Preparation of garlic powder with high allicin content by
using combined microwave–vacuum and vacuum drying as well
as microencapsulation
Yu Li
a,b
, Shi-Ying Xu
b,*
, Da-Wen Sun
c,*
a
College of Food Science and Technology, Henan Agricultural University, Zhengzhou, Henan 450002, PR China
b
State Key Laboratory of Food Science and Safety, School of Food Science and Technology, Southern Yangtze University, Wuxi, Jiangsu 214036, PR China
c
Biosystems Engineering, University College Dublin, National University of Ireland, Earlsfort Terrace, Dublin 2, Ireland
Received 23 November 2006; received in revised form 26 February 2007; accepted 26 February 2007
Available online 7 March 2007
Abstract
Garlic powder with high allicin content was prepared using microwave–vacuum and vacuum drying as well as microencapsulation in
order to protect alliinase activity throughout the human stomach and improve the ratio of alliin transforming into allicin. The results
showed that the optimal microwave–vacuum drying condition was drying for 3 min under the microwave output power 376.1 W, then
282.1 W for 3 min, followed by 188.0 W for 9 min, and finally for 3 min under the output power 94.0 W. The thiosulfinates retention after
drying was 90.2%. Following drying, garlic powder was microencapsulated by modified fluidized bed technique. Scanning electron micro-
scope revealed good integrity and core materials that were embedded in microcapsules. Studies on the release kinetics of microencapsu-
lated garlic granulates in vitro using simulated intestinal fluid indicated that release of garlic powder could be controlled in intestine by
passing through human stomach conditions.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Garlic; Allicin; Thiosulfinates; Microwave–vacuum drying; Garlic powder; Microencapsulation
1. Introduction
Since ancient times, garlic has been used worldwide as a
seasoning, spices, and herbal remedies (
Garlic is known to possess a vast variety of biological func-
tions such as antimicrobial (
), antithrombotic (
), and anticancer (
), antioxidant (
Farhath, 1997; Prasad, Laxdal,
), improving immune-system (
), as well as ability to lower
serum lipid and glucose levels (
Lawson, Wang, & Papadimitriou, 2001
) and blood pressure
(
Ali, Al-Qattan, Al-Enezi, Khanafer, & Mustafa, 2000
).
Since the identification of the thiosulfinate allicin in 1944,
many studies have been focused on the thiosulfinates of
garlic. The characteristic flavors of fresh garlic (Allium sat-
ivum) are associated with thiosulfinates (RS(O)SR) and the
volatile substances are formed by the action of the enzyme
alliinase (EC4.4.1.4) on hydrolyzing S-alkyl-substituted
cysteine sulfoxide derivatives to the corresponding alkyl
alkane thiosulfinates, ammonia, and pyruvic acid (
). This enzyme is separated from its natural sub-
strates until the garlic tissue is disrupted. Since the nonpro-
tein amino acid alliin (S-allyl-
L
-Cysteine Sulfoxide) is the
major substrate in garlic, allicin (diallylthiosulfinate) is
the main thiosulfinates and constitutes 60–80% of total
0260-8774/$ - see front matter
Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jfoodeng.2007.02.057
*
Corresponding authors. Tel./fax: +86 510 5884496 (S.-Y. Xu).
E-mail addresses:
(S.-Y. Xu),
(D.-W. Sun).
URLs:
,
(D.-W. Sun).
www.elsevier.com/locate/jfoodeng
Journal of Food Engineering 83 (2007) 76–83
garlic thiosulfinates (
Block, 1992; Lawson, Han, & Han,
). The following shows alliinase-catalyzed reaction of
alliin into allicin, pyruvic acid and ammonia:
Many recent studies have provided strong evidence that
most of these biological functions of garlic are attributed to
allicin. In fact, no compound outside the thiosulfinates (of
which allicin is about 60–80%) has been found that
accounts for a significant portion of the pharmacological
activities of crushed garlic at levels representing normal
human consumption (2–5 g/day). And these biological
effects of thiosulfinates can be related to their strong SH-
modifying and antioxidant properties (
).
In Japan and West countries, garlic products have been
popular and marketed in recent years as healthy foods with
beneficial physiological effects for humans. Consequently,
the majority of the garlic supplements sold today is garlic
powder tablets that are standardized on allicin (
et al., 2001; Ali et al., 2000
). However, the quality of garlic
products is questionable. Many garlic products appear to
undergo harsh processing (
). Although garlic slices dehydrated by freeze
drying (FD) is in excellent quality, it is also one of the most
expensive processes for manufacturing dehydrated garlic
slices due to large capital outlays and high operating costs.
In recent years, microwave–vacuum drying (MVD) has
been investigated as a potential method for obtaining high
quality dried foodstuffs. MVD combines the advantages of
both vacuum drying and microwave drying, and it can
improve energy efficiency and product quality. Several
fruits and grains have been successfully dried by the
MVD (
Erle & Schubert, 2001; Nidhal & Mohammed,
2002; Sham, Scaman, & Durance, 2001
). However, there
is little information available on garlic slices dried by the
use of MVD (
). In this paper garlic
slices were dried using the new drying method – micro-
wave–vacuum drying combined with vacuum drying
(MVD/VD) to produce high potency allicin-containing
garlic powder for healthy foods.
It has been established that garlic powder and granules
can serve as an important nutritional supplement. How-
ever, it has also been found that if garlic powder or gran-
ules were stored for long periods, active ingredients
present in freshly ground garlic were often eliminated or
otherwise rendered inactive. The alliinase is irreversibly
deactivated at the pH level in the human stomach. If taking
garlic powder directly there would be only an insignificant
amount of allicin that can be produced inside the human
body. Therefore, in this study garlic powder was microen-
capsulated and coated by materials, which could resist
human stomach conditions in order to prolong the shelf life
and protect alliinase activity through the stomach. In this
way allicin could be released only in the intestines, resulting
in decreasing the characteristic odor and aftertaste.
2. Materials and methods
2.1. Materials
Fresh garlic bulbs were obtained from local market in
Wuxi, China. The garlic bulbs were subjected to mild pres-
sure by hand to separate them into cloves. The cloves were
peeled, cut into two pieces using a kitchen knife and the
slices were dried as soon as possible.
L
-Cysteine, 5,5
0
-
dithio-bis (2-nitrobenzoic acid) (DTNB) and Hepes were
purchased from Sigma Chemical Co. (St. Louis, MO).
2.2. Drying methods
The fresh garlic slices were put in a thin layer and dried
to a final moisture content of about 5% (wet basis), either
by freeze drying (FD) or microwave–vacuum (MVD),
which was designed and built by the author (
), combined with vacuum drying (VD, DZG-6050,
Senxin Ltd., Shanghai, China). FD (freeze drying) was con-
ducted using a laboratory freezing dryer (FD, LGJ-10, Sih-
uan Ltd., Beijing, China) with the plate temperature at
45
°C and absolute pressure at 10 Pa.
MVD with VD was performed with 120 g garlic slices
each time. There were four levels of output microwave power
used: 376.1 W, 282.1 W, 188 W and 94 W. Following MVD,
VD at 40
°C and absolute pressure at 4 kPa was conducted to
dry the garlic slices to a final moisture content of 5%. During
MVD, the rotation speed of the turntable was 5 rpm.
Response surface methodology (RSM) was used to
design the MVD/VD method to dry garlic slices. RSM data
were submitted to the analysis of response surface by using
Statgraphic Version 5 statistical package (Manugistic Inc.,
Rockville, MD, USA).
2.3. Color measurement
Comminion international de I’E classage (CIELAB) L
*
(whiteness), a
*
(red-green), and b
*
(yellow-blue) values of
dried garlic powder were measured using a colorimeter
(WSC-S, Precision Instrument Ltd., Shanghai, China).
The colorimeter was calibrated with a white standard plate.
Triplicate measurements were performed for each sample.
Allicin
Pyruvate
Alliin
Alliinase
+H
2
O
CO O
-
CH
2
S
CH
2
CH
N H
3
+
O
2 H
2
C=CH
O
N H
4
+
+
2
2
CH
3
C
O
CO O
-
H
2
C=CH CH
2
+
S
S
CH
2
CH =CH
2
Y. Li et al. / Journal of Food Engineering 83 (2007) 76–83
77
2.4. Microencapsulation of garlic powder
The garlic slices dried by MVD/VD were ground into
garlic powder by a pulverizer (DFT-200, Dahai Ltd.,
China) of average diameter in the range of 30–100 mesh.
150 g of dried garlic powder was fluidized in a modified
fluid bed coater (FLP-3, Jiafa Ltd., China). The inlet tem-
perature was adjusted to achieve a product temperature of
30
°C. The garlic powder was then sprayed using with ethyl
cellulose (EC, 3%, w/w) in acetone and isopropanol as sol-
vent at the ratio of 9:1. The speed of spraying was adjusted
in order to obtain a good and homogeneous film on the
garlic powder.
The coated garlic powder by above-mentioned process
was further coated with a second layer of polymer solution,
cellulose acetate phthalate (CAP, 10%, w/w) in acetone and
isopropanol as solvent at the ratio of 1:1. The ratio of EC/
CAP was 2:3. The ratio of garlic powder to wall materials
was 4:1. The encapsulation yield, encapsulation efficiency
of microencapsulating garlic powder was 95.0% and
96.8%, respectively.
2.5. Microstructure determination
The microcapsules were placed on scanning electron
microscope (SEM) specimen holders using a double-coated
adhesive tape (Ted Pella, Redding, CA, USA) and then
coated with a thin layer of gold in a Fine Coat Ion Sputter
JFC 1100 (JEOL Ltd., Akishima, Japan). The surface
structure of microcapsules was observed by a high vacuum
JEOL SEM (JMS-35, JEOL, Ltd., Akishima, Japan) at
10 kV. The microcapsules were fractured using a razor
blade perpendicularly through a layer of capsules attached
to the specimen holder with a double-coated adhesive tape
(Ted Pella, Redding, CA, USA) to observe the structure.
2.6. Determination of thiosulfinates
Modified Lawson’s method (
) was
used for quantitative determinations of total thiosulfinates
in fresh garlic, garlic powder, and microencapsulated garlic
powder.
Fresh garlic cloves were peeled and homogenized in
5 ml/g of Hepes buffer (50 mM, pH 7.5). The homogenate
was allowed to stand at room temperature for 5–10 min to
ensure complete enzymatic conversion to the thiosulfinates.
Garlic juice was obtained by filtrating using filter paper.
The solution of cysteine was freshly prepared in 50 mM
Hepes buffer (pH 7.5). The concentration of cysteine was
determined by measuring the amount of 2-nitro-5-thi-
obenzoate (NTB) formed after reaction with 5,5
0
-dithio-
bis (2-nitrobenzoic acid) (DTNB). All the reactions were
carried out at 26
°C. Cysteine solution (5 ml) was added
into 1 ml distilled water, and 1 ml reaction mixture was
diluted to 100 ml. The diluted solution (4.5 ml) was incu-
bated in a cuvette with 0.5 ml 50 mM Hepes buffer (pH
7.5) containing 1.5 mM DTNB. The absorbance at
412 nm was measured after 15 min (A
0
) using a spectrofo-
tometer (722-ultraviolet/visible spectrofotometer, Shanghai
analysing apparatus Ltd., China). Cysteine solution (5 ml)
was added into 1 ml garlic juice, and the mixture was incu-
bated for 15 min. Reaction mixture (1 ml) was diluted to
100 ml. The diluted solution (4.5 ml) was incubated in a
cuvette with 0.5 ml 50 mM Hepes buffer (pH 7.5)
containing 1.5 mM DTNB. The absorbance at 412 nm
was measured after 15 min (A). Therefore, the concentra-
tion of thiosulfinates can be determined by the following
relation:
C
thiosulfinates
ðmmol=mlÞ ¼ ðDA
412
100Þ=ð2 14150Þ;
where A
412
= A
0
A, 14150 is the molar extinction coeffi-
cient of 2-nitro-5-thiobenzoate, and 2 means half amount
of cysteine reduced denotes the amount of thiousulfinates.
Garlic extract was prepared by homogenizing each prep-
aration of garlic powder with distilled water (1 g/15 ml).
Supernatant obtained by centrifugation at 3000 rpm was
used to determine thiosulfinates with the same method as
described above.
For microencapsulated garlic powder, 10 ml acetone
was added into 1 g microencapsulated garlic powder and
rubbed using a mortar for a few minutes to break the wall,
5 ml distilled water was added into it, mixed and allowed to
stand at room temperature for 1 min. The mixture was cen-
trifuged at 3000 rpm and supernatant was adjusted to
determination of thiosulfinates as the same before.
2.7. Release kinetics of thiosulfinates from
microencapsulated garlic powder
Simulated gastric juice was prepared with 9 ml HCl
added to 1000 ml distilled water and to adjust pH to
1.2. Simulated intestinal juice was prepared with 23.5 ml
H
3
PO
4
added to 950 ml distilled water and pH was
adjusted to 6.8 using 5 ml 4 N NaOH solution, distilled
to 1000 ml. In vitro, thiosulfinates release from microen-
capsulated garlic powder was determined with a horizon-
tal shaker method. Microspheres were placed in bottles
containing simulated intestinal juice without the digestive
enzymes normally found in intestinal fluid at a tempera-
ture of 37
°C. The mixture was shaken in a horizontal
shaker at a speed of 100 rpm. The samples were with-
drawn at predetermined time intervals and assayed
spectrophotometrically.
3. Results and discussion
3.1. Effect of vacuum pressures on inner temperature of
garlic slices
shows the average internal temperature of garlic
slices under three different vacuum pressures during
MVD when microwave output power was 219.7 W. It
can be seen that the internal temperature of garlic slices
increased with the decrease in vacuum pressure. Adopting
78
Y. Li et al. / Journal of Food Engineering 83 (2007) 76–83
a low vacuum pressure can reduce the evaporation temper-
ature. When the vacuum pressure was 4 kPa, the water
evaporation temperature was about 30
°C, while when
the vacuum pressure was 6 kPa, the water evaporation tem-
perature was about 35
°C. Therefore, using 6 kPa can
ensure the temperature of foodstuffs was about 35
°C dur-
ing the early stage of the microwave–vacuum drying. How-
ever, when the vacuum pressure was 8 kPa, the water
evaporation temperature was about 45
°C, which will be
too high to maintain alliinase activity. Therefore, all the
subsequent experiments were performed with the micro-
wave–vacuum drying at 6 kPa.
indicates that there are three drying stages during
MVD process, the first stage is an accelerating drying
stage (preheating period), the second one is a constant rate
drying stage (constant temperature drying stage) and the
third one is a falling rate drying stage (temperature
increasing drying stage). This observation was consistent
with the study conducted by Nidhal and Mohammed
(
). During the initial drying
phase, moisture content of garlic slices was high, and
the absorption of microwave energy for garlic slices was
quick, therefore, the temperature of garlic slices increased
linearly with the water vapor temperature corresponding
to vacuum pressure. This process usually lasts very short
time (about 3 min). As soon as the temperature of garlic
slices reached moisture vaporization temperature, the
inner and surface moisture of garlic would begin to evap-
orate. During the second stage, as the microwave energy
was used to vaporize the moisture in the sample, the tem-
perature of garlic slices was basically kept constant during
this period. During the falling rate drying stage or temper-
ature increasing drying stage, the availability of the inte-
rior moisture of garlic slices was insignificant. Although
the dielectric loss constant of garlic slices and the absorp-
tion of microwave energy decreased as moisture content
decreased, the total absorption energy was still greater
than the required latent heat of evaporation. Therefore,
the temperature of garlic slices increased, resulting in over-
heating of garlic slices and causing the inactivity of
alliinase.
3.2. Optimum drying conditions
Microwave can be absorbed efficiently by water. As
water is depleted, microwave absorption is reduced, leading
to lower thermal efficiency due to significant contribution
of sensible heating during drying. It was reported that
the thermal efficiency could be as low as 30% at the end
of the drying period when moisture content became small,
and the heating process was 100% efficient initially when
the moisture content was high even when vacuum was
not applied (
). As drying progressed
and moisture content decreased, the advantage of VD
became more evident due to the enhanced mass transfer
of the vapor at reduced pressure. Then, the drying effi-
ciency of VD becomes similar to MVD. Therefore,
although MVD was an efficient method for drying food,
it was not be so at the later stage of drying and must be
applied in a controlled environment. Maximum power
must be applied at the early stage of drying and the power
should be decreased as drying progresses to avoid waste of
energy and any damage to the microwave generating equip-
ment, and to get the high quality of foodstuffs. Because
only a limited amount of water was available during the
later stage of the MVD process, the temperature of garlic
was easy to rise and difficult to control, and the alliinase
of garlic becomes very susceptible to temperature. High
temperature could lead to alliinase inactivation and so alli-
inase could no longer hydrolyze alliin to allicin. Therefore,
VD would be an alternative way in this period. This com-
bination of MVD/VD can be an alternative way to obtain
high quality dehydrated garlic. When the moisture content
of garlic slices dried by MVD with absolute pressure at
6 kPa reached to about 15%, vacuum drying at 40
°C with
absolute pressure at 4 kPa would be adopted to continue
the drying of the garlic slices until the moisture content
becomes below 5%.
Four levels of microwave output power, i.e., A =
376.1 W, B = 282.1 W, C = 188.0 W, D = 94.0 W, were
used to dry garlic slices with different time. Thiosulfinates
retention was determined at different drying time for each
output power variables (
).
shows the anal-
ysis of variance for thiosulfinates retention using 3-factor
study. The data in
were used to develop
the following polynomial model:
Thiosulfinates
retention = 87.78
1.55*A 1.70*B +
0.32*C
0.40*D 8.29*A
2
2.34*B
2
1.44*C
2
+ 0.56*
D
2
2.78*A*B 3.35*A*C 3.52*A*D + 0.70*B*C +
1.43*B*D
0.55*C*D.
From
, it can be seen that the model fitted to the
experimental data well (P value was only 0.0003). The
model shows significant interaction effects between the
variables (P < 0.05) and indicates that the contribution of
each variable to the yield could be ranked in the following
order: B > A > C > D.
Response surfaces for the drying process were shown in
, where x and y axes are drying time in minutes. The
optimum drying condition was determined by the software
0
10
20
30
40
50
60
70
80
Drying time (min)
Inter
n
a
l tem
p
.of
g
a
rlic s
lice
s
0
0.5
1
1.5
2
2.5
M
o
is
tu
re
c
o
ntent (
k
g/k
g
db)
Fig. 1. Average internal temp. of garlic slices under three different vacuity
pressure and drying rate under 6 kPa during microwave–vacuum drying at
219.7 W output power.
Y. Li et al. / Journal of Food Engineering 83 (2007) 76–83
79
of response surfaces analyzing for thiosulfinates retention
as follows: drying for 3 min under the microwave output
power 376.1 W, then for 3 min under the output power
282.1 W, after that for 9 min under the output power
188.0 W, finally for 3 min under the output power
94.0 W, and the thiosulfinates retention was 90.2%. This
optimum condition was confirmed by further.
Until now, among different drying methods, FD is still
the best drying method to obtain high quality drying mate-
rials. The comparison of MVD/VD with FD is shown in
, from which it can be seen that FD was much more
time-consuming than MVD/VD, due to the fact that the
latter was typically two or three times more efficient than
the former in terms of water evaporation per unit energy
(
). Garlic slices that dried by
MVD/VD were, without statistical significance, a little
lighter and slightly higher in yellow hue than that of FD
samples, possibly due to the exposure to heat, resulting in
Millard reaction. During the later stage of the MVD/VD,
the Millard reaction could not be totally absent. However,
no significant loss of thiosulfinates occurred during the
microwave–vacuum and vacuum drying and thiosulfinates
retention of MVD/VD was above 90%. This was only
slightly less as compared with 93.6% of FD (
). As
mentioned above, FD has disadvantages in drying garlic
slices, such as expensive equipment, high energy consump-
tion and high cost. Therefore, it is obvious that garlic slices
dried by MVD/VD would be much more competitive in
market value than those dried by FD.
3.3. Microstructure of microencapsulated garlic
Scanning electron microscope (SEM) was employed to
investigate the surfaces and the internal structures of
microencapsules (
). SEM micrographs showed that
the whole surface of the microencapsulated garlic powder
granules were continuous and smooth, and presenting
irregular sphericity. Surfaces of all microcapsules displayed
integrity and compactness, so the wall of microencapsu-
lated garlic powder could protect the core material from
environment. As granulating was not avoidable during
Table 1
Response surface analysis of effect of microwave output power on
thiosulfinates retention
Microwave output power
Thiosulfinates
retention % (w/w)
376.1 W
282.1 W
188.0 W
94.0 W
Time (min)
Time (min)
Time (min)
Time (min)
5
3
15
7
69.9
1
7
5
7
79.7
3
5
5
5
85.7
3
5
10
5
89.2
3
5
10
3
87.9
3
5
15
5
85.3
5
7
5
3
74.6
3
5
10
5
87.8
3
5
10
5
88.9
3
5
10
5
88.6
3
5
10
5
89.4
3
7
10
5
82.9
1
3
5
3
71.6
5
5
10
5
77.1
1
7
15
7
87.6
3
3
10
5
86.3
5
7
15
3
71.3
5
3
5
7
78.2
1
5
10
5
80.2
1
3
15
3
78.9
3
5
10
7
87.1
Table 2
Analysis of variance for thiosulfinates retention – 3-factor study
Effect
Sum of squares
df
Mean square
F-Ratio
P-Value
Model
832.82
14
59.49
26.99
A
4.81
1
4.81
2.18
0.1903
B
5.78
1
5.78
2.62
0.1565
C
1.02
1
1.02
0.46
0.5209
D
0.32
1
0.32
0.15
0.7163
AB
12.32
1
12.32
5.59
0.0560
AC
89.78
1
89.78
40.73
AD
19.88
1
19.88
9.02
BC
3.92
1
3.92
1.78
0.2307
BD
3.25
1
3.25
1.47
0.2703
CD
2.42
1
2.42
1.10
0.3351
Total error
1.57
4
0.39
Total
846.05
20
*
Very significant.
73.5
77.1
80.7
84.3
87.9
Thiosulfinates
Retention(%)
1
2
3
4
5
5
8
10
13
15
376.1W
188W
81.3
83.0
84.7
86.4
88.1
Thiosulfinates
Retention(%)
3
4
5
6
7
5
8
10
13
15
282.1W
188W
Fig. 2. Response surfaces of effect of different microwave output power on thiosulfinates retention (x and y axes are drying time in minutes).
80
Y. Li et al. / Journal of Food Engineering 83 (2007) 76–83
preparation of microcapsules when air suspension method
was used, it is a common character for microcapsules pre-
pared with air suspension method to have irregular form.
Therefore, unlike the original fine powder granules, the
microencapsulated garlic powder exists as pieces of bigger
cohesive solids. There are two reasons to cause garlic pow-
der assembled and adhesive. One reason is that there exist
molecular and electrostatic forces among original fine gar-
lic powder granules and they incline to conglobate during
fluidization process on a fluid bed. The other reason is that
after wall solution was sprayed to the garlic powder, the
powder would adhere together with liquid bridge because
of wall solution having some degree of viscosity. With vol-
atilization of solvent, the adhesive granules would adhere
to each other with solid bridge of wall material. Hence,
the core of granules formed first, and then the core became
larger through two manners: aggregation and coating.
Through aggregation garlic powder and wall material
adhered together forming granules. This causes the surface
of final products rough and the shape of final products
irregular. Coating is a process that the core grew up with
fine garlic powder sedimentation on the core layer by layer
and so it makes the surface of final products smooth and
the shape of final products round. From the micrograph
it could be seen that microcapsules of garlic powder was
smooth on the surface and presented irregular sphericity
(
). Perhaps at the beginning aggregation predomi-
nated in the process of growing larger granules and then
coating followed in the remaining growing process.
3.4. Controlled release of microencapsulated garlic powder
After microencapsulated garlic powder was incubated in
simulated gastric juice at 37
°C for 2 h, thiosulfinates reten-
tion was still above 98%. This result demonstrated that the
wall of the product could resist the low pH and preserve
alliinase activity in gastral cavity. Then the same microen-
capsulated garlic powder was incubated in simulated gas-
tric juice for 2 h, and finally in simulated intestinal fluid
at 37
°C for 1 h. Under ideal conditions, controlled release
of core materials can follow zero-order, one-half order or
first-order kinetics equation.
Fig. 3. Microstructure of microencapsulated garlic powder. Outside structure (left), inside structure (right).
0
20
40
60
80
100
0
10
20
30
40
50
60
time (min)
release amount (%)
Fig. 4. Release kinetic curve of microencapsulated garlic powder.
time (min)
release amount (%)
y = 14.067x
R = 0.9983
2
0
10
20
30
0
0.5
1
1.5
2.5
2
Fig. 5. Release kinetic curve of microencapsulated garlic powder for
drying within the first 2 min.
Table 3
Compared of garlic quality dried by different drying methods
Drying method
Freeze drying
Microwave vacuum drying/vacuum drying
Drying time
(h)
L
*
a
*
b
*
% Thiosulfinates
retention (w/w)
Drying time
(h)
L
*
a
*
b
*
% Thiosulfinates
retention (w/w)
48
72.24
0.62
10.98
93.6
3.8
73.74
1.62
11.90
90.2
Y. Li et al. / Journal of Food Engineering 83 (2007) 76–83
81
show that the release of microencapsulated
garlic powder in simulated intestinal fluid followed the
zero-order linear release within the first 2 min (
), with
regression equation being Y = 14.067X and correlation
coefficient of R
2
= 0.9983, where X is incubated time
and Y is release content of thiosulfinates. From 2 to
50 min, the release of microencapsulated garlic powder
followed
first-order
with
regression
equation
being
Y =
0.0089X 0.2081 and correlation coefficient of
R
2
= 0.9809, as shown in
. From 50 to 60 min, the
release rate increased suddenly. This may be due to the dis-
appearance of the rate-determining step that the solvent of
aqueous solution of the core inside capsule diffused from
high concentration to surrounding outside capsules as a
result of cellulose acetate phthalate wall material crum-
bling. In this period, the release of microencapsulated gar-
lic powder followed zero-order, and the regression
equation was Y = 1.648X
4.08, with correlation coeffi-
cient of R
2
= 0.9985 (
4. Conclusions
Garlic powder was prepared by MVD/VD technology.
This technology can provide high allicin content with the
quality of the finished product as good as the product
prepared by FD. The optimal dry condition was drying
for 3 min under the microwave output power 376.1 W,
then 282.1 W for 3 min, followed by 188.0 W for 9 min,
and finally for 3 min under the output power 94.0 W.
The thiosulfinates retention after drying was about
90.2%. The structure of microencapsulated garlic powder
made by modified fluidized bed technique showed good
integrity and core materials were well embedded in micro-
capsules. The microencapsulated garlic powder could
resist stomach condition and its release could be con-
trolled in intestinal.
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y = -0.0089x - 0.2081
R
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= 0.9809
-0.8
-0.6
-0.4
-0.2
0
0
10
20
30
40
50
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log of residual amount o
f
capsule core
Fig. 6. Release kinetic curve of microencapsulated garlic powder for
drying from 2 to 50 min.
y = 1.648x - 4.08
R = 0.9985
2
60
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47.5
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52.5
55
57.5
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time (min)
release amount (%)
Fig. 7. Release kinetic curve of microencapsulated garlic powder for
drying from 50 to 60 min.
82
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83
Document Outline
- Preparation of garlic powder with high allicin content by using combined microwave-vacuum and vacuum drying as well as microencapsulation
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