Drying characteristics and drying quality of carrot

using a two-stage microwave process

J. Wang

a,*

, Y.S. Xi

a,b

a

Department of Agricultural Engineering, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, China

b

Department of Machinery Engineering, Jinhua College of Science andTechnology, Zhejiang Jinhua 321017, China

Received 19 January 2004; accepted 29 June 2004

Abstract

Little detailed information is available on the alternative of using varying microwave power during drying of food products.

Experiments were made to study microwave drying characteristics and dried product quality. A two-stage microwave power system
using a first and second stage power input for varying times during drying was used. The study focuses on describing microwave
drying characteristics of carrot and discussing the effect of sample thickness, power applied during first-stage (first-stage power),
power applied during the second-stage (second-stage power) and duration of first-stage on b-carotene content, and rehydration
ratio. The dehydration rate increased and the drying energy consumption decreased, as the thickness of the sample decreased, power
level increased and mass load decreased. There were two falling rate periods when using microwave drying of carrot, the first falling
rate period being over moisture contents of 1.0 (d.b.), and the second falling rate period applying at moisture contents less than 1.0
(d.b.). The same water loss will consume more energy and the energy curve was shown to the steeper when the moisture content is
less than 1.0 (d.b.). Slice thickness, first-stage power, second-stage power, and duration of the first-stage affected b-carotene content
and rehydration ratio. The rehydration ratio of the dried products decreased with increase in duration of the first-stage and slice
thickness. b-carotene content decreased with increase of power applied during the second-stage and duration of the first-stage. With
the exception of the effect of first-stage power on b-carotene content and duration of the first-stage on the two quality indicators,
slice thickness, first-stage power, and second-stage power significantly affected the two quality indicators.
2004 Elsevier Ltd. All rights reserved.

Keywords: Microwave; Drying; Carrot; b-carotene; Characteristic

1. Introduction

Drying is one of the oldest methods of food preserva-

tion and it is a difficult food processing operation mainly
because of undesirable changes in quality because the re-
moved of water from a food product using conventional
air drying, may cause serious damage to the dried prod-
uct. Major disadvantages of hot air drying of foods are
low energy efficiency and lengthy drying time during the
falling rate period. Because of the low thermal conduc-

tivity of food materials in this period, heat transfer to
the inner sections of foods during conventional heating
is limited (

Feng & Tang, 1998

). The desire to eliminate

this problem, prevent significant quality loss, and
achieve fast and effective thermal processing has resulted
in the increasing use of microwaves for food drying.
Microwave drying is rapid, more uniform and energy
efficient compared to conventional hot air drying. In this
case, the removal of moisture is accelerated and, further-
more, heat transfer to the solid is slowed down signifi-
cantly due to the absence of convection. Also because
of the concentrated energy of a microwave system, only
20–35% of the floor space is required, as compared to
conventional heating and drying equipment. However,

0260-8774/$ - see front matter

2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2004.06.027

*

Corresponding author. Tel.: [phone]; fax: +86 571

86971139.

E-mail address:

[e-mail]

(J. Wang).

www.elsevier.com/locate/jfoodeng

Journal of Food Engineering 68 (2005) 505–511

microwave drying is known to result in a poor quality
product if not properly applied (

Drouzas & Schubert,

1996

;

Yongsawatdigul & Gunasekaran, 1996

).

For microwave applications, a two-stage drying proc-

ess involving an initial forced-air convective drying, fol-
lowing by a microwave finish drying, has been reported
to give better product quality with considerable saving
in energy and time (

Jeppson, 1964

;

Nury & Salunkhe,

1968

). Water accounts for the bulk of the dielectric com-

ponent of most food systems especially for high mois-
ture fruits and vegetables. Hence, these products are
very responsive to microwave applications and will ab-
sorbthe microwave energy quickly and efficiently as
long as there is residual moisture. Microwave applica-
tion for drying therefore offers a distinct advantage,
i.e., energy absorption proportional to the residual
moisture content. Proteins, lipids and other components
can also absorb microwave energy, but are relatively less
responsive (

Mudgett & Westphal, 1989

). A second

advantage of microwave application for drying of vege-
tables is the internal heat generation. Drying causes the
moisture to recede inwards from the surface. In conven-
tional systems, heat that is applied at the surface has to
be transferred through a moisture-resistant dry layer for
the evaporation of water at the receding waterfront. In a
microwave drying system, the microwave can easily pen-
etrate the inert dry layers to be absorbed directly by the
moisture at the waterfront. The quick energy absorption
causes rapid evaporation of water, creating an outward
flux of rapidly escaping vapour.

In recent years, microwave drying has gained popu-

larity as an alternative drying method for a wide variety
of food products such as fruits, vegetables, snack foods
and dairy products. Several food products have been
successfully dried by the microwave-vacuum application
and/or by a combined microwave assisted-convection
process. These authors included

Kim and Bhowmik

(1995)

for plain yogurt,

Yongsawatdigul and Gunaseka-

ran (1996)

for cranberries,

Lin, Durance, and Seaman

(1998)

for carrot slices,

Drouzas and Saravacos (1999)

for model fruit gels,

Al-Duri and McIntyre (1992)

for

skimmed milk, whole milk, casein powders, butter and
fresh pasta,

Bouraout, Richard, and Durance (1994)

for potato slices,

Tulasidas, Raghavan, and Morris

(1996)

for grapes,

Funebo and Ohlsson (1998)

for apple

and mushroom, and

Ren and Chen (1998)

for American

ginseng roots.

It has also been suggested that microwave energy

should be applied in the falling rate period or at a low
moisture content to finish drying (

Funebo & Ohlsson,

1998

;

Kostaropoulos & Saravacos, 1995

). However,

microwave drying conducted by the altering output
power method has been little reported, i.e., little detailed
information is available on the altering microwave
power drying of food products, such as a two-stage
drying process involving an initial power followed by

an altered drying power. In addition, the energy con-
sumption change for microwave drying has not been
reported.

The objectives of this study were to: (1) describe

microwave drying characteristics of carrot and discuss
the influence of slice thickness, microwave power and
mass load on drying characteristics and energy con-
sumption; (2) determine the effect of slice thickness,
first-stage power, second-stage power and duration of
first-stage on b-carotene content, and rehydration ratio.

2. Materials and methods

2.1. Material

The carrots were hand harvested in 2002 from the

experimental farm in the Department of Horticulture,
Zhejiang University. Carrots (Zhe-Agriculture No. 2,
is a firm and fragile carrot) with an initial moisture con-
tent of 6.87 kg H

2

O/kg dry solids were stored at

4 ± 0.5

C. Prior to drying, samples were taken out of

storage, hand peeled, cut into 1.5, 3, 4.5, 6, 7.5, 9,
±0.15 mm thick slices with a cutting machine. All carrots
used for drying were from the same batch. Initial mois-
ture content was determined using a vacuum-oven for
70

C, 3 kPa, and a heating time of 12 h (GB/T8858-88,

Chinese National Standard).

2.2. Drying equipment

The drying apparatus used consisted of a laboratory

microwave oven (WEG-800A, Jinan, China), which
operated at 2450 MHz. The energy input was microproc-
essor controlled from 10 to 1000 W at 10 W increments.
The outlets were provided on the left upper side of the
oven to allow introduction of temperature sensors, while
another input provided at the right top of the oven to
allow the introduction of airflow and a thermocouple.
The

dimensions

of

the

microwave

cavity

were

445

· 420 · 285 mm. The microwave oven was operated

by a control terminal, which could control both micro-
wave power level and emission time (1 s–100 h).

2.3. Drying procedure

Factors investigated in microwave drying were micro-

wave power intensity (120, 160 and 240 W) and sample
load (100 g, 200 g and 300 g). One dish, containing the
sample, was placed on the centre of a turntable fitted in-
side (bottom) the microwave 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 reflected
microwaves onto the magnetron. The drying was per-
formed according to a preset power and time schedule.

506

J. Wang, Y.S. Xi / Journal of FoodEngineering 68 (2005) 505–511

Moisture loss was recorded at 5 min intervals during
drying by taking out and weighing the dish on a digital
balance. For water vapour removed, an outlet fan was
setup in the microwave oven. An outlet air velocity of
1 m/s was used for the experiment. When the material
reached a constant weight, equilibrium moisture content
was assumed to be reached. Attention was paid to en-
sure that the sample was not charred.

2.4. Drying indices

2.4.1. Rehydration ratio

The dried samples were manually ground and imme-

diately loaded (about 10 g each) into small aluminum
sample dishes. 500 ml of distilled water was transferred
into a glass jar and a tripod was also placed in the jar.
The dishes were placed on the tripod in the jar which
was then tightly closed and kept at 20

C for equilibra-

tion. The dishes were periodically weighed until equilib-
rium was reached. The rehydration percentage, was used
to express the rehydration of the dried potato (

Farkas &

Singh, 1991

;

Jayaraman & Das Gupta, 1990

;

Lewicki,

1998

).

Rehydration ratio

¼

Mass after rehydration

Mass before rehydration

2.4.2. b-Carotene content

The b-carotene content of carrot slices was analyzed

using high performance liquid chromatography (

Bureau

& Bushway, 1986

;

Lin et al., 1998

). The wavelength of

the detector was set at 470 nm. A Vydac 5 lm
210TP54column 250 cm

· 4.6 cm (Anspec, Ann Arbor,

MI) and a solvent system, methanol-BHT stabilized
tetrahydrofuran (THF) 90/10 (v/v), were used for the re-
verse-phase separation of carotenes. The b-carotene of
carrot was calculated by comparison with carotene
standards and expressed as micrograms of carotene
per gram of sample on a dry weight basis.

3. Results and discussion

3.1. Dehydration characteristics

3.1.1. Effect of sample thickness

The moisture content versus time curves for micro-

wave drying of carrot samples as influenced by thickness
are shown in

Fig. 1

. As the thickness of the sample in-

creased, the time required to achieve a certain moisture
content increased and water loss slowly. For example,
the drying times for reaching about 0.18 kg H

2

O/kg

dry solids moisture content of 3, 5 and 7 mm thick sam-
ples were about 80, 86 and 95 min. The drying rate was
calculated at different times and plotted against moisture
content as shown in

Fig. 2

. It was seen that the dehydra-

tion rate was higher in the thinner samples at the same
moisture content. A constant rate period was not ob-
served in drying of the carrot samples. Hence, the entire
drying process occurred in the falling rate period in this
study. However, there were two falling rate periods
using microwave drying of carrot, the first falling rate
period under at moisture contents above approximately
1.0 (d.b.), the second falling rate period at moisture con-
tents less than 1.0 (d.b.). This agrees with the report that
the drying of banana takes place in the falling rate per-
iod (

Maskan, 2000

). The moisture content 1.0 (d.b.) cor-

responds to the moisture content of the inflexion point
where the high dehydration rate is transformed into a
low dehydration rate.

When considering influence of the internal structure

changes on carrot drying, it must be considered that
there are three possible ways for the movement of water
in and out of cells (

Wang & Chao, 2002

): trans-mem-

brane transport (through plasmalemma membrane
boundaries), symplastic transport (via cytoplasmic
strands or plasmodesmatas) and the cell wall pathway.

Tyree (1970)

had reported this last as the preferred path-

way for smaller non-ionic species like water and

Molz

and Ikenberry (1974)

had concluded that a significative

Fig. 1. Moisture content versus time curves for sample thickness (mass
load 200 g, drying powder 160 W).

Fig. 2. Dehydration rate versus moisture content for sample thickness
(mass load 200 g, drying powder 160 W).

J. Wang, Y.S. Xi / Journal of FoodEngineering 68 (2005) 505–511

507

portion of the water flux traversing a plant tissue could
occur in the cell wall. During the first drying period,
moisture content is greater in carrot and the movement
of water is mostly by trans-membrane transport and the
cell wall pathway. The dehydration rate is higher, but
deceased rapidly with moisture content. In the second
drying phase, the moisture content is lower in carrot
and the movement of water is mostly symplastic trans-
port way and the dehydration rate is lower.

The moisture content versus electrical energy con-

sumption curves for microwave drying of carrot slices
at the load 200 g and 160 w are shown in

Fig. 3

. It can

be seen that the electrical energy consumption curves
of the three thicknesses were different. When the carrot
samples were dried to the same lower moisture content
(the same moisture content were lost), the thick sample
used the more energy than the thin sample. This reflects
the low rehydration rate and long drying time for thick
samples.

During the microwave drying, the internal heat will

be generated in the vegetable, internal moisture move
easily to surface and the dehydration rate will increase.
However, increase in the thickness will increase the en-
ergy required for transfer. It is because of volumetric
heating, generating high pressure inside the samples, re-
sulted in more boiling and more bubbling of the samples
(

Wang, Zhang, Wang, & Xu, 1999

).

In

Fig. 3

, the same water lose will consume more en-

ergy and the curve become steeper when the moisture
content is less than 1.0 (d.b.). In the second falling rate
period, moisture content is lower in the carrot, the
movement of water is mostly symplastic transport and
more energy was consumed. The moisture content 1.0
(d.b.) corresponds to the moisture content at the inflex-
ion point where the first falling rate period is trans-
formed into the second falling rate period.

3.1.2. Effect of microwave power

The effect of changing the power output in the micro-

wave oven on the dehydration characteristics of 2 mm
thick carrot samples is shown in

Fig. 4

. It is shown that

the dehydration rate was increased at higher power lev-
els at the same moisture content. The results indicated
that mass transfer is rapid during the larger microwave
power heating because more heat is generated within
the sample, creating a larger vapour pressure differential
between the centre and the surface of products (

Lin

et al., 1998

).

It was also shown that two falling rate periods existed

when using different microwave power levels on carrot,
the first at moisture contents greater than 1.0 (d.b.),
the second at moisture contents less than 1.0 (d.b.). The
observed initial acceleration in drying may be caused
by an opening of the physical structure allowing rapid
evaporation and transport of water (

Kostaropoulos &

Saravacos, 1995

).

Efforts were made to study the effect of the power

output on energy consumption on 2 mm thick carrot
sample.

Fig. 5

shows that the relationship between en-

ergy consumption and moisture content. Unexpectedly,
energy consumption is different for the three power lev-
els for the same moisture loss. The lower microwave dry-
ing power consumes the most energy when the same
moisture content was lost. One of many reasons might
be that the drying time is longer under lower power
and results in the increase in energy consumption.

Fig. 3. Energy consumption versus moisture content for sample
thickness (mass load 200 g, drying powder 160 W).

Fig. 4. Moisture content versus time curves for power output (mass
load 200 g, sample thickness 2 mm).

Fig. 5. Energy consumption versus moisture content for power output
(mass load 200 g, sample thickness 2 mm).

508

J. Wang, Y.S. Xi / Journal of FoodEngineering 68 (2005) 505–511

Similar to effect of slice thickness, the water loss will

consume more energy and the curve become steeper
when the moisture content is less than 1.0 (d.b.).

3.1.3. Effect of mass load

The effect of changing mass load in the microwave

oven on dehydration characteristics of 3 mm thick carrot
samples and power input 200 W is shown in

Fig. 6

. It is

seen that the dehydration rate increased with smaller
mass load, because mass transfer is fast and there is
more rapid vapour generation within the small mass
sample. It was also shown that there were two falling
rate periods using different microwave power, the first
at moisture contents above 1.0 (d.b.), the second at
moisture contents less than 1.0 (d.b.).

Fig. 7

shows that the energy consumption changes

with the moisture content. The energy consumption in-
creases with decrease in the moisture content. The en-
ergy consumption also increases with the greater mass
load because the more water needs to be removed. When
the moisture content is greater than 1.0 (d.b.), the energy
consumption curve is liner and they become steeper at
moisture content less than 1.0 (d.b.).

3.2. Effect of the alternative factor on dried indices

3.2.1. Effect on characteristic

To investigate effect of changing the microwave dry-

ing power on dried product quality, drying was con-
ducted using a two-stage drying process involving an
initial power input following by a changed power input.
The factors considered were first-stage power (power ap-
plied during first stage), second-stage power (influence
of mass load and drying power were united and consid-
ered), duration of first-stage and slice thickness. The ef-
fect of these factors on b-carotene content, and
rehydration ratio were conducted and analyzed.

The effect of slice thickness on b-carotene content and

rehydration ratio is shown in

Fig. 8

(first-stage power

1.0 kW/kg, second-stage power 2.0 kW/kg, duration of
first-stage 16 min). Rehydration ratio of the dried prod-
ucts and b-carotene content both decreased with in-
crease of slice thickness. It may be because great
volumetric heating, generating high pressure inside the
carrots, resulted in boiling and bubbling of the samples
and rehydration ratio of dried products and b-carotene
content decreased.

The effect of first-stage power on rehydration ratio of

dried carrot is shown in

Fig. 9

(sample thickness 4 mm,

second-stage power 1.0 kW/kg, duration of first-stage
12 min). It was shown that the rehydration ratio of the
dried-product at a power of about 2 kW/kg was smaller
than at any other power. When first-stage power is
0.5 kW/kg, the rehydration ratio was greater.

b

-carotene content change with first-stage power

shows the opposite trend to the effect on rehydration
ratio (

Fig. 9

). b-carotene content of dried carrot was

greatest at about first-stage power of 2 kW/kg than at
any other power.

Rehydration ratio increased with increase in second-

stage power when the power was less than 2.6 kW/kg but
it decreased above this power (

Fig. 10

). When the sec-

ond-stage power is too high, water loss is fast and the

Fig. 6. Moisture content versus time curves for mass load (drying
power 200 W, sample thickness 3 mm).

Fig. 7. Energy consumption versus moisture content for mass load
(drying power 200 W, sample thickness 3 mm).

Fig. 8. Effect of slice thickness on b-carotene content and rehydration
ratio (first-stage power 1.0 kW/kg, second-stage power 2.0 kW/kg,
duration of first-stage 16 min).

J. Wang, Y.S. Xi / Journal of FoodEngineering 68 (2005) 505–511

509

dried sample shrinks quickly. This may result in the
rehydration ratio decrease. The b-carotene content of
the dried product decreased with increase of second-
stage power. This is due to the sample temperature in-
crease when the latter power is high.

The effect of duration of first-stage on b-carotene

content and rehydration ratio is shown in

Fig. 11

(sam-

ple thickness 6 mm, first-stage power 1.5 kW/kg, second-

stage power 1.0 kW/kg). The rehydration ratio of dried
products decreased with increase in duration of first-
stage, but b-carotene content rose. This implied that
the duration of first-stage is conflicting for the two dried
product quality indicators.

3.2.2. Level of the difference

To evaluate the effect of slice thickness, first-stage

power, second-stage power, and duration of first-stage
on b-carotene content and rehydration ratio, the follow-
ing values were selected: first-stage power (0.5 kW/kg,
2 kW/kg, 3 kW/kg), second-stage power (0.5 kW/kg,
2 kW/kg, 3 kW/kg), duration of first-stage (8 min,
16 min, 28 min), and slice thickness (1.5 cm, 4.5 cm,
9 cm). Statistical analyses of experiment results were per-
formed using the SAS/STAT Procedure (

SAS Institute

Inc, 1999

).

The statistical analyses of the main effects and two-

factor interactions on the two indicators are summarized
in

Table 1

. The magnitudes of the F-values indicate the

relative importance of the effects.

Slice thickness and second-stage power significantly

affected the two quality indicators (

Table 1

). Based on

the magnitudes of the F-values, it appears that, sec-
ond-stage power more significantly affected b-carotene

Fig. 9. Effect of first-stage power on b-carotene content and rehydra-
tion ratio (sample thickness 4 mm, second-stage power 1.0 kW/kg,
duration of first-stage 12 min).

Fig. 10. Effect of second-stage power on b-carotene content and
rehydration ratio (sample thickness 6 mm, first-stage power 2.0 kW/kg,
duration of first-stage 20 min).

Fig. 11. Effect of duration of first-stage on b-carotene content and
rehydration ratio (sample thickness 6 mm, first-stage power 1.5 kW/kg,
second-stage power 1.0 kW/kg).

Table 1
F-values from STAT/ANOVA on the main effects and interaction

Source

Degrees of freedom

b

-Carotene content

Rehydration ratio

Slice thickness

2

11.92

*

13.15

*

First-stage power

2

1.49

3.39

*

Second-stage power

2

15.62

*

3.57

*

Duration of first-stage

2

1.13

1.09

Slice thickness

· first-stage power

4

1.65

0.96

Slice thickness

· second-stage power

4

6.00

*

3.29

*

First-stage power

· second-stage power

4

0.16

0.30

First-stage power

· duration of first-stage

4

0.95

1.04

Second-stage power

· duration of first-stage

4

1.45

1.31

*

Significant at the p 6 0.05.

510

J. Wang, Y.S. Xi / Journal of FoodEngineering 68 (2005) 505–511

content. First-stage power significantly affected rehydra-
tion ratio, but had a small effect on b-carotene content.
Duration of first-stage did not significantly affected the
two indicators.

The interaction between slice thickness and second-

stage power did significantly influence the two quality
indicators. Other interaction did not significantly affect
the two indictors.

4. Conclusions

(1) As the thickness of the sample decreased, power

level increased and mass load decreased, the dehy-
dration rate increased and the drying energy con-
sumption decreased.

(2) There were two falling rate periods using microwave

drying of carrot, the first falling rate period at mois-
ture contents greater than 1.0 (d.b.), the second at
moisture contents less than 1.0 (d.b.). The same
water loss will consume more energy and steeper
curves were shown when the moisture content is less
than 1.0 (d.b.).

(3) Slice thickness, first-stage power, second-stage

power, and duration of first-stage affected b-caro-
tene content and rehydration ratio. The rehydration
ratio of the dried products decreased with increase
of duration of first-stage and slice thickness, b-caro-
tene content decreased with increase of second-stage
power and duration of first-stage.

(4) With the exception of the effect of first-stage power

on b-carotene content and duration of first-stage on
the two quality indictors. Slice thickness, first-stage
power and second-stage power significantly affected
the two indictors.

Acknowledgement

The authors acknowledge the Chinese Specialized

Research Fund for the Doctorate of High Education
through project 20020335052.

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