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

Longer shelf-life, product diversity and substantial

volume reduction are the reasons for popularity of dried

fruits and vegetables, and this could be expanded further

with improvements in product quality and process ap-

plications. These improvements could increase the cur-

rent degree of acceptance of dehydrated foods in the

market.

Kiwifruits have very short shelf-life because of soft-

ening and vitamin loss during storage even at refriger-

ated conditions (O'Connor-Shaw, Roberts, Ford, &

Nottingham, 1994; Agar, Massantini, Hess-Pierce, &

Kader, 1999). In order to extend their shelf-life, kiwi-

fruits, like most fresh fruits, need preservation in some

form. A growing resistance of consumers to the use of

chemicals for food preservation and the increasing

popularity of high quality fast-dried foods with good

rehydration properties are now leading to a renewed

interest in drying operations. From a technological

point of view, dehydration is often the ®nal step in an

industrial food process and determines, to a large extent,

the ®nal quality of the product being manufactured

(Sereno & Medeiros, 1990).

The most common drying method employed for food

materials to date was hot air drying. But there are many

disadvantages of this method. Among these are low en-

ergy eciency and lengthy drying time during the falling

rate period. This is mainly caused by rapid reduction of

surface moisture and consequent shrinkage, which often

results in reduced moisture transfer and, sometimes, re-

duced heat transfer (Feng & Tang, 1998). Several inves-

tigators of drying have reported that hot air drying, hence

prolonged exposure to elevated drying temperatures, re-

sulted in substantial degradation in quality attributes,

such as colour, nutrients, ¯avour, texture, severe shrink-

age, reduction in bulk density and rehydration capacity,

damage to sensory characteristics and solutes migration

from the interior of the food to the surface (Bouraout,

Richard, & Durance, 1994; Yongsawatdigul &

Gunasekaran, 1996; Feng & Tang, 1998; Maskan, 2000).

In recent years, microwave drying o€ered an alter-

native way to improve the quality of dehydrated prod-

ucts. Usually, drying is not induced by dielectric heating

alone, but most microwave drying systems combine

microwave and conventional heating. The heating may

take place in separate operations or simultaneously.

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-

ture contents below 20% (Mudgett, 1989; Giese, 1992).

Journal of Food Engineering 48 (2001) 177±182

www.elsevier.com/locate/jfoodeng

E-mail address: [e-mail] (M. Maskan).

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 1 5 5 - 2

Therefore, essentially for economic reasons, it has been

suggested that microwave energy should be applied in the

falling rate period or at a low moisture content (where

conventional drying takes a long time) to ®nish drying.

Microwave drying has been used in drying of herbs (Gi-

ese, 1992), potato (Bouraout et al., 1994), raisins (Ko-

staropoulos & Saravacos, 1995), apple and mushroom

(Funebo & Ohlsson, 1998), diced apples (Feng & Tang,

1998), carrots (Prabhanjan, Ramaswamy, & Raghavan,

1995; Litvin, Mannheim, & Miltz, 1998; Lin, Durance, &

Scaman, 1998) and banana (Maskan, 2000).

Little data currently exists on processing (minimally

processing, packaging, storage etc.) of fresh cut kiwi-

fruits to extend the shelf life. Therefore, the objective of

this study is the comparison of the microwave, hot air

and hot air-microwave ®nish drying methods for the

processing of kiwifruits in respect to drying, shrinkage

and rehydration characteristics obtained by the three

drying techniques.

2. Materials and methods

2.1. Material

Kiwifruits (Actinidia deliciosa) used in this study were

purchased from a local market. The whole samples were

stored at 4  0:5°C before they were used in experiments

in order to slow down the respiration, physiological and

chemical changes (O'Connor-Shaw et al., 1994). The

initial moisture content of kiwifruits was found to be

4.55 kg H

2

O=kg dry solids. Prior to drying, samples

were taken out of storage, peeled with a sharp vegetable

peeler, and cut into 5:03  0:236 mm thick and

40  0:812 mm diameter slices with a cutting machine.

At least ten measurements of the thickness were made at

di€erent points with a dial micrometer; only slices that

fell within a 5% range of the average thickness were

used. All kiwifruits used for drying were from the same

batch. The equilibrium moisture content was assumed to

be the ®nal moisture content of each run.

2.2. Drying equipment

A programmable domestic microwave oven (Arcßelik

ARMD 580, TURKEY), with maximum output of

700 W at 2450 MHz. was used. The oven has the facility

to adjust power (wattage) supply and the time of pro-

cessing and was ®tted with a turntable. The hot air

drying experiments were performed in a pilot plant tray

dryer (UOP 8 tray dryer, Arm®eld, UK). The operation

mode of this dryer was explained in detail elsewhere

(Maskan, 2000).

2.3. Drying procedure

(1) Hot air drying. The dryer was operated at

an air velocity of 1.29 m/s, parallel to the drying surface

of the sample, 60°C dry bulb and 27°C wet bulb tem-

peratures. The study was carried out at constant sample

thickness and at constant temperature. Moisture loss

was recorded by a digital balance (Avery Berkel,

CC062D10ABAAGA) at 10 min intervals during drying

for determination of drying curves. Kiwifruits were

dried until equilibrium (no weight change) was reached.

(2) Microwave drying. Di€erent microwave power

intensities (210, 350, and 490 W) were investigated in

microwave drying at constant sample thickness of

5.03 mm. It was observed that (our preliminary tests)

charring and sample boiling occurred at 350 and 490 W

power. Therefore, only the 210 W power level was

chosen in this study. One glass petri dish (7:1 cm

diameter  1:2 cm deep), containing the sample, was

placed at the centre of the oven turntable in the micro-

wave cavity during treatment for even absorption of

microwave energy. The presence of the turntable was

necessary to achieve the optimum oven performance and

to reduce the levels of re¯ected microwaves onto the

magnetron (Khraisheh, Cooper, & Magee, 1997). The

drying was performed according to a pre-set power and

time schedule. Moisture loss was measured by taking

out and weighing the dish on the digital balance peri-

odically. When the material reached a constant weight,

equilibrium moisture content was assumed to be

reached. Attention was paid to ensure that the sample

was not charred.

(3) Air followed by a microwave ®nish drying. Drying

was carried out by a combination of hot air-microwave

techniques. A 5.03 mm thick kiwifruit sample was dried

by hot air initially for 135 min, then, dried by microwave

(at 210 W). This point corresponds to a moisture con-

tent of about 1.2 kg water/kg dry solids. It is a critical

Notation

k

drying constant (min

ÿ1

)

MR

moisture ratio

r

2

coecientof determination

S

shrinkage

SE

standard error

t

drying time (min)

V

volume of sample (ml) at any time

V

0

initial volume of sample (ml)

W

d

weight of dried sample (g)

W

t

weight of rehydrated sample (g) at any time

X

moisture content (kg H

2

O=kg dry solids) at any

time

X

e

equilibrium moisture content (kg H

2

O=kg dry solids)

X

0

initial moisture content (kg H

2

O=kg dry solids)

178

M. Maskan / Journal of Food Engineering 48 (2001) 177±182

point, because there was a sudden change in colour

parameters (results were not shown) up to this point

when the samples were dried by microwave alone (for

about 10 min). The objective of using this combined

system was to see if hot air pre-drying a€ects the drying

characteristics and maintains the colour quality of ki-

wifruits. The details of colour change were presented in

another study (Maskan, 2001).

The following di€usion model (Eq. (1)) was used to

describe drying kinetics of kiwifruits rather than using

Fick's di€usion model. The latter model could not be

applied because of severe shrinkage in sample dimen-

sions during drying.

MR ˆ

X ÿ X

e

X

0

ÿ X

e

exp…ÿkt†:

…1†

2.4. Shrinkage and rehydration capacity

For shrinkage and rehydration studies, similar drying

experiments, given in Section 2.3, were conducted sep-

arately using the same microwave, hot air and micro-

wave ®nish drying conditions. Measurement of

shrinkage and rehydration times were established from

preliminary assays.

Volume changes due to sample shrinkage were mea-

sured by a water displacement method as described by

(Sjoholm & Gekas, 1995). Measurements were made as

quickly as possible (less than 30 s) to avoid water uptake

by samples. The shrinkage/volume change of the sam-

ples was expressed as a bulk shrinkage ratio of sample

volume at any time to initial volume

S ˆ

V

V

0

:

…2†

Rehydration experiments were performed by im-

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 4±5 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:

weight gain …%† ˆ

W

t

ÿ W

d

…

†

W

d

100:

…3†

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,

little shrinkage, and an attractive colour. In this study,

the e€ect of drying method on drying rate, shrinkage

and rehydration of kiwifruits was evaluated.

The drying curves of kiwifruits dried by various

methods are shown in Fig. 1. It is evident from exam-

ination of these curves that the drying of kiwifruits by

microwave is much faster than drying by hot air or hot

air-microwave combination. For example, the time to

reach about 0.39 kg water/kg dry solids was 24.0, 225.0

and 136.5 min, respectively. In fact, the drying time for

the sample was reduced to about 89% of the hot air

drying time with the microwave drying and about 40%

with the hot air-microwave combination drying method.

This indicated that the drying time requirement was

signi®cantly reduced as microwave was introduced.

Maskan (2000) found that the hot air-microwave ®nish

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

various drying techniques. (Bouraout et al., 1994;

Prabhanjan et al., 1995; Lin et al., 1998; Funebo &

Ohlsson, 1998). These authors explained that the shorter

drying time under microwave heating conditions could

be due to the additional energy input, rapid heat pene-

tration by microwave and forced expulsion of gases.

Drying rates were calculated as quantity of moisture

removed per unit time per unit dry solids (kg water/kg

dry solids/min). The drying rates of kiwifruits were given

in Figs. 2 and 3. From an examination of these ®gures it

is obvious that the drying occurred mostly in the falling

rate period (with two periods), regardless of drying

conditions. Although the kiwifruits have high moisture

content, an expected constant rate period was not

observed in the present study. In Fig. 2, the slope of

the second falling rate line was steeper than that of ®rst

falling rate line (moisture content  1:3 kg water=kg

dry solids) and hence drying rate decreased rapidly dur-

ing last stages of drying. Two falling rate periods were

Fig. 1. Typical drying curves for kiwifruit slices (5:03  0:236 mm

thick).

M. Maskan / Journal of Food Engineering 48 (2001) 177±182

179

also obtained with the microwave drying (Fig. 3) with

no constant rate period. The ®rst falling rate period was

from the beginning of drying to about 1.9 kg water/kg

dry solids and was followed by a rapidly decreasing

second falling rate period. The reason for existence of

two falling rate periods may be due to case hardening

which acts as a barrier to moisture migration during

prolonged drying. The case hardening is because of the

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

Gogusß (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

drying rate curves, examined in this study, for kiwifruits

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 13±14 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 r

2

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 r

2

value were 0.999 for hot

air section and 0.001 for microwave section which in-

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

e€ect of drying method on shrinkage of kiwifruits was

shown in Fig. 4. The shrinkage of kiwifruit samples was

85%, 81% and 76% for microwave, hot air and hot air-

microwave drying, respectively. Hot air drying pro-

moted a moderate shrinkage, which was explained by

the fact that a long drying time gives more time for the

product to shrink (Ratti, 1994). The shrinkage was the

Table 1

Nonlinear regression analysis results of one parameter di€usion model (Eq. (1)) for drying of kiwifruits

Parameter

Drying method

Air

drying

Microwave

drying

Hot air-mircrowave drying

Air section

Microwave section

k

0.0103

0.1798

0.0094

0.0208

SE ()

0.0002

0.0305

0.0001

0.0058

r

2

0.988

0.997

0.999

0.0010

Fig. 2. Drying rate curves for kiwifruits dried by hot air (60°C and

1.29 m/s).

Fig. 3. Drying rate curves of kiwifruit slices under various conditions.

180

M. Maskan / Journal of Food Engineering 48 (2001) 177±182

least by combined drying method. However, there was a

higher and rapid shrinkage of samples dried by micro-

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

pattern of typical drying curves (Fig. 1), with high

shrinkage initially and gradual levelling o€ towards the

end of drying so that the ®nal size and shape of samples

were ®xed before drying was completed. Similar beha-

viour has been observed by Ratti (1994) on drying of

potatoes, apples and carrots, Sjoholm and Gekas (1995)

on apple drying and Wang and Brennan (1995) on po-

tato drying.

During reconstitution of dehydrated products the

amount and rate of water absorption determines to a

considerable extent the sensorial properties and prepa-

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 in¯u-

enced by processing conditions, sample pretreatment

and composition (Feng & Tang, 1998).

The rehydration capacities of kiwifruit samples dried

with di€erent methods were calculated by Eq. (3) and

presented graphically. The rehydration curves of dried

kiwifruits obtained at 50°C were shown in Fig. 5. Mi-

crowave dried kiwifruit slices exhibited lower rehydra-

tion capacity and faster water absorption rate than the

other two drying methods. This may be due to changes

in the structure/texture of the samples during microwave

drying because of temperature rise (greater than air

temperature, 60°C. Higher water absorption capacity

was expected for microwave dried sample (Drouzas &

Schubert, 1996; Feng & Tang, 1998; Lin et al., 1998)

because of short drying duration. The rehydration ca-

pacity of samples dried with hot air-microwave com-

bined method had the highest value. This method

improved the rehydration capacity of kiwifruits. These

results are in agreement with shrinkage data (Fig. 4)

showing that a less shrunk structure had higher capacity

to absorb water when reconstituted.

4. Conclusion

From the above results it was concluded that micro-

wave and microwave assisted heating reduced the drying

times by 89±40%. The one-parameter di€usion model

adequately described the hot air and microwave drying

data. The hot air-microwave ®nish dried products ex-

hibited less shrinkage, hence, they had better rehydra-

tion characteristics. Experiments with kiwifruit slices

showed that hot air-microwave ®nish drying can be used

for preservation of high quality products.

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