Energy Consumption and Colour Characteristics of Nettle Leaves during Microwave, Vacuum and Convective Drying (Ilknur Alibas).pdf

ARTICLE IN PRESS

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: ialibas@uludag.edu.tr

(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 850W) 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 50mm [Hg]) and drying temperature (50 and 75 1C) were considered. Drying periods ranged from 4 to 6,

30 to 120 and 35 to 65min 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 850W.

Published by Elsevier Ltd

1. Introduction

pottage, a tea made from the leaves has traditionally

been drunk ( Chevallier, 1996 ).

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 ( Alibas-Ozkan et al.,

2005 ).

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 ( Bouraout et al., 1994 ;

Yongsawatdigul & Gunesekaran, 1996 ; Feng & Tang,

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 ( Akgul, 1993 ). 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 ( Allardice,

1993 ), amino acids ( Martinez-Para, et al., 1980b ),

ascorbic acid ( Martinez-Para & Torija-Isasa, 1980 ), rare

carbohydrates ( Martinez-Para et al., 1980a ), and several

mineral elements ( Weiss, 1988 ). It is also known that

nettle has anti-oxidant, antimicrobial, anti-ulcer and

analgesic properties ( Gulcin et al., 2004 ). Shoots of

nettle cooked as a potherb are added to soups and can

also be dried for winter use ( Facciola & Cornucopia,

1990 ). Although the plants are used principally in

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I. ALIBAS

1998 ; Lin et al., 1998 ; Drouzas et al., 1999 ; Maskan,

2000, 2001 ).

The use of microwave rays in the drying of

agricultural products such as grains ( Adu & Otten,

1996 ; Walde et al., 2002 ), vegetables ( Litvin et al., 1998 ;

Lin et al., 1998 ; Alibas, 2006 ; Alibas-Ozkan et al., 2005 )

and fruits ( Tulasidas et al., 1997 ; Funebo & Ohlsson,

1998 ; Kadlec et al., 2001 ) has become widespread

because it minimises the decline in food quality and

provides a rapid and an effective head distribution in the

material ( Dı´ az et al., 2003 ), which leads to energy

savings ( Feng, 2002 ). 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 ( Mas-

kan, 2001 ).

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 ( Maskan, 2001 ).

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 ( Torringa et al., 2001 ).

Vacuum drying is a drying technique which is used for

drying of various products, retaining colour and vitamin

content ( Methakhup et al., 2005 ). 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 ( Pere & Rodier, 2002 ). Better product

quality with respect to traits such as taste, ﬂavour and

rehydration can be retained via high-degree vacuum

treatment ( Drouzas & Schubert, 1996 ). 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 ( Montgomery et al., 1997 ). 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 ( Tsami et al., 1998 ).

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

003) 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

05 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,

50Hz and 2900W. 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

1000W. 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 1m/s air

velocity. The area on which microwave and convective

drying is carried out was 327mm by 370mm by 207mm

in size, and consisted of a rotating glass plate with

280mm diameter at the base of the oven. Glass plate

rotates for 5min 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, 50Hz, 35 A and

800W. 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 300mm by 200mm by 250mm 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.

(

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497

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 500W.

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 50mm [Hg] and two different

temperature regimes at 50 and 75 1C, and the trials were

realised under the combinations of 50 1C–20mm [Hg],

50 1C–50mm [Hg], 75 1C–20mm [Hg] and 75 1C–50mm

[Hg]. A laboratory type greasy vacuum pump (Carpa-

nelli MMDE80B4, Italy) was used in the vacuum drying

with operating conditions were 220/240 V, 50/60Hz

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 5min 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 ( Soysal, 2004 ; Alibas, 2006 ). 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 kWh precision.

Drying process continued until the moisture content of

nettle fell down to 01

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

( Soysal, 2004 ):

C ¼

a 2 þ b 2

(2)

a ¼ tan 1 ðb=aÞ

(3)

2.4. Data analysis

0005 on dry basis.

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

2.2. Data analysis and empirical drying model

3.1. Drying curves

The following common semi-empirical Page’s equa-

tion [Eqn (1)] was used to describe the thin-layer drying

kinetics of nettle leaves ( Soysal, 2004 ; Alibas, 2006 ):

Moisture–time diagram of nettle along the drying

period on dry basis is given in Figs 1–3 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 6min depending

on the microwave power level. A marked decline was

noted in the drying period of leaves with the increasing

microwave power level ( Prabhanjan et al., 1995 ;

Drouzas & Schubert, 1996 ; Funebo & Ohlsson, 1998 ;

Soysal, 2004 ). The required time for microwave drying

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 ;andn is the dimension-

less exponent.

M R ¼ X X e

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I. ALIBAS

Microwave drying time, min

Vacuum drying time, min

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 , 850W; m, 750W; & , 650W;—,

500W

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 20mm [Hg]; m,751C and 50mm [Hg]; & ,501C

and 20 mm [Hg];—, 50 1C and 50mm [Hg]

The convective drying process which reduced the

nettle leaves moisture contents from 441 db to moisture

content of 010 db took 30–120min, depending on the

applied temperature. As the temperature was increased,

the drying time of leaves was signiﬁcantly reduced

probability (P o 001) ( Demir et al., 2004 ; Mwithiga &

Olwal, 2005 ; Menges & Ertekin, 2005 ). 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 ( Methakhup et al., 2005 ).

Drying time at 50 1C temperature was found as 55 and

65min for 20 and 50mm [Hg], respectively, and at 75 1C,

it was found as 35 and 45min for 20 and 50mm [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 50mm [Hg] vacuum value with

the longest drying period was 186 times higher

compared with the drying process realised at 75 1C and

20mm [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 50mm [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 1Cwith20

and 50mm [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.

0 10 20 30 40 50 60 70 80 90 100 110 120

Convective drying time, min

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; & ,751C;—,

50 1C

at 500W was 15 times longer than that at 850W. 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 850W 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 ( Soysal, 2004 ).

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499

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.

3.3. 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

( Table 1 ) and the ﬁtness is illustrated in Figs 1–3 ,

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 850W in

microwave drying, 09994 at 75 1C in convective drying

and 09976 (75 1C–20mm [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 ( Soysal, 2004 ;

Alibas-Ozkan et al., 2005 ). The drying constant k values

increased with the increasing temperature at all tem-

perature values but 75 1C( Mwithiga & Olwal, 2005 ).

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 20mm [Hg]

vacuuming rate gave the best result (09999) with respect

to coefﬁcients of determination.

3.2. Energy consumption

The energy consumption values obtained during

microwave, convective and vacuum drying of nettle

leaves are given Fig. 4 . 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 850W microwave levels among all drying

methods. Energy consumption at this level was

006 kWh. The highest value in all drying methods

regarding energy consumption was noted in vacuum

drying process consisting of 50 1C temperature and

50mm [Hg] vacuuming rate, with 081 kWh. There was

a13 5—fold difference between the highest (50 1C-

50mm [Hg]) and the lowest (850W) energy consump-

tion values.

850 W

750 W

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

650 W

500 W

50 mm[Hg], 50 ° C

Drying method

k (NS)

SEE( 7 ) (NS)

R 2(NS)

50 mm[Hg], 75 ° C

Microwave drying

850W

01865 20276 001603

09982

750W

01472 20911 001219

09990

20 mm[Hg], 50 ° C

650W

01171 20335 001185

09990

500W

00925 19988 001291

09988

20 mm[Hg], 75 ° C

Vacuum drying

75 1C–20mm [Hg] 00373 12348 001721

125 ° C, 1 m/s

09976

75 1C–50mm [Hg] 00345 12155 001148

09989

50 1C–20mm [Hg] 00368 11492 000396

09999

100 ° C, 1 m/s

50 1C–50mm [Hg] 00354 11302 000665

09996

75 ° C, 1 m/s

Convective drying

50 1C

00697 11383 000841

09995

50 ° C, 1 m/s

75 1C

00483 11520 000633

09997

100 1C

00251 12006 000826

09994

0 . 2

0 . 4

0 . 6

0 . 8

125 1C

00261 10446 000348

09999

Energy consumption, kWh

Column mean values with different superscripts are signiﬁcantly

different.

Probability P o 001, NS not signiﬁcant.

Fig. 4. Energy consumption during the drying of nettle leaves at

three different drying methods

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