Journal of Food Engineering 39 (1999) 117ą122
Microwave/vacuum drying of model fruit gels
*
A.E. Drouzas, E. Tsami , G.D. Saravacos
Department of Chemical Engineering, Zografou Campus, National Technical University, 157 80 Athens, Greece
Received 5 May 1998; accepted 10 October 1998
Abstract
Combined microwave (MW)/vacuum drying of fruit materials has a promising potential for high-quality dehydrated products. A
better knowledge of the drying kinetics of fruit products could improve the design and operation of e�cient dehydration systems.
A laboratory MW/vacuum drier was used for drying kinetics experiments with model fruit gels, simulating orange juice con-
centrate. The system was operated in the vacuum range of 30ą50 mbar and MW power of 640ą710 W. The distribution of the
electromagnetic �eld in the cavity of the oven was determined from the drying rate of samples, placed at 5 di�erent locations.
The drying rate was determined by periodic weighing of the sample. The rate constant (K) of the single-layer model of drying was
estimated by regression analysis of the experimental data. An empirical model is proposed for estimating the drying constant (K) as
a function of the absolute pressure and the MW power of the system. Ó 1999 Elsevier Science Ltd. All rights reserved.
Nomenclature more expensive than vacuum drying and it is not eco-
nomical for fruit products. Heat transfer in vacuum and
K drying constant (1/min)
freeze drying may be enhanced by infrared or micro-
K0 drying constant corresponding to pressure
wave radiation. Microwave (MW) or dielectric energy
P0 and MW power Q0 (1/min)
has the advantage of higher penetration in the material
m, n empirical constants (ą)
and its preferential absorption by the water molecules
P, P0 absolute pressure (mbar)
Q, Q0 MW power output (W) (Drouzas & Schubert, 1996). The potential of MW en-
t drying time (min)
ergy to drying has found only limited applications
X, X0 moisture content (kg/kg dry matter)
(Schi�mann, 1995). Improved dehydrated potato and
apples were obtained by MW drying (Huxsoll & Mor-
gan, 1968). Recent research has shown that pre-treat-
ment of food materials with MW energy increases
1. Introduction
substantially the air-drying rate (Kostaropoulos &
Saravacos, 1995; Drouzas, Tsami & Saravacos, 1997).
Dehydration of fruit materials, especially fruit juices,
The improved drying rate is ascribed to the development
is a di�cult food processing operation, mainly because
of a porous structure of the food material, which facil-
of undesirable changes in quality of the dehydrated
itates the transport of moisture (Marousis, Karathanos
product. High temperatures and long drying times, re-
& Saravacos, 1991).
quired to remove the water from the sugar containing
The MW and high frequency (HF) energy have two
fruit material in conventional air-drying, may cause se-
advantages of high penetration in the solid material and
rious damage to the Żavor, color and nutrients of the
preferential absorption by the water molecules. How-
dehydrated product. Vacuum drying has been proposed
ever, they have the disadvantage of non-homogeneous
to overcome these problems, especially with orange and
distribution in the processing cavity, creating problems
other fruit juices. However, the vacuum-drying process
of non-uniform heating (Risman, Ohlsson & Wass,
is expensive, due to high capital and operating costs.
1987; Ohlsson, 1990; Schubert, Gruneberg & Walz,
Expensive vacuum equipment is required, and heat
1991). The temperature distribution in the cavity is in-
transfer in vacuum may limit seriously the drying rate.
Żuenced by the composition and the dielectric properties
Freeze drying, which yields products of higher quality is
of the food material, and its location in the oven. The
energy absorption by a sample is also a�ected by the
*
Corresponding author. E-mail: [e-mail] presence of other materials in the cavity (Kraisheh,
0260-8774/99/$ ą see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 0 - 8 7 7 4 ( 9 8 ) 0 0 1 3 3 - 2
118 A.E. Drouzas et al. / Journal of Food Engineering 39 (1999) 117ą122
Cooper & Magee, 1997). The sugar content of agar gels (c) The sugar solution (b) at 70�C was added with
may have a signi�cant e�ect on the absorption of MW constant stirring to the gelatinous sugar pectin solu-
energy (Padua, 1993). tion (a).
Although combined MWąvacuum drying has found The hot gel was poured quickly into petri dishes with
some application in the dehydration of fruit juices, more a diameter of 5 cm, where gelling occurred instanta-
research and development is needed before the process is neously. The mean thickness of the gel in the petri dish
used in large commercial scale. In particular, the e�ect was 5 mm.
0
of vacuum and MW power on the drying kinetics should The Brix of the samples of the gel was measured
be known quantitatively, so that the drying system can with an Abbe refractometer, and the initial moisture
be optimized from the cost and quality standpoints. content was determined, using the vacuum oven
method (AOAC, 1984) at 70�C, 25 mbar for 24 h. The
rest of the samples were preserved in a refrigerator
2. Experimental procedure ( 4�C).
2.1. Apparatus
2.4. Drying experiments
In the present study two MW ovens SHARP IEC 705
with di�erent power outputs, 800 and 700 W, both at One petri dish, containing the gel, was weighed using
2450 MHz, were used. For each of the ovens there was a an electronic balance and it was placed in a �xed posi-
glass vacuum desiccator, in which the samples being tion inside the vacuum desiccator, which was sealed air-
dried were placed, as well as a vacuum pump for the tight, and the desired pressure and radiation was ap-
application of the vacuum (Drouzas & Schubert, 1996; plied.
Kiranoudis, Tsami & Maroulis, 1997). The vacuum The sample was radiated for 10 s, using the full power
system included a pressure regulator and cooling unit of the oven, which corresponds to constant radiation,
for condensing and cooling the water vapor at low then the power was paused for 30 s, resumed again for
temperature ( 25�C). A conventional air drier was also 10 s, paused again for 30 s, and �nally resumed again for
used. 10 s. In this way boiling and bubbling of the gel was
High-moisture samples of pectin gel were used avoided.
(X0 � 4ą7 kg water/kg dry basis), in order to detect any The sample was weighed using a Mettler AE 160
di�erences in the drying rate at the initial stage of drying. electronic balance, it was placed once again into the
desiccator and the same procedure was repeated (eight
2.2. Materials and methods times) until the drying was completed. The total dura-
tion of the radiation was 4 min. When samples were
The material used was a pectin gel (model concen- weighed, the radiation was paused and the vacuum was
trated orange juice) with the following composition in % released inside the desiccator. The amount of the dry
(w/w): solids of the dried sample was determined using the
vacuum oven method (AOAC, 1984). The whole pro-
cedure was repeated three times.
Glucose 14.2
For comparison of the color changes during drying,
Fructose 15.8
Sucrose 27.6
the MW-drying apparatus was operated at atmospheric
Citric acid 1.2
pressure, using the same sample material and the same
Pectin 2.8
MW power.
Water 38.4
Samples of the gel were also dried in a laboratory
tunnel air drier at a temperature of 60�C, relative hu-
100 g
midity 15% and air velocity 4.5 m/s. At frequent inter-
vals (10 min) the sample was weighed in an electronic
Mettler scale for the determination of the drying curve
2.3. Preparation of the pectin gel and comparison of the air drying with the vacuumąMW
drying.
(a) About 1/3 of the sugars was mixed with the pectin The procedure was repeated three times.
and the mixture was added to water at 40ą50�C with The MW energy distribution in the oven was de-
constant stirring to ensure complete homogeneity of termined indirectly at atmospheric pressure, by placing
the solution. samples of the model gel in 5 �xed locations in the
(b) The remaining 2/3 of the sugars was added to the cavity and estimating the drying rate (Fig. 1). The
pH bu�er solution (pH � 3) and boiled until it be- same �xed positions were used in all experimental
came very viscous. work.
A.E. Drouzas et al. / Journal of Food Engineering 39 (1999) 117ą122 119
where (P,P0) and (Q,Q0) are, respectively, the operating
and reference pressure and MW power output. The
empirical constants K0, m and n can be estimated by
nonlinear regression of the experimental drying data
(Kiranoudis et al., 1997).
3. Results and discussion
3.1. MW-energy distribution
A signi�cant variation of the drying rate was ob-
served for samples placed in di�erent locations in the
MW-oven cavity (drying curves of Fig. 2). The di�er-
ences in drying rate were caused by the uneven (multi-
mode) distribution of the electromagnetic energy in the
cavity (Risman et al., 1987; Ohlsson, 1990).
Fig. 1. Location of samples in the MW cavity for estimation of the
MW-energy distribution.
Locations (1) and (4) showed the highest and lowest
drying rates, corresponding to ``hot'' and ``cold'' spots
in the oven. At three locations (2,3,5) the absorbed MW-
2.5. Isotherms and color of product
energy seemed to converge as the drying of the samples
progressed (after about 6 min). These locations were
Samples of dried gel were also used for the determi-
used for placement of the samples in the subsequent
nation of the moisture sorption isotherm at 25�C, using
MW-drying experiments.
a Rotronic-hygroscop BT apparatus attached to a water
In drying applications, the complex modeling of MW
circulator. The dried in MW-oven samples were placed
distribution in the oven can be simpli�ed (Ohlsson,
above water in desiccators and they were humidi�ed to
1990; Kraisheh et al., 1997). The wavelength of MW at
di�erent levels of relative humidity which were then
2450 MHz is 12 cm in the air, which is close to the di-
determined with the use of the apparatus mentioned
mensions of the laboratory MW ovens. The penetration
above. Equilibration of the samples was ensured by
length of MW power at low moisture contents estimated
leaving them in the apparatus for long enough time to
from Lambert's absorption law, is about 30 cm, which is
reach constant weight. Each sample was weighed after
higher than the normal thickness of the food pieces
each measurement and, using the vacuum oven method,
being dehydrated. Thus, attenuation e�ects of foods at
the dry mass of each sample was recorded and the
low moisture content can be neglected.
equilibrium moisture content was calculated.
The absorbed MW energy has been found to increase
The color of pectin gels dried under vacuum and
linearly with the diameter of the food material (Kraisheh
without the use of a vacuum was determined with the
use of the Hunter Lab program and apparatus. Three
replications were made, and the mean of three mea-
surements of each replication is reported.
2.6. Modeling of MW vacuum drying
The drying curves were prepared by plotting the
moisture content X (kg moisture/kg dry matter) vs. time
t (min). Assuming the thin-layer theory of drying, the
drying rate can be expressed by the equation
X Xeą=X0 Xeą � exp Ktą; 1ą
where K is the drying constant and X0, X, Xe are the
moisture contents at the beginning, after time (t) and at
equilibrium. For vacuum drying it can be assumed that
Xe � 0.
The drying constant (K) can be expressed by the
following empirical model
Fig. 2. Drying rates of samples of pectin gel at di�erent locations (1, 2,
K � K0P=P0ąnQ=Q0ąm; 2ą 3, 4, 5) in the MW-oven cavity (atmospheric pressure).
120 A.E. Drouzas et al. / Journal of Food Engineering 39 (1999) 117ą122
et al., 1997). However, the absorbed energy decreased Pretreatment of fruit and vegetable materials with
nonlinearly with the loading of the oven. Thus, the en- MW energy has been found to increase the drying rate
ergy distribution in an MW oven can be made more during the early stages of air drying (Drouzas et al.,
uniform by changing the load pattern in the cavity. 1997). However, the drying rate of sugar-containing
food materials was reduced in the last stages of drying,
3.2. MWąvacuum drying evidently due to the collapse of the porous structure,
created earlier in the drying process. It is evident that
Combination of MW heating and vacuum drying vacuum drying maintains the porous structure through-
resulted in acceleration of the drying rate of model fruit out the drying process, reducing sharply the required
gels. The experimental pectin gel of 38.4 moisture con- drying time.
tent dried to less than 3% moisture within 4 min (Fig. 3).
By comparison, similar samples of pectin gel required 3.3. E�ect of pressure and MW power
more than 8 h to reach a moisture of about 10% in an air
drier at atmospheric pressure and 60�C (Fig. 4). The The optimum operating pressure in vacuum-drying
high sugar content of the gel caused shrinkage and processes depends on the process economics and the
collapse of the gel structure during air drying, resulting quality of the dried products. High vacuum yields nor-
in low transport rate (di�usion) of water and prolonged mally better quality but the equipment and operating
drying time. costs may be too high for most food products. Most
The MW energy and vacuum drying created a very vacuum-drying operations use the pressure range of 30ą
porous structure (pu�ng) of the gel samples, facilitating 50 mbar, in which water evaporates from the liquid
the transport of the water vapor. Evaporation of water phase and product pu�ng takes place. Freeze drying
within the sample is accelerated by the preferential ab- requires pressures lower than 5.33 mbar (evaporation
sorption of microwave energy by the water molecules.
Fig. 3. MWąvacuum drying curve of model pectin gel (P � 40 mbar, Fig. 5. E�ect of pressure (P) and MW power (Q) on the drying rate
Q � 710 W). constant (K).
Fig. 6. Moisture adsorption isotherm of MWąvacuum dried pectin gel
Fig. 4. Atmosphere air-drying curve of model pectin gel at 60�C. (25�C).
A.E. Drouzas et al. / Journal of Food Engineering 39 (1999) 117ą122 121
Table 1
Color parameters of dried pectin gel
L a b
MWąVac MWąAir MWąVac MWąAir MWąVac MWąAir
51.56 23.97 3.51 7.08 14.94 10.60
49.50 21.60 3.84 6.08 15.17 8.40
50.60 21.40 3.20 5.92 14.43 7.20
from the frozen state) and long drying times, increasing is a characteristic color change of high-sugar food ma-
considerably the cost of the process. terials, heated at high temperatures.
The drying rate constant (K), estimated from Eq. (1),
was found to increase signi�cantly as the pressure (P)
was reduced from 50 to 30 mbar (Fig. 5). A signi�cant
4. Conclusions
increase of the drying rate constant was observed when
the MW power output (Q) was increased from 640 to
The drying rate constant of the thin-layer model of
710 W.
drying of a model fruit gel was found to increase with
Regression analysis of the experimental data of dry-
increasing MW-power output and decreasing absolute
ing rate (K) vs. pressure (P) and MW power output (Q),
pressure in vacuum drying. Due to the uneven distri-
using Eq. (2), yielded the following empirical constants:
bution of the MW energy in the MW oven the location
m � 0.698, n � 0.318 and K0 � 0.857 1/min, for ref-
of the material in the cavity should be speci�ed. The
erence P0 � 40 mbar and Q0 � 710 W.
color of the MWąvacuum dried fruit gel was signi�-
cantly lighter than the color of the MWąair dried
3.4. Moisture sorption isotherms
product at atmospheric pressure.
Fig. 6 shows a typical moisture adsorption isotherm
at 25�C of a sample of vacuum-dried pectin gel. The
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