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Effect of drying techniques on extraction of Astragalus pieces.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2011; 6: 301–304
Published online 29 March 2010 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.433
Short Communication
Effect of drying techniques on extraction
of Astragalus pieces
Di Qianqian, Yang Junhong* and Zhao Jun
School of Mechanical Engineering, Tianjin University, Tianjin 300072, China
Received 28 September 2009; Accepted 24 November 2009
ABSTRACT: To investigate the effect of different drying techniques on the quality of Chinese herb medicine (CHM),
drying experiments of Astragalus pieces after rehydration were carried out, and four different kinds of drying techniques
viz microwave drying, freeze-drying, vacuum drying and hot-air drying, were examined in this paper. The microstructure
of dried samples was characterized by means of scanning electron microscope (SEM), and the molecular weight
distribution of Astragalus pieces extract was characterized by means of ultrafiltration. Results show that dehydrated
rate by microwave drying is five times faster than those by others three drying techniques. The effective component
content in the extract of sample dried by microwave drying, with the molecular weight of less than 4000 Da, is higher
than those by other three drying techniques. Observations of the SEM images show that the microstructure of sample
dried by microwave drying is much similar to that of raw Astragalus pieces, indicating that microwave technique helps
keep the penetrability of inner transmission tissues and thin-wall cell tissues, and benefits to the extraction of effective
components of Astragalus pieces.  2010 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: drying method; Astragalus pieces; molecular weight; extract microstructure
INTRODUCTION
Drying is an important step in processing of Chinese
herb medicine (CHM) pieces and it is the foundation
of CHM storage, preservation of quality and clinic
curative effect. Some papers concentrated on the effects
of drying methods on physical properties of plant
products.[1 – 3] Drying parameters also have influences
on product qualities such as effective constituent content
and chemical constitution.[4 – 6]
According to the concept of holism of traditional
Chinese medicine theory, an appropriate extraction
separation technique for traditional Chinese medicine
should make the separated product possess a plenty of
components in a certain molecular weight section.[7,8]
Generally, the molecular weight of most effective
components is smaller, and that of some ineffective
constituents such as cellulose, amylum and protein is
larger. Astragalus is one of the widely used species
of traditional CHM, and the particular component,
Astragalus saponin with a molecular weight of less than
4000 Da is considered as one of the main effective
components in Astragalus.
*Correspondence to: Yang Junhong, School of Mechanical
Engineering, Tianjin University, Tianjin 300072, China.
E-mail: yangjunhong@tju.edu.cn
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
In order to develop the suitable drying technology of CHM pieces, four different drying techniques,
microwave drying, hot-air drying, vacuum drying and
freeze-drying, were tested to examine the effects on
product qualities of Astragalus pieces. The ultrafiltration membrane technique was applied to characterize
the molecular weight distribution of aqueous Astragalus
extract, and the environment scanning electron microscope (SEM) was applied to characterize the microstructure change of Astragalus pieces.
MATERIALS, APPARATUSES AND METHODS
Materials and apparatus
Astragalus pieces were provided and identified by Tianjin Medicine Research Institute, China. Distilled water
was used as the extraction solvent. Membranes were the
0.45 µm microfiltration membrane and 4000 Da ultrafiltration membrane.
Main apparatus include microwave fast reaction system (Shanghai Yiyao Analysis Instrument Limited
Company, WF-4000, China), vacuum drying apparatus (The Fourth Instrument Factory of Dalian, Model
668, China), electric heating blast oven (Tianjin Taisite
Instrument Company, 101-1, China), vacuum freezedrying apparatus (Tianjin University of Commerce,
302
D. QIANQIAN, Y. JUNHONG AND Z. JUN
Asia-Pacific Journal of Chemical Engineering
Table 1. Drying methods and operating conditions.
Drying methods
Microwave drying
Freeze-drying
Vacuum drying
Hot-air drying
Operating conditions
Power: 400 W
Cooling temperature:
−40 ◦ C
Vacuum pressure: 10 Pa
Shelf temperature at 40 ◦ C
Vacuum pressure:
20 mmHg
Heating temperature: 40 ◦ C
Temperature: 50 ◦ C
Drying
times (h)
2
10
10
<12
China), refrigerator (Gree, KFR-35GW/K, China), SCM
cup ultrafilter (Shanghai Institute of Applied Physics,
Chinese Academy of Sciences, China), electronic balance (Shanghai METTLER-TOLEDO, 0.1 mg, China),
and SEM (Hitachi X-650, Japan).
Figure 1. Dehydration curves of the sample of Astragalus
freeze-drying; vacuum
pieces ( microwave drying;
drying; hot-air drying).
Drying experiments
Astragalus aqueous extract in the dried sample were
determined.
Thirty grams of raw Astragalus pieces was fully rehydrated in distilled water. Drying experiments of the
samples were carried out with four different kinds of
drying techniques to determine the dehydration curves.
Operating conditions and drying times are listed in
Table 1.
Microstructure of the dried samples
The microstructures of parenchyma cell located in
the inner layer of dried samples of Astragalus pieces
with different drying methods were observed under the
scanning electron microscopy (SEM; Hitachi X-650,
Japan).
Effective component contents in the dried
samples
Ten grams of the dried samples were soaked in 100 ml
of distilled water for 2 h. The extract liquid was
obtained in the microwave fast reaction system for
10 min at 400 W, 110 ◦ C. Afterwards, such extract
liquid was filtrated by means of the microfiltration
membrane (0.45 µm) and the ultrafiltration membrane
with the molecular weight of 4000 Da in sequence
under the circumstance of nitrogen gas at 0.2 MPa.
In the end, 5 ml of filtrate was taken and dried
in the electric heating blast oven until the mass of
the sample remains constant. The molecular weight
distributions and the effective component contents of
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
°
RESULTS AND DISCUSSIONS
Figure 1 shows the dehydration curves with four different kinds of drying techniques. It is seen that drying
time by microwave drying is much shorter than those
by other three drying techniques; freeze-drying and vacuum drying have almost the same drying time of about
10 h, and drying time by hot-air drying is the longest.
Moreover, hot-air drying does not reach a residual water
content of the sample as low as those of the former three
drying techniques.
Figure 2 shows the SEM images of parenchyma cell
located in the inner layer of the samples dried with four
different kinds of drying techniques, as well as that of
the raw Astragalus piece. It is seen that the sample
dried by microwave drying is similar to that of the
raw pieces, while the other three drying techniques all
make the microstructure of parenchyma change greatly.
The cytoplasm of parenchyma by freeze-drying and
hot-air drying clings to the cell wall closely. The one
by vacuum drying is separated obviously with the cell
wall.
The microstructure characteristics of the samples
dried by different drying techniques are related to the
dehydration mechanism during the drying process. For
the freeze-drying, expansion in moisture volume during change of liquid water into ice in sample, followed by ice sublimation, results in a loose structure and poor mechanical property. With regard to
the vacuum drying, the water in sample is also easily evaporated into vapor, resulting in the shrinkage
Asia-Pac. J. Chem. Eng. 2011; 6: 301–304
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF DIFFERENT DRYING TECHNIQUES ON ASTRAGALUS PIECES
Table 2. The content of solute with some range of
molecular weight in Astragalus water extracts and its
relative percentage.
Less than 4000 Da
Drying
technique
(a) Initial sample
(b) Microwave drying
(c) Freeze-drying
(d) Vacuum drying
Microwave drying
Freeze-drying
Vacuum drying
Hot-air drying
Less than
0.45 µm
content (mg)
Content
(mg)
Relative
percentage
(%)
1762
1872
1826
1648
1440
1248
1360
1198
81.7
66.7
74.5
72.7
of the cytoplasm of parenchyma. For the hot-air drying, the surface convective transfer of heat and mass
is the main mechanism of sample dehydration; characteristics of thin-wall cell looks like that by freezedrying with stronger mechanical property. Concerning
the microwave drying, microwave heating mechanism
is different from traditional conduction, convection and
radiation of heat.[9] The outstanding characteristic is
inner heating mechanism. There is no obvious cytoplasm flowing inside parenchyma cell, which results
in well-preserved structure parenchyma cell (shown in
Figure 4-b) and maintaining the permeability of moisture transfer path of sample dried by microwave drying
technique.
Table 2 shows the contents of solute with certain range of molecular weight in Astragalus aqueous
extracts and their relative percentages. The extract content of sample dried by microwave drying, with the
molecular weight of less than 4000 Da, is higher than
those by other three drying techniques, and is 1.2 times
as much as that by hot-air drying. The relative percentage of small molecule effective constituent in extract
dried by microwave drying is also higher than those
by other three drying techniques, which has important
meanings to the modernization research and development of CHM.
CONCLUSIONS
(d) Hot-air drying
Figure 2.
SEM images of
parenchyma of Astragalus pieces
dried by different drying methods: (a) initial sample (b) microwave drying; (c) freeze-drying;
(d) vacuum drying and (e) hot-air
drying.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Four different kinds of drying techniques were used
to investigate the effects on the quality of Astragalus
pieces, a CHM. Results show that dehydrated rate of
microwave drying is much shorter than those of others
three drying techniques. The microstructure characteristics and quality of the dried sample by different drying
techniques are related to the dehydration mechanism
and its quality. Microwave technique is helpful to keep
the penetrability of inner transmission tissues and thinwall cell tissues during the drying process of sample,
which is beneficial to the subsequent extraction of effective components within Astragalus pieces. Therefore,
the microwave drying technology is advantageous to
Asia-Pac. J. Chem. Eng. 2011; 6: 301–304
DOI: 10.1002/apj
303
304
D. QIANQIAN, Y. JUNHONG AND Z. JUN
the dehydration of Astragalus pieces, not only for its
shorter drying time, but also for easy extraction of its
effective components.
Acknowledgements
We thank Researcher Zhang Tiejun of Tianjin Institute
of Pharmaceutical Research for providing the identification of Chinese herb.
Asia-Pacific Journal of Chemical Engineering
[2] R. Omidbaigi, F. Sefidkon, F. Kazemi. Flavour Fragnace J.,
2004; 19, 196–198.
[3] B. Raghavan, L. Rao, M. Singh, K. Abraham. Nahrung, 1997;
41, 159–161.
[4] B. Joanna, M. Marek, B. Wioletta. J. Food Eng., 2007; 81,
306–312.
[5] L.H. Chin, C. Wenlug, M.W. Yih, Y.T. Chin. Food Chem.,
2003; 83, 85–92.
[6] D.F. Ren, Z.H. Mao, J.Z. Wang. Chin. J. Chem. Eng., 2004; 12,
822–825.
[7] J. Zhou. J. Tradit. West. Med., 1998; 2, 67 (in Chinese).
[8] L.W. Guo, W.Q. Jin. Film Sci. Technol., 2002; 22, 46–49 (in
Chinese).
[9] M.K. Krokida, Z.B. Maroulis. Drying Technol., 1999; 17,
449–466.
REFERENCES
[1] S. Ingvald, A.F. Odilio, M.E. Trygve, C.C. Ingrid. Chin.
J. Chem. Eng., 2004; 12, 814–817.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2011; 6: 301–304
DOI: 10.1002/apj
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