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Eur Food Res Technol (2003) 218:68–73
DOI 10.1007/s00217-003-0791-3
ORIGINAL PAPER
Taner Baysal · Filiz Icier · Seda Ersus · Hasan Yıldız
Effects of microwave and infrared drying on the quality
of carrot and garlic
Received: 17 March 2003 / Revised: 18 August 2003 / Published online: 31 October 2003
Springer-Verlag 2003
Abstract The effects of microwave and infrared drying
on the quality of carrot and garlic were studied and
compared with the effects of conventional hot air (tray
drier for carrot and fluid bed drier for garlic) drying. The
quality of carrot and garlic were evaluated by instrumen-
tal and sensory analysis. Rehydration, moisture content,
water activity, particle density, bulk density, porosity and
colour values were obtained for microwave, infrared and
hot-air dried vegetables. In addition, total moisture
content versus time was represented by drying rate curves
of carrot and garlic samples. Finally, free moisture
content versus drying rate were compared for the three
different drying methods.
Keywords Microwave · Infrared · Hot air · Carrot ·
Garlic · Drying
Introduction
The dehydration technique is probably the oldest and the
most important method of food preservation practiced by
humans. The removal of moisture prevents the growth
and reproduction of microorganisms which cause decay,
and minimizes many of the moisture-mediated deteriora-
tive reactions. It brings about substantial reduction in
weight and volume, minimizing packaging, storage and
transportation costs and enables storability of the product
under ambient temperatures [1]. Owing to changing
lifestyles, especially in the developed world, there is
now a great demand for a wide variety of high quality
dried products with emphasis on freshness and conve-
nience.
Carrot and garlic are cultivated widely in Turkey and
both are important to the dried vegetable industry. The
quality of the product is important in carrot and garlic
dehydration processes. During dehydration many changes
take place; structural and physic-chemical modifications
effect the final product quality, and the quality aspects
involved in dry conservation in relation to the quality of
fresh products and applied drying techniques. Hot-air
drying is the most widely used method for production of
dehydrated vegetables. Major problems associated with
air dehydration are considerable shrinkage caused by cell
collapse following the loss of water, poor rehydration
characteristics of the dried product and unfavourable
changes in colour, texture, flavour and nutritive value
caused by drying [2].
In recent years, the improvement of quality retention
of dried products by altering drying process and/or pre-
treatment has been a research goal. In this respect, novel
drying methods including vacuum, infrared, microwave,
freeze- and osmotic drying of carrots [3, 4, 5, 6, 7] and
garlic [8, 9, 10, 11, 12, 13, 14, 15] have been the subjectof
extensive research.
In this paper the effects of microwave and infrared
drying on the quality of carrots and garlic were studied
and compared with the effects of conventional hot-air
drying.
Materials and methods
Carrot and garlic samples were obtained locally. Carrot (Nantes
variety) samples were stored overnight at 4€1 C and garlic
samples were stored at ambient temperature 20€2 C before
processing. The processing schemes for carrot and garlic are given
in Figs. 1 and 2, respectively.
The carrots were mechanically peeled, washed and sliced into
3€1-mm thick pieces. The sliced samples were than blanched at
98€1 C for 3 min. After draining the carrot samples were divided
into 3 batches and dehydrated by using the microwave, infrared and
hot-air drying methods.
The garlic samples were peeled, sliced into 3€1-mm pieces and
dehydrated by using the microwave infrared and hot-air drying
methods.
T. Baysal ()) · F. Icier · S. Ersus · H. Yıldız
Food Engineering Department, Engineering Faculty,
Ege University, I
˙zmir, Turkey
e-mail: taner@food.ege.edu.tr
Tel.: +90-232-3884000/3043
Fax: +90-232-3422792
Microwave treatment
200 g of carrot and garlic samples were placed in a 27.8 cm-
diameter tray and submitted to microwave treatment in a Vestel
Goldstar, ER 535 MT model oven (max power output, 1.2 kW,
2450 Mhz). The oven was modified with an aspiration system for
draining inside air.
During the drying period the microwave applied maximum
power level for 60 s and power off for 15 s for carrot, and power on
for 45 s and power off for 15 s for garlic.
Far infrared dehydration (FIR)
A laboratory scale infrared dryer was used for infrared dehydration
of the samples (OHAUS MB 200, OHAUS Corp., Florham, USA).
Samples were placed in dishes (12 cm diameter 3 cm height)
and a time-temperature programme was chosen according to pre-
analysis and applied as follows for carrot and garlic:
Temperature (C) Time (min)
Carrot 105 15
100 30
95 40
Garlic 85 20
80 100
85 10
Hot-air drying
Carrot samples were dehydrated in a tray drier (Armfield Ltd.,
Ringwood Hampshire, England) at 70 C with 0.86 m/sec air
velocity and garlic samples were dehydrated in a fluidized bed
dryer (Sherwood Scientific, England) at 50 C with 0.93 m/sec air
velocity. Samples were dehydrated until they reached equilibrium
state, i.e. constant weight.
Drying curves were determined by periodic weighing of the
mass of carrot and garlic during the drying period.
After dehydration, quality was assessed instrumentally by
sensory techniques. All treatments were duplicated and a factorial
randomized block design was used for statistical analysis.
Instrumental product analysis
Dry matter content
The AOAC method of total solids in fruit and vegetable products
[16]was used. Dry matter content was calculated on the basis of
fresh weight.
Colour
L (lightness), a (redness), b (yellowness) colour values of the
samples were measured using a spectral photometer (Datacolour,
textflash, USA). After standardization L, a, b values were measured
for fresh and dehydrated products. Colour values (dE), colour
intensity (chroma, dC) and hue angle were calculated according to
Eqs. (1-3):
dE¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
LLref
2þaaref
2þbbref
2
hi
rð1Þ
dC¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
aaref
2þbbref
2
hi
rð2Þ
Hue angle ¼tan1b=aðÞ ð3Þ
Water activity (aw)
Water activity were measured using a Testo 610 relative humidity
and temperature measurement device [17]. The results were
calculated according to Eq. (4).
aw¼ERH
100 ð4Þ
Rehydration
The rehydration properties were determined by water immersion at
25€1 C [18].
Fig. 2 Garlic drying process
Fig. 1 Carrot drying process
69
Bulk density and shrinkage
A glass measured cylinder (500 ml) was used for bulk density
measurements. A known amount of sample (M) was poured into the
cylinder and the volume was evaluated by reading the scale of
cylinder (V1) [19, 20]. Loose bulk density (rbl) was calculated
using Eq. (5). The cylinder with the same sample was tapped 20
times on a smooth, soft surface from a height of 10 cm and the
volume of the sample was evaluated by reading the scale (V2) [21,
22]. Tapped bulk density (rbt) was calculated using Eq. (5).
rb¼M
Vg=ml ð5Þ
(For loose and tapped bulk density calculation V1orV2must be
used instead of V*).
The degree of shrinkage (sb) can be calculated from the bulk
density of fresh (rb,o) and dried (rb,dry) product, and from the
moisture contents of fresh (X0) and dried material (Xdry) on a dry
matter basis according to Eq. (6) [17]:
sb¼rb;0
rb;dry
Xdry þ1
X0þ1
ð6Þ
Particle density
A known amount of sample (Mp) was immersed in a known volume
of carosen solution (Vc) in a measuring cylinder and the final
volume (Vf) was read off the scale of the cylinder. The particle
density of the sample was calculated according to Eq. (7) [23].
rp¼Mp
VfVcg=ml ð7Þ
Porosity
Porosity was calculated by using Eq. (8) [23]. In Eq. (8), for loose
and tapped porosity calculation rbl or rbt must be used instead of
rb*.
Porosity ¼1r
b
rpð8Þ
Sensory evaluation
The appearance and colour of the dehydrated samples were
evaluated by a sensory panel of seven assessors. The scoring
used by the panel followed the method given in [24]: from 10
(very good) to 1 (not accepted).
Conversion of data to rate of drying curve
Data obtained from drying equipment are usually obtained as
total weight (W) of the wet solid at different times (t) over the
drying period. If Ws is the weight of the dry solid in kilograms,
Xt ¼WWsðÞ=Ws kg total water=kg dry solidðÞ
For the given constant drying conditions, the equilibrium
moisture content Xe (Kg equilibrium moisture/kg dry solid) is
determined and the free moisture content Xf in (kg free water/kg
dry solid) is calculated for each value of XT.
Xf ¼XTXe
The drying rate (R) is calculated for each drying time increment
using:
R¼Ws DXfðÞ=ADtðÞkg water=m2h
where A is the exposed surface area for drying in m2[25].
Statistical analysis
One way analysis of variance (ANOVA) and least significant
difference (LSD) criterion were used for interpreting the data.
Results and discussion
Drying rate is defined as the amount of water removed per
unit area and time (kg H2O removed/m2h) and drying
curves are shown in Figs. 3,4 for carrots and Figs. 5, 6 for
the garlic samples. Microwave and infrared drying
directly increased the temperature of the samples without
Fig. 3 Drying rate (R) versus time curve of carrot samples dried by
different methods: IR, infrared; HA, hot air; MW, microwave
Fig. 4 R versus free moisture content (Xf) curve of carrot samples
dried by different methods
Fig. 5 R versus time curve of garlic samples dried by different
methods
70
heating the air. This heating effect was the cause of the
high drying rate. Owing to the effect of microwave drying
on the vaporization of the free moisture, the total drying
time was considerably reduced. In microwave drying, the
volumetric heat generation in the wet sample due to the
directly transmitted and absorbed energy by the water
molecules results in higher interior temperatures, thus
reaching the boiling point of water substantially faster
than would be possible in convective drying
As shown in Fig. 3, the drying rate of the hot-air
dehydrated carrot samples was constant until 120 min
with a value of 1 kg H2O/m2h. However there was an
immediate increase in the drying rate during the first
20 min for microwave- and infrared drying (1 kg H2O/
m2h to 2 kg H2O/m2h for microwave, 2 kg H2O/m2hto
4kgH
2O/m2h for infrared), then it decreased linearly
over time. The shortest drying times were observed for
microwave- and infrared drying of carrot samples. The
drying rate decreased during the drying of garlic for all
drying methods. The shortest drying time of garlic was
observed for hot-air dried samples.
The reason for the lower drying rates in microwave
drying may be due to a decrease in the dielectric
properties of the samples as a result of the decreased
moisture content [25]. Because of the higher initial
moisture content of carrot, the drying rate was higher than
for garlic and the drying time was shorter during
microwave drying.
The decrease in the total moisture content of the
samples over time was determined for all drying methods
as reported in [25, 26, 27, 28, 29]. The change in the
drying rate, as the result of the reduction of the free
moisture content of samples during drying, occurred
during the falling rate period in all drying methods. This
has also been reported in [2, 14, 30, 31].
The effects of microwave, infrared and hot-air drying
methods on the quality of sliced carrot samples are shown
in Table 1.
The microwave dehydrated carrot sample had signif-
icantly higher dry matter content in the final product,
followed by the infrared and hot-air dried samples. But no
significant differences were found between the hot-air and
infrared dried samples.
L, a, b values of dehydrated carrot samples were in the
same range and were not significantly different except for
b values of the hot-air dried carrot sample. Hue angle
values indicate the degree of browning. In other words
increasing yellowness and/or decreasing greenness result
in high hue angles [32]. The hue angles were not
significantly different for all samples. But the total colour
value (dE) and colour intensity (dC) of the hot-air
dehydrated sample resembled the fresh product (Table 1).
But it has to be noted that the final moisture content was
not the same for the products of the different drying
methods. And changes in colour values were probably not
only due to browning, but could also be attributed to
effects such as shrinkage and changes in reflective
characteristics during dehydration [33].
Water activity has a direct relationship with the
equilibrium moisture content [1]. In this respect, the
highest water activity value was found for hot-air dried
carrot samples which had the highest moisture content.
The water activity of microwave dried samples was found
to be lowest, but not significantly different from the
infrared dried carrot samples.
The rehydration capacities of carrots dehydrated by
different drying methods are given in Fig. 7.
During the 115-min rehydration process sample
weights increased. The highest rehydration capacity was
observed for infrared dried samples (8.95 g H2O/g)
followed by the microwave dried (8.38 g H2O/g) and hot-
air dried samples (7.96 g H2O/g).
The loose bulk density, tapped bulk density, and loose
and tapped porosity values were found to be highest for
hot-air dried samples but no significant differences were
found between the drying methods (Table 1). Particle
density of hot-air dried carrot had significantly higher
values and the microwave dehydrated samplehad the
Table 1 Effects of drying methods on some quality characteristics
of carrot
Analysis Carrot
Raw Hot air Microwave Infrared
Dry matter (g/100 g) 13.13c84.02b92.87a88.82b
Colour
L 56.49a54.94a47.98a50.06a
a 22.77a17.46b14.22b16.24b
b 26.93a17.44b13.67c14.94c
Hue angle 49.60a45.00b43.90b42.50b
dE – 10.98b17.93a15.09a
dC – 10.87b15.78a13.65ab
Water activity – 0.75a0.58b0.64b
Loose bulk density
(g/ml) 0.43a0.14b0.11b0.13b
Tapped bulk density
(g/ml) 0.46a0.16b0.13b0.14b
Particle density 1.04a1.40b0.70d1.12c
Shrinkage – 1.85b2.17a1.96ab
Loose porosity 0.59b0.90a0.81a0.88a
Tapped porosity 0.56b0.89a0.84a0.88a
a–c means with different supercript(s) within rows are significantly
different (P<0.05)
Fig. 6 R versus Xf curve of garlic samples dried by different
methods
71
lowest value. Shrinkage of this sample was found to be
significantly higher than in the air and infrared dried
samples. It has been shown that significantly different
bulk density values of a product can be caused by
variation in particle size or dry matter content [34].
Shrinkage also has a relationship with moisture content of
the product [35]. As mentioned previously the microwave
dehydrated samples had the highest dry matter content.
This could have had a direct effect on the shrinkage and
density values. It has been reported that the bulk density
of microwave dried material is lower than that of
conventionally dried material especially in the case of
carrot and potato. And the porosity of microwave dried
material was also higher than that of air-dried material
[34].
The effects of different drying methods on the quality
of garlic samples are given in Table 2.
The highest dry matter content was found for the hot-
air dried garlic sample and the infrared dehydrated sample
had the lowest. But there weren’t any significant differ-
ences between the dehydrated samples.
The hot-air dried sample had the highest L value which
did not significantly differ from the raw material. The L
value of the microwave dehydrated sample was found to
be significantly different from that of the raw material L
value but not different from the hot-air dried sample. The
lowest L value was observed for infrared samples.
a and b values were found to be highest for infrared
dehydrated samples, whereas the hot-air dried samples
had the lowest. a/b values. And the hue angle values of
the samples showed that the hot-air dried sample resem-
bled the fresh product the most but was not significantly
different from the microwave dried sample. Total colour
(dE) and chroma (dC) values of the infrared dried sample
were found to be the highest and significantly different
from the microwave and hot-air dehydrated samples.
The water activity, bulk and tapped bulk density,
particle density and shrinkage values of microwave,
infrared and hot-air dehydrated garlic samples were not
found to be statistically different.
The rehydration capacity of dried garlic is given in
Fig. 8. The highest rehydration capacity was found for the
hot air dehydrated sample (2.21 g H2O/g). The microwave
and infrared dried samples rehydration capacities did not
show any significant difference, and the absorbed water
amounts were 2.06 g H2O/g and 2.049 g H2O/g after
90 min for infrared and microwave dried samples,
respectively.
The effects of drying methods on the general appear-
ance of carrot and garlic samples were not statistically
significant (data not shown). But it was observed that the
colour of the hot air dried garlic samples gave better
results.
Conclusion
During dehydration the colour of food changes but it was
found that the colour of dried carrot resembled the fresh
product the most after hot air drying, whereas its
rehydration capacity was the lowest: infrared dehydrated
carrot had the best rehydration capacity.
But when the dry matter content of the dried carrot was
evaluated, it was clear that microwave dehydrated carrot
sample had significantly higher dry matter content in the
final product with the shortest drying time and signifi-
cantly higher shrinkage.
Table 2 Effects of drying methods on some quality characteristics
of garlic
Analysis Garlic
Raw Hot air Microwave Infrared
Dry matter (g/100 g) 32.50b90.10a88.65a87.05a
Colour
L 77.54a74.24ab 73.73b68.92c
a 1.87c2.83bc 3.71b6.27a
b 21.83ab 18.77b22.46a23.65a
Hue angle 85.10a81.42b80.60b75.10c
dE – 4.60b4.28b9.85a
dC – 3.21a1.94c4.76a
Water activity – 0.49a0.49a0.47a
Loose bulk density
(g/ml) 0.44a0.28b0.32b0.31b
Tapped bulk density
(g/ml) 0.47a0.29b0.33b0.32b
Particle density 0.87a0.69b0.68b0.71b
Shrinkage – 1.04a0.94a0.98a
Loose porosity 0.46b0.59a0.53a0.56a
Tapped porosity 0.49b0.58a0.52a0.55a
a–cmeans with different supercript(s) within rows are significantly
different (P<0.05)
Fig. 8 Rehydration capacity of dried garlic
Fig. 7 Rehydration capacity of dried carrot
72
The results show that the quality characteristics of
dried carrot differs according to the drying method used.
In order to reduce the drying time resulting in a
favourable impact on the end product quality, more
research is needed on a combination drying process where
microwave or infrared is used for pre-dryingfollowed by
conventional hot-air dehydration.
In the garlic dehydration process, no significant
difference was found between the hot-air (fluid bed
dryer), microwave and infrared drying methods except
colour.
Hot-air or microwave drying processes can be used for
colour protection instead of infrared drying, where colour
is an important parameter.
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