Journal of Food Engineering 48 (2001) 177ą182
www.elsevier.com/locate/jfoodeng
Drying, shrinkage and rehydration characteristics of kiwifruits during
hot air and microwave drying
Medeni Maskan
Department of Food Engineering, Engineering Faculty, University of Gaziantep, 27310 Gaziantep, Turkey
Received 24 March 2000; accepted 11 September 2000
Abstract
Hot air, microwave and hot air-microwave drying characteristics of kiwifruits (5:03 0:236 mm thick) were investigated. Drying
rates, shrinkage and rehydration capacities of these drying regimes were compared. The drying took place in the falling rate drying
period regardless of the drying method. Drying with microwave energy or assisting hot air drying with microwave energy resulted in
increased drying rates and substantial shortening of the drying time. Shrinkage of kiwifruits during microwave drying was greater
than hot air drying. Less shrinkage was observed at hot air-microwave drying. Microwave dried kiwifruit slices exhibited lower
rehydration capacity and faster water absorption rate than the other drying methods studied. Ó 2001 Elsevier Science Ltd. All rights
reserved.
Keywords: Drying; Kiwifruits; Rehydration; Shrinkage; Air; Microwave
1. Introduction The most common drying method employed for food
materials to date was hot air drying. But there are many
Longer shelf-life, product diversity and substantial disadvantages of this method. Among these are low en-
volume reduction are the reasons for popularity of dried ergy e�ciency and lengthy drying time during the falling
fruits and vegetables, and this could be expanded further rate period. This is mainly caused by rapid reduction of
with improvements in product quality and process ap- surface moisture and consequent shrinkage, which often
plications. These improvements could increase the cur- results in reduced moisture transfer and, sometimes, re-
rent degree of acceptance of dehydrated foods in the duced heat transfer (Feng & Tang, 1998). Several inves-
market. tigators of drying have reported that hot air drying, hence
Kiwifruits have very short shelf-life because of soft- prolonged exposure to elevated drying temperatures, re-
ening and vitamin loss during storage even at refriger- sulted in substantial degradation in quality attributes,
ated conditions (O'Connor-Shaw, Roberts, Ford, & such as colour, nutrients, Żavour, texture, severe shrink-
Nottingham, 1994; Agar, Massantini, Hess-Pierce, & age, reduction in bulk density and rehydration capacity,
Kader, 1999). In order to extend their shelf-life, kiwi- damage to sensory characteristics and solutes migration
fruits, like most fresh fruits, need preservation in some from the interior of the food to the surface (Bouraout,
form. A growing resistance of consumers to the use of Richard, & Durance, 1994; Yongsawatdigul &
chemicals for food preservation and the increasing Gunasekaran, 1996; Feng & Tang, 1998; Maskan, 2000).
popularity of high quality fast-dried foods with good In recent years, microwave drying o�ered an alter-
rehydration properties are now leading to a renewed native way to improve the quality of dehydrated prod-
interest in drying operations. From a technological ucts. Usually, drying is not induced by dielectric heating
point of view, dehydration is often the �nal step in an alone, but most microwave drying systems combine
industrial food process and determines, to a large extent, microwave and conventional heating. The heating may
the �nal quality of the product being manufactured take place in separate operations or simultaneously.
(Sereno & Medeiros, 1990). Microwave drying, like conventional drying, is caused
by water vapour pressure di�erences between interior
and surface regions which provide a driving force for
moisture transfer. It is most e�ective at product mois-
E-mail address: [e-mail] (M. Maskan). ture contents below 20% (Mudgett, 1989; Giese, 1992).
0260-8774/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 155- 2
178 M. Maskan / Journal of Food Engineering 48 (2001) 177ą182
Notation V volume of sample (ml) at any time
V0 initial volume of sample (ml)
Wd weight of dried sample (g)
k drying constant (min 1)
Wt weight of rehydrated sample (g) at any time
MR moisture ratio
X moisture content (kg H2O=kg dry solids) at any
r2 coe�cientof determination
time
S shrinkage
Xe equilibrium moisture content (kg H2O=kg dry solids)
SE standard error
X0 initial moisture content (kg H2O=kg dry solids)
t drying time (min)
Therefore, essentially for economic reasons, it has been 700 W at 2450 MHz. was used. The oven has the facility
suggested that microwave energy should be applied in the to adjust power (wattage) supply and the time of pro-
falling rate period or at a low moisture content (where cessing and was �tted with a turntable. The hot air
conventional drying takes a long time) to �nish drying. drying experiments were performed in a pilot plant tray
Microwave drying has been used in drying of herbs (Gi- dryer (UOP 8 tray dryer, Arm�eld, UK). The operation
ese, 1992), potato (Bouraout et al., 1994), raisins (Ko- mode of this dryer was explained in detail elsewhere
staropoulos & Saravacos, 1995), apple and mushroom (Maskan, 2000).
(Funebo & Ohlsson, 1998), diced apples (Feng & Tang,
1998), carrots (Prabhanjan, Ramaswamy, & Raghavan, 2.3. Drying procedure
1995; Litvin, Mannheim, & Miltz, 1998; Lin, Durance, &
Scaman, 1998) and banana (Maskan, 2000). (1) Hot air drying. The dryer was operated at
Little data currently exists on processing (minimally an air velocity of 1.29 m/s, parallel to the drying surface
processing, packaging, storage etc.) of fresh cut kiwi- of the sample, 60�C dry bulb and 27�C wet bulb tem-
fruits to extend the shelf life. Therefore, the objective of peratures. The study was carried out at constant sample
this study is the comparison of the microwave, hot air thickness and at constant temperature. Moisture loss
and hot air-microwave �nish drying methods for the was recorded by a digital balance (Avery Berkel,
processing of kiwifruits in respect to drying, shrinkage CC062D10ABAAGA) at 10 min intervals during drying
and rehydration characteristics obtained by the three for determination of drying curves. Kiwifruits were
drying techniques. dried until equilibrium (no weight change) was reached.
(2) Microwave drying. Di�erent microwave power
intensities (210, 350, and 490 W) were investigated in
2. Materials and methods microwave drying at constant sample thickness of
5.03 mm. It was observed that (our preliminary tests)
2.1. Material charring and sample boiling occurred at 350 and 490 W
power. Therefore, only the 210 W power level was
Kiwifruits (Actinidia deliciosa) used in this study were chosen in this study. One glass petri dish (7:1 cm
purchased from a local market. The whole samples were diameter 1:2 cm deep), containing the sample, was
stored at 4 0:5�C before they were used in experiments placed at the centre of the oven turntable in the micro-
in order to slow down the respiration, physiological and wave cavity during treatment for even absorption of
chemical changes (O'Connor-Shaw et al., 1994). The microwave energy. The presence of the turntable was
initial moisture content of kiwifruits was found to be necessary to achieve the optimum oven performance and
4.55 kg H2O=kg dry solids. Prior to drying, samples to reduce the levels of reŻected microwaves onto the
were taken out of storage, peeled with a sharp vegetable magnetron (Khraisheh, Cooper, & Magee, 1997). The
peeler, and cut into 5:03 0:236 mm thick and drying was performed according to a pre-set power and
40 0:812 mm diameter slices with a cutting machine. time schedule. Moisture loss was measured by taking
At least ten measurements of the thickness were made at out and weighing the dish on the digital balance peri-
di�erent points with a dial micrometer; only slices that odically. When the material reached a constant weight,
fell within a 5% range of the average thickness were equilibrium moisture content was assumed to be
used. All kiwifruits used for drying were from the same reached. Attention was paid to ensure that the sample
batch. The equilibrium moisture content was assumed to was not charred.
be the �nal moisture content of each run. (3) Air followed by a microwave �nish drying. Drying
was carried out by a combination of hot air-microwave
2.2. Drying equipment techniques. A 5.03 mm thick kiwifruit sample was dried
by hot air initially for 135 min, then, dried by microwave
A programmable domestic microwave oven (Arcelik (at 210 W). This point corresponds to a moisture con-
�
ARMD 580, TURKEY), with maximum output of tent of about 1.2 kg water/kg dry solids. It is a critical
M. Maskan / Journal of Food Engineering 48 (2001) 177ą182 179
point, because there was a sudden change in colour the e�ect of drying method on drying rate, shrinkage
parameters (results were not shown) up to this point and rehydration of kiwifruits was evaluated.
when the samples were dried by microwave alone (for The drying curves of kiwifruits dried by various
about 10 min). The objective of using this combined methods are shown in Fig. 1. It is evident from exam-
system was to see if hot air pre-drying a�ects the drying ination of these curves that the drying of kiwifruits by
characteristics and maintains the colour quality of ki- microwave is much faster than drying by hot air or hot
wifruits. The details of colour change were presented in air-microwave combination. For example, the time to
another study (Maskan, 2001). reach about 0.39 kg water/kg dry solids was 24.0, 225.0
The following di�usion model (Eq. (1)) was used to and 136.5 min, respectively. In fact, the drying time for
describe drying kinetics of kiwifruits rather than using the sample was reduced to about 89% of the hot air
Fick's di�usion model. The latter model could not be drying time with the microwave drying and about 40%
applied because of severe shrinkage in sample dimen- with the hot air-microwave combination drying method.
sions during drying. This indicated that the drying time requirement was
signi�cantly reduced as microwave was introduced.
X Xe
Maskan (2000) found that the hot air-microwave �nish
MR � exp ktą: 1ą
X0 Xe
drying reduced the convection drying time of bananas
by about 64.3%. Similar results were obtained by dif-
ferent authors on drying of fruits and vegetables by
2.4. Shrinkage and rehydration capacity
various drying techniques. (Bouraout et al., 1994;
Prabhanjan et al., 1995; Lin et al., 1998; Funebo &
For shrinkage and rehydration studies, similar drying
Ohlsson, 1998). These authors explained that the shorter
experiments, given in Section 2.3, were conducted sep-
drying time under microwave heating conditions could
arately using the same microwave, hot air and micro-
be due to the additional energy input, rapid heat pene-
wave �nish drying conditions. Measurement of
tration by microwave and forced expulsion of gases.
shrinkage and rehydration times were established from
Drying rates were calculated as quantity of moisture
preliminary assays.
removed per unit time per unit dry solids (kg water/kg
Volume changes due to sample shrinkage were mea-
dry solids/min). The drying rates of kiwifruits were given
sured by a water displacement method as described by
in Figs. 2 and 3. From an examination of these �gures it

(Sjoholm & Gekas, 1995). Measurements were made as
is obvious that the drying occurred mostly in the falling
quickly as possible (less than 30 s) to avoid water uptake
rate period (with two periods), regardless of drying
by samples. The shrinkage/volume change of the sam-
conditions. Although the kiwifruits have high moisture
ples was expressed as a bulk shrinkage ratio of sample
content, an expected constant rate period was not
volume at any time to initial volume
observed in the present study. In Fig. 2, the slope of
V
the second falling rate line was steeper than that of �rst
S � : 2ą
V0 falling rate line (moisture content � 1:3 kg water=kg
dry solids) and hence drying rate decreased rapidly dur-
Rehydration experiments were performed by im-
ing last stages of drying. Two falling rate periods were
mersing a weighed amount of dried samples into hot
water at 50�C for 50 min. At 10 min intervals the sam-
ples were drained over a mesh for 30 s and quickly
blotted with the paper towels 4ą5 times gently in order
to eliminate the surface water and then reweighed. The
rehydration capacity, described as percentage water
gain, was calculated from the sample weight di�erence
before and after the rehydration as follows:
Wt Wdą
weight gain %ą � 100: 3ą
Wd
3. Results and discussion
When deciding which process conditions produce the
best quality dried products, it is necessary to compare
drying time and quality parameters. Good quality was
de�ned as fast rehydration capacity, low bulk density,
Fig. 1. Typical drying curves for kiwifruit slices (5:03 0:236 mm
little shrinkage, and an attractive colour. In this study, thick).
180 M. Maskan / Journal of Food Engineering 48 (2001) 177ą182
high surface temperature, migration of soluble solids to
the surface of the sample and build-up of such soluble
materials at the surface as the water evaporates (Feng &
Tang, 1998). This results in complex physical and
chemical changes in the surface layer. Thereafter, sur-
face cracking occurs, tissues split and rupture internally
forming open structures due to the temperature rise in
the sample, so that moisture transfer is allowed with a
greater rate from the cracks and an open structure
formed (Maskan & Ibanoglu, 1998). One of the reasons
for the greater drying rates (steeper slope) in the second
falling rate period has been reported by Maskan and
 
Gogus (1998). They stated that it may be due to cell wall
�
destruction and, therefore, the resistance to moisture
di�usion decreases within the sample. The pattern of
Fig. 2. Drying rate curves for kiwifruits dried by hot air (60�C and
drying rate curves, examined in this study, for kiwifruits
1.29 m/s).
was similar to that reported for apple purees (Moyls,
1981), carrots (Prabhanjan et al., 1995), sultanas (Ka-
rathanos & Belessiotis, 1997), and bananas (Maskan,
2000). Microwave heating resulted in greater increases in
drying rates (Fig. 3), almost 13ą14 times higher com-
pared to that with hot air drying at the start of the
drying process.
Data from the hot air, microwave and combined hot
air-microwave drying were used to test the applicability
of Eq. (1). The parameter k was evaluated using non-
linear regression (Table 1) and the �tness was illustrated
in Fig. 1. The analysis yielded excellent �t with high
values of r2 for hot air drying (0.988) and microwave
drying (0.997). On the other hand, the model did not �t
all the data of the combined drying method. Therefore,
it was applied to the hot air and microwave sections
separately. In this case, the r2 value were 0.999 for hot
air section and 0.001 for microwave section which in-
Fig. 3. Drying rate curves of kiwifruit slices under various conditions.
dicated a very poor �t for the latter.
Shrinkage took place during drying by all the drying
methods tested and it was calculated using Eq. (2). The
also obtained with the microwave drying (Fig. 3) with
e�ect of drying method on shrinkage of kiwifruits was
no constant rate period. The �rst falling rate period was
shown in Fig. 4. The shrinkage of kiwifruit samples was
from the beginning of drying to about 1.9 kg water/kg
85%, 81% and 76% for microwave, hot air and hot air-
dry solids and was followed by a rapidly decreasing
microwave drying, respectively. Hot air drying pro-
second falling rate period. The reason for existence of
moted a moderate shrinkage, which was explained by
two falling rate periods may be due to case hardening
the fact that a long drying time gives more time for the
which acts as a barrier to moisture migration during
product to shrink (Ratti, 1994). The shrinkage was the
prolonged drying. The case hardening is because of the
Table 1
Nonlinear regression analysis results of one parameter di�usion model (Eq. (1)) for drying of kiwifruits
Parameter Drying method
Air Microwave Hot air-mircrowave drying
drying drying
Air section Microwave section
k 0.0103 0.1798 0.0094 0.0208
SE ( ) 0.0002 0.0305 0.0001 0.0058
r2 0.988 0.997 0.999 0.0010
M. Maskan / Journal of Food Engineering 48 (2001) 177ą182 181
Fig. 4. Shrinkage of kiwifruit slices during drying.
Fig. 5. Rehydration behaviour of dried kiwifruit slices at 50�C.
least by combined drying method. However, there was a showing that a less shrunk structure had higher capacity
higher and rapid shrinkage of samples dried by micro- to absorb water when reconstituted.
wave. It is because of extensive heat generation, accel-
erating removal of water from the tissues in the sample
by microwave. In all processes shrinkage followed the
4. Conclusion
pattern of typical drying curves (Fig. 1), with high
shrinkage initially and gradual levelling o� towards the
From the above results it was concluded that micro-
end of drying so that the �nal size and shape of samples
were �xed before drying was completed. Similar beha- wave and microwave assisted heating reduced the drying
times by 89ą40%. The one-parameter di�usion model
viour has been observed by Ratti (1994) on drying of
 adequately described the hot air and microwave drying
potatoes, apples and carrots, Sjoholm and Gekas (1995)
on apple drying and Wang and Brennan (1995) on po- data. The hot air-microwave �nish dried products ex-
hibited less shrinkage, hence, they had better rehydra-
tato drying.
tion characteristics. Experiments with kiwifruit slices
During reconstitution of dehydrated products the
showed that hot air-microwave �nish drying can be used
amount and rate of water absorption determines to a
considerable extent the sensorial properties and prepa- for preservation of high quality products.
ration time. The rehydration characteristics of a dried
product are used as a quality index and they indicate the
physical and chemical changes during drying as inŻu-
References
enced by processing conditions, sample pretreatment
and composition (Feng & Tang, 1998).
Agar, T. I., Massantini, R., Hess-Pierce, B., & Kader, A. A. (1999).
The rehydration capacities of kiwifruit samples dried
Postharvest CO2 and ethylene production and quality maintenance
with di�erent methods were calculated by Eq. (3) and of fresh-cut kiwifruit slices. Journal of Food Science, 64, 433ą440.
Bouraout, M., Richard, P., & Durance, T. (1994). Microwave and
presented graphically. The rehydration curves of dried
convective drying of potato slices. Journal of Food Process
kiwifruits obtained at 50�C were shown in Fig. 5. Mi-
Engineering, 17, 353ą363.
crowave dried kiwifruit slices exhibited lower rehydra-
Drouzas, A. E., & Schubert, H. (1996). Microwave application in
tion capacity and faster water absorption rate than the
vacuum drying of fruits. Journal of Food Engineering, 28, 203ą209.
other two drying methods. This may be due to changes Feng, H., & Tang, J. (1998). Microwave �nish drying of diced apples in
a spouted bed. Journal of Food Science, 63, 679ą683.
in the structure/texture of the samples during microwave
Funebo, T., & Ohlsson, T. (1998). Microwave-assisted air dehydration
drying because of temperature rise (greater than air
of apple and mushroom. Journal of Food Engineering, 38, 353ą367.
temperature, 60�C. Higher water absorption capacity
Giese, J. (1992). Advances in microwave food processing. Food
was expected for microwave dried sample (Drouzas &
Technology, 46, 118ą123.
Schubert, 1996; Feng & Tang, 1998; Lin et al., 1998) Karathanos, V. T., & Belessiotis, V. G. (1997). Sun and arti�cial air
drying kinetics of some agricultural products. Journal of Food
because of short drying duration. The rehydration ca-
Engineering, 31, 35ą46.
pacity of samples dried with hot air-microwave com-
Khraisheh, M. A. M., Cooper, T. J. R., & Magee, T. R. A. (1997).
bined method had the highest value. This method
Shrinkage characteristics of potatoes dehydrated under combined
improved the rehydration capacity of kiwifruits. These
microwave and convective air conditions. Drying Technology
results are in agreement with shrinkage data (Fig. 4) International, 15, 1003ą1022.
182 M. Maskan / Journal of Food Engineering 48 (2001) 177ą182
Kostaropoulos, A. E., & Saravacos, G. D. (1995). Microwave pre- Mudgett, R. E. (1989). Microwave food processing. Food Technology,
treatment for sun-dried raisins. Journal of Food Science, 60, 344ą 43, 117ą126.
347. O'Connor-Shaw, R. E., Roberts, R., Ford, A. L., & Nottingham, S. M.
Lin, T. M., Durance, T. D., & Scaman, C. H. (1998). Characterization (1994). Shelf-life of minimally processed honeydew, kiwifrui,
of vacuum microwave, air and freeze dried carrot slices. Food papaya, pineapple and cantaloupe. Journal of Food Science, 59,
Research International, 4, 111ą117. 1202ą1206,1215.
Litvin, S., Mannheim, C. H., & Miltz, J. (1998). Dehydration of Prabhanjan, D. G., Ramaswamy, H. S., & Raghavan, G. S. V. (1995).
carrots by a combination of freeze drying, microwave heating and Microwave-assisted convective air drying of thin layer carrots.
air or vacuum drying. Journal of Food Engineering, 36, 103ą111. Journal of Food Engineering, 25, 283ą293.
 
Maskan, M., & Go gus, F. (1998). Sorption isotherms and drying Ratti, C. (1994). Shrinkage during drying of foods. Journal of Food
�
characteristics of mulberry (Morus alba). Journal of Food Engi- Engineering, 23, 91ą105.
neering, 37, 437ą449. Sereno, A. M., & Medeiros, G. L. (1990). A simpli�ed model for the
Maskan, M., & Ibanoglu, S. (1998). Drying behaviour of infrared prediction of drying rates for foods. Journal of Food Engineering,
dried tarhana dough. In Proceedings of Food Engineering Congress 12, 1ą11.

(pp. 171ą177). Turkey: Gaziantep. Sjoholm, I., & Gekas, V. (1995). Apple shrinkage upon drying. Journal
Maskan, M. (2000). Microwave/air and microwave �nish drying of of Food Engineering, 25, 123ą130.
banana. Journal of Food Engineering, 44, 71ą78. Wang, N., & Brennan, J. G. (1995). Changes in structure, density and
Maskan, M. (2001). Kinetics of colour change of kiwifruits during hot porosity of potato during dehydration. Journal of Food Engineer-
air and microwave drying. Journal of Food Engineering, 48, 169ą ing, 24, 61ą76.
175. Yongsawatdigul, J., & Gunasekaran, S. (1996). Microwave-vacuum
Moyls, A. L. (1981). Drying of apple puree. Journal of Food Science, drying of cranberries: Part II. Quality evaluation. Journal of Food
46, 939ą942. Processing and Preservation, 20, 145ą156.