Comparative decomposition kinetics of neutral monosaccharides by microwave and induction heating treatments

Article abstract of DOI:10.1016/j.carres.2013.04.013

The stabilities of five neutral monosaccharides (glucose, galactose, mannose, arabinose, and xylose) were kinetically compared after the molecules were submitted to microwave heating (internal heating) and induction heating (external heating) under completely identical thermal histories by employing PID (proportional, integral, and derivative) temperature controlled ovens and homogeneous mixing. By heating in water at 200 C, the rate constants for the decomposition reactions varied from 2.13 × 10^-4 to 3.87 × 10^-4 s^-1 for microwave heating; however, the values increased by 1.1- to 1.5-fold for induction heating. Similarly, in a dilute (0.8%) sulfuric acid solution, the decomposition rate constants varied from 0.61 × 10^-3 to 2.00 × 10^-3 s^-1 for microwave heating; however, the values increased by 1.5- to 2.2-fold for induction heating. The results show that microwave heating imparts greater stability to neutral monosaccharides than does induction heating. The undesirable decomposition of monosaccharides at the surface boundary of reactor walls may have increased the probability of monosaccharide decomposition during induction heating.

Full text of DOI:10.1016/j.carres.2013.04.013

Carbohydrate Research 375 (2013) 1–4
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Note
Comparative decomposition kinetics of neutral monosaccharides
by microwave and induction heating treatments
a
b
b
c
Shuntaro Tsubaki a, , Kiriyo Oono , Ayumu Onda , Kazumichi Yanagisawa , Jun-ichi Azuma
⇑
a Oceanography Section, Science Research Center, Kochi University, Akebono-cho 2-5-1, Kochi City, Kochi 780-8520, Japan
b Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi City, Kochi 780-8520, Japan
c Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The stabilities of five neutral monosaccharides (glucose, galactose, mannose, arabinose, and xylose) were
kinetically compared after the molecules were submitted to microwave heating (internal heating) and
induction heating (external heating) under completely identical thermal histories by employing PID (pro-
portional, integral, and derivative) temperature controlled ovens and homogeneous mixing. By heating in
water at 200 °C, the rate constants for the decomposition reactions varied from 2.13 Â 10À4 to
3.87 Â 10À4 sÀ1 for microwave heating; however, the values increased by 1.1- to 1.5-fold for induction
heating. Similarly, in a dilute (0.8%) sulfuric acid solution, the decomposition rate constants varied from
0.61 Â 10À3 to 2.00 Â 10À3 sÀ1 for microwave heating; however, the values increased by 1.5- to 2.2-fold
for induction heating. The results show that microwave heating imparts greater stability to neutral
monosaccharides than does induction heating. The undesirable decomposition of monosaccharides at
the surface boundary of reactor walls may have increased the probability of monosaccharide decompo-
sition during induction heating.
Received 15 February 2013
Received in revised form 11 April 2013
Accepted 12 April 2013
Available online 19 April 2013
Keywords:
Decomposition kinetics
Neutral monosaccharides
Microwave heating
Induction heating
Ó 2013 Elsevier Ltd. All rights reserved.
Lignocellulosic and algal biomass resources are promising
renewable feedstocks for producing bioenergy and bio-based
materials for the establishment of sustainable industries. These
biomass feedstocks are predominantly composed of polysaccha-
rides; therefore, the advanced handling of sugars has become very
important. One of the main schemes of biomass processing is to
hydrolyze polysaccharides into fermentable monosaccharides,
which can be converted into ethanol and lactic acid. At the same
time, the extracted polysaccharides can be used to produce binder
for particle boards,1 films, and coatings,2 and bio-based plastics.3
However, due to the high recalcitrance of biomass structures, the
requirement of any severe pretreatment is to make cellulase more
accessible to cellulose.4 Most biorefining processes require ele-
vated temperatures above 150 °C, which makes the process time
longer and leads to undesirable energy consumption. Therefore,
fast-heat-transferring technologies such as electromagnetic wave
heating provide a promising approach to biorefining processes.
Microwaves are electromagnetic waves with frequencies be-
tween 300 MHz and 300 GHz. Microwaves are widely used in tele-
communication, radar, as well as heating devices in domestic
kitchens. Dipoles, electrolytes, conductive materials, and magnetic
materials are effectively heated due to dielectric heating and
induction heating under an electromagnetic field. These unique
heating properties have previously been used to improve the reac-
tion rate, selectivity, and product yield of many chemical reac-
tions.5 Microwave heating has also been successfully employed
in biorefining processes such as pretreatments for lignocellulosic
materials,6 food waste processing7–9 and the extraction of value-
added products.10–12
However, the mechanism that induces the effects of microwave
irradiation on biomass components is still unknown; for example,
standing waves in microwave reactors generate inhomogeneous
reaction temperatures, which result in the overestimation of
microwave effects.13 For instance, Mijovic et al. reported that no
significant difference could be observed between microwave and
conventional heating treatments for epoxy-amine reactions.14
Therefore, it is very important that a precise comparison of the ef-
fects of microwave and other conventional processes on biomass
components having the same thermal history be performed. Re-
cently, we succeeded in employing induction heating in a conven-
tional oven that exhibited the same rapid thermal history as that
obtained by microwave ovens.15 In the case of microwave heating,
the reactant and reaction media are heated from within, while in
the case of induction heating, reactants were heated from without
through the conduction of heat from the reactor wall. The results
regarding the application of these heating methods to hydrolyze
cellobiose indicate that higher selectivity of glucose production
could be obtained by microwave heating, while induction heating
⇑
Corresponding author. Tel./fax: +81 88 844 8927.
E-mail addresses: stsubaki@kochi-u.ac.jp, shuntaro.tsubaki@gmail.com (S. Tsu-
baki).
0008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carres.2013.04.013

2
S. Tsubaki et al. / Carbohydrate Research 375 (2013) 1–4
produces a greater amount of secondary decomposed materials,
suggesting a difference in the stability of the hydrolyzed products
generated by these methods. In this study, therefore, we performed
a thorough comparative investigation of the stability of five neutral
monosaccharides submitted to microwave and induction heating
treatments.
The degradation behaviors of five neutral monosaccharides in
water are shown in Figure 1A. The prolonging of the reaction time
to approximately 30 min at 200 °C caused the decomposition of the
monosaccharides due to autohydrolysis. When the concentrations
of the monosaccharides (X) in terms of Àln(1 À X) were plotted
against reaction time (s), good linear positive correlations were ob-
tained. Therefore, the decomposition of the monosaccharides fol-
lowed first-order kinetics, the decomposition rate constants (k)
are listed in Table 1. The decomposition rate constant of monosac-
charides under microwave irradiation varied from 2.13 Â 10À4 to
3.87 Â 10À4 sÀ1 depending on the monosaccharide type. The most
stable monosaccharide under microwave heating in water was gal-
actose, followed by glucose. On the other hand, the decomposition
rate of xylose was the highest among the five sugars. Previously,
enthalpy–entropy compensation was proposed to determine the
degradation properties of hexoses and pentoses under hydrother-
mal conditions.16,17 A similar decomposition tendency was ob-
served during induction heating (Fig. 1A) and demonstrated
similar first-order decomposition kinetics. The decomposition rate
constants (k) for induction heating varied from 2.39 Â 10À4 to
4.38 Â 10À4 sÀ1. By comparing the decomposition rate constants
between the microwave and induction heating treatments, the lat-
ter show values that are 1.1–1.5 times greater than those of the
former.
The production of 5-HMF and furfural was determined by HPLC
as an indicator of decomposition, and the results are shown in Fig-
ure 1B. After 30 min of reaction, approximately 40–50 mg of 5-
HMF was produced from hexose, and 60–80 mg of furfural was
produced from pentose. Heat-labile monosaccharides such as xy-
lose and mannose produced larger amounts of furfural and 5-
HMF, respectively, relative to the amounts produced by the other
monosaccharides. The amount of furfural produced from pentose
by induction heating was, on average, 1.6- to 1.7-fold greater than
that produced by microwave heating, whereas the amount of 5-
HMF produced from hexose was slightly lower (1.1- to 1.4-fold
greater on average).
The pH values of the monosaccharide solutions after both heat-
ing treatments gradually decreased with reaction time, as listed in
Table 3. The production of organic acids, such as levulinic and for-
mic acids,18 reduces the pH of such solutions. Induction heating
yielded a lower pH than microwave heating, following a trend sim-
ilar to that observed for monosaccharide decomposition and 5-
HMF/furfural production. The overall results indicate that the sta-
bility of monosaccharides induced under microwave heating is
higher than that induced by induction heating.
Subsequently, dilute acid treatment (0.8% sulfuric acid) was
conducted at 180 °C according to the methods of Saeman,19 and
the degradation behaviors of the neutral monosaccharides are
shown in Figure 2A. The decomposition rate constants (k) are listed
Table 1
Kinetic parameters of decomposition of neutral monosaccharides under hydrother-
mal conditions (200 °C) in water using microwave and induction heating
Neutral monosaccharide
Microwave heating
Induction heating
k (sÀ1
)
R2
k (sÀ1
)
R2
Figure 1. Stability of neutral monosaccharides under hydrothermal conditions
(200 °C) in water. (A) Residual monosaccharide rate, (B) 5-HMF/furfural yield.
Glucose
Galactose
Mannose
Arabinose
Xylose
2.31 Â 10À4
2.13 Â 10À4
2.86 Â 10À4
2.61 Â 10À4
3.87 Â 10À4
0.953
0.974
0.979
0.996
0.998
3.02 Â 10À4
2.39 Â 10À4
4.28 Â 10À4
3.00 Â 10À4
4.38 Â 10À4
0.933
0.987
0.983
0.988
0.998
in Table 2, which are well fitted by first-order kinetics. The decom-
position rate constants achieved by microwave heating varied from
0.61 Â 10À3 to 2.00 Â 10À3 sÀ1. Similar to the results obtained for

S. Tsubaki et al. / Carbohydrate Research 375 (2013) 1–4
3
Previously, Saeman reported that the decomposition rate con-
stants of neutral monosaccharides in 0.8% sulfuric acid were
0.40 Â 10À3 to 1.2 Â 10À3 sÀ1 by using a glass-bomb-type reactor.19
The rate constants achieved in this study were higher, most likely
due to rapid energy transfer by microwave and induction heating.
However, the stabilities of the monosaccharides against dilute sul-
furic acid treatments followed the same order indicated in the pre-
vious report: glucose > galactose > mannose > arabinose > xylose.
The presence of sulfuric acid in the hydrolysis media promoted
the decomposition of the neutral monosaccharides by a factor of
2.54–5.17 in the case of microwave heating, whereas an increase
by a factor of 3.54–8.77 was achieved in the case of induction
heating.
The amount of 5-HMF produced by the sulfuric acid treatment
of hexoses increased with reaction time up to 10 min; however,
longer reaction times resulted in the loss of 5-HMF due to second-
ary reactions. Induction heating in sulfuric acid solution produced
1.15- to 1.83-fold (in average) higher amounts of 5-HMF than
microwave heating (Fig. 2B), but the yield was much smaller than
that produced by heating in water, indicating that 5-HMF is unsta-
ble under strong acidic condition. On the other hand, the amount of
furfural produced from pentoses was up to 60–70 mg, comparable
to the data obtained for heating in water. Induction heating
showed slightly higher (1.02- to 1.14-fold on average) production
of furfural than microwave heating.
The overall results indicate that higher monosaccharide stabil-
ity was achieved under microwave treatment than under induction
heating, regardless of the presence of dilute sulfuric acid. A similar
tendency was observed for the hydrolysis of cellobiose.15 This
could be due to differences in the mode of energy transfer between
microwave and induction heating: internal heating and external
heating. In induction heating, nonselective heat transfer at the sur-
face boundary of the reactor walls may have resulted in a higher
probability of the unexpected decomposition of monosaccharides
into fructose, erythrose, 1,6-anhydroglucose, glyceraldehyde, dihy-
droxyacetone, and saccharinic acid through epimerization and the
degradation of the sugar backbones.20 On the other hand, inside of
a reactor, microwave radiation selectively heats substances from
within and does not show overheating effects at reactor walls,
leading to lower damage to monosaccharides. Therefore, the inter-
nal heating property of microwave heating was concluded to have
an advantage over external heating with respect to the production
of monosaccharides from biomass, which could allow for effective
decomposition in hydrothermal and dilute sulfuric acid
pretreatments.
1. Experimental
1.1. Microwave heating treatments
Microwave heating was performed using a START D multimode
microwave oven (Frequency; 2.45 GHz, max output; 1 kW, Mile-
stone Inc., Shelton, USA) and HPR-100 TFM reactor (100-mL closed
Table 2
Kinetic parameters of decomposition of neutral monosaccharides in dilute sulfuric
acid (180 °C) using microwave and induction heating
Figure 2. Stability of neutral monosaccharides in dilute sulfuric acid (180 °C). (A)
Residual monosaccharides rate, (B) 5-HMF/furfural yield.
Neutral monosaccharides
Microwave heating
Induction heating
k (sÀ1
)
R2
k (sÀ1
)
R2
Glucose
Galactose
Mannose
Arabinose
Xylose
0.61 Â 10À3
0.75 Â 10À3
1.09 Â 10À3
1.18 Â 10À3
2.00 Â 10À3
0.999
0.998
0.981
0.998
1.000
1.07 Â 10À3
1.23 Â 10À3
2.05 Â 10À3
2.63 Â 10À3
3.08 Â 10À3
0.979
0.971
0.831
0.949
0.995
treatments in water, hexoses were more stable than pentoses. The
decomposition rate constants achieved by induction heating ran-
ged from 1.07 Â 10À3 to 3.08 Â 10À3 sÀ1, which were 1.54- to
2.23-fold higher than those achieved by microwave heating.

4
S. Tsubaki et al. / Carbohydrate Research 375 (2013) 1–4
Table 3
cooled by a fan; the cooling time was the same as that required
for the microwave experiments (15 min). The initial concentra-
tions of the neutral monosaccharides and volumes of the reactants
were the same as those used in the microwave experiments.
pH values after hydrothermal decomposition of neutral monosaccharides
Reaction time (min)
Microwave heating
Induction heating
Glucose
0
5
10
20
30
4.48
3.71
3.46
3.25
3.17
4.28
3.55
3.33
3.1
1.3. Analysis
The concentrations of monosaccharides, 5-hydrolxymethylfurf-
ural (5-HMF) and furfural after heating treatments were deter-
mined by using HPLC (Hitachi, Ltd, LaChrom Elite System, Tokyo,
Japan) equipped with a Shodex SC1011 column (Showa Denko
K.K., Tokyo, Japan, 8.0 Â 300 mm) and RI detector operated at
80 °C using water as an eluent at a flow rate of 1 mL/min. The con-
centrations of monosaccharides, 5-hydrolxymethylfurfural (5-
HMF), and furfural after dilute sulfuric acid reactions were deter-
mined by using HPLC (Hitachi, Ltd) equipped with a Shodex
SH1011 column (Showa Denko K.K., 8.0 Â 300 mm) operated at
50 °C using 1 N H2SO4 as an eluent at a flow rate of 1 mL/min
and an RI detector. The pH values of the reactant solutions after
hydrothermal treatment were measured by a SevenGo pH meter
(Mettler Toledo, LLC, OH, USA).
3.09
Galactose
0
5
10
20
30
4.16
3.58
3.44
3.24
3.15
3.9
3.51
3.27
3.15
3.12
Mannose
0
5
10
20
30
4.12
3.6
3.46
3.25
3.16
4.02
3.54
3.33
3.16
3.08
Arabinose
0
5
10
20
30
4.77
3.64
3.41
3.21
3.12
3.91
3.43
3.33
3.13
3.07
Acknowledgments
Part of this study was supported by the Program to Disseminate
the Tenure Tracking System of the Ministry of Education, Culture,
Sports, Science and Technology. The Japanese Government, and
Adaptable and Seamless Technology Transfer Program through
Target-driven R&D, Japan Science and Technology Agency
(AS242Z03337N).
Xylose
0 min
5 min
10 min
20 min
30 min
4.08
3.49
3.33
3.13
3.07
3.87
3.4
3.33
3.13
3.07
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