Microwave drying characteristics of spinach

I. Alibas Ozkan

a,*

, B. Akbudak

b

, N. Akbudak

b

a

Uludag University, Faculty of Agriculture, Department of Agricultural Machinery, 16059 Bursa, Turkey

b

Uludag University, Faculty of Agriculture, Department of Horticulture, 16059 Bursa, Turkey

Received 12 May 2005; accepted 25 October 2005

Available online 20 December 2005

Abstract

Spinach leaves (Spinacia oleracea L. cv. ‘‘Meridian’’) with 50 g weight and 9.01 humidity on dry basis were dried in microwave oven

using eight different microwave power levels ranging between 90 and 1000 W, until the humidity fell down to 0.1 on dry basis. Drying
processes were completed between 290 and 4005 s depending on the microwave power level. Energy consumption remained constant
within the power range of 350–1000 W, whereas 160 and 90 W resulted in significant increase in energy consumption. In this study, mea-
sured values were compared with predicted values obtained from PageÕs thin layer drying semi-empirical equation. The best quality in
terms of colour and ascorbic acid values were obtained in the drying period with 750 W microwave power. Microwave power of 750 W
for 350 s produced the least energy consumption and the energy requirement for drying was only 0.12 kW h.
2005 Elsevier Ltd. All rights reserved.

Keywords: Ascorbic acid; Colour; Dehydration; Microwave drying; Spinach

1. Introduction

Spinach (Spinacia oleracea L.) is a cool season annual

vegetable. It is a popular vegetable that is eaten raw, boiled
or baked into various dishes. Spinach is low in calories and
is a good source of ascorbic acid (vitamin C) (

Toledo,

Ueda, Imahori, & Ayaki, 2003

). Ascorbic acid is an impor-

tant nutrient in vegetable. It is a hydro-soluble vitamin and
more sensitive to heat, oxygen, light and considered to be
highly sensitive to losses during processing (

Soysal & So¨y-

lemez, 2005; Yanishlieva-Maslarova, 2001

). Spinach is a

vegetable which rapidly perishes after harvest and which
is consumed only in the product season. Drying is the
one of the storage methods, which has the capability of
extending the consumption period of spinach, yet main-
taining its vitamin 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 microorganisms, enzymes or ferments
in the material are suppressed via drying (

AlibasßO

¨ zkan &

Isßık, 2001

).

Different drying methods are used in the drying of fruits

and vegetables. Hot-air drying is the most common method
in the drying of foodstuffs. However, this method leads to
serious injuries such as the worsening of the taste, colour
and nutritional content of the product, decline in the den-
sity and water absorbance capacity and shifting of the sol-
utes from the internal part of the drying material to the
surface, due to the long drying period and high tempera-
ture (

Bouraout, Richard, & Durance, 1994; Drouzas,

Tsami, & Saravacos, 1999; Feng & Tang, 1998; Lin,
Durance, & Seaman, 1998; Maskan, 2001; Yongsawatdigul
& Gunasekaran, 1996

). The use of microwave rays in the

drying of products has become widespread because it
minimizes the decline in quality and provides rapid and,
effective heat distribution in the material as well (

Dı´az,

Martı´nez-Monzo´, Fito, & Chiralt, 2003

). Furthermore,

high quality product is obtained via microwave drying in
addition to the decline in drying period and energy conser-
vation during drying (

Feng, 2002

).

0260-8774/$ - see front matter

2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2005.10.026

*

Corresponding author. Tel./fax: [phone].
E-mail address:

[e-mail]

(I.A. Ozkan).

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 78 (2007) 577–583

Microwave drying creates an effect for moisture transfer,

leading to a water vapour pressure gradient between the
surface and internal part of the material, as in the conven-
tional drying methods (

Maskan, 2001

). Microwave energy

applications in the drying of vegetables have several advan-
tages including the shortening of drying time, a homoge-
nous 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 res-
piration, lowered product temperatures when combined
with vacuum drying, reduction in the loss of water-soluble
components and energy savings (

Torringa, Esveld, Scheewe,

van den Berg, & Bartels, 2001

).

Microwave drying techniques have proved to be effective

for a number of agricultural products such as herbs (

Giese,

1992; Karting, Lu¨cke, & Lassnig, 1994

), potato (

Bouraout

et al., 1994

), soybeans and white beans (

Adu, Otten, &

Brown, 1994; Adu & Otten, 1996

), grapes (

Tulasidas,

Ratti, & Raghavan, 1997

), apple and mushroom (

Feng &

Tang, 1998; Funebo & Ohlsson, 1998

), carrot (

Lin et al.,

1998; Litvin, Mannheim, & Miltz, 1998

), banana (

Maskan,

2000

), kiwifruits (

Maskan, 2001

), yellow pea (

Kadlec et al.,

2001

), wheat (

Walde, Balaswamy, Velu, & Rao, 2002

),

parsley (

Soysal, 2004

).

The aim of this study was to (i) evaluate the efficacy of

microwave drying technique for spinach, (ii) compare the
measured findings obtained during the drying of spinach
with the predicted values obtained with PageÕs thin layer
drying semi-empirical equation, and (iii) determine the
changes in the ascorbic acid and colour values of the prod-
uct after drying.

2. Materials and methods

2.1. Materials

Plants of fresh spinach (Spinacia oleracea L. cv.

‘‘Meridian’’) used in the drying experiments were provided
from Karacabey county of Bursa. They were stored at a
temperature of 4 ± 0.5

C until the drying process (

Soysal,

2004

). Four different samples, each being 50 g were kept in

the drying oven at 105

C for 24 h, after which the mois-

ture content of spinach fell down to 9.01 ± 0.08 on dry
basis.

2.2. Microwave drying technique

Microwave energy is capable of polarizing substances.

The electrons in the polarized substance were in motion
due to the conversion of electromagnetic energy embedded
in the substance into kinetic energy. Electrons bump into
each other during this electron movement and their energy
is converted to heat energy as a result of friction. Thus, the
moisture was removed from the product in the microwave
oven.

2.3. Drying equipment and drying method

Drying treatment was performed in a domestic digital

microwave oven (Arcelik MD 592, Turkey) with technical
features of

230 V, 50 Hz and 2900 W. 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 area on which microwave drying is car-
ried out was 327

· 370 · 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 direc-

tion of 360

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.

Drying trial was carried out at eight different microwave

generation power being 1000, 850, 750, 650, 500, 350, 160
and 90 W. The spinach leaves to be dried were 50
(±0.09) g in weight and selected from the uniform, and
healthy plants. Three different drying trials were conducted
at each microwave generation power and the values
obtained from these trials were averaged and the drying
parameters were determined. Rotating glass plate was
removed from the oven periodically (every 30 s) during
the drying period, and the moisture loss was determined
by weighing the plate using digital balance (Alsep EX
2000A, Germany) with 0.01 g precision (

Maskan, 2000;

Soysal, 2004

). All weighing processes were completed in

10 s during the drying process. Energy consumption of
microwave oven was determined using a digital electric
counter (Kaan, Type 101, Turkey) with 0.01 kW h preci-
sion. Drying process continued until the moisture content
of spinach fell down to 0.1 ± 0.009 on dry basis.

The following common semi-empirical PageÕs equation

(Eq.

(1)

) was used to describe the thin layer drying kinetics

of spinach leaves (

Sharma & Prasad, 2001; Soysal, 2004

),

where M

R

is the moisture ratio; X is the moisture content

db; X

e

is the equilibrium moisture content db; t is the time

in min; k is the drying constant in min

1

; and n is the

dimensionless exponent. The equilibrium moisture content
(X

e

) was assumed to be zero for microwave drying

(

Maskan, 2000

)

M

R

¼

X

X

e

X

0

X

e

¼ expðkt

n

Þ

ð1Þ

2.4. Ascorbic acid

Ascorbic acid was determined by exposing fruit samples

to extraction with oxalic acid (0.4%) and then reading and
calculating the absorbency values at 520 nm in the spectro-
photometer

(Shimadzu

UV-120-01)

(Shimadzu

Co.,

Duisburg, Germany) (

Holden, 1976

).

2.5. Colour parameters

Leaf colour was determined by two readings on the

two different symmetrical faces of the leaf in each

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I.A. Ozkan et al. / Journal of Food Engineering 78 (2007) 577–583

replicate, using a Minolta CR 300 colorimeter (Konica-
Minolta, Osaka, Japan), calibrated with a white standard
tile.

2.6. Data analysis

The research was conducted using randomized plots fac-

torial experimental design. Determination of the investi-
gated components was carried out in three replicates.
Mean differences were tested for significance by using an
LSD (MSTAT) test at 5% level of significance.

Non-linear regression analysis was performed using

NLREG (NLREG version 6.3) to estimate the parameters
k and n of semi-empirical PageÕs equation (Eq.

(1)

). Regres-

sion results include the coefficients for the equation, stan-
dard error of estimate (SEE (±)) and coefficient of
determination R

2

.

3. Results and discussion

3.1. Drying curves

Moisture-time diagram of spinach along the drying per-

iod on dry basis is given in

Fig. 1

. As seen in

Fig. 1

, a

reduction in drying time occurred with the increasing
microwave power level. The time required for the lowering
of moisture content of spinach levels to 0.1 level, from 9.01
on dry basis varied between 290 and 4005 s depending on
the microwave power level. A marked decline was noted
in the drying period of leaves with the increasing micro-
wave power level (

Drouzas & Schubert, 1996; Funebo &

Ohlsson, 1998; Prabhanjan, Ramaswamy, & Raghavan,
1995; Soysal, 2004

). The drying time obtained in the drying

process using 90 W microwave power levels was 13.81
times longer than those in 1000 W. The drying time

0

1

2

3

4

5

6

7

8

9

10

0

240

480

720

960

1200 1440 1680 1920 2160 2400 2640 2880 3120 3360 3600 3840 4080

Drying Time, s

Mo

is

tu

re

C

ont

e

n

t (

db)

Fig. 1. The drying curve of spinach leaves on dry basis; +, 1000 W; j, 850 W; m, 750 W; h, 650 W; , 500 W; s, 350 W; , 160 W; –, 90 W.

0

1

2

3

4

5

6

7

8

9

0

240

480

720

960

1200 1440 1680 1920 2160 2400 2640 2880 3120 3360 3600 3840 4080

Drying Time, s

Moisture quantity lost by the material for every 30 s (g)

Fig. 2. The quantity of moisture loss from the spinach leaves in every 30 seconds of the drying period; +, 1000 W; j, 850 W; m, 750 W; h, 650 W;

, 500 W; s, 350 W; , 160 W; –, 90 W.

I.A. Ozkan et al. / Journal of Food Engineering 78 (2007) 577–583

579

reduced by 33 and 17 times in the drying treatment realised
at 50 and 75

C temperatures and at 1 m/s air velocity com-

pared with the drying treatment realised at 1000 W micro-
wave power.

During the drying of 50 g spinach leaves at eight differ-

ent microwave power, a total of 44.5 (±0.07) g of weight
loss occurred from each drying sample. The quantities of
moisture removed from the material in every 30-s time per-
iod of drying cycle at eight different microwave power lev-
els are given in

Fig. 2

. Maximum value of moisture

removed from the material at 1000 W microwave power
(8.21 g) was obtained between 120 and 150 s of the drying
period. About 60% of the drying process was completed in
the 150th second when the maximum evaporation rate was
recorded. The value of maximum evaporation between
240th and 270th seconds of the drying period at 90 W
microwave powers was determined as 0.66 g. At this point,
7.91% of the drying process was completed.

The drying rates (kg (H

2

O) kg

1

(DM) min

1

) obtained

in unit time under different microwave power levels are
given in

Fig. 3

. Depending on the drying conditions, aver-

age drying rates of spinach leaves ranged from 0.045 to
0.802 kg (H

2

O) kg

1

(DM) min

1

for the output power

between 90 and 1000 W, respectively. 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.
The drying rates increased with the increasing microwave
power levels. Therefore microwave power level has an
important effect on the drying rates. Our results are in
agreement with previous studies (

Funebo & Ohlsson,

1998; Maskan, 2000; Sharma & Prasad, 2001; Soysal,
2004

).

The energy consumption values obtained in the drying

trials carried out at eight different microwave power levels

(

Fig. 4

). Energy consumption at all levels within the power

range of 1000 and 350 W in which the drying process lasted
for 290–560 seconds was determined as 0.12 kW h. The
drying energy consumption rates were determined as 0.16
and 0.26 kW h for the power values of 160 W (with drying
period of 1530 s) and 90 W (with drying of 4005 s, respec-
tively). As a result, the energy consumption in the drying
processes carried out at low microwave power levels yield-
ing longer drying period was determined to be in higher
rates.

3.2. Modelling drying data

Microwave drying kinetics of spinach (S. oleracea L. cv.

‘‘Meridian’’) leaves were described using the drying data.
Non-linear regression technique was used to estimate the
parameters k and n of semi-empirical PageÕs equation

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.

8

0

1

2

3

4

5

6

7

8

9

Moisture Content (db)

Drying Rate (kg water /kg DM*min)

Fig. 3. Drying rates of the spinach leaves at different microwave power
levels; +, 1000 W; j, 850 W; m, 750 W; h, 650 W; , 500 W; s, 350 W;

, 160 W; –, 90 W.

0

0.05

0.1

0.15

0.2

0.25

0.3

1000

850

750

650

500

350

160

90

Microwave Power Level, W

Energy Consumption, kW h

Fig. 4. Energy consumption during the drying of spinach leaves at different microwave power levels.

580

I.A. Ozkan et al. / Journal of Food Engineering 78 (2007) 577–583

(Eq.

(1)

) for a given drying condition (

Table 1

) and the fit-

ness is shown in

Fig. 5

. The model gave an excellent fit for

all the experimental data points with values for the coeffi-
cient of determination of greater than 0.9943 and the stan-
dard error of estimates lower than 0.018035 (90 W). It is
determined that the value of the drying constant k
increased with the increase in microwave power. This data
points out that with increase in microwave output power
drying curve becomes steeper indicating faster drying of
the product. As a result, measured moisture ratio values
and estimated moisture ratio values were found similar to
each other.

3.3. Ascorbic acid

The ascorbic acid levels in various microwave power cat-

egories of spinach were compared with the levels in the cor-
responding fresh sample in

Table 2

and differences were

observed in ascorbic acid values of spinach. Reduction in
the ascorbic acid levels of the samples subjected to micro-
wave drying was recorded depending on time. Ascorbic
acid values gave the lowest results (23.30 mg 100

1

g

1

)

in the power level of 90 W, the longest drying period.
Ascorbic acid values of the spinaches dried in the energy
levels of 160 W (with 25.67 mg 100

1

g

1

) and 350 W

(25.70 mg 100

1

g

1

) were found lower than those in higher

energy levels. The differences between the values were also
found significant statistically (

Table 2

). Although there

were not very notable differences statistically between
500 W and higher powers with respect to ascorbic acid val-
ues, the best ascorbic acid values were obtained at 750 W
power with 43.09 mg 100

1

g

1

and at 650 W power with

43.57 mg 100

1

g

1

. Microwave cooking treatment of

broccoli was realised in a study by

Zhang and Hamauzu

(2004)

, and significant losses were determined in the ascor-

bic acid values with prolonging the treatment period. Also,
it was determined in a study on asparagus that drying per-
iod as well as drying methods leaded to ascorbic acid losses
(

Nindo, Sun, Wang, Tang, & Powers, 2003

).

3.4. Colour parameters

The results of colour parameters obtained from drying

processes of various microwave power categories are pre-
sented in

Table 3

for L (brightness), a (redness), b (yellow-

ness) values, respectively.

Table 3

shows the reduction in

brightness value L of the dried spinach leaves. Significant

Table 1
Non-linear regression analysis results of semi-empirical PageÕs equation
(Eq.

(1)

) for microwave drying of spinach leaves under various microwave

power

Microwave
power (W)

Drying rate
constant (k), min

1

Exponent (n)

**

SEE (±)

R

2

1000

0.1283

2.125639

0.01880

0.9976

850

0.1279

2.034130

0.015477

0.9983

750

0.0899

2.052521

0.019679

0.9971

650

0.0796

1.989679

0.016745

0.9977

500

0.0531

2.064432

0.018232

0.9971

350

0.0436

1.903894

0.019389

0.9962

160

0.0159

1.629115

0.018357

0.9968

90

0.0107

1.287173

0.018035

0.9943

SEE (±), standard error of estimate; R

2

, coefficients of determination.

**

Means with same letter do not show significance at P < 0.01.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

10

20

30

40

50

60

Drying Time, min

MR

70

Fig. 5. Moisture ratio versus time, comparing experimental curve with the predicted one through semi-empirical PageÕs equation (Eq.

(1)

) for spinach

leaves under various microwave powers; +, 1000 W; j, 850 W; m, 750 W; h, 650 W; , 500 W; s, 350 W; , 160 W; –, 90 W; —— predicted.

Table 2
Effect of microwave drying on ascorbic acid content in spinach

Microwave power (W)

Ascorbic acid content (mg 100

1

g

1

)

Fresh

50.18 ± 1.36a

*

90

23.30 ± 1.93c

160

25.67 ± 0.94c

350

25.70 ± 1.62c

500

42.86 ± 1.61b

650

43.57 ± 1.24b

750

43.09 ± 1.88b

850

42.68 ± 1.50b

1000

41.79 ± 2.89b

*

Means with same letter do not show significance at P < 0.05.

I.A. Ozkan et al. / Journal of Food Engineering 78 (2007) 577–583

581

L value losses occurred in the dried spinach leaves com-
pared with the fresh ones. The greatest loss in brightness
was determined in 1000 W energy application, whereas
higher values were obtained in the power levels of 750,
650 and 500 W compared with the other treatments. ‘‘a’’
and ‘‘b’’ values obtained were similar to the results
obtained from L values. The colour values closest to that
of fresh spinach values were obtained in the drying pro-
cesses made using the energy levels of 750, 650 and
500 W. These results are consistent with those of

Chua

and Chou (2005)

. They investigated the colour change in

carrot at microwave powers of 100, 300 and 500 W, and
determined the lowest colour change at 500 W microwave
powers. According to

Schiffmann (1995)

, high moisture

bio-products undergoing microwave drying have the
advantage. Microwave drying pushes liquid into the sur-
face and the liquid is usually converted into the vapour.
This process results in drying without causing surface over-
heating phenomena. Therefore, in terms of surface colour
degradation, preservation of the product colour was good.
It is estimated that the products are subjected to high tem-
perature with the increasing power levels during microwave
drying. Therefore, the product colour is adversely affected
in the drying processes realized high microwave powers
(

Chua & Chou, 2005

). Also, in our study, the low values

measured in L, a, and b colour criteria at 1000 W micro-
wave power are supported by the results obtained in the
previous research above.

4. Conclusion

Microwave drying period of spinach leaves lasted

between 290 and 430 s at the microwave powers at 1000
and 500 W, respectively, while the energy consumption
was constant (0.12 kW h). Ascorbic acid loss in the product
dried at power levels equal to or more than 500 W was less
than those below 500 W. However, the colour criteria
assessments showed that drying at 500 and 850 W pro-
duced the best brightness, redness and yellowness parame-
ters. We concluded that 750 W is the optimum microwave
power level in the microwave drying of spinach with
respect to drying time, energy consumption, ascorbic acid
level and colour criteria.

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Table 3
Comparison between microwave output powers for colour parameters during spinach drying (L: brightness, a: redness, b: yellowness, C: chroma, a

: hue

angle)

Microwave output power (W)

L

a

b

C

a

Fresh

41.98 ± 0.99

*

14.83 ± 0.82

16.69 ± 0.53

22.33 ± 0.56

131.62 ± 0.82

90

31.74 ± 1.71

8.94 ± 0.45

12.12 ± 0.37

15.06 ± 0.39

126.41 ± 1.93

160

33.64 ± 1.03

9.80 ± 0.78

13.26 ± 0.64

16.49 ± 0.82

126.47 ± 1.08

350

32.24 ± 1.16

10.32 ± 0.75

12.91 ± 0.97

16.53 ± 1.00

128.64 ± 1.30

500

35.99 ± 0.39

11.28 ± 0.89

14.50 ± 0.65

18.37 ± 0.35

127.88 ± 1.34

650

35.58 ± 2.20

10.45 ± 0.72

13.30 ± 1.13

16.91 ± 1.12

128.16 ± 0.92

750

36.00 ± 1.02

10.86 ± 0.39

14.51 ± 1.03

18.12 ± 0.66

126.81 ± 1.92

850

34.15 ± 1.16

9.53 ± 0.19

13.43 ± 0.72

16.47 ± 0.58

125.36 ± 1.17

1000

29.90 ± 2.25

6.93 ± 0.16

8.92 ± 0.90

11.30 ± 0.80

127.84 ± 1.87

*

Values are means ± SEE.

582

I.A. Ozkan et al. / Journal of Food Engineering 78 (2007) 577–583

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