ARTICLE IN PRESS
LWT 39 (2006) 247 255
www.elsevier.com/locate/lwt
Far-infrared and microwave drying of peach
Jun Wang , Kuichuan Sheng
Department of Agricultural Engineering, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, PR China
Received 14 August 2004; received in revised form 3 February 2005; accepted 3 February 2005
Abstract
Little detailed information is available for the far-infrared and microwave drying characteristics on peach and far-infrared
combined with microwave drying on other food products. Experiments were conducted to study microwave and far-infrared
dehydration characteristics and two-stage drying process involving far-infrared following microwave drying on peach. As
microwave drying power and infrared drying power increased, dehydration rate of peach increased and whole drying energy
consumption decreased. Peach experienced two falling rate periods when dried with microwave drying or far-infrared drying, and
the first falling rate period under moisture content of peach more than 1.7 (dry basic, d. b.), the second falling rate period under less
than moisture content 1.7 (d. b.). The same water loss will consume more energy and the steeper curve of energy versus moisture
content were obtained when the moisture content is less than 1.7 (d. b.). However, differed from microwave drying, an accelerating
dehydration rate period existed in the initial period of far-infrared drying. The effects of infrared drying power, microwave drying
power and exchanging moisture content at former far-infrared drying converting into latter microwave drying (three factors) on
energy consumption rate and sensory quality (two indices) are significant. The interaction effect of infrared drying power and
exchanging moisture content on two indices is significant. The effects of second-order of microwave drying power and of interaction
between infrared drying power and microwave drying power on energy consumption rate were not significant. The effects of second-
order of exchanging moisture content and of interaction between exchanging moisture content and microwave drying power on
sensory quality were not significant.
r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
Keywords: Microwave; Drying; Peach; Infrared; Quality; Characteristic
1. Introduction Infrared radiation has significant advantages over
conventional drying. These advantages are higher
Major disadvantages of hot air drying of foods are drying rate, energy saving, and uniform temperature
low energy efficiency, quality loss and lengthy drying distribution giving a better quality product. Therefore,
time during the falling rate period (Boudhrioua, Infrared drying can be used as an energy saving drying
Giampaoli, & Bonazzi, 2003). Because of the low method. At present, many driers use infrared radiator to
thermal conductivity of food materials, heat transfer improve drying efficiency, save space and provide clean
to the inner sections of food during conventional working environment, etc. ( Ratti & Mujumdar, 1995;
heating is limited. The desire to eliminate this problem, Yamazaki, Hashimoto, Honda, & Shimizu, 1992).
prevent significant quality loss, and achieve fast and Attempts have been reported on application of
effective thermal processing has resulted in the increas- infrared drying of agricultural materials. With inter-
ing use of infrared and microwaves for food drying. mittent infrared and continuous convection heating of a
thick porous material, the drying time can be reduced to
222:5 times less compared to convection alone while
Corresponding author. Tel.: [phone];
keeping good food quality and high energy effciency
fax: [phone].
E-mail address: [e-mail] (J. Wang). (Dostie, Seguin, Maure, Ton-That, & Chatingy, 1989).
0023-6438/$30.00 r 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.lwt.2005.02.001
ARTICLE IN PRESS
248 J. Wang, K. Sheng / LWT 39 (2006) 247 255
Far infrared drying of potato achieved high drying rate (Al-Duri & McIntyre, 1992; Wang, Xiong, & Yu, 2004;
with infrared heater of high emissive power (Masamura Wang & Xi, 2005).
et al., 1988). The drying rate was also reported to In recent years, microwave drying has gained
increase when the electric power supplied to the far popularity as an alternative drying method for a variety
infrared heater was increased and, consequently, the of food products such as fruit, vegetable, snack food and
temperature of the sample was also observed to be high. dairy product. Several food products have been
Far infrared and near infrared drying using three types successfully dried by the microwave-vacuum application
of granular bed and their quantitative comparison to and/or by a combined microwave assisted-convection
hot air drying from the viewpoint of the heat transfer process. The researchers included Kim and Bhowmik
has been reported by Hashimoto, Hirota, Honda, (1995) for plain yogurt, Yongsawatdigul and Gunase-
Shimizu, and Watanabe (1991). karan (1996b) for cranberries, Lin, Durance, and
Infrared is important in drying technology, but it is Scaman (1998) for carrot slices, Drouzas and Saravacos
not a panacea for all drying processes. It penetrates and (1999) for model fruit gels, Al-Duri and McIntyre (1992)
produces heating inside the material being dried, but its for skimmed milk, whole milk, casein powders, butter
penetrating power is limited (Wang & Sheng, 2004). and fresh pasta, Bouraout, Richard, and Durance (1994)
Microwave drying is more rapid, more uniform and for potato slices, Tulasidas, Raghavan, and Norris
more highly energy efficient compared to conventional (1996) for grapes, Funebo and Ohlsson (1998) for apple
hot air drying and infrared drying. In this case, the and mushroom, and Ren and Chen (1998) for American
removal of moisture is accelerated and, furthermore, ginseng roots, Prothon et al. (2001) for apple and Feng
heat transfer to the solid is slowed down significantly et al. (2000) for blueberries.
due to the absence of convection. And also because of It has also been suggested that microwave energy
the concentrated energy of a microwave system, only should be applied in the falling rate period or at a low
20 35% of the floor space is required, as compared to moisture content for finish drying (Kostaropoulos &
conventional heating and drying equipment. However, Saravacos, 1995; Funebo & Ohlsson, 1998). Microwave
microwave drying is known to result in poor quality may be advantageous in the last stages of air drying.
product if not properly applied (Yongsawatdigul & Because low efficient portion of a conventional dry-
Gunasekaran, 1996a; Drouzasm & Dchubert, 1996). ing system is near the end, two-thirds of the time may
For microwave applications, a two-stage drying be spent, the last one-third of the moisture content
process involving an initial forced-air convective drying, (Al-Duri & McIntyre, 1992).
following by a microwave finish drying, has been However, the far-infrared and microwave drying
reported to give better product quality with considerable characteristics on peach and far-infrared combined with
saving in energy and time (Feng & Tang, 1998). Water microwave drying for food were little reported, i.e. little
accounts for the bulk of the dielectric component of detailed information is available on the alternative
most food systems especially the high moisture fruit and microwave power drying on food products, such as a
vegetable. Hence, these products are very responsive to two-stage drying process involving far-infrared follow-
microwave applications and will absorb the microwave ing microwave drying.
energy quickly and efficiently as long as there is residual The objectives of this study were: (1) to study far-
moisture (Feng, Tang, & Cavalieri, 2002). The micro- infrared and microwave dehydration characteristic of
wave application for drying, therefore, offers a distinct peach and discuss the influence of drying power on
advantage, i.e. high-energy absorption proportional to dehydration characteristic and energy consumption; (2)
moisture content. Proteins, lipids and components can to determine the effect of exchanging moisture content
also absorb microwave energy, but are relatively less (the moisture content of breaking point that former far-
responsive (Mudgett & Westphal, 1989). A second infrared drying convert into latter microwave drying),
advantage of microwave application for drying of infrared drying power and microwave drying power on
vegetables is the internal heat generation (Wang, Zhang, sensory quality, rehydration ratio and energy consump-
Wang, & Xu, 1999). In a microwave drying system, the tion rate; (3) to obtain optimizing combination of drying
microwave can easily penetrate the inert dry layer to be parameter for sensory quality and energy consumption
absorbed directly by moisture in food parts. The quick rate.
energy absorption causes rapid evaporation (boiling) of
water, creating an outward flux of rapidly escaping
vapor (Feng, Tang, Cavalieri, & Plumb, 2001). In 2. Materials and methods
addition to improving the rate of drying, this outward
flux may help to prevent the collapse (shrinkage) of 2.1. Material
tissue structure, which prevails in most conventional
air drying techniques. Hence better rehydration char- Ripe peach (a firm yellow Chinese peach, Zhe-
acteristics may be expected in microwave-dried products Agriculture No. 2, was usually used to process into
ARTICLE IN PRESS
J. Wang, K. Sheng / LWT 39 (2006) 247 255 249
fruit-food) were selected for all drying experiments and recorded at 5 min interval during drying by measured
peach with an initial moisture content of 9.36 kg H2O/kg and weighing the dish on the digital balance (JY10001,
dry solid (dry basic, d. b.) was hand harvested on 1000, 70.01 g).
14 July 2003 from the experimental farm in the Depart- An experimental dryer with far-infrared heat source is
ment of Horticulture, Zhejiang University, and stored shown schematically in Fig. 2 (Wang, 2002). The far-
at 470.5 1C within 5 days. The peach was 75 83 mm infrared heaters were made of SIC (carbonic silicium)
endlong length (length from stem to calyx) and and operated at 220 V, and at a maximum power of
70 80 mm  Horizontal diameter . Prior to drying, peach 2.4 kW. The drying chamber was made of a vertical
was pitted, halved, and sliced into strip 5 570.2 mm. plywood column of 450 mm 450 mm cross section,
Initial moisture content was determined using a 600 mm high. The insulated walls were formed from
vacuum-oven for 70 1C temperature, absolute pressure aluminized steel with a 50 mm thick adiabatic layer
3 kPa, and heating time 12 h (GB/T8858-88, Chinese between them. The sample tray, made of wire mesh, was
National Standard). Initial mass was 300 g for drying. elevated parallel to the far-infrared heater and the
distance between the heater and the peach sample was
maintained constant at 160 mm throughout the experi-
2.2. Drying equipment
ments. The radiation intensity was varied by regulating
the voltage and hence the output of the heater.
The drying apparatus consisted of a laboratory
The far-infrared intensity is usually expressed as
microwave oven (WEG-800A, Jinan, China, Fig. 1),
radiation power per unit area (kW/m2). Before drying,
which operated at 2450 MHz. The energy input was
food samples were placed on the wire mesh tray
microprocessor controlled from 10 to 1000 W at 10 W
(450 mm 450 mm), and the initial mass per unit area
increments. The outlets were provided on the left
was calculated. Thus, far-infrared intensity radiated on
upper side of the oven to allow the introduction of
peach is given as power per mass (kW/kg, power to
airflow. The dimensions of the microwave cavity were
initial mass of peach). Drying experiments were
445 420 285 mm. The microwave oven was operated
conducted using two replicates at three radiation
by a control panel, which could control both microwave
intensities (0.50, 0.70 and 1.00 kW/kg). For water
input power level and emission time (1 s 100 h). Sliced
vapour taken out, the outlet fan was setup in the drying
peaches with different mass were dried under various
oven (Fig. 2). The outlet air velocity of 1 m/s was used
microwave power, and microwave drying intensity was
for the experiment.
expressed into power per mass (initial mass).
The microwave or far-infrared was applied until the
Factors investigated in microwave drying were the
weight of the sample reduced to a level corresponding to
microwave power intensities (0.50, 0.70 and 1.00 kW/kg)
a moisture content of about 0.1 d. b.
using two replicates. One dish, containing the sample,
was placed on the centre of a turntable fitted inside
2.3. Drying indices
(bottom) the microwave cavity. To remove water
vapour, the outlet fan was setup in the microwave oven.
2.3.1. Sensory quality
The outlet air velocity of 1 m/s was used for the
The sensory evaluation of intact dried peach was
experiment. The drying was performed according to a
carried out by a panel of five trained judges and six
preset power and time schedule. Moisture loss was
Fig. 1. The scheme diagram of microwave drying equipment. Fig. 2. The scheme diagram of far-infrared drying equipment.
ARTICLE IN PRESS
250 J. Wang, K. Sheng / LWT 39 (2006) 247 255
untrained judges (include: the authors, post-graduates sample mass), and microwave drying power (0.88
and food Lab. assistants). The panelists were asked to 3.32 kW/kg). The matrix for the QORD optimization
indicate their preference for each sample, based on the experiment is summarized in Table 2. The QORD had
quality attributes of visual colour, external twist and air eight experimental points in a cube (run No. 1 8), six
bubble (Table 1). The yellow attributes of sample star points with an axial distance of 1.285 (run No.
surface was denoted as 4 score, no twist on external or 9 14), 2 replications at the central point of the design
no air bubble in sample was denoted as 3 score. The (run No. 15 16) for experimental error determination. A
significant of sensory attributes is 2 for visual color, and full second-order polynomial model of the type shown in
1 for twist on external or for no air bubble. Eq. (1) was used to evaluate drying indices (response
variable, Y) as a function of dependent variable (actual
level, Z) namely exchanging moisture content (denoted
2.3.2. Energy consumption and energy consumption rate
by subscript 1, Z1), infrared drying power (denoted by
During drying, the electrical energy was consumed.
subscript 2, Z2), microwave drying power (denoted by
The electrical consumption consumed was recorded with
subscript 3, Z3) and their interactions.
ammeter. Before drying, the no-loading consumption
was measured while the turntable is rotating and the Yźb0þb1Z1þb2Z2þb3Z3þb11Z2þb22Z2þb33Z3
1 2 2
outlet air velocity of 1 m/s was used. During drying, the
þb12Z1Z2þb13Z1Z3þb23Z2Z3. ð1Þ
whole electrical energy was measured. The energy
Analysis of variance (ANOVA) for treatment main
consumption for sample drying was thought as the
effect was conducted using the SAS software (SAS,
whole electrical energy minus the no-loading consump-
1999). Multiple comparison of means were by the
tion.
Duncan s multiple range test (DMRT). All statistical
During whole drying process, the consumption energy
significance was determined at the 10% significacne level
was related to quantity of lost water and expressed as
(Po0:1).
energy consumption rate that the unit lost water (kg)
consumed the electrical energy value (kWh):
Energy consumption rate
3. Results and discussion
Consumed energy value
ź .
Value dehydrated moisture content
3.1. Dehydration characteristics
2.4. Experimental design 3.1.1. Effect of microwave power
The effect of changing the microwave power on
To investigate the far-infrared drying combined with dehydration characteristic is shown in Fig. 3. Drying
microwave drying on quality of dried peach, drying rate increased with microwave power level at the same
process was divided into two stages involving far- moisture content. The results indicated that mass
infrared drying followed by microwave drying. transfer within the sample was more rapid during the
The quadratic orthogonal regression design (QORD) higher microwave power heating because more heat was
(Khuri & Cornell, 1989) was employed in this study. The generated within the sample (Lin et al., 1998). A
QORD consisted of a three-factored factorial with five constant rate period was not observed in drying of
levels. The factors were exchanging moisture content peach samples. The entire drying process for the samples
(i.e., moisture content of breaking point that former occurred in the range of falling rate period in this study.
far-infrared drying convert into latter microwave drying, This agree with the finding of Feng, Tang, and Cavalieri
0.858 2.143 d. b., moisture loss was recorded at 5 min (1999) that not constant rate in drying of apple with a
interval during drying by measured and weighed the microwave and spouted bed dryer. The drying curve has
dish on the digital balance, the weight of the sample two falling rate periods: the first falling rate period
reduced to a level corresponding to a moisture content), under moisture content above about 1.7 (dry basic, d.
infrared drying power (0.88 3.32 kW/kg for initial b.), the second falling rate period under moisture
Table 1
Evaluated scale for sensory quality (Y2, fully scale 14)
Attribute Evaluated value Significant
Visual color (l1) Yellow (4) Slight yellow (3) Snuff color (2) Brown (1) 2
External twist (l2) No twist (3) Slight twist (2) Twist (1) 1
Air bubble (l3) None (3) Smaller air bubble (2) Bigger air bubble (1) 1
Total: Y2ź2l1+l2+l3.
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J. Wang, K. Sheng / LWT 39 (2006) 247 255 251
Table 2
Selected factors and their levels for the first factorial design with the QORD design
Run Standardized coded levels (Actual value)
Exchanging moisture Infrared drying Microwave drying Sensory Energy consumption
content power power quality (S) rate (kWh/kg)
1 1(2.0) 1(3.0) 1(3.0) 6 2.182
2 1(2.0) 1(3.0) 1(1.0) 12 2.994
3 1(2.0) 1(1.0) 1(3.0) 9 2.485
4 1(2.0) 1(1.0) 1(1.0) 7 3.142
5 1(1.0) 1(1.0) 1(3.0) 8 2.394
6 1(1.0) 1(1.0) 1(1.0) 12 2.744
7 1(1.0) 1(1.0) 1(3.0) 9 2.983
8 1(1.0) 1(1.0) 1(1.0) 5 3.357
9 1.285(0.858) 0(2.0) 0(2.0) 11 2.911
10 1.285(2.143) 0(2.0) 0(2.0) 10 2.69
11 0(1.5) 1.285(0.715) 0(2.0) 7 2.971
12 0(1.5) 1.285(3.285) 0(2.0) 8 2.485
13 0(1.5) 0(2.0) 1.285(0.715) 8 2.779
14 0(1.5) 0(2.0) 1.285(3.285) 9 2.495
15 0(1.5) 0(2.0) 0(2.0) 12 2.533
16 0(1.5) 0(2.0) 0(2.0) 12 2.437
In Fig. 4, the same water loss will consume more
energy and a slightly steeper curve were shown when the
moisture content is less than 1.7 (d. b.). In the second
falling rate period, moisture content is smaller in peach,
the movement of water is mostly symplastic transport
way and more energy was consumed when the same
water lost (Wang & Chao, 2002, 2003). The moisture
content 1.7 corresponds to the moisture content at the
inflexion point where the first falling rate period
transformed into the second falling rate period.
3.1.2. Effect of far-infrared drying power
The effect of far-infrared power on the dehydration
characteristics is shown in Fig. 5. Similar to microwave
Fig. 3. Dehydration rate versus moisture content for microwave
drying,  0.50 kW/kg, K 0.70 kW/kg, m 1.00 kW/kg.
drying, the dehydration rate increased with infrared
power level at the same moisture content. The results
indicated that mass transfer was more rapid during the
larger power heating because more heat was generated
content less than 1.7. This is similar with the earlier within the sample. However, the dehydration rate at the
reports on the drying of banana (Maskan, 2000) and same power level was far lower than during microwave
apples (Feng, Tang, Mattinson, & Fellman, 1999). The drying.
moisture content 1.7 (d. b.) responses the moisture We observed two falling rate periods when using far-
content of the inflexion point where the high dehydra- infrared drying on peach as well. However, differed
tion rate transformed into the low dehydration rate. from microwave drying, an accelerating dehydration
Efforts were made to study the effect of the power rate period existed in the initial period during far-
input on energy consumption. Fig. 4 shows relationship infrared drying. This is consistent with Mongpraneet,
between energy consumption and moisture content. Abe, and Tsurusaki (2002) and Wang (2002) finding far-
Unexpectedly, energy consumption is different for three infrared drying characteristics on onion. During falling
power inputs at the same initial mass with the same period, the first falling dehydration at moisture contents
moisture lose. The lower microwave drying power the greater than 1.7 (d. b.), the second falling dehydration at
more energy consumes. One reason might be that the moisture contents less than 1.7 (d. b.).
drying time is longer under lower power and result in the Fig. 6 shows relationship between energy consump-
increase of energy consumption. tion and moisture content. Energy consumption is
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252 J. Wang, K. Sheng / LWT 39 (2006) 247 255
Differed from microwave drying, energy consumption
curves were consists of three parts during the far-
infrared drying. These responses to three dehydration
stages. The curve is steeper in initial part than in the first
falling period, which corresponds to the accelerating
dehydration rate period. The energy consumption is
greater in far-infrared drying than in microwave drying,
this might have been caused by the factor that the drying
time is longer in far-infrared drying and the more heater
energy was taken out by outlet-vent air.
Similar to the effect of microwave drying, the water
loss will consume more energy and the curve become
steeper when the moisture content is less than 1.7 (d. b.).
Fig. 4. Energy consumption versus moisture content for micro-
wave drying (Initial mass 300 g),  0.50 kW/kg, K 0.70 kW/kg,
m 1.00 kW/kg.
3.2. Models of influence of main factors on the drying
indices
3.2.1. Equation of influence
The effect of exchanging moisture content, infrared
drying power and microwave drying power on sensory
quality and energy consumption rate (two indices) were
investigated using response surface analysis. Regression
models were generated and the parameters that were not
significant were dropped from the regression equation.
Regression analysis showed that the effect of experi-
mental variables on the two indices were significant.
The significant level, Po0:05; indicates the suitability
of the second-order polynomial to predict the two
indices (energy consumption rate and sensory quality).
Fig. 5. Dehydration rate versus moisture content for far-infrared
Eqs. (2) (3) were employed in this study.
drying,  0.50 kW/kg, K 0.70 kW/kg, m 1.00 kW/kg.
For energy consumption rate, the followed equation
was obtained (significantly Pź0:029):
Y ź5:231 1:792 Z1 0:850 Z2þ0:054
1
Z3þ0:540 Z2þ0:092 Z2þ0:188
1 2
ð2Þ
Z1 Z2 0:187 Z1 Z3.
For sensory quality, the equation is as following
(significantly pź0:087):
Y ź 13:895þ2:440 Z1þ13:703 Z2þ8:636
2
Z3 1:763 Z2 1:759 Z2 1:220
2 3
ð3Þ
Z1 Z2 2:000 Z2 Z3.
In the above two equations, the effect of three factors
on two indices were significant. The interaction effect of
infrared drying power and exchanging moisture content
Fig. 6. Energy consumption versus moisture content for far- on two indices was also significant. The effects of
infrared drying (Initial mass 300 g),  0.50 kW/kg, K 0.70 kW/kg,
second-order of microwave drying power and of
m 1.00 kW/kg.
interaction between infrared drying power and micro-
wave drying power on energy consumption rate were,
however, not significant. The effects of second-order of
different for the three power levels at the same moisture exchanging moisture content and of interaction between
loss. The lower drying power consumes the more energy exchanging moisture content and microwave drying
when the same moisture content was lost. power on sensory quality were not significant.
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J. Wang, K. Sheng / LWT 39 (2006) 247 255 253
3.2.2. Objection optimizing calculation Effect of microwave drying power on two indices was
In order to investigate the three-factor optimizing shown in Fig. 9. Energy consumption rate decreases
combination, it is necessary that the two indices were linearly with the increase of microwave drying power
calculated for the maximum value or the minimum value (Y1ź2:99620:269 Z3). Sensory quality is highest
being obtained. at power 1.231 kW/kg (Y ź10:054þ2:853 Z3
2
Optimization was conducted by employing canonical 1:159 Z2).
3
analysis (Khuri & Cornell, 1989) and the range of the
variable (within the experimental range) was considered
to obtain the maximum of sensory quality and minimum
4. Conclusions
of energy consumption rate. The three-factor optimizing
combination was obtained by using the SAS software.
(1) As microwave drying power and infrared drying
The calculation results were shown in Table 3.
power increase, dehydration rate of peach increases
For two indices, three-factor optimizing combination
and drying energy consumption decreases.
is different. Exchanging moisture content is slightly
lower for sensory quality than for other index. Infrared
drying power is near for two indices. Microwave drying
power should be higher for energy consumption rate
and lower for sensory quality.
3.2.3. Effect of each factor on the drying indices
To analyse effect of each factor on two indices (energy
consumption rate and sensory quality), substituting two
other optimizing parameters into Eqs. (2) (3), the
models of effect for each factor on two indices were
obtained and shown in Figs. 7 9.
Effect of exchanging moisture content on two indices
was shown in Fig. 7. It was represented that relationship
Fig. 7. Effect of exchanging moisture content on two indices.
between exchanging moisture content and sensory
quality was linear (Y ź12:74321:086 Z1) and the
2
sensory quality decrease with increase of exchanging
moisture content. Energy consumption rate was lowest
at exchanging moisture content 1.730 kW/kg
(Y1ź3:729 1:869 Z1þ0:540 Z2). This is consis-
1
tent with the inflexion that the first falling period
transformed into the second falling period, and micro-
wave drying should be applied in the second falling
period.
Effect of infrared drying power on two indices was
shown in Fig. 8 and the curves were quadratic
polynomial (Y1ź2:863 0:525 Z2þ0:092 Z2;
2
Y2ź2:929þ10:195 Z2 1:763 Z2). Sensory qual-
2
ity is highest and energy consumption rate is lower at
infrared drying power 2.891 and 2.860 kW/kg, respec-
Fig. 8. Effect of infrared drying power on two indices.
tively.
Table 3
The optimizing parameter combination for three drying indices
Indices Exchanging moisture content Infrared drying power Microwave drying power Optimizing value of
the objective indices
Coded Actual Coded Actual Coded Actual
value value value value value value
Sensory quality 1.285 0.858 0.8914 2.891 0.769 1.231 11.810
(score)
Energy consumption 0.459 1.730 0.860 2.860 1.285 3.285 2.113
rate (kW/kg)
ARTICLE IN PRESS
254 J. Wang, K. Sheng / LWT 39 (2006) 247 255
Research Fund for the Doctor of High Education
through project 20020335052.
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