Examining of athermal effects in microwave-induced glucose/glycine reaction and degradation of polysaccharide from Porphyra yezoensis

Article abstract of DOI:10.1016/j.carbpol.2013.04.033

Many reports claim the existence of athermal effects in microwave-induced reactions, and this challenge the assumption that the thermal effect (heating) is the sole factor in microwave heating. Therefore, microwave-induced Maillard reaction of d-glucose/g

Full text of DOI:10.1016/j.carbpol.2013.04.033

Carbohydrate Polymers 97 (2013) 38–44
Contents lists available at SciVerse ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
Examining of athermal effects in microwave-induced glucose/glycine
reaction and degradation of polysaccharide from Porphyra yezoensisଝ
Cunshan Zhoua,b,c, Xiaojie Yua,b,c, Haile Maa,b,c,∗, Shulan Liua, Xiaopei Qina,
Abu El-Gasim A. Yagouba, John Owusua,d
a School of Food and Biological Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China
b Jiangsu Provincial Research Center of Bio-process and Separation Engineering of Agri-products, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013,
China
c Key Laboratory for Physical Processing of Agricultural Products, Jiangsu University, 301 Xuefu Road, Zhenjiang 212013, China
d School of Applied Science and Technology, Koforidua Polytechnic, P.O. Box 981, Koforidua, Ghana
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 March 2013
Received in revised form 26 March 2013
Accepted 12 April 2013
Available online 22 April 2013
Many reports claim the existence of athermal effects in microwave-induced reactions, and this chal-
lenge the assumption that the thermal effect (heating) is the sole factor in microwave heating. Therefore,
microwave-induced Maillard reaction of d-glucose/glycine and degradation of polysaccharide from Por-
phyra yezoensis (PSPY) were investigated. Browning reactions were monitored by measuring heating
rate, UV-absorbance and brown color, UV–vis and synchronous fluorescence spectra, GC/MS analysis and
intrinsic viscosity of degradation. Heating of d-glucose/glycine solution produced brown compounds
which were detected at A420, and the intermediate products, 2-acetylfuran and 5-methylfurfural, whose
fluorescence intensity evidenced their formation. Maximum emission of synchronous fluorescence spec-
tra of samples were at 430–440 nm and 370–390 nm. Both microwave and water bath heating did not
cause any compositional changes in the Maillard reaction products. All data failed to show any significant
athermal effects of compositional changes in the Maillard reaction products. It can be inferred that some
of the reports suggesting the existence of athermal effects, which could ascribe to the different set-up
obtained in not well temperature controlled microwave heating systems.
Keywords:
Maillard reactions
Microwave
Glucose
Glycine
Polysaccharide
© 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
1. Introduction
In conventional thermal processing, energy is transferred to
the material through convection, conduction, and radiation of heat
Domestic microwave ovens have been widely in use from the
beginning of the 1950s. Since the first study on the use of a
microwave system for pasteurization of milk (Hamid, Boulanger,
Tong, Gallup, & Pereira, 1969), several works on microwave milk
treatment, mainly on microbiological aspects, have been reported
(Villamiel, Corzo, Martínez-Castrob, & Olano, 1996). A report in
1986 demonstrated that microwave energy is also suitable for
accelerating organic reactions (Giguere, Bray, Duncan, & Majetich,
1986). This has led to a growing interest in the microwave-
assisted processes as tools in the synthesis of organic and inorganic
substances.
from the surfaces of the material. In contrast, the microwave energy
is delivered directly to materials through molecular interaction
with electromagnetic field. In heat transfer, energy is transferred
as a result of thermal gradient between two points, but microwave
heating involves the transfer of electromagnetic energy to thermal
energy, and thus it is a kind of energy conversion rather than heat
transfer. This difference in the way energy is delivered can result
in many potential advantages to using microwaves for processing
of materials (Thostenson & Chou, 1999).
Many researchers have reported on the non-thermal
phenomena that have been broadly termed “microwave effects”.
Examples of the microwave effect include enhanced reaction rates
of thermosetting resins during microwave curing (Marand, Baker,
& Graybeal, 1992) and faster densification rates in ceramics sinter-
ing (Janney & Kimrey, 1991). Although there is considerable debate
over the existence of microwave effects, many papers present
unexpected results that do not seem to be a consequence of reduced
thermal gradients which is within microwave processed materials.
Critics of the microwave effect often claim that differences can
be attributed to poor temperature measurement and control of
ଝ
This is an open-access article distributed under the terms of the Creative Com-
mons Attribution-NonCommercial-No Derivative Works License, which permits
non-commercial use, distribution, and reproduction in any medium, provided the
original author and source are credited.
∗
Corresponding author at: School of Food and Biological Engineering, Jiangsu
University, 301 Xuefu Road, Zhenjiang 212013, China. Tel.: +86 511 88790958;
fax: +86 511 88780201.
E-mail address: mhl@ujs.edu.cn (H. Ma).
0144-8617/$ – see front matter © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbpol.2013.04.033

C. Zhou et al. / Carbohydrate Polymers 97 (2013) 38–44
39
experimental conditions that result in systematic error. The exist-
ence (or non-existence) of microwave effects continues to be an
area of considerable debate and research.
Large numbers of experiments, both academic and industrial,
have been conducted over the past few decades to study the
enhancement of reaction rate under microwave heating not only
for endothermic reactions but also for various classes of reactions.
Numerous studies indicated that the overall progress of chemical
reactions can be enhanced by using microwave heating (Cherbanski
& Mogla, 2009; de la Hoz, Diaz-Ortiz, & Moreno, 2005; Kappe, 2008;
Leonelli & Mason, 2010). However, due to difficulties associated
with the measurement of local temperature/concentration within
the reacting domain, especially under reaction conditions, previ-
ous experimental studies lack quantitative correlation between
observed enhancement of reaction rate and microwave power
absorption within the reacting media.
Absorption of microwave may be highly non-uniform depend-
ing on the dimension and dielectric properties of the reacting
medium (Bhattacharya & Basak, 2008), which may cause dra-
matic improvement in the local reaction rate leading to observed
enhancement of the overall reaction rate. In the absence of
detailed information regarding the local reaction rate induced by
microwave, the observed acceleration of chemical reaction was
often speculated to be athermal effect of microwave radiations on
the chemical reactions (Herrero, Kremsner, & Kappe, 2008). This
necessitates detailed theoretical analysis of the microwave heat-
ing pattern and its “per se” effect on the local reaction rate in
order to quantify as well as provide better understanding about
the observed enhancement of reaction rate under microwave radi-
ations in the absence of experimental data.
Although the enhancement of chemical reactions by microwave
irradiation is known, few studies have been carried out on chem-
ical changes produced as a result of Maillard reaction caused by
microwave heating in a simplified model system. Non-thermal
effects are claimed to change the chemical, biochemical, or the
physical behavior of some systems (Shazman, Mizrahi, Cogan, &
Shimoni, 2007) while the temperature controlled and all other
parameters remain unaltered. The possibility of an athermal effect
was tested with small molecular (simplified) reaction and macro-
molecule degradation in a very well controlled, high radiation
intensity system in the paper. The purpose of this work was to
test the effect of microwave heating on Maillard reaction in a
d-glucose/glycine model system and the degradation of polysac-
charides.
2. Material and methods
2.1. Experimental set-up
The experiments were carried out in an 800 W domestic mod-
ified microwave system (WP800SL23-2, Galanz, China) with a
frequency of 2450 50 MHz. The schematic diagram of experimen-
tal set-up for the modified microwave system (MG08S-2B, Huiyan,
China) is shown in Fig. 1A. The experimental set-up consisted of a
three-necked glass flask, containing the reaction solution, placed in
the microwave cavity (350 mm × 215 mm × 330 mm) or in a water
bath in case of the control test. An overhead stirrer was placed
inside the reaction solution through a central hole at the top of
the three-necked glass flask. The microwave power was selected at
a linear adjustable level (0–800 W) and for various exposure times
(0–99 h).
Fig. 1. (A) Schematic diagram of the experimental system: (1) oven window, (2)
condenser, (3) stirrer, (4) infrared temperature detector, (5) microwave generator,
(6) microwave power controller, (7) platinum temperature sensor, and (8) three-
necked glass flask; (B) the relationship between anode current and microwaves
power in the microwave system; (C) the temperature increase of water in different
volumes by microwave irradiation.
generator is adjusted to an appropriate current. Measurement of
process temperature was done by platinum temperature sensor
connected to data acquisition system (temperature accuracy of
0.5 ◦C), and this was linked to proportional-integral-derivative
(PID) controller (TCG-6061, HUAYOU) for continuous recording of
data. The platinum temperature sensor was inserted into the reac-
tion solution and an infrared temperature detector was held just
above the reaction solution. However, the infrared temperature
Different microwave powers were produced by altering the
anode current supply of the microwave generator transformer
(Fig. 1B). Thus, in order to generate a microwave with a desired
microwave power density, the anode current of the microwave

40
C. Zhou et al. / Carbohydrate Polymers 97 (2013) 38–44
detector (Cole-Parmer Inc. 08406 series, IL, U.S.A) with an accuracy
of 1 ◦C was used for further reassurance. During experiments, the
readings of platinum temperature sensor and the infrared temper-
ature detector almost agreed with a difference of 1 or 2 ◦C.
L × I.D. 30 m × 0.25 mm, df 0.25 ␮m), combined with an Agilent-
5973 mass spectrometer equipped with an electron ionization (EI)
and quadruple analyzer. The temperatures of the ion source and
interface were set at 200 and 300 ◦C, respectively. A split injec-
tion (injection 1 ␮L, split ratio 40:1, v/v) and injector temperature
of 220 ◦C were employed and a mass scan range was from 40 to
300 amu. The electron energy was set at 70 eV. The oven tempera-
ture was programmed from 40 ◦C to 250 ◦C at a rate of 10 ◦C/min.
Total runtime was 30 min. Helium was used as a carrier gas with
initial pressure of 5 psi (Lee et al., 2012).
2.2. Maillard reaction
In order to test the formation of Maillard reaction products,
equimolar solutions (0.5 mol/L) of d-glucose and glycine were pre-
pared separately in a phosphate buffer (0.1 mol/L, pH 5.0), filtered
(0.2 ␮m, Schleicher & Schuell) and then mixed together for the
experiment as described by Shazman et al. (2007). The test solution
was heated by microwave or by the water bath (control). At pre-
determined heating time, the sample was withdrawn and cooled
immediately in ice water prior to analyses. The drop in tempera-
ture upon cooling resulted in the virtual termination of the reaction.
Each reaction mixture was prepared, heated and analyzed at least in
triplicate. The collected samples were lyophilized in a freeze dryer
(ALPHA 1-2, Martin Christ Inc., Osterode, Germany). The sample
was dissolved in acetone to prepare a solution with concentration of
1 mg/mL (w/v). Then 1 ␮L of this solution was injected into GC/MS
for analysis.
2.8. Determination of intrinsic viscosity
Intrinsic viscosity is an important index for studying degra-
dation of large biological molecules, because it is relative to the
change of molecular weight and molecular configuration (Cyrille
& Marc, 1989). The intrinsic viscosity [ꢀ] of PSPY was determined
by an Ubbelohde capillary (type Ф0.5–0.6 mm, 0.01187 mm2/s2) at
25 0.1 ◦C as in the method of our previous publication (Zhou, Yu,
Zhang, He, & Ma, 2012). The [ꢀ] value was determined by the mean
intercept of Huggins and Kraemer plots (Young & Lovell, 1991).
2.9. Statistical analysis
2.3. The degradation of polysaccharide from Porphyra yezoensis
All analyses were run in triplicate. Analysis of variance (ANOVA)
was performed and means comparison were done by Duncan’s
multiple range test at P < 0.05. Analysis was performed using a SPSS
package (SPSS 8.0 for windows, SPSS Inc., Chicago, IL).
Porphyra yezoensis was purchased from Nantong Lanbo Industry
Co. Ltd. (Jiangsu, China). Polysaccharide from P. yezoensis (PSPY)
was prepared as described in our previous study (Zhou & Ma, 2006).
The effect of microwave intensity (60 and 240 mA), with or without
H2O2 solution, on the degradation of PSPY (0.5%, w/w) at 85 ◦C was
examined.
3. Results and discussion
3.1. Temperature–time profiles of the microwave system
2.4. Measurement of browning
In order to inspect the heating characteristics of microwave sys-
tem, different volumes (20, 30, 40 and 60 mL) of distilled water
(the initial temperature was 17 ◦C) were prepared in the three-
necked glass flask, and the microwave anode current was set at
120 mA. The temperature–time profile of the heating characteris-
tics of the microwave system is shown in Fig. 1C. As can be seen
from Fig. 1C, the time lag of the microwave system was about
3–4 s. t-Test analysis revealed that there was no significant differ-
ence in terms of the rate of increase in temperature among water
of 20, 30 and 40 mL (linear regression analysis: 20 mL, slope of
1.56 0.13, r = 0.9902; 30 mL, slope 1.46 0.15, r = 0.9959; 40 mL,
slope of 1.35 0.12, r = 0.9989). However, the t-test indicated that
the rate of increase in temperature for the water volume 60 mL
was significantly lower than those for 20, 30 and 30 mL (linear
regression analysis: 60 mL, slope of 1.05 0.14, r = 0.9929). In addi-
tion, the results showed that the difference between 20 and 60 mL
had P = 0.0099, 30–60 mL (P = 0.0258), and 40–60 mL (P = 0.0479), at
least 0.05 levels above the difference. Accordingly, linear relation-
ship between temperature and microwave irradiation-time was
established.
The absorbance of Maillard reaction products of the samples
was measured according to the method of Ajandouz, Tchiakpe, Ore,
Benajibas, and Puigserver (2001). A 10-fold diluted aqueous sam-
ple solution was prepared and the browning intensity measured
at 420 nm, using a Cary 100 UV-Vis spectrophotometer (Shimadzu,
Kyoto, Japan).
2.5. Ultraviolet visible spectroscopy analysis
Light absorption measurements were made on a Shimadzu
double-beam spectrophotometer; model UV-2450 at 25 ◦C using
quartz cuvettes of 1 cm path length. The absorption spectra of
sample solutions were recorded in the wavelength range of
190–700 nm against deionized water as a blank.
2.6. Synchronous fluorescence spectra
All synchronous fluorescence (SyF) measurements were made
with a Hitachi F-4500 fluorescence spectrophotometer (Tokyo,
Japan) equipped with a plotter unit and a 1 cm quartz cell. The SyF
spectra were recorded with excitation between 200 and 800 nm
and with the following settings: 5 nm excitation and emission slits
width; wavelength difference between the excitation and emission
monocromators of 50 nm; scan rate of Super. The spectra were dig-
itized at every 1 nm throughout the spectral range (Esteves da Silva,
Machado, & Oliveira, 1997).
3.2. The effects of microwave radiation on Maillard reaction
The chemistry involved in Maillard reaction is complex. There-
fore, we suggest that, if any of the many stages of this process
are athermally affected by microwave radiation, we may expect a
detectable effect on the formation of the final color pigments. Thus,
using d-glucose/glycine solutions, we explored this possibility by
heating these solutions in a similar manner, using both microwave
and conventional water bath. Fig. 2A and B shows the results of a
triplicate measurement made at absorbance of 420 nm. A signifi-
cant difference in the absorbance readings of solutions heated by
the microwave and the water bath was observed (Fig. 2A). Possible
2.7. Gas chromatography–mass spectrometric analysis
GC/MS analysis was performed with an Agilent 6890 N gas chro-
matography instrument (SUPELCOWAX®10 Capillary GC Column,

C. Zhou et al. / Carbohydrate Polymers 97 (2013) 38–44
41
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(A)
(A)
4.0
Maillard reaction at 90ºC
Microwave
60 mA
3.5
Maillard reaction at 90ºC
3.0
Water Bath
Microwave
180mA
1.395
120mA
180mA
240mA
Water bath
as control
2.5
292nm
60 mA
2.0
1.5
1.0
0.5
0.0
-0.5
120mA
240mA
1.004
0
30
60
90 120 150 180 210 240 270 300
200
300
400
500
600
700
Time/min
(B) 1.0
0.9
Wavelength/nm
(B)
550
500
450
400
350
300
250
200
150
100
50
Water bath
90ºC
0.8
Maillard reaction at 90ºC
Water Bath
Microwave
60 mA
91ºC
92ºC
0.7
0.6
0.5
120mA
180mA
0.4
0.254
0.3
0.2
0.1
0.0
-0.1
-0.2
0
0
30
60
90 120 150 180 210 240 270 300
Time/min
-50
200
300
400
500
600
700
800
Wavelength/nm
Fig. 2. The effect of different microwave anode currents (A) and water bath tem-
peratures (B) on the development of brown colors due to Maillard reactions.
Fig. 3. Spectra of UV absorption (A) and synchronous fluorescent scanning (B) at
different microwave anode currents.
explanation of the phenomenon was ascribed to variations or slow
drift found in the readings of the spectrophotometer (Shazman
et al., 2007). There was no consistent pattern in these results; e.g.
sometimes the microwave heating seems to produce slightly higher
readings and sometimes vice versa. Since the drift could not be
accounted for quantitatively, another test was conducted where the
solutions were heated at temperatures of 91 and 92 ◦C in a water
bath (Fig. 2B). Here too, the results suggest differences between
microwave heating at 90 ◦C and water bath heating at 90, 91 and
92 ◦C.
It has previously been shown in model systems (Leclère &
Birlouez-Aragon, 2001; Masaki, Okano, & Sakurai, 1999) and in bio-
logical samples (Murthy & Sun, 2000) that fluorescent products
from Maillard reaction have a maximum excitation at wavelengths
between 340 and 370 nm, and that the wavelengths of maximum
emission are between 420 and 450 nm. The fluorescence from
these products is clearly distinguishable from that of tryptophan
in proteins (for which the wavelengths for maximum excitation
and emission are 290 and 336 nm, respectively). Synchronous
fluorescence spectra were thus selected to analyze the samples
(Fig. 3B). The wavelengths of maximum emission of fluorescent
products were at 370–390 nm and 430–440 nm, which agreed with
the above-mentioned range (420–450 nm). However wavelengths
between 370 and 390 nm were not detected in the previous papers
of the above-mentioned authors. Although the chemical structures
of the fluorescent products were only described for a few of those
compounds, all the available structures indicated that they always
have one or more nitrogen atoms originating from the reactant
amino-compounds.
d-Glucose/glycine solution is among the most frequently used
model systems. The intermediate Maillard reaction products
identified in aqueous d-glucose/glycine solutions include quinox-
alinone, alkyl-substituted pyrazinones, 5-hydroxy-1,3-dimethyl-
2(1H)-quinoxalinone (Keyhani
Matni, Pare Belanger, 1997), pyrrole-like or furanone-like
& Yaylayan, 1997; Yaylayan,
&
compounds (Bailey, Ames, & Monti, 1996), and 2-acetyl-6-hydroxy-
7-hydroxymethyl-1,5,6,7-tetrahydro-4H-azepinone (Ames, Bailey,
& Mann, 1999).
Woodward’s rules, named after Robert Burns Woodward
and also known as Woodward–Fieser rules are several sets
of empirically derived rules which attempt to predict the
wavelength of the absorption maximum (ꢁmax) in an UV–vis spec-
trum of a given compound. Absorption maximum of furaneol
(2,5-dimethy-4-hydroxy-3(2H)-furanone): Maternal conjugated
two dilute base 215 + 35(␣-OH) + 30(␤-OCR) + 12(␤-CH3) equal to
292 nm (Figs. 3A and 4).
It is assumed that furfural is formed during the catalytic degra-
dation of glucose in the presence of amino compounds. According
to the assumption, the enol form of 3-deoxy-d-erythrohexosulose
appeared during the degradation that was followed by dehydration,
oxidation and disproportionation (Olsson, Pernemalm, & Theander,
1978). There were also other compounds in large amounts

42
C. Zhou et al. / Carbohydrate Polymers 97 (2013) 38–44
R1
R2
O
3(2H) furanone
HO
O
R3
O
O
O
HO
O
Homofuraneol
Furaneol
2-ethyl-4-hydroxy-5-methyl-3(2H)-furanone
2, 5-dimethy-4-hydroxy-3(2H)-furanone
O
O
O
O
HO
O
Mesifurane
Norfuraneol
4-methoxy-2,5-dimethyl-3(2H) furanone
4 hydroxy-5-methyl-3(2H) furanone
2, 5 -dimethy-4-hydroxy-3(2H)-furanone: Maternal conjugated two dilute base 215 + 35(α -OH) +
30(β-OCR) + 12(β-CH3) = 292 nm
Fig. 4. Structures of Maillard reaction products at the maximum absorption of 292 nm.
experimentally detected in the systems tested. In our experiments,
the 2-acetylfuran and 5-methylfurfural, Maillard reaction products,
were detected and that agreed with the reports of other authors
about the key-compounds of the Maillard reaction (Shen, Tseng, &
Wu, 2007). It should be emphasized that in our study, the GC/MS
investigation was not aimed at the identification and quantifica-
tion of new and/or minor compounds in model systems, but only
major constituents were considered. There were no changes in
composition of Maillard reaction with microwave and water bath
heating (Fig. 5). These observations are identical to those under con-
ventional Maillard reaction, which might suggest that microwaves
seem not to have affected the reaction path thermodynamically but
have affected the reaction in a kinetic way.
processing of materials. Microwaves may be able to initiate chem-
ical reactions not possible in conventional processing (Venkatesh
& Raghavan, 2004). So many reports claim the existence of ather-
mal effects in microwave-induced reactions (Herrero et al., 2008).
But, all tests here failed to produce a significant difference between
the microwave and conventional bath heating methods. As the
controversy, the study of Gedye (1997) is of great interest. In his
experiments, which had a very careful and well temperature con-
trol, Gedye did not detect any microwave athermal effects. The
security in terms of applications of microwave in food industry is
positive.
3.3. The effects of microwaves on degradation of polysaccharide
Maillard reaction is a form of non-enzymatic browning, resulted
from a chemical reaction between an amino acid and a reducing
sugar, which is very common in agriculture and food processing.
Microwave heating has vast applications in the field of food
processing (including drying, pasteurization, sterilization, thawing,
tempering, and baking of food materials) over a period of several
decades (Chandrasekaran, Ramanathan, & Basak, 2013). In conven-
tional thermal processing, energy is transferred to the material
through convection, conduction, and radiation of heat from the
surfaces of the material. In contrast, microwave energy is deliv-
ered directly to materials through molecular interaction with the
electromagnetic field. The difference in the way energy is delivered
can result in many potential advantages to using microwaves for
Fig. 6A illustrates that under the experimental conditions, even
in the larger anode current (240 mA), microwave itself did not cause
the degradation of PSPY. However, even in microwave with small
anode current (60 mA), microwave can assist H2O2 in the degra-
dation of PSPY (Fig. 6B). Meanwhile, the effects of microwave on
the degradation of PSPY were verified with the Maillard reaction
results. Linear regression analysis of Fig. 6B shows that, under these
conditions there was a good linear dependence (r > 0.98) between
intrinsic viscosity and degradation time for both water bath and
microwave heating. The degradation process rule of PSPY is simi-
lar to microwave degradation of ␭-carrageenan, which contains a
sulfate and 3, 6-anhydrogalactose unit in its structure similar to

C. Zhou et al. / Carbohydrate Polymers 97 (2013) 38–44
43
4.0
(A)
Abundance
30000
3.5
3.0
2.5
2.0
1.5
20000
10000
0
30000
20000
10000
Microwave at 240mA, 85ºC
Water bath at 85ºC
0
1.0
0
30
60
90
120
30000
Time/min
20000
10000
4
(B)
0
y=3.5768-0.0216x
r=-0.9833
30000
3
20000
10000
2
1
0
y=3.2121-0.0290x
r=-0.9816
0
30000
20000
10000
0
Microwave at 60mA, 85 ºC with 0.5 % H2 O2
Water bath at 85 ºC with 0.5 % H2 O2
0
15
30
45
60
75
90
5
10
15
20
25
30
Time/min
Time/min
Fig. 6. The effects of microwave and water bath heating without H2O2 (A) and with
H2O2 (B) on the intrinsic viscosity of polysaccharide from Porphyra yezoensis.
Fig. 5. GC/MS chromatogram of Maillard reactions (acetone solubility components)
at different microwave anode currents and water bath heating.
4. Conclusions
The results demonstrated the penetration effect of microwave
heating, and this occurred in a reaction solution of volume 60 mL
from temperature–time profiles in the microwave system. The
PSPY degradation produced two major intermediate products, 2-
acetylfuran and 5-methylfurfural, whose formation was detected
from their fluorescence intensity. Maillard reaction intermedi-
ate products were observed, evidenced by A292 and fluorescence
intensity. Further development of browning was noticeable at
A420. Maximum emission of synchronous fluorescence spectra at
370–390 nm was not detected in previous works. All data failed to
show any significant athermal effects of compositional changes in
the Maillard reaction products.
that of PSPY. Therefore, a degradation consistency between the
two polysaccharides was observed. Interestingly polysaccharide
sulfate for either microwave degradation or acid–base hydroly-
sis have similar degradation regularity (Hjerde, Smidsrød, Stokke,
& Christensen, 1998). As effects of microwaves on degradation
of polysaccharide, the resulting degradation curves were practi-
cally identical in Fig. 6A. Thus, we suggest that the differences
in Fig. 6B, as compared to bath heating, may due to the two
heating systems with different heating procedures, and cannot be
attributed to athermal effects. An extensive review reflecting this
debate about athermal effects was published by the Royal Society
of Canada (Krewski et al., 2001). The comparison of these studies
in Krewski’s review is difficult, if not impossible, due to the lack
of common experimental conditions. Extensive researches have
been conducted and found athermal effects are always associated
with microwave radiation (Bohr & Bohr, 2000; de Pomerai et al.,
2003; Pagnotta, Pooley, Gurland, & Choi, 1993). However, many
other studies could not detect any athermal effect (Meissner &
Erbersdobler, 1996; Welt, Steet, Tong, Rossen, & Lund, 1993). Simi-
larly, under the well temperature controlled conditions, the results
of the present study cannot support the hypothesis of athermal
effects induced by microwave radiation. Some of the reports sug-
gesting the existence of athermal effects could ascribe to not using
well temperature controlled microwave heating systems.
Acknowledgements
The work was funded by National Natural Science Foundation of
China (Nos. 31071502, 30940058, 31170672, 31271966), National
High Technology Research and Development Program 863 (No.
2013AA102203), Special Fund for Agro-scientific Research in the
Public Interest (No. 201303071), Jiangsu Government Scholarship
Fund for Study Abroad, the Foundation for the Eminent Talents
and Research Foundation for Advanced Talents of Jiangsu Univer-
sity (12JDG074) and the Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions.

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C. Zhou et al. / Carbohydrate Polymers 97 (2013) 38–44
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