Research Journal of Chemical Sciences 
______
______________________________
______
____
 
ISSN 2231
-
606X
 
 
 
Vol. 
2
(
4
), 
57
-
60
, 
April 
(201
2
)
 
 
Res.J.Chem.Sci.
 
 
 
International Science Congress Association
 
 
 
 
    
57
 
Short Communication
 
Microwave Drying 
o
f Onion Slices
 
 
Kalse S.B., 
Patil M.M.
 
and
 
Jain 
S.K.
 
Dept
. 
of Processing and food Eng
.
, College of Tech
. 
and Eng
.
, Maharana Pratap
 
University of Agriculture and Technology,
 
Udaipur
,
 
INDIA
 
 
Available online at: 
www.isca.in
 
(Received
 
8
th
 
October 
201
1
, revised
 
6
th
 
January 2012
, accepted
 
27
th
 
January 2012
)
 
 
Abstract
 
White onion slices were dried using microwave drying technique. The effect of process parameters during microwave 
dehydration such as effect of various power levels (0.25, 1.00, 1.50 and 2.25 kW) on mass reduction, water loss and diffusivi
ty 
were studied. 
It was found that the mass reduction and water loss increased with increase of power level. The moisture 
diffusivity varied in the range of 6.491 × 10
-
09 
to 6.491 ×10
-
08 
m²/s. The drying times of onions slices by microwave drying at 
0.25, 1.00, 1.50 and 2.
25 kW level were 15.86, 6.78, 5.3 and 3.2 hrs. Quality of dried product in respect to colour, rehydration 
and water activity was superior.
 
 
Keywords
: Microwave drying, colour value, Onion dehydration
.
 
 
 
Introduction
 
India (21%) and China 
(19.3%) is the largest producer of onion 
(
Allium cepa L
.) by area as well as by total production. Onions 
compared with other fresh vegetable are relatively high in food 
energy, intermediate in protein content and rich in calcium and 
riboflavin. The post ha
rvest losses of this vegetable are quite 
serious in our country so drying or dehydration should be 
carried out to increase its shelf life but 
c
hallenge in fruits and 
vegetables drying are to reduce the moisture content of the 
product to a level where micro
biological growth will not occur 
and simultaneously keep the high nutritive value. 
 
 
Onions are dried from initial moisture content of about 82 per 
cent to 6 per cent or less sufficient for storage and processing
1
.
 
V
-
12 is the special variety of white onio
n which is used for 
dehydration purpose due to its high solid content (17
-
18 per 
cent). Tray drying
 
brings the product to undesirable changes in 
colour, texture, flavor and loss in nutritive value
2
. Hence, a new 
method of drying in combination of osmosis (
in which partial 
dehydration of the fruit is brought about by immersing them in 
osmotic solution using controlled heat transfer applications and 
then drying) could be a good solution
3
. 
 
 
Osmosis of onion is done in salt solution because the 
reconstitution 
factor for salt
-
treated samples is greater than that 
for sugar
-
treated product
4
. 
Osmotic dehydration is effective 
even at ambient temperature, so that damage of texture, color, 
and flavor of food from heat are minimized
5
.
 
Microwave drying 
has been shown to
 
have low energy consumption
6
. Heating is 
more rapid because the surface
-
to
-
centre conduction stage is 
largely eliminated
7
. Microwaves have been shown to result in 
faster drying of materials and satisfying certain quality 
constraints of product for corn
8 
a
nd raisins
9
.
 
 
Material and Methods
 
Materials
: 
The good quality white onions (cultivar V
-
12) were 
selected and cut with a sharp stainless steel knife into circular 
slices of approximate 4 mm 
+
 
0.1 mm thickness. 
 
 
Measurement of initial Moisture Content
: 
The
 
moisture 
content of the fresh as well as osmotically dehydrated onion 
samples was determined by using AOAC (1984) method. 
 
 
Microwave Drying Characteristics
: 
Water loss
: 
The water 
loss (WL) is defined as the net weight loss of the fruit on initial 
weight basis and will be estimated as 
 
WL = 
 
 
Mass reduction
: 
The weight reduction (WR) can be defined as 
the net weight loss of the fruit on initial weight
 
basis.
 
WR =  
 
Microwave drying of onion
: 
Main switch control put at ON 
condition which gives the power to blower for cooling the 
magnetron and then switched ON the filament switch of the 
magnetron which gives 100 per cent voltage to 
filaments after a 
delay of 120 seconds then known percentage of initial moisture 
content and known weighed sample of onion were taken and 
uniformly spread on the turntable inside the microwave cavity, 
for an even absorption of microwave energy. Now power l
evel 
and time were pre
-
set through a control panel provided on front 
side of dryer. 
 
 
Moisture Diffusivity
: 
The moisture diffusivity of the samples 
was estimated by using the simplified mathematical Fick’s 
diffusion model. 
 
Research Journal of Chemical Sciences 
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_
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______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
57
-
60
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
58
 
 
 
A general form of above eq
n 
could be written in semi
-
logarithmic form, as follows.
 
 
 
Experimental values of the effective diffusivity are typically 
calculated by plotting experimental drying data in terms of ln 
(
M
R
) versus drying time and the slope of the curve was a 
measure of the 
moisture diffusivity.
 
 
Quality evaluation
: 
In order to determine quality of osmotic 
dehydrated onion samples had been evaluated on the basis 
several parameters such as, colour, rehydration and water 
activity. 
 
 
Colour measurement
: 
The most common techniqu
e to assess 
the colour is Hunter lab colorimeter used in the present. 
 
 
Water activity
: 
A digital water activity meter was used for 
measuring water activity of dehydrated onion samples.
 
 
Rehydration ratio
: 
The rehydration ratio of dried onion 
samples was d
etermined by putting 10 g of samples with 1000 
ml of distilled water in beaker. It was allowed to rehydrate for 5 
h at 20 
0
C temperature. 
 
 
 
Specific energy consumption
 
 
 
Results and Discussion
 
Microwave drying 
of onion slices
: 
The weight of the onion 
samples was taken after every 15 min intervals for the first one 
hour of the experiments. Afterwards the weight was noted after 
30 min intervals till the sample attained constant weight. The 
drying times of onions s
lices by microwave drying at 0.25, 
1.00, 1.50 and 2.25 kW power level were 15.86, 6.78, 5.3 and 
3.2 hrs respectively. Graph plotted as moisture content v/s 
drying time (
f
ig
ure
 
1).
 
 
Moisture Diffusivity of onion slices
: 
Moisture ratio plotted as 
ln (MR) v/s drying time (
f
ig
ure
 
2). The variation in (MR) with 
drying time for each case was found to be linear. The slope 
became steeper with increase in microwave power level. 
Moisture diffusivities were calculated from the slop
es of these 
straight lines
10
,
11
.
 
It was observed that moisture diffusivity 
increased with microwave power level in microwave drying 
processes .The moisture diffusivity varied in the range of 6.491 
× 10
-
09 
to 6.491 ×10
-
08 
m²/s during microwave drying of oni
on 
samples.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig
ure
-
1
 
Drying curve for fresh onion slices by microwave drying
 
 
 
Microwave drying of onion slices
 
0
 
10
 
20
 
30
 
40
 
50
 
60
 
70
 
80
 
90
 
100
 
0
 
50
 
100
 
150
 
200
 
250
 
300
 
350
 
400
 
450
 
500
 
550
 
600
 
650
 
700
 
750
 
800
 
850
 
900
 
950
 
1000
 
Drying time (min)
 
Moisture content, per cent
 
0.2
 
1.0
 
1.5
 
2.25
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
57
-
60
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
59
 
Effect of process parameter on quality
: 
The quality of 
dehydrated onion slices was evaluated on the basis of colour as 
well as water activity. The L* (lightness) increased for the 
onion slices when power level was increased from 0.25 kW to 
1.50 kW, meaning that sample became lighter in colour, 
H
owever, L
-
value reduced further at 2.25 kW level possibly 
due to exposure of product to high temperature result in 
darkening of dried product (
t
able 2).
 
 
Water activity
: 
Water activity was determined as a measure of 
storage stability using a Hygrolab
-
3 wat
er activity meter for all 
samples. The water activity of all samples were around 0.46
-
 
0.61 which is recommended to avoid microbial growth and 
enzymatic reactions
12
.
 
 
Specific Energy Consumption
: 
Specific energy consumption 
was determined using equation 
and the data are summarized in 
table 5.
 
 
The specific energy consumption was found more at 1.50 kW 
(35.39) power level.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig
ure
-
2
 
Variation in ln
 
(MR) with drying time at different power levels for microwave drying of onion slices
 
 
Table
-
1
 
Effective moisture diffusivity of onion slices during microwave drying
 
Sr. no
 
Microwave Level
 
Diffusivity (m
2
/s)
 
R
2
 
1
 
0.25
 
6.49114 x 10
-
09
 
0.9112
 
2
 
1.00
 
1.29823
 
x 10
-
08
 
0.946
 
3
 
1.50
 
4.86835 
x 10
-
08
 
0.9374
 
4
 
2.25
 
6.49114 
x 10
-
08
 
0.9409
 
 
Table
-
2
 
Lightness (L* values) of microwave dried onion slices
 
Drying method
 
Mw Density
 
L*
 
a*
 
b*
 
Microwave Drying
 
0.25
 
33.11
 
7.23
 
3.97
 
1.00
 
41.53
 
6.82
 
4.71
 
1.50
 
41.61
 
7.15
 
5.95
 
2.25
 
38.48
 
8.54
 
6.30
 
 
Table
-
3
 
Rehydrational characteristics of microwave dried onion slices
 
Sr.no
 
Drying Techniques
 
Microwave Level 
(kW)
 
Rehydration ratio
 
(RR)
 
Coefficient of rehydration
 
(COR)
 
1
 
Microwave Drying
 
0.25
 
3.74
 
0.71
 
2
 
1.00
 
3.94
 
0.75
 
3
 
1.50
 
4.18
 
0.80
 
4
 
2.25
 
4.21
 
0.80
 
-
7
 
-
6
 
-
5
 
-
4
 
-
3
 
-
2
 
-
1
 
0
 
0
 
100
 
200
 
300
 
400
 
500
 
600
 
700
 
800
 
900
 
1000
 
Time (min)
 
lnMR
 
0.25 kW
 
1.0 kW
 
1.5 kW
 
2.25 kW
 
Research Journal of Chemical Sciences 
______
_
_
_______________________________
______________
_
____
 
ISSN 2231
-
606X
 
 
Vol. 
2
(
4
), 
57
-
60
, 
April 
(201
2
)
 
  
Res.J.Chem.Sci
 
International Science Congress Association
 
 
60
 
Table
-
4
 
Water activity (a
w
) of microwave dried onion slices
 
 
Table
-
5
 
Specific energy consumption of Osmo
-
microwave and microwave dried onion slices at different power levels
 
Sr. No
 
Microwave Level
 
Drying Time (min)
 
SEC (MJ/Kg 
water
)
 
1
 
0.25
 
952
 
17.66
 
2
 
1.00
 
407
 
23.20
 
3
 
1.50
 
318
 
35.39
 
4
 
2.25
 
192
 
32.05
 
 
Conclusion
 
Based on the results of the investigation, the following 
conclusions were drawn:
 
The moisture diffusivity varied in the 
range of 6.491 × 10
-
09 
to 6.491 ×10
-
08 
m²/s during microwave 
drying of onion samples.
 
 
Microwave drying efficiency of onion 
w
as in the range of 17.43 to 34.93 (MJ/Kg 
water
) at different 
power levels 
 
The values of rehydration ratio of microwave 
dried sample ranged from 3.74 to 4.21. 
 
Microwave power level 
had a significant effect on the rehydration ratio, colour and 
water activi
ty of dried samples.
 
 
Notations Used
: 
 
= mass of slices after time 
θ
, g
, 
  
= 
initial mass of slices, g
 
, 
WR = weight reduction
,     
 
= mass 
of slices after time 
θ
, g
 
, 
  
= initial mass of slices, g
, 
 
= 
water content as a fraction of mass of slices at time 
θ
.,
  
=water content as a fraction of initial mass of slices,       
D = 
d
iffusion coefficient, M = 
m
oisture content g water p
er g dry 
matter
, 
X= characteristic dimension i.e. distance from the center 
of the line and
, 
T= 
t
ime elapsed during the drying 
, 
SEC =   Sp. 
e
nergy 
c
onsumption (J/kg water)
, 
ton = 
t
otal power
-
on time (s)
, 
P = 
m
icrowave power input (W)
, 
m = 
i
nitial mass (kg)
, 
Mi = 
i
nitial moisture content (ratio, wet basis)
, 
Mf = 
f
inal moisture 
content (ratio, wet basis)
 
 
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3181
 
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,
 
Microwave and dielectric drying
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,
  
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,
 
Microwave Processing 
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England 
(1986)
 
Sr. No
 
Drying Technique
 
Power Level (%)
 
Water activity (a
w
)
 
1
 
Microwave Drying
 
0.25
 
0.616
 
2
 
1.00
 
0.536
 
3
 
1.50
 
0.595
 
4
 
2.25
 
0.610
 
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