Microwave Application in Vacuum Drying of Fruits (Drouzaf, H. SchuberP).pdf

Joumul of Food Engineeting 28 (1996) 203-20’)

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Research Note

Microwave Application in Vacuum Drying of Fruits

A. E. Drouzaf & H. SchuberP”

” Department of Chemical Engineering, National Technical University, CR 15780. Athens,

Greece

” Institute of Food Process Engineering, University of Karlsruhe, Karlsruhe. Germany

(Received 5 February 1995; accepted 18 June 1995)

ABSTRACT

Microwave vacuum dying of banana slices was investigated experimentally.

This type of dying procedure is preferable to conventional dying techniques in

order to avoid product degradation due to high temperatures encountered in

convective dying. The dying process was examined by introducing pulse-

generated microwave power in banana samples. The material temperature was

monitored. Temperature peaks in the last stages of dying indicated that dying

could be favoured if temperature was maintained below a maximum level, so

that the final product should not be burned by hot spots during microwave

dying, This procedure produced dehydrated products of excellent quality as

examined by taste, aroma, smell and rehydration tests. Copyright 0 1996

Elsevier Science Limited.

INTRODUCTION

Dehydration operations have been used for decades in chemical and food processing

industries for efficient long-term preservation of final products. The basic objective

in drying food products is the removal of water from solids to a level at which

microbial spoilage is avoided. A major disadvantage concerning conventional drying

operations is the thermal degradation of important Aavour and nutritional

substances. In order to prevent significant loss of these substances, vacuum drying is

successfully used instead. In this case, the removal of moisture is accelerated and,

“Author to whom correspondence should be addressed.

203

204

A. E. Drouzas, H. Schubert

futhermore, heat transfer to the solid phase is slowed down significantly due to the

absence of convection. Microwave (MW) technologies for rapid heat transfer during

drying and other food processing have recently found important applications

(Rosenberg & Boegl, 1984, 1987; Giese, 1992). Vacuum microwave drying of foods

acts, in many cases, as a complementary method to the conventional ones (Attiyate,

1979; Kock, 1989). However, a major disadvantage is the high cost of energy which

is utilized in such applications. This comes as a consequence of the low efficiency of

Magnetron devices (typically 50%) and the scattering of radiated energy to the

product (Schubert et aZ., 1991).

This high cost of energy dictates that microwave vacuum techniques could be used

only in cases where drying of final products has to meet high quality specifications

or as a supplementary drying method for further product quality improvement.

In this paper, the vacuum microwave drying of bananas is studied and both the

experimental method and the results are presented.

PRINCIPLES

Microwaves cover the field of the electromagnetic spectrum ranging from O-3 to 300

GHz, corresponding to wavelengths ranging from 1 mm to 1 m (between radio

waves and infrared radiation). In most European countries the allowable frequency

for the use of microwaves is 2.45 GHz, corresponding to 12.2 cm wavelength in

vacuum. For industrial purposes and under special authorization the frequency of

915 MHz is allowed.

As far as microwave heating or drying of foods is concerned (frequencies around

2456 MHz) it is essential to point out the following properties of microwaves

(Meinke, 1986).

Microwaves are reflected by conductors (metals)

Microwaves are capable of polarizing the material molecules that absorb them

(change of dipole orientation within them).

Microwaves can penetrate glass and polymers without significant energy loss.

A prerequisite for the absorption of microwave energy by the material is the

existence of substances containing dipoles or dipolic regions (polar centres)

(Grueneberg et al., 1992). The heat produced in the product is directly transferred

by means of conduction mechanisms throughout the entire mass of the radiated

material. This phenomenon greatly reduces the time required for complete drying

by more than 30% when compared to conventional methods (Attiyate, 1979; Kock,

1989) and leads to a substantial improvement of the final product quality.

EXPERIMENTAL

PROCEDURE

A domestic microwave oven (Sharp 5VlZW) was used for the purposes of the

present investigation and is depicted in Fig. 1. A glass vessel containing the material

to be dried was put into the interior of the oven. Vacuum conditions were

maintained by means of a vacuum pump. Temperature in the bulk of the material

(banana) slices and in the interior of the oven was monitored by means of optical

thermosensors (ASEA-FT 1000) connected through an A/D converter card to a PC.

Microwave application in vacuum drying of fruits

205

1. Microwave oven

2. Banana sample

3. Optical thetmo-sensors

4. Vacuum pump

5. Computer

Fig. 1. Experimental apparatus.

Optical thermosensors were used instead of conventional thermocouples because

the latter absorb microwave energy and produce erroneous temperature indications

(Schubert et al., 1991). Pressure was maintained at a desirable level by means of a

pressure controller. The Magnetron power was kept constant for a period of 10 s,

followed by a pause of 20 s. The procedure repeated several times. All experiments

were carried out in the Institute of Food Process Engineering, University of

Karlsruhe, Germany.

DISCUSSION

The material moisture content deviation for several experiments under different

pressure levels and for the same Magentron power is presented in Fig. 2. No

significant variation was observed as far as the drying rate under different pressure

levels is concerned. However, the final product quality varied significantly. The worst

results were obtained for high values of pressure. Experiments under different

Magnetron levels were also carried out. The results are presented in Fig. 3. As

expected, an increase of power increases the drying rate. In this case, the product

quality deteriorates as Magnetron power level is increased. In this case, high power

levels overheated and sometimes burned the final product. Therefore, the optimum

material temperature should be maintained during microwave drying. The quality of

the final product varies with temperature, pressure and radiation levels and thus

special care should be taken in order to optimize the entire process.

The variation in material and oven temperature is presented in Fig. 4. In this

figure, temperature peaks represent the active Magnetron periods, followed by the

206

A. E. Drouzas, H. Schubert

¡ Embar

¦ 25mbar

h 2.:-

o 5Omb.w

A 2OOmbar

r 8

1.5 -

A 3OOmbar

fi A

0.5-

280 w Power ¦ B

‘g

Time (mid

Fig. 2. Evolution of material moisture content for experiments under different pressure

levels and for Magnetron power level of 280 W.

radiation pauses during the experimental procedure, as previously described. The

drying curve is also included in the same figure. From this, it can easily be seen that

at the last temperature peak the material moisture content has been reduced to

40% of its initial value. At this point the material is largely dried and its specific

heat, involved in the conductive heat transfer, is decreased sharply, and therefore

the material temperature takes a maximum value. The temperature peaks were

observed at different time periods during each individual experiment and the

temperature value measured was always greater than the boiling point of water at

the corresponding pressure conditions (see Fig. 5). An explanation for the

temperature rise within these profiles is the elevation of the boiling point due to

soluble substances in the dried regions. Futhermore, during these experiments

temperature was measured in the geometrical

centre of the material studied. Due to

-15ow

-n-280

-850

30 mbar Pressure

z” 0

Time @in)

Fig. 3. Evolution of material moisture content for experiments under different Magnetron

power level and pressure level of 30 mbar.

lOOmbar

Microwave application in vacuum drying of fruits

l-40

120

0.8

100

2 80

g60

0.6

g 40

0.4

0.2

loo

200

300

400

So0

Time (s)

Fig. 4. Evolution of material and oven temperature monitored in each experiment

f 4o

Time 0)

Fig. 5. Temperature profiles for optimum drying.

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