ANALYSIS OF MICROWAVE DRYING VARIANTS AND THEIR EFFECT ON THE QUALITY OF THE DEHYDRATED PRODUCT

2nd Mercosur Congress on Chemical Engineering
4th Mercosur Congress on Process Systems Engineering


ANALYSIS OF MICROWAVE DRYING VARIANTS AND THEIR EFFECT
ON THE QUALITY OF THE DEHYDRATED PRODUCT
R. Rodríguez, J. I. Lombraña*1
Departamento de Ingeniería Química - Universidad del País Vasco (Spain)


Abstract. Drying is one of the most important unit operations of food processing; being necessary its study for
either the quality evaluation of dried product or its effect on the operational costs. The quality and structural
change of mushrooms dried under microwave energy was studied in this work combining different operational
conditions related to pressure and temperature control. Two different experimental sets were studied: in the first
one, the zone for temperature control was varied and in the second one, two different kinds of heating were
investigated. One based on applying only microwave heating and, the other one, using a moderate heating of
microwave helped by convective heating. Mushrooms were chosen as test material and, once cut up into thin
slices, were dried in a guide cavity applying microwave power at 2.45 GHz. Temperature control was carried out
at different locations from the edge whereby microwaves were introduced. In the drying experiments, the
temperature of a given location was controlled using operational heating cycles in which the heating device is
operated on/off depending on if temperature is below/above the set temperature. The quality of the dehydrated
mushrooms was studied by two different techniques: sorption isotherms (Halsey and B.E.T. equations) and
scanning electron microscopy (SEM). The results show that the best quality in the dehydrated product
corresponded to the sample in which the temperature control zone was at the farthest location from the edge
whereby microwaves penetrated. On the other hand, the samples dried by microwave-convective heating provide
better results than those obtained with only microwave heating.

Keywords: Drying, Microwave heating, Quality


1. Introduction

Drying is one of the oldest known methods for food preservation (Cohen and Yang, 1995, Photon et al., 2001)
with minimal damaging effect on quality. An important advantage in the drying of food is the reduction of the weight
that enhances transport costs and handling. The main point in food drying is the reduction of the moisture content to
a level that usually ranges from 1 to 5%, which avoid microbial spoilage and undesirable enzymatic reactions (Vera-
Mercado et al., 2001).
There are several dehydration techniques, such as: solar drying, osmotic drying, acoustic drying, etc. Microwave
technology is a relatively new and exciting field and the literature concerning this technology is rapidly growing. The
use of high frequency electromagnetic energy covers a very broad spectrum of interests, including biological effects,
material dielectric properties, cooking, baking and defrosting. Microwave is an efficient way to supply energy; heat
is generated directly inside the product by the friction of the solvent molecules upon themselves, so there is no
external heat transfer resistance. This is distinctive from conventional conductive drying methods in which energy is
supplied, at the surface of the product, and then penetrates inside by thermal diffusion (Lombraña et al., 2001).

*To whom all correspondence should be addressed.
Address: Departamento de Ingeniería Química, Facultad de Ciencia y Tecnología. Universidad del País Vasco. P.O. Box. 644,
48080 Bilbao, Spain. E-mail: iqploalj@lg.ehu.es

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There are only a few established applications of microwave heating at the industrial level (Berteli and Marsaioli,
2005). In spite of many excellent reasons for adopting microwave energy, acceptance of microwave heating has been
slow due to the economic reasons and lack of understanding of interactions between materials and electric field.
Microwave drying or combined microwave/convective drying is used in several industrial food processing
applications in place of conventional hot air drying to reduce drying time and to improve food quality (Hoover et al.,
1996). Nevertheless, to conduct the process adequately, it is necessary to know the effect of operating conditions
such as: pressure, temperature control either on the process kinetics or on the product quality. Consequently, all these
aspects were here investigated on the dehydration of mushrooms, using microwave and convective heating. The
quality was study by two different techniques: sorption isotherms (B.E.T and Halsey equations) and SEM
The food quality can be measured by sorption isotherms, which relate the equilibrium moisture content and
water activity (aw) at a given temperature and pressure. The isotherms provide a way to describe the hygroscopic
properties of foodstuffs (Delgado and Sun, 2002). Moreover, if the driving force for mass transfer at the product
surface is assumed to be the difference in partial vapor pressure of water between the surface of the food and the air,
the sorption isotherm can also be used in the calculation of mass transfer rates. Optimization of drying processes and
prediction of self-life stability requires the knowledge of sorption isotherm correlations and the evaluation of mono-
layer moisture content (Sahbaz et al., 1999).
Several mathematical models have been proposed to describe the sorption isotherms for food materials (Shivhare,
2004), B.E.T and G.A.B equations appear to be the most popular food isotherm equation and provide the mono-layer
moisture content. The Halsey modified equation fit adequately the sorption isotherms in a relative moisture interval
of 0.1-0.8 (Menkov and Dinkov, 1999).
Another method to analyze the quality of the dehydrated samples is Scanning Electron Microscopy (SEM), which
is commonly used to investigate the structural changes at cell wall level (Tu, 2000).

2. Materials and Methods

Mushrooms (Agaricus bisporus) were obtained from the same supplier at a local market. They were cleaned, cut
into parallelepiped pieces and placed in the sample holder, of which more details will be given in the results section,
to arrange them conveniently along the waveguide axis direction inside the monomode cavity of the MW drier.

2.1. Equipment
The installation consist of a magnetron emitting at a frequency of 2.45 GHz, with a maximum microwave power
of 300 W, that feeds the microwave power through a waveguide to the monomode cavity. This cavity is inside a
chamber that gives the possibility of working below the atmospheric pressure. In Figure 1, one can see the top-side
view of the drier with the control panel and vacuum chamber where the fundamental elements can be found:

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waveguide, monomode cavity and water-load, which absorbs the fraction of transmitted energy through the sample.
The installation is completed with a condenser and a vacuum pump, here not indicated. The system presents a data
acquisition system OPTO22 with the application software, Factory Floor that controls and records the pressure,
temperature, mass and microwave power data during the drying process.
0.0 g


Fig. 1. Schematic representation of the microwave vacuum drier

The mushroom pieces were placed in a sample holder above a scale to measure the mass during the process. The
scale (Sartorius, model PT-1200) was connected with the control panel and computer by an RS232.
The temperature was measured by three fiber-optic probes; one was placed inside the sample (Tin), another on the
surface (Tsur) and the last one inside the monomode cavity to measure the ambient temperature (Tair). The
temperature of a given location was controlled using operational heating cycles in which the heating device was
operated on/off depending on if temperature was below/above the set temperature.
The installation presents a pressure regulation system operated by an electronic valve connected to a pressure
sensor by Leybold (model PIRANI PG-3).
The energy power (incident, reflected and transmitted) was measured by three diodes (National Electronics, CIM-
D’OR and Carlo Gavazzi model EDM 35) placed in the corresponding zone of the waveguide.
Two sets of experiments were carried out; in the first one, the zone for temperature control was varied, attending
to the location from the edge whereby the microwaves were introduced. In these experiments samples were dried at
150 W, at a set temperature of 80ºC. During the first half and hour, pressure was lowered until 2 mm Hg while the
rest of the process was carried out at atmospheric pressure. The control temperature in all experiments was the inner
one, usually higher than the outside one and in this way the sample temperature was controlled within an interval of
±5 ºC.
In the second experimental set, two different kinds of heating were investigated, one using only microwave
heating supplier (240W), at a set temperature of 50°C and 30 mmHg of pressure. Similar to the above experiments,

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the first half and hour the pressure was maintained at 2 mmHg. In the second type of heating, MW heating was
applied moderately (120 W) helped by convective heating (hot air) and set temperature was fixed at 50ºC.

2.3. Water Sorption Isotherm
Sorption isotherms were determined with a Novasina RTD-33 TH-2 multi-channel system (Mathias, Spain)
which can perform temperature-controlled aw measurements. Sorption isotherms for all experiments were carried out
at 2 degrees. Calibration was first made at 25ºC by using salt saturated solutions of 11%, 53% and 90% relative
humidity. Moisture content was determined with an HR73 Halogen Moisture analyzer (Mettler Toledo, Spain). The
sample was placed inside and was dried at 150 degrees until constant mass.
Samples were introduced inside the apparatus into five different relative humidity values (salt saturated solutions)
at increasing concentration from 11% to 90 %. Sample’s mass was measured until constant mass.
The isotherm equations used to fit the data are B.E.T and Halsey (Sandoval and Barreiro, 2002) and the
correspondent equations are those that are shown in Eq. 1 and 2 respectively.

?
K ? 1
1
=
? +

(1)
X
1
( ? ? )
X K
X K
e
1
1

?
?
? ? exp(A + B *T ) ?
? = exp

(2)
?
?
c
X
?
?
?
e
?


A non-linear regression analysis was used to calculate the best values of constants in the equations. The goodness
of fit as applied to experimental data was evaluated through the mean relative percentage error (P%) between the
predicted (Xth) and experimental (Xexp) moisture contents. The relative error is defined as follows:

100
exp
P%
? X ? X
=
n
th
(4)
i=
n
1
X exp

2.4. Scanning Electron Microscopy (SEM)
The Scanning Electron Microscopy was used to analyze the internal structure of mushrooms after drying.
Samples were firstly frozen using liquid N2, after that, were fractured and cut up into thin films, which were hydrated
with a salt solution (NaCl 0.8g/L, 5 minutes) to maintain the osmolality. The structure of the samples was fixed with
glutaraldehyde in 2.5% phosphate buffer for 4 hours (Kessel and Shih, 1976, Speilberg et al., 1993). Later on, the
samples were dehydrated by successive extractions with alcohol solutions at increasing concentrations (50, 70, 96
and ethanol absolute), the samples were washed three times for ten minutes at each alcohol solutions. Finally, a
solution of hexamethildisilazane was used for 20 minutes in order to reach total dehydration. Then, samples were

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glued to the holder, sputter-coated with gold and examined with a EXL and WDXJEOL microscope (JEOL 6400
with EDX Link). A x1000 magnification was used in all SEM observations.

3. Results and discussion

3.1. Effect of Temperature Zone Control
As the drying conditions are extremely important in the determination of the adequacy of a process, the effect of
the temperature zone control on the quality of the dehydrated samples was studied in the present work. Figure 2
shows a representation with dimensions of the inner zone of the sample holder, made of Teflon, where the mushroom
pieces were introduced. The sample holder walls present holes where the vapor comes off. Three 0,5 mm diameter
fiber-optic probes were inserted in the mushroom pieces at 50, 100 and 150 mm from the edge of MW penetration.
200 mm
Mushroom
piece
MW
22 mm

x 50mm x 100mm x 150mm
5 mm
Insertion of temperature probes

Fig. 2. Sample holder and the three locations for the insertion of fiber-optic probes

The temperature profiles obtained along the drying process for two of the three locations studied are shown in
Figure 3. Here, three temperature profiles are plotted: the temperature of the mushroom piece (inner, Tin), that of
surface (Tsur) and the temperature of the air inside the vacuum chamber (Tair). As a consequence of control the
temperature, the value of Tin oscillates around 80 ºC but with deviations that can get to be of ± 8 ºC in some instants.
100
100
A B
80
80
60
60
40
40
T (ºC)
20
T (ºC)
20
0
Tin
0
Tin
-20
Tsur
-20
Tsur
Tair
Tair
-40
-40
0
1500
3000
4500
6000
0
2000
4000
6000
8000
Time (sec)
Time (sec)


Fig. 3. Temperature profiles obtained for the temperature control at 100mm (A) and 150mm (B) from the edge.


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As seen above, the inner temperature is higher than the surface one and depending on the position the differences
between them are more marked. Those differences were less important in the experiment carried out as farther the
250
50mm
100mm
200
150mm
d.s)
g 150
a
ter /
100
W
g
X (
50
0
0
25
50
75
100
Water activity (%)
position from the left edge (Figure 3 B) and could be a significant factor responsible of the changes in the internal
structure that influences in the quality of the sample.
Fig. 4. Sorption isotherms for dried product obtained with different temperature control positions.

Figure 4 represents the experimental absorption values for mushroom samples in the three locations studied.
According to the classification of Brunauer (Brunauer et al., 1940) into five general types of sorption isotherms,
sorption curves obtained are of type III.
The moisture capacity of the sample is related to the specific surface of it. In Figure 4, the best obtained value
corresponds to the experiment made controlling the temperature at the farthest location from the power supplier. This
result was deduced after analyzing the temperature profiles once experimental moisture values were fitted to BET
and Halsey sorption models. The corresponding parameters and the percentage error are shown in Table 1.

Table 1. Parameters obtained for B.E.T and Halsey sorption models.
B.E.T Halsey
Experiment
K X1 P% A B C P%
50mm 33.33 24.17 13.8 0.210
0.075 6.210 2.39
100mm 46.89 7.47 13.94 0.084
0.025 3.056 10.66
150mm 38.86 53.6 15.59 0.538
0.044 2.925 8.65

The quality of the samples was also analyzed by SEM. Figure 5 shows the micrographies obtained for the fresh
sample (A) and that corresponding to the three experiments (B, C and D). The micrographies corroborate the
isotherms results commented above. Two factors have been taken into account on analyzing the micrographies, one
is the thermal level and the second one is the vapor flux or drying kinetics. The thermal level effect on the sample

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structure can be seen by the shrinkage of the sample. The structure suffers an important contraction as the process
temperature increases, particularly at low vapor fluxes. Attending to the vapor flux, it seems that a high flux value
provokes the opening of holes in a excessively shrank structure or dragging the cells walls when the product has not
suffered shrinkage.
The set temperature for these experiments was the same (80 ºC), but the temperature was controlled inserting the
fiber-optic probe at different locations. This fact influences the global temperature of the sample that is different for
each of the three cases. Consequently the sample’s structure is also different, obtaining an opener and more
homogeneous structure in the third case as it can be seen in Figure 5D. This situation agrees with the high values of
water sorption found for the dried product obtained in this case. Spite of certain shrinkage observed, this is moderate
and the vapor flux does not cause dragging of material. An important part of original cells were maintained, giving
rise to a general structure very similar to that of the fresh product.

A
B


C
D


Fig. 5. Sample micrographies (x1000) of fresh (A) and dried mushroom obtained depending on temperature control
position: 50mm (B), 100mm (C) and 150mm (D) from the edge.

The structure of the fresh sample can be seen in Figure 5A, where the structure appears homogenous in cell size.
Figure 5B shows the structure consequence of the control temperature at 50 mm location and offers a non-
homogenous structure completely shrank in many zones and with great openings in others, due to the exit of vapor
flux in a very shrank structure. In the structure obtained when drying was controlled at 100mm (Figure 5C), there
were no signals of contraction but the dragging of material seems considerable; practically all the wall cells were
broken by the vapor flux. These results of case C can be probably explained because of intermittent violent vapor
flux that at least in certain moments of the drying exits as a consequence of heating and cooling succession derived
from temperature control (see former Figure 3).

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3.2. Effect of the Drying Process
Some authors (Berteli, 2005) studied the influence of applying a combined method of drying, concluding that the
combination of the dielectric plus forced convection heating, introduced after sample critical moisture content had
been reached, exhibited a synergistic effect over the drying process. When MW is used as the sole heating source the
applied power was high, consequently a succession of very short periods of heating and cooling take place which
gives rise to a violent water removing. On the contrary, in the MW-convective drying heating and cooling periods
are wider and the effect on the structure is considerably minor.
250
MW
MW-Conv
200

.
s)
d
g 150
r
/
a
te

W 100

X (g
50
0
0
25
50
75
100
Water activity (%)

Fig. 6. Sorption isotherms for MW and MW-convective experiments

Figure 6 shows the sorption isotherms for the two processes, from its analysis it can be seen that both of them
present almost the same sorption values at high relative humidity, while those corresponding to the mono-layer are
appreciably different, being three times higher in the process carried out with the two types of heating (Table 2). As
in the former experiments the Halsey sorption model was also probed for searching the corresponding parameters
that appear also in Table 2, together with percentage error values of fitting.

Table 2. Parameters obtained for B.E.T. and Halsey fit for the two experiments.
B.E.T. Halsey
Experiment
K X1 P% A B C P%
MW 15.18
8.8 9.46
0.104
0.098
7.030
16.444
MW-Conv 37.79 33.67 16.05 0.394 0.018 1.969 8.598

Figure 7 shows the structure obtained after the drying treatment in both cases. In Figure 7A the structure for the
microwave experiments is shown, this structure present heterogeneity with parts with appreciable shrinkage and
other parts with extirpation of matter leading to the formation of big holes due to the elevate vapor flux. Structure

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shown in Figure 7B is more homogeneous and although there has been shrinkage, the sample has suffered no
significant extirpation. Once, the sorption isotherms can be explained as a consequence of these micrographies.
A
B

Fig.7. Micrographies: dried with microwave heating (A) and with microwave-convective heating (B).

4. Conclusions

The sorption isotherms of mushrooms are of type III form, and the temperature zone control has an important
influence as a result of the change of the structure associated to the level and temperature fluctuations so as to the
drying rate. The Halsey equation was found to describe accurately the sorption isotherms in the relative humidity
range of 10-95% with of mean relative percentage error values within 2.39-16.44 %. The experiments carried out to
analyse the effect o position for temperature control showed that the best results corresponded to temperature-
controlled samples at 150 mm from the edge. This fact has been probed through the high specific surface values
obtained according to the monolayer sorption BET model.
On the other hand, a combination of MW and convective heating with moderate thermal levels yields good
quality dried products that gave also good values of moisture sorption in the monolayer. When only MW was used as
heating source, the moisture sorption in the monolayer was three times lower. The higher thermal level and stronger
temperature oscillations associated to the control temperature, in this case, would explain the poorer quality in the
dried product. The SEM micrographies corroborate the results obtained with the sorption isotherms.
Consequently, MW convective drying at atmospheric pressure seems to be the most suitable option but
complemented with a temperature control in the farthest position (150 mm), which has been probed to give better
quality results.

Nomenclature

? = Relative humidity.
A, B and C = Parameters of Halsey equation.
K = Constant related to the mono-layer adsorption heat.
m = is related to the curvature.
MW = Microwaves.
n = number of points.
P% = Mean relative error percentage.

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T = Temperature (K).
t = time (sec).
t0 = Time correspondent to reach the initial moisture, if the experiment began at initial moisture equal to 0 (sec).
Tair = Air temperature (K).
Tin = Inside temperature (K).
Tsur = Surface temperature (K).
X = Moisture content (gWater/100g dry solid).
X1 = Humidity correspondent to the mono-layer (gWater/100g dry solid).
Xe = Equilibrium moisture (gWater/100g dry solid).
Xexp = Experimental moisture content (gWater/100g dry solid).
Xth = Theorical moisture content (gWater/100g dry solid).

References

Berteli, M. N., Marsaioli Jr, A. (2005). Evaluation of short cut pasta air dehydration assisted by microwaves as compared to the
conventional drying process, Journal of Food Engineering, 68, 175.
Brunauer, S., Deming, L. S., Deming, W. E. (1940). On a theory of the Van der Waals adsorption of gases, Journal of Food
American Chemical Society, 62, 1723.
Cohen, J. S., Yang, T. C. S. (1995). Progress in food dehydration. Trends in Food Science & Technology, 6, 1, 20.
Delgado, A. E., Sun, D. W. (2002). Desorption isotherms for cooked and cured beef and pork, Journal of Food Engineering, 51,
163.
Hoover, M. W., Markantonatos, A., Parker, W. N. (1996). UHF Dielectric Heating in Experimental Acceleration of Freeze-
Drying of Foods, Food Technology, 20, 6, 103.
Kessel, R. G., Shih, Ch. Y. (1976). La microscopía electrónica de barrido en biología. Ed. DOSSAT (Madrid). Springer Verlag
(Berlin).
Lombraña, J. I., Zuazo, I., Izkara, J. (2001). Moisture Diffusivity Behaviour During Freeze Drying Under Microwave Heating
Power Application. Mujumdar, A. S., (ed). Drying Technology, 19(8), 1613.
Menkov, N. D., Dinkov, K. T. (1999). Moisture sorption isotherms of tobacco seeds at three temperatures. Journal of Agriculture
Engineering Research, 74, 261.
Prothon, F., Ahrné, L. M., Funebo, T., Kidman, S., Langton, M., Sjoholm, I. (2001). Effects of Combined Osmotic and
Microwave Dehydration of Apple on Texture, Microstructure and Rehydration Characteristic. Lebensmittel-Wissenschaft und-
Technologie, 34, 95.
Sahbaz, F., Palazoglu, T. K., Uzman, D. (1999). Moisture sorption and the applicability of the Brunauer-Emmet-Teller (B.E.T)
equation for blanched and unblanched mushrooms, Nahrung, 3, 5, 325.
Sandoval, A. J., Barreiro, J. A. (2002). Water sorption isotherms of non-fermented cocoa beans (Theobroma cacao), Journal of
Food Engineering, 51, 119.
Smith, R. F. (1979). Microwave-hot air drying of pasta, onions and bacon. Microwave Energy Applications Newsletter, 12, 6, 6.
Speilberg, L., Evensen, O., Bratberg, B., Skjerve, E. (1993). Evaluation of five different immersion fixatives for light microscopic
studies of liver tissue in Atlantic salmon Salmo salar. Diseases of aquatic organisms, 17, 47.
Tu, K., Nicolai, B. and De Baerdemaeker, J. (2000). Effects of relative humidity on apple quality under simulated shelf
temperature storage. Scientia Horticulturae, 85, 217.
Vega-Mercado, H., Góngora-Nierto, M., Barbosa-Cánovas, G. (2001). Advances in dehydration of foods. Journal of Food
Engineering, 49, 271.

Acknowledgements


The authors are grateful to the Universidad del País Vasco (UPV/EHU) for the research grant and to the Basque
Government for the financial support of the study through the INTEK program during the years from 2002 to 2004.


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