (007 kW h). The energy
consumption at 850 W microwave levels with the short-
est drying period (4 min) was determined as 006 kW h.
The most suitable methods after microwave drying
method with respect to energy consumption are
convective-drying and vacuum-drying methods, in
decreasing suitability order. Drying period lasted for
30–120 min in convective-drying method, depending on
temperature level (125–50 1C). The drying period in
vacuum drying method ranged from 35 to 65 min,
depending
on
vacuum-temperature
combination
(50 1C–50 mm [Hg] and 75 1C–20 mm [Hg]). Results
closest to those of fresh products with respect to colour
criteria were obtained from microwave drying method,
and this was followed by vacuum and convective drying
methods, in order. The best values of colour criteria,
drying time, energy consumption and drying rate were
obtained at 850 W microwave power levels.
References
Adu B; Otten L (1996). Diffusion characteristics of white beans
during microwave drying. Journal of Agricultural Engineer-
ing Research, 64(1), 61–69
Akgul A (1993) Spices Science and Technology. Association
Food Technology, Publ. No. 15. Ankara, Turkey
Alibas I (2006). Characteristics of chard leaves during
microwave, convective, and combined microwave-convec-
tive drying. Drying Technology, 24, 1425–1435
Alibas-Ozkan I; Akbudak B; Akbudak N (2005). Microwave
drying characteristics of spinach. Journal of Food Engineer-
ing, 78(2), 577–583
Allardice P (1993). A–Z of Companion Planting. Cassell
Publishers Ltd, London
Bouraout M; Richard R; Durance T (1994). Microwave and
convective drying of potato slides. Journal of Food Process
Engineering, 17, 353–363
Chevallier A (1996). The Encyclopedia of Medicinal Plants.
Dorling Kindersley, London
Chua J K; Chou K S (2005). A comparative study between
intermittent microwave and infrared drying of bioproducts.
International Journal of Food Science and Technology, 40,
23–39
Demir V; Gunhan T; Yagcioglu A K; Degirmencioglu A (2004).
Mathematical modelling and the determination of some
quality parameters of air-dried bay leaves. Biosystems
Engineering, 88, 325–335
Dı´az G R; Martı´nez-Monzo´ J; Fito P; Chiralt A (2003).
Modelling of dehydration-rehydration of orange slices in
combined microwave/air drying. Innovative Food Science &
Emerging Technologies, 4, 203–209
Doymaz I; Pala M (2002). Hot-air drying characteristics of red
pepper. Journal of Food Engineering, 55, 331–335
Drouzas A E; Schubert H (1996). Microwave application in
vacuum drying of fruits. Journal of Food Engineering, 28,
203–209
Drouzas A E; Tsami E; Saravacos G D (1999). Microwave/
vacuum drying of model fruit gels. Journal of Food
Engineering, 39, 117–122
Facciola S; Cornucopia A (1990). Source Book of Edible
Plants. Kampong Publications, Vista
Feng H (2002). Analysis of microwave assisted fluidized-bed
drying of particulate product with a simplified heat and
mass transfer model. International Communications in Heat
and Mass Transfer, 29, 1021–1028
Feng H; Tang J (1998). Microwave finish drying of diced apple
slices in a spouted bed. Journal of Food Science, 63,
679–683
Funebo T; Ohlsson T (1998). Microwave-assisted air dehydra-
tion of apple and mushroom. Journal of Food Engineering,
38, 353–367
Gulcin I; Kufrevioglu O I; Oktay M; Buyukokuroglu M E
(2004). Antioxidant, antimicrobial, antiulcer and analgesic
activities of nettle (Urtica dioica L). Journal of Ethnophar-
macology,, 90(2–3), 205–215
Kadlec P; Rubecova A; Hinkova A; Kaasova J; Bubnik Z; Pour
V (2001). Processing of yellow pea by germination,
microwave treatment and drying. Innovative Food Science
and Emerging Technologies, 2, 133–137
Lin T M; Durance T D; Seaman C H (1998). Characterization
of vacuum microwave air and freeze dried carrot slices.
Food Research International, 4, 111–117
Litvin S; Mannheim C H; Miltz J (1998). Dehydration of
carrots by a combination of freeze drying, microwave
heating and air or vacuum drying. Journal of Food
Engineering, 36, 103–111
Martı´nez-Para M C; Fidanza F; Torija-Isasa M E (1980a). La
ortiga en la alimentacio´n [English tradition IV] Fibra
alimentaria. Anales de Bromatologı´a, 32, 109–118
Martı´nez-Para M C; Fidanza F; Torija-Isasa M E (1980b). La
ortiga en la alimentacio´n [English tradition V] Estudio de la
proteı´na. Anales de Bromatologı´a, 32, 309–314
Martı´nez-Para M C; Torija-Isasa M E (1980). La ortiga en la
alimentacio´n [English tradition III] Ascorbic acid. Anales de
Bromatologı´a, 32, 295–298
ARTICLE IN PRESS
ENERGY CONSUMPTION AND COLOUR CHARACTERISTICS OF NETTLE LEAVES
501
Maskan M (2000). Microwave/air and microwave finish drying
of banana. Journal of Food Engineering, 44, 71–78
Maskan M (2001). Drying, shrinkage and rehydration
characteristics of kiwifruits during hot air and microwave
drying. Journal of Food Engineering, 48, 177–182
Menges O H; Ertekin C (2005). Mathematical modelling of
thin layer drying of Golden apples. Journal of Food
Engineering, 77(1), 119–125
Methakhup S; Chiewchan N; Devahastin S (2005). Effects of
drying methods and conditions on drying kinetics and
quality of Indian gooseberry flake. LWT—Food Science
and Technology, 38(6), 579–587
Montgomery S W; Goldschmidt V W; Franchek M A (1997).
Vacuum assisted drying of hydrophilic plates: static drying
experiments. International Journal of Heat Mass Transfer,
41, 735–744
Mwithiga G; Olwal J O (2005). The drying kinetics of kale
(Brassica oleracea) in a convective hot air dryer. Journal of
Food Engineering, 71, 373–378
Pere C; Rodier E (2002). Microwave vacuum drying of porous
media: experimental study and qualitative considerations of
internal transfers. Chemical Engineering and Processing, 41,
427–436
Prabhanjan D G; Ramaswamy H S; Raghavan G S V (1995).
Microwave assisted convective air drying of thin layer
carrots. Journal of Food Engineering, 25, 283–293
Schiffmann R F (1995) Microwave and dielectric drying. In:
Handbook of Industrial Drying (Mujumdar A S (ed), pp
345–372. New York, USA
Soysal Y (2004). Microwave drying characteristics of parsley.
Biosystems Engineering, 89, 167–173
Sumnu G; Turabi E; Oztop M (2005). Drying of carrots in
microwave and halogen lamp–microwave combination
ovens. LWT—Food Science and Technology, 38, 549–553
Torringa E; Esveld E; Scheewe I; van den Berg R; Bartels P
(2001). Osmotic dehydration as a pre-treatment before
combined microwave-hot-air drying of mushrooms. Journal
of Food Engineering, 49, 185–191
Tsami E; Krokida M K; Drouzas A E (1998). Effect of drying
method on the sorption characteristics of model fruit
powders. Journal of Food Engineering, 38, 381–392
Tulasidas T N; Ratti C; Raghavan G S V (1997). Modelling of
microwave drying of grapes. Canadian Agricultural En-
gineering, 39(1), 57–67
Walde S G; Balaswamy K; Velu V; Rao D G (2002). Microwave
drying and grinding characteristics of wheat (Triticum
aestivum). Journal of Food Engineering, 55, 271–276
Weiss R F (1988). Herbal Medicine. Beaconseld Publishers,
London
Yongsawatdigul J; Gunasekaran S (1996). Microwave-vacuum
drying of cranberries: part II, quality evaluation. Journal of
Food Processing and Preservation, 20, 145–156
ARTICLE IN PRESS
I. ALIBAS
502
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