Journal of Food Engineering 44 (2000) 71ą78
www.elsevier.com/locate/jfoodeng
Microwave/air and microwave �nish drying of banana
Medeni Maskan
Department of Food Engineering, Engineering Faculty, University of Gaziantep, 27310 Gaziantep, Turkey
Received 16 July 1999; accepted 6 December 1999
Abstract
Banana samples (4:3 0:177; 7:4 0:251 and 14 0:492 mm thick) were dried using the following drying regimes; convection
(60�C at 1.45 m/s); microwave (350, 490 and 700 W power) and convection followed by microwave (at 350 W, 4.3 mm thick sample)
�nish drying. The drying of banana slices took place in the falling rate drying period with convection drying taking the longest time.
Higher drying rates were observed with the higher power level. Microwave �nish drying reduced the convection drying time by
about 64.3%. A physical model was employed to �t the experimental data and gave good �t for all experimental runs except mi-
crowave �nish data. Microwave �nish dried banana was lighter in colour and had the highest rehydration value. Ó 2000 Published
by Elsevier Science Ltd. All rights reserved.
Notation
rious damage to the Żavour, colour, nutrients, reduction
k drying constant (min 1)
in bulk density and rehydration capacity of the dried
MR moisture ratio
product (Lin, Durance & Scaman, 1998; Drouzas,
MW microwave
Tsami & Saravacos, 1999). Major disadvantages of hot
r2 coe�cient of determination
S.E. standard error air drying of foods are low energy e�ciency and lengthy
t drying time (min)
drying time during the falling rate period. Because of the
Wd weight of dried sample (g)
low thermal conductivity of food materials in this peri-
Wt weight of rehydrated sample (g) at any time
od, heat transfer to the inner sections of foods during
X moisture content (kg H2O/kg dry solids) at any time
conventional heating is limited (Adu & Otten, 1996;
Xe equilibrium moisture content (kg H2O/kg dry solids)
Xo initial moisture content (kg H2O/kg dry solids) Feng & Tang, 1998). The desire to eliminate this prob-
lem, prevent signi�cant quality loss, and achieve fast
and e�ective thermal processing has resulted in the in-
1. Introduction creasing use of microwaves for food drying. Microwave
drying is rapid, more uniform and energy e�cient
Banana is one of the important high sugar containing compared to conventional hot air drying. In this case,
tropical fruit crops grown commercially in many coun- the removal of moisture is accelerated and, furthermore,
tries. It is very susceptible to deterioration and consid- heat transfer to the solid is slowed down signi�cantly
erable amounts of this fruit is wasted due to the lack of due to the absence of convection. And also because of
e�cient preservation techniques that are unique to ba- the concentrated energy of a microwave system, only
nana. An alternative method seems to be drying to ob- 20ą35% of the Żoor space is required, as compared to
tain a stable banana for further use. conventional heating and drying equipment. However,
Drying is one of the oldest methods of food preser- microwave drying is known to result in a poor quality
vation and it is a di�cult food processing operation product if not properly applied (Yongsawatdigul &
mainly because of undesirable changes in quality of the Gunasekaran, 1996a; Adu & Otten, 1996; Drouzas &
dried product. High temperatures and long drying times, Schubert, 1996).
required to remove the water from the sugar containing It has also been suggested that microwave energy
fruit material in conventional air drying, may cause se- should be applied in the falling rate period or at a low
moisture content for �nish drying (Prabhanjan, Ra-
maswamy & Raghavan, 1995; Kostaropoulos & Sarav-
E-mail address: [e-mail] (M. Maskan). acos, 1995; Funebo & Ohlsson, 1998). The reason for
0260-8774/00/$ - see front matter Ó 2000 Published by Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 167- 3
72 M. Maskan / Journal of Food Engineering 44 (2000) 71ą78
this is essentially economic. Due to high cost, microwave shorter drying times compared with hot air drying
can not compete with conventional air drying. However, alone.
microwaves may be advantageous in the last stages of The objectives of this study are to: (1) compare drying
air drying. Because the least e�cient portion of a con- characteristics, colour and rehydration of banana sam-
ventional drying system is near the end, when two-thirds ples dried by hot air, microwave and hot air followed by
of the time may be spent removing the last one-third of a microwave �nish drying; (2) determine the drying
the moisture content (Al-Duri & McIntyre, 1992). constants, using a simple di�usion model (Eq. (1)) and
assess the e�ect of selected parameters such as drying
temperature, sample thickness and microwave power.
X Xe
MR � � exp ktą: 1ą
2. Microwave principles and application to drying of foods
Xo Xe
For microwave drying it can be assumed that Xe � 0.
Microwaves are electromagnetic waves in the fre-
quency range of 300 MHz to 300 GHz (equivalent to a
wavelength of 1ą0.01 m), generated by a magnetron-
3. Materials and methods
type vacuum tube. Electromagnetic energy at 915 and
2450 MHz can be absorbed by water containing mate-
3.1. Material
rials or other ``lossy'' substances, such as carbon and
some organics, and converted to heat (Khraisheh,
Ripe bananas (Musa species) with an initial moisture
Cooper & Magee, 1997a). Because the waves can pene-
content of 3.1 kg H2O/kg dry solids were obtained from
trate directly into the material, heating is volumetric
a local supermarket and stored at 4 0:5�C. Prior to
(from the inside out) and provides fast and uniform
drying, samples were taken out of storage, hand peeled,
heating throughout the entire product. The quick energy
cut into 4:3 0:177; 7:4 0:251; 14 0:492 mm thick
absorption by water molecules causes rapid evaporation
and 30 0:901 mm diameter slices (where shows
of water (results in higher drying rates of the food),
standard deviation of measurements) with a cutting
creating an outward Żux of rapidly escaping vapour. In
machine. At least 10 measurements of the thickness were
addition to improving the rate of drying, this outward
made at di�erent points with a dial micrometer; only
Żux can help to prevent the shrinkage of tissue structure,
slices that fell within a 5% range of the average thickness
which prevails in most conventional air drying tech-
were used. All bananas used for drying were from the
niques. Hence better rehydration characteristics may be
same batch.
expected in microwave dried products (Prabhanjan et al.,
1995).
In recent years, microwave drying has gained popu- 3.2. Drying equipment
larity as an alternative drying method for a wide variety
A programmable domestic microwave oven (Arcelik
�
of food products such as fruits, vegetables, snack foods
ARMD 580, TURKEY), with maximum output of
and dairy products. Several food products have been
700 W at 2450 MHz. was used. The oven has the facility
successfully dried by the microwave-vacuum application
to adjust power (wattage) supply and the time of pro-
and/or by a combined microwave assisted-convection
cessing. The hot air drying experiments were performed
process. These authors included Kim and Bhowmik
(1995) for plain yogurt, Yongsawatdigul and Gunasek- in a pilot plant tray dryer (UOP 8 tray dryer, Arm�eld,
UK). The dryer (Fig. 1) consisted of a proportional (P)
aran (1996a) for cranberries, Lin et al. (1998) for carrot
controller controlling the temperature. Air was drawn
slices, Drouzas et al. (1999) for model fruit gels, Al-Duri
into the duct through a mesh guard by a motor driven
and McIntyre (1992) for skimmed milk, whole milk,
axial Żow fan impeller whose speed can be controlled in
casein powders, butter and fresh pasta, Bouraout,
Richard and Durance (1994) for potato slices, Prab- the duct.
hanjan et al. (1995) for carrots, Tulasidas, Raghavan
and Norris (1996) for grapes, Funebo and Ohlsson 3.3. Drying procedure
(1998) for apple and mushroom, and Ren and Chen
(1998) for American ginseng roots. The drying regimes were as follows:
Another group of researchers proposed a two-stage (1) Hot air drying: The dryer was operated at an air
drying process involving an initial forced air convective velocity of 1.45 m/s, parallel to the drying surface of the
drying, followed by a microwave �nish drying (Prab- sample, 60�C dry bulb and 30�C wet bulb temperatures.
hanjan et al., 1995; Feng & Tang, 1998). Microwave The three sample thicknesses were studied at constant
application has been reported to improve product temperature. Moisture loss was recorded at 10 min inter-
quality such as better aroma, faster and better rehy- vals during drying for determination of drying curves
dration, considerable savings in energy and much by a digital balance (Avery Berkel, CC062D10ABAAGA).
M. Maskan / Journal of Food Engineering 44 (2000) 71ą78 73
Fig. 1. Schematic diagram of the hot air drying equipment (not to scale).
Bananas were dried until equilibrium (no weight change) colour and rehydration was performed (within one week
was reached. after the drying).
(2) Microwave drying: Factors investigated in micro-
wave drying were microwave power intensity (350, 490 3.4.1. Colour
and 700 W) at constant sample thickness of 4.3 mm, and Sample colour was measured before and after drying
sample thickness/load (4.3 mm/3.56 g, 7.4 mm/5.60 g by a HunterLab ColorFlex, A60-1010-615 model col-
and 14.0 mm/11.85 g) at constant microwave power ormeter (HunterLab., Reston, VA). The colour values
output of 490 W. One glass petri dish (7.1 cm diameter were expressed as L (whiteness/darkness), a (redness/
1:2 cm deep), containing the sample, was placed on the greenness) and b (yellowness/blueness). And also, the
centre of a turntable �tted inside (bottom) the micro- total colour di�erence from the fresh bananas DE, as
wave cavity during treatment for even absorption of de�ned the following, was used to describe the colour
microwave energy. The presence of the turntable was change during drying:
q��������������������������������������������������������������������������
necessary to achieve the optimum oven performance and
2 2 2
DE � L o L � a o a � b o b ; 2ą
to reduce the levels of reŻected microwaves onto the
magnetron (Khraisheh et al., 1997b). The drying was
where subscript ``o'' refers to the colour reading of fresh
performed according to a preset power and time
banana, L ; a and b indicate brightness, redness and
schedule. Moisture loss was measured by taking out and
yellowness of dried samples respectively. Fresh banana
weighing the dish on the digital balance periodically.
was used as the reference and a larger DE denotes
When the material reached a constant weight, equilib-
greater colour change from the reference material.
rium moisture content was assumed to be reached. At-
tention was paid to ensure that the sample was not
3.4.2. Rehydration
charred.
The dried samples were manually ground and im-
(3) Air followed by a microwave �nish drying: A4.3 mm
mediately loaded (about 0.5 g each) into small alumi-
thick banana sample was dried at 60�C and 1.45 m/s air
nium sample dishes. 100 ml of distilled water was
velocity to 1.25 kg H2O/kg dry solids moisture content,
transferred into a glass jar and a tripod was also placed
the point where drying slows down. Then, sample was
in the jar. The dishes were placed on tripod in the jar
taken out and dried in the microwave oven. Some pre-
which was then tightly closed and kept at 20�C for
liminary tests conducted on partially air dried sample
equilibration. The dishes were periodically weighed until
resulted in burning of sample at high microwave power
equilibrium was reached. The rehydration ratio was
levels. Hence, a microwave power of 350 W was selected
determined by
for �nish drying purpose.
Wt Wdą
Weight gain %ą � 100: 3ą
Wd
3.4. Quality evaluation
For quality evaluation, similar drying experiments 3.5. Statistical analysis
were conducted separately under the same microwave,
hot air and microwave �nish drying conditions. Drying Analysis of variance (ANOVA was conducted to de-
ANOVA)
was terminated when moisture content reached about termine the e�ect of variable factors on drying param-
0.1 kg H2O/kg dry solids. After the drying tests, samples eters using Statgraphics software (1991). Least-squares
were kept in air tight glass jars until measurements of multiple range test was performed to di�erentiate the
74 M. Maskan / Journal of Food Engineering 44 (2000) 71ą78
signi�cant e�ect of drying methods on drying rate. The
parameter of non-linear model (Eq. (1)) was calculated
by the NLIN procedure of the SigmaPlot (Scienti�c
Graph System, version 4.00, Jandel).
4. Results and discussion
4.1. Hot air drying
The moisture content versus time curves for hot air
drying of banana samples as inŻuenced by thickness are
shown in Fig. 2. Obviously, as the thickness of the
sample increased, the time required to achieve a certain
moisture content increased. For example, the drying
Fig. 3. Drying rate curves of banana slices dried by hot air (60�C and
times for reaching about 0.1 kg H2O/kg dry solids
1.45 m/s).
moisture content of 4.3, 7.4 and 14 mm thick samples
were about 482, 610 and 777 min respectively at 60�C air
constant rate period in their study. A high initial drying
temperature. These results were in agreement with pre-
rate, with higher rates at lower thicknesses, was ob-
vious literature studies (Yusheng & Poulsen, 1988;
served (Fig. 3). All the samples tended to dry slowly at
Madamba, Driscoll & Buckle, 1996; Maskan & Ibano-
the last stages of drying, presumably due to collapse
glu, 1998). The drying rate was calculated at di�erent
(shrinkage) of the banana structure resulting in low
times and plotted against average moisture content as
transport rate of water and prolonged drying time
shown in Fig. 3. A constant rate period was not ob-
(Kostaropoulos & Saravacos, 1995).
served in hot air drying of banana samples at 60�C.
Hence, the entire drying process for the samples oc-
curred in the range of falling rate period in this study.
4.2. Microwave drying
However, it might be possible to have a short constant
rate period using lower temperatures such as 40ą50�C.
The e�ect of changing the power output in the mi-
It has been reported that almost all of the drying of
crowave oven on the moisture content curve of 4.3 mm
biological products takes place in the falling rate period
thick banana sample is shown in Fig. 4. At all power
(Madamba et al., 1996). However, Mowlah, Takano,
levels, drying curves were steeper and tended to end at
Kamoi and Obara (1983) have found both constant and
about the same time. The observed initial acceleration of
falling rate periods in dehydration of bananas at 60�Cin
drying may be caused by an opening of the physical
an air circulated oven. Air in the oven is saturated, by
structure allowing rapid evaporation and transport of
the time, and forms a thick �lm around the food that
water (Kostaropoulos & Saravacos, 1995). The ANOVA
ANOVA
prevents e�ective separation of the evaporated moisture
showed no e�ect of the intensity of power on moisture
from the food. This may be the reason for existence of a
Fig. 4. Drying curves of banana slice (4:3 0:177 mm) dried by mi-
Fig. 2. Air drying curves for banana slices (air at 60�C and 1.45 m/s). crowave method with di�erent microwave power levels.
M. Maskan / Journal of Food Engineering 44 (2000) 71ą78 75
loss (P > 0:05). Similar results were obtained by Walde, karan, 1996b). Due to that, the 14 and 7.4 mm thick
Balaswamy, Sivaswamy, Chakkaravarthi and Rao samples spread on the bottom of the dishes as a thin
(1995) for microwave drying of gum karaya and layer, a large drying surface area formed hence, drying
Yongsawatdigul and Gunasekaran (1996b) for micro- accelerated (data were not shown). Only the thin sample
wave-vacuum drying of cranberries. However, several (4.3 mm) maintained its shape without spreading and it
investigators have reported the e�ect of power output on took much time to dry this sample compared to the
drying time of food materials (Al-Duri & McIntyre, others. The drying times were found to be about 18, 13
1992; Prabhanjan et al., 1995; Drouzas & Schubert, and 10 min at 490 W for 4.3, 7.4 and 14 mm thick
1996). Fig. 5 shows the e�ect of drying conditions on the samples to reach a moisture content of about 0.1 kg
drying rate. Although high moisture foods can be ex- H2O/kg dry solids.
pected to have a period of constant rate drying, this was
not observed in the present study under any of the test
4.3. Microwave �nish drying
conditions. The total drying times required to reach a
�nal moisture content of about 0.1 kg H2O/kg dry solids
Banana sample (4.3 mm thick) was hot-air dried at
were 13, 18 and 27 min at 700, 490 and 350 W, respec-
60�C initially, then microwave energy was applied (to
tively. These results show that the drying time of the
the point where conventional drying is very slow) for
4.3 mm thick sample was shortened from 482 min by hot
�nish drying. The drying rate versus average moisture
air drying to the 13ą27 min range when dried by mi-
content curve was presented in Fig. 6. It can be seen that
crowave energy. ANOVA results showed a signi�cant
ANOVA
microwave �nish application increased the drying rate
di�erence (P < 0:05) between drying rates of hot air and
signi�cantly (over 0.8 kg water/kg dry solids/min). The
microwave drying techniques. Average drying rate over
same sample has an initial drying rate value of about
the drying periods was 0.0088 kg water/kg dry solids/
0.035 kg water/kg dry solids/min (Fig. 3) when air dried,
min for air dried, and between 0.0880 and 0.2027 kg
0.4 kg water/kg dry solids/min when microwave dried at
water/kg dry solids/min for microwave dried 4.3 mm
350 W microwave power output (Fig. 5). This applica-
thick banana sample in the 350ą700 W power range,
tion also reduced the drying time from 482 to 172 min
respectively. The results indicated that mass transfer
(64.3% reduction in drying time) when dried by air and
within the sample is rapid during microwave heating
�nish dried respectively.
because heat is generated within the sample, creating a
large vapour pressure di�erential between the centre and
4.4. Modelling drying curves
the surface of products (Lin et al., 1998).
E�orts were made to study the e�ect of the sample
In this work, Eq. (1) was employed as a physical
thickness (4.3, 7.4, 14 mm) on drying at constant power
model for description of the drying processes, rather
output (490 W). In contrast to hot air drying, the thicker
than a mathematical model (Prabhanjan et al., 1995;
sample dried more rapidly than the thin one. It is be-
Madamba et al., 1996; Drouzas et al., 1999). Drying
cause of sudden and volumetric heating, generating high
data were used to test the applicability of this model.
pressure inside the bananas, resulted in boiling and
The parameter k together with S.E. and r2 were evalu-
bubbling of the samples (Yongsawatdigul & Gunase-
ated using nonlinear regression. The results are
Fig. 5. Drying rate curves of banana slice (4:3 0:177 mm) under Fig. 6. Drying rate of banana slice (4:3 0:177 mm) dried by hot air
various microwave power levels. (60�C and 1.45 m/s) followed by microwave at 350 W power output.
76 M. Maskan / Journal of Food Engineering 44 (2000) 71ą78
tabulated in Table 1. The model gave good �t for all the red colour (low a value) than microwave drying. The
experimental runs with r2 values greater than 0.91. It did discoloration during drying may be related to nonen-
not adequately �t the whole data of microwave �nish zymatic browning (Feng & Tang, 1998). No di�erences
drying experiment. Hence, the �tting was done on hot were observed between b values of air and microwave
air and microwave �nish drying parts separately. In the dried samples. Microwave drying also caused colour
case of hot air drying, the drying constant (k) estimated darkening. The di�erences between colour values
from Eq. (1) was found to increase signi�cantly as the (L ; a ; b ) of microwave dried samples were not sig-
thickness was reduced from 14 to 4.3 mm. However, the ni�cant (P > 0:05). The change in colour values was not
opposite case was observed when banana was dried by dependent on the microwave power intensity. This is not
microwave at constant power level (490 W). The reason in agreement with the observation of Funebo and
for this was the spreading of the sample as explained Ohlsson (1998). The colour of the microwave �nish
previously. This may be an advantage in terms of dried sample was much lighter (higher L value) than the
shortening drying time, if the �nal shape/size is not air and/or microwave dried product. At the same time
important for the product. A signi�cant increase of this drying process decreased the redness (a value) and
constant k was observed when the microwave power was increased yellowness (b value).
increased from 350 to 700 W. The highest value of k was The total colour change (DE) values, which takes into
observed in the case of microwave application which account changes in redness and yellowness, were also
indicated that it o�ered a minimum resistance. compared. Microwave �nish drying caused little, and air
drying the highest colour change among the drying
methods. Drying temperature and time are important
parameters for colour change during drying. The lower
4.5. Colour
colour degradation of microwave �nish dried banana
Table 2 shows the results of colour measurements of may, therefore, be due to the substantial reduction in
drying time. It is clear that microwave �nish drying
fresh and dried bananas. Preferred colours are those
maintained the colour quality of the fresh bananas com-
closest to the original colour of fresh sample. Air drying
pared to the hot air and microwave drying methods alone.
resulted in a darker slice colour (lower L value) and less
Table 1
Nonlinear regression analysis results of Eq. (1)
Parameter Drying method
Air MW Air-MW �nish
Thickness (mm) MW powera (W) Thicknessb (mm) Air part MW
part
4.3 7.4 14 700 490 350 4.3 7.4 14
K 0.0086 0.0059 0.003 0.268 0.188 0.141 0.188 0.387 0.469 0.0059 1.319
S.E. ( ) 1 10 4 9 10 5 5 10 5 5 10 3 4 10 3 4 10 3 1 10 2 2 10 2 1 10 2 9 10 5 3 10 2
r2 0.992 0.991 0.987 0.995 0.994 0.994 0.916 0.978 0.998 0.988 0.999
a
Sample thickness is constant (4:3 0:177 mm).
b
MW power is constant (490 W).
Table 2
Colour parameters of fresh and dried banana slices (4:3 0:177 mm)
Parameter Fresh banana Drying method
Air MW (W) Air-MW �nish
700 490 350
L* 71.01 40.815 53.25 55.13 52.88 62.08
a 3.96 6.32 9.95 9.46 11.70 5.97
b 21.75 14.35 13.37 13.85 16.46 18.00
DE 0.00 31.17 20.53 18.56 20.41 9.89
M. Maskan / Journal of Food Engineering 44 (2000) 71ą78 77
It had little e�ect on the colour and rehydration ca-
pacity of �nished products as compared to the hot
air and microwave drying methods.
4. Constant drying rate period was not observed under
any of the test conditions.
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