Drying kinetics and quality of vacuum-microwave dehydrated
garlic cloves and slices
Adam Figiel
*
Institute of Agricultural Engineering, Wrocław University of Environmental and Life Sciences, Chełmon
´skiego Str. 37/41, 51-630 Wrocław, Poland
a r t i c l e
i n f o
Article history:
Received 31 December 2008
Received in revised form 1 March 2009
Accepted 2 March 2009
Available online 12 March 2009
Keywords:
Garlic
Vacuum-microwave drying
Shrinkage
Texture
Water absorption
Color
Volatile oils
a b s t r a c t
Whole garlic cloves, halved and sliced were subjected to vacuum-microwave (VM) drying at three micro-
wave power levels: 240, 480 and 720 W. The process of drying was described by a sigmoid function. Slic-
ing the cloves prolonged the time of drying. An increase in microwave power resulted in increased drying
rate. The temperature of garlic samples measured in a vacuum flask amounted to 50 °C till the moisture
content of 0.6 g/g dry matter. Further drying of garlic was associated with temperature increase to about
70 °C. Decreasing moisture content, till 0.6 g/g dry matter, was accompanied by decreasing relative vol-
ume of the dried material. On exceeding this value no further change in volume of the slices was
observed, though for whole cloves and halves a marked increase in volume was observed. Lowering of
moisture content below 0.6 g/g dry matter and increasing microwave power caused an increase in garlic
cloves compressive strength. Increasing the degree of the material subdivision and microwave power
resulted in increased water absorption capacity. Drying the garlic samples with the VM made the color
brighter, shifting it towards red and blue, compared to fresh garlic cloves. The best retention of volatile
oils was observed for garlic slices dehydrated with microwaves at 720 W.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Drying of plant material is often described using a conventional
model of drying (
). Based on that model, drying
of the non-shrinking material has a linear character up to a certain
point K (first drying period), and an exponential character beyond
that point (second period). However, in many cases the first drying
period does not at all occur (
Drouzas et al., 1999; McMinn et al.,
), or its existence is assumed just to sim-
plify description of the drying process usually described by the two
term exponential model (
). This is especially true
of materials that are difficult to dry. The morphological structure
of garlic skin protects its cloves against loosing water to the med-
ium (
Nussinovitch and Hershko, 1996
). The drying time of the con-
vective technique can be shortened by using higher temperatures
which increase moisture diffusivity (
) and by
cutting the material into small pieces (
). In-
creased drying temperature entails higher costs and may cause bio-
chemical changes that degrade the dried product quality; whereas
subdividing the material is an additional process that results, espe-
cially under industrial conditions, in mass losses and lowering of
the product quality (
The drying time can be greatly reduced (
) and the quality of finished product insured (
) by applying the microwave energy to the
dried material. Lowered pressure can additionally shorten the dry-
ing time. The vacuum-microwave (VM) method is not yet common
in the industry (
Attiyate, 1979; Vega-Mercado et al., 2001
), though
investigations are conducted on drying, among others, carrots (
), cranberries (
), strawberries (
), tomatoes (
) and even shrimp
). Little efforts have so far been made to dry garlic
using this method (
). Lowered pressure induces faster
evaporation of water from the material at relatively low tempera-
ture. However, an uncontrolled irradiation with microwaves can
cause a jump in temperature of the material at the end of drying
and thus compromise its quality (
Warming up the material from within with microwaves in vacuum
restricts the volumetric shrinkage and produces a porous structure
of the product (
The texture of a dried material is usually estimated using
compressive strength tests (
). It turns
out that rehydration of a dried material may furnish valuable infor-
mation on the material structure fixed by drying (
). In the case of garlic a color alteration and reduction
in volatile oil may also be of interest.
Therefore the aim of the studies presented was to determine the
effect of microwave power on the VM method drying kinetics of
0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2009.03.007
*
Tel.: +48 71 3205730; fax: +48 71 3482486.
E-mail address:
Journal of Food Engineering 94 (2009) 98–104
Contents lists available at
Journal of Food Engineering
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j f o o d e n g
whole cloves and also cloves cut into halves and slices. The aim was
also to perform a quantitative analysis of the dried material based
on estimation of drying shrinkage, compressive strength test,
absorption of water from air of high humidity, color and volatile oils.
2. Materials and methods
2.1. Materials
Fresh garlic (Allium sativum L.) bulbs of Chinese origin were pur-
chased from a local wholesaler. The garlic bulbs had initial mois-
ture content of 1.9 g water/g dry matter and uniform size of
55 mm height and 70 mm diameter. The vacuum oven method
was used to determine the moisture content of the garlic cloves.
The cloves were smashed and mixed with dry sand in a glass vessel
before putting them into oven for six hours. The operating temper-
ature was 70 °C. The garlic cloves were hand peeled and part of
them were halved or sliced into 4 mm pieces just before
dehydration.
2.2. Vacuum-microwave drying
Garlic samples (60 g) were subjected to VM drying in a SM-200
dryer (Plazmatronika, Wroclaw, Poland) that had two magnetrons
of 1200 W combined power and a revolving drum of approximately
0.18 m radius and 0.27 m length. Three levels of microwave power
were applied: 240, 480 and 720 W. Garlic was dried with 30 s
microwave pulses followed by 5 s breaks. Investigations conducted
by
showed that pulse drying of cranberry is
more effective than continuous drying. The absolute pressure in
the drum revolving at 6 rev/min was from 4 to 6 kPa. Excessive low-
ering of pressure, necessitating the use of more expensive vacuum
pumps (
), did not result in significant shorten-
ing of the drying time (
). The kinetics of garlic drying
was determined on the basis of mass losses of 60 g portions of garlic
with previously estimated moisture content. With successive por-
tions of fresh material longer and longer drying times were applied.
Therefore, the number of measurement points necessary for drying
curve fitting amounted to the number of fresh material portions.
For quality assessment garlic samples were dehydrated to the safe
moisture content of 0.11 g water/g dry matter.
2.3. Convective drying
Garlic slices (240 g) spread as a single layer on a tray were
dehydrated to moisture content of 0.11 g water/g dry matter using
pilot convective drier designed and made in the Agricultural Engi-
neering Institute of Wroclaw. The air velocity and temperature
were 3 m/s and 70 °C. The influence of air velocity on garlic slices
drying kinetics at the range over 3 m/s is almost negligible (
). Convection-dehydrated slices were con-
sidered as comparative samples for assessment of the quality of
VM dehydrated samples.
2.4. Temperature measurement
Garlic samples just after taking them out of the vacuum-micro-
wave dryer were placed into a 350 cm
3
vacuum flask with a ther-
mocouple inside. First, a steep increase and then slow decrease in
temperature was observed. The maximum recorded temperature
was taken as mean temperature of samples placed in the vacuum
flask. It is expected that the local temperature within material dur-
ing drying was higher. However, a direct measurement in the dry-
ing chamber under vacuum is not practically possible because the
measuring elements are heated by the microwave emission.
2.5. Relative volume
Relative volume of the dried material (V
R
) was determined by
calculating the ratio of garlic volume after drying (V) to garlic vol-
ume before drying (V
0
). The volume of fresh and dried garlic was
measured with a graduated cylinder. The cylinder was filled in
turns with flax seeds of 72 g mass, that corresponds to 100 cm
3
vol,
and garlic samples and then it was gently shaken to obtain smallest
volume. Due to the shape and smooth surface of flax seeds the free
spaces between garlic particles could be filled during light shaking.
2.6. Compressive strength
The puncture resistance of dried garlic cloves was measured
with an Instron 5544 strength testing machine (Instron, High Wy-
combe, UK). A rod of 2.5 mm diameter was used to measure the
force required to penetrate individual garlic gloves. The values of
punch force reported are the mean of 10 measurements conducted
at crosshead speed of 25 mm/min. Breaking stress value was calcu-
lated from the equation:
r
max
¼
4 F
max
p
d
2
r
ð1Þ
2.7. Water absorption
Mean moisture content of garlic samples subjected to quality
assessment was 0.11 g water/g dry matter. However, in spite of
Nomenclature
a, b, c, d function parameters
AC
absorption capacity (g/g)
d
r
rod diameter (mm)
F
max
punch force (N)
K
critical point of drying process
L
*
degree of lightness
a
*
degree of redness
b
*
degree of yellowness
m
A
mass of water absorbed from humid air (g)
m
D
mass of water removed during drying (g)
R
2
coefficient of determination
T
temperature (°C)
t
time (min, h)
u
moisture content (g water/g dry matter)
V
R
relative volume of the dried material (cm
3
/cm
3
)
V
volume after drying (cm
3
)
V
0
volume before drying (cm
3
)
VM
vacuum-microwave
w
whole
h
half
s
slice
Greek symbols
a
significance level
r
max
breaking stress (MPa)
Superscript
a, b, c, d, e, f, g, h, i, j indicate significant differences
A. Figiel / Journal of Food Engineering 94 (2009) 98–104
99
mixing the material in the microwave field, individual samples
contained slightly different quantities of water. In order to avoid
the effect of nonhomogeneous sample drying on the results of
the rehydration test, both whole and sliced cloves dried at three
microwave power levels were additionally dried in a vacuum drier
at 40 °C and 4 kPa absolute pressure for 12 h to reach the dry mass.
Here, it was assumed that the conditions of extra drying are mild
enough not to affect the structural changes that occurred during
the VM drying. The samples of ca. 1.5 g weight were placed in a
WK111
340
GmbH chamber at 21 °C and 95% relative humidity.
The curves of absorption capacity were determined at certain
intervals for 90 h from the weight of the samples kept in the cham-
ber compared to the initial weight that corresponded to dry mass.
Samples were weighed each time, after taking out of the chamber,
in plastic containers on a balance with 0.001 g accuracy. Absorp-
tion capacity AC expressing the degree of water restoration in
dry material resulting from absorption of water vapour relative
to water content before drying was calculated from an equation
similar to that proposed by
Le Loch-Bonazzi et al. (1992) and Lew-
AC ¼
m
A
m
D
ð2Þ
Each experimental point was the result of three replications.
2.8. Color measurement
Color of fresh garlic and dried samples was evaluated by a
Minolta Chroma Meter CR-200 Reflectance System. The device is
a tristimulus colorimeter that measures four specific wavelengths
in the visible range, specified by the Comission Internationale de
l’Esclairage (CIE). Samples before measurement were diminished
using an electric mill. The results were expressed as L
*
(white-
ness–darkness), a
*
(red–green), and b
*
(yellow–blue) values. The
measurements of color were repeated three times.
2.9. Flavour strength
The volatile oil content of garlic samples was determined with a
Dering apparatus according to
standard. Ten grams
of diminished garlic sample was extracted in boiled water of
300 cm
3
for 3 h. The oils when combined with xylene increased
its volume measured with a graduated pipe that was part of the
extraction vessel. The volatile oils in the samples were expressed
as g oil/kg dry matter.
2.10. Statistical analysis
The results obtained in the study were subjected to statistical
analysis. Standard deviations were estimated by means of Micro-
soft Excel 97. Table Curve 2D Windows v. 2.03 enabled mathemat-
ical modelling with the best determination coefficient. The results
obtained in rehydration and color estimation tests were evaluated
by statistical analysis with the use of the Statistica v. 7.1. Homoge-
neous groups were determined with the Duncan’s multiple range
test (at significance level
a
= 0.05). In order to find out if the differ-
ences in the mean values estimated were statistically significant,
the one-way analysis of variance was applied.
3. Results and discussion
3.1. Drying kinetics
Based on the experimental results obtained (
), it was found
that the decrease in moisture content u of whole garlic cloves,
halves and slices during drying by the VM method can be described
by a sigmoid equation:
u ¼ a þ
b
1 þ e
tc
d
ð3Þ
Though its form is more complicated than in the conventional
model of drying, it precisely reflects the drying kinetics within
the entire time interval, independently of the microwave power
and material subdivision. The representation of the rate of drying
is also precise, especially in the initial period, owing to differenti-
ation of the sigmoid function proposed (
). Parameters a, b, c
and d of that function are given in
. It should be noted that
Fig. 1. Drying curves for whole garlic cloves (w), halves (h) and slices (s) under VM
drying condition at microwaves power 240, 480 and 720 W.
Fig. 2. Drying rate curves for whole garlic cloves (w), halves (h) and slices (s) under
VM drying condition at microwaves power 240, 480 and 720 W.
100
A. Figiel / Journal of Food Engineering 94 (2009) 98–104
parameter a is the asymptotic value of water content during dry-
ing. That value can be a bit lower than zero and is thus not reached
during drying. Parameter b is a theoretical interval of moisture
content values. Parameter c corresponds to the time coordinate
of the inflexion point in the drying curve. That point can thus be
treated as a critical point K that divides the drying process into a
period of increasing drying rate (above K) and decreasing drying
rate (below K). Thus the point K constitutes an extremum in the
drying rate function (
). During drying slices at 720 W the K
point does not exist (c = 0.7), i.e. drying goes on with decreasing
rate. The d parameter is the time constant of the drying process.
The greater the d the longer is the drying process. With greater
microwave power the rate of drying increased and the drying time
decreased.
A decrease in moisture content of all garlic samples from 1.9 to
0.11 g water/g dry matter lasted about 25 min at 240 W of applied
power and was approximately three times shorter at 720 W
(
). Drying up slices of the material with a convective method
at 70 °C lasted as much as 170 min. Convectively dried garlic slices
were treated as control samples for the quality assessment of VM
dried product reported in the following chapters. In an investiga-
tion by
drying with the convective
method of whole garlic cloves of smaller 1.6 moisture content with
similar drying parameters took about 5 h. With a conventional
method subdivision of the material causes a shortening of drying
time (
). This is especially true of vegetables
whose external cover makes removal of water difficult (
). However, earlier studies have shown that slicing the
cloves prolongs the time of drying by the VM method (
). Moreover, the investigations done by
showed that the time of drying banana and po-
tato slices with the microwave method is the longer the thinner are
the slices (in contrast to the convective method). The results ob-
tained by
proved that samples of larger
diameters absorb larger microwave power at given radiation con-
ditions. Smaller thickness of the material may reduce the absorp-
tion of microwave energy if the penetration depth, related to the
dielectric properties, is too large (
). The
drying curves in
indicate that the degree of subdivision did
not alter the drying time of garlic cloves with the VM method.
The supposition that microwaves generate higher temperature in
whole cloves than in sliced garlic being under vacuum needs more
consideration.
3.2. Temperature
It was found that the temperature in whole and sliced cloves
measured in a vacuum flask after removal from the drying cham-
ber was similar and amounted to 50 °C up to 0.6 water content
(
). Further drying of garlic till 0.11 g water/g dry matter was
connected with temperature increase to about 70 °C.
The shape of temperature curve obtained during VM drying of
carrot slices (
) was almost the same as those re-
ported here.
stated that when a
material is largely dried and its specific heat, involved in the con-
ductive heat transfer, has decreased sharply, the material temper-
ature takes a higher value. The temperature course in the vacuum
flask as dependent on water content in the material is represented
with an exponential function:
T ¼ 50:6 þ 25:9 e
u
0:167
ð4Þ
That function was fitted to all the experimental points, except
the four obtained at 720 W microwave marked with circle in
. In the course of the experiment it turned out that overdrying
of garlic cloves at highest power was accompanied by violent oxi-
dation. Possibly it is not the mean temperature of samples mea-
sured in the vacuum flask but temperature distribution within
the tissue (
), which is closely connected with
pressure distribution inside the cellular structure that is decisive
Table 1
Effect of drying conditions on parameters a, b, c and d of the functions describing the drying kinetics, relative volume, compressive strength and water absorption of garlic
samples.
Drying conditions
Microwave power (W)
Drying kinetics
Relative volume
Compressive strength
Water absorption
u ¼ a þ
b
1þe
tc
d
V
R
¼ a b u þ c u
2
r
max
¼ a þ b e
u
c
AC ¼ a ð1 e
t
b
Þ
a
b
c
d
R
2
a
b
c
R
2
a
b
c
R
2
a
b
R
2
w
240
0.013
2.27
7.96
4.92
0.99
0.763
0.363
0.259
0.99
1.34
3.17
0.109
0.98
0.295
58.98
0.99
480
0.165
2.83
3.48
3.44
0.99
0.794
0.366
0.254
0.98
1.34
4.05
0.141
0.99
0.290
51.17
0.99
720
0.053
2.71
2.09
2.17
0.99
0.802
0.375
0.255
0.98
1.23
5.52
1.59
0.99
0.295
43.95
0.99
240
0.243
3.58
4.01
9.85
0.99
0.592
0.124
0.188
0.97
0.294
41.93
0.99
h
480
0.246
3.22
3.14
4.54
0.99
0.683
0.252
0.225
0.98
0.290
34.21
0.99
720
0.172
4.03
0.249
3.47
0.99
0.705
0.276
0.236
0.97
0.298
29.84
0.99
240
0.160
3.01
6.44
8.46
0.99
0.521
0.087
0.092
0.99
0.304
29.53
0.99
s
480
0.176
2.80
3.98
3.91
0.99
0.667
0.054
0.124
0.99
0.297
27.31
0.99
720
0.251
4.74
0.70
4.03
0.99
0.678
0.084
0.136
0.99
0.301
22.57
0.99
Convection
0.293
25.49
0.99
Fig. 3. Temperature of whole garlic cloves (w), halves (h) and slices (s) measured in
a vacuum flask after VM drying at microwaves power 240, 480 and 720 W.
A. Figiel / Journal of Food Engineering 94 (2009) 98–104
101
for drying kinetics of a material of various degree of subdivision. A
direct measurement of temperature and pressure inside a material
during drying by the VM method is very complicated and verifica-
tion of the theoretical considerations by
is thus dif-
ficult. Nonetheless, it might be true that the morphological
structure of whole garlic cloves (
), which
makes water release during drying difficult, favours generation of
a larger intracellular pressure during microwave heating at low-
ered external pressure.
3.3. Relative volume
Decreasing moisture content till about 0.6 g water/g dry matter
was accompanied by decreasing relative volume of the dried mate-
rial. On exceeding this value no further change in volume of the
slices was observed, though for whole cloves and halves a marked
increase in volume was visible (
). It should, however, be noted
that each measurement was done after taking the sample out of
the drier.
The changes in relative volume were described with a square
function (
). The shrinkage was greater at 240 W microwave
power and moisture content over 0.6 g water/g dry matter, com-
pared with samples dried at 480 and 720 W that had similar values
of relative volume. Slices dried at 240 W till 0.11 g water/g dry
matter had the smallest relative volume of all the samples dried
with the VM method. It was, however, by 0.1 g water/g dry matter
greater than the relative volume of slices dried with the convective
method. In this method a linear shrinkage behaviour of food mate-
rials was reported (
Lozano et al., 1983; Sjoholm and Gekas, 1995
),
whereas during air-microwave drying two shrinkage periods and
also higher relative volume were observed (
).
3.4. Texture
Based on the results of compressive strength tests on garlic
cloves (
), it was noticed that lowering of moisture content
to 0.6 g water/g dry matter did not cause considerable change in
the breaking stress
r
max
, which was 1.25 MPa independently of
the microwave power applied. Lowering of moisture content below
0.6 g water/g dry matter caused an increase in
r
max
according to an
exponential function (
). It can thus be noted that increasing
microwave power increases the compressive strength of the dried
material, though the large values of standard deviations diminish
significance of the differences observed. The increase in garlic
cloves strength caused by lowered moisture content below 0.6 g
water/g dry matter is connected with increase in relative volume
(
). Since lowered water content below 0.6 g water/g dry mat-
ter was accompanied by increase in temperature (
), it should
be assumed that 0.6 g water/g dry matter is a critical moisture con-
tent. Apparently, before reaching the critical moisture content the
microwave power is mainly absorbed by the water present in the
intercellular space, which evaporates readily guaranteeing con-
stant, rather low temperature. Meanwhile the water yet in the cells
and cell walls insures their elasticity and thus tendency of the
material to shrinkage as the turgor pressure weakens. On exceed-
ing the critical moisture content, due to the lack of water in the
intercellular spaces, the microwaves are absorbed only by the
water that still remains in the cells and walls. Evaporation of that
water is much more difficult and hence the increase in pressure in
the cells is accompanied by temperature increase. The pressure
developed in the cells causes their walls to expand, enabling the
material inside to gain in volume. This phenomenon is called puff-
ing. Removal of water from cellular walls results in their increasing
stiffening and thus in fixation of the puffing effect. Stiffening of the
walls results, of course, in increased compressive strength of the
material.
3.5. Water absorption
The absorption capacity AC of water from humid air by garlic
samples placed in a climatic chamber (
) is described by the
exponential function:
AC ¼ a ð1 e
t
b
Þ
ð5Þ
Parameter a is an asymptotic value of AC while b is the time
content of the process (
). Increasing degree of dry material
subdivision results in increased AC, which can be explained by in-
Fig. 4. Relative volume of whole garlic cloves (w), halves (h) and slices (s)
dehydrated by VM method at microwaves power 240, 480 and 720 W.
Fig. 5. Compressive strength of whole garlic cloves dehydrated by VM method at
microwaves power 240, 480 and 720 W.
102
A. Figiel / Journal of Food Engineering 94 (2009) 98–104
creased surface for water absorption from the surrounding air. The
rehydration ratio of microwave dried carrot slices increased also
with decreasing slice thickness (
). Increased
power of the microwaves also induce increase in AC, while in the
case of slices the differences caused by power increase disappear
in the course of absorption (no significant differences in AC were
observed for the last ten hours of rehydration test). This might
be due to vicinity of the saturation state for slices, which were
characterized by an absorption dynamics that was stronger than
that of halves or whole garlic cloves. The rehydration properties
of microwave-convection dehydrated carrot cubes were also im-
proved when higher microwave powers were applied (
).
The dynamics of water absorption by slices induced by the con-
vection method was similar to the one exhibited by slices dried
with 480 W of microwave power. The effect of power on AC is dif-
ficult to explain on the basis of drying shrinkage analysis only
(
). It seems that smaller shrinkage should favour water
absorption. Nonetheless, respective samples dried with the same
power are characterized by similar shrinkage. Apparently, the
quicker process of drying driven by larger power resulted in fixa-
tion of such changes in the cellular structure that stimulated water
absorption from humid air.
3.6. Color assessment
Assuming the parameter L
*
, a
*
and b
*
values determined for
fresh material as a basis for color assessment, it was found that
drying with the VM method made the color brighter, shifting it a
little towards red and a bit more towards blue; whereas drying
slices with the convective method lessened the color brightness,
shifting it markedly towards red and yellow (
). Regarding
only the samples dried with the VM method, it was found that of
brightest color were whole cloves dried at 720 W, and darkest
were slices dried at 240 W. Chopping the cloves into slices caused
the dried material color to shift towards red, the effect being the
largest in the case of slices dried at 720 W, whose color was the
most shifted towards blue as well. Comparing the results pre-
sented with color assessment of garlic dried with the microwave
method only (
), it can be stated that low-
ered pressure during microwave irradiation results in brighter col-
ors shifted towards blue.
3.7. Volatile oils content
On the basis of the results presented in
it can be stated
that the smallest loss in volatile oils during VM drying occurred
with microwaves of the largest 720 W power. Retention of volatile
oils was also favoured by increased subdivision of the dried mate-
rial. The content of volatile oils in fresh material was 17.3 cm
3
/kg
dry matter In slices dried at 720 W it fell to 15.8 cm
3
/kg dry matter,
while in whole cloves dried at 240 or 480 W it amounted barely to
9.5 cm
3
/kg dry matter The content of volatile oils of 12.2 cm
3
/kg
dry matter in slices dried with the convective technique was
slightly smaller than that in whole cloves at 480 W and slightly lar-
ger than the one found in whole cloves dried at 720 W. It turns out
that in garlic dried by the microwave-convective method till mois-
ture content 0.06 g water/g dry matter the content of volatile oils is
markedly lower and amounts to only 5.6 cm
3
/kg dry matter (
4. Conclusions
The decrease in moisture content of whole garlic cloves, halves
and slices during drying by the VM method can be described by a
sigmoid function. For the process of drying a critical point of mois-
ture content of 0.6 g water/g dry matter was found. Beyond that
point an increase in the dried garlic samples temperature was no-
ticed and no further decrease in volume of garlic slices was ob-
served, though for whole cloves and halves a marked increase in
volume was visible as an effect of puffing. The increase in volume
of whole cloves was associated with an increase in breaking stress,
Fig. 6. Absorption capacity of whole garlic cloves (w), halves (h) and slices (s)
dehydrated by VM method at microwaves power 240, 480 and 720 W.
Table 2
Effect of drying conditions on the quality attributes of dehydrated garlic samples.
Drying conditions
Microwave power (W)
Color parameters
Volatile oils content (g/kg d.m.)
L
*
a
*
b
*
w
240
71.58
a
1.68
b
13.2
f
9.54
480
72.03
b
2.41
c
16.41
a
8.84
720
74.77
c
2.33
d
16.9
ab
11.67
240
70.74
d
0.97
e
16.32
de
12.04
h
480
72.57
e
1.92
f
14.65
d
12.15
720
72.98
f
2.18
g
15.99
ae
14.61
240
67.46
g
0.32
h
18.29
c
13.37
s
480
71.52
a
1.37
a
17.57
bc
12.77
720
71.56
a
1.35
a
17.5
bc
15.84
Convection
59.8
h
7.2
i
28.87
g
12.2
Fresh garlic
63.74
i
2.68
j
21.58
h
17.32
a, b, c, d, e, f, g, h, i, j: different superscripts within columns indicate significant differences (Duncan test, p < 0.05).
A. Figiel / Journal of Food Engineering 94 (2009) 98–104
103
as estimated in strength compressive tests. Increasing microwave
power increased the breaking stress as well and additionally in-
creased the drying rate and water absorption capacity. Slicing the
garlic gloves also increased the absorption capacity and prolonged
the time of VM drying despite of the degree of subdivision. How-
ever, drying time of sliced cloves was approximately 10 times low-
er compared to convective drying. Drying of garlic samples with
the VM made the color brighter, shifting it towards red and blue,
compared to fresh garlic gloves and to slices dehydrated in convec-
tion. The best retention of volatile oils was observed for garlic
slices dehydrated with microwaves of the largest power.
Acknowledgements
This work was supported by the Polish Ministry of Science and
Higher Education under Grant No. N312 031 32/2036.
References
Attiyate, Y., 1979. Microwave-vacuum drying: first industrial application. Food
Engineering 51 (2), 78–79.
Ayabe, M., Sumi, S., 1998. Establishment of a novel tissue culture method, stem-disc
culture, and its practical application to micropropagation of garlic (Allium
sativum L.). Plant Cell Reports 17, 773–779.
Condori, M., Echazu, R., Saravia, L., 2001. Solar drying of sweet pepper and garlic
using the tunnel greenhouse drier. Renewable Energy 22, 447–460.
Cui, Z.-W., Xu, S.-Y., Sun, D.-W., 2004. Microwave-vacuum drying kinetics of carrot
slices. Journal of Food Engineering 65, 157–164.
Drouzas, A.E., Schubert, H., 1996. Microwave application in vacuum drying of fruits.
Journal of Food Engineering 28, 203–209.
Drouzas, A.E., Tsami, E., Saravacos, G.D., 1999. Microwave/vacuum drying of model
fruit gels. Journal of Food Engineering 39, 117–122.
Durance, T.D., Wang, J.H., 2002. Energy consumption, density, and rehydration rate
of vacuum-microwave and hot-air convection-dehydrated tomatoes. Journal of
Food Science 67 (6), 2212–2216.
Figiel, A., 2006. Drying kinetics and drying shrinkage of garlic subjected to vacuum-
microwave dehydration. Acta Agrophysica 7 (1), 49–58.
Gunasekaran, S., 1999. Pulsed microwave-vacuum drying of food materials. Drying
Technology 17 (3), 395–412.
Henderson, S.M., 1974. Progress in developing the thin layer drying equation.
Transactions of the ASAE 17, 1167–1172.
Kaensup, W., Chutima, S., Wongwises, S., 2002. Experimental study on drying of
chilly in a combined microwave-vacuum-rotary drum dryer. Drying Technology
20 (10), 2067–2079.
Khraisheh, M.A.M., Cooper, T.J.R., Magee, T.R.A., 1997. Microwave and air drying I.
Fundamental considerations and assumptions for the simplified thermal
calculations of volumetric power absorption. Journal of Food Engineering 50,
207–219.
Khraisheh, M.A.M., McMinn, W.A.M., Magee, T.R.A., 2004. Quality and structural
changes in starchy foods during microwave and convective drying. Food
Research International 37, 497–503.
Krulis, M., Kuhnert, S., Leiker, M., Rohm, H., 2005. Influence of energy input and
initial
moisture
on
physical
properties
of
microwave-vacuum
dried
strawberries. European Food Research Technology 221, 803–808.
Le Loch-Bonazzi, C., Wolff, E., Gilbert, H., 1992. Quality of dehydrated cultivated
mushrooms (Agaricus bisporus): a comparison between different drying and
freeze-drying processes. Lebensmittel Wissenschaft und Technologie 25, 334–
339.
Lewicki, P.P., 1998. Some remarks on rehydration of dried foods. Journal of Food
Engineering 36, 81–87.
Lewicki, P.P., Jakubczyk, E., 2004. Effect of hot-air temperature on mechanical
properties of dried apples. Journal of Food Engineering 50, 307–314.
Lin, T.M., Durance, T.D., Scaman, C.H., 1998. Characterization of vacuum-microwave,
air and freeze-dried carrot slices. Food Research International 31 (2), 111–117.
Lin, T.M., Durance, T.D., Scaman, C.H., 1999. Physical and sensory properties of
vacuum-microwave dehydrated shrimp. Journal of Aquatic Food Product
Technology 8 (4), 41–53.
Lozano, J.E., Rotstein, E., Urbicain, M.J., 1983. Shrinkage, porosity and bulk density of
foodstuffs at changing moisture contents. Journal of Food Science 48, 1497–
1502.
Madamba, P.S., Driscoll, R.H., Buckle, K.E., 1996. The thin layer drying characteristic
of garlic slices. Journal of Food Engineering 29, 75–97.
Maskan, M., 2000. Microwave/air and microwave finish drying of banana. Journal of
Food Engineering 44, 71–78.
Maroulis, Z.B., Kiranoudis, C.T., Marinos-Kouris, D., 1995. Heat and mass transfer
modelling in air drying of foods. Journal of Food Engineering 26, 113–130.
McMinn, W.A.M., Khraisheh, M.A.M., Magee, T.R.A., 2003. Modelling the mass
transfer during convective, microwave and combined microwave-convective
drying of solid slabs and cylinders. Food Research International 36, 977–983.
Nussinovitch, A., Hershko, V., 1996. Gellan and vegetable coatings. Carbohydrate
Polymers 30, 185–192.
Pabis, S., Jaros, M., 2002. The first period convection drying of vegetables and the
effect of shape-dependent shrinkage. Biosystems Engineering 81 (2), 201–211.
Pezzutti, A., Crapiste, G.H., 1997. Sorptional equilibrium and drying characteristic of
garlic. Journal of Food Engineering 31, 113–123.
Polish Standard, 2001. Spices, Condiments and herbs-determination of volatile oil
content. PN – ISO 6571 (in Polish) in accordance with ISO 6571:1984 (in
English).
Prabhanjan, D.G., Ramaswamy, H.S., Raghavan, G.S.V., 1995. Microwave-assisted
convective air drying of thin layer carrots. Journal of Food Engineering 25, 283–
293.
Prakash, S., Jha, S.K., Datta, N., 2004. Performance evaluation of blanched carrots
dried by three different driers. Journal of Food Engineering 62, 305–313.
Sacilik, K., Unal, G., 2005. Dehydration characteristic of Kastamonu garlic slices.
Biosystems Engineering 92 (2), 207–215.
Sanga, E.C.M., Mujumdar, A.S., Raghavan, G.S.V., 2002. Simulation of convection-
microwave drying for a shrinking material. Chemical Engineering and
Processing 41, 487–499.
Sham, P.W.Y., Scaman, C.H., Durance, T.D., 2001. Texture of vacuum-microwave
dehydrated apple chips as affected by calcium pretreatment, vacuum level, and
apple variety. Journal of Food Science 66 (9), 1341–1347.
Sharma, G.P., Prasad, S., 2001. Drying of garlic (Allium sativum) by microwave-hot
air combination. Journal of Food Engineering 50 (2), 99–105.
Sharma, G.P., Prasad, S., 2002. Dielectric properties of garlic (Allium sativum L) at
2450 MHz as function of temperature and moisture content. Journal of Food
Engineering 52, 343–348.
Sharma, G.P., Prasad, S., 2004. Effective moisture diffusivity of garlic cloves
undergoing microwave-convective drying. Journal of Food Engineering 65,
609–617.
Sjoholm, I., Gekas, V., 1995. Apple shrinkage upon drying. Journal of Food
Engineering 25, 123–130.
Sunjka, P.S., Rennie, T.J., Beaudry, C., Raghavan, G.S.V., 2004. Microwave-convective
and microwave-vacuum drying of cranberries: a comparative study. Drying
Technology 22 (5), 1217–1231.
Szarycz, M., 2001. Mathematical modelling of microwave-convection drying of
agricultural raw material on the example of apples (in Polish). Zeszyty Naukowe
Akademii Rolniczej we Wrocławiu, Rozprawy CLXXXIII 420, 2–79.
Vega-Mercado, H., Gongora-Nieto, M.M., Barbosa-Canovas, G.V., 2001. Advances in
dehydration of foods. Journal of Food Engineering 49, 271–289.
Wang, J., Xiong, Y.-S., Yu, Y., 2004. Microwave drying characteristics of potato and
the effect of different microwave powers on the dried quality of potato.
European Food Research Technology 219, 500–506.
Wang, J., Xi, Y.S., 2005. Drying characteristic and drying quality of carrot using a
two-stage microwave process. Journal of Food Engineering 68, 505–511.
Watada, A.E., Ko, N.P., Minott, D.A., 1996. Factors affecting quality of fresh-cut
horticultural products. Postharvest Biology and Technology 9, 115–125.
Witrowa-Rajchert, D., 1999. Rehydration as an Index of Changes Occurring in Plant
Tissues During Drying (in Polish). Academic Press, Rozwoj SGGW, Warsaw,
Poland. p. 80.
Yongsawatdigul,
J.,
Gunasekaran,
S.,
1996.
Microwave-vacuum
drying
of
cranberries: Part II. Quality evaluation. Journal of Food Processing and
Preservation 20, 145–156.
104
A. Figiel / Journal of Food Engineering 94 (2009) 98–104
Document Outline
- Drying kinetics and quality of vacuum-microwave dehydrated garlic cloves and slices
Wyszukiwarka
Podobne podstrony:
Effect of drying conditions on the quality of vacuum microwave dried potato cubes
Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum microwav
Drying kinetics and drying shrinkage of garlic subjected to vacuum microwave dehydration (Figiel)
Drying kinetics and drying shrinkage of garlic subjected to vacuum microwave dehydration (Figiel)
Drying kinetics and rehydration characteristics of microwave vacuum and convective hot air dried mus
Effect of vacuum microwave drying on selected mechanical and rheological properties of carrot
Drying kinetics and quality of potato chips undergoing different drying techniques
Headspace Volatiles and Physical Characteristics of Vacuum microwave, Air, and Freeze dried Oregano
Microwave Drying of Parsley Modelling, Kinetics, and Energy Aspects
Microwave drying characteristics of potato and the effect of different microwave powers on the dried
Drying characteristics and drying quality of carrot using a two stage microwave process
Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying
Microwave–vacuum drying kinetics of carrot slices (Zheng Wei Cui, Shi Ying Xu, Da Wen Sun)
Effect of Drying Techniques and Storage on Mulberry (Morus alba) Quality
Accelerated Drying of Single Hardwood Boards by Combined Vacuum Microwave Application
Influence of different microwave seed roasting processes on the changes in quality and fatty acid co
Use of exponential, Page’s and diffusional models to simulate the drying kinetics of kiwi fruit
więcej podobnych podstron