Salt-assisted acid hydrolysis of starch to D-glucose under microwave irradiation

Article abstract of DOI:10.1016/S0008-6215(00)00311-6

The effect of inorganic salts on the hydrolysis of starch in a microwave field was investigated and it was found that some inorganic salts can effectively accelerate the acid hydrolysis of starch. The yield of D-glucose reached 111 wt% (equal to the theoretical yield).

Full text of DOI:10.1016/S0008-6215(00)00311-6

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Carbohydrate Research 331 (2001) 9–12
Salt-assisted acid hydrolysis of starch to
microwave irradiation
D
-glucose under
Li Kunlan, Xia Lixin, Li Jun, Pang Jun, Cao Guoying, Xi Zuwei *
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road,
Dalian 116023, People’s Republic of China
Received 12 November 2000; accepted 14 November 2000
Abstract
The effect of inorganic salts on the hydrolysis of starch in a microwave field was investigated and it was found that
some inorganic salts can effectively accelerate the acid hydrolysis of starch. The yield of -glucose reached 111 wt%
equal to the theoretical yield). © 2001 Published by Elsevier Science Ltd. All rights reserved.
D
(
Keywords: Salt-assisted; Acid hydrolysis of starch; Microwave irradiation; -Glucose
D
1
. Introduction
showed that starch (10%) in dilute hydrochlo-
ric acid (0.5 M) was completely hydrolyzed
with 5 min microwave irradiation without for-
mation of colored byproducts. Some investi-
gators noted that the presence of a large
number of ions, which can frequently enhance
effects of dielectric loss and microwave cou-
The effect of microwave heating has long
been studied in polymer chemistry and is used
industrially in vulcanization processes. How-
1
ever the application of microwave heating in
organic chemistry appeared only at the end of
the 1980s, with the pioneering work of Gedye
and Gigere. Since then, microwave chem-
istry has become a rapidly developing branch
in organic synthesis.
Microwave dielectric heating dissipates heat
inside the medium and raises the energy of the
molecules rapidly, and this heating mode is
thus quite different from conventional heat-
ing. Consequently, under microwave dielectric
heating, more molecules become energized,
and this usually results in higher reaction rates
and greater selectivity.
11
pling, could produce superheating. The addi-
2
,3
tion of salts to solvents can increase their
conductivity and has a dramatic influence on
12
their rate of heating. The present study was
conducted to examine the effect of inorganic
salts on the acid hydrolysis of starch under
microwave irradiation. We found that inor-
ganic salts could be coupled with microwave
irradiation to accelerate the reaction rate, and
that different kinds of salts manifest different
effects. The yield of
D-glucose obtained
reached 111 wt% (equal to the theoretical
yield) after the addition of inorganic salts.
When compared with conventional heating,
the reaction rate for the hydrolysis of starch
to glucose was accelerated 100 times under
microwave irradiation.
Some papers have reported the hydrolysis
4
–10
of starch in a microwave field.
Yu et al.
*
Corresponding author. Fax: +86-411-4684746.
E-mail address: huangw@ms.dicp.ac.cn (X. Zuwei).
0008-6215/01/$ - see front matter © 2001 Published by Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(00)00311-6

10
L. Kunlan et al. / Carbohydrate Research 331 (2001) 9–12
2
. Experimental
The vessel was sealed and heated in an oil-
bath at 145 °C for 5 h. The solution was then
Materials and equipment.—Soluble starch
brought to pHꢀ7 with 5% (w/w) NaOH. The
was purchased from the Shanghai Chemical
Reagents Plant, and a domestic microwave
oven of 2.45 GHz was used.
Methods.—Reaction of the starch with hy-
drochloric acid under microwa6e irradiation. In
a 70 mL thick-walled Teflon with a screw-cap
transparent to microwave radiation, 200 mg
of starch granules (water content was 11.6%)
was suspended in 2 mL of 0.05% (w/w) HCl,
and 1 mL of a solution of inorganic salts
amount of
D
-glucose was determined by glu-
cose oxidase analysis.
The percent yield of
D
-glucose was calcu-
lated by the following equation
mg
D
-glucose
%
yield=
×100
200 mg×(1−11.6%) starch
3
. Results and discussion
Table 1 gives the results of acid hydrolysis
−
2−
containing 0.15 mol/L [Cl ] (or [SO4 ]) was
added. The container could withstand pres-
sures up to 1.2 MPa. The reactor was irradi-
ated in a microwave field for different lengths
of time. The solution was then brought to
pHꢀ7 with 5% (w/w) NaOH. The amount of
of starch in the presence of metal halides in a
microwave field. Compared with the results of
Entry 1, which showed that the yield of -glu-
D
cose was 78.4% in 600 s without added metal
halides, the result of other entries show that
all metal halides studied accelerated the acid
D
-glucose in the resulting solution was deter-
1
3–15
mined by glucose oxidase analysis.
Reaction of the starch with HCl with oil-bath
heating at 145 °C. In a glass vessel, 200 mg of
starch granules (water content 11.6%) was sus-
pended in 2 mL of 0.05% (w/w) HCl, and then
hydrolysis of starch, and the yield of
cose was higher than 90%, except for AlCl
which gave a yield of only 25.6%. Among
D
-glu-
,
3
these metal halides, LiCl, BaCl , and FeCl
2
3
were the most effective catalysts for starch
1
mL of a solution of inorganic salts contain-
− 2−
ing 0.15 mol/L [Cl ] (or [SO4 ]) was added.
hydrolysis in a microwave field, and BaCl2
showed the best yield of 111.0%
120 s, indicating that starch was converted
completely into -glucose without any by-
D-glucose in
Table 1
The yield of
D-glucose in the acid hydrolysis of starch in the
D
a
presence of metal halides under microwave irradiation
products. It suggests that ‘selective heating’ in
a microwave field causes all of the starch
molecules to become hydrolyzed in a short
time, and efficiently inhibits decomposition of
c
Entry Metal halide
Yield of D-glucose (wt%)
1
20 s
180 s 240 s 600 s
78.37
105.8 85.1
96.0 81.1
99.2 60.4
96.5 83.4
91.1 90.0
111.0 70.1
45.1
the
D
-glucose product. This is significantly
b
1
0
0
0
different from conventional heating. The low
2
3
4
5
6
7
8
9
0
1
2
LiCl
NaCl
KCl
MgCl2
CaCl2
24.0
88.5
61.9
88.0
59.0
21.7
108.6
0.1
60.0
83.9
89.2
efficiency of AlCl is probably because it can
form H[AlCl ] with HCl, leading to a decrease
3
4
of ion concentration, which in turn decreases
the microwave coupling of the system.
BaCl2
The results suggest that the metal halide’s
ability to promote the hydrolysis of starch is
due to the salt’s ability to cause superheating
of the solution. Microwave heat involves a
direct interaction with certain classes of ab-
sorbing molecules. This direct absorption can
lead to a localized introduction of energy to a
region from the remote microwave source and
raises the solution temperature. It is well es-
tablished that the superheating effects caused
by microwave irradiation can lead to tempera-
FeCl3
AlCl3
NaBr
KBr
KI
25.6
4.4
1
1
1
92.3 69.3
90.1 82.6
77.7 48.8
a
Reaction was carried out with 200 mg of soluble starch,
.00 mL of 0.05% (w/w) hydrochloric acid, and 1 mL of 0.15
2
−
mol/L [Cl ] under 800 W of microwave power irradiation.
b
With 1.00 mL of H O to replace the salt solution to
2
maintain the reaction volume.
c
The theoretical yield of
D-glucose is 111%.

L. Kunlan et al. / Carbohydrate Research 331 (2001) 9–12
11
Table 2
The yield of
completed in a short time in the presence of
metal halides.
D-glucose in the acid hydrolysis of starch in the
a
presence of metal halides with conventional oil-bath heating
The yield of
D-glucose decreased dramati-
c
cally with 240 s of microwave irradiation be-
cause of higher temperature reached by the
Entry
Metal halide
Yield of D-glucose (wt%)
b
1
2
3
4
5
6
7
8
9
77.8
94.6
91.3
88.5
95.9
76.3
71.4
95.0
61.9
91.3
95.6
97.0
solution. Under these conditions,
D-glucose is
LiCl
NaCl
KCl
MgCl2
CaCl2
not stable at the higher temperature, and un-
dergoes oxidation and decomposes rapidly. In
practical applications, it may thus be difficult
to control the reaction time for the maximum
BaCl2
yield of
D-glucose.
FeCl3
Table 2 summarizes the results obtained
with conventional oil-bath heating. It is clear
AlCl3
NaBr
KBr
KI
10
11
12
that metal halides, except CaCl , BaCl and
2
2
AlCl , are effective catalysts for starch hydrol-
3
ysis. When compared with Table 1, these re-
sults suggest that microwave dielectric heating
can shorten the reaction time, enhance the
chemical yield, and improve the selectivity. In
a
Reaction was carried out with 200 mg of soluble starch,
2
0
.00 mL of 0.05% (w/w) hydrochloric acid, and 1 mL of
−
.15mol/L [Cl ] in an oil-bath at 145 °C for 5 h.
b
With 1.00 mL of H O to replace the salt solution to
2
maintain the reaction volume.
the case of BaCl , the large difference of cata-
2
c
The theoretical yield of
D-glucose is 111%.
lytic activity between conventional heating
and microwave dielectric heating is considered
to result from the ‘salt-assisted’ effects in a
microwave field. Barium chloride, which is
capable of strong coupling with microwaves,
can lead to much higher rates of heating than
in the conventional method.
Compared with conventional heating, mi-
crowave dielectric heating is an efficient proce-
dure which shortens the reaction time
100-fold.
Table 3
The yield of
D
-glucose in the acid hydrolysis of starch in the
a
presence of metal sulfates under microwave irradiation
c
Entry Metal sulfate
Yield of D-glucose (wt%)
120 s
180 s 240 s 600 s
b
1
0
0
0
78.37
2
3
4
5
6
Na SO4
MgSO4
8.0
7.7
7.0
10.0
88.9
12.6
7.8
6.9
66.8
88.9
9.1
4.4
4.7
50.9
80.7
2
ZnSO4
Further investigations were made on the
effect of metal sulfates in a microwave field.
As listed in Table 3, Na SO , MgSO , and
Al (SO )
2
4 3
Ce(SO4)2
2
4
4
a
ZnSO did not accelerate the acid hydrolysis
Reaction was run with 200 mg of soluble starch, 2.00 mL
of 0.05% (w/w) hydrochloric acid and 1 mL of 0.15 mol/L
4
of starch, and the highest yield of
D
-glucose
2
−
[
SO ] under 800 W of microwave power irradiation.
4
was only 12.6% in the presence of Na SO .
2
4
b
With 1.00 mL of H O to replace salt solution to maintain
2
However, when Ce(SO ) was employed, the
4
2
the reaction volume.
c
yield of
D
-glucose reached 88.9%, and thus
The theoretical yield of
D-glucose is 111%.
Ce(SO ) facilitates hydrolysis of starch more
4
2
than other sulfates in a microwave field. These
results suggest that the catalytic activities of
sulfates are lower than metal halides, and the
presence of sulfates, such as Na SO , MgSO
ture 10–30 °C in excess of the conventional
boiling point of the solvent. Furthermore, the
presence of salts in polar solvents frequently
enhances dielectric loss effects and microwave
coupling of the solvents. Consequently, super-
heating effects can be magnified by the addi-
tion of such ions as NaCl. Also it is possible
that localized superheating effects could lead
to a small but significant increase in the reac-
tion rate. The hydrolysis reaction can thus be
2
4
4
and ZnSO , even inhibit the acid hydrolysis of
4
starch. The reason is probably that sulfate
buffers the pH and decreases the acidity of
0.05% HCl because the hydrogen sulfate ion is
a weak acid
Table 4 shows the results with metal sulfates
under conventional heating. A maximum yield

12
L. Kunlan et al. / Carbohydrate Research 331 (2001) 9–12
Table 4
The yield of
yield of
D
-glucose reached 111 wt% with use
D-glucose in the acid hydrolysis of starch in the
of the metal halides, LiCl, BaCl , and FeCl ,
a
2
3
presence of metal sulfates with conventional oil-bath heating
with 120–180 s of microwave irradiation. The
difference between metal halides and sulfate
salts illustrates that the properties of the salts
are very important in affecting the hydrolysis
reaction in a microwave field.
c
Entry
Metal sulfate
Yield of D-glucose (wt%)
b
1
2
3
4
5
6
77.8
0
2
Na SO4
MgSO4
0
ZnSO4
Al (SO )
Ce(SO4)2
5.6
24.9
95.6
2
4 3
References
a
Reaction was run with 200 mg of soluble starch, 2.00 mL
of 0.05% (w/w) hydrochloric acid and 1 mL of 0.15 mol/L
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2
4
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.
1
ysis is accelerated significantly by the addition
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1
5
1
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drolyzed to
D
-glucose completely in a short
time by adding metal halides. The maximum